Slurry Impeller Erosion

[NRaaum]

We have a 1750 rpm 1600 gpm 314 ft HD horizonatl open impeller slurry pump that is eroding impellers every 300-400 hrs. The slurry is bottom ash from a coal plant. I just started at this company and have yet to seen a failed impeller, but I am told that the fins basically erode away to the point where the pump is unable to maintain prime. The pump is pumping from a sump where the level fluctautes, consequently the pump routinely pumps below the suction pipe (8ft) and loses prime. I suspect that at 8 ft the pump may also be running into cavitation.
The mechanics have tried many different materials on the impeller with little change. They are leaning towards installing a new pumping system, is there anything that should be tried before looking at a new pumping system? If not what could be done in new pumping system to eliminate this?

 

[mog69]

You could look at metal spraying a carbide or other hard facing layer onto the impeller. You don't say what material the impeller is made from, so the technique might not be appropriate, but the advantages are that the applied heat is very local, and there is a large range of alternative compounds to choose from to get the right properties at the surface.

mog69

 

[arunmrao]

Slurry pump impeller wear is an acceptable phenomenon. You can decide on the material if it is NiHard or high chrome castirons. Perhaps introducing a few carbide inserts in a some locations might help .

 

[jet1749]

8 foot of suction lift from a sump should not present any problems for a centrifugal pump, and a properly designed suction piping system should not be causing cavitation problems. Loss of prime however will most certainly be an issue as the suction inlet becomes exposed. As suggested already, the erosion of metal components of a slurry pump are inevitable. What is open to interpretation is the acceptable level of erosion. I make your pump to have an impeller diameter of about 17.75" if the 314 ft of head is the shut off figure, that gives you an impeller tip velocity of 135ft/sec. My references make that figure the maximum velocity for pumping dirty water, medium slurries 110ft/sec, higher concentrations are 100ft/sec. I have had favourable results from using rubber lined centrifugal slurry pumps particularly Warman A/H series on volcanic ash duty. Also bear in mind the need to keeep pipeline velocities low to prevent pipe erosion, particularly around elbows and other fittings. Conversely pipe velocities need to be high enough to prevent slurries settling out.

 

[NRaaum]

Thanks for the replies. We have tried NiHard and some high chrome castings neither have hepled much. I'm leaning towards jet1749's comments. It does seem that the velocity of the impeller is too high. I don't know what the relationship between erosion and blde velocity, but I suspect it would be exponential. We are looking at Warman A/H pumps but I think we still need to address the issue of velocity if we want to greatly reduce impeller wear. Would it make sense to put two pumps in series so that both could be run slower but still produce the same head? Also the pump does periodically empty the sump dry (3-5X daily) will running the pump dry like this greatly affect impeller wear?

 

[dik]

Is the high rate of wear due to abrasion? or cavitation?

Dik

 

[NRaaum]

dik,
I have not seem a failed impeller yet (just started this job two weeks ago) I am told that the blades erode away until the pump is unable to develop the head needed and looses prime. Supposedly there is very little pitting so we do not suspect cavitation as the leading cause of failure.

 

[jet1749]

I would presume running two pumps in series would give the head you require without running at excessive impeller velocities. Bear in mind the second pump needs to be able to handle the (suction) pressure delivered from the first, particularly as the seal chamber pressures on pump two will be substantially increased. Probably more of an issue if you plan to use a packed gland. The issue of running the pumps dry raises new problems not really related to erosive wear. These have to be addressed by either preventing that happening or mitigating the consequences. Prevention could be by level control with simple float switches etc or mitigation by something like load monitors fitted to the motors which trip the pump when the absorbed kw drops below a set level when loss of prime starts to occur. It would also be advisable to fit a clean water feed to the mechanical seals to ensure a fluid film at the faces, this will be especially important if dry running occurs.

 

[svanels]

Take a look at

http://www.practicalpumping.com/articles.php

I have seen to many damage on pumps to enter the debate of abrassion x erosion x cavitation

Lately I had an impeller of cooling water pump completely destroyed. After we opened it we wondered how we didn't catch it with the flowmeters or pressure indicators. Theoretically sound piping practices, pump below the cooling tower basin, amperage was climbing, vibration increasing.
Then we found a 2 inch hole on the suction just below a chemical injection quill...

 

[DDRSS10]

I have worked with centrifugal pumps that were used for pumping taconite slurry, and crushed hard rock overburden slurry, two of the most abrasive things out there. I am not sure of the gpm, but these were 1000-2500hp pumps that would erode a 350-600lb high mn cast steel impeller by 70% in as little as 90hrs continious running. These pumps were gravity fed so were never subject to losing prime. In your case it sounds like 300-400 hrs is doing pretty good. I agree with mog69 that you could flame spray a wear surface on them but that is very expensive, the shop I work for pays $40 a pound for that type of metalizing wire and we charge $165 an hour for application. Which can be very tricky on an impeller.

Nick

"Speed costs money boys, how fast do you want to go?"

 

[boo1]

Try one of the Metco TiCarbide powders, apply with HVOF process. We use applicators Thermal Spray and Machine in Norfolk VA and A&A Compay in South Plainfield, NJ

 

[arunmrao]

I have had opportunities to witness surface hardness increase by thermal spray or hard facing techniques both in the case of a slurry pump impeller and parts used in a VSI(vertical shaft impactor). In both these instances there was no dramatic increase in life compared to increase in costs (The life was a fraction of that obtained by conventionally cast wear metals). Also the deposits got eroded quite fast exposing a soft base metal.

 

[rob768]

you might consider high manganese steels. They work-harden considerably and are often used for these applications.

 

[sprintcar]

Have you contacted the pump manufacturer? Try their Tech folks instead of Sales. They may have other alloys or suggest perhaps a different impeller. I just read where GIW has a new impeller material so you might try them:

http://www.giwindustries.com

Timing has an awful lot to do with the outcome of a rain dance.