Galling: The Hidden Enemy of Heat Exchangers
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.
