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Chemical and Process Engineering Resources

Valve Sizing and Selection

Dec 05 2011 03:20 PM | Chris Haslego in Fluid Flow ****- Share this topic:
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Sizing flow valves is a science with many rules of thumb that few people agree on. In this article I'll try to define a more standard procedure for sizing a valve as well as helping to select the appropriate type of valve. **Please note that the correlation within this article are for turbulent flow.

Step #1: Define the System


The system is pumping water from one tank to another through a piping system with a total pressure drop of 150 psi. The fluid is water at 70 °F. Design (maximum) flowrate of 150 gpm, operating flowrate of 110 gpm, and a minimum flowrate of 25 gpm. The pipe diameter is 3 inches. At 70 °F, water has a specific gravity of 1.0.
Key Variables: Total pressure drop, design flow, operating flow, minimum flow, pipe diameter, specific gravity

Step #2: Define a maximum allowable pressure drop for the valve


When defining the allowable pressure drop across the valve, you should first investigate the pump.
What is its maximum available head? Remember that the system pressure drop is limited by the pump. Essentially the Net Positive Suction Head Available (NPSHA) minus the Net Positive Suction Head Required (NPSHR) is the maximum available pressure drop for the valve to use and this must not be exceeded or another pump will be needed. It's important to remember the trade off, larger pressure drops increase the pumping cost (operating) and smaller pressure drops increase the valve cost because a larger valve is required (capital cost). The usual rule of thumb is that a valve should be designed to use 10-15% of the total pressure drop or 10 psi, whichever is greater. For our system, 10% of the total pressure drop is 15 psi which is what we'll use as our allowable pressure drop when the valve is wide open (the pump is our system is easily capable of the additional pressure drop).

Step #3: Calculate the valve characteristic


Attached Image: valve1.gif
For our system:
Attached Image: valve2.gif
At this point, some people would be tempted to go to the valve charts or characteristic curves and select a valve. Don't make this mistake, instead, proceed to Step #4!



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13 Comments

Great Post! Very helpful.
Thanks
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MAGICAL_RUPI
Jun 09 2012 07:06 AM
Magnificent post !!!
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ecovarrubias
Aug 14 2012 10:37 AM
Congratulations... Great Post!!!! Thanks
Thanks for the info
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Emad Elgebesy
Aug 28 2012 05:56 AM
Hi, Chris
I'm Emad Elgebesy. Senior process engineer in Worley Parsons. First of all, thanks for your clear efforts on the community, And im willing to share my experiance in this regard.
In Worley Parsons, we did the valve calculation for, single phase & two phase. The above mentioned calculation did not demonstrate which phase is, But i think this liquid phase valve calculation only.

After calculating the Cv, Supplier should verify the strock range (10% to 90%) is commnly used, noise level& body size as well.
Finally, there is many free control valve software of FirstVue do regirous calculation

Regards
Emad
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Chris Haslego
Aug 28 2012 11:57 AM
Thanks for your feedback. You are correct, this article only addresses valve sizing for liquid phase design.
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khaerullah ibnu umar
Dec 28 2012 03:50 AM
so when will you post about valve sizing for vapor phase??
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ashish dubey
Feb 02 2013 01:03 PM

thanks for the info Chris

Dear Chris,

 

If the vendor did not supply the valve chart, is there any we can get the control valve opening by using calculated Cv and rated Cv? 

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Chris Haslego
Feb 17 2014 12:39 PM

Dear Chris,

 

If the vendor did not supply the valve chart, is there any we can get the control valve opening by using calculated Cv and rated Cv? 

 

Cv charts are generally made for experimental data.  If your valve supplier doesn't have a Cv chart for a valve that they sold to you...find a new valve supplier...that is not acceptable.

Chris,

Nice article!

I'd like to make a suggestion for a follow-up article. There are many times that we size liquid control valves in systems where the source pressure comes from a gas in the system instead of a pump (I am thinking of scrubber dump valves). In these situations, after sizing the liquid valve, we often need to determine the "gas blowby" rate, which is how much gas would flow through the valve if the liquid level control loop failed and left the valve wide open. This is often required when sizing pressure safety valves further downstream in the system. It would be great to add an additional step to this article to calculate the maximum gas flow through the selected liquid control valve.

Regards,

Paul

Great job