# How to use flow rates (GPM) and pressure drop to determine if a valve is usefull

1. Jan 21, 2009

### MoonKnight

I need to determine if a valve can handle a flow rate of 50 GPM, this valve being: 2/2 Way, Normally closed, w/ 2-pilot control, and coupled solenoid.

I also need to determine pressure drop...

it is a burkhert model - 457361D

the valve specs indicate:

Cv = 35.1

pressure measurement = 0 -145 psi

if anyone can help, or push me in the right direction, thanks...

2. Jan 21, 2009

### FredGarvin

The Cv is your main source of information. The Cv is, by definition, the amount of flow through the valve with a delta P across it of 1 psi. Most valve suppliers will give you a curve of Cv vs. % open. You can use the following equation to calculate the flow through given the following equation:

$$Cv = Q \sqrt{\frac{SG}{\Delta P}}$$

3. Jan 30, 2009

### proinwv

The equation given is generally correct as long as there is no choking, and the flow is laminar.

The ISA standard S75.01.01 gives extensive information on the subject. Also, the valve manufacturer should provide sizing information that will take that into account.

www.ostand.com

4. Jan 30, 2009

### proinwv

ooops I meant to say the flow must be TURBULENT.

5. Jan 30, 2009

### stewartcs

I've never heard of a restriction on the flow being turbulent for that equation to be valid. Where did you reference that from?

CS

6. Jan 30, 2009

### proinwv

It is part of the ISA S75.01.01 This is "the" standard of the valve industry.

Most references ignore this at their peril. Choking occurs when the pressure in the vena contracta within the valve drops to the vapor pressure of the liquid and vaporization occurs, preventing further flow increases, unless the inlet pressure is increased.

Turbulent flow rather than laminar or transitional will pass the amount indicated by the equation that was quoted earlier. Otherwise the equation must be modified by a valve reynolds number factor which is =<1. Turbulent flow occurs when the valve reynolds number is 10,000. This is calculated by the equations in the ISA standard.

If these factors are not checked for signifcant errors can occur in the calculation of flow or delta p.

7. Jan 30, 2009

### proinwv

This might be of some use in understanding what I was trying to say, as it relates to turbulent flow.

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8. Feb 2, 2009

### MoonKnight

thanks for your help guys... I found what I needed a while ago, but your responses were appreciated