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Hydraulic thrust K factor rule of thumb?

Hydraulic thrust K factor rule of thumb?

Hydraulic thrust K factor rule of thumb?

We repair vertical motors, sometimes see premature thrust bearing failure that needs to be explained. Assessing the load requires K factor data on the pump that is often unknown. A recent example: 800hp 1800rpm motor rated 10,300lbs downthrust - on a site visit we recorded production of 2500gpm at 56hz (1688rpm) using 625hp, has a 2-7/16 inch lineshaft, 18 inch discharge head, no nameplate. The operator is certain the pump is set around 750 feet, but has no record of what bowls are installed. Shafting alone weighs nearly 12,000lbs so seems obvious there's a problem, question is what motor downthrust rating is really needed.

In the Pentair online electronic catalog (very cool, with multispeed curves and lots of data), using hypothetical conditions (2500gpm, 720TDH), I find a 7 stage Fairbanks 17MS bowl unit that produces that flow & head at 1688rpm using 625hp, pretty much an exact match. It has a K factor of 21.11lbs/ft (x720 = 15,199), rotating element weight of 455lbs; adding those to the shaft weight (11,910) totals 30,853lbs downthrust.

So if that happens to be the bowl, our motor needs a 31,000lb downthrust rating (triple what it has now) - does that seem right?

To verify I searched for other manufacturer's online selectors, didn't really find anything. I expect a real pump engineer would just look at catalog bowl data (where the K factor is always shown per individual bowl at a given speed), do the math to see outcomes with multiple stages at different speeds and heads.

I understand (a little) how the K factor is all over the map depending on the relative axial/radial aspects of the impeller design (simple Bernoulli equations, maybe I'll solve some later, just for fun . . .). Meantime, are we on the right track here, or might there be some other bowl unit down there that could produce this same flow and head at 5 lbs/ft, or 40 lbs/ft?

Most pump stations we see these days have VFDs and flowmeters, so it's easy to collect a lot of data (flows and horsepowers at different speeds for testing), which leaves guessing at the pumping water level (though sometimes that's known also) and maybe setting length (though mostly the operators have a pretty good idea of that, having paid by the foot). Is there a rough rule of thumb for K factor estimation in applications like this?

RE: Hydraulic thrust K factor rule of thumb?

None of our deep well pumps are that large. The closest example we have is rated to 1500 gpm at 700 feet of head at 1760 rpm. Our line shaft is 1-15/16".
That one is 7 stages and is down 460 feet. The normal thrust is about 12,000 lbs with a maximum at dead head of about 14,000 lbs and a minimum at end of curve of about 10,000 lbs. Our motor has a thrust bearing rated for 15,000 lbs continuous down thrust. Given the larger and longer line shaft, it seems perfectly reasonable to me that you could have a thrust load of 30,000 lbs.

Johnny Pellin

RE: Hydraulic thrust K factor rule of thumb?

Think it is a given that in the case you have described, the thrust bearing download is well under-rated.
That's assuming the motor is carrying the shaft and hydraulic load and not a seperate thrust assembly at the discharge head.

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.)

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