Selection of Impeller material

[sowhatso]

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...

 

[CH5OH]

cast iron works good on fresh neutral water,
would prefer bronze if any salt (sea water is involved)

 

[SJones]

Nickel aluminium bronze isn't that soft but it would be more helpful to have a fuller specification of the pumped water.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[JJPellin]

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 3600 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

 

[impeller1]

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?

 

[Artisi]

If the sand content is minimal I would opt for duplex SS maybe Cd4Mcu or similar - this is very abrasive tolerant, in my opinion 3xx series SS are too soft for abrasive conditions.

 

[lonovane]

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 1400 LG2 and this will be the most cost effective. BS 1400 CT1 is more expensive but has a higher hardness and abrasion resistance.
If the sand content is negligible - bronze is the best answer.

 

[EdStainless]

Before you head for 316 or a duplex stainless look at a Ni-Resist. These are austenitic cast irons that have good corrosion resistance, fairly good strength, and they are easily castable. These are very common in deep well pumps.
I agree with the comments that for common service Bronze is a good choice. With CA6 being an upgrade from there.

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Plymouth Tube

 

[MJCronin]

EdStainless..

I am curious about your "austenitic cast irons"

I have never heard this before. Can you give us more details ?

Perhaps an internet reference ?

 

[Artisi]

Typing austenitic cast irons into Google resulted in 11,900 hits, should be enough detail there to keep you occupied for a few hours.

 

[macmil]

I the situation mentioned, consider a urethane coating on the standard impellers. It can improve service life significantly.