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Pump Minimum Flow Control Philosophy Query

pump minimum flow centrifugal pump flow control valve

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#1 rs20170808@gmail.com

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Posted 23 October 2018 - 08:21 PM

Hi, 

 

I have a query regarding the control of pump minimum flow lines. We have a flow transmitter at pump discharge and a control valve on the min. flow line. The control is set such that as soon as min. flow is seen by the transmitter (around 90gpm) the control valve will open and recirculate the flow back to the tank. 

 

My question is when the control valve opens to recirculate the flow, how much flow will go in the main line? or is it assumed the min. flow will come only when the main line is blocked and pump goes back on its curve?

 

What action needs to be taken when there is min. flow condition? 

 

Now, if we assume that the blockage downstream is opened..how will the control valve start to close?

 

Rs

 

Pump%20miniflow%20-%20FE_CV.jpg


Edited by rs20170808@gmail.com, 23 October 2018 - 08:34 PM.


#2 Rahimzadeh

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Posted 23 October 2018 - 10:01 PM

Dear:

For preventing pump overheating  and kept away the pump from pump surging , we need min flow circulation .

 In case discharge blockage removed , the control valve will begin to reduce min circulation.



#3 rs20170808@gmail.com

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Posted 23 October 2018 - 11:44 PM

Hello sir, 

 

Just trying to understand the control better: For example if the pump has normal flow of 275 gpm..i assume when there is 275 gpm at pump discharge the control valve is closed. Now lets say there is blockage and flow reduces to min flow i.e. 90 gpm...the control valve should open and send this entire flow back to pump inlet vessel. Now say the blockage is removed and flow start increasing say it goes from 90 to 120 gpm..in that case ..does the control valve still keep sending 90 gpm back to inlet or does it fully close and send 0 flow to inlet..how is the control valve supposed to throttle for min flow? sorry if my question are very basic. 

 

rs



#4 rs20170808@gmail.com

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Posted 23 October 2018 - 11:45 PM

Hello sir, 

 

Just trying to understand the control better: For example if the pump has normal flow of 275 gpm..i assume when there is 275 gpm at pump discharge the control valve is closed. Now lets say there is blockage and flow reduces to min flow i.e. 90 gpm...the control valve should open and send this entire flow back to pump inlet vessel. Now say the blockage is removed and flow start increasing say it goes from 90 to 120 gpm..in that case ..does the control valve still keep sending 90 gpm back to inlet or does it fully close and send 0 flow to inlet..how is the control valve supposed to throttle for min flow? sorry if my question are very basic. 

 

rs



#5 fallah

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Posted 23 October 2018 - 11:49 PM

Rs,

 

The logic is to be such that:

 

1) As long as the flow in the main discharge line is equal to or greater than the specified minimum flow; the minimum control valve is closed and there is no re-circulation back to the suction drum.

 

2) At once and in any case the flow in the main discharge line would be lower than the specified minimum flow; the control valve being opened such that the summation of the flow in the main discharge line and the flow in the re-circulation line shall be, at least, equal to the specified minimum flow.

 

3) At the complete blockage of the discharge line; the control valve is to be full open and passing flow is equal to the specified minimum flow, hence by gradual removing the blockage the control valve would proportionally be closed till the flow in the main discharge line to be equal to the minimum specified flow then the control valve is completely be closed.



#6 rs20170808@gmail.com

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Posted 24 October 2018 - 12:20 AM

Hi Fallah, 

 

Thanks for your reply..Just for understanding I have assumed some values below:

 

1. Lets say my pump min flow recommendation from vendor is 90 GPM. Lets say I set the flow controller set point at 110 GPM. When 110 GPM comes control valve should partially open and send some flow back to suction vessel..say for e.g. 30 GPM, then main line flow is 80 GPM and recirculation flow is 30 GPM - Is this understanding correct??

 

2. Now lets say discharge side starts getting more blocked and the resistance increases...flow will decrease as per pump curve..now control valve should open more and start sending more flow back to the suction vessel (Say 60 GPM to recirculation and 30 GPM to main line)...is this correct in your opinion?

 

3. Ultimately when the discharge side is fully blocked ..no flow in main line...all flow through control valve to pump suction vessel (maybe = 90 GPM)



#7 fallah

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Posted 24 October 2018 - 12:28 AM

Rs,

 

1. Yes, of course you can set the CV at 90 gpm...

 

2. Yes

 

3. Yes



#8 rs20170808@gmail.com

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Posted 24 October 2018 - 12:48 AM

Thanks Fallah.

One more ques. the flow meter should be located before the control valve line - right? so that flow detection is summation of the flow in recirc line and main discharge line. When the flow detected here is lower than specified min flow...then control valve will come into action and some flow will be recirculated and some will be in main discharge line and so on...?



rs



#9 fallah

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Posted 24 October 2018 - 12:55 AM

Yes...



#10 Estiqaatsi

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Posted 22 November 2020 - 06:24 AM

Rs,

 

The logic is to be such that:

 

1) As long as the flow in the main discharge line is equal to or greater than the specified minimum flow; the minimum control valve is closed and there is no re-circulation back to the suction drum.

 

2) At once and in any case the flow in the main discharge line would be lower than the specified minimum flow; the control valve being opened such that the summation of the flow in the main discharge line and the flow in the re-circulation line shall be, at least, equal to the specified minimum flow.

 

3) At the complete blockage of the discharge line; the control valve is to be full open and passing flow is equal to the specified minimum flow, hence by gradual removing the blockage the control valve would proportionally be closed till the flow in the main discharge line to be equal to the minimum specified flow then the control valve is completely be closed.

 

Dear Fallah,
the adjustment mechanism is not clear to me. If the control valve installed on the product line adjusts to have a certain rate, let's say below the minimum flow, the valve on the recirculation line will have to adjust to recirculate the difference, ensuring that the flow rate at the pump discharge is always at least equal to the minimum flow. But how does the valve on the recirculation line regulate if the only input it receives comes from a flow measurement that it kept constant?



#11 fallah

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Posted 22 November 2020 - 07:23 AM

 

Dear Fallah,
the adjustment mechanism is not clear to me. If the control valve installed on the product line adjusts to have a certain rate, let's say below the minimum flow, the valve on the recirculation line will have to adjust to recirculate the difference, ensuring that the flow rate at the pump discharge is always at least equal to the minimum flow. But how does the valve on the recirculation line regulate if the only input it receives comes from a flow measurement that it kept constant?

 

 

Hi,

 

As long as the flow through the discharge line is kept constant, either below or higher than the specified minimum flow, the control valve on the recirculation line is fixed in partially open or closed position but if there would be a partial or full blockage at downstream due to, let say, valve closure the flow measurement will activate the control valve on the recirculation line to adjust accordingly.



#12 Estiqaatsi

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Posted 22 November 2020 - 08:15 AM

 

 

Dear Fallah,
the adjustment mechanism is not clear to me. If the control valve installed on the product line adjusts to have a certain rate, let's say below the minimum flow, the valve on the recirculation line will have to adjust to recirculate the difference, ensuring that the flow rate at the pump discharge is always at least equal to the minimum flow. But how does the valve on the recirculation line regulate if the only input it receives comes from a flow measurement that it kept constant?

 

 

Hi,

 

As long as the flow through the discharge line is kept constant, either below or higher than the specified minimum flow, the control valve on the recirculation line is fixed in partially open or closed position but if there would be a partial or full blockage at downstream due to, let say, valve closure the flow measurement will activate the control valve on the recirculation line to adjust accordingly.

 

 

Thank you very much. In the meantime I had thought about it too! The recirculation valve can have different degrees of opening at the minimum flow, depending on the flow rate on the product line. The valve remains stationary in its position until there is a change in the pump discharge flow rate, which signals the need to move to another working point. It is not the flow but the variations in flow that set the control system "in motion".



#13 zurncompany

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Posted 30 November 2020 - 07:09 AM

Yes...Pressure reducing valve 112-500XL is lead free device which is ASSE, IAPMO, CSA certified. This water pressure regulator valve has a direct acting integral by-pass design which prevents build-up of excessive system pressure caused by thermal expansion.






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