Materials for Impeller/Shaft

Each of these metals might be compatible (noncorroding) with respect to the pumped fluid, but in combination there might be electrolytic corrosion according the relative positions of two or more metals on a galvanic scale.

I would actually ask a prospective vendor or two for their advice.

It would be helpful if you told us what you were pumping. We don't know who wrote the spec either. Do the pumps you get now work?

Come on. If you want an answer, give us something to work with.

The pumps are required to pump sweet water for fire fighting. The specification was written by Halcrow - we are the consultant for the project. Pumps are in procurement; suppliers we have recommending that we go with the bronze shaft

Welcome to the insanity. I am assuming that you are the original poster. If the pump is designed to be used for a fire pumping operation, I would suggest you use a pump designed for that purpose. In the states, I would suggest Waterous, Hale, or Darley. I am sure there are other brands that are manufactured closer to you. All of those manufacturers manufacture cast iron housings along with bronze impellers and SS impeller shafts. Also available are bronze housings. I would suggest you search fire pumps, and compare your specs to the offerings from a major fire pump manufacturer. Chances are that your specs were written around a specific pump manufacturer's offerings. Good luck. If you need more fire pump manufacturers, give me a couple of days to finish a project, and i will search a international fire magazine for other manufacturers.

Contractor has proposed Aurora 2500GPM with manufacturer standard shaft carbon steel with bronze shaft sleeve. Neither Aurora nor Patterson could comply with our Spec on this point

Thanks. I think that's the bottom line on this

Bronze shaft? Crazy.

It sounds like they cast the shaft and impeller as an integral unit and then machine the shaft to fit the bearings in the housing. This avoids dissimilar metal problems, but brass is weak compared to steel and if something breaks the impeller you must do a major job to fix it. I have seen impellers with a bearing on one side and a detachable shaft bolted on on the other and a bolted on steel panel to open to replace the impeller. It also depends on how much room you have to swap pumps, many people simply swap pumps, then fix to old one to be the spare. Cost is also a factor. If this is a huge installation, it might not pay to have the cost of a spare pump if they fail every 10 years. Then if it takes 2 months to get a new pump and production = zero, a spare makes more sense. A lot depends on the power level and the fluid pumped as well.

As others have said, more data

Bronze, depending on the alloy is really not much weaker than cast iron, especially after the cast iron has been corroding for a while. it also stands up to cavitation better than cast iron and does not tend to promote marine growth.

Firewater may be stagnant for long periods so there can be algae and such. Corrosion loves to form under such growth.

And I was referring to the impeller and housing, not the shaft.

Bronze can be pretty strong, depending on the alloy and the processing. Consider Toughmet 3 from Brush Wellman, and AMPCO 45.

If the pump manufacturer is not offering those materials on their pump, it would not seem to be cost effective to have the shaft(s) custom manufactured for this single application.

Bob C, I'm sorry if I wasn't clear on my intent. I wasn't making a materials suggestion for the pump manufacturer, just trying to point out that there are a number of high strength bronzes out there on the market and that you should not necessarily turn your nose up at bronze without knowing WHICH bronze you are discussing. You might be surprised, the properties run a pretty wide gamut.

No offence taken on this end. I find I am suprised at least once a week by something I read here. Sometimes I get suprised by what I see in the mirror.

This documnet from NIDI can give you some good information on material selection for pumps:

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

and also this from NPL

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

S

corrosion prevention & corrosion protection

Worked as a Mechanical Equipment Specialist for many years on refineries, chemical plants etc. Typical water pumps for simple cooling water circulation were CI housings with Bronze (CDA 836) or CI impellers, bronze shaft sleeves with a steel (typically 4140) shaft.

Bronze would never be specified as a shaft material even though a continuously cast Aluminum Bronze would probably have the necessary strength. Stainless steel is also not an exceptionally strong material and I would agree with the comment that with too many different materials you run into electrolysis problems.

The bronze shaft sleeves pretty much cover all the wetted areas of the shaft and protect the steel shaft from corrosion.

Bronze pump casings were pretty much reserved for smaller applications due to the high cost. Aluminum bronze pump casings were mostly used in acid water applications due to their corrosion resistance. I have installed cooling tower pumps in excess of 20,000 GPM and they were CI/CI because the refineries pretty much control the water quality in those.

Higher strength shaft requirements were often met through the use of a 17-4 ph shaft if the HP requirements exceeded the capability of 4140 as I recall.