Article by Mike Prevost (VP Specialty Services – Ohmstede Industrial Services, Inc.) and Scott Hamilton (CEO and Founder – Hex Technology)
By following guidelines spelled out in ASME PCC-1, utilizing certified and experienced bolting personnel, and implementing some shared best practices based on years of field experience, this article provides useful insights and suggestions for consideration related to minimizing leaks on heat exchangers.
- Probability of Failure (PoF), or the likelihood of leaking
- Consequence of Failure (CoF), of the potential for harm/damage if a leak occurs, or if the equipment is taken offline
Teamwork with an inherent burden on assemblers
Joint assembly is a team effort that involves many more people than just the crew putting their hands on the flange itself. The two groups of people with the most influence on the quality of the outcome are Engineering and Assembly teams.
- The bolting industry has been trained for decades to think bolt stress is more critical than gasket stress. Improper or unbalanced gasket load is responsible for the great majority of all flange leaks.
- Appendix O is a comparatively new way to calculate bolt load based on the resulting gasket stress.
- Because Appendix O of PCC-1 has only been around for 10 years, not all design codes have been updated recently or thoroughly enough to use it.
What’s a good heat exchanger joint assembly process?
The right amount of gasket compression
Bolts (a.k.a. studs) have an elastic property, which allows them to work as springs to compress the flanges together on the gasket. When load is applied, it stretches the bolt, which then tries to return to its original length. This creates compressive force across the joint members. See how a fastener acts as a spring to compress the gasket between two flanges in the diagram below (see figure 3).
Bolted flange joint assemblies create a “Clamping Force” across the joint surface that applies gasket stress or compression sufficient to maintain a seal during normal operating conditions.
Torqueing and tensioning are two different methods for achieving an axial load on the bolt. Torqueing uses the turning of the nut to stretch the bolt. Tensioning uses different tooling that stretches the bolt without requiring the turning of the nut to achieve the bolt load, as illustrated above (see figure 4).
Gasket selection is also essential. A Double Jacketed gasket has been the staple for heat exchanger gaskets for years. However, we have seen that Kammprofile gaskets have produced better results in most cases. Manipulating the area of the Kammprofile gaskets allows for proper gasket stress. By following the ASME PCC-1 (Appendix A) to train bolting personnel, owners can dramatically improve the likelihood of achieving leak-free exchangers.
Hairpin Heat Exchangers
Bolted flange joints on Hairpins are often not so well designed as they ought to be and are prone to leakage as a result. Manufacturers often prescribe higher torque values to make up for this deficit.
Plate Heat Exchangers (PHE)
Plate heat exchangers require rubber gaskets, which cannot withstand the amount of gasket stress that a Kammprofile or a Spiral Wound Gasket (SWG) can withstand.
These rubber gaskets are relatively easy to over-compress; and as a result, it is easy to damage them. Therefore, assembly teams must obtain proper torque values from the manufacturer and work across the body of the exchanger in a crisscrossed pattern.
Hairpin Heat Exchangers
Bolted flange joints on Hairpins are often not so well designed as they ought to be and are prone to leakage as a result.
Manufacturers often prescribe higher torque values to make up for this deficit.
Air-Cooled (a.k.a. “Fin Fan”) Heat Exchangers
Typical maintenance on an Air-Cooled Heat Exchanger includes removing plugs, replacing them, and then reinstalling them into the header box. This process is trickier than it sounds.
The #1 rule for assemblers here should be: Do NOT use an impact wrench for assembly or disassembly. It is very easy to strip threads on these units, and experience proves they are more prone to galling. Proper lubrication should be considered a must on all assemblies.
Shell and Tube sheet Heat Exchangers
- Flange circumferential variation (T1), or the “rollout” on a flange. This refers to variations in height around the flange sealing surface.
- Radial variation tolerance (T2),or what’s commonly referred to as “high-low.” T2 is the difference in height from the inside to the outside of a sealing surface.
- Pass partition tolerance, which refers to height differences on any of the pass partitions within the flange.
There are multiple complex methods for measuring these variations. However, the differentials are so small that it is difficult, if not impossible, to determine if the issue is within or outside of the accepted range with the naked eye. Therefore, if an assembly crew has any reason to believe there’s an issue with these surfaces, the best practice is to speak with a supervisor and bring in a machining tool for a proper analysis.
About the authors
Ohmstede Industrial Services, Inc.can provide ASME PCC-1 trained personnel for bolted flange joint assemblies for exchangers and piping. Mike Prevost has over 30 years of experience in the oil and gas business. He has worked with bolted flange joint connections for well over 15 years.
Hex Technology, LLCis a technology-based company specializing in the science and training of bolted flange joint connections. Scott Hamilton has a passion for bolting and for making bolted flange joint training accessible to everyone. He has a staff of experienced and committed people who bring the field to the classroom – the training is down to earth, practical, and technically accurate.
For more information email Scott@hextechnology.com.