Unusual pump casing erosion situation

[p92]

Here's an unusual situation...

1. Centrifugal pump in slurry service. Hardened alloy material.
2. Short outage to briefly do maintenance elsewhere---nothing done with this pump.
3. When pump starts up, the pressure drops dramatically. within days.
4. Removal of casing shows concentric rings all around suction, almost to edge of casing. Impeller, however, looks essentially undamaged. Some machine marks still visible.
5. New casings show same erosion, not quite as fast, but much faster than ever before. Same materials of construction used over last two years without issue.
6. Slurry does not seem to have changed.

Any thoughts on why this could be???

 

[Artisi]

Some photographs would be a great help.


Naresuan University
Phitsanulok
Thailand

 

[PUMPDESIGNER]

Hello Artisi and p92,
Photos would be best.
Just post 'em up somewhere and give us the link.
Put 'em up high res if possible so that we can download them and zoom.

PUMPDESIGNER

 

[p92]

Mmmmm...well, technically, I'm not supposed to send any photos (even harmless ones like this) without a signed form for each company I send it to. I can only imagine how many forms I'd have to sign for a Web posting... =)

What would you be looking for in particular? There doesn't seem to be any pittting in the circular grooves on the inside of the casing. The grooves are concentric. There are more of them than can be easily counted. The materials people say that it looks like erosion is the primary issue, not corrosion. I just don't know what the reason behind the erosion would be, other than the slurry's being different or there being recirculation somehow.

Is there a possibility that quality control issues with the casing can cause recirculation?

 

[Artisi]

What model and size of pump are we talking about - is it open or closed impeller, what are the materials of construction, what is the slurry.

WHAT HAS CHANGED RECENTLY, if you used to get 2 years life from casing.

No connection between quality control and recirculation, recirculation is a function of pump hydraulics.
Have you check material of casing against spec - is it as hard as it is supposed to be.




Naresuan University
Phitsanulok
Thailand

 

[p92]

4" suction, 3" discharge, 10.25" open impeller. Ferralium 255 material of construction. Chemical slurry, so it's not like a rock slurry or something.

The "what has changed recently?" is the part that has gotten me confused. It doesn't look like anything has changed. We took a short outage, not touching those pumps at all during the outage, and the readings I've looked at (pressure, temperature, level, etc.) look the same after the event as compared to before.

One thing that has changed is that, normally, there is a recirculation line that bleeds off the main line so pressure doesn't get too high. Right now, it is plugged---but it plugged after our issues started, not before.

There's no connection at all between quality control and recirculation? I've heard of people trying to weld repair their casing and getting unusual circulation patterns, so I thought quality control might be coming into play here.

I've material-checked the metal, but not the hardness yet. I didn't check the hardness because the first casing that had damage was one we had run on for several months, so it seemed unlikely that it was softer. However...

We have Pump 1 and Pump 2 as in-line spares. Pump 1 first had the casing issues, but it's always been Pump 2 since then, for the next three casings. (Normally, we would run on Pump 1 all the time, but it has developed a habitual leaking problem, so we run on it only intermittently.) I dismissed hardness as an issue at first. But now I think that maybe Pump 1 was an anomaly and Pump 2 may follow more of a pattern (like being not as hard as before).

Any more thoughts? Thanks for the feedback so far...

 

[Artisi]

The only wild card thought I have at the moment is some hard solids in the slurry. If the open impeller front clearance is a bit wide - you may have solids jamming between the impeller and the casing - this is the only thing I can see that will give you concentric grooves worn into the casing.

Naresuan University
Phitsanulok
Thailand

 

[Ethan317]

I think that the suction vortex has the most predictable flow pattern, allowing the slurry to "phonograph" your suction inlet. You may or may not see the same wear pattern within the casing because of the more complex flow pattern due to pressure differentials within the casing.

Suction diffuser?

 

[p92]

Hi Ethan317,

I may not have stated the situation as clearly as I thought. The suction nozzle actually looks pretty clean. It's once you get inside the casing that you see the "phonographing." It goes as far as the outside diameter of the impeller.

You think a suction diffuser would still be the way to go in this case, or would that not help this particular situation?

 

[Ethan317]

p92,

The suction diffuser would probably not help in that case. I threw it out there for fun. I'm curious about that recirc line that's plugged. Let's not confuse internal casing recirculation with an auxiliary recirculation piping plan.

Ethan

 

[btrueblood]

p92,

It looks like there might be a pattern hidden in your posts: does the wear show up more on pumps that are started/stopped more frequently? I could imagine a situation where the slurry segregates slightly during flow stoppages, and the resulting "clumps" of material could do a lot of damage until the flow is up and humming along again.

Ben T.

 

[p92]

The pump actually runs continuously. When we switch over to the in-line spare to make repairs to a pump that is having issues, we flush out the troubled pump ahead of time.

We thought we might have had a clump when the first pump had its issues, but having it for four other pumps after that is less plausible.

p92

 

[KASTNER]

Hi

Im an After Sales Expert for all kind of Power Plant and Industry Pumps, Valves, Stainers and so on. To check the real damage reason some photos would be helpful. Please, enclose also the techical datas of the pump together the photos like (Medium incl. PH value, Volume, High, Suction and Discharge pressure, Temperature).

With the help of this information it should be possible to have a better look to the problem.

Bye and greetings from Germany
Jochen