20 replies to this topic

[Root]

Hi,

Why design engineers are using selected CV of control valve instead of calculated CV, I have gone through design calculation of one of control valve, its calculated CV is 120 on 110% design and design engineer selected 195. This control valve is giving 100% flow on 20% valve opening, my question, is this justified and really save energy cost.
In argument, higher CV will reduce less pressure and reduce the energy cost. can any one explain it more on selection of higher CV.

Toor

[Zauberberg]

Hello Toor,

This concept doesn't make sense to me, unless there are some other issues which are not known to us. Normally you want to have control valve open between 20-75% (depending on valve type it can be as low as 10%) in continuous mode of operation, because it gives smooth and stable control.

On the other hand, choosing Cv = 195 in case when 110% of design flow results in Cv = 120 will result in very small % valve opening during normal operation, and perhaps in unacceptably low % opening in case of turndown. That will promote/accelerate valve erosion and damage, and it will lead eventually to a poor and unstable flow control.

We have had the same issue with one of our control valves here in QG, and finally it has been decided to replace the valve with a smaller trim. The valve was operating between 3-10% opening most of the time which has led to damage and premature failure. The gate valves upstream of this particular control valve had to be partially closed in order to have some control over the flow.

[Root]

Yes our valve opening is very less. only 20% on 100% flow and with 50% during upset time we are fcing problem and getting tripped due to pump safety. I agree with you control must give max flow on 70% opening but I never seen this valve even go more than 21%.
Have you any idea about energy saving concept due to using of high CV?

Toor

[Zauberberg]

Control valve is never an energy-saving option, regardless of the Cv. It just consumes the required pressure drop in order to put the pump on its curve and control the flow - if we are speaking about the fixed speed pump and fixed pressure downstream of the valve. The only difference is how much any of those two valves will be % open for any given flow rate (which poses valve reliability issues), but energy saving is zero in both cases. See attached.

Attached Files

•  Pumps & Valves.pdf   77.02KB   280 downloads

[Root]

If pressure drop through the valve is less by using high CV this will not have any effect on energy saving (operating cost of pump)?
Thanks
Toor

[Zauberberg]

Oversized control valve is not a design option for me. It is another headache created by design engineers who have never been in the real, operating plant.

In order to have good and stable control, pressure drop is required. It is not an option, but the fact, necessity.

[chemsac2]

Toor,

Valve sizing does not affect pressure drop. Pressure drop is not decided by control valve but the system. Control valve with Cv of 120 or 195, pressure drop would be same for a given flow in a given system.

Only % opening and controllability would change with different control valve Cv values.

Control valve does not have pressure drop of its own, but merely connects two pressure systems.

Regards,

Sachin

[Zauberberg]

Toor's concern is an undersized control valve, i.e. requiring more pressure drop to achieve desired flow. But, as said, as long as you are within your design flow rates for a Cv = 120 control valve, there will be no excessive head/energy wastage in the sense that you will NOT need a bigger pump. Your engineer has mixed the two terms.

[fallah]

Toor's concern is an undersized control valve

Undersized or Oversized?

[Zauberberg]

Hello Fallah,

If the valve is undersized (or if the actual flow will be higher than the flow anticipated during design stage), it will require more pressure drop (= higher pressure upstream) in order to achieve higher flow rate demand. In that case, a higher-head pump will be required. But, as long as the actual flow is within design limits/design Cv, there are no concerns.

[fallah]

Hello Fallah,

If the valve is undersized (or if the actual flow will be higher than the flow anticipated during design stage), it will require more pressure drop (= higher pressure upstream) in order to achieve higher flow rate demand.

Thanks Zauberberg,

I think the above highlighted should be lower.Please correct me if i am wrong.

Regards

Edited by fallah, 08 August 2010 - 05:16 AM.

[Zauberberg]

Fallah,

Let's consider two cases for clarification purpose. Let's also say that the valve design DP is e.g. 3.0 bar in both cases.

1) Design Cv = 120, at maximum design flow the valve is open 65%. At turndown flow, the valve is open 20%.

2) Design Cv = 195, at maximum design flow the valve is open 40%. At turndown flow, the valve is open 5% only.

Now, if the actual flow will never be higher than e.g. 110-120% of design flow, the valve (1) will be working perfectly. If we use the valve (2) in same application, it will be working fine as well at high flow rates, but it will erode if operating for prolonged time in turndown conditions, due to low % opening. Energy consumption by the pump (assuming fixed-speed motor) will be identical in both cases.

The question anticipated by Toor's colleague (but in a wrong way) is - what if for some reason the flow requirement increases up to 200% of design flow rate? Then the valve (1) will become too small (undersized), and it will need higher pressure upstream (= higher-head pump) in order to push that flow rate through the valve, while the valve (2) will still be adequate. That is why I said that undersizing the valve will call for more pump head. But it will not call for more head if it has been designed for adequate flow in the first place. So, if the design flow has been choosen correctly, the option (1) is the right one.

[SSWBoy]

I may have misinterpreted something here, but say the design CV for a control valve is 120, at rated flow for a pump circuit you do not wish to exceed Cv/Cvmax of 0.8, so this dicates that the control valve cv must be at LEAST 150.

Same as pipe sizes, control valves come in distinct sizes therefore for preliminary sizing the engineer should select a control valve with CV >150, from memory this is true of a 4" which has CV of 195. a 3" control valve has a CV of 110 therefore would not be suitable.

Although it would be nice to select a control valve with CV of 150 surely specifying bespoke solutions such as this would be $$$ compared to a standard offering from a control valve manufacturer.

[Zauberberg]

No matter which design standard you follow, choosing a control valve that will operate with 5% opening at turndown conditions is a serious mistake, and it will lead to plenty of operational problems.

If the range of flows (turndown/maximum) is too wide to be succesfully managed with a single control valve, one can consider a split-range type of control with having two flow control valves in two parallel lines, with different size (Cv). I have seen this configuration working perfectly exactly in such cases.

Best regards,

[Root]

I may have misinterpreted something here, but say the design CV for a control valve is 120, at rated flow for a pump circuit you do not wish to exceed Cv/Cvmax of 0.8, so this dicates that the control valve cv must be at LEAST 150.

Same as pipe sizes, control valves come in distinct sizes therefore for preliminary sizing the engineer should select a control valve with CV >150, from memory this is true of a 4" which has CV of 195. a 3" control valve has a CV of 110 therefore would not be suitable.

Although it would be nice to select a control valve with CV of 150 surely specifying bespoke solutions such as this would be $$$ compared to a standard offering from a control valve manufacturer.

SSWBoy,

Process Dynamics and contorl by Dale E. Seborg suggest CV/CV max at 0.7 or 70% for optimum control but here design engineer selected 195 for 4" valve. I agree with Zauberberg during upset time control is very difficult and if you calculate vale lift for 120m^3/hr this valve open less that 21% when I have delta pressure 20psi. now what and how you can explain for safety reason during upset time when we need control 50% feed of 120m^3/hr.
Toor

[SSWBoy]

No matter which design standard you follow, choosing a control valve that will operate with 5% opening at turndown conditions is a serious mistake, and it will lead to plenty of operational problems.

If the range of flows (turndown/maximum) is too wide to be succesfully managed with a single control valve, one can consider a split-range type of control with having two flow control valves in two parallel lines, with different size (Cv). I have seen this configuration working perfectly exactly in such cases.

Best regards,

With an Equal % valve at approx 50% of rated CV the valve will be shy of 80% open. This is analogous to our case where we have a desired CV of 120 and the rated is 195.

At turndown (say 40% of operation) the required CV will be 48. This represents say 25% of the original rated CV. The valve will be 65% open.

I do not see where you are getting 5% from? Perhaps if you used a fast opening valve for the application?

SSWBoy,

Process Dynamics and contorl by Dale E. Seborg suggest CV/CV max at 0.7 or 70% for optimum control but here design engineer selected 195 for 4" valve. I agree with Zauberberg during upset time control is very difficult and if you calculate vale lift for 120m^3/hr this valve open less that 21% when I have delta pressure 20psi. now what and how you can explain for safety reason during upset time when we need control 50% feed of 120m^3/hr.
Toor

0.7 is a guide, companies vary from service to service. E.g. based on normal flow you might take 0.5, maximum flow 0.7 and pump rated flow 0.9, then you tabulate each of cv's for required case and determine which is the dictating case.

Getting back to the 'design engineer selected 195 for 4" valve' imagine you are sizing a line, target velocity is say 2m/s and this could be achieved with a 3.24" line do you specify this? No. You go one size up and select 4" line, this is analagous to this case where 3" valve would not be sufficient so you select a 4" CV. EQ% valves tend to have a rangeability based on CV of 30:1 so for a 4" valve you could reasonably control down to a CV of 7 (though i would not recommend this..) which coincides with a valve lift of 10%

Edited by SSWBoy, 08 August 2010 - 03:52 PM.

[S.AHMAD]

Dear Toor
If you choose linear valve characteristic you may get 20% opening. However, for the same Cv selected, you may get 80% by using equal percentage. It is common practice that the installed Cv is higher than the calculated Cv but then choose the right valve characteristic so that the valve opening is around 70% for good control at maximum flow.

Energy saving could be realized from the pump not from the control valve Cv. That is for bigger Cv it means less pressure drop across the valve then we may select lower pump impeller size that mactch the system hydraulic. For more details you can refer to page 465 of ALBRIGHT'S CHEMICAL ENGINEERING HANDBOOK.

S. AHMAD

[Root]

Dear Toor
If you choose linear valve characteristic you may get 20% opening. However, for the same Cv selected, you may get 80% by using equal percentage. It is common practice that the installed Cv is higher than the calculated Cv but then choose the right valve characteristic so that the valve opening is around 70% for good control at maximum flow.

Energy saving could be realized from the pump not from the control valve Cv. That is for bigger Cv it means less pressure drop across the valve then we may select lower pump impeller size that mactch the system hydraulic. For more details you can refer to page 465 of ALBRIGHT'S CHEMICAL ENGINEERING HANDBOOK.

S. AHMAD

Dear S. Ahmad,
The valve is linear only, 'ie' valve lift remained 20%.
Tank you for your input.
Toor

[Root]

SSWBoy,

You calculate CV on these condition then you will understand it, Flow 106 & 56 M63/hr, SP.Gr 0.683 and Temp is 60^0C. Take Three condition for DPv , 20, 68,245psi. also calculate valve lift.
Hope will agree with me.

Thanks
Toor

[SSWBoy]

SSWBoy,
You calculate CV on these condition then you will understand it, Flow 106 & 56 M63/hr, SP.Gr 0.683 and Temp is 60^0C. Take Three condition for DPv , 20, 68,245psi. also calculate valve lift.
Hope will agree with me.
Thanks
Toor

Not to answer a question with a question but....

The condition of 245psi differential pressure is at what flow? The reason I ask is that I assume that this is at a low flow condition and represents (presumably) the case where control valve dP >> system dP. To then go to a situation where you have a control valve dP of 20psi indicates to me that there is an increase in system pressure drop (purely due to piping and equipment losses) of 225psi. What I'm getting at is that the control valve pressure drop needs to be a sensible fraction of the total pressure drop in the system, otherwise we get into a case (such a this) where by the valve is wide open at one flow and wide closed at another point. Typical practice is to take a control valve as a minimum of 10psi or 33% of the dynamic pressure drop. So for this instance you might specify a pressure drop of 100psi at normal flow, true this results in added pumping power but you need the control valve dP to be a reasonable proportion of the total system pressure drop.

If possible in your reply could you indicate the system pressure drop (excluding control valve). I could be barking up the wrong track but something seems intrisicaly wrong where there is a range in dP from 245 to 20psi.

Best regards.

[Root]

SSWBoy,
You calculate CV on these condition then you will understand it, Flow 106 & 56 M63/hr, SP.Gr 0.683 and Temp is 60^0C. Take Three condition for DPv , 20, 68,245psi. also calculate valve lift.
Hope will agree with me.
Thanks
Toor

Not to answer a question with a question but....

The condition of 245psi differential pressure is at what flow? The reason I ask is that I assume that this is at a low flow condition and represents (presumably) the case where control valve dP >> system dP. To then go to a situation where you have a control valve dP of 20psi indicates to me that there is an increase in system pressure drop (purely due to piping and equipment losses) of 225psi. What I'm getting at is that the control valve pressure drop needs to be a sensible fraction of the total pressure drop in the system, otherwise we get into a case (such a this) where by the valve is wide open at one flow and wide closed at another point. Typical practice is to take a control valve as a minimum of 10psi or 33% of the dynamic pressure drop. So for this instance you might specify a pressure drop of 100psi at normal flow, true this results in added pumping power but you need the control valve dP to be a reasonable proportion of the total system pressure drop.

If possible in your reply could you indicate the system pressure drop (excluding control valve). I could be barking up the wrong track but something seems intrisicaly wrong where there is a range in dP from 245 to 20psi.

Best regards.

Ok pump Discharge pressure is 36.5 barg (Design Pressure is 34barg) and Product separator pressure is 21 barg, CFE inlet pressure is 28 barg.
Go for 106 amd 56m^3/hr and use 195 CV then you will see the valve lift (46m^3/hr is trip value but for safety 56m^3/hr).
Thank you for time and inputs in advance.
Toor