Flow control Valve Selection

[ivoryhorseman]

Hi all,

I have been doing some calculation to select an air operated valve for one of the rigs i am working on.

the pressure at the inlet of the valve is 44 psi. which is constant, however the outlet pressure needs to varied from 30 to 5 psi.

this led me calculate the Fl required(the Fl factor to avoid chocked flow).
I was stuck with when it comes to calculating the Fl rated, which i have understood is based on the trim style of the valves. Is this somthing which is present on the valve catalogue or somthing i need to calculate.

the criteria is that if the rated Fl is larger than the required Fl then ther Coefficient of flow=

similialry if the rated Fl is smaller than the required Fl then it has a different Cv fomulae.

the issue is the required Fl varies for the different outlet pressure and at one point it becomes lessser than rated Fl(I got hold some catagoue which had a rated Fl value).

How can I select a valve when the Cv value varies with the pressure variation across the valve.

I have attached the calculation i have done for your reference. Kindly help

 

[ivoryhorseman]

please find second page

 

[gerhardl]

What practical task are you trying to solve? Is this a theoretical test?

This is not a comment on your calculations, but a practical way to find a suitable valve.

Normally, the simplest way to size a control valve is to look at the exctreme limits. For instance highest pressure drop and lowest required flow at this delta P, normal/medium flow and medium pressure drop, and lowest pressure drop and highest required flow at this low delta P (at least). If the valve can manage the outer limits, all other will fit, its only a matter of good and correct control system, installation, layout and suitable valve type and quality.

You do not mention liquid type and temperature, viscosity etc (??)

Next question is which valve type normally would be preferred selected for this application.

If variations are extreme, you will probably need a special valve, perhaps a caged valve or needle valve (German: Ringkolbenventile), or other specials. With required outer limits its more easy to check if an exsisting valve on the market can manage the job.

 

[MattC1234]

"How can I select a valve when the Cv value varies with the pressure variation across the valve"

Simple, you select a control valve. The control valve will open or close to respond to a change in signal from the control room (or manually) to control the downstream pressure, by means of increasing or decreasing the amount of flow allowed through the valve.

A control valve will have the lowest pressure (FL) recovery of valves, with a pressure drop this small (relatively) it is unlikely that choked flow will occur. A control valve, depending on trim type will have a pressure recovery value from between 0.8 and 0.999.

Based on your all your process conditions, the valve manufacturer will select a trim with a sufficent pressure recovery value for the process.