Pump station performance curves vs. station start-up performance 1

[vnardone]

Subject: Pump performance curves vs. pumps performance
Author: vnardone
Date: 7/9/2002
When installation of a triplex sewage pumps station and start up is performed, is it normal that the readings at the start up do not meet exactly the required flow capacities that were submitted as approved certified curves that were in fact accepted?
How do you explain why the pumps are not meeting the specificaitons? The pump station is in fact doing it's job and has been for over a year, but the testing results are close but not meeting what the engineers are requiring as per the specifications,is this normal?
I would think that the circumstances such as the wet well, what's coming thru the pumps and "other environmental factors" would have a direct effect on the readings taking at start-up.
Do the pumps normally meet the curve at the start up?, even though it is not in a "controlled enviornment" when certified and do many variations have to be taken into consideration such as suction and discharge lines being clogged, wet well depth and the distance between the pump station and readings being taken from are a distance away?
The system is working and has been doing it's job for over a year, yet the engineers are concerned that the pumps are not meeting the required disign conditiond outlined in the specifications. The pumps did meet the specifications according to the certified curves submitted and accepted. Now, they are either they are over exceeding or meeting just under.
ANY INPUT? Should the station be rejected based on this?
THANKS

 

[fredb]

Dear vnardone,

There are a number of different ways of looking at this but basically if you accept anything other than specified performance you are essentially getting less than you paid for.

Now it may be that there are legitimate reasons for pumps not meeting spec. but they should be well documented (no guessing allowed) and then your engineer should make an assessment on the potential downside effects, and if they can be accepted with no present and/or future problems, then maybe the contractor gives a rebate for the failure to meet spec.

Alternatively you could just tell the contractor that the pumps don't meet spec and your not going to pay him for this work until they do, and let him solve the problem.

Bearing in mind all the above, be advised that there can be legal issues involved here and you should consult with a good Contracts Attorney before proceeding with any decision. For instance, you may be liable if you or the engineer accept pumps that don't meet spec unless you have the blessing of the owner of the station.

 

[vnardone]

Hi Fred B.
Thank you for your answer, however, please understand this is a Triplex Sewage Pump station that has been successfully been up and running for over a year. I have had nothing but problems with the engineers on the job. They requested a
"certain" pump manufacturere and did not get the pump they originally requested....and it has caused friction since day one. That and the fact that I am a woman I think, but I won't go there, lol. I just want this job to come to some end!


The problem is that the pump manufacturer is stating the pumps performance do in fact meet the specifications, and perform as stated. The Engineers are stating they are not.

Now, the specifications for this Triplex Sewage pump station show pump performance to be as follows
1285GPM @ 120 FT @ 1750 RPM @ 75% EFF @ 51 BHP
1900 GPM @ 80 Ft @ 1750 RPM @ 73% EFF @ 54 BHP
600 GPM @ 55 Ft.@ 1150 RPM @ 66% EFF @ 13 BHP

The testing on site at start up shows
Pump #1, 1,302 GPM @ 112.93 FT .
#2 1,302 @ 111.94 FT.
#3 1,302 @ 107 FT.

We have gone back and tested 3 times trying to make these engineers satisfied. The Pump Manufacturer is stating the pumps do in fact meet the specification and the certiifed curves that were accepted upon approval of the submittal.

My problem is I have the Engineers on one hand telling me the pumps failed and the Pump Manufactureron the other telling me they do per Hydraulic Institue standard B. And, who have come out 3 times to retest the pumps and stating they do in fact meet the specifications. At the last testing we pulled out pump #3 and sent it back to the factory for testing again. It was found to have a damaged impeller (something went thru the pump) and performed at 1185 GPM @ 119 TDH, which the manufacturer stated that it closeley meets the published performance curve and the design condition of 1285 gpm@ 120 Ft, bear in mind though, the impelller was badly damaged.

The two matching pumps had been certiifed tested at the factory proopr to the delivery to the site and accepted by the engineers. The closest test points to desgin condition for these peumps were respectively 1305gpm @ 120 Ft. and 1287 gpm @ 117 Ft., thus one pumps was just slightly less than design but within the Hydraulic Institute standear "B" and tthe other just over disign condition. Thus, the manufacutre contends that these original certified tests performed on a test stand under controlled environmental conditions, bear witness ot the pumps meeting the specification, and specified disgn conditions. They stand by their orginial certified testing of the these pumps that were clearly accepted by the engineers. these pumps are desnged to pumps sewage,not garbage or construction debris, were the voulme of heavy debris to continue to pass into the station wet well, we highly recommend the installation of a communutor or grinder, at least a a bar screen. (because of the damaged impeller).

Also be be considered prior to removing the 3rd pump for re-testing at the factory, these pumps have been on line for over a year. During the course of constructio the statio did experience such heavy flow conditions and the first two pumps installed did in fact keep up with the flow to the station, demonstrating that the pumps were more than suitable for the service intended. The majority of time of operation, the pumps operate singly, and most often at reduced speed and head . This normal reduced flow operation of the pumps is easly met with thest pumps and the installed VFD's. (18 pulse drive system).

So it is the Pump Manufacturers contention that the pumps supplied did and do meet the specificationa and perform as stated. The pump returned for retesting can be fitted with a new impeller, which is badly damaged due to something coming thru the the pipes on site, and returned to the site for installation.

The Engineers are telling me they are not meeting the spec.

So....what do you suggest I do?

Thanks.

 

[fredb]

hello again

If you have the pump curves you should be able to use these to determine if they meet specs. Each pump operating singly should match a flow and head point along the pump curve for a given impeller size, plus or minus some tolerance. Your Engineers should be able to demonstrate to you using the pump curves and/or calculations why the pumps do not meet the spec they wrote.

 

[vinayakb50]

Dear vnardone,
Hello, this is an eternal, interesting question you have raised. No, it is not normal for a pump not to achieve its specified performance. However, a systematic approach is required to determine success or failure of the pump.

First & foremost, pl. clarify whether the purchase contract specified successful site testing of pumps or only laboratory testing. Pump manufacturers usually avoid committing to site tests as you have rightly pointed out that the test conditions at site are not controlled. Also, measuring instrumentation at site has to be on par with that found in laboratories.

If your company has not insisted on successful performance test at site as pre-condition for acceptance of pumps in purchase contract, then nothing much can be done. However, if you did insist on the same, you may have to refer to the performance curves provided by the manufacturer and check whether the test point lies within the acceptable H-Q zone and with what efficiency/ power consumption. Your engineers will be able tell you if the pump passes within acceptable
tolerance limits on head, discharge, efficiency and other performance parameters as specified by the applicable standards.

I may also caution you that pumps, particularly solids handling pumps such as sewage pumps, are liable to suffer deterioration in performance over a period of time due to wear & tear. Hence it is always advisable to make up your mind on accepting the pumps on the basis of site tests in the initial stages of operation( just after commissioning).

I hope, with the above data you can take an informed decision on the matter.

Please let me know what you decide and the outcome of your decision.

 

[piffer]

It is hard to imagine that the manufacturer cannot verify the pump perfomance at the test site, especially since they did a factory performance test on each unit. Even if a calibrated flow meter isn't available, a pressure measurement and motor input power measurement can be correlated and compared to the factory test curve. You will need to know the motor efficiency at the loading condition to complete this field check.
Another aspect to consider is the engineer's calculated system head curve and "duty point." It may be off due to their "assumptions" and/or some unknown phenomema (piping restriction?).
The last point is with the pump manufacturer. If a duty point was specified, was the pump efficiency @ duty point specified? If it was, and no inference to HI, Level B was made, the manufacturer is required to meet the specified performance (flow, head, efficiency) with NO NEGATIVE TOLERANCE- - -HI, Level A is required (unless HI/B is specified). I would reject the pumps if the manufacturer cannot meet/exceed the specified duty condition.

 

[pumper]

Do the pumps have a common discharge header. For some reason pumps, #2,3, are running left to the design point. Less flow and more head, Plot the curve from the running points to the design points. This will tell you wheather or not the pumps are capable of running @ design point. this should give you the answer, "if" it is a pump problem or a system problem. My guess with out seeing a curve, is a system problem because of the testing you have done. Industry standards give a one year warranty so your probably not going to be able to go back to the MFR. at this point. Look at any change between the test facility and the plant site. Were the pumps tested separate or as a unit. Consider elevation, npsha, pipe and fittings, submergence, product, sump design. Any thing that might have changed. In a common discharge header pumps will act against each other or take the path of least resistance.

Send all the information you have and the answer should be fairly easy to find.

Pumper

 

[PUMPDESIGNER]

All the correct issues were brought up except a few.
I personally have found that factories do lie, and sometimes they are simply wrong either in their testing or in their instrumentation. This is hard for some of us to accept, but unfortunately true.

We have found factories to be outright liers, and in large ways too because few engineers will trouble themselves to stop it.
One of the largest pump manufacturers in the world misrepresents their pumps frequently. In the past year we had a pump purchased and tested from this very large manufacturer. The results were so far from the factory representation that a second identical pump was purchased from a different distributor, and the results were the same.

I have not yet figured out how all this is done, but the possibilities are limitless. RPM, instrumentation, etc.

Simply put, only thorough analyses will solve your problem. Someone somewhere is wrong. Can your company treat this as an investigation and find out where the short fall is?

Also, I feel for your situation, the system seems to be working and yet ...

However, I have seen so many jobs go awry and fall short of a specification simply because people cut too many corners and then were let off the hook by engineers just wanting to get away from the project. Also, I have seen contactors trying to "wear out" the engineers by avoiding the obvious requirement. Just keep avoiding, just keep them on the phone, just keep them going, and sooner or later the engineers just wear out. Richard Neff
Irrigation Craft

 

[gregcase]

Vanrdone,
Several important topics have been addressed in the above emails. It is a fact that pump companies (generally marketing departments) embelish the truth to put themselves above a competitor. The Hydraulic Institute standards clearly define test methods and the reporting of test data. Non member companies often do not comply. You may also find that member companies are not rigorously adhereing to the standards.

Before you get into the leagal fray, make sure your assesment of your equipment was performed correctly. Do your measurements with callibrated gages, at the correct locations on the pump. This means suction and discharge. Verify the calibration of your flow meters. Make sure that you check the operating speed of the unit. Check the water temperature.

I spent many years as the Engineering manager for several pump manufacturers and have been running a consulting company for the last four that is engaged in pump trouble shooting. I have ssen many problems stemming from the factory, but at least double that number stemming from users not fully understanding how pumps are tested. Make sure yourtests meet HI standards as close as you can and then go after the Contractor/Distributor/Factory.