How long centrifugal pump can work on minimum flow for commissioning purpose? 5

[Nick4684]

Normally we are doing 4hrs MRT (mechanical run test) for commissioning of pumps.
Now I have pump, 400Kw- 350m3/hr, (min. flow 100m3/hr)- 50bar discharge pressure and 260m head
any idea for how long can I run it for MRT on minimum flow line to check pump overall performance, vibration, temperature?

 

[1503-44]

Do you have a recirculation line, flowing to a tank, or some other means to maintain 100m3/h flow through the pump? Not running deadhead, you should be able to maintain as low as 20% of rated pump flow indefinitely.

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[LittleInch]

That's a decent sized pump so if you dead head or go below your 100m3/hr for even a few minutes ( 1-2) then you will generate a lot of heat.

what are you pumping as 260m head and 50 bar means it is 0.5 SG or you have a big inlet pressure??

But as mr 44 says, if the vendor has quoted 100 m3/hr as the min continuous flow which sounds right (usually ~30% of full flow) then it will work like this for as along as you want. Clearly you won't be able to measure what it does at full flow so vibrations etc may be higher or lower at full flow.

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[Artisi]

What's the point of checking temp. and vibration at minimum flow, unless it will be continually run at this point: temp, vibration, flow, head and power input should be checked at its operating duty, anywhere else is fairly useless, other than giving a reference for a reduced condition due to an operating problem.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[LittleInch]

Depends on what you want to check, but checks at 100%, 80, 60 & 40 of rated flow are good points to see if your pump is good over a range of flows. And 110% if you can.

But if it's going to be run at one more or less constant flow then just look at that duty point.

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[1503-44]

We had 400m3/h, 140 bar pumps running at Site 6. They would recirculate all day at 20% 80m3/h, but dead head them and you could cook 1/2 a camel in 15 minites. You can probably recirculate for quite awhile at 30-60%. At higher flow rates >60%, you're going to find flowing into a pipeline or a tank very useful.

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[TugboatEng]

Minimum flow? My definition of minimum flow would be the minimum flow that the pump can operate at indefinitely. If you're operating below minimum flow you're going to need to watch temperature as you don't want to overheat your seal or run in to cavitation as the fluid heats.

 

[Artisi]

Minimum flow on larger units is usually by design, it's the point where suction conditions are deteriorating as flow onto the impeller blades is other than ideal, leading to vibration by recirculation, unbalanced Axial and radial loading, NPSH considerations etc, it has nothing to do percentages of BEP conditions. Smaller units tend to be more forgiving due to much lower power input (usually) and manufacturers have a good idea what minimum flow is acceptable based on testing and installation experience.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[1503-44]

If they don't tell you 20-30%, they're not telling the truth, or if they are, buy a different pump.

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[1503-44]

Nick, TugBoatEng is correct. He gave me an idea. I can do that. Let's just put it in the simulator. You post the pump Make & Model, the Q-H & either power-Q or Efficiency-Q curve and tell me the BEP speed, power and flow ratings. Also need to know what you're pumping and the driver type and control arrangements. Is it constant speed electric, VFD-electric, or internal combustion VSD, and it is on RPM, flow, or discharge pressure control. Then we'll find out how fast it heats up flowing to a tank/pipeline, or recirculating while at various speed, or various flow rate or discharge pressure settings. I forgot that I did the same analysis for those pumps I mentioned above.

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[horacio Torres]

At minimum flow, you can run the pump as long as you want. At minimal flow you can check the vibration, bearing behaviors, or mechanical test.

To test performance or overall efficiency, it should be done  100 % of the loan for a long period of time-24 hr at least.

Horacio

 

[Nick4684]

Thank you all for your useful response,
Sorry for delay to response; I had some personal problem

Medium is water, constant speed electric, on Flow Control, it is recirculating to the suction pipe and full suction pressure is available- suction pressure is 20 bar .
The reason of testing on minimum flow is I need to demonstrate pump operation during commissioning and before start up, since there is not possibility to run pump with full flow I will run it on minimum flow just to ensure the operation of system and logic check to ensure there is no abnormal operation or leakage issue.

 

[LittleInch]

If you're doing the test by recirculation direct to the suction with no forward flow then the temperature will climb quite rapidly for a pump of that size.

I would guess you have about 5 minutes max.

To do this normally you either need a cooler or go back into a big tank

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[Nick4684]

@ LittleInch
Thanks for your advise, I did MRT for almost the same size pump/ same arrangement on minimum flow for app. 2hrs without any issue, ( I checked pump temperature every 10');
but according to our procedure and client request, MRT should be done in 4hrs, however, as "Artisi" mentioned above, maybe there is no point for running pump on min. flow for 4hrs;
I am thinking to deviate again from procedure and go for 2hrs.

 

[LittleInch]

Well ok, two hours going round in a loop - must be quite a big loop... you must be putting 30-40kW of heat into the water even at min flow with a pump that big surely?

No forward flow at all?

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[1503-44]

With constant speed electric drive, how are you going to control flow to 100m3/h. Do you have flow control going into suction?
I think no forward flow counts as dead head.

I had a pump similar to yours laying around in the virtual pipeline warehouse.

Assuming you have a temporary tank, that you can flow 300m3/h to that and recycle 100m3/h to suction with only a 1C rise in temp.
100m3/h to suction and 300 to tank or pipeline will not raise temperature more than 1C
200m3 to suction, 200 to tank will get you +2C
250 to suction, 100 to tank will put you at +4C
275 to suction, 50 to tank +6C
300 to suction, 25 to tank +10C
More than 300 to suction will constantly raise temperature.

> Dead head, I'll give you 9 minutes to get to 250F (120C).
In 15 minutes you get a firewater deluge.

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[LittleInch]

When I say no forward flow, what I mean is that the line downstream of the bypass line has no flow. Not dead head.

It usually only takes a small amount of forward flow to prevent temperature rise, or a decent sized loop or tank.

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[1503-44]

Yeah, I know. But it doesn't matter, as thermodynamically it amounts to the same thing; the heat has nowhere to go. Yes, that's what I told him in the first post. Just 20% "minimum flow" (forward flow) is more than enough to keep heat from building up. In the sim, you only get a couple of degrees build up at 100m3/h. You could do that 24/7. In fact you can easily recycle 50% BEP flow.

What I don't understand is what Nick thinks operating at minimum flow means. Min flow for me is the flow leaving the pump and recycle lines, going down the pipeline, to tank, etc.; the flow NOT being recycled. Does it seem to you that he thinks it is something else? Nick, the fluid spinning in the pump plus that being recycled back to suction is not minimum flow. Minimum flow in the case of a constant speed pump is recycling as much as possible while only allowing a minimum flow to leave the pump & recycle to go out to a pipeline or tank. Minimum flow in a VSD pump is the same, that going into a pipeline or tank, the difference being that you don't need a recycle to do it, you just turn the pump speed down lower to get that minimum flow. That is a big difference. The VSD is not wasting a lot of power at low flow. A constant speed is usually wasting a lot of power when on recycle, unless your recycle line is so big you can do it with no load/head. Could you please explain how you will make min flow?

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[LittleInch]

I read "Medium is water, constant speed electric, on Flow Control, it is recirculating to the suction pipe and full suction pressure is available- suction pressure is 20 bar ." to mean the pump is just recirculating the entire flow coming out of the pump at the min continuous flow.

If this is the case I'm quite surprised a pump of that size won't start to heat up within less than an hour.

But then we can't see the system or details of what exactly is going on....

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[1503-44]

Same. So that's dead head while recycling a bit. No water is going anywhere, except back to suction.
He's only got 9min of doing that with a typical arrangement. I say typical, because if you recycled back at 400m3/h, the pump would not have any load, so you could go on for awhile doing that, but that's not the way most systems are configured. I doubt that a recycle line and valve are size for 400m3/h with bascially no pressure drop.

Usually you want to recycle while you load or unload near highest delivery head, so recycles are not sized for zero pressure drop, so the diameters and flow capacities are towards the 50-150m3/h, rather than 400 BEP flow. So, there is usually some significant power being expended when on recycle, just do to the recycle's pressure drop. So therefore it heats up. At full BEP recycle, maybe you aren't going to be making much heat, due to the low head. Could that be the plan?

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