Mechanical (pump design)

[Thibo]

I have a condensate extraction pumps, a condensate temperature at pump inlet is 80 deg.celcius, we frequently (3 monthly averangely) encounter shaft breakages, we are convinced that the cause is cavitation due to a lot of other reasons and the more suitable solution would be to install a vertical pump howevder, there are restrictions, i just need an advice to prolong a running time, can using a more tough shaft materail assist in extending the opreating duration before the next shaft breakage? Since my theory is that the condensate is too hot on inlet which results in an increase of vapour pressure which causes a lot of loading and unloading of the impeelr each cycle and eventually the shaft breaks just on the shaft thread. if yes, how can I go about determining the forces/stresses induced on the shaft during cavitation?

 

[TenPenny]

Where is the shaft breaking?

With recurring shaft breakages, my first suggestion is to look at flow rates, the vast majority of recurring shaft break issues I've had were due to low flow running. And verify flow rates over several days; we had one case where it was insisted that the pump ran at Xgpm, after quite a lot of investigating, we found out that, 'well, during the night shift we run at Ygpm for about three or four hours. Every night.'

 

[JJPellin]

It seems very unlikely for cavitation to cause repeated shaft fracture. I would examine the fracture surface for indications of the failure mode. For high cycle fatigue in bending, I would look for internal or external misalignments or high side loads from belt drives. For torsional failures, I would look for torsional resonance or overload. You didn’t provide enough information for us to provide a meaningful answer. If you are not sure why the shaft is breaking and all you do is make the shaft stronger, I would expect something else to break.

Johnny Pellin

 

[GPRnD]

I agree with JJPellin in that cavitation is an uncommon cause for shaft fatigue.

Before we all launch into possible root causes, is it possible to share some information that might help focus discussions:

[ul]
[li]Where the pump is operating relative to BEP and NPSH margin[/li]
[li]What the vibration spectrum looks like[/li]
[li]Where the fracture is located (pump X sections and photos really help)[/li]
[li]What is the shaft material[/li]
[/ul]

Any info at all is really appreciated

 

[Artisi]

Are you using OEM shafts or el cheapo from the local pirates?
As JohhnyP has pointed out - not much info to go on.
More info - pump data, curve etc, pump duty - actual, not from the design data sheet, pictures of the installation and breakages would help.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Ziggypump]

Pump cavitation destroys impellers, haven't ever heard of it causing problems to the shaft. Look up pump cavitation online and look at the pictures.

 

[LittleInch]

your description (loading and unloading of the shaft) sounds much more like some sort of process issue involving slugging and surging rather than cavitation which is much more local impact on the impellor, which destroys the impellor, but not normally the shaft.

"we are convinced that the cause is cavitation" - please explain.

you might be getting a lot of vibration causing fatigue damage on the shaft, but if it is cavitating to that extent then your damage will be on the impellor, not the shaft.



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[TenPenny]

As others have said, I cannot imagine how cavitation can cause shaft damage, and I'm curious about how that conclusion was reached.

 

[desertfox]

Hi Thibo

Can you provide pictures of the broken shafts?

 

[ghoryme]

Shaft failure at unloaded end of shaft in multistage or double –suction pumps may be a fatigue failure in tension resulting from high cycle stresses induced in the shaft when the pump is operated in the discharge recirculation zone. Axial stresses can be reduced by increasing pump out put or installing recirculation line to bypass sufficient flow to move pump total flow rate beyond the point where damaging discharge reciculation occurs.
Other reason of high stress is wear ring clearance increase during life of pump increasing net thrust .Likewise unequal ring wear leads to uncertain changes in the thrust
( Ref: Pump Hand Book 3 rd Edition by Igor J. Karassik etc.)

 

[Artisi]

Looks like the OP has got lost or found the answer.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)