5 replies to this topic

[MFAHOS]

Hello

Can anybody help me predicting the travel of a control valve knowing that the following data are available:

1- Flow condition: flow rate, pressure drop of the system & pressure drop of the control valve.
2- Valve flow characteristics & Rated CV.

Can the inherent flow characteristics chart be used to determine the lift ? explanations are welcome.


Thanks

[djack77494]

When you say that you know the valve flow characteristics and the valve's rated Cv, I take that to mean that you know the relationship between the Cv and the %lift. So if the valve's Cv is say 25, at its full open position, then you would know that to get the valve's Cv to 12.5 you might need a lift of about 80% if you had a valve with equal percentage characteristics, a lift of 50% with a linear valve, and a lift of just over 25% with a quick opening valve. (For a specific family of valves, these values would vary somewhat.) Typically, you might have a plot of %Maximum Cv vs. %Lift.

You know the relationship between flowrate and the control valve's Cv - by definition Cv is the capacity in gpm of 60 degreeF water that will through a valve with a 1 psi pressure drop. So our fully open valve will pass 25 gpm of water with a 1 psi dP. When its Cv is halved, for example with 80%Lift and an equal percentage valve, it will pass 12.5 gpm. If fully open and the valve's pressure drop is raised to 4 psi, then it will pass 50 gpm because the dP vs. flowrate relationship is of the square root of dP. I hope this offers you a bit of explanation about the operation of a control valve. It's hard to put some of this in words, so I'd suggest you follow up with some suitable vendor's literature such as the Control Valve Handbook from Fisher/Emerson. Lacking contact with valve vendors, you could try a textbook or the internet.

[Jiten_process]

Hello

Can anybody help me predicting the travel of a control valve knowing that the following data are available:

1- Flow condition: flow rate, pressure drop of the system & pressure drop of the control valve.
2- Valve flow characteristics & Rated CV.

Can the inherent flow characteristics chart be used to determine the lift ? explanations are welcome.


Thanks

Assuming you are using globle type control valve with equal percentage trim characteristic, below is the equation you can use

L = 100*[1+ln(CV/CVmax)/lnR]

where L= %opening of valve
R= rangeability (ratio of max controllable flow to min. controlable flow)
Cvmax = valve selected Cv

For linear opening, it is the simple ratio of Cv at desired flow to valve selected Cv

good luck...

[MFAHOS]

Thanks djack & Jiten for the fast reply. your information was very helpful.

Then to estimate the lift, all we have to do is to calculate the ratio CV/CVmax and use the chart shown in the figure (Assuming that the Horizontal axis is for lift and the vertical for CV ratio).

Thanks.

[djack77494]

Looks like you've got it. Good luck.

[Jiten_process]

Thanks djack & Jiten for the fast reply. your information was very helpful.

Then to estimate the lift, all we have to do is to calculate the ratio CV/CVmax and use the chart shown in the figure (Assuming that the Horizontal axis is for lift and the vertical for CV ratio).

Thanks.

GR8....but just one thing keep in mind that thse graph is assuming inherent characterstics but practically it is not.

good luck...