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Control Valve Hunting


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#1 ADR

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Posted 11 April 2019 - 08:45 AM

Hello All

 

I would like to have your thoughts on this one.

 

I am currently designing a dosing system which consists of a storage tank, centrifugal pump and 3 dosing points. The pump draws seawater from the tank and discharges it to the dosing points as shown on the attached sketch.

 

As you will see from the attached sketch I would like to maintain a constant pressure in the header line and control the flow rate at each injection point by means of flow control valves. I don't foresee any problem during dosing scenario 1 as the other dosing points 2 & 3 will be isolated using the ON/OFF valves . But during the other two scenarios, the flow will be equally shared between the streams with the help of the flow control valves installed in each stream.

 

My question is, having three control valves in parallel cause control valve hunting issue? If so, is there a way to mitigate the problem? Alternatively, if there is a better way to design the system please let me know.

 

P.S. Assume that the pipe size, length, material, valves in the dosing lines are identical.

 

Many thanks

Allen 

 

 

 



#2 thorium90

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Posted 11 April 2019 - 09:07 AM

What sketch?

 

 

I believe you just need a pressure regulating valve on each dosing line to maintain the sufficient back pressure on the control valve and prevent hunting.


Edited by thorium90, 11 April 2019 - 11:23 AM.


#3 thorium90

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Posted 11 April 2019 - 11:34 AM

I see what you mean. This is a typical flow header split flow control. The method of control that should be applied is known as Most Open Valve (MOV) Control.

Unfortunately this method of control is abit difficult to explain in just words, but I will try..

 

Basically, the most open valve logic is to have one of the control valves become the most open valve (MOV) at a specified max opening, say 85% open, and then allow the other control valves to seek their position to meet their own flow requirements. When a control valve that is not the MOV is calculated to be have become a greater opening than the current MOV, then that valve becomes the new MOV, and the previous MOV will be able to adjust and start closing. This logic gets continuously checked and updated and therefore allows multiple otherwise separate but hydraulically linked processes to be controlled by an integrated controller, which can then ensure systemwide stability and eliminates hunting.

 

Otherwise, if this is too complex, then might I suggest, buy two more pumps? The flow and pressure you have mentioned is not large, so the pump will not be expensive.


Edited by thorium90, 12 April 2019 - 09:07 AM.


#4 ADR

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Posted 11 April 2019 - 01:37 PM

Nice one!! thanks.

 

I will discuss about the MOV control with my colleagues and investigate the possibility of adding pumps.






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