What caused this?

[SteveWag]

I was recently asked to investigate a loss of capacity problem with a drinking water pumping facility. I found two split case pumps with a shop drawing capacity of 800 GPM @ 214 feet each. Only one pump can run at a time (electrical interlock). Both pumps have a flooded suction with water service about 5 feet above the impeller, from a tank about 4 feet away. The curve indicates a required NPSH of 26 feet at 800 GPM. The compound suction gauge has always indicated a positive pressure. Tests indicated the pump (only one was operable) was only capable of reaching 85% of shut-off. I ordered a new rotating assembly. While I was there, I noticed the Operator had disabled the check valve (propped it open) in order to get some more flow from the pump. I cautioned him that a power failure would cerate havoc due to reverse flow.

Sure enough we had a failure last weekend. The pump now would not run and was torn down.

The below link is what I found yesterday:

http://www.gswagner.com/RSPumps/RS14.jpg

I assume the broken impeller was due to the high-speed anti rotation of the pump as the 1,000,000 gallons of stored water emptied through the pump. The pump was operating quietly when I was doing my tests, no vibration, rotation of the motor was correct.

Now that the assembly was out of the pump I was able to inspect the impeller:

http://www.gswagner.com/RSPumps/RS1A.JPG

What caused this?

The shaft sleeves are Bronze same as the impeller:

http://www.gswagner.com/RSPumps/RS3.jpg

The sleeve (next to the spring extending to the impeller) shows no sign of damage.

Nor do either wear ring:

http://www.gswagner.com/RSPumps/RS4A.JPG

Any comments?
Thanks
Steve

 

[relentlessratt]

i"m not as experienced as you guys but I don't think this is cavitation. The area of damage extends beyond the the normal areas exhibited by cavitation damage. Would cavitation cause material removal on the outside of the impeller shroud and near the wear rings?

I think there may be something in the water causing a general corrosion mechanism.

 

[dabluffrat]

I agree... cavitation didn't do this damage to the shroud and impeller trailing edge. Much more likely corrosion

Sounds like you were only able to reach 85% of shut off because the check valve was forced open. You were probably further out then 800 GPM as well, but had this been cavitation damage, you'd experience a long history of vibration and system issues with this pump, which doesn't sound like the case.

The problem with capacity is likely assoc. with the worn impeller.


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