Cavitation, Why?

[791018]

I found many pitted on our Maxidur 3 blade Dredgepump Impeller, also at surface of inner pump casing. Standard pressure at suction ( refer to iHC standard) is average between 4 to 5 bar, and its achieved. Pitted holes due to cavitation cause the impeller running out of balance, and lately will damaged the impeller shaft in the gearbox. we've done the stability test with the orifice, and its ok. Suction and discharge pressure was also balanced in their range. Pipeline distance was also within the range. what other factor should i check?

 

[MortenA]

That the vapour pressure of the fluid is as expected?

Best regards

Morten

 

[Latexman]

Any dissolved gases?

Good luck,
Latexman

 

[TBP]

Is it possible that it's corrosion pitting, and not cavitation damage?

 

[791018]

in drdging works, there's a different pressure when flushing of water and pumping of slurry. the pressure was within the specification. if it was corrosion pitting, it should be distributed uniformly. the impeller was installed 6 month ago. normally, when pumping of slurry or sand, the impeller could not corroded, it should worn. i'm very sure, its about cavitation.

 

[25362]

Thread378-142556 and attached links may bring you some answers, in particular when dealing with slurries containing settling scree, shingle and gravel larger than [½] inch.

 

[bulkhandling]

791018,

If you have got your answer, please post it here!

Or, more detailed data about your application would be helpful. Such as suction pipe size, length, pipe fittings, minimum and maximum suction head, rated flowrate, slurry density, particle size, solids percentage, pump size, pump material, discharge pipe size, length, static head, and pump curve.

If your pump has frequent start-stops, suggest you check the run-out conditions. At the beginning of a start, the discharge head is very small so that pump will run-out with a high flowrate for a short time. At this flowrate, the NPSH required is normally much higher than your design operating point, and the friction loss in the suction pipe is also much higher, so a surge happens.

 

[791018]

hello frenzz
after got some hint from u guys, we upgrading our pump and piping system. i'm now in process to monitor the improvement.
before this, our pump running at 1550rpm and it create a suction pressure at 6 - 7 bar at 6 bar discharge pressure. the minimum flow rate we got is about 0.8 cu.m/sec. we were dredge a fine at +-60% mix with +-40% of water. the material size is range between 50micron to 2mm.
after some checking, i found our discharge pipe is larger than our suction, about 536mm:511mm. its 255mm difference. can i says his just can give a small effects to produce cavitation in our pump? any suggestions?

 

[791018]

hello frenzz
after got some hint from u guys, we upgrading our pump and piping system. i'm now in process to monitor the improvement.
before this, our pump running at 1550rpm and it create a suction pressure at 6 - 7 bar at 6 bar discharge pressure. the minimum flow rate we got is about 0.8 cu.m/sec. we were dredge a fine at +-60% mix with +-40% water. the material size is range between 50micron to 2mm.
after some checking, i found our discharge pipe is larger than our suction, about 536mm:511mm. its 25mm*** difference. can i says his just can give a small effects to produce cavitation in our pump? any suggestions?

*** correction to previous post

 

[Artisi]

a few photo's would help our evaluation of your problem

Naresuan University
Phitsanulok
Thailand

 

[791018]

i'll try to post an image. However, let me firts monitor the improvements of new installed pump casing and suction/discharge pipe.
TQ

 

[bulkhandling]

The following link is to a cavitation article that may be helpful:

http://www.mcnallyinstitute.com/01-html/1-3.html

 

[Artisi]

probably wise to post some photo's as soon as you can, fitting new impeller and casing won't solve the problem.

Naresuan University
Phitsanulok
Thailand

 

[bulkhandling]

Agree with Artisi. And the bigger discharge pipe than suction pipe does not necessorily cause the problem of cavition although a bigger suction pipe size may ease the problem.
I do not quite get the meaning of "it create a suction pressure at 6 - 7 bar". Does the system have a negative suction head - suction elevation lower than the pump center line? Where did you measure the suction pressure? Did you do NPSHa calc and check against NPSHr? What are the data?

 

[791018]

bulkhandling
the pump is used at a cutter suction dredger where the suction head is varies due to elevated suction. thats why the pressure assumed to be negative (so my pessure is about -6 to -7 bar). I'll try do do the NPSHa calc and i'll comeback with the data.

 

[Artisi]

Seems to be some confusion with the inlet side numbers - the suction pressure can not be minus 6 or 7 bar.
I think you should describe the pump configuration and installation so we an assist.
Also what material is Maxidur?

Naresuan University
Phitsanulok
Thailand

 

[791018]

maxidur is the model of the pump. actually the material used is a chromium carbide cast iron. see thread330-144660 for details.
I agree with you, the suction pressure cannot be minus. Just because there is two gauge to display suction pressure and discharge pressure, suction gauge must be noted with minus sign to differ it from discharge pressure gauge. Sorry for any cunfusion

 

[bulkhandling]

From the pictures posted, I believe it's a pump problem, not a metallurgy problem. The system configuration has to be modified.

Strongly suggest you provide the following:
Pump suction elevation (range)
Pump center elevation
Pump discharge elevation
Suction pipe size (known), length, if any other major fittings
Pump size, pump curve (you can photo it)
Dicharge pipe size (known), length.

Pump curve is necessory with the NPSH requirement.

Pump may constantly running at run-out condition (far end of curve) or close to dead-head condition.

 

[Artisi]

I assume the pics shown on thread 330-144660 are the pump your talking about.
All I see in these pics is normal hi-hard casing wear - the cracks though are something else - and can only make the following observations
1. a large solid passingt hrough the pump and jamming against the case.
2. the pump running against a blocked suction or discharge resulting in boiling the water in the case - resulting in thermal shock if the blockage suddenly clears.

Naresuan University
Phitsanulok
Thailand

 

[791018]

i not yet post a pictures of my impeller which had a pitted on it. I'll try to post it ASAP.