Dezicification of bronze Impellers

[mapleleafs]

Has anyone experienced dezincification of pump impellers? We started our water treatment plant 5 years ago and now have to replace all our impellers on our vertical turbine pumps. The original material of the impellers is a brass alloy 875 (15 % Zinc)and the manufacturer is now recommending a tin alloy 903. We are also trying to find out why this happened. Are water at the time of startup was ph 6.8 , chlorine free 1.0 mg/l and an alkalinity of 10 mg/l. It appears the damage was done in the first two years of operation as we lost 12 % flow. We started using a zinc pyrophosphate for corrosion control after 2 years and pump production seemed to stay the same as if the damage stoppewd occurring. We did coat two impellers on one of are small pumps with Belzona last year instead of replacing the impellers but the larger pump impellers are not worth trying to save. Any comments would be appreciated.

 

[smckennz]

Hi Mapleleafs
the manufacturer seems to be heading in the right direction; bronze is a far better material for impellers.
Do you have sacrificial anodes fitted and is there an earth on the pump shaft? Both are minor points but may be worth looking at. Stray currents can cause havoc at times.

Cheers

Steve

 

[abcpump]

Are you sure this is not an issue of lead free vs leaded bronze? I remeber something about chlorine reacting with lead free bronze and leaching the zinc ions causing pitting and eventual deterioration of the lead free bronze impellers. Ask your manufacturer about alpha nickel aluminum bronze or equal construction

 

[semo]

I think abc is correct. I've seen three instances now where the zinc alloy bronze impeller has been eaten. Two projects I designed where the impeller was eaten within 1 year. Another community I recently ran across complaining of having to routinely replace the impellers.

Two instances, the manufacture had another bronze alloy to use in chlorinated water. The other instance a cast iron impeller was used. In my humble opinion, most pump manufacturers dealing with drinking water know about this problem now; but, the sales reps don't always know. You might have to argue with the pump manufacturer though. I did as they denied ever having it happen before. As we got into the discussions they did let it slip that other communities had the same problem. They just didn't want to take blame during the warranty period.

You have to be sure, at least in the shop drawing stage, that the "bronze" impeller, that is standardly specified, is suitable for chlorinated water on high service or transfer pumps.

And this problem does revolve around the lead regs instituted by EPA.

 

[PUMPDESIGNER]

Are you sure you are seeing deterioration due to chemical alone?
In these kinds of services incipient cavitation can be a severe problem, and you may not hear it at all.

If incipient cavitation is a problem, then there is a well known combination of the two (Karassick, et al), chemicals are given access to fresh metal as the oxide layer is constantly stripped away by incipient cavitation.

PUMPDESIGNER

 

[semo]

I agree cavitation can be a factor or culpret. The first time I saw this, I thought cavitation could be a culpret; but, the conditions did not appear to be correct for cavitation. In addition when the pump impeller was pulled, I noted the manufacturer had painted numbers on the impeller with epoxy paint. Those letters were intact and infact in the area around the lettering, the impeller looked like one of those laser cut woodworks where the text is raised and the surrounding area is depressed where it has been cut.

This helped lead me to the cause of dezincification. I would expect the epoxy coating to deteriorate if cavitation were the cause.

The new impellers have been in place 4 years now and are not showing any signs similar to the old ones. They have not been removed; but, their performance is still 100 percent. The first impellers had a quickly deteriorating performance which is what lead to their removal and examination.

But, as you are stating, this may not be the same as other cases. Each one should be examined based upon its own installation.

 

[wellman]

Your problem may be a process called elecrolosis. tis generally hapens in a pump where the two diffent materials come into contact such as a bronze impeller and a cast iron bowl. There is usally considerable damage to the bowls as well. I would also suggest trying cast iron impellers. The nickel aluminum bronze might also work but are quite expensive. It might be cheaper to conyinue to replace standard impellers every few years.

 

[wellman]

Your problem may be a process called elecrolosis. tis generally hapens in a pump where the two diffent materials come into contact such as a bronze impeller and a cast iron bowl. There is usally considerable damage to the bowls as well. I would also suggest trying cast iron impellers. The nickel aluminum bronze might also work but are quite expensive. It might be cheaper to conyinue to replace standard impellers every few years.