Tolerances & Relief Valve Testing

Packdad,

It is "implied" that, in order to accurately set your relief valve within the specified tolerances identified in ASME Section VIII, you will need to take into account (allow for) the pressure gauge accuracy.

When you say "take into account", do you mean that the instrument accuracy must be subtracted from the valve setpoint accuracy, or do you mean that the instrument accuracy must simply be less than or equal to the setpoint accuracy? (In other words, +/- 3% versus +/- 2.5% using the example above)

in order to set your valve within the ASME specified tolerances, you would need to, at a minimum, subtract the equipment uncertainty(accuracy) from the specified tolerance. In this case if the specified tolerance was +/-3%, and the gauge uncertainty was .5%, then the set pressure should be set within +/- 2.5%so that the valve will be set within the 3% allowance.

Keep in mind that there are other factors that will affect accuracy. Test stand volume, gauge size and range, location of pressure tap to valve inlet(for water leg), etc...

AMSE PTC 25-2001 is the definitive reference for Pressure Relief Valve Testing. PTC 25 addresses measurement uncertainty. The National Board of Boiler & Pressure Vessel Inspectors as ASME Designee for the ASME Code Symbol Stamp Programs (V, UV, HV, NV) for Pressure Relief Valves has made this issue a priority for PRV Manufacturers and Assemblers for the past several years. Their approach has been to "require" ASME Certificate Holders to limit their actual test pressure tolerance to less than the ASME allowable tolerance. That is, the Certificate Holder has made a committment in their QC Program to use only +/- 2% rather than 3% of Set Pressure.
I have a different view of measurement uncertainty which comes at the issue from an instrumentation angle rather than as a Code Issue. ASME PTC 19 is a document on Instrumentation which calls for a minimum 4:1 accuracy ratio between the Calibration Standard and the Test Object. In other words, a Test Gauge should be at least 4 times more accurate than the PRV being Tested. For ASME Sec. VIII Applications, the minimum tolerance is +/- 2 psi from 15 psi to 70 psi. This means the Test Gauge needs to be accurate to +/- .5 psi or 4 times > 2 psi. If you "do the numbers", a 0 to 200 psi Test Gauge with .25% accuracy will read +/- .5% of Full Scale or .5 psi. Therefore, a 200 psi gauge would be sufficiently accurate to test any PRV set between 15 and 175 psi. Again, the minimum recommended accuracy ratio is 4:1, but the gauge will become increasingly more accurate as the PRV Set Pressure increases. For example, at 150 psi the tolerance is +/- 3% or 4.5 psi which is a 9:1 accuracy ratio. I hope this long explanation is helpful.

J. Alton Cox