pH and abrasives effects on choosing pump construction

[nalmeter]

We have several self-priming stainless centrifugal pumps that have been running for 15 years and have given us very little trouble. I do not have nuts and bolts flying into the impeller, but I do have a Manganese Dioxide powder as well as plastic pellets (and the occasional plastic trash in any wastewater system). We treat this wastewater for pH, hence the pH can swing from 6-9, rarely as low as 4 and sometimes as high as 10 or 11. What material of construction would be most appropriate here since I need to upgrade these pumps? My first thought is to go with what has worked for years, however, if I can save some money by going with ductile iron or carbon I would prefer that route. The flow is ~180 GPM @86 FT TDH.
Also, in order to improve my NPSHA (suction pressure is only 7-8 PSIA)would moving the suction strainer to the discharge be unwise in that I could plug/ruin the impeller/pump?

 

[profengmen]

nalmeter,
15 years pumping the crap you are is a long time. It ain't broke........

 

[ccfowler]

You may want to give due consideration to non-metallic pumps. Be sure to clearly state all expected conditions to the pump manufacturers and to require performance and durability guarantees.

Suction strainers are called that for the obvious reason. If NPSHa is a problem, can the pump elevation or the suction pipe size be changed?

 

[nalmeter]

How close to the required NPSH is "too close". If I have 16 FT and the pump requires say 12-14 should I look at a different design?

 

[ccfowler]

nalmeter,

The 12-14 ft NPSHr probably represents pump operation with 3% head loss to cavitation, so operation at 16 NPSHa does not present much of a favorable margin. I would want to consider other pumps and system configurations if possible.

 

[nalmeter]

Fortunately the flowrate range I'm looking at 200-250 GPM and the pump sizes do not require NPSHr much past ~7-8 ft. Thanks for the input. This is one area in which I'm still getting a feel for.