Well Pump Problem 1

[islandeng]

Hello all,

I am an engineer for a small island in the southern West Indies (about 140 miles north of Venezuela). We have been having a continual problem pumping brackish water to our watermaker.

The splines on the spline shaft shear off, allowing the motor to run at no-load speed until either the thermal protection kicks in or the motor burns out. This problem has happened three times, and the pump/motor combination has lasted between 3 months and 3 weeks. Our pump supplier is telling us it is an installation problem, however he also does not recognize the splines are shearing off…so I don’t trust him (meaning he only agknowledges the motor is burned out, even though the splines are obviously sheared).

The motor is a Grundfos (this has also happened with a Franklin motor) 2.00 hp, 60 Hz, MS 4000. The pump is rated for 25 GPM and 218 feet of head (according to the plate on the side of the pump. It is a 4 inch submersible model. The pump is made of 304 SS, as the pump supplier suggested it would work for our brackish water. However I feel the next pump should be made of 904L SS, to better handle the salt content.

The well is a 6 inch diameter 120 foot deep well with a 10 inch bullnose at the bottom. The pump must pull the water up the 120 vertical feet, then along a 1800 foot horizontal run through 2.5 inch PVC to an open atmosphere tank. As I stated before the water is brackish, I don’t have a salinity meter (yet) but I am currently assuming the salt content is about 75% that of seawater (could be more, could be less). The average temperature of the water is 89 deg. F.

I have spoken with the tech support at Grundfos, and they listed three potential problems for the repeating failure, frequent start-stop cycles (which we don’t have, as the pump constantly runs), pump overloading (possible, but the pump should be big enough), and misalignment between the pump and the motor (manufacturing defect occurring 3 times in a pump with an excellent reputation?). So my question after this lengthy problem statement:

1.) What is the cause of this problem? Is it simply the salinity content of the water, and we need to use the 904L ss? Could I drop a zinc down with the pump (we have a lot of boats here, so we have a lot of extra zincs)? Is the pump undersized for the application? I have been through the length of the pipe and there is no blockage.

2.) Are there any quick solutions? This is becoming more important, as our water supply is dropping and, well, that’s a bad thing. More importantly, long term solutions – is this a common problem with Grundfos MS4000? Should we look into other manufactures, or suppliers?

Thank you all for your help. I have I have given enough information.

 

[islandeng]

25362,

There is some slight corrosion, ie surface rust in the crevises of the splines. It is what I call "rust dust", the type of rust that with a moderate scrubbing comes off easily. I didn't bring this to corrosion engineering forum because my material choices are so limited (basically wither 304 or 904L) that I think the adequate choice is clear, 904L.

Pumpdesigner,

What you say about thrust is very interesting, I hadn't thought about that. I will look at the support system (pump hanging from a simple cable) to see if there could be some issues with vertical slippage. According to the good tech support staff at Grundfos USA, there is very little difference between the Franklin 4" motor and the Grundfos 4" motor. So I would estimate that if Franklin has a problem with thrust bearings so does Grundfos. If you don't mind an off subject question, what is his application that he requires 100 4" low hp motors which can handle some thrust?

Thanks again to both of you for your suggestions.

-Loyal

 

[25362]

Crevice corrosion in salty water is stronger than pitting in the sense that it happens at lower temperatures.

SS 904L is good for pitting at just below 30[sup]o[/sup]C. What would be its resistance against crevice-type corrosion ? I couldn't tell, but the corrosion experts certainly would be able to give you useful advice.

 

[25362]

I forgot to mention that the wastage of the splines could be a combined effect of chemical attack and a concomitant mechanical weakening, as suggested by Pumpdesigner.

 

[islandeng]

Thanks 25362, I just posted this on the corrosion forum. We'll see what i get! I think i mentioned the temperature in the well is 89 deg F, which is about 31 or 32 deg C, so it is right around the 904L limit....

 

[PUMPDESIGNER]

islandeng,
That guy who buys 100 4" pumps at a time and puts his own thrust bearings into them manufactures fountains. He mounts the motors horizontally, which according to Franklin is a big no-no on 4" motors because they have limited thrust capacity. Sooo, he makes his own thrust bearings for them. I find that strange, I suppose I would try a better motor with roller thrust bearings before I would make my own. But this guy has been doing this for years and years.

Also, I think this guy runs the motors at, near, and possibly over capacity 24/7, which would wear through any cheap little thrust bearings pretty quickly.

PUMPDESIGNER

 

[hydrae]

Here is my two cents
No check valves on the pump discharge. When the pump is shut off for any reason the water in the pipe will flow backwards until it drains the line, since this is a long line the water will backspin the pump and motor during the drain back. If the pump is re-started while the motor is spinning backwards the motor will produce enough torque to shear the shaft at the splines, this will happen so fast the motor protection devices will not engage to prevent damage.
Some grundfos pumps have a built in check valve in the pump if this valve is now stuck open the above may be what is happening.

Hydrae

 

[islandeng]

Hydrae,

Thanks for your $0.02. I will install a check valve for the next round, however the pump was never stopped in the 3 weeks of operation which led to the pumps failure. There are a lot of less than ideal designs here, which, I believe, all contributed to the failure. Again thanks for your input.

-islandeng

 

[hydrae]

Islandeng

Have you measured the salinity yet? if not you can weigh the five gallon bucket with the brackish water then clear water to see the difference, (you can use a bathroom scale) remember the H(ft)*Q(gpm)/3960 = hp needs to be adjusted for the density of the liquid being pumped if diffenent than water.
How clean is the power to the motor, does the voltage. amperage or Hz vary? single or 3 phase, this can also affect the performance of the motor. Franklin submersible motors do not work well above the Service factor rating as typical air cooled motors do.

Hydrae

 

[islandeng]

Hydrae,

I have not yet measured the salinity of the water. We have a salinity meter coming in on our next shipment, which was suppossed to arrive last week, however it is stuck in customs (this happens a lot), and hopefully will arrive today or tomorrow....

I was actually using the formula Power = (Q * H * Gamma) / neta. I used properties of salt water I found on the internet and in some textbooks.

The power coming to the motor is decently clean, three phase. The voltage doesn't vary enough for us to pick it up on our voltmeters, and the phase stays constant at 59 Hz. We don't have an ammeter on the line so I don't know what the amperage is doing, but i would expect it to remain constant, as we run a lot of motors on the island and most of them last a very long time.

Hope I answered your questions.

-islandeng

 

[islandeng]

I am beginning to think the solution is to use a 904L pump & motor, and give the system some backpressure. I spoke with a new supplier yesterday and he mentioned that the pumps, which are stacked impellers, need to push against some head or backpressure, or the impellers will float around vertically. This could mirror the thrust problems Pumpdesigner mentioned. comments?

thanks,
islandeng

 

[coastpumpguy]

Hi to all
I've just registered here to reply to this post.

I have seen many spline failures in 4,6 & 8" submersible motors which were caused by a lack of lubrication at the time of coupling the wet end (pump) to the motor.
Both franklin and grundfoss reccomend lubricating the splines. If it is allowable for the application use pure nickel anti sieze or a moly paste.
You will allway have some missalingment with commercial quality formed pumps and the micro motion resulting in the coupling / spline interface will cause rapid wear without lube.
2) The shaft and spline dimensions (strength) remain the same up to 10 HP.
3) If my memory is working today this shaft will be 17-4 ph and the pump coupling is either 329 or 304. Don't quote me it's late.
4) Grundfoss makes these pumps in titainium for corosive applications and they may be within your budget.
5) Your motor is rated for 1500 lbs of downthrust. All of the 4" submersible motors for groundwater wells are rated around this value. they do not accept upthrust. These units are equipped with kingsburry style thrust bearings which typically outlast the rest of the installation except if they are ramped up to slowly with a vfd.

I hope this info is helpfull.

coastpumpguy

 

[Kawartha]

Islandeng,

Hi also to PUMPDESIGNER and D23.

Sorry if I offend anyone with the following comments, I haven't had time to review all of the comments.

I think Coastpumpguy has come to the same kind of conclusion that I have. Several years ago, many of the couplings for submersible pumps were either solid or dog type couplings. These were far less susceptible to erosive wear because the interference fit was much greater than with the spline design. These types of couplings were not as flexible as the newer spline design. The spline coupling design allows greater flexibility at the coupling, and means that less accuracy is required during manufacturing.

It is not a common problem, however if you are pumping liquids which contain very hard abrasive material, it will enter the coupling housing resulting in grinding the rather small spline sections. This is because the flexible coupling is continuously shifting and abrasive material between the metal parts will result in grinding action. I don't think lubrication will help to solve this problem because the abrasives will still enter into the coupling housing.

Perhaps you should ask your supplier if they can provide a dog type or solid coupling. These will also wear, but you should extend you life significantly.

Regards,

Richard