Fraction of Gasket Load Remaining after Relaxation, ϕg, for Sheet Gaskets
Tightening your bolted flange assembly and walking away for good may not work well for all gaskets. In fact, gaskets with a low Fraction of Gasket Load Remaining after Relaxation, ϕg, may need to be retightened after relaxing.
A 0.7 default value for the Fraction of Gasket Load Remaining after Relaxation, ϕg, is often used in ASME PCC-1 Appendix O calculations. This default value may not accurately reflect the true Gasket Relaxation Fraction of the gasket material, especially for ⅛” thick sheet gaskets. Many times thick sheet gaskets relax more than anticipated.
Some gasket materials, like PTFE, may retain only around 50% of their original gasket stress a week after assembly. This is why retightening back to the intended Target Assembly Gasket Stress,SgT is a great idea. Retightening a week after original assembly can help minimize the chances of the gasket “walking” during operation and reduce the occurrence of leaks.
Gasket Area in PCC-1 Appendix O
Can you calculate gasket area?
Out of all of the complexities regarding modern gaskets, Gasket Area should not be something that designers are still getting wrong.
And yet here we are…
I’ve met Owner/Operators who love to order gaskets with a larger outer diameter than the raised face outer diameter so they can “see the gasket”. That’s fine enough but using that outer gasket diameter in ASME Appendix 2 Flange and ASME PCC-1 Appendix O calculations is not. Please be careful not to include the gasket area outside of the raised face diameter.
Another common calculation mistake arises when the gasket diameter fits within the flange raised face diameter. Many times designers use the gasket raised face diameter instead of the actual gasket diameter. This is usually because the raised face diameter is the “default” value in the spreadsheet they are using. Please be careful to use the actual gasket area when it resides within the flange raised face diameter.
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Fraction of Gasket Load Remaining after Relaxation - Phi, g (ϕg)
When I think about phi, g, I think about relaxation. But have we been too relaxed about phi, g?
ASME PCC-1 Appendix O allows a default Fraction of Gasket Load Remaining after Relaxation (ϕg) of 0.7 if no data is available. I have found that this statement leads designers to use the “default” value for too many gasket types. 0.7 is too high for some PTFE gaskets and other gaskets with high compressibility and can lead to leaky flanges over time. Please consider your gasket material before selecting the Fraction of Gasket Load Remaining after Relaxation (ϕg) needed in the ASME PCC-1 Appendix O calculation.
Did you know that the Gasket Load Remaining after Relaxation (ϕg) may be lower for thicker gaskets? Please consider the possibility of using a lower Gasket Load Remaining after Relaxation (ϕg) value when increasing compressible gaskets from 1/16” to 1/8” thickness.
Unsure what “default” Fraction of Gasket Load Remaining after Relaxation (ϕg) your internal spreadsheets are using? Metalmark Engineering offers on-site training to teach your team how to use modern software solutions.
Minimum Gasket Seating Stress (Sgmin-S)
Missing the Minimum Gasket Seating Stress (Sgmin-S)? No problem!
It’s true that not all gasket manufacturers provide each of the gasket variables needed to perform the ASME PCC-1 Appendix O checks. Here’s a trick I like to use if I don’t have the Minimum Gasket Seating Stress (Sgmin-S). I use a simple substitution, cancel out a few terms, and then I start printing money. Lots of it.
And now you can too!
I like the Target Assembly Gasket Stress (Sgt) to govern the Selected Assembly Bolt Stress (Sbsel). In this case you may not need the Minimum Gasket Seating Stress (Sgmin-S). This is because it is assumed that the Target Assembly Gasket Stress (Sgt) is greater than the Minimum Gasket Seating Stress (Sgmin-S). Without the bolts or flange to govern the Target Assembly Gasket Stress (Sgt), there is no way for the Minimum Gasket Seating Stress (Sgmin-S) to fail the ASME PCC-1 Appendix O check.
Stop Blaming the Gasket for Bad Flange Designs
Stop blaming the gasket manufacturer for your bad flange designs.
I’ve noticed the gasket manufacturers always get the blame for leaky flanges. But did the designer even consider that the flange could be governing the bolted flange joint assembly? Or did they just grab an off-the-shelf ASME B16.5/16.47 flange and assume everything would happen to work out?
There are many cases for ASME B16.5/16.47 flanges where the Target Assembly Gasket Stress, SgT, is difficult to achieve without exceeding the Maximum Permissible Bolt Stress Prior to Flange Damage, Sfmax. This means that the flange may yield and start leaking before the gasket ever reaches its optimal stress condition for sealing!
There is not much the gasket manufacturer can do for your damaged flange.
Please refer to Table O-4.2-1 inside ASME PCC-1 Appendix O for a list of ASME B16.5/16.47 governed by the Maximum Permissible Bolt Stress Prior to Flange Damage, Sfmax.
Please refer to WRC 538 to better understand which flange stresses are governing the Maximum Permissible Bolt Stress Prior to Flange Damage, Sfmax in your designs.
Change Gasket with Service Change
Thinking about changing the service of existing equipment or piping? Don’t forget the gaskets!
I’m not a big fan of changing from methane service to hydrogen service, but I also understand that changes like these are becoming more and more popular as new construction costs rise. There may be some cost savings converting existing equipment and pipelines compared to building new. However, differences in the service conditions, such as molecule size, can have a big impact on gasket selection.
Please note differences in the PCC-1 Appendix O definitions for the new gaskets including:
Target Assembly Gasket Stress, SgT
Maximum Permissible Gasket Stress, Sgmax
Minimum Gasket Operating Stress, Sgmin-O
Minimum Gasket Seating Stress, Sgmin-S
Changing the gasket Target Assembly Gasket Stress, SgT directly impacts the Selected Assembly Bolt Stress, Sbsel in ASME PCC-1 Appendix O so a complete PCC-1 Appendix O recalculation is required!