Cv question...

hvacfreak

I was wondering...if I know the pressure drop across a device ( in this case it is a 2 inch ball valve ), and I have the Cv rating of the device...can I get an actual GPM figure from that ? Here's my " example " , not sure if this is correct...

Cv = 217 , measured pressure drop is 5 psi


217 gpm @ 1 psi...so 217 / 5 = 43.4 GPM

Any help appreciated. -M

Steamhead (in transit)

I Think

you multiply instead of dividing. So if your valve has a 217 Cv, at a 5 psi delta P, you'll have a 1085 gpm flow...assuming it's just warm water.

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hvacfreak

Yeah...I tried that ...the number just looked to crazy. This is on a cooling tower ( open system )...I measured 24 psi right before this valve and 19 psi just after. I'll check the Nibco site and confirm that this Cv is based on US Gal / PSI.

Steamhead (in transit)

Ooops, You're right

It's a bit more involved: Cv = gpm(sp grav./delta P)to the 1/2 power. That's for water. Steam's a different formula.

It's about 487 gpm.

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DanHolohan

If the Cv is 217

then the flow rate at a 5-psi pressure differential will be about 480 gpm. My old eyes aren't as good with the System Syzer as they once were, but that's pretty close.

hvacfreak

whoa...thanks guys

I need to digest this I guess. The piping is 4 inch reducing to 2 inch heat exchanger connections.

Here's the valve info ( page 108 ) F-510 ball valve 2 inch.

http://www.nibco.com/assets/BallValve.pdf

I'm sure you guys are right...just hard to imagine ( I watched the water flow from the top of the tower today for awhile ). Thanks again. - M

bob_46

Dan- old

Talk about old Dan. When I read the post I reached for the "old" Syzer. On the back is the distributers name and phone number. See if you recognize it. Bornquist, Inc. SPring 4-2800 7050 Lehigh Avenue, Price $2.00. Regards bob