Question on CV

Go to this link and download the Fisher Control valve handbook, it's free.


Man, that's an ugly URL. The Cv is only constant as long as the valve is fully open. The valve open and closes to maintain a pressure, or flow or temperature or whatever variable you are controlling and that changes the Cv.

Cv is a fuction of geometry, therefore it is constant.

For compressible (gas) flow the flow is a function of the upstream pressure and the rate of the upper pressure to the downstream. The function is highly non-linear.

For incompressible (liquid) flow the flow rate is the funcion of the square root of the pressure difference between the upstream and the downstream pressures.

Hey tchaiket

In Crane 410 "Flow of Fluids", the formula for C_v is given as C_v=Q.G/(&[ignore]Delta[/ignore];P)^1/2 where Q is the flow rate.

I think this answers your question; C_v IS dependent on pressure changes and flow rate.



Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

I am sorry but you missed the physics. The fact that the formula is written this way does'nt mean that Cv is a function of the pressure. The physical correct way is
Q=Cv......

The formula that you present is convenient when one wants to find the actual Cv from the measurement of the flow rate and pressure.

tchaiket, For liquids, Ql=cv*(dP/spGl)^1/2. For gases,
Qg=59.64*cv*P*(dP/P)^1/2*(520/spGg*T)^1/2. For full explantion of the nomenclature, see the Fischer Control Valve handbook site.

From the above equations, it becomes appparent that the cv is dependant on flow and pressure. Such that, if the cv is held constant and the pressure changes, the flow changes. If the cv is held constant and the flow changes, the pressures changes. Get the picture?

Hope this helps.
saxon

I think that Patricia Lougheed and saxon are both mistaken.

The theoretical formula is Cv=d*d*15 where d is the orifice diameter in inches. For example for a 0.01" diameter Cv=0.0015.

Again, Cv is a geometrical property of the equivalent orifice of the valve. And therefore is constant.

Cv is flow coefficient of the valve : It is flow in US Gallons per minute of Water at 1 psi pressure drop across the valve or any other piping component.

Cv is dependant on the geometry and cross section of the flow path. More the flow area more will be the Cv value. Eg: Smaller DN size valves will have smaller Cv value and Cv value increases as the valve size increases.

Also we can estimate the water flow at higher pressures by multiplying Cv value with the sq.root of the pressure drop in Psi.

Crane "Flow of Fluids " is a very good reference. Also you can access to their web site.
BROWSE!

israelkk

I think it's a case where everyone's "right", depending on what they're doing. The formula you gave is appears to be for designing a valve; that never changes for a given valve.

However, consider what happens when that particular valve is partially shut: the "equivalent diameter" of the orifice is reduced from the full open size so C_v goes down. Similarly, if the pressure or flow change on a control valve, the C_v is going to change along with them.

And you're right that the formula from Crane is actually expressed as Q=C_v√ΔP/G; however one of those nice math properties is that you can rearrange things on either side of "=" so you can solve for whatever property you need. And saxon is right in giving the formula for compressible fluids.

Patricia Lougheed

Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.

Amen Patrica.

ANSI/ISA S75.01 Standard: Flow Equations for Sizing Control Valves" does not recognize the Cv defined by Fisher.

The "Cv method", a well accepted method by all valve manufacturers, was first introduced in 1944 by Masoneillan International, Inc. [Ref: Corripio, Priniciples and Practice of Automatic Process Control.]

Of course 1944 was well before most of us were born.