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Asme Sec Viii Blowdown Limits

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#1 SawsanAli311


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Posted 28 March 2020 - 08:36 AM

Dear All,

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

ii- increase tightness of the pilot operated valves as the force applied over the seat increases as it is imposed on a greater area.




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