Submersible Pump 1

[fmn]

On one of our facilities we have an influent well fed by force main with discharge handled by a submersible pump. The pump wears out frequently due to running too much. From the elevation at which the pump takes in influent to that elevation at which the influent discharges downstream is 4'.

The pump in question is spec'd to require 12' of head on the discharge side. It is thought that by adding pipe to create the 12' of head needed, the pump would then operate in an efficient manner and would not wear so quickly.

Two questions arise from the proposed solution:
1) Is the solution plausible?
2) Or, would it be possible to find a pump that would operate in the given situation with only the existing 4'elevation change?

Thanks for any comments, questions, suggestions, etc.

 

[dicksewerrat]

You may want to swith to an 'axial flow' style pump. They are designed for low head-high discharge.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bimr]

Would recommend that you go to the pump supplier and get a recommendation.

Adding unnecessary head to a pump will not make the pump more efficient, it will make the pumping inefficient.

 

[PELS]

Is the wet well large enough to install 2 pumps? They could be wired to turn on alternately, thus minimizing the pump cycling time by one half. So if the pump is coming on every 15 minutes, having 2 pumps will reduce each pump to every 30 minutes.

Also you should analyze the design storage depth in the wet well. This is the depth between the top of the pump motor (Pump Off" elevation) and the elevation of the "Pump On" float switch. Pumps that come on too frequently could be due to not having enough storage depth in the wet well. A deeper storage depth yields less pump activation.

Do you have any vertical flexibility to adjust the "Pump On" float switch elevation? If the "On" elevation can be raised, this will increase the storage depth to minimize pump cycling time.

Also some pump manufacturers have pumps that can allow the motors to be exposed to air for specified periods of time. This allows you to lower the "Pump Off " elevation and thus increase the storage depth.

Finally, if your pump is running for extended periods of time, it could be that it is undersized to remove the design flows efficiently. A pump with higher HP and speed will evacuate the wet well quicker and maximize the storage fill duration time and thus reduce the frequency of pumping. Proper pump sizing needs analysis of these factors:

1.TDH or Total Dynamic Head
2.The Hydraulic slope (Hazen Williams equation)
3.Friction loss
4.Expected flows
5.Wet well diameter
6.Manufacturer's pump performance curves

These are the things I would look at first.

 

[Valvecrazy]

Pumps do not wear out from running to much. They are made for continuous duty and will actually last longer if they run 24/7 than if cycling on and off.

What is the cause of failure?

The only way running to much can cause a failure is if you are pumping abrasives. Are the impellers and shaft worn from abrasives?

Not enough head can cause a failure from upthrust. Are the impellers worn out on the top side?

Cycling on and off to often can cause a broken shaft, stripped out coupling, loosened impellers, overheating of the motor, and other problems. Does the motor burn up or does the pump just wear down and stop moving enough fluid?

Determining the actual cause of failure can help you find the solution.

 

[BigInch]

Running anything off BEP has its penalties. You are only operating at 33% of BEP Head conditions. Unbalanced forces are most likely high and affecting the bearings. You don't state the nature of the failures. Do you have excessive bearing wear?

http://virtualpipeline.spaces.msn.com

 

[fmn]

Thanks everyone for your replies. I will try to address each below. I apoligize if I miss anything.

1) Axial flow will collects rags too easily
2) Pump suppliers are not much help with this since the configuration demands a low head/high discharge pump and there do not seem to be many, if any, of these that will work for the situation.
3) The well does have two pumps in it and both would be replaced should we find the appropriate type available.
4) I beg to differ in regard to pumps not wearing out from running too much. On wastewater facilities pumps are replaced continually from the incredible variety of matter flowing through the pumps. The combination of running continually and abrasives does them in quite well.

Our need and request is simple, to find a quality low head/high discharge pump that will operate continously for long periods that will handle abrasives.

 

[BigInch]

You're correct in every case, except running at 33% BEP.

http://virtualpipeline.spaces.msn.com

 

[Valvecrazy]

It is still not that the pump runs all the time that wears it out. It is the abrasives. You need an impeller that can handle abrasives. Maybe an open type impeller would help. I have also heard about a super cold treatment for the impeller that helps.

 

[Artisi]

Regarding your comments re the pumps "wearing out" -it stands to reason that if you are pumping solids etc you can expect the pumps to "wear out"- the secret is to find pumps that are designed to handle abrasive materials and give you the pump performance required,

What is the flow rate for these pumps, this will assist in making some meaningful comment and what pumps do you have installed at the moment?

 

[cvg]

your original post seems to indicate that you believe the pump to be operating outside of it's most efficient range and thus wearing out. Later post seems to indicate that you now feel it is abrasive effluent that is causing the wear. Suggest you first check a worn pump to determine which is the case. Is it failing because of the abrasives, or from running overspeed / unbalanced forces caused by pumping off the end of the pump curve?

The solution to the abrasives may be twofold. Better screening and better quality impeller and casing.

Solution to the pump curve issue may be to throttle a valve, reduce the size of the discharge line or to replace the bearings with something a little beefier that can handle the thrust.

 

[rcooper]

It sounds like you are specifying an archemedian screw pump. They last for ever and will pump anything. They are low head devices and can have huge capacities.

 

[semo]

If you operate the pump too far to the right on it's curve, you can cause cavitation which will destroy the pump. You need to look at the pump curve along with your system curve to find a pump that will operate properly. Like abrasives, cavitation will also eat at the impellers. You need to make sure what the problem actually is before you can fix it.

For abrasives, you can get impellers designed for abasives (such as ceramic coated impellers). They cost more; but, will last longer.

If you can't find a pump with a good curve, you can throttle the flow as CVG stated. A good valve for this is a pinch valve as it will not have protruding edges to catch material when it is partially closed.

 

[ltho98]

Pumps don't wear out from running constantly, they wear out from cycling, heat, and overcurrent and undercurrent condtions.

First off, do you have a pump protector. (Franklin electric makes a good one)

Second, an axial flow pump is what you should probably look into. (Grundfos is what i recommend for all my clients)

And on top of that is your pump operating in its efficiency peak?