Unhappy with the FAT of a Pump 1

[ivanngwh]

I purchased a centrifugal pump (duty=333L/s at 130m head) coupled with a 3.3kV 710kW 1500rpm motor.
In the factory test, the manufacturer is going to verify the pump performance curve. I expect them will use the 3.3kV motor I purchased to perform the test. But they used a smaller motor of 380V 250kW 1000 rpm to do the test. The Factory explained that using a smaller motor with reduced speed to do the test is allowed according to BS EN ISO 9906-2012.
I checked this Standard.
Clause 5.7.2 do specify the speed of rotation (of the test) shall be within the range of 50% to 120% of the specified speed.
Clause 4.4.3 do specify each test point to be recalculated to the rated speed using the affinity laws.
It seems that the Factory is doing the right thing but it is really out of my expectation because the motor under test is really small(250kW), only 35% of the original motor rating (710kW).
What my "thought" is that they are playing toys or doing school experiments but not doing a proper industrial FAT for a real product.

The Factory declared that testing with a 3.3kV motor is not practical because 3.3kV utility supply is not available at his district.
The argument arise because the Contract do not clearly specify the pump performance test shall be carried out using the purchased 3.3kV motor.
Can anyone give me some technical knowledge /experience /advice on such kind of un-expectation?
I want to assure whether the Factory is cheating me or not.
Thanks

 

[Artisi]

Of course the power will reduce, power input at the test speed will reduce at the cube of the speed change -as per the affinity rules.

710 x (1000/1500)^3 = 210 kW , therefore 250kW is more than sufficient.

So don't worry all is in order.

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]

Chalk this one down to experience and make sure that what you THINK is in the contract or specification is what is actually written there.

It is very common for a pump supplier to use a "slave" motor and not the purchased motor due to power issues or voltage supplies. If you want a complete train test then be prepared to pay for it and carefully check the proposal actually sent back by the vendors as they can vary the specification or scope of supply that way.

I can fully understand why you see this as somehow cheating the system, but according to your notes above they will meet the specified code.

I must admit that is a new one on me - to reduce the speed to use a smaller motor....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JJPellin]

I very much dislike reduced speed testing. I consider the NPSH test to be completely invalid. The affinity laws are intended to be used for relatively small changes in speed or impeller diameter. I don't believe you get good accuracy if testing at 50 percent of rated speed. I have accepted tests on 50 Hz power for my 60 Hz pumps, but that is about as far as I am willing to go. Our purchase specifications require advance notice and approval for any reduced speed testing. If the standards applicable to you allow for it, you may have no choice. I would recommend you change your specifications to discourage it. I consider this lazy on the part of the pump manufacturers. If they sell a pump, they must be able to test it. If a given test facility can't do the test, they should move it to a facility that can.

I do not require all testing be performed with my motor. If they want to do the testing with a test motor at the same speed, that is fine by me.

Johnny Pellin

 

[Artisi]

A question for those querying reduced speed testing, can you please explain how pumps of a few thousand HP or kW rating are tested? The answer is reduced speed or even reduced physical dimensions testing.

Having worked for one of the major players some years back, many very large pumps are model tested or at reduced speed with the the subsequent performance testing scaled to full size.
You will also find that a particular impeller / volute design is identical for a large range of pumps with vastly different physical sizes..

Added: having just checked I find there are now a couple of large power test facilities available capable that weren't available in my time with large pumps - however this doesn't change that reduced speed or size are not valid for scaled up performance

So for this particular pump /test /reduced speed to a reputable pump designer / pump company is normal, acceptable and will give very accurate results.

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.)

 

[JJPellin]

There are certainly some circumstances where a reduced speed test is acceptable or even necessary. For pumps in very low specific gravity service, a full speed test on water might exceed the pressure rating of the discharge flange (for example). I may be spoiled by virtue of where I am located. I have witnessed full speed shop tests of pumps up to 1000 HP. Since the vast majority of our pumps are much smaller than that, I have really only had a problem getting a full speed test two or three times. For industries that use very, very large pumps, this may be common.

We require NPSH testing for the vast majority of our new pumps. Please correct me if I am wrong, but you can't do an NPSH test at reduced speed.

Johnny Pellin

 

[JJPellin]

Another options I have used is to accept a full speed test with a smaller diameter impeller to stay within the capabilities of the test facility. That allowed them to complete a full, proper NPSH test. It just cost me an extra impeller which I had no use for after the test.

Johnny Pellin

 

[TugboatEng]

JJ, one bit of speculation is that very large horsepower pumps are selected conservatively and installed in engineered systems so NPSH is unlikely to become a factor. It's not Joe Schmo grabbing a pump off the shelf and matching all piping to the inlet and outlet diameters of the pump.

 

[Artisi]

Tugboat: excellent comment, I would also add that the major players, Weir, Sulzer, KSB, IR etc have enough first hand design/ test data to accurately predict the performance of their standard pumps for all normal applications, for application pushing the envelope such as marginal NPSHa/r, H/Q, power limitations testing might be valid.
For me on-sight verificaction of required duty is more to point and useful.


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.)

 

[ivanngwh]

Dear Artisi, LittleInch, JJPellin, TugboatEng,
Thank you very much for your fruitful advices.
I understand more on the technical aspect and the trade practice now, with all your helpful replies.

 

[Artisi]

great to get some positive feed back - I'm sure we all agree.

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.)

 

[JJPellin]

I work in an oil refinery. Our primary processes are distillation, reforming, hydro-treating and de-sulfurizing. All of these processes take place in the vapor phase, at boiling point or very close to boiling point. Our boiler feed water systems all run with deaerators which operate very close to boiling. Most of our centrifugal pumps operate within about 10 feet of NPSHr. We have about 1600 centrifugal pumps. We have full speed test data, including NPSH testing for the vast majority of these. We often have situations where the pump performance on the test stand does not meet our needed operating point and it has to be changed. We have had to change impeller diameters, under-file, vane profile, etc. on the test stand. I have not personally experienced it, but I have heard of pumps which failed for vibration on the test stand because they were experiencing standing wave acoustic resonance in the long cross-over channel in an axial split, multi-stage pump and had to change the vane count on one or more impellers to solve it. I would much rather find and fix all of these problems on the test stand rather than in the field. I will continue to insist on full speed (when practical) performance testing, including NPSH testing for all new API pumps.

Johnny Pellin

 

[Artisi]

Johnny - as I said, pushing the envelope on performance as in your plant operation -I would agree that testing be done as close as possible to site conditions - my comment was in more general terms where / when inexperienced consultants / end users etc specify unnecessary hydraulic testing for run of the mill applications where the pumps are probably selected and impeller trims decided on using standard published curves and experience.

For instance - once had a pump rejected by a "consultant" who required a witnessed performance test for a small water supply installation, the pump was rejected because the efficiency advised was 82% at the duty point - on test the duty point was 84.5% - of course the end user was very smartly advised to get themselves a new consultant.
The same end user (local municipal authority) issued another 20 page spec - including full hydraulic testing for a construction type dirty water submersible pump available from the local hardware store. We politely sent the document back with a note saying to contact the hardware shop a few hundred yards down the road from their office.

On another issue, I had a quick look at NPSHr testing with reduced impeller - briefly read one paper that suggested an impeller reduction could show an increase of NPSHr - any comment or experience?

Blade pass frequency can be a problem - in one case we trimmed the impeller dia. instead on at 90deg to the axis we machined at an angle of 45deg. it solved the frequency problem.

Ian

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.)