Equal % valves - Part 1 - What is the equation?
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- Опубліковано 1 сер 2024
- 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:
docs.google.com/spreadsheets/...
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...”
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For Equal Percentage valves, try instead: Cv = R^(x-1) for x > 0, where R is a fitting parameter.
Hey Bart! Thanks for this, I've checked and you are correct apart from 1 small correction: Cv = Cv_max × R^(100-x). Thanks a lot for this one! To convert to this form from what I show in the video I see that m = - ln(R) and c = ln(Cv_max) + 100ln(R). I have added this to the sheet I link! Appreciate it.
@@ProcesswithPat I should have mentioned that in my equation x is a fraction between 0 and 1 (instead of 0% to 100%). And the resulting expression is a normalized Cv curve also with values between 0 and 1. Multiply those resulting values by Cv_max to revert back to a non-normalized curve. I have worked with an equal percentage valve provider, and the expression above was provided, with different values of R.
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’ll tell Pete you said thanks ;) No problem though! Am glad it’s useful, that’s why I made it!
@@ProcesswithPat Sorry for misspelling Pat. ☺️
Best explanation i’ve seen yet and truly the only one that has made sense yet. Thx so much for making this!
Glad it makes sense! I did the video because this is something that wasn’t clear to me either.
Hey mate, this video was excellent. The explanation, the video quality, everything. Keep making them!
Really appreciate man!
Very well explained
Really good topics for junior engineers in general, especially those working in plants and even design
Keep making these videos man
Nice video as always. Good explanation 👍
Waiting for part 2.
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!!!
Glad to hear it! Good luck with the course!
Thank for sharing
Beautiful explanation.
Thank you.
Wanted to know pressure drop across equal % control valve.
Many thanks for your detailed and clear explanation. One question please - are the equal % valves same to the logarithmic characteristic ones?
perfect!!!!
Subscribed
Thanks
Where and which process required this Equal persentage valve
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.
Hey there! You can work out the flow rate of the gas from the Cv, once you e calculated it, only if you have the pressure drop of the valve using the equation that defined Cv for gases.
Choked flow is a special condition achieved when the critical pressure ratio is achieved across the valve. The critical pressure ratio is only a function of the gas in question (ratio of Cp to Cv).
@@ProcesswithPat Hey!! I do have Pressure differential value across the choke valve. With this information, how can I calculate the flow through the choke valve. Please share some useful references on this.
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?
Hey, thanks a lot! So we are on the same page - adding reducers will increase dP. I do not understand the part of your argument where you say "consequently, means that the control valve can do less DP". What exactly are you imagining? A different valve, or the same one?
When people say capacity drops they mean that if you have two IDENTICAL systems where the valve characteristics and Cv in both systems are the same but the only difference is one system has reducers on the valve and the other doesn't, then the flow through the system with the reducers will be lower due to higher dP. I think that's what is meant by "the valve capacity is lower".
What you are saying is that because there is additional dP due to the reducers one would need to install a larger valve, one with a greater Cv/capacity, in order to maintain the exact the same flow through the system.
@@ProcesswithPat thanks for your reply. You got my point. If I want to keep the same flow, then reducers will cause more DP in the piping system, which means a bigger valve or alternatively the DP of the control valve is lower. We agree on that, right? Regarding the other statement, the flow is not constant, so more restrictions means lower flow can pass through, according to Poisseuille law. Do you agre
Yeah I agree with what you’re saying it’s just that when you say lower dP OR larger valve - to me those are almost the same thing. The way you get lower do is with a higher Cv valve.
@@ProcesswithPat I agree with you! Nice discussion. Btw, do you have any good reference about the percentage of DPvalve comparing to the DP of piping system??
Have you watched part 2 of the equal percentage valve video where I talk about why they are used? There I discuss this exact point, although I do not remember whether I gave an actual reference.
nice
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.
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
I'm not sure I understand your question.
Equal percentage must be use positioner valve or can be only actuator?
Hey Aby! I would not say that they MUST use a positioner. In my head the use of a positioner on an actuator is independent of the valve trim used.
Thanks you sir
May i have the sizing valve program please
The link to the sheet I use is in the description 👌
Um Cv=e^(mx+c) can't be solved for zero either.