Impeller corrosion 2

[kbs]

I have a 5hp end suction centrifugal pump with a 6" bronze impeller. The impeller has serious galvanic corrosion evident. This pump is part of a large decorative fountain. The pump draws water from a holding tank at the same level as the pump and pumps the water up approx. 12' to the fountain outlets. Water travels back to the tank from the overflows. We suspect electrolysis and wonder if placing sacrificial rods in the holding tank will prevent, or slow down at least, this corrosion. The possibility of corrosion from cavitation has been addressed. Stainless impellers are not available for this model. Thanks for any help.

 

[PUMPDESIGNER]

I would first need to agree with your diagnosis.
What is the appearance of the damage?
Those types of pumps are very often improperly specified by unkowledgeable people and run into serious cavitation problems, which will tear up an impeller quickly with pits and holes.

If you think cavitation may be the problem give us more info. Give us NPSHR for the pump at the flow rate it is operating at, hp, rpm, make and model, etc.

PUMPDESIGNER

 

[Artisi]

I'm with Pumpdesigner, it is more than likely cavitation damage that you are seeing - unless of course it is an "el-cheapo" pump unit and the impeller material in just rubbish brass and not bronze, you might then have some material degragation.

 

[kbs]

Thanks for responding. The pump is a Berkeley model B5WPS with a standard bronze 6" impeller. NPSHR from the manufacturer is 10' at 500gpm. The tank is at sea level and there is negligible friction loss as the tank is 5' from the pump inlet. The outlet piping from the tank to the pump is visible as the top of the tank is open. There is no indication of any vortex from the suction. We did find a badly deteriorated gasket on the strainer access lid and suspect air was entering at this point. I believe this will solve our cavitation problem. The deterioration of the impeller is pits and holes and looks much like pictures I've seen of cavitation deterioration. Again, thanks for your input.

 

[PUMPDESIGNER]

kbs,
Thank you for reporting back what you found.
Don't go running off yet however, you have not figured out the problem.

Air cannot cause cavitation under any circumstances, in fact air will reduce cavitation and sometimes cure it.

I do not have the time right now to explain, but I will later.

PUMPDESIGNER

 

[Artisi]

I'm still with PUMPDESIGNER - air entrainment is not causing the damage that you are reporting. The next step is for you is to establish what the flow rate is and what is the total discharge head, it is important that you establish this so you can compare it to the performance curve. It could that the pump is operating at the extreme end of curve and the pump is suffering from suction cavitation.

Do not assume that the pump is operating where you think it is - check it.

 

[kbs]

Thanks for your help. How do I measure the suction pressure properly? As I mentioned, the tank water level is approx. 24" above pump C/L. The tank is open to atmosphere. I have a tapping in the suction line about 6" from the pump inlet. I should mention there is a basket strainer in the 5' of suction piping between the tank and the pump. The strainer seems clear. The inlet piping is 6" PVC with one 90degree bend before the 6" pump inlet. Would throttling the pump discharge line help if low suction pressure is the problem? Thanks, again, folks.

 

[stewbaby]

Might be helpful...Once you've nailed down your existing performance as recommended, Go to

http://www.berkeleypump.com

and there you can use there interactive pump curve to see where you fall on the curve.

 

[PUMPDESIGNER]

If suction cavitation is your problem you will in fact reduce it sharply by reducing flow rate.

Do you have noise at full flow? Cavitation sounds like small hard particles going through the pump, very distinctive and very different than air going through a pump.

To diagnose cavitation just turn the flow down, if the noise goes away then you have suction side cavitation.

A few notes:
Intake line is critical on that pump. You must have a clean flow through your intake line to properly feed the impeller. There should be a straight length of pipe the same exact size as the pump inlet connection, probably on your pump this straight run should be about 40" perhaps slightly more. In that straight run there should not be any fittings, reducers, valves, etc. You must provide a clean flow of water straight into the impeller eye to produce even loading onto the impeller and shaft.

Further away from the pump check for any high spots that may cause an air lock which would reduce flow, these high spots may be built in (pipe rises and falls), concentric reducers or bushings installed in the horizontal position, or restrictive check valves.

Restrictive check valves are a particular problem, many suppliers for some reason will not stock or supply high flow check valves for pumps (naw, not a price problem again), in fact, this is one of the largest crimes on pumps, using cheap restrictive check valves, and installing them close to the pump.

Check for anything in the line that would cause high turbulence. Excessive turbulence in the line can reduce the flow rate sharply enough to cause low NPSHr.

You may have to increase the size of the intake line (not at the straight run into the pump however), and all the components in that line.

I believe you have sufficient NPSHr easily available but perhaps the intake line has excessive losses.

PUMPDESIGNER

 

[kbs]

The pump strainer has been removed and I slowed the flow down. I believe low suction pressure was the problem. Thanks for all your help.

 

[MJCronin]

kbs,

You state that the pump is part of a large decorative fountain.

Is there some type of water treatment that is being added to the fountain.....Swimming pool chemicals perhaps ?????

Is the system being "shocked" periodically with chemicals ?

There is a solid reason that swimming pool pumps and systems do not use brass or bronze components. Brass/bronze etc. are not good materials in some caustic environments.

Ask the maintenance people what chemicals find thier way into the system........

I suggest that you consider the replacement of the pump with a plastic/FRP model suitable for swimming pool service.


MJC

"There comes a time in the affairs of man when he must take the bull by the tail and face the situation." W.C. Fields

 

[kbs]

mjc

The fountain is outdoor with no chemical or treatment of any kind. We have a heavy annual rainfall in this area which I believe keeps the fountain water neutral. I feel fairly certain the problem has been cavitation from low suction head as indicated by the noise from the volute
area.

Thanks for your input.