6 replies to this topic

[andybud]

Hi all,

 

I am designing a system with 3 x 50% pumps (produced water), each pump is approximately 100 kW.

 

Should each pump have a dedicated recycle, or should there be a common recycle which all use? 

 

What are the advantages to both? Are there any rules of thumb regarding size of pumps, service etc which would help make the decision?

 

best regards

 

Andrew

[Bobby Strain]

You want to have a check valve on each pump discharge. And a flow meter on each pump discharge. It's probably best to provide a recycle control valve on each pump recycle. If you want to be cheap, you can use a single valve for all three. And sacrifice reliability.

 

Bobby

[fallah]

Andrew,

 

Although in safety/reliability standpoint it's better having dedicated recycle for each pump, in most cases just one common recycle is being considered for several parallel pumps mostly in order to reduce the CapEx...

[Technical Bard]

The problem with a single valve is how to know that it is giving you the desired flow.  Let's do a thought experiment.  Let's say that the pumps have a minimum flow of 30% of the normal point.   Each pump normally flows 100%, so with two pumps running you have 200% of a single pump flow.  To protect the pumps, the minimum flow recycle needs to be sized for 30% of EACH pump, or 60% of the normal point of a single pump.  

 

If you are running only one pump, then the minimum flow recycle will be fully open at 60% of the flow of a single pump, so you are protected, but not very efficient.

 

If you foresee a scenario where they would run ALL the pumps, then the total flow is 300%.  If the flow is restricted through the main line, the minimum flow valve will flow a maximum of 60% of a single pump, or only 20% of EACH pump, which is below the minimum flow value for the pumps and damage could occur.  Now, that said, the minimum flow control valve probably has excess capacity in it, and you can likely get above the actual minimum flow point by changing the controller limits.  But operations would need to do that based on how many pumps they are running.

 

This is why most major companies specifications don't like common recycles.  You need to balance the risk with the frequency or likelihood of running all three pumps.

[andybud]

Thank you all for your replies, they were appreciated and of much help.

 

Andrew

[thorium90]

I don't believe the recycle needs to be sized for anything significantly more than the minimum flow of one pump if a suitable setup and control philosophy is adopted.

 

Say you have 3x50% pumps and each pump has a nominal flow of 100cmh and a minimum flow of 30cmh.

The common recycle just needs to recycle 30cmh and all remaining scenarios will be by common discharge valve throttle and turning off excess pumps.

Flow demand: 0 to 30cmh. 1 pump with recycle 30cmh

Flow demand: 30 to 100cmh. 1 pump with discharge throttle and recycle 0cmh

Flow demand: 100 to 200cmh. 2 pump with discharge throttle and recycle 0cmh. The 2nd pump starts when demand is just above 100cmh so each pump would then be 50cmh so no requirement for recycle.

 

Lets explore further and see how the scenario works if a third pump is somehow required.

Flow demand: 200 to 300cmh. 3 pump with discharge throttle and recycle 0cmh. The 3rd pump starts when demand is just above 200cmh so each pump would then be 67cmh so no requirement for recycle.

 

Therefore all that is required is a common recycle valve with control capability, common discharge valve with control capability, common flow meter and recycle flow meter.

The most favourable control which eliminate the extra recycle line and valve is to have all pumps with VFD. The common flow meter controls the speed of all the pumps. The minimum flow will be much lower and in the vast majority of cases, redundant, making the recycle line unnecessary.

If you anticipate that the amount of time requiring full flow: ie: 200cmh based on the above example is not alot, and most of the time partial flow is required, then running at partial speed will be much more favourable for you.

Edited by thorium90, 04 December 2018 - 09:40 AM.

[Technical Bard]

The challenge with what Thorium90 is saying is that the control system would rarely be setup to automatically turn pumps OFF when the flow drops.  The operators would need to do it, and in an upset scenario where they are focussed elsewhere, my scenario where you have both pumps running with a blocked discharge and a 30 m3/h spillback with each pump at 15% of it's normal point would not be advised.