Leaking Discharge Cover- Strange Damage

[wastewaterguy]

I have a very interesting problem. All of the pumps discussed are the same model close-coupled (bloc) pumps from the same manufacturer. Two pumps (A & B) for moving wastewater were installed 9 years ago and have served very well with simple routine maintenance since 1995.

Pump B had bearing damage so in November 2003 two new complete pumps (C & D) were added to the two older pumps in the plant, to facilitate bringing down pumps A & B for maintenance. [See pictures of old pump B discharge cover, inside and outside] ***


After 25 days of operation, new pump C began leaking at the discharge cover. The damage of pump C is shown in a picture ***

The pump was replaced by pump E. 15 days later pump D also began leaking.
The discharge covers where removed from pump C D and E and are photographed here ***.

I could repeat the endless story here, but suffice to say here is the problem in a nutshell.
The two old pumps ran for over 9 years with no problems. The 4 new pumps lasted less than 40 before the discharge covers began to leak.

All of the pumps are the same model, but clearly the design of the discharge plate is different between the old models and the new models as shown in the pictures***. I also suspect the material of which the pump cover is made is different. But the damage looks very strange to me. Any suggestions or ideas??

Here are the pictures

http://uk.f2.pg.photos.yahoo.com/ph/wastewaterguy_357/album?.dir=/95a1

 

[quadtracker]

The design change in the cover could have affected the hydro-hydraulics flow characteristics within the pump. It looks like severe erosion. Does the impeller have wear rings, if so what is the clearances between the case cover and the impeller wear ring, maybe they're being washed out and the discharge flow is eroding the case cover. Your right, the manufacturer may have changed the metal material as well. What does the manufacturer recommend to resolve the problem?