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Set Pressure Tolerance and Blowdown Definitions

Set Pressure Tolerance and Blowdown Definitions

Set Pressure Tolerance and Blowdown Definitions


I would like to enquire on the below key items addressed in ASME SEC VIII Div.1 UG-126 and Non Mandatory Appendix-M :

a- what is the significance of the set pressure tolerances mentioned as max +/- 3% for set points above 70 Psi? Is this tolerance associated with the testing of the PSV and assigning its set point specially when it is a conventional PSV where its set point is compensated in the spring setting? Is this tolerence affected by the gauge used in the test? Will this tolerance affect the setting of the set pressure in the 1st place. I read that this tolerance doesn't affect the difference between the set pressure and blowdown.
b- ASME also discusses the differential to set pressure which is the blowdown stating it shall have a minimum of 7% for set pressure above 1000 Psi, with a set pressure tolerance of +/-3%. Is the value of 7% blowdown applicable to pilot operated valves also.. pilot operated valves can achieve a small blowdown down to 2% to 5%, however, the question is :'' do these small blowdown mandate having soft seat material which may not be compatible with the service composition and temperature''?
c- In fact, nonflowing pilot operated are used to allow exceeding the 3% inlet pressure loss while sizing the remote sensing line.. this surely is promoted by the small blowdown which can be achieved with or even without soft seat material.. do you agree?

d- we tend to install pilot operated for systems where that operating pressure to set pressure ratio is more than 90%.. this means that the pilot valve is subject to very close tolerance to its set pressure, do you agree this is driven by:

i- small blowdown achieved by the pilot operated valves which means the PSV will be able to remain close even at high operating pressure > 90% of PSV set pressure
ii- increase tightness of the pilot operated valves as the force applied over the seat increases as it is imposed on a greater area.

Additionally, in order to mitigate the cycling prone chattering (relatively low frequency chattering) for the case of oversized PSV .. if my PSV is already the smallest in size as D orifice.. would you recommend using modulating pilot operated PSVs in order to restrict the lift of the PSV such that the pressure at the inlet of the PSV would not fall below the closing pressure.

Do you agree that using pilot operated PSVs for mitigating high PSV inlet pressure losses would not be attractive unless the pilot valve is modulating non flowing PSV with the presence of blowdown seat which prevents flow inside the valve during relieving.


RE: Set Pressure Tolerance and Blowdown Definitions

Hi. I've just come across your post while searching under the blowdown topic.

If you still require comments/answers, please repost in the safety relief valve forum.

Per ISO-4126, only the term Safety Valve is used regardless of application or design.

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