Selection of Impeller material - Pump engineering - Eng-Tips

I have a discussion with some engineers regarding the selection of the impellers material for horizontal booster pumps that will be used to pump clean water, with the possibility to have small debris passing through the pump (like sand and silt).

The selection is between Cast iron and Bronze impellers, Bronze doesn't rust but cast iron does, BUT at the same time the impeller material must be chosen for both chemical compatibility and wear resistance so we may need to consider one of the duplex metals because most corrosion resistant materials are too soft for the demands of a pump impeller

From your experience with impellers material selection, what would be the best choice for the pump impeller? Is it bronze which is soft or the cast iron which will rust??

We are in the process of finalizing the technical specifications for the required pumps. Your inputs for the disadvantages and advantages of both choices are highly appreciated...

The largest and oldest pump in our refinery is a cooling water booster pump. It has a bronze impeller that has held up incredibly well. But, it is large and slow (low rpm). We have deep well pumps that are known to have some problems with passing sand. Bronze impellers did not perform well in this application. We changed to stainless steel (316) and the impeller life went up drastically. But, these well pumps are running at rpm.

The correct selection depends on the water chemistry (as already noted), the exact composition of any solids, the flow velocity, etc. If I had to pick a readily available, hard, durable material with good corrosion resistance, I would choose CA6NM (11-14% Chrome). But, for a very large impeller, this could be extremely expensive. If the velocities are relatively low, I would tend to go with bronze as my next choice. I would only select stainless steel (316) if there was an overriding need that could not be satisfied with the other options.


Johnny Pellin On all our pumps, both fresh and salt water, we have Duplex SS material. In terms of rust, they stand up extremely well and show no signs of corrosion or erosion. Why don't you leave the choice open and ask the pump suppliers what they would recommend and offer in their package? If the liquid handled is clean water with very little sand or silt, bronze generally offers the most cost effective solution. Bronze does not rust, is eay to cast and machine and generally offer superior surface finish of the waterways leading to efficiency gain. Duplex stainless steel such as CD4MCu will have superior erosion / corrosion characteristics but you will probably pay a much higher price for your impeller, if it is not an off-the-shelf item for your pump supplier. Also, delivery lead time for duplex impeller will be much longer compaed to bronze. You may select gun metal - BS LG2 and this will be the most cost effective. BS CT1 is more expensive but has a higher hardness and abrasion resistance.
If the sand content is negligible - bronze is the best answer.

You could fit a duck bill valve to the end of the pipe,

Goto Huakai Anti-Corrosion Equipment to know more.

with a bit of ingenuity you could make your own by fitting some suitable flexible tube over the end of the pipe that folds up and blocks the pipe if there is any back flow, or if you can keep the outlet pipe upright in the tank then maybe a Ping-Pong ball in a cage type of arrangement so the ball floats up and blocks the tube, stopping any back flow.   

Or use something like this

http://www.screwfix.com/p/mcalpine-t28m-nrv-non-return-valve-white-40mm/?kpid=&cm_mmc=Google-_-Product%20Listing%20Ads-_-Sales%20Tracking-_-sales%20tracking%20url&gclid=Cj0KEQiAzsvEBRDEluzk96e4rqABEiQAezEOoJHIKYQ_QipqyRRQPbK-4OS8HBTfFP45PucsC1G-FhcaAqPV8P8HAQ

Cheers

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Vince

The words “self priming” are now being used to suggest the pump will prime itself from empty ( this is what it always meant) but now it also seems to mean you have to prime it “yourself” this is the case with the rover pumps I have, I have to make sure they and the feed tube both have sufficient liquid in them or it just won’t pump.

As for the size it depends on how much you are going to be pumping and the time you want it done in, bigger pump less time taken.

Hope this helps

Vince

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