pump and system curves 5

[solidspaces]

the performance of our chilled water pumps is attached.
• There are four pumps on duty (the pump selection and duty point performance is on page 1 of the attachment).
• We obtained readings of each pump with all the four pumps running. (Curve is on page 2 of the attachment).
• We also took the readings of each pump without operating the other pumps (Curve is on page 3 of the attachment).
• The pressure measured on the common discharge header with all 4 pumps running is 75psi.
• Are the attached pump curves drawn correctly? If yes, the pumps are not performing as they should both when run one at a time or when all 4 are run in parallel, and how does this compare with system head

 

[LittleInch]

There's something a little strange going on, but can you send us a sketch of the piping layout for e.g. pump 4 showing all elbows, valves (whats a DRV??) and most importantly where the pressure readings are taken from. It looks to me like there is some sort of pressure loss going on downstream of the pump between the pump exit and the place where the readings are taking place. If you havve any sort of control valve between the pump and the presusre reading, then even fully open then can have a significnat pressure drop, but you need to attach the piping diagram, even as a sketch.

How did you vary the flow rate for one pump and for all pumps to get the data?

see also the other recent posts on similar subjects which might help, e.g.

http://www.eng-tips.com/viewthread.cfm?qid=343897

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Artisi]

At first look it seemd that the individual pumps are down on performnce, however in saying that it is important to also say that duplicating factory performance results with on-site testing is virtually impossible, that is asssuming the pumps were factory tested and not just selected from published curves.

Looking a bit further at the figures on table 4, flow could well vary between individual pumps but discharge pressure has to the same on each pump if pumping into a common pipeline - that is unless there is any major difference in pipe / connection congfiguration between the pump discharge and the connection into the common pipework.

Without running any check on the performance figures you supplied, the pumps could well be operating within the pump performance test code in terms of performance.




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]

Also when you're running four pumps together, you're also getting a 5% volt drop (380 vs 400) compared to a single pump or multi pumps no flow. Input power seems to be a bit lower than the duty point (I assume the current listed is a single phase of a 3 phase supply). Try a bigger cable.... Where was voltage measured? at the motor terminals or from the switchboard as there could be other losses going on for the electrical supply side, especially when four units are pumping together.

I'd still like to see the set-up to see what is going on, but artisi is right - what you really need are the actual test curves for each unit, but testing conditions in a factory are different to reality, e.g. have you calibrated your presusre transmitters / guages - test ones are cerified guages, ditto the flow measurement devices. A few percent out on each one and you're some way to making up the difference.

What temperature is the water in the pumps - is this before or after the chillers? Doesn't make a lot of difference but if temp is above 40 C can make a couple of percent difference in density.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[solidspaces]

Reply to the points raised by Little Inch) Our readings of flow can be deemed to be correct, because the discharge of each pump was tallied with the readings far away from the pump where there is no turbulence or fittings, and everything adds up.
The pressure gauges of all pumps are giving nearly the same readings, and the sonic flow meter readings tally as explained above.
The temperature of the water going in and out of the pump is 15 degrees celcius.
(Reply to Artisi):
Could you please review the accuracy of the curve (by running a check on the supplied performance figures). If the curves are drawn correctly from the pressure and flow readings, it would then indicate an over-trimmed impeller as being the reason of deviation from the pump characteristic curve.

 

[LittleInch]

Ok, but where were the pressure readings taken from ? Is there anything between the pump discharge and the pressure guage ? Was the voltage taken at pump discharge?

I know this is basic, but are the pumps going round in the right direction? Who checked?

Do you have actual test results for each pump ?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[solidspaces]

Thanks much for the reply.
There is no fitting between the pump discharge and the pressure gauges.
The voltages are the average of the three phases and it is a pretty stable power supply. The readings were taken from inside the Control Panel adjacent to the pumps.
The pump curve (Page 1 of the attachment) is the factory supplied curve.
The hand drawn curve superimposed on this curve was plotted from the field data of pressures and flows.

 

[BigInch]

You say your discharge pressure at common point on the header is 75 psiG. Your pump discharge pressures are all 92 to 96 psig. How are you losing the 17-21 psi from pump's discharge flanges to the common point?

When running your pumps individually, Pumps 3, 2 and 1 have a progressively greater suction pressure drop (relative to pump 1) at low flows. That indicates an imballanced arrangement for suction piping. You should feed into the center of the suction header, but it looks like you are feeding all pumps from the far end of the suction line and each pump draws a lower suction pressure according to their position on the header and as head losses increase down the header. BAD.

Somehow they tend to equal at around 800 gpm. There seems to be some control action going on.

Your pumps running individually all have a discharge pressure of around 90 psig at a flowrate of around 800 gpm and yet you have about the same discharge pressures when running all four pumps together. You have not explained how your system curve stays at 75 psig at the common point on the header even though you supposedly have 4 times as much flow running through it. Some unexplained control action is going on, or you have 4 pumps each feeding through separate closed loops, or maybe you are running first one chiller, than 2 chillers, then 3 chillers, then 4 chillers.

Explain your system and include a proper system diagram before you go asking any more questions. I've wasted enough of my time with this guessing game, as have other kind people above.



Independent events are seldomly independent.

 

[Artisi]

Agreed, far too many unknowns which I have seen on many occasions. At the end of the it is probably better remove one pump from the system and test it in a certified test facility - no arguments then, either the pump complies or it doesn't.

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.)

 

[chicopee]

I think that the pumps are getting old so how old are they?

 

[ccfowler]

My first concern is knowing the exact details of the piping system arrangement particularly looking at any quirks that may cause differences for individual pumps. For example, do space constraints impose the need for multiple bends in close proximity for only some of the pumps? Are bends affecting some pumps sharper than others? Are there check valves that may be opening differently for different pumps? Are there differences in the type, proximity, or configuration of isolation valves for the different pumps?

Have all of the pumps seen similar amounts of usage? Are some relatively new while others have been rebuilt? Are all of the motors nominally original, or have some been re-wound?

How clean is the piping system internally? Is it possible that there could be some "debris piles" at some places in the piping system?

Obviously, the list of such possible issues can be nearly endless without more direct knowledge of the specific system.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[solidspaces]

I am grateful for the meaningful feedback. I will try to upload the arrangement of the pumps in the next day or two. Meanwhile, (ccflower), the pumps are brand new and the network is not only new, it has been flushed and chemical treated only a couple of weeks ago. The configuration and layout of the pumps in parallel is essentially the same. I agree that there might be something peculiar that we are missing, so please bear with me till I post the sketch. Thanks again.

 

[BigInch]

Here's your basic data in XL.
All data should be posted like this, so we don't have to do it for you.
Sometimes we might need to do a few basic calculations, at least to check your work, before we waste time with details.

your dishcarge head into the system appears more or less consistant.
That's why I said it may be somthing with the suction, which varies a lot, esp at low flows.
Your system curve is constant even though flow is probably nearly 4 times more. We need to know why. You can't do anything with pump system design without knowing what is going on with the rest of the system. Pumps are only 1/2 the story. The other half, piping and controls as applicable, are as essential as the pump curves.

Independent events are seldomly independent.

 

[LittleInch]

I know its basic stuff, but can you confirm the pumps are going round in the right direction - it's a very common fault with underperforming pumps with 3 phase motors.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Artisi]

It's all pretty basic, you can't get anything more basic than a simple end-suction water pump. Applying them correctly is more the challenge.

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.)

 

[BigInch]

Nothings running backwards. In fact it looks rather normal for a poorly designed suction system and running 4 chillers.

I think the mistake he's making is not adding suction head to get the system discharge curve. The pump curve needs to be superimposed on the system curve. He's doing the reverse and wondering why his discharge head is low. But I refuse to play guessing games. Sit tight.

Independent events are seldomly independent.

 

[solidspaces]

To: BigInch and the other kind contributors
The spreadsheet and the curves are super.
The sketch will soon be underway.
More later. Many thanks

 

[BigInch]

Thank you.
I am curious to see what else you've got for us.

Independent events are seldomly independent.

 

[solidspaces]

To: All dear consultants
Here it is finally. The attachment gives the general arrangement of pumps.
Waiting anxiously for feedback.
What intrigues me is that the regardless of installation deficiencies, are'nt the pumps constrained to operate according to their curves. The data suggests they are not following their curves, which I thought was either an incorrectly drawn curve of an over trimmed impellor.
Thanks

 

[LittleInch]

Where, exactly, were the pressure tappings you used for your readings. Can you mark on so that we can see where they are or describe using the marked valves. I think like others we suspect that the readings are being affected by some thing, either suction or discharge.

For info can you explain how the double valve works.

At the end though are you getting less flow than you should? Your pump discharge is 90 psi, whilst discharge header is 75psi so I guess the control valves are kicking in on some sort of signal. Do you actually have a problem or are you just curious about the readings?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way