ASME 31.4 / 31.8 conversion 2

[linford]

ASME 31.8 mentions conversions from lines previously covered by 31.4 into dual-use or gas service lines. Furthermore OPS regulations also discuss conversions from liquid to gas service. I can't really find any discussion on the opposite conversion (from lines covered by 31.8 to a 31.4 line). While this question may seem stupid, if a line was designed to 31.8 would it necessarily comply with 31.4? 31.4 is generally considered a looser standard, no? Anybody have a good overview look at the essential differences between 31.4 and 31.8 (besides, of course, the product moved)?

 

[EEG]

The main thing you need to look at is the Pipe Wall calculation. The thickness of B31.8 piping varies with population density as accounted for with Basic Design Factor F. B31.4 has no such requirement. Chances are if the B31.8 piping was design for Class 1, it will not meet B31.4. If the B31.8 pipig is Class 4 it should easily meet B31.4.

 

[linford]

Thanks eeg for your input. I've read both documents and was able to pin down what you said and a few other points, if anybody out there ever needs this...

My piping was designed for class 3, but exceeded those requirements because it was a metering skid. The thickness on the walls was much greater than required by the design pressure (the flanges are the most restrictive component). And the only reason why 31.8 walls need to be less thick is the lower safety factor for class 1 areas.

Class two and above have identical (.72) or lower safety factors (31.4 has a constant safety factor of .72, although I believe it is called something different, lord knows why).

Besides that, the hydrotest time for a 31.4 system is 4 hours vs the 2 hours for 31.8 (which I found surprising). The test pressures are greater for 31.8 (especially for class 3) but the duration is shorter. Thankfully we tested our skid well above what we needed it for.