Thermodynamic question

Doug Murphy_2

Hello all,

I have an interesting question that possibly someone might be able to help me with.

In a PRV (Pressure Reducing Valve) what is the process that is being performed? Is it throttling or is it doing work (on the spring)?If it is work being done can this be shown on a Mollier diagram. (Following the line of constant superheat.)
Any insight would be helpful. Opinions area welcome.
If you know where the answer is (text) please point me in the right direction.

Thanks
Doug Murphy

Dan Law_3

PRV

What application are you speaking of? Water, air, steam, refrigeration?

Tony Conner

Steam PRVs...

...are usually a spring & diaphragm arrangement, and throttle to maintain the downstream pressure. Temperature control valves do the same throttling, but they're looking at maintaining a temperature. The action is the same though, and most manufacturers use the same main valve body for both applications, just different pilots. You can get work out of this pressure drop, and that's what some places do with back-pressure steam turbines. You need a pretty steady, year round load of about 10,000 #/hr to make this pay, though.

You'll get some superheat downstream from a PRV, and you can calculate this, but to do it, you need to know the steam quality upstream. This value is not usually available, but very few (if any) saturated steam systems have totally dry steam. What you'll typically get in practice, is very high quality steam a few feet downstream of the PRV. I once installed a thermometer downstream of a 125 PSIG to 10 PSIG PRV and the steam temp would wander with the load. Sometimes you'd see quite a bit of superheat (higher load), and at low loads, very little or none at all.