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PSV Sizing for a PD Pump

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Ian19

Ian19

Mechanical
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Feb 28, 2012
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We have a Positive Displacement Pump on a very old system which requires a new PSV. We have no information on the previous PSV and it has been machined during maintenance over the years. The valve must be set to crack at 17 MPA, the manufacturer has sent us a new valve which is physically about 1/4 of the size of the previous valve although it is set to relieve at 17 MPA. Question is what other parameters need considered given that the orifice through the valve is smaller and inlet and outlet are also smaller.
 
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Please don't take this the wrong way, but why are you not asking the manufacturer these questions? 1/4 the size is huge! For example, 1" vs. 4". Flow is porportional to diameter squared. 1/16th the flow? Wow!!! Are they working off a make, model. and serial number? They should be almost identical, or about the same size. Something is wrong somewhere!

Good luck,
Latexman
 
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Any such pump that I have ever had to specify required the PSV to be sized for full pump flow at its set pressure. If you have pump nameplate data then that ought to be all you need.

Regards,

SNORGY.
 
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The difference in apperat size could e.g. be if the original was a spring loaded and the new was a pilot. But you should do as Latexman says: Ask the vendor!

Best regards

Morten
 
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Is the reliving rate similar to [or larger than] the PD pump's pumping rate? If so, its fine. Otherwise, ask ASAP! Somebody at the mfr may have transposed a number. It happens.
 
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This is speculation but I have run across systems where a relief valve between a pump and a vessel was sized for scenarios that protected both.

It may well be that that the pump alone only requires a small PSV, but another relief scenario is depending on the rate the large PSV provides. A indication this might be the case on older systems is the lack of block valves between pump and vessel, close physical proximity with minimal piping runs, etc...


Take a system approach when evaluating relief valves and sudden size changes such as what you are experiencing here.


 
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