Flow control valve sizing specification

[Indy]

Good morning,
Could anyone point me in the right direction ( literature/webpage for data sheet template) on how to best specify/ size a flow control valve.

Thanks.

 

[ashtree]

The Fisher Control Valve Handbook is an excellent reference and should be available online for download.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[Indy]

Thanks for this.Do you happen to know where I can find a worked example to show the procedure from start to finish?

Thanks

 

[RVAmeche]

The Fisher handbook and other manufacturer resources are great starting points and definitely worth looking into to get a basis.

You should model the hydraulic system the valve will be in, and analyze the various different scenarios to ensure that the Cv and pressure drop across all the scenarios are adequate. While doing this model, you'll have to make some preliminary choices of valve type/size (2", 3", globe, v-port ball valve, etc).

Other basic considerations are type of actuator (air operated, electrical, hydraulic?), P-T check of soft goods (all soft goods have a graph of pressure/temperatures they can be used in), valve materials, extended bonnets for hot services, cavitation review, etc.

 

[mk3223]

It's an engineering job to design and size the control valve with the combination efforts from Process and Instrument, and the Valve Vendor will propose a proper valve with a suitable actuator with the additional control elements as requested in the design.
Consider to hire the experienced Engineer to do it for you, and learn the actual process from it.

 

[Indy]

Again thanks for the replies. I’ve got a copy of the Emerson book. Am I correct in assuming that it would be the norm for the process engineer to calculate the initial cv value based on the process conditions?

Thanks

 

[RVAmeche]

You can "calculate the initial Cv" using the standard equation and expected dP/flow rate to get a very general idea of what you'd be looking at. But it's really best practice to model the system with some sort of flow analysis program.

Losses in fittings/pipe/equipment are typically not constant and vary at different rates. These losses will affect where the system curve meets the pump curve, or what speed the VFD is set to, depending on your system of course. It's best to use a program that tracks and adjusts all these values so you can focus on other things, as well as being more accurate.

 

[LittleInch]

The art of a control valve data sheet is in finding the "normal" flow and pressures and also the extremes, usually low flow high DP and high flow low DP. Sometime you can have two "normal" flow cases with different flows and pressures.

Then seeing if this is possible within the same valve. The vendors have the design software to do this very easily. Different vendors will come up with different sized valves depending on their own range of valves. Let them at it.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Indy]

Thanks for this. So in the data sheet are the operating cases ( min/norm/max flow, pressure) and the vendor will provide CV value and a choice of valve based on their products.

Thanks

 

[LittleInch]

Yes, but it is useful to know what they are giving you.

You normally want a valve operating between 20 to 80% from min to max.
Vendors will also give you sound levels.

More than that and you are probably asking too much of the valve.

Same thing with shut off - lots of people ask for tight shut off but in a control valve that's not a good idea and rarely works well in reality.

Use a control valve to control flow and an isolation valve to isolate.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.