Use of 150# SS Fittings B31.3 2

[AWH]

Hello,

I'm new to B31.3 and am trying to clarify the use of SS threaded pipe fittings. Can anyone clarify what ASME/ANSI/MSS standard covers the use of 150lb cast stainless steel threaded pipe fittings?

Regards,
Allen

 

[bvi]

Iron threaded fittings would be Class 150. Steel threaded fittings would be per ASME B16.9, and would be 2000# to 9000#.

 

[donf]

They're covered by MSS SP-114.

http://www.mss-hq.org/price_list.htm

Good luck using threaded SS pipe and fittings. My experience is that they're notorious leakers.

Donf

 

[AWH]

Thank you Donf for the information. Do you have any knowledge of MSS SP-114 fittings being acceptable for B31.3 normal service?

Allen

 

[moltenmetal]

Leakage with threaded systems is entirely a matter of how good the piping designer and pipefitter are and what kind of sealant system is used. If an appropriate sealant is used, properly-assembled threaded piping is appropriate for B31.3 normal fluid service up to and including 2"NPS.

Commonsense on the part of the person selecting the fitting method used for the service is key- that goes for all piping systems, but it goes double when threaded piping is selected.

Clearly, if you have significant vibration, wide/frequent temperature cycling, the tendency for crevice corrosion, or solvents which will affect the sealant chosen, or if minor leakage will result in catastrophe if it ever occurs, you're better off with welded/flanged piping. You've got to trade those issues off against the (greatly) reduced cost of fabrication and the increased flexibility to future change offered by threaded piping.

Stainless adds a complication not encountered to the same degree with malleable iron fittings and steel pipe: galling during installation. Choose the wrong sealant system or install the stainless pipe/threaded fitting combination improperly and you will get leaks because the pipe/fittings gall before they're tightened adequately.

As to 150# stainless fittings, be careful to ensure that you use the correct design T/P range for these fittings- it's quite limited. There are both 150# cast and 1000# forged fittings in stainless (with similar dimensions) and they have widely different design T/P ranges.

One area of concern is the lack of mill test reports for 150# fittings. The best you can get around here is lot certificates of compliance, which may not satisfy inspectors/regulators in some jurisdictions.

 

[bvi]

They are not listed in ASME B31.3 so they would have to be qualified as unlisted components. Suggest using B16.9 fittings if it is a B31.3 system.