Pump Well Problem

[islandeng]

Hello,

I have been having a problem with the splines shearing on a shaft connecting a 2 hp motor to a pump. The pump is located in Brackish-salt water. It was suggested that I bring the forum to corrosion engineering and see what you materials guys can come up with. For full pumping/mechanical information please visit the pump engineering forum. I will outline what I think the important information regarding corrosion;

The water is brackish, I don't know the exact salinity, but a good estimate is that it is 75-80% seawater.

The current pump is made from 304 Stainless Steel, however the next pump will be made from 904L Stainless. Those are our only two material choices, so hopefully one of those two materials will work for us. I was thinking of dropping a Zinc down with the pump, similar to how one treats galvanic reactions on boats. What are your thoughts? Thanks in advance for your help.

-islandeng

 

[rnd2]

What you may have here is relatively warm salty water with very little dissolved oxgen. That combination will attack 304.

 

[EdStainless]

904L will be marginal in that environment. There is no way that 304 or 316 would survive.
Large submergibles use Monel K500 or 625 for shafting and bolts. That is largely driven by strength issues.
In concept using anodes sounds good, but the consumption rate will probably be very high.
Will all of the wetted parts be 904L? If so it might work fine. Make sure that there aren't some stray parts that are 304 or they will corrode very fast.
On parts that can be coated, like pump shells, you should consider it. You can use a Zn rich two layer epoxy system. The Zn provides some protection when you get surface damage.

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Corrosion never sleeps, but it can be managed.

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[islandeng]

Thank you both for your comments. To elaborate on the problem, there are no other signs of corrosion with exception to the small amount on the spline shaft. I am not sure what all the materials are, the pump is manufactured by Grundfos (along with the motor), so I would hope a well known company in well (fresh and salt) water pumping would sell their products with adequate materials.

One person I spoke with who has experienced this problem before suggested lubricating the spline shaft with Di-electric grease...Any thoughts on this?

thanks again,

-islandeng

 

[rnd2]

Is this correct?
There is definetly no similar problem with this model pump in other locations. You have checked this with the manufacturer and they have confirmed this in writing.
A number of the same model pumps have been tried at this location and all have failed.
The failure of the pumps is caused by spline failure due to corrosion.
There is very little to no corrosion on the pump other than the splines and spine shaft.

 

[islandeng]

Rnd2,

I have gotten mixed answers, some people have said there have been lots of problems with this pump, and other people (especially the manufacturer) have said there are no problems with this pump, however none of it has been in writing, except through a few emails. The manufacturer suggests getting a more powerful pump and using the 904L SS (and has done so in writing).

We have tried three of the exact same pumps and all three have failed at the spline shaft. On the first two the motors also burned out because they ran at a no-load speed for a while (duration no known) before a problem was noticed. But the original failure was a wearing or shearing of the spline shafts, then the motor ran at no-load speed (because it could not transmit load).

I'm not sure why the splines are failing. I feel corrosion could be one of the factors, as there is a little surface rust in the crevices of the splines. But there is no pitting, and i think 3 weeks is pretty quick for corrosion to take hold. That's why I posted here, was to find out if corrosion could have caused the quick failure.

There is very little corrosion on the pump, most of the pump looks brand new, except for the splines on the spline shaft which were worn and show a small amount of corrosion.

Hope that helps clarify. rnd2. What is the feeling about using a pump made of Titanium? I have heard that is resistant to corrosion with salt water and low 02.

-islandeng

 

[EdStainless]

What type of power are these pumps on? Spline failures are unusual. Yes, the splines are more suseptable to crevice corrosion and the loads are high. This sounds like a corrosion/fatigue issue maybe. If the power isn't clean the motor will pulse. Or if there are valves in the outlet side that open and close quickly you can get the same pounding of the splines.
Since there is no corrosion anywhere else I would look to mech/hyd issues causing abuse of the splines.

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Corrosion never sleeps, but it can be managed.

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[unclesyd]

If you have the information come back with a description of the fracture surface on the shafts.
Was the fracture straight across the shaft or was it on a diagonal?
Where was the origin of the fracture, in the spline proper or at the transition from spline to shaft?

Anything like this would be quite helpful.

If you have Dye Penetrant do a penetrant test of the broken shaft. If you don't have access to penetrant you can spray the shaft with penetrating oil, let it set 15 minutes, wipe off all the excess oil and give it a light dusting of chalk dust. Look immediately for any areas that wet the chalk, especially look for thin branching lines.

 

[griffengm]

islandeng,
Please excuse if the following is too basic but I have to ask.
First, are the splines to which you refer at the connection between the pump and motor?
Second, what does the wear on the splines look like? One face of all splines? On some splines and not others? What does the mating spline look like after a failure?
Third, has anyone noticed that these pumps are noisier than others in your facility or that they are uncomfortable to work near?
Fourth, if the pump is shut down after running for a while, is the spline area warm or hot to the touch?
From your description, methinks you have an alignment problem. BT,DT.

Griffy

 

[rnd2]

"........some people have said there have been lots of problems with this pump.........."
Contact these people again and determine exactly what their complaints were. You can then eliminate what is not relevant to your specifics. If their complaints match yours and you can document it, present the information to the manufacturer. If their complaints do not you can eliminate a general problem effecting that model pump.
On the surface, aside from mis-alignment, and that would have to have happened three times?? it does seem that perhaps a pump with more "grunt" might be needed.
I would follow the manufacturers recommendation.

 

[islandeng]

EdStainless,

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. All the valves on the system are mechanically operated, so I doubt they are opening and closing very quickly. Thanks for your questions.

unclesyd,

The failures are straight across on all splines. The failure occurs in the middle of the spline on the motor side and through the entire spline on the pump side. The splines are very shiney, and the failure looks a lot more like wear than impact fracture or shear. There is no evidence of chips, or shavings. The shaft really looks like it was in serrvice for a few decades and was just worn down...which is odd since it was only in service for three weeks. I haven't done the penetrant test yet (need to find chalk dust or similar powder) but will get back when i have done it. Hope I answered your questions. Thanks.

griffingm,

The splines are the conection between the pump and the motor (sometimes i forget to include the basic information which is very important). The wear, described above, is on the face of all splines. The mating surface displays the same type of wear, but to a lesser extent. It is difficult to say if the pumps are loud, they are submersible, so we leave them in the well unless they fail. On that same logic, we cant tell if the splines get hot during work, but we don't cycle the pump, to avoid this exact problem. Thanks griffy.

Rnd2,

The people who have noted problems with this pump have problems with the motors burning out, or with galvanic reactions if they spent the $$ for a 904L Grundfos pump, and their supplier sold them a mating Franklin motor made of 304 SS. We shouldn't have that problem, as we used 304 for both ends. We did burn out a motor, but that was because it was allowed to run at no load for a too long after the splines failed. Thanks for your advice.

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 the impellers will float vertically around. That vertical movement could cause premature failure in the splines, as typically one doesn't design splines to handle a lot of sliding between the surfaces. comments?

thanks,
islandeng

 

[EdStainless]

Small multistage cent pumps use floating stages. There is a rather narrow range of flow/pressure that these pumps work best in. You need to try hard to keep in the range. maybe this unit is mis-sized.
The other issue with these pumps is that they have very little suction capability. They require quite a bit of submergance below the fluid level or they will not get enough flow in the intake and cavitation will follow. It may be in these pumps, since they are not heavy and solid (the impellers are either stamped from sheet or are plastic) that this results in severe vibration of the shaft. Look at the edges of the impellers and at the impeller hubs for signs of wear.
These are great little pumps that usually work very well. Give you sizing and instalation a good look.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

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