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Oversizing pumps and motors

Oversizing pumps and motors

Oversizing pumps and motors

In the old days we often used to oversize water supply pumps - and then ended up adding orifice plates to bring them onto their duty curve - over the past 20 years I have instead evaluated worst case hydraulic scenarios (maximum and minimum) and ensured that the selected pumps best meet the duty range. I assumed everyone was doing the same.

I have recently received designs of a pump station from a leading international consultant - the designs include VSD and FS pumps. For the VSD pumps the designer has allowed for pumps to run to 110% speed - this has substantial impacts on power demands, motor cooling etc ( some 25% more power required ) For the Fixed speed pumps the designer has added 10% to the pump sizing. (In addition ambient temperature is 55 degree C so there is a substantial de-rating factor but that is a motor oversizing it does not increase the power or temperature gain). Pumps are between 1 and 2 MW capacity so an extra 25% is a huge excess

My question is - is it still typical to oversize pumps and motors ?

RE: Oversizing pumps and motors

Generally adding 10% margin for head is sufficient to have safely operation. Although It depends on pump curve. To be checked pump curve is inevitable to choose pump for your system.

RE: Oversizing pumps and motors

Why are they adding margins - covering their arse, just in 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.)

RE: Oversizing pumps and motors

It is not good practice to oversize pumps and motors because you will pay an energy inefficiency penalty for the lifetime of the facility.

RE: Oversizing pumps and motors

Pump technology and associated data has with the advent of computers and programmes together with prior knowledge reached a point where installation, hydraulics and pump selection can now be predicted very accurately. Over sizing large pumps is an exercise in stupidity and would indicate lack of professional expertise.

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.)

RE: Oversizing pumps and motors

I have to agree with bmir. I don't understand the thinking of your "international" consultant. It is bizarre to say the least. I hope your "international" consultant is experienced with VSD (VFD??) drives. A lot of consultants throw out the use of VFD's as a cure all for everything. Very few of those consultants ACTUALLY KNOW what the effects of harmonics backfeed into system can have on other equipment. They talk about it like they know but they do not know

RE: Oversizing pumps and motors

The other problem with using VFDs as a 'fix' is that you can still end up with a system that operates well off of BEP. They forget that the head isn't linear with speed.

I used to work in downhole oilfield pumps. We would work hard to size them just a few % over capacity.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Oversizing pumps and motors

I agree that use of VFDs is sometimes seen as an easy way out of a varying requirement for flow, but not sure what you mean by "110% speed"? VFDs are supposed to be able to vary rotational speed, usually in the range 40 htz to 70 htz depending on the load cases.

~I always think there is too much emphasis and the "easy" way out to use extreme design cases together as the design point instead of working to a 95% rule, e.g. max flow on the hottest or coldest day / conditions with rough pipe and lowest inlet head etc

So sure, the pump will be able to provide the design flow in any conditions, but it means 80-90% of the time it's oversized and running inefficiently or needs throttling back.

So is it common - yes from a consultants point of view because if they undersize it a bit and it doesn't meet the test conditions / performance test all hell breaks loose and they might not get paid. Design consultants basically don't normally care about OPEX, their involvement / financial reward has absolutely nothing to do with actual performance / OPEX costs or even CAPEX, providing their estimate is a bit higher than it actually turns out to buy and build.

Only when you get operators who will have to live with the costs and also have a deep understanding of what is really needed and can make the necessary compromises / judgments as to how fine to cut the margin do you get pumps which are nailed on.

A longer winded way of saying what Artisi said above in one sentence....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Oversizing pumps and motors

Not all centrifugal pumps are created equal, and hence you can't have a one size-fits all selection method.

Pumps below 2 hp are frequently oversized because the primary requirement is head, not flow. Operation at low efficiency (well to the left of BEP) is the only option you have. The benefits of a centrifugal pump over a PD pump in some of those applications warrant wasting a little energy.

Oversizing is sometimes required to prudently account for unknowns in the system curve. If you don't know the hydraulics of your discharge side with precision, and you can't wait until you have all that information before you need to buy the pump (because the thing isn't sitting on someone's stock shelf, but rather has to be made to order for you), some oversizing is inevitable unless you want to run the risk of a disaster, i.e. a pump which is undersized despite having its max diameter impeller fitted. Of course, if your'e careful, you can either do an impeller trim or add a VFD to shift the operating point later to reduce the impact of the need to guess high.

You need to be careful about what happens to the BEP as you manipulate the speed and the inmpeller trim. It is a myth that a VFD always saves energy, though a VFD which replaces a throttling valve typically does.

RE: Oversizing pumps and motors

This 110% speed business has a profound effect on motor life, NPSHr, bearings, vibration etc. It is better to stick within the published parameters of pump operation. If somehow you are short on capacity just stick in a bigger impeller.

RE: Oversizing pumps and motors

Probably the most common root cause of pump problems is having had too much, or too many safety margins applied to design flow and/or head. Engineers have been making this ridiculous mistake for so long and so often that they must be in cahoots with the Aftermarket Services groups.

I always thought the best approach, if someone insists on a CYA plan, was to make sure the design impeller trim is X% less than full impeller trim for that pump. Of course that might require buying a bigger pump in the first place.

As said above, engineers have the tools nowadays to trust their design.

If you can't trust your calculations, then the job is too big for you.

RE: Oversizing pumps and motors

As said above, engineers have the tools nowadays to trust their design.

I think the tools (pencil and calculator) that we used yesterday are just as good as the tools that we have today. Be it yesterday or today all we have to throw into the mix is a brain that is trained to think...lol...

RE: Oversizing pumps and motors

Thanks for the responses interesting consensus that with current computing capability we can evaluate the potential range of operation and evaluate were the system meets the pump duty range at extremes and we should be selecting a pump that best meets the potential operating range. I am now retired and Have taken on this assignment to try and rescue the design, I have spent much of my career sorting out hydraulic systems were too many safety factors have been applied and this a good example. In fact you really cannot have safety factors in hydraulics. I agree that sometimes a fixed speed pump operating at its BEP with a FCV will be more efficient and less costly (considering capital and OPEX) than a VFD). Also running a pump at 110% speed (10% above synchronous speed for a fixed speed pump) is pretty stupid bearing life will be substantially reduced.
Thanks for he discussion it confirms the view that "Engineers now have the tools to confirm their design"

RE: Oversizing pumps and motors

Bris; the comment about running pumps above syn. speed will have a detrimental effect on bearing life isn't always true nor a warranted comment in all cases. In 50 Hz countries using 60 Hz designed pumps isn't a problem as they would be running 20% below design - further, running a design check of speed against load / deflection / NPSH etc. for all pumps could well negate any concerns.

A company that I worked for in Aust. for many years were manufacturing US 60Hz designed pumps, for interested I ran some L10 calcs. on bearing life under worst case conditions to find that life cals. came out approaching 1,000,000 hrs and in some cases exceeding this figure. Based on this it was reasonable to suggest that bearing failures would occur thru' lack of maintenance rather than life failure. Using this knowledge, allowed us to counter an opposition bid by offering 10 year warranty on workmanship and bearing life on a major pumps contract (naturally, fair wear and tear and lack of maintenance were excluded). We had full confidence in the product based on many years of after sales contacts with many users and close inspection of any in-plant problems our customers experienced - we were awarded the contract and slept easy without any concerns of any major claims for bearing failures.

Guess this is a long way round to saying bearing life should be fully reviewed under all operating conditions - not a blanket statement based of some unknown and un-proven belief.

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.)

RE: Oversizing pumps and motors

It is incredible how many people, as mentioned in an earlier post, STILL add 'cover my arse' margins...

The amount of pumps we get called to where pumps are nowhere near operating on the curve is astounding and more worryingly the amount of people running plants who don't understand pumps and system curves even more worrying. We have been running quarterly 'pump training' courses and feedback from these is really really good, most candidates not realising consequences of running pumps outside the acceptable envelope of the performance curve.

As mentioned earlier, VSD drives are a fantastic addition to any process system, but not always the answer to the problem and often seen as a solution to protect the designer.

Ash Fenn


RE: Oversizing pumps and motors

Don't put total faith on computer designs, therefore, hand calculations is also a counter check to computer results.

RE: Oversizing pumps and motors

I totally agree...garbage in....garbage out. Years ago Fairbanks Morse (FM) was having trouble with a certified witness pump performance curve for a big 4000 HP municipal water treatment plant high lift 100% stainless steel pump in the Toronto area (yes...stainless steel...including the casing, pump base frame...LOL...I took a really good look because I'll never see another one like that for the rest of my life! smile) . The big big brand name consultant and FM had a lot of back and forth discussions amongst themselves. The Owner asked me to go to Kansas City to witness the test. I double checked calibration of equipment etc. when I was there. On Saturday, sure enough the pump did not perform according to the predicted curve. On Sunday, I did my own hand calculations in their boardroom on NPSHa available. I went shopping while FM continued their tests. When I came back, FM were then trying to explain away why they weren’t getting the predicted curve...something about 4th order curve fitting....When the FM engineer started to talk about the 4th order curve fitting...yikes!!!...it brought me back to my engineering student days and struck terror in my heart!.... LOL.

I advised FM that they did not have enough NPSHa to get proper points the further to the right one tested the curve. Basically the pump was cavitating more and more the further to the right of the curve that we tested and that is why they could not get the desired performance curve. It was a classic cavitation pump curve. I showed them my hand calculations on the boardroom whiteboard using various values from, ironically, my laptop saved FM hydraulics handbook. The results showed that we could never do a proper witness test because FM's physical setup required too much suction lift and as a result we were cavitating. Funny thing is that we could not hear the cavitation. I can always remember the look on the engineer's face when I went over the calculations with him on the whiteboard. It was like a lightbulb went off in his head. He ran up to his office and confirmed the error on his excel spreadsheet.

That Sunday afternoon, I passed the pump for delivery to Toronto. We tested the pump on site with the proper physical and hydraulic conditions. The pump curve was fine. FM spent a lot of money and heartache trying to figure out why the pump was not performing. They almost gave up. Had they did that the monetary damages would have been really bad for them. At the end of the day the bleeding of money was stopped. The morrow of the story is to check your calculations. These calculations take about 15 minutes.

BTW, the steaks and BBQ are great in Kansas City. I wish I was there right now!

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