Submersible Pumpset Thrust Bearing Vs Check Vave

[friend81]

We have a 60HP, 440V, 60Hz Submersible Pumpset installed in a borewell at 210m depth. The delivery pipe is of 6" diameter & after 15 days of operation the current of the pumping unit surged to 156A from its 95A.

After dismantling the motor we found that the thrust bearing of the motor is completely damaged(made of Graphite Carbon & SS 420 hardened Segments). When contacted our supplier for guarantee they replied that the failure is because of lack of intermediate Non-return valve in 210m of delivery pipe which caused high thrust in the pump operation. I would like to know what is the relation between an intermediate Non Return Valve & Thrust Bearing failure as already the pump has a inbuilt one. If the Pump valve don't have the capacity to withstand the column pressure then, it has to break before the Thrust bearing gets damaged.

Anyone can explain the theory please?

Thanks in advance

 

[MartinSr00]

Perhaps it has something to do with the kinetic response.

Say, for example, you had a check valve half way up the pipe, and that maximum thrust was on pump start-up. Let's assume pump thrust is proportional to pump discharge head. When you shut the pump off, the pressure in the line between the pump and the check valve would be, say, 125m of head. As the pump started, and the water column opened the check valve, the pump would have had time to reach operating speed.

Another possibility is that if you don't have a check valve, when the pump shuts off, the pump will turbine, running in reverse. This may cause lubrication starvation in the bearings (like if a shaft driven lube pump is turning in reverse), or, since the pump will have no load, it may overspeed as it runs away.

This goes beyond your original question:

Ah, but this is just conjecture. Check the installation literature and see lack of such a valve voids the warranty. If it does, it sucks to be you. If it doesn't, they should honor the warranty -- notwithstanding the truth or fiction of their story. Then your problem is a legal one and not a technical one. Probably no lawyers on this forum, but a nasty letter from one may clear it up.

We all tend to work in ways that further our own self-interest. The pump vendor may conclude that a good non-return valve story is cheaper than a new or repaired pump. It's in your self interest to not give them a nickel that they're not entitled to. Check it out: (1) The truth or falsity of their story and (2) whether their warranty excludes coverage when such a valve is lacking.

 

[CH5OH]

the reason for this premature failure is the shock wave created from starting up the pump.Good working practice for a centrifugal pump, with regards to this shockwave, is to start up the pump with a closed discharge valve (as close mounted to the pump as feasable.The shorter the lenght of discharge pipe, the smaller the shock wave.In this case a deepwell pump with an open length of over 200m for sure will break the oil film on the thrust bearing.Mounting a check valve prevents the pressure wave from reaching the pump.

 

[friend81]

Thanks Mr.Martin

You comment on kinetic response has something to do with Newtons Third law.

The manual only recommends the installation of intermediate valve but didn't mention anything about void of warranty. The supplier also agreed to repair the pump free of cost.

 

[friend81]

Mr.CH50H

Since the pump capacity is more than the borewell yield the pump was always operated with a 20% closed discharge valve.

thanks for your attention

 

[jonr12]

If the bearing segments are broken, which I doubt, then shock may have caused the problem. I don't think an additional check valve will help. An additional check valve will make the shock worse, if you put it an even distance from the top and bottom of the string like 50/50. If the segments are just worn down, not broken, then it is a lack of flow problem. Either you need your discharge valve closed to 25% or more because you are pumping air, or you need a flow inducer because the flow is not going directly past the thrust bearing before entering the pump.

 

[MartinSr00]

I'm not sure shock is the right word. If you have a 200m water column above you, it will take twice as much horsepower for the same flow to move it. I'm not sure if motor inrush current will be higher (because it's locket rotor anyway, I think), but it will likely persist longer.

In addition, when designing a pump for thrust, the designers typically treat the impellers as axial pistons. Since a pump impeller has pressure on each side of it, with differentials because of the various wear ring diameters, this part is irrespective of pump pressure.

One part isn't. If you start the pump against a 210m column, this portion will have twice the thrust as with a 105m column. I suspect what happens with a check valve half way up, is when you shut off the pump, this portion drains back into the well, making this section unfilled. When you start the pump, it's like starting it with the vavle wide open discharging into atmosphere. When the water hits the first check valve, halfway up, and it opens, the pressure will double. But by this time, everything's running, and the ability of the thrust bearings to carry maximum thrust is there -- especially if your thrust bearing is hydrodynamic (like a kingsbury bearing).

 

[friend81]

as per the manual bearings are Kingsbury Type water lubricated & the worn out or broken part is the Graphite part & not SS segments.

The pump is operated using a auto-transformer starter & starts with 65% of its nominal voltage then 80% & then gets its 100% voltage. The interval between each taps is 3sec.

The pump has an inbuilt check valve & an external check valve is provided in the ground level that is 210m from installation depth

 

[friend81]

the thrust load capacity marked in the nameplate is 45500N.

 

[thewellguy]

Friend81,

Several things for you to check.

1) The auto trans on this is a bad idea. From what you say it takes 9 sec for the motor to get to full power. During that time you have no idea if or how fast the motor is turning. All the manufactures that I have seen require that the motor reach true minimum speed usually 1/2 rpm in 2 or 3 sec so that the thrust bearing lifts and you get the water film on in the bearing. If not you just grind up the thrust bearing.

2) Is the pump capable of pumping more water then the well produces? If so and the pump is not throttled back to keep roughly 3m of water over the intakes, this can destroy the bearing. The pump will pump the bore "dry" and then the well recovers and picks up another shot of water. This causes a shock load on the bearings and destories it.

3) How many times a day is the pump started?

4) I have never installed a return valve in a water well so I have no idea what the manufactuer is talking about.

5) Another thing that happens that can present like this is that there is not enough water flow past the motor. All water well pumps use the flow of water pass the motor to cool the motor. IF there is not enough flow, the motor heats up and I have seen bearings damange due to this