Centrifugal Pump Impeller Failure

[JJPellin]

I have seen a problem with two of our centrifugal pumps and am curious if anyone has seen this before or has an explanation for possible cause. First, I should provide some information about the pumps and the service. These are single-stage, overhung, centrifugal pumps that operate at 3600 rpm. They have enclosed impellers with three curved vanes. They have wear rings on the head and case side and three large balance holes. They are very typical API pumps built in about 1999 by a major manufacturer. The impellers are 14.125 inch diameter. The maximum impeller diameter for this model is 15.625 inch. The impellers, heads and cases are all cast 316L Stainless Steel. The shrouds on these impellers are about 3/16 to 1/4 inch thick.

The pumps are in lean amine service at about 140 °F. They are typically operated at about 1000 gpm and 820 feet of head. The product specific gravity is 0.98 at normal operating temperature. The suction specific speed is just over 13,000 in US units.

The problem was see on the bench today has occurred three times in 10 years of operation. The shrouds on the impeller are bulged out between the vanes to the point where cracks are developing in the corner where the vane meets the shroud. The first time this occurred, it was on the spare pump that rarely runs. The impeller was replaced in kind since none of us had ever seen this before and we had no explanation for the cause. The second time also occurred on the spare pump about 4 years later. This time, the damage had progressed to catastrophic failure on the head side. At this failure, we replaced the impeller with one we had designed to incorporate short half-vanes at the OD to support the shrouds half way between the full-vanes.

The third failure occurred on the main pump that normally runs 24/7. With the current failure, the shrouds are bulged out about 1/8” and cracks are just visible at the corner where each vane meets the case-side shroud. The bulging looks like it is about the same on both shrouds. The impeller in our shop right now also has significant cavitation damage in the eye on the visible side of the vane back about 3/4” from the leading edge. I don’t believe we saw any cavitation damage on the previous two failures.

I am interested to know if anyone else has seen failures of this type. I would also, very much like to hear your opinions as to possible cause.


Johnny Pellin

 

[Artisi]

Johnny,

A mystery by the sounds of it!

 

[rmw]

The shrouds are bulging because the vanes are trying to straighten out.

Try that statement on for size.

rmw

 

[JJPellin]

rmw,

That would match better if the bulge was occuring at the vane locations. That is not the case. The bulge is occuring between the vanes and the bulge is greatest the further you are away from the vanes. I can't picture how this could be based on the vanes trying to straighten.

Johnny Pellin

 

[tr1ntx]

Well, the "cracks" turned out to be grooves eroded into the impeller, so these "bulges" aren't the result of erosion on the outer face, are they? I mean, what you're looking at as an outward bulge is not really the original width, is it, and then the thin area is where material has been eaten away?

I've seen some strangely "deformed" looking impellers come out of slurry pumps and when you set it next to a new one, you can see that it is actually the situation of a huge amount of material has been eroded.

 

[rmw]

JCP, in the scenario that I envisioned by popping that question, the bulging would have to be away from the vanes and in the center of the span between them.

It would also explain why the addition of the half vane which acts as a stiffener stops the bulging.

I can't pull up a specific memory (yet - it takes longer now) but I must have seen something like this with hot fans somewhere.

rmw

 

[pumpman50]

I've been in the pump business for 45 years and have only seen this problem once where shrouds have bellied out. It was due to failure of the discharge non return valve causing a violent flow back of the discharge product. We were amazed at the time how this had occurred.

As the vanes are thin and relativly unsupported due to only having the 3 vanes, distortion could probably occur in this manner but only if there is a severe backward flow of liquid. There would certainly be enough kinetic energy available in the system you describe for this to happen.

 

[PumpSmart]

TWO TIMES I HEAR SOMETHING LIKE THAT
ONE TIME WAS STRESS CORROSION CRACKING BECAUSE H2S CONTENT, RELATED INFORMATION YOU WILL SEE IN API610 STANDARD AND NACE.
SECOND TIME WAS BECAUSE VANE PASS FREQUENCY SYDROME.