Control valve pressure drop - Best method

[sheiko]

Hello,

I have heard of three alternate methods (vs the traditional one consisting of allowing 50 to 25% of the system frictional pressure drop excluding itself to the control valve) to determine the allowable pressure drop across a control valve during the design stage:

1/ Connell's one using a formula that everyone knows

2/ A method consisting of assigning a minimum pressure drop (10 or 15 psi) at its maximum expected design flowrate and at a upper opening limit 80%. This method has been presented by Frank Yu in "Easy way to estimate realistic control valve pressure drops" issued August 2000 in HP.

3/ Also, in section 3.7 - chapter 3 of the very good free ebook available at

http://www.lightmypump.com/pump_book.htm

It is recommended to assume a pressure head drop across the valve of 10 ft of fluid at its maximum expected design flowrate and at a upper opening limit 90%.

My feeling is that the Connell's one is more rigorous as the notion of controllability is incorporated in the equation, but in the article corresponding to the second method, Mr YU concludes that his method is better than the Connell's one (from an economical point of view namely)...

A/ Then i would like to know if some experienced engineers could tell me which one they would recommend and why?
AND their feeling on the third method?

B/ Also, I would like to know the origin of the methods 2/ and 3/, i.e. why 10-15 psi or 10 ft of fluid? why these values?



"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[sheiko]

Attached is the reference related to method 2/

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[sheiko]

Attached is the reference related to method 2/

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[JLSeagull]

The application may dictate some design issues. For example a steam letdown valve might not follow any common guidelines.

 

[JimCasey]

Method 3 MIGHT work in selected applications, but it sounds more like drop expected with an on-off valve.

1/3 the system pressure drop at the valve at max flow is the rule-of thumb I was taught. That gives effective control. And "System pressure drop" is almost NEVER the same as the system pressure. JLSeagull's reference to a steam blowdown valve being the exception, where upstream is pretty much constant and downstream is atmospheric.

Note that As the flow is dropped back the system pressure drop decreases as the square of the flowrate, and the pressure backs up on the pump curve, so you're not backing up the flow much before the valve is dropping essentially ALL of the system flow.

Yet I see data sheets every day where the inlet pressure is specified as the same pressure regardless of flowrate, and the outlet pressure is specified as the same pressure regardless of flowrate, and only the flowrate changes. Those numbers cause me concern because they will NOT reflect reality.

 

[vpl]

Sheiko

I noticed that you posted this same question in another forum (see thread378-225325 . I've included the reply provided by Big Inch so that everyone can see his response:

Everything you need to know is in the Fisher control valve manual.

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[BigInch]

Thank you VPL.

I'm really getting tired of answering this question. Now 3 times. Each in different threads and each in different forums and ... each thread keeps disappearing.

Any reasonable method that can be applied in a general sense across the board has to be in variable terms. I've found many applications where a pressure drop of 10 ft of fluid wouldn't be enough to move the needle to the next graduation line. If you must use something, use 10% of the maximum pressure drop, but as mentioned previously, CV sizing is highly application dependent, so hard "rules of thumb" IMO are pretty useless. It depends on the process stability and the turn down ratio you need to bring any deviations back to the set point.

A pump recycle valve for starts & stops is a classic example. Pump starts with low head, but that's when you need to recycle all the pump's flow only to find that when high head finally develops, you only need to recycle a small amount of flow. 100% flow with little pressure drop and 10% flow with 90% pressure drop is a completely reversed scenario than what you might typically expect. The only way it works is if the 100% pump flow you need at no dP and the flow you need at max dP both happen to fit within the turndown ratio of the valve, often at its full pressure range.

I've been using the Fisher Manual to size CVs for 30+ years and don't see any reason to change now.

"I think it would be a good idea."
- Mahatma Gandhi (1869-1948),
when asked about Western civilization