Pump corrosion

[thewellguy]

I saw a new one today that has me stumped. I pulled a line shaft water lube turbine because it was off in prefromance. The pump is 5 years old. The iron pipe, stainless shaft, and bronze spider bearing all looked fine. When we took the pump apart we found that the bronze inpellers have "roited" in half. The vains were gone and the impellers were in 2 pieces. This is a 8 stage pump and 6 of the impellers were in 2 pieces. The pump is from the worlds largest manifacture. It pumps well water and according to the customer they have not had any other problems. Any thoughts?

 

[Artisi]

For a start have a material analysis done on the remains of the impellers - although this seems unlikely for pumps from the worlds largest manufacturer it sounds like poor quality bronze for the impellers.

 

[coastpumpguy]

When you say thr impellers are in two pieces and the vanes are gone, does this mean the front and back shroud are still relatively whole with the rear (upper)shroud still connected to the shaft,or have the impellers come apart through the hub (nice section view)?

What size bowls are they and what speed?

What is the operating duty?

What is the Ph & conductivity of the ground water?

If this unit operates continuously and it's a 3600rpm unit this could be normal vane thinning. 40K Hrs is in the realm for very thin vanes for a smaller unit with "active" water.

It is very likely that the spiders are not the same alloy as the impellers and are operating exposed to far lower velocities, even if they were the same alloy, the corrosion/erosion would be at a very different rate.

If the water is relativley conductive and the Ph is high or low, you could have a nice galvanic cell with the impellers being anodes to the 416 shaft and collets.

Bronze / brass if it contains zinc can be subject to dezincification which looks like the material "rotted".


CPG.

 

[Artisi]

coastpumpguy
The OP did say rotted not corroded/eroded. However, I agree completely with the dezincification if brass impellers but unlikely considering OP specified bronze and from memory there isn't any zinc in bronze.
A metalurgical analysis is needed as a first step to establish what the impeller composition is.

 

[bk19702]

Material analysis first is a good step. Was there any PMI conducted at the time of purchase? I'd check for that documentation, if it exists. 5 years old from the world's largest manufacturer with this type of failure does not sound promising.