What is Hot Bolting?

Hot bolting refers to the technique of replacing bolts on a live pipeline or flange system. This process is conducted one bolt at a time and is typically employed only when no other maintenance options are feasible.

Critical Criteria for Hot Bolting

Before performing hot bolting, several essential criteria must be met to ensure safety and effectiveness:

Operating Pressure: The operating pressure must be below 75% of the Maximum Allowable Working Pressure (MAWP) in accordance with ANSI B16.5, considering the operating temperature of the piping or process system.

Flange Configuration: The flange should have a minimum of eight bolts to ensure structural integrity.

Temperature Range: The process temperature should be maintained between 4°C and 71°C.

System Support: All flanges and associated equipment must be adequately supported to prevent excessive vibration or pulsation.

Safety Precautions during Hot Bolting :

Monitoring for hydrocarbons is essential throughout the hot bolting operation. The procedure closely resembles the sequence used for tightening bolts. Specific safety checks include:

Gasket Area Inspection: Ensure that the gasket area shows no signs of leakage.

Condition of Components: Verify that piping, flanges, and bolts are not severely corroded, which could compromise their integrity.

Tightness of Existing Bolts: All existing flange bolts and nuts should be tight and of the appropriate size and grade.

Hydrocarbon Monitoring: Continuous monitoring for hydrocarbons is essential during the hot bolting operation.

Hot Bolting Procedure

The hot bolting procedure follows a sequence similar to that of tightening bolts. Important steps include:

Leakage Inspection: Ensure that the gasket area is free from any signs of leakage.

Component Condition: Check that piping, flanges, and bolts are not severely corroded, which could compromise their integrity.

Tightness of Existing Hardware: All existing flange bolts and nuts should be tight and of the correct size and grade.

Considerations for Bolt Replacement

When deciding whether to reuse or replace bolts, the decision largely depends on the cost and degree of corrosion. Careful reusability is advisable for large-diameter, expensive bolts. Conversely, smaller, more common materials may be replaced as a standard policy.

Real-World Applications

Hot bolting is practised in various high-temperature and high-pressure environments. For example:

Operating at High Temperatures: We have successfully hot bolted at temperatures of 1250°F with a pressure of 250 psig and routinely on polymer lines at 600°F and 1500 psig.

Production Unit Overhaul: Our processes require dismantling production units during overhauls, which involves removing thousands of studs, primarily B-16 5/8″ to 1/2″, and significant B8 Cl2 materials.

Stud and Bolt Reusability :

In our operations, components using H11 Socket Head Cap Screws (SHCS) are changed and reused on a set schedule of about 15 days. Each line has 48 of these components with 98 SHCS. Given that we have 17 lines, the reuse of studs and bolts is nearly essential.

While the studs are not formally inspected, we employ a screening process known as “Daging. ” We add two nuts to each stud and apply a colloidal graphite lubricant. The studs are then run through a pyrolysis furnace to remove any existing coating, followed by a Daging bath.

Quality Control and Rejection Rates :

After the Design process, studs and nuts are reassembled with full nut engagement on both ends. Remarkably, less than 1% of students are rejected during this process. Light-duty mechanics recover most rejections, and it’s rare for issues to arise when reassembling the piping. Some studs in service are over 40 years old, including Crane Alloy studs from the 1950s.

Clarifying Hot Bolting vs. Hot Torqueing :

It’s important to clarify that “hot bolting” and “hot torquing” are distinct operations. Hot bolting involves replacing or servicing bolts, while hot torquing refers to retightening bolted connections under operating conditions. Both processes require careful execution, particularly regarding the correct tightening sequence. Mistakes during hot torquing can lead to significant issues.

Calculating Maximum Operating Pressure for Hot Bolting:

When performing hot bolting, calculating the maximum operating pressure is crucial. While you can use an ASME Section VII calculator (like Mr. Pedersen’s), simply redistributing the stress across remaining bolts after removing one may not provide an accurate representation.

To determine the proper spacing and bolt stress, you might consider simulating the removal of half the bolts. However, this method can yield overly conservative estimates regarding bolt stress related to MAWP. Focus on identifying the limiting factor between bolt stress and seating pressure for accurate assessments.

Industry Practices and Perspectives

While some may view the practice of removing one bolt at a time from a pressurized vessel as dangerous, it is a standard procedure in the industry. Many large companies have developed specific protocols for hot bolting, ensuring safe execution.

Typical scenarios for hot bolting include:

Routine Maintenance: During turnaround maintenance, bolts may be removed and inspected.

Corrosion Repair: If corrosion is found on a bolt, it can be cleaned and lubricated before reinstallation.

Replacement of Inadequate Bolts: When bolts are too short or not of the correct grade, replacement becomes necessary.

Conclusion

Hot bolting is a crucial maintenance procedure in various industrial applications. While it requires strict adherence to safety protocols and criteria, it can be effectively performed when no alternative solutions are available. Understanding the associated risks and implementing careful monitoring can help ensure successful hot bolting operations.

FAQs

Q. 1 Is hot bolting safe?

Ans: Yes, it is safe if done according to established procedures and safety protocols.

Q.2 How often should hot bolting be performed?

Ans: Hot bolting should be performed as needed, particularly when bolts need to be maintained or replaced.

Q.3 What should I monitor during hot bolting?

Ans: Continuous monitoring of hydrocarbons and checking for any signs of leakage or corrosion are essential during the process.

Q.4 Can hot bolting be done on any type of piping?

Ans: Hot bolting is generally suitable for flanged systems that meet specific criteria, including pressure and temperature limits.

Q.5 What are the alternatives to hot bolting?

Ans : If feasible, consider options like depressurizing the system or using other maintenance techniques that do not involve working on a live line.

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Er. JOGINDER CHAUHAN

Hi, all power engineers. I am Joginder Chauhan(B.Tech(Mech.Engg.)BOE/E.M.) I am the founder of the website www.askpowerplant.com. I am also a power plant engineer with over 12+ years of experience in the power sector. I learned about power technologies like AFBC, CFBC, TRAVELLING GRATE, PULSATING GREAT, PF BOILER, WHRS, and many more during this period. My motive for this site is to share my power sector knowledge with every person who belongs to this field; on this website, I share information related to boiler operation, boiler maintenance, turbine operation & maintenance, boiler & turbine question answers, D.M plant, and all topic related to power plant sector please join this website for latest updation regarding power plant ………..thanks ( JOGINDER CHAUHAN )

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