Centrifugal pump curve falls short of factory test 3

[sboswell]

against the factory testing that is..

This is a drop of a substantial amount. close to 20 gpm difference at 30ft of discharge head which is the lowest our piping system will go.

pump shutoff measured here and at the factory is 47 ft.
NPSH is 2ft, 2.0hp, 1750rpm, pump is a 6.125" impeller, 1" discharge dia, 1.5" suction dia. factory BEP Flow is 55 gpm. our field tested rpms during operation have all been better than 1750rpm. we've changed out the suction piping to a few different configurations tank + valve to a stand pipe and 3" continuous suction piping leading up to the pump. flowrates and pressures have been double and triple checked with calibrated equipment onsite and several different gauges and styles of flowmeters.

I though a pump curve can only change due to loss of rpm or a suction side starvation. We've also vibration tested the pump and checked its impeller clearances. Any help is greatly appreciated.

Sam Boswell

 

[QualityTime]

...thanks BI...

Sboswell has advised that he has manipulated the factory pump test results....that may explain the questionable results that are in the factory test data table. They are questionable because the numbers just don't add up properly. You and I are both in agreement in questioning all of the test data results.

You and I are both suspecting that there is NPSHa/cavitation problems here. Even though the test data is suspect the drop in the performance curve is an indicator of lack of NPSHa which leads to cavitation. Sboswell should do some calculations to check it out as i previoulsy suggested. Furthermore he should have been able to have heard the classic sounds of cavitation when he was doing his field tests.

 

[BigInch]

Could also still be a backwards rotating pump, or wrong voltage. One thing for sure, its major trouble with the numbers, the column labels and the units. In general field testing data must be treated more seriously so everyone that reads the test at a later date will know exactly what data was being taken and not become confused about what was, or how it was being calculated. Suction "Pressure" in Feet, just doesn't work for me.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[BigInch]

Nor should it be "modified". Do that on some other spreadsheet. Leave field data intact.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[sboswell]

Ok, posting the original factory performance testing and my latest run of field tests as well as a process diagram. I have worked around pumps that had cavitation problems before so i can assure you this is not occurring. The pump runs extremely quiet at times its hard to tell if its even on. NPSH required is 2.0' which is equal to 2.0' of water in our storage tank. We usually operate with the tank at a water height at least doubling this. These pumps have also been vibration tested and noise tested successfully with demanding standards. As far as rotation direction goes the impeller vanes are spreading out in a clockwise rotation, I will check the pumps again to be sure they are rotating in the counter clockwise direction.

the people on this board are a credit to humanity. I should have joined a lot sooner, thank you guys so much.

-Sam

 

[sboswell]

Oops. I meant to say that the pump runs quiet, and at times it is hard to tell if it is even on. by that I do not mean it changes much in loudness.

 

[BigInch]

I don't think there's anything wrong with the pump. You're making shutoff head, but falling off significantly after 30 gpm with what looks to be mostly a function of increasing velocity. That's what it should do, but not so much. I'm going to bet on a problem with the suction piping. I don't like the looks of those elbows directly in front of the pump suction intake; those should be 10 diameters away from the pump. Can you give us some pipe lengths in that suction piping spoool?

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[sboswell]

CCW rotation is correct for the pump & i've verified this. I've attached pictures of the area. there is 2" piping downstream of the process diagram i uploaded; roughly 100 ft of it going up 6ft in elevation and dumping into a tank. I've also used this tank to confirm flow numbers using graduated marks along its clear sight glass.

BigInch,
we've modified the suction piping with a few different designs at the manufacturers recommendation. we piped the entire line with 3" with a bell reducer going into the pump, where it was 2" before, you can see this in the attached picture on the far pump. the bell could've been set further back but i think we have more than enough distance from the elbow on the original setup (shown in background)
I agree with you that there shouldnt be anything wrong with the pump. would you bet its a factory test mixup.

 

[sboswell]

bumped the pumps backwards in the CW direction. max flow with valves fully open was 26gpm, i could hear cavitation so I turned the pump off. I'm sure they are going in the correct direction. Based on a few things going on with the discharge piping its occured to me to simplify the piping between the pump and the discharge gauge. I'll be taking out the rubber expansion piece and replacing it with a 1" flgxflg nipple with tap for a gauge. Its looking as though maybe there is a large pressure drop between the pump outlet and where i have my gauge currently. if its large enough it would shift the curve in the right direction.

 

[TenPenny]

I'm assuming that you are looking at the suction end of the pump when you refer to CW and CCW, because by standard terminology, the 3196 is CW rotation, as determined by standing where the motor is, looking at the coupling end of the pump.

There is an arrow in the bearing frame, near the coupling guard to confirm.

 

[sboswell]

I think I have solved the issue. The discharge gauge was too far downstream of the pump and between the pump there was a 90 degree elbow, a rubber expansion joint and a bushing to 2" pipe. There were too many drag inducing components which came into play between the pump and the disch gauge. I will post my new results in a few minutes. Everything seems to be plotting on the curve.

 

[QualityTime]

Hi;

Factory Performance Tests

I can’t figure out the factory performance test results. The TDH calculation does not make sense. Also I notice that the performance test was at SG=1 and the as built performance at rated speed the SG=1.2. What is the SG of your product? But let’s assume it is correct.

Filed Test #3 and #4

I corrected your results and you can see it on the appended spreadsheet in yellow boxes. You can see the formulas in the spreadsheet. The equation to compute total dynamic head is as follows:

TDH for a
Flooded condition = reading on discharge pressure gauge - reading on suction gauge + delta v2/2g

The shut in head for the factory test, test 3, test 4 is 46.5 ft, 44.04 ft and 45.43 ft is close enough using the instruments that you have. Both site test curves start to drop off on increasing flow. Have you checked your suction piping for debris or checked inside the pump impeller for debris? The shape of the curve is a classic curve for something blocking the flow in the suction piping

Have you checked the performance for the other pump?

Can you see any vortexes at the pump suction at different water levels

Appended is my spreadsheet that shows how i did the calculations

 

[Artisi]

Which of the two pumps shown in your photo are you having problems with, is it the one in the foreground?

If it is the one in the foreground with the concentric reducer on the inlet this could be causing some problems with performance.

 

[QualityTime]

Hi;

I am resending this with the spreadsheet and graph. The last version did not have the graph

Factory Performance Tests

I can't figure out the factory performance test results. The TDH calculation does not make sense. Also I notice that the performance test was at SG=1 and the as built performance at rated speed the SG=1.2. What is the SG of your product? But let's assume it is correct.

Field Test #3 and #4

I corrected your results and you can see it on the appended spreadsheet in yellow boxes. You can see the formulas in the spreadsheet. The equation to compute total dynamic head is as follows:

TDH for a
Flooded condition = reading on discharge pressure gauge - reading on suction gauge + delta v2/2g

The shut in head for the factory test, test 3, test 4 is 46.5 ft, 44.04 ft and 45.43 ft is close enough using the instruments that you have. Both site test curves start to drop off on increasing flow. Have you checked your suction piping for debris or checked inside the pump impeller for debris? The shape of the curve is a classic curve for something blocking the flow in the suction piping

Have you checked the performance for the other pump?

Can you see any vortexes at the pump suction at different water levels

 

[QualityTime]

Look at this trouble shooting chart....again...looks to see if you have a blccked suction or something blocking the impeller.....

 

[QualityTime]

The concentric reducer at the pump suction nozzle is not a good design. Air collects at the top of the horizongal suction piping and acts like an air blockage to reduce flow


What liquid are you pumping? Does it off gas?

 

[Artisi]

There is 1 problem I do not see in the list, air pocket trapped in the top of concentric reducers lowering NPSHa. Certainly well worth a check.

 

[sboswell]

Ten Penny, yes the CCW rotation I called out is from the suction side looking at the face of the impeller. thanks for setting me straight, i was not sure which side is the proper side to call the rotation from. It is rotating CW from the coupling side looking at the pump. We ran it backwards today and there was some bad noises coming from it. definitely cavitation and wide open flow of only 28 gpm (47 gpm normally)

Quality time, could the strange data from the factory be explained by the TDH was total dynamic head rather than total differential head? I did not see them taking into account their suction side head (ie they were not subrtracting the number from their discharge head. They only added the velocity head to the discharge head and went ahead and plotted that against their capacity.

Yes, our test medium so far has only been water. we will eventually be running hydrochloric acid (SG 1.2) through the pump. No gassing.

Thank you for weeding through my results, I should have posted them in an editable format. I see what you did there with the delta v2/2g, that spreadsheet is excellent.

Pump #2 in the background runs roughly the same with no difference to the pump in the foreground. I should have stated that earlier. all the changes we have made have done nothing to help our situation out. Once I've answered all the questions I'll explain what did help.

The impeller was clean and clear as was all the piping going into it. The concentric reducer was just a temporary idea suggested by the manufacturer it was changed to that from the setup shown in the background. I could see why it would be a better idea to have an eccentric reducer there. We'll probably do that in a final design if one is needed along that horizontal run. The vortice issue was one that the factory had brought up as well. My coworker and I had tossed around the idea of using clear pvc pipe and injecting food coloring.

At some point yesterday I decided that I wanted a pressure gauge even closer to the pump discharge outlet, so we removed the rubber expansion joint and fitted a hard piped flanged nipple with a gauge. We ran the pump for a few minutes and noticed a drop in pressure from this newly placed gauge to the original gauge. Its clear to me now that the discharge pressure gauge cannot be downstream of any major drag inducing elements, the rubber expansion joint we installed was constricted to .85" diameter and had a rough internal layout. As the pictures show, downstream of the flex coupling we have a 90 degree elbow, 1" Tee, a 1x2 bushing & 2" tee where the old gauge is located.

After the new gauge location was fitted the pump was started; after settling for awhile the new gauge read 17 psi at the fully open condition. The gauge situated in the old location read only 10psi. We've since taken 4 points and they all graph far beyond our old tests, lying directly on top the factory curve. As soon as I get back to the office I'll post the results.

Thanks for all your help in this matter!

 

[sboswell]

forgot the attachment. in summary to my last post, which is basically answering all of the last few questions, I've gotten the pump to run on the curve by eliminating drag inducing elements between the pumps discharge and the original discharge gauge location. The elements are still there I've just inserted a gauge closer. So far the readings I've taken were up to 7psi higher towards the end of the curve. These points land very precisely on top of the factory test curve now. I now see that it makes sense for the curve to meet the shutoff head pressure and slowly fall off the factory tested curve given that there is enough obstructions between the pump and the disch gauge to create an increasing amount of drag as the flow increases in velocity. The system is still responsible for not meeting the tail end of the 66 gpm. I'm confident now that the pump will meet the full capacity of 66gpm after removing some of the discharge pipings drag inducing elements and using smoother transitions.

 

[Artisi]

Another one of life's pump lessons for pumps "that don't work", get all the basics sorted out correctly before starting down the academic track.

A standard mass produced pump that has been round for years and properly tested over and over will perform near curve everytime provided it is installed correctly and any testing is undertaken with correctly positioned and accurate measuring equipment.

In my 30 years in the pump industry, not once have I seen a factory tested pump which "doesn't meet the curve" on a site test and returned for re-test fail the re-test. On a number of occassions where liquidated damages etc were involved I had pumps tested on other test rigs just so there wasn't any bias with results.

 

[katmar]

Sorry to jump in so late. I'm happy to see the problem has been solved but just some comments I would like to make for future reference.

It seems that the discharge velocity head was calculated for the field tests based on a 1" pipe, but the static pressure was measured in the 2" section of pipe. You can't add the two to get the total head. This is adding apples and oranges.

The original pressure gauge was mounted on a reducing tee, with the inlet flow via a reducing bush. You could make a crude venturi ejector this way and I wonder if the gauge wasn't actually seeing something of a venturi effect? But the expected pressure drop through the 1" elements upstream of the gauge pretty much account for the "missing" head and this venturi effect is probably small.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com