blowdown and set pressure drift

[gmax137]

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??

 

[Latexman]

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