Control valve CV basics

[Phil121324]

Hi,

Im a little confused with control valve sizing,

I have a control valve with CV of 475 (approx 1800 litres per min), at the plant the flow through the control valve is up to 300m3/hr, or 5000 litres per min. Is it possible the valve is correctly sized. I will be looking at the control valve handbook and getting a better understanding of the parameters, however, 5000 is more than 1800 .....am i missing something?

thanks

Phil

 

[hacksaw]

The Cv rating, historically is based on water tests with 1.0 psi pressure differential.

All your missing is the process data! The bane of all instrument engineers.

Your installed sizing depends on the service conditions the process design mandates.

 

[TiCl4]

For incompressible flow...

Q = K * Cv * SQRT( dP / s.g.)

Q = Flow through the valve, gpm
K = 1 when all variables are in the units listed here
Cv = Valve coefficient, unitless
dP = pressure differential across the valve, psi
s.g. = liquid specific gravity, unitless

Your flow through the valve is a function of pressure drop across the valve, which is, in turn, a function of the system loss. Friction losses will increase with increasing flow, leaving less available dP for the valve.

To correctly size a control valve, you'll need to set up a pressure-loss calculations for the upstream/downstream piping/equipment as a function of flow (Q). This may also involve setting up expression for a pump curve (Pressure as a function of Q) if liquid flow in the system is provided by a pump. Simultaneously solve those equations with the equation above for Q with Cv as a specified (i.e. known) variable to determine Q at a given Cv for your system.

The extended Bernoulli's equation is the governing equation for this calculation.