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blowdown and set pressure drift

blowdown and set pressure drift

blowdown and set pressure drift

(OP)
I'm trying to understand how steam safety valves work.  The ones I'm looking at are large spring safety valves (6 inch inlet, around 800,000 lb/hr capacity) with adjustable blowdown.  Now my understanding of blowdown is this:  if the set pressure is say 1000 psi, and the blowdown is 5%, then once the valve opens on high pressure, it will close when the pressure drops to 1000(0.95) = 950 psi.  Now if the same valve (still set  for 5% blowdown) opens at a lower pressure (say 3% low) does it still close at 950 psi, or will the blowdown now be 1000(0.97)(0.95) = 921 psi?  I guess I don't really understand how the adjustments to set pressure and blowdown actually work.

Or, am I overdoing it here, are these settings really so precise??

RE: blowdown and set pressure drift

I'd say the PSV that actually opens at 970 psi (3% low) with a 5% blowdown, will reseat at about 920 psi, and, yes, you are carrying too many significant figures for these type valves in my opinion.

Good luck,
Latexman

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