Single Stud Replacement: An (ineffective) method to deal with galling

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.

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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.

  1. 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].

  2. 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.)

  3. 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).

  4. 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.

  5. 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.

  6. 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.

  7. 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].

  8. 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.

  9. 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.

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