Flange face finish

[anning]

We're installing some new 2500# piping in hydrogen service and we have several RFWN joints. ASME B16.5 2009 states that the flange faces shall have a surface finish from 125 to 250 AARH but our plant specifications state that the finish must be 125 AARH. The gaskets we're using are 1/8" spiral wound 304 SS with flexible graphite filler and CS centering and inner rings. Does anyone know whether the finer surface finish provides better sealing? Historically this piping has proven to be difficult to seal properly so we want to ensure we're doing all we can to attain the highest quality joints.

 

[austsa]

For such high pressure piping, I would highly recommend RTJ flanges & gaskets, to ensure good sealing quality.

 

[TGS4]

anning - your 125 AARH is within the range provided in B16.5. I would tend to more coarse flange facings myself, but that may just be a personal preference.

What are you doing to ensure that the flanges are assembled properly. Despite austsa's comment, Class 2500 B16.5 flanges do indeed work, but you need to do them right. You are following ASME PCC-1 assembly instructions, right?

 

[anning]

After doing some research, it appears that the requirement of 125 AARH is a holdover from the past. The 1988 edition of B16.5 specified a flange face surface finish of 125-500 AARH and it wasn't until the 1996 edition that it was changed to 125-250 AARH. I expect the special finish was a requirement to stay away from 500 AARH finishes. It seems that companies have moved away from the special finish requirement as the code no longer allows the coarser finishes.

Yes, we follow PCC-1. It makes sense to me that the quality of the joint would be far more sensitive to flange alignment, proper bolting, gasket and seal face condition and cleanliness and pipe stresses and vibration.

Thanks for your help.