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BPVN (Mechanical)
4 Jun 08 20:09
Hi,
From where, i can find reference in the Code(ASME VIII div. 1) about the required accuracy of Dial Pressure Gauges(on the range or 'span'),Except UG-102(b).
Thanks in advance
BPVN
gr2vessels (Mechanical)
4 Jun 08 22:40
BPVN,
There is no specified accuracy required for those pressure gauges except your own specification, calling for a recognized gauge manufacturer. For a range not larger than double of the test pressure, the accuracy of those gauges are deemed acceptable. I wonder if any AI or even insurance companies would have a specification or the like on these requirements.
The ambiguous issue is the interpretation of the "calibration" and re-calibration intervals. Would you trust the recommended 12 month recalibration time on a Chinese gauge, risking to blow up your vessel? Would you recalibrate every three months all the 24 gauges in your inventory, just in case one is out of calibration? Is it acceptable to use a gauge unchecked for 12 months, assuming acceptable accuracy without any appropriate test? Would the AI accept the gauge's log book, to prove the re-calibration done 11 months and 25 days ago? What are the legal ramifications of a bad gauge, having a good logbook, but no test since re-calibration?
Any other comments?
cheers,
gr2vessels
JLSeagull (Electrical)
5 Jun 08 7:53
The accuracy of the typical 4.5-inch diameter process pressure gauge is Grade 2A or +/- 0.5% of the full scale range (span) excluding friction.  The bottom 5% of most gauges is pretty sloppy due to friction error, etc.  Perhaps as a generalization, lower ranges and smaller gauges are less accurate than larger gauges and higher ranges.  ASME B40.100, Pressure Gauges and Gauge Attachments applies accuracy grades for gauges.

The sequence of ASME B40.100 accuracy ranges from most accurate to least accurate are 4A at 0.1% and 8.5-inch diameter, 3A at 0.25%, 2A at 0.5%, 1A at 1%; then A, B, C and D at diminshing accuracies.
 
JoeTank (Structural)
6 Jun 08 9:48
For pressure relieving devices ASME states a tolerance of 2 psi for vessel MAWPs up to 70 psi.  A 3% tolerance is stated for higher pressures.

Joe Tank

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