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# Cv value

Member Posts: 3

• Member Posts: 3

This is probably an easy question for most, but here goes. If I have a valve with a Cv of 10 how much will it increase the head pressure of the system?
• Member Posts: 60
Don't listen to me!

If i read prim second pumping made easy, pages 59-63 right, Cv=10 means that you need to move 10 gals of water through the valve to create a 1 psi dif in pressure through the valve =2.31'head
If you move 8 gallons of water through it, head loss =square of flow rate=8*8=64psi drop accross the valve=147.84 feet of head. and if you press 15 gallons through it 15*15=225psi accross the valve=519.75 feet of head.

Doesn't make sense to me though, how can this be when 10 gallons through it makes 1 psi drop but 8 and 15 gals make more? Dans example was Cv of 1, it gets more complicated when it isn't that simple though. Great question, looking forward to someone with the answer.
• Member Posts: 24
If

you have a valve with a Cv rating of 10 that means 10gpm's going thru valve will create a pressure drop of 1psi.Therefor the higher the Cv (Coeffecint of volume)number the less restriction to flow.Some valves are more restrictive than others.Example gate valve less restrictive than globe valve.The more restrictions in system the harder your pump has to work to overcome these restrictions and deliver the required gpm.Higher head pumps can create more pressure to deliver their required gpm.Of course pumps have to be picked according to the system they will be installed in,size of pipe,length of pipe,# of and type of fittings all these things are taken into account and a system resistance curve is plotted and matched to the pump curve to decide which pump will deliver required gpm.Somebody else can help me out here.I'm still learning.
• Member Posts: 89
Your making this too hard

Drop is the square of the ratio of the flows. the Cv 10 with 8 gal is 0.8*0.8 = 0.64 drop and for 15 Gal it is 1.5*1.5 or a drop of 2.25 psi. Hope this helps.
• Member Posts: 610
and...

same thing happens w/ oil burner nozzles: a 1.0 GPH (at 100 psi) nozzle at 140 psi will flow: squareroot(140/100) times 1.0 GPH or 1.18 GPH, and at 200psi it will flow squareroot(200/100)*1.0 = 1.41 GPH.

Note: you use squareroot because you are starting w/ the pressure drop (the pressure is 0 after the oil has left the nozzle) to get the flow, but if you start with the flow (as in th eoriginal question) and want the pressure drop then you use the square
• Member Posts: 3

So if I have this right, I can move 10 gpm through the valve and get only 1 psi. I got that, if I am reading correct 1 psi is = to about 2.5' of head. Sound about right? I have been doing hydronic heating for a while, but am just now starting to get into the nitty gritty of it. The Cv to head thing has gotten me so far. Thanks for the input.
• Member Posts: 60

the square root part wasn't in the book. It only dealt with squaring it for increased resistance.

you helped me much! Interesting enough, I've been reading about it the past week, and here it is popped up on a post! Didn't realize how little I knew.
• Member Posts: 71
CV flow calc

Use this attached small excel file and the cacculations will be easy
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