×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Tolerances & Relief Valve Testing

Tolerances & Relief Valve Testing

Tolerances & Relief Valve Testing

(OP)
When you test a relief valve to verify its setpoint, do you count instrument accuracy and setpoint tolerance simultaneously?  For example, if you (or the OEM) have relief valve with a rated setpoint tolerance of +/- 3%, and you measure inlet pressure with a gauge that is accurate to +/- 0.5%, do you look for the actual popping pressure to be within +/- 3% or +/- 2.5%?

If the code were to specify separate tolerances for the setpoint AND for the test instrument, it would seem to make some sense that you would not "add" the tolerances together during the test.  But is this a true assumption?  And what if the code does NOT specify a separate test instrument tolerance?

RE: 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.  

RE: Tolerances & Relief Valve Testing

(OP)
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)

RE: Tolerances & Relief Valve Testing

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...

RE: Tolerances & Relief Valve Testing

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
www.delucatest.com

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources