Cavitation damage to casing on suction side - Why?
Cavitation damage to casing on suction side - Why?
(OP)
I am investigating three treated water pumps (in the same installation) that have experienced significant cavitation damage to the pump casing on the suction side. The pumps typically operate with about 8m NPSH, have a cast iron casing and bronze impeller. There is no noticable cavitation damage to the impeller.
The pumps are conventional dual-suction split casing centrifugal water pumps from a reputable manufacturer. The inlet conditions are not the best, with three 90-degree bends (in the same direction) in quick succession leading to the pumps. Individually, the pumps are operating at about 500L/s at 30m head.
The pumps are operating at a specific speed (in US units) of approximately 3000, and a suction-specific speed of around 8000.
My initial thoughts are that this is due to a combination of the relatively high suction-specific speed combined with the poor inlet conditions, but I am puzzled by the fact that there is damage to the casing but not the impeller.
Any thoughts?
The pumps are conventional dual-suction split casing centrifugal water pumps from a reputable manufacturer. The inlet conditions are not the best, with three 90-degree bends (in the same direction) in quick succession leading to the pumps. Individually, the pumps are operating at about 500L/s at 30m head.
The pumps are operating at a specific speed (in US units) of approximately 3000, and a suction-specific speed of around 8000.
My initial thoughts are that this is due to a combination of the relatively high suction-specific speed combined with the poor inlet conditions, but I am puzzled by the fact that there is damage to the casing but not the impeller.
Any thoughts?





RE: Cavitation damage to casing on suction side - Why?
You're right about the piping configuration, not the best layout. That kind of configuration could be asking for prerotation, but I thought that usually leads to radial load problems, not casing cavitation.
Where's your pump operating with respect to the best efficiency point? Might be a matter of low flow recirculation in the casing causing the cavitation.
RE: Cavitation damage to casing on suction side - Why?
RE: Cavitation damage to casing on suction side - Why?
Look to modify the impeller, or get the correct pump since explaining the mistake will only get harder every time the owner repairs the pump.
BobPE
RE: Cavitation damage to casing on suction side - Why?
I haven't actually measured the head, but at a tapping point about 1m from the inlet to the pump I can get a reasonable fountain (at least 4-5m) if I open the valve on the tapping with the pump running.
The BEP is about 410L/s @ 41m. Typical operation 520L/s @ 31m, so about 25% to right of BEP. I agree with your comment on "bringing it back", but probably we are looking to pump replacement.
One other issue (I was trying to minimise complication on the original post!) is that there is also an abrupt contraction on the inlet taper where the contractor apparently found he had a DN600 to 450 taper discharging to a DN400 suction and just decided to bolt them together (don't ask me how, exactly!)
Agree with choosing the right pump! That's why I'm here! Some better design and workmanship wouldn't have gone astray either!
RE: Cavitation damage to casing on suction side - Why?
I have to admit though, I'd expect cavitation to show up in the impeller, though bronze is more cavitation resistant than cast iron. Maybe they're both experiencing the cavitation, but it's mild enough the bronze is able to handle it without any damage? Where exactly in the casing is the damage? Is it just inside the suction nozzle, or between the impeller and casing near the impeller eye?
RE: Cavitation damage to casing on suction side - Why?
BobPE
RE: Cavitation damage to casing on suction side - Why?
BobPE, there is a short length (about 500mm) of DN400 leading to the suction flange of the pump after the contraction. The contractor tried to reduce the contraction problem by tapering the cement mortar lining in the DN400, obviously with limited scope for any success.
The contraction loss has been factored into recent hydraulic calculations, but I cannot say whether this was done when it was originally designed.
Scipio, the suction piping bends are in different planes, but rotate the flow the same way. There is general agreement that there is significant pre-rotation of the flow, and this would be compounded through the conservation of rotational momentum in the reduction from DN600 to DN400.
The cavitation damage is predominantly outside and adjacent to the static wear ring.
It does seem to me from these discussions, however, that there is a relatively high-velocity and highly rotational "jet" into the suction side of the pumps - hardly good conditions but difficult to quantify the likely effects.
My problem is trying to quantify the effects so that the same mistake is not made with the new pumps, which are required to meet the same duty. I am considering some slower pumps (960RPM) with 600mm suction which would reduce the inlet velocity and pre-rotation, but cost a lot more money. If I can quantify the effects, then I can have greater confidence in the selection of the replacement pumps.
RE: Cavitation damage to casing on suction side - Why?
You are not drip feeding us LOL....We love this stuff as you can tell and we like asking questions in order to feel you out as well as help you....
Take care
BobPE
RE: Cavitation damage to casing on suction side - Why?
It is usually safe to assume cavitation by the damage you describe. However I am curious if you have proved this to yourself. One very quick test is: If you can hear the cavitation at full flow, reduce flow and see if the cavitation noise decreases or stops. Many centrifugal pumps become very quiet at shut off when they were obviously cavitating under max flow.
PUMPDESIGNER
RE: Cavitation damage to casing on suction side - Why?
Does this shed any light?
RE: Cavitation damage to casing on suction side - Why?
BobPE
RE: Cavitation damage to casing on suction side - Why?
I am puzzled by this. Look forward to a resolution, perhaps can learn something. What about chemical damage BobPE, think that could be it. (Just kidding).
PUMPDESIGNER
RE: Cavitation damage to casing on suction side - Why?
Some additional particulars:
1/ The cavitation damage is all on the suction side and predominantly on the inside and outside of the wear ring support spigot, although there is also significant damage to the flow splitter.
2/ Site tests have shown that under normal operating conditions NPSHA-NPSHR is about 2.0m, so although it is low it is probably not a direct cause of cavitation.
I have heard suggestions that the cause might be recirculation to the right of the BEP. Recirculation on the left of the BEP appears to be well understood - eddies occurring in the impeller because of the mis-match between vane angle and speed/flow at that particular point where the radial flow can't "keep up" with the blades. What I have read suggests that there are no problems to the right of BEP, but it has been suggested to me that the reverse conditions occur when the flow is well to right of BEP, with the blades unable to "keep up" with the radial flow with the result that recirculation also occurs but on the other side of the blade.
The resulting eddies on the inlet side of the pump, exacerbated by low NPSH, might be the cause of the problem.
Does this hypothesis have any basis?
RE: Cavitation damage to casing on suction side - Why?
You could have minor "cavitation" problem, not necesssarily a NPSHA/NPSHR consideration but flow separation or other problem already well addressed - with this being the case, it is possible that you have the insidious problem of cavitation - corosion- erosion especialy if there are any fines in the pumped liquor.