6 replies to this topic

[Jiten_process]

Hello everyone

I have a question.

For liquid application there is one practice generally followed that PD across control valve shall be 10-15% of line pressure drop or 5 MWC, whichever is max.

Now for control valve used in steam lines what is the thumbrule for allowing PD across control valve?

Awaiting for reply....

[ankur2061]

Jiten,

I made a company standard called "Specification for Process Design Criteria" for a reputed oil and gas company of the middle east and for the PD criteria for control valves the following is what I have incorporated:

Quote "Pressure Drop Criteria

For initial sizing purpose of control valves in pumped liquid lines, a general guideline is to consider a pressure drop of 30% of the total system pressure drop or 10 psi whichever is higher.

For control valves on gas lines the minimum pressure drop specified shall not be less than 3 psi at design flow unless otherwise dictated by specific process requirements." Unquote

Hope this helps.

Regards,
Ankur

[djack77494]

Jiten,
My approach has always been to look at underlying principles. You must select a control valve that can control. Sounds obvious, but have you ever heard that stated before? In a pumped circuit with a single control valve on its discharge, the control valve must be capable of causing significant movement along the pump's curve. That is how you will vary the controlled variable (flowrate). I've heard 30% or one third as very general guidelines or rules of thumb. That's fine, if you know how to apply them. Apply them in the early design stages of the project when you don't know what the final pump curve will look like. When you get the curve, then revisit this topic and confirm that the control valve will do an adequate job. I like to keep the control valve between 15% and 85% open. Much outside this and it's hard to call it control. Similar principles apply in other situations though I fail to see any "boundary crossing" when you switch to the vapor phase.

BTW, I have no idea what MWC means and I suspect most other readers also don't know. May I suggest as an extremely helpful practice that we spell out the meaning of our acronyms the first time they are used.
Thanks,
Doug

[Jiten_process]

Jiten,
My approach has always been to look at underlying principles. You must select a control valve that can control. Sounds obvious, but have you ever heard that stated before? In a pumped circuit with a single control valve on its discharge, the control valve must be capable of causing significant movement along the pump's curve. That is how you will vary the controlled variable (flowrate). I've heard 30% or one third as very general guidelines or rules of thumb. That's fine, if you know how to apply them. Apply them in the early design stages of the project when you don't know what the final pump curve will look like. When you get the curve, then revisit this topic and confirm that the control valve will do an adequate job. I like to keep the control valve between 15% and 85% open. Much outside this and it's hard to call it control. Similar principles apply in other situations though I fail to see any "boundary crossing" when you switch to the vapor phase.

BTW, I have no idea what MWC means and I suspect most other readers also don't know. May I suggest as an extremely helpful practice that we spell out the meaning of our acronyms the first time they are used.
Thanks,
Doug

Djack,
Thanks a lot for your insight of control valve. Well, We are using unit system that defines PD in Meter Water Column (MWC)(generally 10MWC = 1bar). We calculate all the pressure drop in terms of Meter Water Column (MWC) for the ease only, as pump curves usually found on water only.

Now, For control valve I am always prefer my operating flow operated at 60% (I see, we get best controllability in this range). For min. or max. whicheverr is more important i prefer to keep it 25% (min.) and 80-85% (max.). The reason of more off range is i want to keep scope of error which could have been made.

Secondly my question still persist, thumbrule which you state too, is for liquid fluid. What about steam? Many times it happens that system upstream and downstream already defined, but in case where system downstream pressure not defined (as of now for case of steam only), what PD i should allow? One thing, we should keep in mind that whatever characteristics we see in vendor software or our spreadsheet is inherent not practical.

Ankur,
For control valves on gas lines the minimum pressure drop specified shall not be less than 3 psi at design flow unless otherwise dictated by specific process requirements."

Your answer is open to other end also. you say PD should not be less than 3psi, but then how much i should take more than 3psi that you havent stated. I am asking some generally good practice as we use for liquid flow control valve.

[katmar]

I am 100% with Doug on this one. You must look at the overall system and how its pressure drop (and pressure generation) behaviour changes with flow rate, and not look at the valve in isolation.

Also we need to be aware that many of these rules of thumb arose during the days of yore when instrumentation was pneumatic and slow acting. I recently came across a gas control situation where a simple butterfly valve was controlling rock steady at about 10% open. The rules of thumb would say this is impossible. With modern ultra fast self tuning control loops the ground rules have changed. Speak to the supplier of your controller and get their input.

[ankur2061]

Jiten,

As Doug and Katmar have indicated there are a variety of possibilities with the modern control instrumentation available. The boundaries for control have broadened and earlier limitations have been overcome.

However, as I mentioned when you are doing initial sizing calculations you need to start somewhere. Inherently when a system is being designed for the first time, there is a lot of missing information and you need to do a lot of assumptions for system design. Rules of thumb were precisely meant for this purpose based on years of accumulated experience by professionals and experts. One of the main focus for rules of thumb was that in no way a system designer falls short on design i.e undersizing the system.

Now for the answer to your precise question about gas lines. As you are aware that dynamic losses in gas systems are comparatively much lower than liquid systems. Hence considering a 3 psi delta P for initial sizing calculations will make sure that you are not short on available system drop. There might be specific processes where a higher delta P may be required but then such processes would be more of an exception than the rule.

Control pressure drop is also related to the type of opening characteristics of the control valve such as equal percentage, linear or quick-opening. You should consult control valve manufacturers with your system design. Most valve manufacturers are now able to fulfill the exact demands of the system design and give suggestions for system design improvement involving control valves.

Regards,
Ankur.

[Jiten_process]

Jiten,

As Doug and Katmar have indicated there are a variety of possibilities with the modern control instrumentation available. The boundaries for control have broadened and earlier limitations have been overcome.

However, as I mentioned when you are doing initial sizing calculations you need to start somewhere. Inherently when a system is being designed for the first time, there is a lot of missing information and you need to do a lot of assumptions for system design. Rules of thumb were precisely meant for this purpose based on years of accumulated experience by professionals and experts. One of the main focus for rules of thumb was that in no way a system designer falls short on design i.e undersizing the system.

Now for the answer to your precise question about gas lines. As you are aware that dynamic losses in gas systems are comparatively much lower than liquid systems. Hence considering a 3 psi delta P for initial sizing calculations will make sure that you are not short on available system drop. There might be specific processes where a higher delta P may be required but then such processes would be more of an exception than the rule.

Control pressure drop is also related to the type of opening characteristics of the control valve such as equal percentage, linear or quick-opening. You should consult control valve manufacturers with your system design. Most valve manufacturers are now able to fulfill the exact demands of the system design and give suggestions for system design improvement involving control valves.

Regards,
Ankur.

You are right, actually we are at the beginning stage only. so as i would need it.

We are generally using Equal% characteristics for liquid and steam application, frankly speaking i dont know the exact reason but in our company we have been using it since long and it is working well. May be precise and good control range could be the reason as per my understanding.

neways, thanks a lot to all for sharing ur experience.