What does pump run back refer to? 3

[fotosaurus]

Hi - I'm new to to pump selection and engineering (as well as the forum). I have heard and read the term "pump run back" and am looking for an explanation of the term. A specific instance of use is "When the recirculation valve closes the pump will initially run back on its curve to provide the required flow rate". I understand as the valve closes the system head will increase but it seems this would cause the flow to decrease, not increase. Is the statement incorrect or is there something I'm missing? What exactly is meant by the term "run back"?

Thanks for your help,
Mark

 

[vpl]

If you browse some of the threads on this particular forum you'll come across entries by BigInch. He frequently posts pictures of pump curves.

"Running back on the pump curve" means that the pump operation will move to the left on the curve under the situation being described.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[Artisi]

Be careful with this type of terminology as "run back" could mean "run out" a term I use to describe a situation where the head acros the pump has been reduced allowing the pump to run-out on the curve to a higher flow rate.
It could also mean that the pump runs backwards when shut down without any NRV in the pipeline as the charged discharge line empties back thru the pump.

 

[TenPenny]

In your example, 'run back on its curve' means exactly what vpl said, closing the valve means a higher head to overcome, so the operating point moves to the left on the pump curve, in order to meet the higher head requirement, and therefore the flow is less, exactly as you said.

Nothing in that sentence you quoted suggests that the flow will increase, the use of the phrase 'provide the required flow rate' suggests that closing the valve is being done to DECREASE the flow rate as required.

 

[BigInch]

As 10p states, I also believe your description of the scenario was written by someone that made the logical assumption that, when running with the recirculation valve open, you were doing that because the system curve would not allow you to run into the system with that flowrate, so therefore you HAD to be recirculating. Why recirculate otherwise. If you could flow into the system, you should be doing that. If you were recirculating for the fun of it, its costing you money.

Thus the flowrate you were at during recirculation was higher than what you could do as you open the discharge valve and close the recirculation valve.

A typical scenario would be starting a diesel driven pump against a very high system head; pumping up the face of a steep slope to a city with a 1500 ft msl elevation. The pump would probably have a discharge check valve to prevent backflow from the higher elevations whenever the pump was turned off. Therefore, when starting the pump, no flow would be discharged until the pump discharge pressure reached the pressure on the check valve. Something that would probably take most of the diesel's rpm to achieve. During diesel rpm runup when below normal operating speeds, you couldn't flow into the system until you reached check valve backpressure and you would have to be recirculating during that time. The second that the check valve opens, the system is at zero flow and the pump momentarily moves to zero flow too, until it can accelerate the system flow from zero flow and reach its new flowrate. I would never want to disappoint Patricia, so here's the pump curve diagram. The blue dots move from lower left to top right, deviating to the left when rpm reaches check valve opening pressure. I should make one of those animated .gif pictures of this some day... soon.


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"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[fotosaurus]

Thanks to everyone for your answers. I believe I read too much into the 'provide the required flow rate' statement and assumed it meant an increase in flow. As the control valve closes, I expect to see flow diverted from the recirc loop to the actual demand loop.

 

[BigInch]

Right, but a lot of people don't think it might decrease in the process of building up the head you need to flow into the mains. If for some reason you happened to be recirculating at a higher head then what you needed to go into the mains, as might happen on start-up when the pump rpms continue to run up with a slowly opening manually operated discharge block valve, it could easily go the other way, but it seems that scenario was not being considered by the author.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[JohnGP]

Ah BigInch, now you're talking about a different curve, if you are considering the pump speed increasing.

I think the initial query has been admirably explained, but like Artisi, when I first saw the title, my thought reaction was that the query related to a pump running backwards. But there you go.

 

[BigInch]

Not necessairily increasing speed; that was just a typical case when it might be doing so. It could happen anytime the pump was recirculating at a TDH greater than open system flow (that could be at a constant rpm) and the discharge block valve opened.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[JohnGP]

Fair enough, I misunderstood the point you were making in the prior post.