Extending the Life of a Wastewater Pump Impeller

I would check to see the pumps are operating at their designed speed. Excess shaft speed will cause excessive tip speed and wear the impellers quickly.

Some pumps can not handle entrained gases and will wear quickly. The gas could be air, gaseous water, or ??

I would talk to pump manufacturers about MOC recommendations. Just search Globalspec or google pump MOC or pump impeller material. If nothing in your application has changed over time, the original application work may have been off leading to excessive wear.

I switched one pump from cast iron impeller to plain old 316 stainless impeller and more than doubled the life.

How about some pictures.

Abrasives or cavitation come to mind.

The problem may be pump cavitation.

If you know the type of pump and impeller size. It does sound like cavitation.

(also being wastewater, possibility of abrasive?)

Before changing pumps be sure the original pump is sized correctly and can handle the waste.

Check the pump curve, and make sure NSPH (Net Positive Suction Head) is correct on the intake.

Heres another... NSPH Definition

This should get you on your way.

Good luck,

p911

PS, if it is incorrect, I'm just throwing this out, after its corrected, you may have over current issues with your motor. We'll cross that bridge when and if we do?

Cavitation is not at all likely, this is a SUBMERSIBLE pump in water service according to the OP. It already has 10m + immersion depth of NPSHa. Not many pumps would need more than that!

USE SEMI OPEN OR OPEN IMPELLER OF SS.

REGARDS

IRSHAD

you stated that "in the recent past"..... did you make any change in the discharge piping (pipe section, number of valves, discharge point elevation?) or the process?

Without knowing all the details, the few things that come to mind are:

1) the pump is operating way out to the right of its Best Efficiency Point (draw the system curve and the pump curve and see where they meet)

2) you have grit (sand) in your water. Consider switching to a recessed impeller type of pump, or better, find the source of the grit and get rid of it.

3) unlikely, but possible: check the content of chlorides, pH, HS, chlorine (?).

For handling abrasives like primary settling clarifier's sludge, we use WEMCO C pumps (here in New York City, USA). They have a recessed impeller and work by creating a vortex in the volute, so the particles never really "touch" the impeller. The efficiency is kind of low, but they last for a long time. I know Flyght and ABS also make similar pumps (the ABS impeller, I believe, is called "Contrablock").

If you have low pH (if <5.5 may be a real issue) that may be a problem with some impeller materials, but the type of impeller sounds more important. When you talk to the manufacturer, make sure you mention the pH.

Good luck with your application, and please, let us know about the outcome.

M.

www.stlmetallizing.com

Your first step has to be to determine whether you are suffering from "Erosion corrosion" or pure abrasive wear. The solutions are very different.

If abrasive wear is your problem then you need "harder" material, if erosion corrosion is the problem then you need to select the material based on "velocity corrosion" resistance.

If abrasion is your problem then Cast iron is about as poor a selection as you can get. Cast Steel would be substantially better and then the 300 series and then the 400 series better still. After that you get into the really hard stuff that is very difficult to work with and thus expensive.

It would be necessary to know exactly the composition of the "wastewater." High chloride concentration could rule out stainless steel, and as someone else said it's necessary to make a failure analysis of the failed impellers to understand the mechanisms of the damage.

This paper from NIDI can help you in selecting the correct material:

Materials for Saline Water, Desalination and Oilfield Brine Pumps

http://www.stainless-steel-world.net/pdf/11004.pdf

Guide to Good Practice in Corrosion Control Pumps and Valves

http://www.npl.co.uk/upload/pdf/pumps.pdf

Hope this helps

S

corrosion prevention & control