Galling: The Hidden Enemy of Heat Exchangers
Heat exchanger maintenance often comes with a frustrating challenge: bolts and nuts that seize up during disassembly. This seizure is usually caused by a phenomenon called thread galling – essentially a form of “cold welding” between the threads.
Why Heat Exchangers Experience Bolt and Nut Galling (Seized Fasteners) and How to Prevent It
Heat exchanger maintenance often comes with a frustrating challenge: bolts and nuts that seize up during disassembly. This seizure is usually caused by a phenomenon called thread galling – essentially a form of “cold welding” between the threads. Galling is extremely common on bolted joints and is actually the primary culprit behind most “seized” fasteners in industry. When galling strikes on a heat exchanger’s bolts, it can lead to costly downtime, dangerous removal methods, and damaged hardware. In this post, we’ll deep-dive into why heat exchangers are prone to bolt and nut galling, and what can be done to prevent this problem (including a look at new solutions like the Velocity Washer, which has become a popular fix on heat exchanger applications). Maintenance engineers and plant operators will learn the causes of galling under cyclical heat and high pressure, and practical strategies to avoid seized fasteners in the future.
Understanding Bolt Galling and Seized Fasteners
Thread galling is a form of adhesive wear where metal surfaces in contact begin sticking together and literally fuse. In bolted connections, galling happens when the male and female threads slide against each other under heavy pressure, causing microscale welding and tearing of the metal surfaces. The result is that the nut and bolt bind together as if they were one piece – hence the term “seized” fastener. Galling often occurs suddenly during tightening or loosening, as friction builds heat and causes material transfer between the threads.
Certain metals are especially prone to galling. Stainless steel fasteners, for example, form a protective oxide film that can break down under the high pressure of tightening; the exposed metal then friction-welds to the mating surface, seizing the threads in place. Other alloys like aluminum and titanium are also notorious for galling, particularly when paired with similar materials. In general, softer, ductile metals gall more easily, but even harder, heat-treated materials, such as 4140 can gall on many heat exchanger and reactor applications. Galling is further aggravated by lack of lubrication – without a lubricant to serve as a barrier, metal-to-metal contact generates much more friction and heat, accelerating the adhesive sticking process.
In short, galling = adhesion + friction under load. When a bolt galls, its threads can lock up and refuse to turn (or worse, shear off if force is applied). What started as routine disassembly can quickly turn into a nightmare of frozen nuts and bolts. Heat exchanger crews often discover this the hard way: a bolt that “won’t budge at all” due to galling has effectively cold-welded itself, and no amount of torque will loosen it. The next sections explain why heat exchangers provide a perfect storm of conditions for galling to occur.
Why Heat Exchangers Are Prone to Galling
Several factors in the operating environment of heat exchangers make thread galling more likely on their bolting:
High Temperatures: Heat exchangers frequently operate at elevated temperatures (hundreds of degrees Fahrenheit). At these temperatures, metal surfaces are more reactive and any protective coatings or oxides on the fasteners can break down. In fact, studies show galling risk roughly doubles once temperatures exceed about 350 °F, and above 800 °F the tendency to gall can increase up to five-fold as oxide layers rapidly degrade. The metal threads also weaken and deform more easily when hot, making it easier for them to smear and stick together.
Thermal Cycling: Heat exchangers undergo repeated heating and cooling cycles during start-ups, shutdowns, and normal operation. This cyclical thermal expansion and contraction causes the bolts and nuts to expand, contract, and rub against each other microscopically over time. Thermal cycling has been found to increase galling propensity by about 50% compared to steady-state conditions. Essentially, every heat-up and cool-down is an opportunity for threads to gall a little more, especially if the fastener lubricant gets burned off or if there’s slight movement at the joint.
High Pressure & Heavy Loads: Heat exchangers (especially in petrochemical and power applications) often operate under high internal pressures, meaning their flange bolts are torqued to very high tension to maintain a seal. The heavy clamping force creates extremely high contact stress between the male and female threads. If an attempt is made to turn a nut while that load is still present, galling can occur almost immediately. In fact, galling is known to occur when surface pressures exceed roughly 30,000 psi on the threads – a condition easily met by large bolted flanges at high pressure. The high preload essentially squeezes the thread surfaces into each other, promoting adhesion if there’s any relative movement.
Material Selection: The bolting material for exchangers is sometimes a galling-prone alloy. Standard carbon steel studs (like B7 grade) are less prone to galling due to their hardness, but many exchangers (especially in corrosive or high-temperature service) use stainless steel or nickel-based alloy fasteners for their corrosion and heat resistance. Unfortunately, austenitic stainless steels (e.g. 304, 316) have a very high galling susceptibility. Using identical materials for nut and bolt (e.g. stainless-on-stainless) can make galling even more likely, because the similar hardness and metallurgy encourages adhesion. Heat exchanger bolts that are not zinc plated or coated in some way also have metal-on-metal contact which can encourage seizing.
Frequent Maintenance and Reuse: Heat exchangers typically require periodic opening for cleaning, inspection, or gasket replacement. This means repeated assembly and disassembly of the same bolts. Each time a bolt is re-tightened and loosened, especially if done in less-than-ideal conditions, the wear and tear on the threads accumulates. Any small galling that occurred during a previous maintenance can worsen the next time. Over multiple cycles, threads become ever more prone to seize. Additionally, fasteners may not be perfectly clean or may have slight damage from prior use, increasing friction. The more a particular stud/nut pair has been through heat and pressure cycles, the higher the chance that the next removal attempt ends with a seized nut.
Harsh Operating Conditions: Beyond heat and pressure, exchangers might be subject to vibration (from nearby equipment or fluid flow) and environment factors. Vibration can cause microscopic movement between threads under load, contributing to galling as well. Likewise, high humidity or corrosive chemicals can degrade lubricants and lead to corrosion products on threads, effectively removing lubrication and adding roughness. All these conditions – high temperature, high pressure, cyclic stresses, and more – combine to make heat exchanger bolting a prime candidate for galling issues. It’s no coincidence that fasteners in high-temperature, high-pressure applications are far more likely to gall and seize.
Consequences of Galling: Seized Bolts, Downtime, and Damage
When a heat exchanger’s bolts gall and seize, the consequences for maintenance can be significant. First and foremost is the lost production time. A bolted exchanger cover that should come off in an hour might turn into a multi-hour (or multi-day) ordeal if multiple nuts seize on the studs. Mechanics often describe a galled nut as feeling like it “locked up solid” after only a quarter-turn, essentially freezing in place. At that point, standard wrenches or hydraulic torque tools often can’t budge it – or risk twisting the stud off entirely.
Common outcomes of severe galling include: damaged threads, broken fasteners, weakened joint integrity, and extremely difficult removal. In a heat exchanger, a single seized bolt can prevent the opening of the channel or bonnet, delaying the whole maintenance schedule. Crews may resort to dangerous removal techniques such as cutting the nut or bolt with a torch or grinder. This “hot work” not only adds time (setting up fire safety, waiting for parts to cool, etc.) but also introduces hazards – open flame in a petrochemical unit or near insulation and gaskets is a recipe for potential fires or injuries. As one industry article noted, galled fasteners on a reactor flange forced technicians to torch-cut the nuts, adding 4–5 hours to the procedure and requiring an open flame in a confined area.
There’s also the cost of replacing hardware. A galled stud/nut pair is usually unusable after removal (threads may be torn or fused). If you have to cut it off, obviously you’ll need new bolts and nuts of the correct specification on hand. If a stud breaks off in a threaded hole (in exchangers with tapped holes), extraction becomes even more complex and can risk damaging the exchanger itself. All of this means longer downtime and higher costs. Galling-related delays can be very expensive in lost production – it’s not uncommon for refinery or plant maintenance managers to cite seized bolts as a major schedule risk during turnarounds.
In summary, galling in heat exchanger bolts isn’t just a minor annoyance; it’s a serious roadblock to maintenance efficiency and safety. It leads to seized (stuck) bolts that require extra labor and time to remove, safety risks from using brute force or hot cutting methods, and potential damage to equipment and fasteners. Plant personnel often dread discovering a galled bolt, because they know it can derail an otherwise well-planned outage. Preventing galling in the first place is therefore critical – and fortunately, there are ways to do so.
How to Prevent Galling on Heat Exchanger Bolts
Preventing thread galling (and thereby avoiding seized nuts and bolts) involves both proper procedures and sometimes special hardware or coatings. Here are several best practices to minimize galling on heat exchanger fasteners:
Use Proper Thread Lubrication: Applying a high-quality anti-seize lubricant to bolt threads and nut faces before assembly is one of the most effective ways to prevent galling. Lubricants create a film that reduces direct metal-on-metal contact and friction. For high-temperature exchanger service, choose lubricants designed for heat (for example, molybdenum disulfide or graphite-based anti-seize, high-temp nickel anti-seize, etc.). These can withstand the heat cycle better and continue providing lubrication. A small amount of lubricant on clean threads can greatly reduce the chances of adhesion. (Be sure to follow the torque adjustment guidelines when using lubricated bolts, as lubricants will change the torque-tension relationship.)
Slow Down the Fastening Speed: Galling is aggravated by fasteners being tightened or removed at high speed. Spinning a nut quickly with an impact gun generates excessive friction heat in the threads. Instead, torque down nuts slowly and evenly, especially on large heat exchanger studs. During removal, avoid rapid unthreading under load – use controlled, smooth turns. The slower the relative motion, the less heat and the lower the galling tendency. In practice, this may mean using hand torque wrenches or low-speed pneumatic tools instead of high-speed tools on final breakout or makeup.
Use Hardened or Coated Fasteners/Parts: Whenever possible, mix the material pairing to avoid similar metals in full contact. For example, pairing a stainless steel stud with a coated heavy-hex nut (or a nut of a slightly different alloy like Nitronic 60) can reduce galling tendency. Using nuts or washers with special anti-galling coatings (PTFE, moly, zinc plating, etc.) is also beneficial. These coatings not only add lubricity but often serve as sacrificial layers to prevent metal adhesion. In general, a harder surface against a softer one is less likely to gall than two soft, similar metals together. If standard B7 carbon steel studs can be used instead of stainless in a given exchanger application, they are less prone to galling due to higher hardness – though corrosion considerations must be accounted for. Always maintain the needed strength and corrosion resistance, but keep galling in mind during fastener selection.
Choose Coarse Threads and High-Quality Threads: Fasteners with coarse threads are less likely to gall than fine-threaded ones in galling-prone materials. Coarse threads have a bit more clearance and less total contact surface area, which helps reduce friction. If you have a choice (for custom studs or replacement bolts), opt for coarse thread series for exchanger applications. Also, prefer rolled threads over cut threads – rolled threads have smoother surfaces, whereas cut or machined threads are rougher and can create more friction hotspots during tightening. High-quality, smoothly formed threads will always perform better in terms of galling resistance.
Maintain Clean, Damage-Free Threads: Dirt, debris, or nicks on threads greatly increase friction and the chance of galling. Always clean bolts and nuts before reuse, and inspect for any galling on the threads from previous use. If a stud has damaged threads or prior galling damage, it’s wise to replace it before it causes a seizure. During installation, ensure nuts spin freely by hand for a few turns – this confirms there’s no cross-threading or burrs. Following proper torque sequences (such as those in ASME PCC-1 for flanges) also helps by incrementally loading bolts rather than one getting fully loaded (and potentially galled) at once.
Avoid Over-Tightening and Lock Nuts: Stick to the specified torque or bolt stress values. Over-torquing can not only stretch and weaken the fastener, but also create excessive friction that invites galling. Similarly, be cautious with prevailing-torque lock nuts (e.g. stainless steel lock nuts or nylon insert lock nuts on stainless bolts) – these inherently add friction by design and thus are much more likely to cause galling if not well-lubricated and turned slowly. If galling is a big concern, it may be better to avoid these locking nut types on heat exchanger service and use other locking methods that don’t introduce thread friction.
Consider Tensioning Methods (No Rotation): One fundamental way to eliminate thread galling is to avoid turning the nut under load altogether. Hydraulic bolt tensioners, for instance, stretch the stud axially so the nut can be run down with very low friction, and then the tension is released – this method tightens bolts without the high-friction twisting of threads. Similarly, for loosening, tensioners can relieve the load before unthreading the nut. These techniques, while requiring specialized tools, prevent the rubbing under pressure that causes galling. In some cases, using tensioning instead of torque tightening on heat exchanger bolts has successfully averted galling problems. (As we’ll see next, there are also innovative washer devices that achieve a similar effect by mechanically removing the load during breakout.)
By implementing the measures above – lubrication, controlled tightening, proper fastener materials, and so on – plants have significantly reduced galling issues. However, even with best practices, the extreme conditions around heat exchangers can still defeat traditional methods (for example, lubricants might burn off over time at 800°F, or a rush during shutdown might lead someone to zip a nut off too fast). In recent years, a new solution has emerged specifically to tackle stubborn galling in bolted joints: the Velocity Washer. This device is proving to be a game-changer for heat exchanger maintenance by guaranteeing no galling and no seized nuts, even under cyclical heat and high pressure.
Velocity Washer: Eliminating Galling on Heat Exchangers
One of the most promising technologies to solve galling is the Velocity Washer, a patented mechanical washer that allows nuts to be loosened without any thread friction under load. The Velocity Washer is installed just like a regular washer under the nut, but it contains a clever internal design (two stacked, crenelated pieces) that comes into action during bolt breakout. When you are ready to loosen the nut, you simply turn it about 12 degrees counter-clockwise – just a tiny fraction of a turn – and the Velocity Washer “pops” to collapse its stack, instantly removing all clamping load from the bolt. In essence, the washer absorbs the bolt stretch, so the nut is no longer squeezed against the threads. At that point, the nut can be spun off freely by hand or with a normal wrench, with zero risk of galling since there is no significant pressure between the thread surfaces as you turn it.
By taking the pressure off the threads before turning the nut, the Velocity Washer completely eliminates the galling mechanism. There’s no more metal adhesion or tearing, because the two surfaces are not forcefully pressed together during rotation. This technique has proven remarkably effective – field installations have reported a 100% success rate in preventing galling on bolted joints using Velocity Washers. Essentially, no matter how high the temperature or pressure the exchanger saw in service, the nuts come off without seizing because the washer ensures the first turn breaks the tension and after that it’s a low-friction removal.
For maintenance engineers, the implications are huge: no more torch cutting seized nuts, no more crew hours wasted on one stubborn bolt, and no more damaged studs. Breakout times are dramatically improved. One case study in a petrochemical plant saw the removal time for a reactor flange (similar bolting to a heat exchanger channel) drop from about 5 hours (with many galled nuts) down to just over 1 hour when Velocity Washers were used. All galling was eliminated, and since no hot torches were needed, the job was also much safer. In another example, a refinery that installed Velocity Washers on large process exchanger channel-head bolts gained an estimated 112 extra hours of uptime because they no longer experienced galling-related delays during turnarounds. These are significant time savings in an industry where every hour of downtime can cost tens or even hundreds of thousands of dollars.
Heat exchangers have become one of the top applications for Velocity Washers due to the severe galling issues common on their flanges. A lead reliability engineer from one plant noted they installed Velocity Washers on two troublesome blowdown exchangers, where historically they would have to torch-cut large seized studs every time. With the new washers, “the studs spun off just like shown in the promotional videos,” and they were even able to re-use the studs and nuts instead of scrapping them. This kind of success has led many sites to adopt Velocity Washers on heat exchanger channel covers, reactor feed/effluent exchangers, and other high-temperature flange joints. The improved safety (no more “hot bolting” with live torches, no pinched fingers from wrench struggles) and the reduced labor are immediate benefits. In fact, by eliminating the galling roadblock, plants can disassemble equipment on schedule every time, with far less uncertainty – a major win for outage planning.
It’s worth noting that Velocity Washers do not require special tools or changes to bolting procedures. They are designed to fit wherever a standard hardened washer would go, and standard torquing methods are used for tightening the nuts. The magic is only in the removal: that small 12° turn to activate the washer’s load release. Because of their high load capacity (made of alloy steel often stronger than the stud itself) and temperature capability (standard units up to ~850 °F, with higher-temp alloys available), these washers are well-suited to the demands of heat exchanger service. Many companies in refining and chemicals have now deployed Velocity Washers on critical exchangers, reactors, and high-pressure flanges to ensure galling doesn’t cause unplanned hiccups. In other words, this simple hardware change – “just by changing your washer” as the manufacturer says – can solve a decades-old maintenance headache.
Conclusion
Bolt and nut galling is a technical issue with very real practical consequences for anyone maintaining heat exchangers. We’ve seen that the combination of high heat, high pressure, and repeated cycling in these units creates a perfect environment for threads to seize up (cold-weld) due to galling. Once a fastener galls, it can halt maintenance progress, introduce safety risks, and drive up costs. The good news is that by understanding the causes, plant engineers can take proactive steps to avoid galling in the first place. Ensuring proper lubrication, choosing the right fastener materials and thread forms, and using correct assembly techniques will go a long way toward preventing most galling incidents. For the most challenging scenarios, advanced solutions like the Velocity Washer now exist to completely eliminate galling and seized nuts on critical exchanger flanges. This technology has already been embraced as a top application in refineries and petrochemical plants to make heat exchanger maintenance faster, safer, and more predictable.
In essence, heat exchangers experience bolt galling because of the severe service they endure – but we no longer have to simply accept seized bolts as a fact of life. By implementing the preventative measures discussed and leveraging new tools when appropriate, maintenance teams can ensure that the next time an exchanger is opened up, the nuts will come off smoothly without a fight. No more cursing at “welded-on” nuts or reaching for the cutting torch – with the right approach, galling and seizing can be kept out of the heat exchanger maintenance equation. The result is less downtime, lower costs, and a safer work environment, which is a win for everyone involved in keeping the plant running efficiently.
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References:
Sild, Siim. “Galling – What Is It, How It Works & Prevention.” Fractory Engineering Blog, 31 Oct. 2023.
Chemical Engineering Magazine – Psimas, L., Britton, R., & Bakshi, S. “Critical Flanges: Advanced Washers Eliminate Galling and Hot Work,” Nov. 1, 2022.
Spex. “Everything You Should Know About Thread Galling.” Spex Industries Blog, 2023.
Atlantic Fasteners. “How to Eliminate Thread Galling – 7 Common Solutions.” Tech Tips, Atlantic Fasteners Co., 2021.
Nord-Lock Group (Japan). “Causes and Solutions for Threaded Fastening Troubles: Galling (Seizing).” Mono.Ipros.com, Jul. 29, 2021.
Velocity Bolting. “Velocity Washer Prevents Galling – Case Study.” VelocityBolting.com, 2023.
Disclaimer:
Portions of this article were generated with the assistance of ChatGPT, a large language model developed by OpenAI. The content is provided for informational purposes only and does not constitute professional, legal, financial, or academic advice. The views expressed do not necessarily reflect those of the author, and readers are encouraged to independently verify any information presented.
The AI-generated content has been reviewed and edited for clarity and accuracy where appropriate.
Why Galling Occurs on Threaded Fasteners
Thread galling is a frustrating problem for engineers working with bolted joints. If you’ve ever had a nut suddenly seize on a bolt during tightening, you’ve witnessed galling. In severe cases, the nut and bolt are effectively “cold welded” together – removal becomes impossible without cutting the fastener off. Such seized fasteners can lead to costly downtime, as equipment must be disassembled with torches or splitters, causing schedule delays and lost production. This article provides an engineering overview of why galling occurs on threaded fasteners, how the galling process works, and – most importantly – how to prevent galling (with the Velocity Washer emerging as the number one solution).
What is Thread Galling?
Thread galling is a form of adhesive wear that occurs when two metal surfaces in sliding contact effectively stick together due to friction and material transfer. In the context of fasteners, galling refers to the seizing or locking of mating threads (bolt and nut) during installation or removal. It is often compared to a cold-welding process: as the bolt and nut are tightened under high pressure, portions of the metal surfaces shear off and fuse together, bonding the threads. Once a fastener has seized from galling, it can no longer turn and usually cannot be removed without destroying the bolt or nut (e.g. by cutting the bolt or splitting the nut).
In less severe cases of galling, the threads may only be mildly damaged (light scoring) and the assembler might notice increased resistance – if tightening is stopped early, the fastener can sometimes be backed off. However, if galling continues, it will strip or weld the threads completely. The end result is often a ruined bolt and nut that must be scrapped - but that is the least of the concerns. It leads to standby time, safety issues, and lost production as well. Galling is commonly observed in stainless steel fasteners (it’s sometimes even called “stainless steel seizing”), but it can affect other materials like steel, titanium and aluminum alloys as well.
Why Does Galling Occur on Threaded Fasteners?
Galling doesn’t happen under normal low-stress contact – it requires a specific combination of conditions that frequently occur in bolted joints. The process is driven by adhesion and friction between the mating threads under high load. Here’s a closer look at why threaded fasteners are prone to galling:
High Contact Pressure and Friction: When a nut is tightened on a bolt, the flanks of the male and female threads are pressed together with tremendous force (especially as the bolt reaches high preload). The surface contact occurs primarily at microscopic high points (asperities) on the metal threads. As the nut turns, these asperities slide against each other under pressure, producing friction. If friction is high, significant heat can be generated in the thread interface. The combination of high pressure and heat causes local adhesion – the asperities can begin to shear and lock together, rather than sliding smoothly. In essence, small bits of one thread weld onto the other. This creates even more friction and a cascading effect: the longer you continue turning under these conditions, the more material transfers and binds, until the threads seize completely.
Removal of Protective Oxide Films: Many metals used in fasteners (notably stainless steels, as well as aluminum and titanium) are protected by a thin oxide layer on their surface. This oxide film normally prevents direct metal-to-metal contact and also reduces friction in the early stages of tightening. However, under high pressure and sliding, the oxide coating can be scraped off at the thread peaks. Once bare, reactive metal is exposed on both the bolt and nut thread surfaces. Exposed fresh metal has a strong tendency to adhere or “grab” the mating surface. In the case of stainless steel, which is a relatively soft and ductile metal, this adhesion quickly leads to galling once the oxide barrier is gone. The metal atoms from one side actually jump to the other side (material transfer), especially as frictional heating softens the material, leading to that galling “lock-up” phenomenon.
Material Properties (Soft, Ductile Metals): The materials of the fastener play a big role in galling propensity. Austenitic stainless steels (like 304 or 316 stainless bolts and nuts) are notorious for galling because they are ductile and work-hardened surfaces can readily bond. They also self-generate oxide layers (as mentioned above) which, once damaged, expose bare metal that likes to stick. Titanium and aluminum fasteners show similar behavior for the same reasons. However, galling can occur on any fastener, including standard low steels, after they have been exposed to things like cyclic heat, which can bake off the flow lubricants and prime them for galling during disassembly.
Thread Design and Condition: The geometry and condition of the threads influence galling. Fine threads (UNF) have more threads per inch and shallower thread depth than coarse threads (UNC). This means more surface contact area and tighter clearances, which can increase friction and make fine-threaded fasteners more susceptible to galling. Additionally, any thread damage or roughness will act like sandpaper and raise the friction between threads. Burrs, nicks, or dirt on thread surfaces create high friction spots that can initiate galling. For this reason, standard practice is to ensure threads are clean and undamaged before tightening. A smoothly rolled thread (as opposed to a cut thread with rough edges) will have fewer asperities to cause trouble. Proper thread fit is also important: threads that are too tight (class 3 fit or poorly toleranced) can have excessive friction. In fact, overtightening a fastener beyond its yield can gall the threads because the threads deform and dig into each other.
Lack of Lubrication: “Dry” assembly of bolts and nuts (with no lubrication) significantly increases the chances of galling. Without lubrication, the coefficient of friction between metal threads is high, leading to more heat and adhesion. Threads that are well-lubricated (with anti-seize compounds, for example) have a much lower tendency to gall because the lubricant separates the surfaces and dissipates heat. Industry standards such as ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly) strongly emphasize proper thread lubrication during assembly to control friction and achieve correct preload. We’ll discuss lubrication more in the prevention section, but it’s worth noting here: a lack of lubricant is a major contributing factor to galling. However, the reality is you can do everything “perfectly” during assembly, including lubricants, but after being in service for months (or years) the threaded fastener is already primed for galling, as the flow lubricants may no longer be present.
Fastener Usage Factors: Certain assembly practices can inadvertently promote galling. One common mistake is using the bolt to “pull” misaligned joints together. If two flange plates aren’t snug and you force them closed by tightening the bolts, you put huge side loads and extra friction on the threads, almost guaranteeing galling. High-speed tightening is another culprit – using an impact gun or spinning a nut down at high RPM generates rapid frictional heating. The faster the installation, the less time there is for heat to dissipate, so temperatures at the thread interface spike, making galling more likely. This is why many manufacturers recommend against using power tools on stainless steel fasteners. Additionally, be cautious with prevailing-torque lock nuts (like nylon insert locknuts or all-metal locknuts). These nuts intentionally add friction to prevent loosening, but that added friction means higher risk of galling during installation. In fact, galling problems are most frequently observed with stainless steel lock nuts. We’ve read a piece from Fastenal’s engineering team that notes nylon insert in stainless locknuts creates so much friction that it significantly increases galling incidence, so they often wax-coat these nuts to reduce friction.
How to Prevent Galling on Threaded Fasteners
Galling may be common – but it’s not inevitable. By understanding the causes, engineers can take preventative measures at both the design stage and during installation to avoid galling. In general, anything that reduces friction, heat, or metal-to-metal adhesion in the joint will help. Below are several proven strategies for mitigating thread galling (with the Velocity Washer as the top recommendation):
Use an Anti-Galling Washer (Velocity Washer): The most effective way to prevent galling is to eliminate the conditions that cause it in the first place – namely, the combination of high pressure and friction while turning the nut. The Velocity Washer is a patented solution designed to do exactly this. It’s a special washer that goes under the nut and temporarily relieves the compressive load on the threads when you start to loosen the fastener. In practice, the Velocity Washer “pops” to break the static friction, allowing the nut to spin off freely without galling. By removing thread pressure during disassembly, it prevents the adhesive wear mechanism from ever getting started. This technology has been deployed on critical bolted flanges and has achieved a 100% success rate in preventing galling across all global installations. In other words, no more seized nuts and no more torch cutting during disassembly. For engineers dealing with chronic galling problems, simply switching to Velocity Washers on the studs can be a game-changer in ensuring bolts come apart smoothly.
Apply Proper Lubrication: Lubricate, lubricate, lubricate – this cannot be overstated as a galling prevention measure. A high-quality thread lubricant or anti-seize compound is one of the simplest and most effective ways to reduce galling risk. Lubricants (such as molybdenum disulfide grease, copper nickel anti-seize, PTFE-based paste, etc.) create a film between the male and female threads that reduces friction by lowering the coefficient of friction and preventing direct metal contact. This in turn keeps the heat down and stops micro-welding. ASME PCC-1 bolting guidelines specifically recommend applying lubricant to both the threads and the nut bearing surface for any critical bolted joint – doing so not only ensures you achieve the correct preload at the specified torque, but also greatly lowers the chance of galling in the process. (Be aware that using lubrication will change the torque-tension relationship: a lubricated fastener will require less torque to achieve the same tension. Adjust your tightening specifications or use the manufacturer’s K-factor recommendations when lubrication is applied. Note: your k-factors will stay the same if you choose Velocity Washer technology, as it is designed to replicate the same friction as a standard hardened washer.)
Slow Down the Installation Speed: Avoid high-speed tightening, especially with galling-prone materials. Frictional heat is a major contributor to galling, so by slowing down the turning of the nut, you allow heat to dissipate and prevent localized hot spots. In practice, this means do not use impact drivers or air wrenches on stainless steel or titanium fasteners if you can help it. Instead, tighten by hand or with a controlled-speed tool, particularly for the final snugging. If you must use power tools to run the nut down, finish the last turns slowly and watch for any binding. This is especially important when using lock nuts or thread locking inserts, which inherently create more friction as they engage. By tightening at a deliberate pace (and even pausing if you feel resistance), you’ll greatly reduce the chance of galling due to heat buildup.
Ensure Clean, Damage-Free Threads: A surprisingly common cause of galling is simply dirty or damaged threads. Always inspect your bolts and nuts before assembly – look for dirt, corrosion, or dings on the threads. Debris or rust can dramatically increase friction and lead to galling. Clean the fasteners with a wire brush or solvent if needed, and chase any deformed threads with a die or tap. If a nut does not run down easily by hand on a bolt, do not force it; that resistance means something is wrong (burrs or misthreading) which could induce galling if you apply high torque. Using new nuts on new bolts is ideal for important joints, as reused fasteners may have picked up damage. In critical applications, some engineers even specify rolled threads for bolts (instead of cut threads) because rolled threads have a smoother surface finish and harden the surface, which helps to minimize friction and galling propensity.
Optimize Fastener Material and Design: If galling is a concern, make smart choices in fastener selection. Material matters – for example, pairing a stainless steel bolt with a hardened, plated steel nut can avoid galling (the hard zinc-plated nut is much less likely to seize on the stainless bolt). In bolted joints that must use stainless steel for corrosion resistance, consider using different grades of stainless for the bolt and nut. Galling is worst when the two mating metals are the same type. Using dissimilar alloys (e.g., a 316 stainless bolt with a 410 stainless or Nitronic 60 nut) can reduce the tendency to gall – though be mindful of any strength or corrosion trade-offs. Thread pitch and fit are another design consideration: coarse threads (e.g., UNC) are more forgiving than fine threads in galling-prone situations. A looser fit (e.g., 2A/2B class) gives a bit more clearance for lubricant and less rubbing of flanks, whereas tight fine threads maximize contact and friction. If your application permits, opt for coarse threads and avoid overly snug fits when using materials like stainless steel. Finally, coatings and surface treatments can be a savior. Applying a low-friction coating or plating to fasteners can dramatically improve galling. Common solutions include PTFE or Xylan® coatings, silver plating (often used on aerospace fasteners to prevent galling), or even a simple wax finish on stainless nuts. These provide a lubricious surface and barrier that keeps the metal from binding. The bottom line: by selecting the right nut/bolt material combination and thread parameters, you can greatly mitigate galling before the wrench even comes out.
In summary, thread galling occurs when the combination of high pressure, friction, and adhesion causes two fastener components to fuse together. It’s a problem most often seen with stainless steel or similar alloy fasteners under heavy load. Fortunately, engineers have a toolbox of solutions to combat galling – from using proper lubrication and installation techniques to choosing better materials and thread designs. The Velocity Washer, in particular, stands out as an innovative solution that tackles galling at its root by removing the harmful friction under load. By implementing these strategies, you can ensure your threaded fasteners tighten smoothly and come apart when they’re supposed to, saving time, money, and headaches in the long run.
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Keywords:
galling, thread galling, galling prevention, stainless steel galling, titanium fastener galling, threaded fastener damage, galling causes, galling solutions, anti-galling washer, Velocity Washer, bolting reliability, ASME PCC-1 galling, bolted joint galling, fastener seizure, cold welding threads, flange joint galling, galling mitigation, galling resistant materials, stainless steel bolt seizure, bolt lubrication galling, anti-seize galling prevention, galling in high torque, galling failure analysis, galling in stainless fasteners, bolting best practices, thread lubricant galling prevention, Velocity Bolting galling solution, galling-resistant washers
References:
Bolt Depot. “Thread Galling.” Fastener Materials and Grades – Bolt Depot Fastener Information. (Accessed 2025) boltdepot.com
Fastenal Engineering Team. “Thread Galling.” The Blue Print (Fastenal Engineering Blog), July 14, 2025.blueprint.fastenal.com
Aztech Locknut Company. “Thread Galling.” Tech Article, citing Fastenal and others, 2016.aztechlocknut.com
ASME PCC-1–2019. “Guidelines for Pressure Boundary Bolted Flange Joint Assembly.” (Emphasis on thread lubrication in bolted joint procedures)velocitybolting.com
Velocity Bolting Inc. “The Galling Phenomenon: What exactly is galling?” Blog post by Ryder Britton, July 14, 2025.velocitybolting.com
Velocity Bolting Inc. “Velocity Washer – Prevent Galling.” Product information page, 2025.velocitybolting.com
Disclaimer:
Portions of this article were generated with the assistance of ChatGPT, a large language model developed by OpenAI. The content is provided for informational purposes only and does not constitute professional, legal, financial, or academic advice. The views expressed do not necessarily reflect those of the author, and readers are encouraged to independently verify any information presented.
The AI-generated content has been reviewed and edited for clarity and accuracy where appropriate.
Hydraulic Bolt Torquing vs. Bolt Tensioning: What is the difference?
Hydraulic Bolt Torquing vs Bolt Tensioning: What is the difference?
Bolted joints are fundamental to the safety and reliability of equipment across industries, from pipelines and pressure vessels to structural steel frameworks. Proper bolt tightening ensures bolts act like springs, stretching elastically to clamp components together and maintain a secure, leak-free joint. Two primary bolt tightening methods are commonly used to achieve the necessary clamping force: bolt torquing and bolt tensioning. This post will explain what each method entails, highlight their technical differences, and compare the advantages, disadvantages, and typical use cases of bolt tensioning vs torquing. We’ll also discuss considerations around safety, precision, and cost-efficiency to help engineers and maintenance professionals choose the right approach for their application.
What is Bolt Torquing?
Bolt torquing is the traditional and most widespread method of tightening bolts. It involves using a wrench or powered tool to apply a specified rotational force (torque) to the nut or bolt head, thereby stretching the bolt through the resistance of friction and material elongation. In practice, as the nut is turned, the bolt’s threads engage and the bolt twists slightly while elongating, developing tension that clamps the joint together. The applied torque is usually measured in units like Newton-meters (Nm) or foot-pounds (ft·lb) and is correlated to an expected bolt preload. Common torque tools range from manual clicker wrenches to pneumatic or hydraulic torque wrenches for larger bolts.
Torquing is favored for its simplicity and accessibility – a single calibrated torque wrench and a set of sockets can cover a wide range of bolt sizes. It is the default bolt tightening method in many fields (including general construction and automotive maintenance) because it requires minimal specialized equipment and technicians are generally familiar with torque procedures. By following specified torque values (often provided by bolt or equipment manufacturers) and using proper techniques, an assembler can achieve a desired level of tension in the bolt. However, it’s important to recognize that the relationship between torque and actual bolt tension can be variable. Only a portion of the input torque (often roughly 10–15%) goes into stretching the bolt; the majority (85–90%) is consumed overcoming friction at the threads and nut face. This means factors like thread lubrication, surface finish, and even the number of times a bolt has been reused play a huge role in how much tension results from a given torque. Proper lubrication and clean mating surfaces are therefore critical when torquing – without them, the required torque to achieve a target tension may be higher, and the achieved preload less predictable. Bolt torquing also leads to the Galling phenomenon - or in other words, stuck nuts and bolts. To solve this problem, look into our Velocity Washer technology.
What is Bolt Tensioning?
Bolt tensioning is a more direct bolt tightening method that applies axial stretch to the bolt without turning the nut. In a typical hydraulic bolt tensioning procedure, a hydraulic tensioner tool (essentially a hydraulic jack or load cell) is fitted over the stud’s protruding threads and pulls the bolt by applying a high-pressure force, stretching it like a spring. Once the bolt has elongated to produce the desired preload, the nut is hand-tightened down to the flange or joint surface. The hydraulic pressure is then released, transferring the load to the now-snug nut and locked-in bolt tension. This method directly creates tension (usually measured in units of force such as kN or pounds-force) in the fastener without most of the energy being lost to friction, since the nut rotation is minimal.
Hydraulic bolt tensioning technology became popular in the 1970s and has seen 50+ years of development. It’s now commonly used for specific, critical applications – especially high-pressure or large diameter bolted joints in industries like oil & gas, power generation, wind energy, and subsea operations. Because tensioning does not induce any twisting on the fastener, it is very useful for long bolts (such as anchor rods or turbine studs) and for situations where torsional stress could be harmful (for example, tensioning anchor bolts in concrete foundations to avoid cracking the concrete bond). To use bolt tensioners effectively, the joint and fastener must be configured to accept them: typically you need a certain length of free thread protruding above the nut (often at least one bolt diameter of extra thread) so the tensioner can grip the stud. The tensioner equipment (hydraulic pump, hoses, and load cell heads) must also fit around the stud and adjacent nuts or components – this can be a design consideration for tight spaces. In summary, bolt tensioning is a precise method of stretching bolts to achieve a specified clamp force, using hydraulic force instead of torque rotation.
Torque vs Tension: Technical Differences in Methodology
While both torquing and tensioning ultimately aim to stretch a bolt to develop clamping force, the mechanics of how the load is applied are very different. In torque tightening, the act of turning the nut creates a combination of bolt elongation and torsional stress. The friction under the nut and along the threads is a key factor – indeed, in a torquing operation a large percentage of the energy is lost to friction, and only a small fraction directly contributes to bolt tension. This inherent friction variability means that achieving an accurate preload via torque is an indirect process; the actual tension in the bolt can scatter significantly due to slight changes in lubrication, surface roughness, thread fit, and other factors. For example, if a bolt’s threads are rusty or dry, much of the applied torque will be eaten up by overcoming thread friction, possibly leaving the bolt under-tensioned even if the torque wrench clicks at the target value. Conversely, a well-lubricated bolt will experience higher tension for the same torque. Engineers account for this by specifying a “K-factor” or friction coefficient in torque calculations, and by insisting on clean, lubricated threads during assembly. Even so, standard torque tightening is generally considered accurate only to within ±20–30% of the desired bolt load in typical field conditions. In critical applications, multiple torque passes and criss-cross tightening patterns are used to gradually and evenly approach the target preload.
In tension tightening, the process is more direct – the hydraulic tensioner pulls on the bolt, inducing pure tensile stretch without any intentional rotation. Because very little torque is used (just a small hand turn of the nut while the bolt is already stretched), frictional effects are minimized. This means the bolt’s achieved preload can closely match the calculated load based on hydraulic pressure, usually within about ±10% of target if done properly. Tensioning also allows for multiple bolts in a joint to be loaded simultaneously (for instance, using several tensioners on a flange at once and pressurizing them together). By tightening many bolts together, the method avoids the “cross-talk” seen in torque tightening – in a torque sequence, when one bolt is tightened, the previously torqued neighboring bolts often relax slightly as the flange or gasket compresses unevenly. Hydraulic tensioning minimizes this effect because an entire group of bolts can be pulled up together, yielding a more uniform distribution of tension. This uniformity is especially beneficial for gasketed joints, reducing the risk of leaks due to uneven gasket compression. Additionally, since tensioning applies force axially, the bolts are not subjected to shear or torsional stresses during tightening. This eliminates the risk of thread galling or twisting off a bolt due to high torque, a failure mode that can occur in torque tightening if a bolt is over-stressed or if the threads seize.
In summary, torque tightening relies on turning force and friction to stretch the fastener, whereas tensioning uses direct axial force. Torque tightening is simpler but can be less predictable in terms of bolt load, while tensioning is more complex equipment-wise but generally achieves a more precise and uniform result. These technical differences lead to distinct pros and cons for each method, as discussed next.
Advantages and Disadvantages of Bolt Torquing
Advantages and disadvantages of tightening using torque.
Bolt torquing offers several clear advantages. It is a flexible and cost-effective method for most bolting needs. With standard tools, technicians can tighten a wide range of bolt sizes, making torquing a versatile choice across many general applications. The equipment is relatively inexpensive and widely available – a basic torque wrench is far cheaper than a hydraulic tensioning pump and jack set, and even advanced hydraulic or pneumatic torque wrenches usually cost less than tensioner systems. The simplicity of the torquing process is another benefit: procedures for torque tightening are well understood in the industry and workers can be readily trained to use torque tools correctly. In many cases, torquing is the only practical choice – for example, on older or space-constrained equipment, there may not be enough clearance around the studs to fit a bulky tensioner, so using a wrench remains the go-to method. Torquing also does not require any extra bolt length; as long as a wrench can engage the nut, the bolt can be tightened (hydraulic tensioners, by contrast, need protruding threads above the nut). This makes torque tools suitable for bolts that are flush to surfaces or in tight assemblies where adding longer bolts for tensioners isn’t feasible. Finally, torque tools can be outfitted with special low-profile attachments or cassettes to get into confined spaces between studs, further improving their applicability where clearance is limited.
On the downside, bolt torquing has notable limitations. Because it relies on turning the fastener, it is inherently slower for large bolted connections – each bolt must be tightened individually and often in multiple passes following a specific pattern to ensure even load distribution. This sequential process can be time-consuming when dealing with flanges that have many bolts. More importantly, torquing is less accurate in achieving a target preload compared to tensioning. The actual clamping force can vary bolt to bolt due to friction differences; even with good practices, the bolt load scatter in a torqued joint could be ±25% or more around the target value. For critical or high-pressure joints, this lack of precision might be “good enough” if it yields an average load in range, but it does leave some bolts tighter or looser than intended. Torque tightening also introduces torsional stress into the bolt – the twisting action can contribute to bolt elongation but also puts shear strain on the material. In extreme cases or if a bolt is weak or a nut seizes, this can lead to bolt torsion and thread galling or damage. Careful use of proper lubrication is required to mitigate thread wear and achieve consistent results. However, the easiest solution is to implement Velocity Washer technology. Additionally, the effectiveness of torquing depends on controlling many variables: the condition of the threads, the presence of lubrication, the surface finish under the nut, and even the speed of turning can all impact the friction and thus the required torque. These factors make torque-based tightening more variable. Lastly, as bolt sizes increase, the amount of torque needed rises dramatically – large diameter fasteners might require such high torque that it becomes impractical or unsafe to apply with standard tools. In such cases (like very big anchor bolts or reactor studs), tensioning or other methods become necessary simply because a sufficient torque cannot be achieved or measured accurately.
Advantages and Disadvantages of Bolt Tensioning
Advantages and disadvantages of tightening using tensioning.
The use of hydraulic bolt tensioning brings a number of technical advantages, particularly for demanding applications. First and foremost is accuracy and consistency: tensioning can achieve a very precise bolt preload with minimal variation between bolts. Because it directly elongates the fastener, the resulting clamping forces are consistent and repeatable, largely independent of friction conditions on the threads. This yields a much tighter bolt load tolerance (often within about ±10% of the target load, as opposed to the wider scatter from torque tightening). In critical assemblies – for example, a high-pressure pipeline flange or a turbine casing – such consistency is invaluable for ensuring all bolts share the load evenly. Another key benefit is that tensioning avoids almost all torsional strain on the bolt. The bolt is stretched along its length and the nut is only turned once the bolt is already elongated, so there’s no heavy twisting force applied. This means there’s far less risk of damaging threads or inducing shear stress in the fastener. It also reduces the possibility of galling (seizing) between the nut and bolt, since the nut rotation under load is very small and done under lower friction conditions. Additionally, hydraulic tensioning allows for simultaneous tightening of multiple bolts, which greatly improves efficiency and joint uniformity on flanged connections. By tensioning, say, 50% or 100% of the bolts at once (using multiple tensioner heads and a manifold of hydraulic hoses), assemblers can avoid the incremental approach required in torquing and minimize elastic interactions between bolts. The result is minimal load cross-talk – the act of tightening one bolt has little effect on the tension of adjacent bolts when all are pulled together. The even, parallel clamping of a flange is much better for gasketed joints, reducing the risk of leaks. Tensioning is particularly advantageous for large-diameter or high-tensile bolts where achieving the required preload via torque would be extremely difficult or time-consuming. In fact, in many oil & gas and power generation applications, hydraulic tensioners are standard for big bolts (such as those above about 2 inches in diameter) because they can deliver the necessary force more quickly and accurately than massive torque wrenches. When done correctly, bolt tensioning provides a very high level of precision and reliability, making it ideal for high-pressure, critical joints (for example, subsea pipeline connectors, reactor vessel studs in nuclear plants, or wind turbine hub bolts) where performance and safety are paramount.
Despite these benefits, there are some disadvantages and practical challenges associated with bolt tensioning. The most obvious is the higher cost and complexity. Tensioning equipment – hydraulic pumps, hoses, and tensioner heads – can represent a significant investment (or rental cost), and using it requires properly trained operators. A bolting crew needs specialized knowledge to calculate required pressure, account for any load losses, and handle the high-pressure hydraulic safety considerations. This means training and experience are crucial; improper use of a tensioner can be dangerous (due to the high forces and pressures involved) and may result in incorrect preload if calculations are off. Another drawback is that tensioners must be custom-matched to the bolt and joint. For each bolt size and series, you need a tensioner of the correct capacity and fit. Factors like the bolt’s diameter, grade, the thickness and diameter of washers, and the available free thread length all determine what tensioner setup is needed. This preparation and measuring add extra steps before tightening can begin, unlike a torque wrench which can often be applied more or less immediately. In joints where studs are very close together or recessed, a bulky tensioner might not physically fit, limiting its use. In such cases, engineers might have to tighten in stages (using a few tensioners moved around in multiple groups) or default to torquing if even staged tensioning won’t work. The need for sufficient stud protrusion is another constraint – if a bolt doesn’t stick out far enough beyond the nut, you simply cannot engage a tensioner on it. This is one reason tensioning is seldom used in typical structural steel construction, where bolts are often flush to the surface for practical reasons. Finally, one technical consideration with tensioning is load loss after tensioning: when the hydraulic pressure is released, a slight drop in bolt load can occur as the nut takes the load and the joint components (gasket, flange faces) compress a bit more. Skilled workers anticipate this by over-pressurizing to a calculated degree or by re-tensioning (multiple cycles) to inch closer to the target load. It’s an added complexity that must be accounted for to ensure the final retained tension is on target. Despite these drawbacks, when the joint is critical enough to warrant it, the improved accuracy and reliability of hydraulic bolt tensioning often outweigh the costs.
Industry Applications and Use Cases
Both bolt tightening methods have well-defined places in industry, and often the choice of torque vs tension comes down to the specifics of the job – joint criticality, bolt size, environment, and available resources. Below are a few industry-relevant examples illustrating when each method is typically used:
Oil & Gas: In the oil and gas industry, bolted flanges on pipelines, pressure vessels, and offshore platforms are common critical joints. For high-pressure connections (such as those in refineries or subsea pipelines), hydraulic bolt tensioning is frequently employed to achieve a uniform, leak-proof seal across all bolts. For instance, large diameter flange bolts on a pipeline may be tensioned simultaneously to ensure even gasket compression and prevent dangerous hydrocarbon leaks. Tensioners are also used on blowout preventers, wellhead connectors, and other critical equipment where bolt failure is not an option. On the other hand, torquing is still used in oil & gas for less critical bolting or smaller sizes – e.g. small-bore piping, valve fastenings, and general plant maintenance – where a calibrated torque wrench can do the job adequately and more quickly. In remote or onshore applications, the practicality of a simple torque tool often wins unless the joint absolutely demands the precision of tensioning.
Power Generation: Power plants (fossil, nuclear, and renewable) contain a mix of bolted joints, from huge turbine casings and generator endbells to heat exchanger joints and support structures. Bolt tensioning is commonly chosen for large, high-stress bolts in turbines, steam headers, and reactor pressure vessels. For example, the giant bolts that hold together a steam turbine casing or a wind turbine tower section are often tightened with hydraulic tensioners to achieve the extremely high preloads necessary for operation. In nuclear power facilities, the need for absolute reliability means tensioning is favored for reactor vessel head studs and other critical flange connections, as it provides maximum preload accuracy and joint integrity. Wind energy also benefits from tensioning for securing turbine blades and tower segments where consistent bolt tension prevents fatigue. Meanwhile, plenty of routine power-plant bolting (such as pump flanges, smaller piping, or equipment mounts) can be handled by torque tightening. Technicians in a plant may use powered torque wrenches for speed on these standard joints, reserving the tensioners for only the most critical assemblies.
Construction and Infrastructure: In civil construction (buildings, bridges, infrastructure), torque tightening remains the predominant method for bolts. Structural steel connections (e.g. bolting steel beams or columns) typically use torque-based specifications or twist-off bolt indicators, because the volume of bolts is high and the crew can quickly install them with standard impact guns and torque wrenches. The simplicity and low cost of torquing is crucial on construction sites where many bolts must be tightened efficiently. Specialized cases in construction do exist for tensioning – for example, large anchor rods in concrete foundations or bridge support cables might be tensioned using hydraulic jacks to avoid imparting torsional stress that could crack the concrete or to precisely set tension in a tendon. One such case is tensioning the anchor bolts of a wind turbine’s concrete foundation; applying torque to those long bolts could twist and damage their grouted hold in the concrete, so hydraulic tensioners are used to stretch them straight up. Generally, however, the limited clearance and budget in typical construction projects means tensioners are rarely used; torquing (or other methods like turn-of-nut tightening and calibrated direct tension indicators) are far more common for building connections.
Maintenance and Other Industries: In maintenance work across industries – whether mining equipment, aerospace assemblies, or manufacturing machinery – the choice between torque and tension often depends on the bolt size and criticality. Small and medium bolts, or non-critical joints, are almost always torqued due to convenience. Sectors like mining and aerospace also use sophisticated torque tools (and even ultrasonic tension measuring devices) to ensure bolts are tightened correctly. When extremely large or safety-critical bolts are involved (for example, holding together heavy mining shovel components or critical aerospace test rigs), tensioning might be brought in. Hydraulic bolt tensioning is a go-to for jobs that demand the utmost precision and have the budget and downtime to accommodate the setup. Meanwhile, bolt torquing is ubiquitous as a quick solution for daily maintenance tasks because it’s versatile and doesn’t require specialized setup for each bolt.
Safety, Precision, and Cost Considerations
When comparing bolt tensioning vs torquing, engineers must consider a balance of safety, precision, and cost-effectiveness for the situation at hand. From a safety standpoint, the primary concern is ensuring the joint is tightened correctly – an under-tightened bolt can lead to catastrophic failures like leaks, blowouts, or structural collapses, while an over-tightened bolt can suffer from yield or fracture. Both torque and tension methods, if followed according to proper procedures, can produce a safe, secure, and leak-free joint. The key is adherence to industry standards (such as ASME PCC-1 for pressure vessels or AISC guidelines for structural bolting), use of calibrated equipment, and trained personnel performing the work. It’s also important to consider the safety of the process itself: torquing generally involves fewer potential hazards (mainly the physical strain or the reaction force on the wrench), whereas tensioning involves handling high-pressure hydraulic equipment. Operators must be trained to set up tensioners correctly and stay clear of the line of fire when hydraulics are under pressure. Misuse of either method can be dangerous – a sudden release of a stuck nut on a high-torque wrench or a hydraulic hose failure on a tensioner can both cause injuries. Thus, worker training and following established bolting procedures are paramount for safety regardless of method.
In terms of precision and joint integrity, hydraulic tensioning has the edge. As discussed, tensioning achieves a more accurate and uniform bolt preload on critical joints. This higher precision translates to better joint performance – gaskets are evenly compressed, and bolts are all sharing loads as intended, which improves the reliability of the assembly. For extremely sensitive or high-risk joints (for example, a flange containing toxic or high-pressure fluids, or a structural splice at the base of a tall tower), the superior consistency of tensioning adds an extra margin of safety in operation. Torque tightening, however, can be sufficiently precise for the majority of applications when done correctly. The reality is that many bolted connections (like standard flanges with good gaskets) have some tolerance for bolt load variation. A well-executed torquing procedure – using a calibrated wrench, proper lubrication, and a controlled tightening sequence – can often achieve bolt loads within perhaps ±15% of target, which is adequate for most non-critical joints. Additionally, modern advancements such as torque-angle tightening and direct tension indicating washers have improved the reliability of torque-based methods. Therefore, while tensioning is the more precise technique, torque remains a practical level of precision for everyday engineering needs, and it’s a trade-off of precision vs. complexity.
Finally, the cost and efficiency factor is often the deciding consideration. Bolt torquing is generally cheaper and more convenient – the tools are less expensive, and most maintenance departments have them on hand. Training for torque tools is straightforward, and the process is quick for small jobs. If a facility has hundreds of routine bolted joints to secure, doing them by torque wrench is usually far faster in total than setting up a tensioner on each one. However, focusing only on the upfront cost can be misleading. For critical joints, a failure or leak later on could incur enormous costs, far outweighing the expense of having done the job with higher precision. For example, a refinery heat exchanger that leaks due to insufficient bolt load might cause unplanned downtime, environmental incidents, or even safety hazards – a scenario where investing in tensioning for a guaranteed result is easily justified. Additionally, on large-scale jobs like refinery turnarounds or wind farm construction, the ability of tensioners to tighten multiple bolts at once can save significant labor time, partially offsetting their setup time and cost. It’s also worth noting the hidden costs of each method: torquing can be physically demanding and might require multiple technicians working in shifts for huge bolts (or using very heavy multiplier wrenches), whereas tensioning might require renting specialized pumps and ensuring all accessories (jacks, bridges, etc.) are available for the bolt dimensions. In budgeting a project, engineers will consider not just the price of tools but also the cost of potential rework or inspection. Often, critical bolting jobs call for tensioning, while standard jobs stick with torquing – but each decision should factor in the joint requirements, the environment, and the consequence of failure.
Conclusion
Bolt torquing and bolt tensioning are both proven bolt tightening methods, each with its own strengths. Bolt torquing vs bolt tensioning is not a matter of one being universally better than the other, but rather which is more appropriate for a given situation. Torquing shines in its simplicity, speed, and low cost for everyday applications, and it remains the workhorse method for the vast majority of bolted joints in construction and maintenance. Hydraulic bolt tensioning, by contrast, offers superior accuracy and uniformity, making it indispensable for high-stakes bolting scenarios – large, high-pressure, or safety-critical joints where precision trumps convenience. Engineers and maintenance professionals should evaluate factors such as joint criticality, bolt size, accessibility, available equipment, and crew expertise when choosing between torque and tension. In many cases, the answer may even be to use a combination: torque tighten less critical bolts and tension the most critical ones. By understanding the technical differences of torque vs tension, and by adhering to industry best practices for each, one can ensure bolted connections are assembled safely, efficiently, and with long-term reliability. Remember, the ultimate goal is achieving the correct bolt preload for a secure joint – whether by the measured twist of a wrench or the controlled stretch of a hydraulic jack, what matters is that the bolts hold tight and keep our equipment and structures safe.
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Keywords: bolt tensioning vs torquing, bolt tightening methods, hydraulic bolt tensioning, torque vs tension, bolt tightening best practices, bolt preload, flange bolting, bolt tightening safety.
References:
Disclaimer:
Portions of this article were generated with the assistance of ChatGPT, a large language model developed by OpenAI. The content is provided for informational purposes only and does not constitute professional, legal, financial, or academic advice. The views expressed do not necessarily reflect those of the author, and readers are encouraged to independently verify any information presented.
The AI-generated content has been reviewed and edited for clarity and accuracy where appropriate.
Single Stud Replacement: An (ineffective) method to deal with galling
A comprehensive guide to single stud changeout (also known as “hot bolting”). We’ll cut to the chase: if you want to avoid all the headaches and safety issues related to single stud changeout, just use Velocity Washers.
The following is a comprehensive analysis of Single Stud Replacement (also know as “hot bolting”). In our opinion, this process is unnecessarily dangerous and can put your workers and plant at risk.
Want to avoid Single Stud Replacement? Use a Velocity Washer.
If you are performing single stud replacement to deal with the galling phenomenon, then Velocity Washers are your answer. Save time, money, and keep your workforce safe - all you need to do is change the washer you use.
If you don’t want to use Velocity Washers to eliminate the need for Single Stud Replacement, then keep reading here:
Maintaining bolted flange joints in critical service (such as oil and gas pipelines and refineries) is a challenging task that often requires innovative approaches. One such approach is Single Stud Replacement, a technique for replacing bolts on a live flanged joint without shutting down the system. Also known informally as hot bolting, this method is a form of online bolt replacement or live flange maintenance. In this detailed guide, we’ll define what single stud replacement is, explain when and why it’s used, outline the step-by-step procedure, describe the tools involved, and highlight essential safety protocols, standards (like ASME PCC-1 and PCC-2), typical flange configurations, and risk mitigation techniques.
What is Single Stud Replacement?
Single Stud Replacement refers to the practice of removing and replacing a single bolt (stud) at a time on a bolted flange joint while the system remains in operation (under pressure). In other words, one bolt is taken out from the flange, a new or refurbished bolt is installed in its place, and it is tightened before moving on to the next bolt. ASME’s official definition (in PCC-1 2019, Appendix B) for “hot bolting” describes it as “the sequential removal and replacement of bolts on flanged joints while the unit is under reduced operating pressure,” generally removing one bolt at a time, relubricating it, reinstalling it (or a new bolt), and retightening to a specified torque[1]. This is exactly the process employed during a single stud replacement.
It’s important to clarify terminology: hot bolting is the commonly used term, but it can be misleading because the procedure isn’t necessarily performed at elevated temperature – “hot” simply means the equipment is live or pressurized, not literally hot[2]. Several related terms exist (and are sometimes confused) such as hot torquing, live bolting, live tightening, and retorquing. Each of these refers to a slightly different activity (for example, merely re-tightening bolts vs. replacing them) and comes with distinct purposes and risks[3]. When the intention is specifically to replace individual studs rather than just tighten them, the correct term is Single Stud Replacement (or replacement)[4]. In this article, we focus on that specific bolt replacement technique.
Typical Use Cases and Rationale
Single stud replacement is typically employed in situations where taking the equipment out of service is undesirable or impractical, yet maintenance of the bolted joint is needed. Common use cases include:
Pre-shutdown maintenance: Performing a single stud replacement campaign just before a planned turnaround or shutdown. By swapping out or servicing studs on live flanges ahead of time, the actual shutdown period can be shortened. For example, cleaning, lubricating, and loosening/replacing each bolt in advance ensures the flange can be opened more quickly during the outage[5]. This is because the galling phenomenon is eliminated (remember: you can solve it much easier by just using Velocity Washers). This practice can improve turnaround efficiency by an estimated 30% by allowing critical pipework connections to come apart faster when the plant is taken offline[6]. In essence, “old for new” bolts are installed before the shutdown, so that during the shutdown you’re not wasting time wrestling with corroded fasteners.
Corroded or damaged bolt replacement: In aging facilities (offshore platforms, refineries, chemical plants, etc.), flange bolts often suffer corrosion or damage over years of service. Replacing severely corroded studs online via single stud replacement can restore joint integrity without an unplanned shutdown. This is a proactive maintenance to prevent leaks or failures – a controlled bolted flange joint maintenance activity to mitigate corrosion issues. A planned replacement campaign is far safer and cheaper than risking a bolt failure that forces an emergency shutdown[7]. It also addresses unknowns like bolts of uncertain remaining strength or unknown tension. By replacing them under controlled conditions, you eliminate the guesswork about whether old bolts can hold until the next outage.
Preventive maintenance and integrity assurance: Even if bolts aren’t visibly corroded, operators may use single stud replacement to gradually upgrade bolts (for instance, installing higher grade fasteners or new gasket materials) while the system stays online. This can be part of an integrity management program to avoid flange leaks. It prevents unknown factors – for example, relieving concerns about unknown gasket behavior or bolt preload after long service – by renewing components in a managed way.
Emergency avoidance: When a flange is found leaking or a bolt is discovered broken during operation, a single stud replacement (or a variant of hot bolting) can sometimes be done to fix the issue without a full shutdown. This is risky and requires strict controls, but in some cases it may prevent a larger unplanned outage. Generally, however, hot bolting is not meant as a reactive emergency fix unless absolutely necessary, due to the dangers involved.
Rationale and Benefits: The primary rationale for single stud replacement is to perform necessary bolt maintenance online to save downtime and enhance safety. By replacing or reconditioning bolts while the equipment is live, you can avoid an unplanned shutdown and preempt failures. Key benefits of a properly executed single stud replacement include:
· Replacing damaged or corroded bolts before they fail, thereby improving joint reliability.
· Preventing unplanned shutdowns by addressing bolt issues during normal operations.
· Enhancing safety by reducing the chance of a hazardous leak or rupture (since weak bolts are replaced in a controlled manner).
· Improving bolted joint integrity and ensuring proper clamp force on gaskets.
· Shortening the duration of planned shutdowns (less time spent dealing with seized bolts during the outage).
· Limiting the number of “containment breaks” (open flange events) because joints can be opened more confidently once bolts are already serviced.
· Increasing maintenance efficiency and turnaround speed for the facility.
· Allowing “old-for-new” bolt replacement so that fresh fasteners are in place when the system is eventually opened.
· Reducing overall maintenance costs by minimizing production loss and extending equipment life.
These benefits make single stud replacement an attractive technique in industries like oil and gas, where even a few hours of downtime can cost millions. By performing live flange maintenance in a carefully managed way, facilities aim to maximize uptime without compromising safety.
Step-by-Step Procedure for Single Stud Replacement
Executing a single stud replacement requires a disciplined, stepwise approach to ensure the flange joint remains intact and leak-free throughout the process. Below is an overview of a typical procedure, step-by-step. Note: This content is provided for informational purposes only and does not constitute professional, legal, financial, or academic advice from Velocity Bolting Inc.
Planning and Assessment: Before any tools touch the flange, thorough planning is essential. A risk assessment and Job Safety Analysis (JSA) should be conducted to evaluate the specific joint and situation. This includes confirming the flange’s design and service conditions, checking bolt condition (if possible), ensuring the correct replacement studs and gaskets (if needed) are on hand, and establishing contingency plans. Operating conditions must be made as safe as possible – for example, reduce the internal pressure if feasible (often to 50% or less of the flange design pressure)[8], and stabilize the temperature and contents of the line. Evaluate if external loads or vibrations can be minimized. Only proceed if the benefits outweigh the risks (per industry guidance) and all stakeholders agree on the plan[9].
Install Flange Supports (if required): In many cases, especially for smaller flanges with few bolts, a hot bolting clamp or similar support device is installed across the flange before any bolt is removed. This device clamps around the flange circumference or across the two flange halves to maintain compressive force on the joint while a stud is out. For example, a lightweight hydraulic clamp may be fitted spanning a pair of adjacent bolts to carry the load when one bolt is removed. If a specialized clamp isn’t available, engineered temporary supports (such as strong-back clamps or backup bridge pieces) might be used, though best practice is to use purpose-built hot bolting clamps[10]. (Note: On four-bolt flanges, a proper clamp is mandatory to prevent loss of gasket load[10] – see more in Safety Protocols section below.)
Select and Loosen the First Stud: Identify the first bolt to replace. Often, technicians choose the most severely corroded or weakest-looking stud first (so that if any problem occurs, it happens on the worst-case bolt). Ensure the remaining bolts are all tight and the clamp (if used) is secure. Using a calibrated wrench or hydraulic torque tool, carefully loosen the target stud’s nut. This can be challenging if the nut is seized; penetrating oil or slight heating might be applied if allowed. In extreme cases, a nut splitter or cutting tool may be needed to get the old fastener off. Only one stud is loosened at a time, and others carry the load in the meantime[1]. As the nut comes loose, be vigilant for any sign of flange separation or leakage (e.g. keep a leak detector spray or sensor on the gasket area).
Remove the Stud: Once the nut is off, remove the stud or bolt from the flange. This may require tapping it out with a hammer if rusted in place (taking care not to damage flange threads or alignment). Immediately clean the bolt holes and flange faces in the area – removing rust, old gasket weepage, or debris that might have accumulated. This cleaning step is a key part of the process historically, as it helps ensure the reassembled joint will have a good seal[5]. Inspect the removed stud; if it was in bad shape (corroded, necked down, cracked), this validates why the replacement was needed.
Lubricate and Insert New Stud: Prepare a new replacement stud of equal material and size (or if reusing the same stud after cleaning, inspect it thoroughly). Apply the specified thread lubricant (as per the procedure or ASME PCC-1 guidelines) to the stud threads and nut bearing surfaces. Proper lubrication is critical to achieve the correct bolt tension when tightening[1]. Insert the new (or cleaned) stud through the flange holes. Start the nut by hand to ensure it isn’t cross-threaded.
Tighten the New Bolt to Specified Torque: Using a torque wrench or hydraulic tensioner, tighten the new stud to the prescribed torque or bolt stress. It’s usually recommended to bring the nut up to tension gradually. For example, one might tighten in increments (e.g. 30%, then 60%, then 100% of final torque) to avoid shock to the gasket. If using hydraulic bolt tensioners, the stud may be strained to the target load and the nut then snugged. Maintain control of the tightening – the goal is to match the preload of the other bolts as closely as possible. ASME PCC-1 bolting procedure guidelines (such as using a star tightening pattern on flanges) are typically followed insofar as applicable, although here we are tightening one bolt at a time instead of a full pattern. Ensure the clamp (if in place) is taking some load as intended while the new bolt is being tightened.
Proceed with Remaining Studs Sequentially: Move on to the next bolt in the sequence. It is good practice to replace non-adjacent studs in a rotation that distributes the work around the flange. For instance, if working on an 8-bolt flange, you might replace a bolt, then move roughly opposite for the next replacement, to keep the gasket load balanced. In a 4-bolt flange, “opposite” means the bolt diagonally across the flange. Following a logical sequence (often outlined in the job procedure) maintains even pressure. Only one bolt should ever be removed at a time. Repeat the process: loosen the next stud, remove it, clean and replace, then retighten. Over the course of the operation, all (or a targeted subset of) studs will be changed out one by one. Figure 3 from a Hydratight case study illustrates a typical sequence on a four-bolt flange: the most corroded bolt was removed first, the second bolt replaced was the one opposite to the first, then the remaining bolts, one by one[11].
Final Verification and Clamp Removal: After all intended studs have been replaced and tightened, it’s important to do a final check. First, if a hot bolting clamp or support was used, gradually release and remove it, making sure the flange remains closed and no gaps open up. Then, perform a uniform tightening pass on all bolts around the flange (including the newly installed ones) to equalize any differences. This often means going around in a criss-cross pattern (per PCC-1 assembly guidelines) and applying the specified torque to each stud to ensure the load is evenly distributed. Monitor the gasket area for any sign of leakage now that the clamp is off and final loads are applied.
Post-Replacement Inspection: With the flange still online, closely observe the joint for a period of time after the procedure. Use leak detection fluid or gas sniffer equipment to ensure no small leaks are present. Check that all nuts are at the correct torque. Often, the procedure will include recording the final torque values or elongation measurements for quality assurance. If everything is tight and leak-free, the single stud replacement operation is considered successful.
Throughout this procedure, communication among the team is crucial. Typically one person is in charge of torquing while another monitors the gasket for leaks. If at any point a leak begins or something seems amiss (e.g. a flange starts to gap or a second bolt starts loosening on its own), the process must be stopped immediately and the system may need to be depressurized to safely re-tighten the joint. Contingency plans developed in the planning stage should be ready to execute if needed (for example, having bolt cages or line stops available, or in worst case, an emergency shutdown plan).
Tools and Equipment for the Job
Performing a single stud replacement safely and effectively requires specialized tools and equipment. Below are the key tools typically used in a hot bolting operation:
Calibrated Torque Wrenches or Hydraulic Tensioners: Precise bolt tightening tools are essential for controlling the bolt preload during single stud replacement. Common choices are hydraulic torque wrenches or pneumatic torque multipliers for applying a specific torque, or hydraulic bolt tensioners that stretch the stud directly. Using these ensures each new bolt is tightened to the correct specification (often derived from ASME PCC-1 bolting guidelines for bolt stress) and compensates for any differences caused by lubrication or thread condition[1]. Accurate control prevents under-tightening (which could leak) or over-tightening (which could crush the gasket or yield the bolt).
Hot Bolting Clamps (Flange Support Clamps): A hot bolting clamp is a temporary mechanical device that attaches to the flange to preserve clamping force while a bolt is removed. These are one of the most important safety devices in single stud replacement. The clamp typically spans the flange joint, gripping both flanges so that the gasket compression is maintained by the clamp’s pressure instead of the missing bolt. They often use hydraulic jacks or screw mechanisms to apply clamping force. Using a clamp greatly reduces the risk of disturbing the joint integrity during the bolt replacement. In fact, industry best practice mandates the use of such clamps for flanges that have very few bolts (e.g. 4-bolt flanges) where removing one bolt dramatically reduces gasket loading[10].
Stud Removal and Installation Tools: To physically remove and install studs, technicians use heavy-duty hand tools and power tools. This can include impact wrenches (air or electric) to initially break loose nuts, slugging wrenches (hammer wrenches) for tight spaces, and nut splitters or portable grinders to cut off nuts that are frozen in place. Special stud extractors might be used if a stud is broken off. During installation, tools like stud alignment pins can help line up the flange holes. All these tools facilitate the safe extraction of old bolts and the placement of new ones without causing damage to the flange or injury to personnel.
Lubricants, Cleaners, and Gaskets: High-quality thread lubricant (anti-seize compound) is a necessary “tool” in bolting work. ASME PCC-1 emphasizes proper lubrication of bolts and nuts to achieve the desired tension at a given torque[1]. The crew will have approved lubricants on hand to apply to each stud/nut during installation. Solvents and wire brushes are used to clean bolt threads and flange surfaces once a stud is removed – cleaning out corrosion and debris ensures the new bolt will seat properly and the nut will turn freely. Although the gasket is not usually replaced during hot bolting (since the joint isn’t opened), it’s important to have a replacement gasket available on standby in case something goes wrong and the flange does have to be separated.
Monitoring and Safety Equipment: During live flange maintenance, safety is paramount. The team will use leak detection equipment like soapy water (bubble solution) or electronic sniffers to continuously check for any gas or vapor escaping from the flange as bolts are changed. Pressure gauges or remote monitoring of system pressure is done to ensure it remains within the safe range (typically <50% design pressure as noted earlier). Everyone on site must wear appropriate PPE – fire-resistant clothing, face shields or goggles, gloves, etc., in case of a spray-out or fire. Sometimes a fire guard is stationed nearby with a fire extinguisher when hydrocarbons are involved. Communications devices are also important so that if any team member observes an anomaly (e.g., a whistling sound from the flange or movement in the clamp), they can immediately alert the others to stop work.
Safety Protocols and Standards (ASME PCC-1, PCC-2, etc.)
Because single stud replacement/hot bolting is inherently a high-risk activity, strict safety protocols must govern its use. The overarching principle is that live bolting on pressurized systems should only be done when absolutely necessary and with full understanding of the risks[9]. Industry standards and guidelines provide criteria and recommendations to ensure safety:
Justification of Need: Guidance from bodies like EEMUA (Engineering Equipment and Materials Users Association) emphasizes that working on live flanged joints should only be considered when the benefits clearly outweigh the risks. “Marginal time savings during shutdowns... should not be considered sufficient incentive for using the technique,” one publication warns[9]. In other words, one should not perform hot bolting just for convenience – there should be a compelling reason, such as preventing a significant economic loss or addressing a serious safety concern, and no viable alternative way (like a short shutdown) to do the work safely.
Pressure and Service Restrictions: A fundamental safety rule is to reduce the internal pressure as much as possible before starting hot bolting. Best practice (referenced in ASME and operator procedures) is to limit operating pressure to around 50% of the system’s design pressure while performing single stud replacements[8]. This reduction provides a larger safety margin in case the gasket experiences any load drop. Additionally, the fluid in the system should ideally be in a stable condition – not rapidly cycling in temperature or pressure. Highly hazardous services (toxic or extremely flammable fluids) warrant extra caution or might be outright prohibited for online bolting, depending on company policy.
Flange Selection Criteria: Not every joint is a good candidate for single stud replacement. Generally, flanges with a higher number of bolts are safer to hot bolt because the load is distributed among many fasteners. As a rule of thumb, many companies require that a flange have at least 8 bolts to consider hot bolting, and even then only if other conditions are acceptable[8]. Flanges with 4 bolts are the most problematic – removing one bolt from a 4-bolt flange means 25% of the clamping force (or more, if that bolt was carrying extra load) is gone, which greatly increases the chance of a leak or blowout. For this reason, performing single stud replacement on a four-bolt flange requires a clamp or additional support and is often avoided unless absolutely necessary[12]. (In the earlier figure, a clamp was used specifically for a 4-bolt flange demo.) Even 8-bolt flanges in low-pressure classes (150# rating) are identified in the industry as potentially “under-bolted,” meaning they don’t have a big safety factor, so hot bolting them is also risky[13]. Each joint must be evaluated individually – factors like flange size, rating, bolt diameter, gasket type, and existing bolt stress all come into play in a formal assessment.
Standards and Guidelines: The activity of hot bolting/single stud replacement is addressed in several engineering standards. ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly) provides general guidance on bolted joint assembly and includes definitions for terms like hot bolting (Appendix B). More specifically, ASME PCC-2, Article 3.11, is devoted to “Hot and Half Bolting Removal Procedures”, which is essentially the detailed recommended practice for single stud replacement on pressure equipment. This article outlines how to do it safely, prerequisites, and cautions. ASME PCC-2 warns that although hot bolting can reduce downtime, it is “potentially hazardous” and thus “caution shall be exercised in [its] planning and execution.”[14]. Following these guidelines means adhering to proper sequences, using appropriate tools, and meeting the conditions (like reduced pressure) specified. In addition to ASME, industry groups (e.g., EEMUA as mentioned, or the API in some recommended practices) and company-specific procedures may impose even stricter rules on when and how hot bolting is allowed.
Trained and Qualified Personnel: Only experienced, qualified bolting technicians and engineers should carry out a single stud replacement. This is not a routine maintenance job for junior staff. A lack of competency can be disastrous – a sobering example cited in industry literature is a 1992 refinery accident in Japan, where improper live bolt tightening (hot torquing) by inadequately trained personnel led to a catastrophic gasket failure, an explosion and fire, and multiple fatalities. Consequently, any crew attempting hot bolting must be well-trained in bolted joint assembly (per ASME PCC-1 training guidelines, for instance) and specifically in hot bolting procedure and hazards. A thorough job safety analysis (JSA) should be completed and reviewed by all participants prior to starting. Supervisors often implement a permit-to-work system for live maintenance, ensuring all checkpoints are satisfied before proceeding.
Continuous Monitoring and Preparedness: During the entire operation, safety protocols demand constant monitoring for any sign of trouble. This includes visual checks of the flange gap, listening for leaks (hissing sounds), and watching pressure indicators. Having a contingency plan is crucial – for example, if a leak starts when a stud is removed, the team should know in advance whether to attempt re-inserting a bolt immediately, whether to tighten neighboring bolts, or whether to evacuate and isolate the line. Often the plan will be to re-install a bolt right away and tighten it if a small leak starts, then reassess. Emergency isolation valves might be identified beforehand in case a major leak or fire occurs, so that section of the plant can be isolated quickly. All these plans are part of the safety preparation.
In summary, compliance with standards and rigorous safety planning are what make single stud replacement a viable technique. By following ASME PCC-1 bolting procedure best practices, adhering to PCC-2’s guidance, and implementing company-specific safety measures, engineers can perform online bolt replacements with minimized risk. Always remember that hot bolting is a last-resort maintenance approach – if there is any doubt about doing it safely, the equipment should be shut down and depressurized for conventional maintenance instead.
Typical Flange Types and Bolt Configurations
Single stud replacement can be performed on a variety of flange types and sizes, but the flange configuration plays a significant role in how the procedure is carried out and how risky it is. Here we discuss the common flange scenarios:
Standard Piping Flanges: Most flanges encountered in the oil & gas and petrochemical industry conform to standards such as ASME B16.5 (for pipe flanges) or ASME B16.47/API 605 (for larger diameter flanges). These can be raised-face (RF) flanges, flat-face, or ring-type joint (RTJ) flanges, all of which use a set of bolts or studs to compress a gasket. The number of bolts on a flange varies with the flange diameter and pressure class – common patterns include 4 bolts (for very small pipe sizes or certain valve bonnets), 8 bolts, 12 bolts, 16 bolts, etc., arranged evenly around the flange. Each bolt pattern has different implications for single stud replacement. As noted, a 4-bolt flange is most vulnerable: removing one bolt drops gasket load significantly and asymmetrically. An 8 or 12-bolt flange provides more distribution; removing one bolt out of 12, for example, usually means the remaining 11 can hold the fort temporarily (provided the gasket is in good shape and pressure is low). Higher-pressure flanges (e.g., Class 600, 900, 1500 in ASME ratings) generally have more bolts and thicker flange rims, which inherently provides a greater safety factor for any one bolt removed[8]. Conversely, low-pressure flanges with minimal bolts are considered “under-bolted” for this kind of operation[13].
Small Equipment Flanges (e.g., Valve Bonnets, Instrument Flanges): In practice, many single stud replacements are done on small flanges like valve bonnet covers, pump casings, or instrument connections, because these often have only a few bolts and tend to corrode quickly. For instance, a valve bonnet might have 4-6 studs. Technicians will use extra caution here; usually a strong-back clamp or support device (as shown earlier) is absolutely required for a 4-bolt bonnet during hot bolting[12]. The clamp effectively turns the 4-bolt flange into a constant-load system temporarily, so that when one bolt is gone the clamp shares the load with the remaining 3 bolts. If an instrument flange (say on a small gauge line) has 4 bolts, one might even consider using a temporary spool to take pressure off or just plan a short system outage if possible, since the risk might not be worth it. Each scenario is evaluated case-by-case.
Large Diameter Flanges: For big flanges (e.g., on pressure vessels, heat exchanger channels, or large pipelines), the bolt count is high (maybe 24 or more bolts). Single stud replacement on large flanges is generally more forgiving in terms of gasket load impact – removing one bolt out of 32, for example, is a small percentage loss of total preload. However, large flanges often have longer bolts and bigger gaskets, meaning the absolute forces are huge, and a lot of energy is stored in the bolt tension. Special attention must be paid to the torque/tension applied so as not to disturb the adjacent bolts. Sometimes, on very large flanges, multiple hot bolting clamps may be used sequentially around the circumference to ensure no part of the gasket loses compression. Also, large flanges might be more likely to have dual pressure seals (like an RTJ gasket plus a sealant injection line), which can provide an extra margin of safety during maintenance – for instance, some systems allow injection of sealant if a small leak starts. Operators may leverage such features when planning an online bolt replacement on large critical flanges.
Heat Exchanger or Vessel Split Flanges: These are flanges that join two halves of a vessel or exchanger, often with many bolts (could be dozens) and sometimes in confined spaces. Single stud replacement here must consider not just internal pressure but also any misalignment forces. If the vessel is large, removing bolts one by one could potentially let one side creep if not evenly supported. Standard practice is to evaluate if the vessel/flange can be safely clamp-supported or if the joint has any jacking screws that can be used to secure it. Typically, if such joints need bolt maintenance, they might be done during shutdowns unless absolutely necessary to do online.
In all cases, knowing the exact flange type and gasket is important. For example, a ring-type joint flange has a metal ring gasket in a groove. These gaskets rely on high stress to seal, and if that stress drops, they can unseal and might not seal again easily. Hot bolting an RTJ flange is especially delicate – the pressure should be very low and the process very controlled, because once an RTJ gasket leaks, you almost certainly have to depressurize to fix it. On the other hand, a soft gasket (fiber, graphite, spiral wound, etc.) might tolerate a little bit of load loss as long as it’s brief and restored quickly. The flange face type (flat vs. raised vs. tongue-and-groove) also can influence how a clamp is attached or how the load redistributes when a bolt is removed.
To summarize, the number of bolts and the flange design determine how you approach a single stud replacement. High bolt-count flanges and robust designs give more margin for error, whereas low bolt-count flanges are risky and demand additional precautions (like clamps and pressure reduction)[12]. Experienced engineers will review the flange’s details and perhaps even perform calculations (using gasket seating stress formulas or finite element analysis) to predict how the joint will behave if one bolt is removed. This analysis guides whether the procedure can be done safely or not.
Common Risks and Mitigation Techniques
Single stud replacement, being a form of live maintenance, carries several inherent risks. Understanding these risks and how to mitigate them is crucial for safe execution:
Leakage or Loss of Containment: The most immediate risk is that removing a bolt causes the flange to start leaking the contained fluid (which could be toxic, flammable, or high-pressure steam, etc.). A leak can range from a minor weep to a full loss of containment or even a violent blowout of the gasket. Mitigation: Preventive measures include reducing system pressure beforehand, using clamps to keep the gasket compressed, and replacing bolts in a sequence that minimizes localized stress drops. Additionally, continuous monitoring for leaks (using soap solution or gas detectors) will give an early warning to re-tighten or re-insert a bolt if a leak starts. A contingency plan (like an emergency shutdown or activation of isolation valves) must be in place for worst-case scenarios[15][16]. Keeping the operation slow and deliberate – e.g. pausing after each bolt replacement to check gasket integrity – can catch problems early.
Bolt Breakage or Stuck Bolts: There is a risk that a corroded stud could break while being turned, or be so seized that it cannot be removed by normal means. A broken bolt could be problematic if part of it is left in the flange (potentially requiring drilling out, which cannot be done live), essentially forcing a shutdown. Mitigation: Careful inspection during planning can identify severely corroded bolts that might not survive turning. These might be left for when the system is depressurized unless a very controlled method (like in-situ machining) is available. Using proper tools and not exceeding torque limits when loosening helps avoid snapping a bolt. If a nut is frozen, using a nut splitter to cut it off is preferable to putting excessive torque that might twist the stud in two.
Adjacent Bolt Overload or Joint Distortion: When one bolt is removed, the remaining bolts take on the extra load. If those bolts were already near yield or the flange is prone to bending, this shift could overload another bolt or warp the flange slightly, causing a leak. Mitigation: Ensure all other bolts are at proper tightness (if one is loose, tighten it before removing another). Sometimes, as part of prep, the flange may be uniformly re-torqued to make sure the load is evenly shared, before starting the one-by-one replacement. The use of a hot bolting clamp greatly mitigates this by carrying much of the load during the swap. Also, doing the swap on a cool system rather than hot can reduce thermal stresses that complicate load distribution.
Gasket Damage: Certain gaskets (especially older ones that have hardened or taken a set) might not rebound well if the load is momentarily reduced. They could start leaking or be permanently compromised by the disturbance. Mitigation: Try to perform hot bolting only on joints with relatively resilient gaskets or ones that were in good condition. If a joint is already suspect (weeping or known to have a damaged gasket), hot bolting may not fix that – in fact, it could make it worse. In such cases, a proper shutdown to replace the gasket might be the only safe solution. If proceeding, keep pressure low and replace bolts swiftly to minimize time that the gasket is under-deflected. In some operations, a very slight internal pressurization increase is done after removing a bolt (by throttling flow) to push the gasket harder against the remaining bolts – but this is a double-edged tactic and generally not recommended without thorough analysis.
Human Error and Communication Failures: As with any complex maintenance, missteps can happen – a worker might remove the wrong bolt or not tighten a new bolt fully, or there could be miscommunication leading to two people loosening bolts on the same flange at once. The consequences in a live system can be severe. Mitigation: Strict procedural adherence is key. Only one bolt at a time, and usually one team working on one flange at a time. Use a checklist to track which bolts have been replaced. Communicate clearly (“Bolt number 3 has been removed and replaced, moving to bolt 6 next”). Supervisors should enforce a one-bolt-out rule – do not even touch a second bolt until the first is fully back in and tight. Having an extra set of eyes (a senior supervisor or third-party inspector) can help catch potential human errors.
Environmental and External Risks: The environment around the flange can pose risks – for example, a tight space could accumulate leaked gas, or a nearby electrical source could ignite a flammable release. Weather can be a factor outdoors (wind dispersing a leak vs. stagnation). External loads, like vibrations from nearby equipment, could trigger a leak while a bolt is out. Mitigation: Ensure good ventilation in the work area or gas detection in confined spaces. If possible, isolate or shut down adjacent vibrating machinery during the hot bolting operation. Remove any ignition sources from the vicinity if dealing with flammable fluids. Set up barriers or exclusion zones around the work area so non-essential personnel are kept away.
Many of these risks were tragically highlighted in past incidents. As mentioned, the 1992 refinery explosion in Sodegaura, Japan was partially due to an improper hot bolting (hot torquing) operation. Lack of proper procedure and oversight can quickly lead to disaster. That is why the risk mitigation techniques for single stud replacement focus heavily on preparation and control. By doing a detailed assessment of the joint beforehand (material conditions, pressure, temperature, external stresses, etc.), one can identify hazards and plan how to avoid them[15]. For example, if analysis shows a flange is likely to leak when a bolt is removed, engineers might decide to weld on temporary support brackets or use multiple clamps, or just decide it’s not worth doing live.
Contingency planning is also a cornerstone of risk mitigation. Teams should ask: “What’s the worst-case scenario if this goes wrong, and what will we do?” If the answer to that is “We can immediately shut a valve and evacuate,” then ensure that valve is manned or automated and everyone knows the escape route. If a leak starts, maybe the plan is to reinstall the old bolt or a new bolt immediately and torque it down – so have that bolt and tools at the ready on the first sign of trouble. In some cases, a sealant injection gun might be prepared (for flanges equipped with injection ports) to seal a minor leak on the fly. Essentially, always have a Plan B (and C) when performing hot bolting. Or: just use Velocity Washers and avoid the need for single stud replacement altogether.
Finally, after completing a single stud replacement, treat that flange with caution until the system can be fully shut down and re-inspected in a controlled state. It’s wise to schedule a follow-up inspection at the next opportunity (the next shutdown) to check the gasket and re-tension all bolts if needed. Hot bolting is a bit of a balancing act – when done properly it can greatly extend the life of equipment and save downtime, but it must be done with respect for the risks involved at every step.
Conclusion
Single stud replacement is a powerful technique in the arsenal of bolted flange joint maintenance, allowing engineers to perform online bolt replacement (hot bolting) on critical equipment with minimal disruption to operations. By clearly understanding what the process entails, why it’s used, and how to do it step-by-step with the right tools, one can appreciate both the benefits and the dangers. Safety is the overriding concern – adhering to standards like ASME PCC-1 and PCC-2 bolting procedures, using proper clamps and equipment, and enforcing strict protocols are non-negotiable when doing live flange maintenance.
When applied judiciously, single stud replacement can prevent unplanned shutdowns, reduce leaks, and maintain joint integrity in aging facilities, all while keeping production running. However, it is not a routine task; it requires experienced personnel, careful planning, and a healthy respect for the potential hazards. The key takeaway for any professional engineer is to always weigh the risk vs. reward: if a single stud replacement can be done safely and offers significant benefit in avoiding downtime or failure, it is a worthwhile procedure – but if there’s any doubt, the safer choice may be to schedule a shutdown and do it the conventional way.
By following the guidance and precautions outlined above, engineers and maintenance teams can execute single stud replacements in a controlled and compliant manner, ensuring that bolted flanged joints remain secure and leak-free both during and after the operation. In the end, meticulous preparation and respect for the process will allow this hot bolting technique to be carried out successfully, maintaining safety and bolted joint integrity[17][18].
Sources:
The insights and procedures discussed in this article are informed by industry standards (ASME PCC-1 and PCC-2), technical case studies[8][12], and guidance from bolting specialists[10]. Always refer to the latest official standards and your company’s engineering practices before performing a single stud replacement.
[1] [2] [3] [4] [6] Hot Bolting and Single Stud Replacement - Enerpac Blog
https://blog.enerpac.com/hot-bolting-and-single-stud-replacement/
[5] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] How to safely, efficiently use hot bolting to combat corrosion - Drilling Contractor
https://drillingcontractor.org/how-to-safely-efficiently-use-hot-bolting-to-combat-corrosion-61018
Disclaimer:
Portions of this article were generated with the assistance of ChatGPT, a large language model developed by OpenAI. The content is provided for informational purposes only and does not constitute professional, legal, financial, or academic advice. The views expressed do not necessarily reflect those of the author, and readers are encouraged to independently verify any information presented.
The AI-generated content has been reviewed and edited for clarity and accuracy where appropriate.
The Galling Phenomenon: What exactly is galling?
Galling is a common mechanical phenomenon encountered during the fastening or disassembly of threaded components.
Galling is a common complication encountered during the fastening or disassembly of threaded components. It can result in damage to the threads or even cause the components to seize. These failures are often costly in terms of manpower, schedule delays, and lost production. Galling is a form of adhesive wear caused by material transfer between metallic surfaces in relative motion. It is driven by adhesion and friction, which can tear the crystal structure of the underlying material.
Threaded fastening, which involves sliding interlocking threads under high load, is particularly prone to galling. This is due to the inherent conditions of the process - such as ductile metals in contact, metal-on-metal sliding, friction, and high compressive loads - all of which not only promote galling but are essential to the operation itself.
When complementary screw threads make contact, the initial mating occurs at the asperities (microscopic high points) of the surfaces. These asperities concentrate stress and energy at a local scale, far exceeding average values. As sliding begins, these concentrated forces lead to plastic deformation, raising local temperatures and energy density. This accelerates adhesion, material transfer, and the formation of protrusions. If these protrusions exceed a critical size, they can breach the oxide layer of the mating thread and deform the underlying ductile material. The result is a plastic flow zone around the protrusion, where galling begins to propagate.
The rate at which energy accumulates in this localized system depends heavily on the size, shape, and material properties of the plastic zone. In contrast to brittle fractures, which generate little heat due to their small plastic zones, the ductility of most common machine screws makes them especially susceptible to galling.
During nut torqueing, axial bolt load contributes directly to energy buildup in the contact zone. As the nut turns and relative motion continues, energy accumulates due to limited heat dissipation - restricted by the small cross-sectional area available for thermal conduction. This results in rising energy density and surface temperature, which in turn alters plastic behavior and further promotes adhesion.
At a critical point, the combination of plastic deformation and sustained contact can create a shared plastic zone between the threads. This zone is characterized by high energy, pressure, and temperature, which can cause the two surfaces to bond. Once this occurs, continued rotation requires significantly more force and may even result in seizure of the nut. Removing a seized nut often requires destructive techniques such as cutting or drilling.
One of the most effective ways to prevent galling is by eliminating compressive thread load before turning the nut. Doing so reduces potential energy and frictional heating at the interface. Tools and systems designed to achieve this - such as hydraulic tensioners, hydraulic nuts, and anti-galling technology like the Velocity Washer - can significantly lower the risk of galling in critical applications.