10 replies to this topic

[panoska]

Hello everyone,

 

I have a question for you, please!

 

I am straggling a bit with the statement below: 

 

"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)? Could you please help me with that?

 

[Pilesar]

I think your argument is correct. Effective control valve Cv is greater in smaller piping compared to the same size control valve in larger piping. I think the meaning is unclear because it depends on what changes. For the same size piping, effective CV is reduced when smaller control valve is placed between reducers compared to using a full-size control valve. So the meaning depends on whether the piping changes or whether the control valve changes.

 

It looks like your quoted statement is from Honeywell concerning ball valves: 

https://digitalasset...051d8835377.pdf

 

Honeywell gives tables showing how the effective Cv changes with pipe reducers on page 14 of the document linked here: 

https://customer.hon...00s/63-2648.pdf

 

The reason for reducers at control valves was addressed here: 

https://www.cheresou...ve/#entry101675

Edited by Pilesar, 16 October 2021 - 02:55 PM.

[panoska]

Thank you so much for your elaborate answer. ????

[panoska]

Hi Pilesar,

 

I would like to ask you something else regarding the control valves:

What should be the rule of thumb related to DP allocated to a control valve?

 

I have found different statements such as:

• Pvalve = 25% system pressure drop (including control valve) or Pvalve = 1/3rd system pressure drop (excluding control valve)

These statements made me a bit confused on how I will evaluate them?

Please see attached the example that I found here: Control Valve Sizing (pdf) – What Is Piping

In this example I calculate the percentage of DP for the control valve, which is equal to1.59/(1.59+0.7+0.7)=50% and not 25% as it is indicated in the rule of thumb. 

Could you enlighten me a bit with this? 

 

 

 

I have found also other statements such as:

For pumped circuit or compressor circuit it is recommended to take the control valve pressure drop from the corresponding pump/compressor calculation.

For pumped circuit, following guidelines can be followed

Control valve pressure drop shall be maximum among

1. minimum 0.7 bar for rated flow case
2. minimum 1 bar for normal flow case
3. 25% of system frictional loss for rated flow case
4. 33% of system frictional loss for normal flow case.

Thank you in advance!

Attached Files

•  Pressure Drop Contol Valve.JPG   52KB   2 downloads

[Pilesar]

I do not routinely size control valves and your rule-of-thumb info is a lot more detailed than I have handy.

My usual application is in evaluating hydraulics for pumped systems with the control valve selection by an instrument engineer.

I looked in my personal reference scribbles and found this: 

'Control valve sizing for liquid pump discharge - assume dP across valve of 10 ft of fluid at 90% valve opening'

I did not supply a reference citation to myself so take that for what it is worth.

 

If I had to size a control valve for real, I would go to one of the valve manufacturer's handbooks like this one:

https://dam.bakerhug...English-pdf.pdf

 

Valve vendors also supply software for control valve sizing.

Edited by Pilesar, 17 October 2021 - 08:17 AM.

[breizh]

Hi,

You may want to study the document attached .

Breizh 

[panoska]

Thank you Breiz for the nice document! I will go through this. ????

[breizh]

Hi,

Consider the document attached to size a control valve (rules of thumb).

Breizh

[Chemitofreak]

Hi,

 

Based on my experience, the control valve size is maximum 1-2 pipe diameter less than the pipe size. 

 

The statement you have mentioned is only true when the other pipe fittings become limiting as compared to the control valve.

 

Regards 

[Chemitofreak]

Hi Pilesar,

 

I would like to ask you something else regarding the control valves:

What should be the rule of thumb related to DP allocated to a control valve?

 

I have found different statements such as:

• Pvalve = 25% system pressure drop (including control valve) or Pvalve = 1/3rd system pressure drop (excluding control valve)

These statements made me a bit confused on how I will evaluate them?

Please see attached the example that I found here: Control Valve Sizing (pdf) – What Is Piping

In this example I calculate the percentage of DP for the control valve, which is equal to1.59/(1.59+0.7+0.7)=50% and not 25% as it is indicated in the rule of thumb. 

Could you enlighten me a bit with this? 

 

 

 

I have found also other statements such as:

For pumped circuit or compressor circuit it is recommended to take the control valve pressure drop from the corresponding pump/compressor calculation.

For pumped circuit, following guidelines can be followed

Control valve pressure drop shall be maximum among

1. minimum 0.7 bar for rated flow case
2. minimum 1 bar for normal flow case
3. 25% of system frictional loss for rated flow case
4. 33% of system frictional loss for normal flow case.

Thank you in advance!

 

In a pump and compressor circuit you increase the pressure of the fluid. In control valve you kill the pressure (a flow control valve also reduces pressure, but the amount of pressure killed is comparatively low).

 

Now, if you kill more pressure in a control valve, then you have to generate more pressure at the discharge of compressor/pump i.e. the motor rating increases i.e. you are generating pressure and subsequently killing it, that is absolute waste of money. The above mentioned rules are there to optimise the system i.e. you do not over consume power and at the same time do not loose on controllability in the control valve.  

[ASalman]

You might be reading the statement you mentioned in the wrong light! From I what I know having a reducer upstream and downstream your control valve will reduce the CV you require for your process, it doesn't necessarily reduce the CV of a valve that's already installed. I suggest having a look at Control Valve Handbook by Fisher - it is available online for free from their own website.