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Well Pump Problem

Well Pump Problem

Well Pump Problem

(OP)
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.

RE: Well Pump Problem

Two problems jump out from your post:

1. Your motor rating is marginal for the service you describe.  I calculate at rated conditions your required HP is 2.2.  I'm assuming 70% pump and 90% motor efficiencies, respectively.  I would have a motor rating of at least 4 HP.

2. 304 SS is the wrong material for brackish water.  Bronze, CuNi, Duplex steel or even cast iron would be a better choice.

In addition, you may be overamping (the spline is acting as a mechanical fuse) your pump on start-up if you don't have a control valve on the discharge side.  Even if you have a manual globe valve to partially close before starting, this would help in keeping the amps down as the system develops back pressure.

Hope this helps and good luck.   

RE: Well Pump Problem

Sounds like the pump may not be the problem, but rather the engineering behind selecting the pump should be looked into.

Obviously we need a lot more info, but from what you gave us, I can tell you need more help to solve this than just the pump supplier.  Grundfos makes an excellent pump and to my knowledge, this model is not problem prone.

I am going to make an assumption here, and you know what they say about assumptions.....I am assuming that before all these problems, you had non-brackish water at your watermaker and things were fine.  I am assuming that you needed a new pump for some reason and called a pump supplier to assist you.  The pump supplier gave you a different pump that what was originally in service.  At this point, I am assuming that you started to get brackish water in your watermaker.

Please let us know if my assumptions are on track and I will continue based on your input, we should all be able to help you here...

take care...

BobPE

RE: Well Pump Problem

(OP)
Jay,

I agree with you, i got a nominal requirement of 2.18 hp when i plugged in the numbers, but the tech support at Grundfos said that was well within their factor of safety for a 2 hp.  And i also think the 904L ss is a better material choice.

Bob,

Close, but a little backward.  Originally we were pumping (and still have the capability to pump) the water out of the harbor.  So it was pure sea water coming into the water maker.  This worked really well until people started anchoring around our pipe inlet, and then we started getting sand, waste, and other stuff into our strainers and filters, which not only shot our filter life (it got to one filter per week!) but it also reduced our water maker output.   So we decided to dig some wells to avoid the anchoring problems.  

The wells never pulled fresh water.  They have consistantly pulled brackish water and have only been in service for a year or two.  Our pump supplier (located in Trinidad) was on site when we dug the wells, and selected this pump then for us.  We have been using this Grundfos MS4000 since the beginning and have been having problems for the last 7 months, where we have replaced the pump 3 times.

Following is some more information about the well, it may help:

The construction of the well is 6 inch diameter PVC pipe.  It is 120 feet deep.  Starting at the bullnose there is 10 foot section of periforated piping, then a 20 foot section of blank pipe, then a 20 foot section of periforated, then the rest is all blank pipe.  The well lies in very sandy soil with some rock (it is called "blue bitch" here, but from what i can tell it is a weak, non-porus, volcanic rock) and some aggrogate (same bluebitch but 1/2" to 2" square chunks).  There is no clay (relatively) and minimal organic material, and no trees within 30 feet of the well.

Thanks to the both of you for replying, if you need any more information, please ask.

-Loyal (EIT)

RE: Well Pump Problem

islandeng...

thanks....assumptions are always dangerous, but for us here, they help us get conversations moving is directions hwere we can help people...

I see you treat brackish water...ok...

jay165's comments on material are very important in this application....it seems like you are aware of that...so if the pump supplier helped you with that decision, get another supplier...

Looking at your pumps system requirements, I, as did jay165, would assume you to have a discharge control valve on your system that allows you to regulate flow????  How do you know you are pumping 25 gpm?  Also, how do you monitor water levels in the wells?  Did you pump test the wells to determine hydraulic characteristics of what flow they can sustain?  I get a dynamic head that is less than the BEP head you gave for the pump of 218 feet of head which means that if you are not regulating flow, you will run the pump off it's curve...

To shear a spline requires a pretty sizable force on a pump like this...the forces can be instantenaous, or cyclical....see if you can answer our additional questions....I think people will have some good ideas....

bobPE

RE: Well Pump Problem

The estimated pump output would be:

205*25/3960 = 1.29 HP

for an overall efficiency of 0.7*0.9=0.63

the motor would have to supply:

1.29/0.63=2.05 HP

To j165: am I right ?

RE: Well Pump Problem

(OP)
Bob,

thanks for the questions, here are some answers that I've obtained through my own observations, or through speaking with our head engineer:

According to the head engineer, we do have a discharge control valve. However it controls the flow direction of the water, not the backpressure in the system (although it will contribute a little backpressure).  The valve diverts the water from exiting into the tank to exiting back into the well.  So basically we have a supply and return line on our system.  This is to allow the pump to continue running regardless of our demand (or lack there of) and thus prevents cycling the pump.  Generally the valve is held at 3/4 open, meaning that, in a perfect world, 3/4 of the flow would go to the tank and 1/4 will return to the well.

The data I gave, 25 gpm and 218 feet of head are stamped onto the pump itself, as maximum permissible (like an either or situation).  We are actually pumping about 12.5 gpm.  This is calculated by a clock and a 5 gallon bucket (timing how long it takes to fill the bucket and then doing the math).  When this was done, the diverting valve was 100% open.

We moniter the water levels in the well with a string (seriously) that shows us where the water is.  We also check for flow bi-hourly.  I don't know if the wells were pump tested, They were installed and running before I was involved in this project.  I would hope they were tested...but frequently "little" details are dropped out here.

I have no clue if the forces are cyclical or instantaneous.  Judging from how the sheared spline looks, I would venture cyclical, but that is only a guess.  I say that because i don't find chuncks of metal, and it doesn't look knurled up, it just looks worn down and fatigued.

Something else that I found interesting is that the motor's spline measures 3/4 of an inch in length.  The pump's collar measures 1/2 inch in length.  So the motor's shaft isn't obtaining the desired contact.  I brought this to the attention of the support department at Grundfos, and they didn't comment on it (no, "we know, it is supposed to be like that" or "really?  that's not correct, something is wrong there.")  What did suprise them is that the motor's shaft is shearing, not the pump's collar.  (To me that made sense from reasons stated above.)  

Thanks for your help thus far.  Again I will answer any follow up questions to the best of my ability.

Cheers,

Loyal

RE: Well Pump Problem

25362:

Where'd the 205 come from? - he said 218' in the original post.  Also I used 1.02 for the sg of brackish water.  Putting those two factors into your equation yields 2.22

RE: Well Pump Problem

Correction: the estimate was indeed done on 218, not 205 gpm.

RE: Well Pump Problem

To j165 you are right. I cannot explain (I wish I could)where did the 205 gpm came from. And you're again right in your last 2.22 HP estimation including efficiencies. How, then, did you arrive at 4 HP for the motor in your previous post ?

RE: Well Pump Problem

It's the next higher even number and I don't like odd numbers?

RE: Well Pump Problem

(OP)
I like your thinking jay165...

So i got a bit curious and actually measured everything out myself.  This is what I found:

Diameter: 2.5 inches
Verticle Head: 130 feet
Other minor losses: 4.3 feet  (length of pvc, two valves, one partially closed, and a couple 90 deg bends).
Desired Flow rate:  25 gpm
Specific Weight of sea water:  10.35 kN/m^3 (i know it's metric...)
Same efficiencies as Jay, cause they sound good.

From this I found that the required power is about 1.4 hp.  A 2 hp pump should be able to do this, correct?  The Factor of Safety is 1.44, but in this situation that should be acceptable...  I'm looking for other, more experienced opinions than mine.

Jay165, I know you aren't a fan of odd numbers, but would a 3 hp be more appropriate, especially considering we have had this continual problem.

Bob, how about you, any new opinions from this data?

I want to recommend using a 3 hp 904L ss Grundfos pump, but I want to make sure that it will solve the problems we have been having.

Thanks to everyone for thier contributions.  

RE: Well Pump Problem

Hello BobPE, nice to see you around some.
Is there a check valve at the pump?
A check valve on top is good, but one at the pump cannot hurt.  If the pump is a Grundfos, it has a check valve built in, like most other 4" itty bitty pumps.  Is the check valve working?

If when the pump shuts off, the check valve(s) do not hold, there is a very hard fall of water back down the hole and some violent reaction at the pump and motor.

Regarding the horsepower and such.  Most pump manufacturers don't ever hesitate to overload a motor, not a problem to them.  You did not state what kind of submersible, but I would guess you have one of the formed stainless sheet metal pumps.  You can get a heavier duty pump, with trimmable impellers, and a submersible motor with true roller thrust bearings.  Lots more money though.

PUMPDESIGNER

RE: Well Pump Problem

(OP)
Pumpdesigner,

Nope, we don't have any check valves, other than the one on the Grundfos.  However since the pump isn't cycling, that shouldn't be a problem...right?  You are correct when i stated it is a formed stainless sheet metal pump.  I like the suggestion of getting the heavier duty pump, however the extra money does present a slight problem...

Thanks for reply and the suggestions.

-Loyal

RE: Well Pump Problem

islandeng,

You did not say how deep the liquid level is in the well.  Circulating water back into the well can carry air to the pump intake.  If you get a large bubble of air into the intake it is very possible to air lock the pump.  If the motor continues to run under that condition the unit will over heat and cause a premature failure.  Are there any indications of heat on your motor(s) or pump(s)?  

A small low volume sub pump is most likely a radial flow impeller that has a very low Ns.  In most cases I would expect the required HP to be a little lower by producing at a lower flow rate rather than at BEP.  You may be better off applying a little backpressure rather than circulating fluid.

Circulating fluid back into a well is not a good design.  Adding additional oxygen to brackish water could lead to scale.  Have you seen any scale on the pump or motor?

On submergible pumps the power supply is very important.  If there is something like an auto-re-closer on the power supply that could explain broke shafts.  If the power blinks for X ms without opening the motor starter it could cause catastrophic torque loading.
  
Hope this helps

D23

RE: Well Pump Problem

(OP)
D23,

Thanks for the comments.  

The water level is 15 feet below the top of the well, which means the water in the well is 105 feet deep.  The pump is physically located 118 feet below the top of the well.

There are no indications of heat on the motor, which is why I don't believe our supplier when he blames the failure on the motor burning out.  In fact in the last spline failure, it was caught quickly enough that the motor still checks out ok.

Not to sound ignorant, what do you mean by "scale" on the pump or motor.  When we pulled the pump and motor out of it was pretty clean with no signs of wear, except fot the splines.

Power supply is the only thing i have not yet checked.  That is one of my projects for tomorrow, so I will let you know what I find out.  Thanks again for your help.

Thank all of you for your help.

-Loyal

RE: Well Pump Problem

d23 had a good idea about power.
Just a simple loose wire, backfeeding relay, or momentary voltage drop allowing the contactor to drop out momentarily, or anything that might interrupt power for even a moment could cause a spline or shaft problem.  The interruption of power could be very small, just milliseconds could be a problem.

The check valve in the pump cannot close fast enough to prevent a fast reversal if power is momentarily interrupted.

I know this is a small pump, but we treat our controls on all pumps as follows for this very reason:

The pump starting contactor is not allowed to be connected directly to any sensor such as a pressure switch or flow switch.  The contactor is locked on and held on by the PLC (or by a time delay if you do not use PLCs).  If a pressure switch or flow switch or any device calls to start or stop the pump, the PLC waits a bit, perhaps around 10 seconds on stop and perhaps about 3 seconds to start.  That way you get no false or fast starts and stops due to momentary blips from a sensor or start button.

PUMPDESIGNER

RE: Well Pump Problem

(OP)
PUMPDESIGNER,

There are no sensors hooked up to this system.  The contactor is linked directly to the power switch which is held on, unless power is cut off, in which case it trips off and stays off.

As I noted before, the failure resembles more of a fatigue failure than an impact failure.  The splines look as if they were worn off (amazing one could wear them off within a span of 3 weeks...) rather than some large impact coming and shearing them off.  I suppose I have been using the wrong terminology in explaining the primary symptom with "shear" as that connotates more of a catastrophic failure.  

Since we don't have any sensors, I cannot know if there had been reduced flow rate as the splines were wearing down.  Please keep in mind that this is an extreemly simple system, with as few parts as possible for failure because that is how the culture works here.  (It took me some time to begin to realize that mindset myself.)

I am beginning to think this is a material problem.  And no one has commented about the length differences of the splines...So I take it that it is not out of ordinary.

Thank all of you for your thoughts, more are appreciated.

-loyal

RE: Well Pump Problem

One could asume that splines mean crevices... and crevices in salty water invite corrosion. I suggest you bring this thread to the corrosion enginering forum.

RE: Well Pump Problem

Not a metallurgist, but 3 months may be long enough for corrosion to have an effect, 3 weeks I cannot see as long enough for corrosion.  I suppose that is why I never considered corrosion.

I do understand island mentality and I do respect it at times when appropriate.

If the problem appears to be wear, consider that the shaft moving up and down within the coupling would wear the splines quickly.  Shock would produce an up and down motion if the thrust bearing is not tight.  I have no direct knowledge about thrust bearings on those little motors, but I do know a guy who uses them in his product.  He buys 100 of those little motors at a time, cuts the ends off, then machines his own thrust bearings into them.  Reason is that he claims those little 4" motors have little or no ability to withstand thrust.  He has been doing this for years.  He has been doing it on the Franklin motors, do not know if he does it on any other brand.

We had to put in a small 4" motor last year.  We were able to get an oil filled 2 hp 4" motor with roller thrust bearings.  Have no idea if that thing will last or not, but we tried to get the best one we could.

PUMPDESIGNER

RE: Well Pump Problem

(OP)
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

RE: Well Pump Problem

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 30oC. 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.

RE: Well Pump Problem

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.

RE: Well Pump Problem

(OP)
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....

RE: Well Pump Problem

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

RE: Well Pump Problem

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

RE: Well Pump Problem

(OP)
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

RE: Well Pump Problem

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  

RE: Well Pump Problem

(OP)
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

RE: Well Pump Problem

(OP)
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

RE: Well Pump Problem

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

RE: Well Pump Problem

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

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