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

 

[solidspaces]

To LittleInch: Thanks much for the troubleshooting exercise.
1. I have attached another sketch that depicts the location of the pressure gauges.
2. The Double regulating valve is simply a balancing valve to set the flow somewhat like the globe valve that throttles the flow. However on this balancing valve, there is a setting that can be locked in place and recorded. From the pressure drop through the valve (picked up form the pressure sensing ports upstream and downstream on the valve body, the flow rate can be read from the Cv charts supplied by the valve manufacturers.
3. The pump is not riding its characteristic curve, particularly at its duty point as can be seen in the hand drawn curve that is superimposed on the pump selection curve. This means that with the fouling of pipes, the pump has no room to be able to handle the increased head requirements of the system with ageing and scaling.
4. By the way the 75 psi that was given as the pressure on the main header was an incorrect reading. The reading on the common header is also 95 psi, so there is nothing enigmatic in that part of the network. Sorry about that error.
5. What is enigmatic is that why are the pumps not following their curves. If the system head is more than anticipated it should simply give less flow, but still remain on its curve. So I just want to rule out the possibility of an over trimmed impeller.
Looking forward to your feedback and of any other interested readers in the forum.

 

[Artisi]

3 questions, 1 comment, 1 statement, 1 recommendation:

1. are the pumps from a reputable manufacturer or are they el-cheapo copies from an unknown source?
2. were they factory tested for performance?
3. have the pressure readings been correct back to the pump centreline, have all velocity heads, height differences etc etc been accounted for in your H/Q curve generation?

comment:
if the impellers were over-trimmed then the CV head wouldn't be exceeded which you are currently reporting, if under trimmed, the H/Q curve would be down by an even amount from CV to end of curve.

Statement:
From the shape of the curve (assuming the first part of the question 1 is in the affirmative) I doubt very much it is a pump problem, appears more likely a system problem or (my belief) a measurements problem.

Recommendation:
Have 1 pump hydraulic performance tested in a certified test-rig, the amount of money spent so far and which looks to be ongoing probably exceeds the cost of a test.

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.)

 

[ccfowler]

I very much agree with Artisi's recent posting.

First, this is NOT a simple arrangement of several nominally identical pumps operating in parallel. That is prevented by the individual "DRV" associated with each pump. Each pump will always operate on its unique characteristic curve, and it will operate at the intersection of its characteristic curve with the piping system's characteristic curve. The individual DRV's serve as independent variables that alter the piping system characteristic curve seen by the individual pumps.

Based on previous experiences, I suspect that piping elbows (and other flow disrupting items) are likely to be uncomfortably close to the pump suctions so that the pumps may not be seeing the well-developed suction flow upon which the published curves are based. It is also likely that the pressure taps may not be seeing the well-developed flow needed for their instruments to operate at their greatest accuracy.

The calibration accuracy of the instruments used must also be taken into account and properly adjusted into the performance evaluations.

It seems likely that the performance curves that you reference are actually just the curves that are published for the generic pump model, and as such, variations of as much as 10% can be anticipated for individual pumps due to manufacturing tolerances and variables. Your information shows a nominal slip speed for an induction motor, but is this the actual shaft speed upon which the characteristic curves are based? It is not unusual for the published curves to be based on a pump being driven at exactly synchronous speed, and it may be necessary to adjust the nominal pump curves to the actual operating speed provided by the motor under the varying head and flow conditions.

Time and again I have seen that what is claimed to be a pump problem will be found to actually be a system problem compounded by an incomplete understanding of the true meaning of the pump's performance curves. The pump (or pumps) provide a seemingly obvious focal point for a misbehaving system, but in reality, the pump(s) almost always will be found to actually be performing according to properly adjusted expectations.


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.

 

[Artisi]

ccfowler, Nicely put and spot-on,especially the points re speed, bends elbows etc.any reputable pump manufacturer if only using their published curves would normally over size the impeller by a small margin to account for any "under-performance", however if the selection was made by the vendor or reselling agent this may not be the case.

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]

I don't see any major problems, other than your suction piping (which isn't major as major things go), within the range of measurement accuracy. You could have a centralized inlet/outlet header, as I mentioned before, but this one works.

For some reason you get a better pressure loss at higher flows. That might be due to proportionally less resistance vs flowrate as you sum the effects of the number of chillers and loops you're turning on. In other words, at low increasing flows your distribution efficiency becomes better, lesser pressure loss/gpm being distributed each time you open a loop in the flow circuit. Nothing particularly unusual about that. For example, twice as much flow area gives you twice, or more than, the previous flowrate at the same, or slightly less, pressure drop, until you get to 750 gpm in each loop, where any further advantage in increasing flow area, without increasing diameter, is lost. That's the velocity where your pipe diameter is being used to their "economic" capacity.

Independent events are seldomly independent.

 

[solidspaces]

So the verdict is in favor of the existing pumps.
I guess down the road when the deposits in the piping increase the friction, at that time we could explore upgrading the impeller.
Thanks to all the respondents.

 

[BigInch]

Not the verdict, just my opinion. I didn't see anything all that unusual, given you are operating it like I said.

Independent events are seldomly independent.

 

[solidspaces]

To BigInch (Petroleum)
I termed it verdict because I sincerely felt it was a learned opinion from someone who has a solid theoretical and field experience.
Thanks again.

 

[BigInch]

Thanks, if that was meant as a compliment.
If it's one vote, it's an opinion. Let's see how the rest of the jury votes.

Independent events are seldomly independent.

 

[ccfowler]

I tend to agree with Biginch. This arrangement may not be performing as you may have wished, but any cures are likely to be more costly than their real value. With good maintenance of the cleanliness and chemistry of the circulating fluid, your system's performance should be able to continue performing in its current manner for a reasonably long time.

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]

ccfowler: Your response is very encouraging indeed.

 

[LittleInch]

I'm just going to do some more small calcs on the data, but I generally agree with the recent points above. You seem to have a system that works, but with no spare capacity.

It's definetly been quite interesting...

My motto: Learn something new every day

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

 

[solidspaces]

LittleInch: Your feedback after double checking the calcs is keenly awaited. Thanks again for your wholesome and constant support from the beginning of this problem till its conclusion.

 

[LittleInch]

Before I do that can you answer / confirm a couple of things - Is the fluid just water or have you added anything e.g. glycol?

If you have what is the SG at your pump temp?

My motto: Learn something new every day

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

 

[solidspaces]

GLYCOL IS NOT ADDED AND SG IS PRETTY CLOSE TO 1 AT THE AVERAGE PUMP TEMP
THANKS

 

[LittleInch]

Ok. As I said at the start of this post, there's something that literally doesn't add up. I suspected that the readings were taken in the wrong place, but there seems to be something systemically not right, but I can't figure out what it is. The no flow pressure and head look ok, but further readings are quite a way off. The vendor looks like it is ksb, a reputable company and any one who can make a pump with nearly 80% efficiency is ok by me.

Assuming the data is correct, and there is a lot of it that all is similar, shows that the hydraulic power required, even based on 79% efficiency at 800 us gpm, is higher than the electrical power in ( v x amps x sqrt 3). so unless you've discovered perpetual motion something is not right here.

I find it difficult to believe that the control valve has no impact on pressure even if fully open.

However your system seems to work even if less than you planned, so take some credit for that.

What can you do now? Use some calibrated guages / transmitters to get some better data, have longer straight lengths before the suction flange (5D min), flow data via a CV is not very accurate (10% at best), check my power figures or take a pump out of line and test it on the bench. Ask the vendor for info and double check that your data pack from the vendor shows you have the impellor that you think you have.

The people posting here on this are top drawer so read them all carefully before deciding what, if anything, you do next.

Good luck and thanks for all the information.

My motto: Learn something new every day

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

 

[solidspaces]

LittleInch: I am impressed. You even figured out the brand! Thanks for the bundle of useful pieces of information.
Thanks to all who favored me with valuable feedback.

 

[solidspaces]

Another, less fancier (but crucial safety) concern is about the underground Pump Room of our decorative fountains. The pumps and their control panels are about 3 meters underground. Unfortunately there is no automatic submersible pump to de-water the underground pump room in the event of leakages from piping or the adjacent water tank. Since there is electrical power and control panel in this underground installation, it could pose an electrical hazard due to flooding. I am searching for precise code requirements and practices of the trade for such systems, and whether the submersible pump system is a mandatory requirement and would appreciate feedback about this point.

 

[Artisi]

I don't know of any code mandating dewatering pump requirements for an underground pump station but would have thought that commonsense would prevail in such a situation.
The design should have allowed for a collection sump with room for a couple of electric sump pumps setup to handle the inevitable problem.

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]

Sounds like the design didn't go through a proper review process (HAZID / HAZOP) which is where this should have been identified and actions taken to install such things. You might find it is in electrical codes about switch gear in which case this post is in the wrong forum....

My motto: Learn something new every day

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