3 replies to this topic

[B.M. Process]

Dear all,

 

We are designing water disposal pumps. The capacity of these pumps is 80 cubic meters per hour with a differential pressure of 120 barg. The pumps draw treated water from a 10 m diameter and 13-meter height tank. To protect the pump against any cases of a minimum flow condition, there are two choices to go for:

 

1. Continuous flow control valve that recycles some flow back to the tank to keep the level as marked in the attached P&ID

2. On/Off Automatic Recirculation Valve (ARC) as shown on the same P&ID.

 

Now, the concern is that if the continuous flow control valve is chosen, the capacity of the pump is very limited and would mean that less amount of the produced water will be disposed to disposal wells. However, ARC valve is chosen, the protection will not be as effective as with continuous control valve.

 

Please refer to the attached P&ID, and let me know your opinion if any.

 

Thanks for your time.

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[fallah]

 To protect the pump against any cases of a minimum flow condition, there are two choices to go for:

 

1. Continuous flow control valve that recycles some flow back to the tank to keep the level as marked in the attached P&ID

2. On/Off Automatic Recirculation Valve (ARC) as shown on the same P&ID.

 

Now, the concern is that if the continuous flow control valve is chosen, the capacity of the pump is very limited and would mean that less amount of the produced water will be disposed to disposal wells. However, ARC valve is chosen, the protection will not be as effective as with continuous control valve.

 

 

B.M. Process,

 

It seems the second option is better one; but why you consider continuous flow control valve and not to consider a control valve is normally closed and would be opened in the cases the discharge flow will go to be lower than minimum flow?

[B.M. Process]

Dear Fallah, 

 

Thanks for your response. For sure, the second option (ARC) is what have in some of the existing pumps and they do work fine. The choice of the control valve was suggested by the client because of some concern upstream that may cause the flow to reduce to some minimum below that is required by the pumps.   

 

The "continuous" control valve mentioned in my previous post, I apologize that it isn't correct because a control valve would open when the flow goes to a minimum depending on the setting, just like you have said. An ARC can also recycle flow back to the tank. However, based on the sensitivity, a control valve would be more sensitive to drop in the flow compared to an ARC valve as the latter entirely uses mechanical means to detect drop pressure.

 

The second choice is, of course, cheaper and less maintenance would be required.

[fallah]

Dear Fallah, 

 

Thanks for your response. For sure, the second option (ARC) is what have in some of the existing pumps and they do work fine. The choice of the control valve was suggested by the client because of some concern upstream that may cause the flow to reduce to some minimum below that is required by the pumps.   

 

The "continuous" control valve mentioned in my previous post, I apologize that it isn't correct because a control valve would open when the flow goes to a minimum depending on the setting, just like you have said. An ARC can also recycle flow back to the tank. However, based on the sensitivity, a control valve would be more sensitive to drop in the flow compared to an ARC valve as the latter entirely uses mechanical means to detect drop pressure.

 

The second choice is, of course, cheaper and less maintenance would be required.

 

B.M. Process,

 

Ok, thanks for your clear explanation; anyway the criteria for selection of ARC valve over CV are as follows:

 

1) In minimum flow recirculation line not higher than almost 8" due to the high cost in larger size...

 

2) In clean service due to potential for plugging... 

 

3) When fine adjustment of minimum flow isn't required because they are not so accurate...

 

4) When the pump curve isn't so flat near the shut off...

 

5) When due to safety issues or high expense of the pump (such as BFW pumps) there should be no dependence to the plant control system, because the ARC valve is fully mechanical