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

 

[MikeHalloran]

The factory curve looks funky.


Mike Halloran
Pembroke Pines, FL, USA

 

[sboswell]

yeah it is a funky curve. what I've received from the factory is a smooth graphical pump curve that I can overlay with the hard data I also received from them. Their raw data is pretty bizarre when plotted against a smooth trendline. nonetheless what could cause the massive loss of flow in our system vs the test center system?

 

[CH5OH]

I noticed strange figures in suct pressure (manufacturer test)
I noticed strange figures in bhp (test 1)
delta pressure=discharge pressure-suction pressure
it is delta pressure or delta head you want to use in a plot

 

[MikeHalloran]

First guess: Temperature. Pumps are tested with water at a specific temperature, or test data are corrected to standard conditions.

Second guess: Measurement uncertainty. I've collected a fair number of pump data sets in the field, well actually in boats that were rolling and pitching, and it never looks particularly good, though rarely as funky as what you've got there.

Third guess: Database problems. I hope the problem is limited to Caterpillar. Their Web-based software retrieves what are clearly the wrong pump curves for their raw water pumps. It appears to be one of those problems where the data was stored in a zero-based 2D array, but retrieved as if it were a one-based array, or complementarily. So you get _a_ pump curve, just not the right one, except for the ones on the ends of the array, in which case you get ... random data from other fields. I have despaired of them ever accepting a bug report.



Mike Halloran
Pembroke Pines, FL, USA

 

[sboswell]

Let me address the strange figures CH50H. suction pressure in the manufacturers test were denoted as being ABS or absolute pressure. I take that to mean it is an actual height of water on the suction side in a stand pipe + 1 atm or 33.9 ft. not sure why they did it that way.

The figures for bhp in test 1 should be omitted. I was playing with some equations for water flow and trying to get horsepower numbers to match the factory test data.

Delta pressure is what I want to use for the pump curve? I'll use this number, except the factory graph does not seem to use delta head pressure. its using the "total head" figures which seem to only factor a combination of the velocity head plus the discharge head. I'll plot everything again using this method to see if I get some kind of an agreement between the two data sets.

Mike Halloran,
Temperature was checked as well, several different times during the day. we were unable to see major differences between the range of 60-80 degree's f. 1-2 psi differences were somewhat normal from two back to back tests. Though we have never reached the 66 gallons per minute at 30 psi that the factory was able to achieve. in fact our best curves are plotted in test 1 and 2. these are far away from the factory's test. the end point of the factory being 66 gpm and our field testing being 44 gpm the factory has a 150% increase. very unusual numbers they've gotten...

should I be concerned about my piping arrangement or turbulent flow characteristics as the factory is telling me?
I can post the P&ID or mechanical drawings if necessary. its basically a 100-150 ft length of pipe without rise, around 8 2" cpvc 90deg elbows, 2" mag flow meter, two 2" diaphragm valves and a 2" ball valve at the very end going into a tank inlet. Upstream is a 8" stand pipe connected to a 3" header that falls 6 ft below the pump centerline and rises up to meet it again 10 feet away. bell reducer from 3" to 1.5" and into the pump. certified gauges and flowmeters downstream. gauges on both sides are within 1' of the pump and have proper air bleed off. heheh i'm at my wits end... Factory is standing by their testing and has insisted it is not a warranty issue err.. basically they've said there is nothing they can do.

 

[QualityTime]

Do the following caculations to determine the break horsepower on all of the values you have in all of the tables. The catalogue curves will give you the pump efficiency that you are looking for to plug into the equation. Compare the calculated bhp to the measured bhp and this will tell you which tests are incorrect. I do have to say that the factory test curve looks very unusual


The brake horse power - bhp - for a pump or fan can be expressed as:

Pbhp = ( ? Q h / 33000 ) / ? (2)

where

Pbhp = brake horse power (horsepower, hp)

Q = volume flow rate (ft3/min, cfm)

? = overall efficiency

 

[TenPenny]

The factory test curve looks pretty much exactly as it should for that pump, if you smooth out those dips (and when you consider how low the pressures are, it's no surprise that the readngs might be 1 or 2 ft off, so it's no worries about that).

Are your flow measurements accurate? What are you using to measure the flow?

 

[sboswell]

Quality time, first off thank you for your response. The calculations are done, using the catalog curves(factory testing efficiency data). they were pretty jumpy so i used a trendline and its equation to get a smooth number to use in my tests. I'll be honest here I can see that my numbers for calculated bhp do not come very close to the 1.3 bhp that the factory obtained at their claimed 66gpm. Can you explain further how I would be able to tell which data set is incorrect?

TenPenny, the factory curve is I'll admit a little bumpy but its understandable that the accuracy can be smudged so i'm not knocking that. we have tested the flow with a fill test of a known volumetric cylinder as well as a calibrated rosemount magnetic flowmeter, in addition to these we've used a pitot tube instrument and a 2" inline horizontal flowmeter purchased from mcmaster. We've taken similar readings from all of these (usually they agree within 3-5 gpm). and pressure gauges, we've been through calibrated and uncalibrated, new old and everything in between. I think the interesting bit is that the test data towards the end of the factory test is so far and away from our field tests (1.5 times) that i'm starting to think the error might be in the form of an HCL to H20 conversion. though that would only make up for 1.2 times the difference.

 

[TenPenny]

Is the pump new? What's the metallurgy?

 

[sboswell]

Yes, these pumps have been only used for testing and are brand new. impeller clearances have also been tested at a few different settings but have stayed at the factory recommended and have not changed during the factory test or our field test. the casing and impeller material are hastelloy C-276 ASTM A494

 

[QualityTime]

Hi sboswell:

The way the data is presented in the factory test and in your field tests don't make sense. An example:

Factory Test Point 2 - if they measured 42.7 ft absolute that means the suction was flooded to 8.74 ft [42.7 - 14.7 x 2.31)]. The velocity head was 0.2 ft. The discharge head was 45.3 ft. Therefore the total dynamic head is 36.76 ft (i.e. 45.3 + 0.2 - 8.74) and not 45.5 ft.

Field Test #1 Point 3 - the flooded suction pressure is 2.84 ft (i.e. 1.23 psi). The velocity head is 2.86 ft. The discharge head is 38.8 ft. Therefore the total dynamic head is 38.8 ft (i.e. 38.8 + 2.86 - 2.84) and not 41.65 ft


I assume all of the pressure readings have been corrected to the centerline of the pump. Also i note that the pump speed for the factory test is different and the test results have to be corrected to a baseline speed.

Without sounding condescending...did the people who did the factory testing and the people who did the field testing have experience in this field?

Also, i don't know what type of pump this is but the efficiencies sure seem low

 

[QualityTime]

Sam:

Appended is some more information from the Fairbanks Morse hydraulics handbook that will help you out.

In my last email i did not answer your question directly but it is difficult to say who is right and who is wrong. The BHP calculation is a proven equation. If the data that is entered is incorrect then the BHP results are incorrect

 

[sboswell]

thats a fair point Quality time, I would say that I am not very experienced in this field. cant speak for the people at the factory. I've corrected them using the following assumptions and removing suction pressure from the discharge pressure. the suction gauge is 1' higher than the cl of the pump and the discharge gauge is 3' higher than the cl of the pump. adding 1' to the suction pressure and 3' to the discharge pressure yeilds a difference of 2' of additional head for the tdh on the field tests. For the Factory tested data I corrected the TDH by first subtracting the suction pressure in abs feet from atmospheric pressure of 33.6' measured at the factory. this value was taken off the total head value which contained simply the addition of velocity head and discharge head values. the correction youve suggested yields a very strange looking pump curve. which is why at first i thought it might be better just plotting the discharge pressure against factory discharge pressure.

also the pump is a centrifugal 6.125" dia horizontal end suction pump. 1" outlet 1.5" inlet

Thanks for your comments

 

[TenPenny]

I would say that the factory test confirmed that the pump is running according to the catalog curve for that unit.

Your mission now is to figure out why your setup isn't giving you what you think it should. Perhaps you could post a piping schematic, there's obviously something wonky.

 

[QualityTime]

Hi;

If I understand it correctly you have massaged the values in the factory test results table???. If so, I would rather see how the pump factory presented the data.

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

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

I would agree with Tenpenny that the factory test confirmed the catalague curve. It would be best to post a schematic of your pump setup showing where the pressure gauges are with respect to the centerline of the pump. In a table, post the suction pressure gauge reading the discharge pressure gauge reading and the corresponding flow for every point you throttled. Do not do any corrections to the readings. I assume you have calibrated pressure gauges and flow meters and they are reading accurately

 

[QualityTime]

Hi

It would be nice if you could post a photo of the installation.

Is it possible that with your piping setup that you do not have enough NPSHa, at for example, 45 usgpm? Do the calculation if you have not done it. The tested pump curve will follow catalogue pump curve as long as you have enough NPSHa. If you don't have enough NPSHa The field tested pump curve will start to droop at the point where you just don't have enough NPSHa. The droop will will get worse as the NPSHa becomes more and more negative.

 

[BigInch]

What 's going on here?

FACTORY TEST

QualityTime, apparently you assume that "Suct. Pres. (ft ABS)" actually means 42.7 psia, but the units are given in [/color red]FEET[/color]. The title of that column should be SUCTION HEAD FT ABS

ALL FACTORY TEST CALCULATIONS APPEAR TO BE CORRECT
however "disch psi" should be "disch press psig".
"disch head" should be "differential head ft"
"total head ft" should be "total differential head ft"
All the numbers are correct, if you assume my definitions.

THE CURVE LOOKS WIERD because you are used to looking at curves plotting differential head, but this is plotting TOTAL DIFFERENTIAL HEAD. Most pump curves do not plot total differential head, they ignore velocity heads. Therefore where the velocity becomes relatively high, you see its squaring conribution to total head as keeping the head at the higher flow ranges than what you are probably used to seeing. When the velocity component is not included in the pump curve, the curve falls off much faster at the higher flowrates.

FIELD TEST 1

I agree with QT and think somebody didn't know what the heck they were doing on this one.

FIELD TEST 1 POINT #3

suction head from 1.23 psiG is 2.84 ft,
differential head from (16.8-1.23) psiG is 35.96 ft
discharge velocity head I take as 2.86 ft,
total differential head is 35.96 + 2.86 = 38.82 ft
(I get the same value as QualityTime there)

Your power calculation is not BRAKE power its HYDRAULIC power. Divide by efficiency to get BHP. I used efficiency values from the mfgr's curve for similar flowrates and found your power consumption is extremely high.

Correcting for HCL assuming it has a SG of 1.2 would make your field test's pressure conversion translate to heads that would be lower than an equivalent water head by 20%, thereby making it worse, ie, not going in the correcting direction and making your power consumption 20% higher.

NOW, what's with the low suction pressures? Perhaps you are cavitating the hell out of this pump, causing the low performance figures you are getting? Nowhere do you ever say what NPSHR is, so I'm not sure, but it looks like a possibility. Your suction pressures are very much lower than the factory's values. A significant portion of NPSHR could be made up from velocity head, so if your's is slower than the factory's there's more of the problem there too.

If this is a new installation, be sure the pump is not running backwards.

"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

 

[QualityTime]

BI,

When I did my calculations for Factory Test Point 2, I definitely used absolute ft and not absolute psi. The table showed the suction pressure was 42.7 ft absolute. Therefore the suction was flooded to 8.74 ft above the centerline of the pump [i.e. 42.7 ft - (14.7 psia x 2.31 ft/psia)]. The velocity head was 0.2 ft. The discharge head was 45.3 ft. Therefore the total dynamic head is 36.76 ft (i.e. 45.3 + 0.2 - 8.74) and not 45.5 ft. I think if you look at the one of the questioner's posts he advised that he changed the appearance of the factory data. The way the data is presented does not make sense. If I remember correctly even the velocity head calculation of 0.2 ft is incorrect.

I would agree with you that velocity head for pumps that have high discharge pressure won't make much difference to the TDH calculation. But for pumps with low head it could be very significant. Any pump testing company will include the velocity head calculation when a witness test is done especially because it is the proper way to do the calculation especially if there is liquidated damages that could be applied

 

[BigInch]

QT, Opps.. yes you did; the 2.31 conversion factor escaped me. Sorry. I'm with you now.

Yes I agree here too. Usually Vhd wouldn't make much difference, but here it is an important proportion of total head, so yes it should be included. I just meant that as a comment towards explaining what some have noted as the strange shape of this curve, as it makes for an apparent hump where velocity head increases rapidly, especially in relation to the fall off of the other head component.

"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