Equal % valves - Part 1 - What is the equation?

What is the equation of an equal percentage valve? What is the Cv of an equal percentage valve that is 50% open? This is something I want to answer before looking at why we would even need a valve that has an equal percentage characteristic.

Link to the sheet with valve characteristics:

00:00 Learning about valve characteristics
02:13 Comparing linear & equal percentage behaviour
05:41 How many data points do I need to define behaviour?
08:04 Deriving the equation
09:43 There are multiple possible equal percentage valves
11:21 Looking at a real equal percentage valve
12:28 Final thoughts

Process with Pat is the place to come for perspective and to ask stupid questions. I want you to leave more knowledgeable, confident, motivated, and most importantly, curious. I also want to invigorate a field that seems tired and uninspiring, at least if you get your perspective from internet forums. These are not lectures. This is a place for you to leave thinking “Oh! That’s why...”

This channel is not only for chemical engineers - anyone who works with processes should be able to find something of value here.

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[bart_seavey]

For Equal Percentage valves, try instead: Cv = R^(x-1) for x > 0, where R is a fitting parameter.

[sofianetidjani]

you are such a gifted engineer and a gifted coach and teacher for this clear, precise and concise explanation. Many thanks indeed Pat, just keep on.

[manojgupta]

Thanks Pete....I have never seen any one else explaining the basics so simply and fluidly....Your 10 min video is equal to watching 10 different videos . Thanks

I really love the way you explain it, and I'm studying a subject whose name is "Process control" and refers to Valve Opening. So, thanks to this video I have more details about my problem. Thank you, sir!!!

[cooldawg]

Best explanation i’ve seen yet and truly the only one that has made sense yet. Thx so much for making this!

[Bruh-vpqf]

Really good topics for junior engineers in general, especially those working in plants and even design

[jameslin]

Hey mate, this video was excellent. The explanation, the video quality, everything. Keep making them!

[xse]

Nice video as always. Good explanation 👍
Waiting for part 2.

[vtbhanukiran]

Beautiful explanation.

Thank you.

Wanted to know pressure drop across equal % control valve.

[markonovicic]

Many thanks for your detailed and clear explanation. One question please - are the equal % valves same to the logarithmic characteristic ones?

[panoskb]

Very nice video as always. I have a question for you somehow related to your topic, I will appreciate if you can give me some insight.

"When control valves are mounted between pipe reducers, there is a decrease in the actual valve capacity."



I do not agree with this and my argument is the following if we add reducers before and after the control valve that means we have more pressure drop in the lines upstream and downstream the control valve, which consequently, means that the control valve can do less DP. Less DP for control valve leads to more Cv (valve capacity)? What is your opinion on that?

[Hiteshmahadik-vy]

Where and which process required this Equal persentage valve

[phaniy]

Hi Pat, thanks for the interesting video. So, please help me out on my query. I can solve the equation for 'm' and 'c' by taking any two points of valve openings. Then, using these values how can I calculate theoretical flow rate of gas (with some water, talking about choke valves) through the valve? Please suggest.

[foodhomeliu]

How to prove the equal percentage equation F=kR^(x-1) from linear equation F=kx where x is opening of valve and k= Cvmax* (delta p/ SG)^0.5

[abychasperindie]

Equal percentage must be use positioner valve or can be only actuator?

[walker]

May i have the sizing valve program please

[panoskb]

Hi Pat, very often I go and watch your videos again. They are so informative and you really try to understand how things work. I would like to ask you a question, which is related to this topic and related to another video you did about pressure drops. I would like to show with a system resistance curve the effect of a control valve on the head of the system. Actually, when the flow rate reduces, which means valve opening reduces, then the head of the system increases. How I could describe this with a head loss term? To be more precise how I can find a relationship between the opening of the control valve and the resistance coefficient L, which will show me this increase in the head of my system?
Sorry for the huge question.

[JasonMitchellofcompsci]

Um Cv=e^(mx+c) can't be solved for zero either.