Impeller damage

[sunloco]

Hi Buddies, we visited a water &sewage water co. The factory includes an inlet pump station and an out pump station. There are 6 split case pumps in each pump station.
The customer told us that the first 4 pumps of each pump station were all damaged at the shroud of the lead edge, They had noticed that the pressure of the discharge of the first 4 pumps in inlet pump station was 13.8 meters and that of the suction was 4 m. there should be no cavitation damage. Who knows what have happened to these pumps ? The pics please see the link below.

http://photo.163.com/photos/sunloco/69422874/

 

[insult2injury]

Upon a quick inspection, it does look like cavitation. Those are the regions where you would typically see cavitation damage on an impeller; however, with the extent of the damage, corrosion may be a factor as well. What is the content of the water/sewage? And what is the material of the impeller?

I2I

 

[Artisi]

You stated there shouldn't be any cavitation -- why do you say that. Just because there is + 4 metres on the inlet side doesn't mean that the pumps are cavitation free.

The photo's certainly seem to show classic cavitation -however, the damage is on underside of the blade (the side you can readily see) which indicates that the pumps are operating at over-capacity.

You need need to establish what the operating point is for each unit in relational to the original selection / selection and where the pumps are operating on their performance curves. You may find the pumps are way out on their curve.

I would also involve the supplier / manufacturer as you might neeed to make some changes to the pump once the duty has beeen established.

Naresuan University
Phitsanulok
Thailand

 

[JJPellin]

It is definately cavitation damage. As noted by Artisi, it is noteworthy that the damage is on the visible side of the vane which would normally be the low pressure side. I would be more suspicious that they were running the pumps at low flow and getting suction recirculation cavitation. But the advice is the same. Establish the flow rate that the pumps see and compare that to the rated flow and best efficiency point from the pump manufacturer.

I have seen similar damage in a water recirculation pump in our coker. In that installation, I found that the operators had pinched down on the minimum flow spill-back line in order to try and get more flow at high demand conditions. However, at another time in the cycle, the destination tank would fill up and the control valve would go closed. The pumps were running at very low flow rate for part of the cycle. In those pumps, the cavitation damage looked like what you are seeing but extended into the impeller hub where it had almost eaten the impeller in two pieces.

If you have a spill-back line, check to make sure it is in operation. If the flow is measured and can be trended, look at the variation in flow rate over time. If the flow cannot be measured, estimate it based on the head produced and the motor amps.

 

[EdStainless]

It really looks like you have some corrosion along with your cavitation.
What is the water quality and material of construction?
By the way, nice photo posting.

There may be a lot of inlet head, but is the flow nice and smooth? There may be too much inlet resriction to run all pumps at once.
When you run tests measure the intake/discharge pressures right at the pump.

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[bingopin]

As mentioned by JJPellin, check for low flow operation. Out of curiosity, what is the suction specific speed?

 

[Artisi]

Just to correct my previous comment - I must have been somewhere else when typing -
The damage on the underside (low pressure)side of the blade is typical of under-capacity NOT over-capacity. This is a mismatch of flow onto the blade where it impinges on the low pressure side as the flow into the impeller does enter into the high pressure side of the impeller as it should.
However, it is still necessary to establish flow rates, head etc so as to compare what is actually happening to what should be happening - without this information everything else becomes guess work.

Naresuan University
Phitsanulok
Thailand

 

[Enkidu]

hmmm - If there really is a significant margin over NPSHA, then I'd suspect a really high suction specific speed, and eye re-circulation - ie, the impeller is operating well below design capacity.
It happens - CI impellers are very susceptible to the damage.

 

[Pumpindex]

I agree with the previous poster as this is a cavitation problem; however, I will also take a look at how long did it took for the impellers to fail, material of construction and solids in the liquid. I think is more of a mixture of problems.

<a href="

http://www.pumpindex.com/">Pumps</a>

 

[JJPellin]

You added another picture since the first time I looked at this. The first picture raises additional issues. First, there appears to be a butterfly valve in the suction line very close to the pump inlet. This is not advisable. The butterfly valve will induce turbulence in the inlet flow and can contribute to cavitation problems. The suctin line passes through the wall and we can't tell from the picture what the configuration looks like after that. It is prefered to have a straight run of pipe into the suction of the pump before the first valve or transition fitting. You have a reducer and a butterfly valve and could have more outside that wall. Since the line is close to the ground, there is a chance that there is a horizontal elbow close by. That would put uneven velocity into the pump suction and make the situation even worse. The damage is cavitation. I assumed it was primarily suction recirculation cavitation from low flow operation. But the addition of the reducer, butterfly valve and possible elbows within a very few pipe diameters of the pump inlet could indicate additional issues. With turbulent flow at uneven velocities coming into the impeller eye, you could get recirculation type cavitation even at normal flow and even with adequate NPSH margin. They need to straigten out that suction line, replace the butterfly valve with a full port gate valve or ball valve and make sure there are no horizontal elblows within 6 or 8 pipe diameters. If the elbows cannot be eliminated, consider flow straightening vanes in the line.