Mechanical (pump design)
Mechanical (pump design)
(OP)
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?





RE: Mechanical (pump design)
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.'
RE: Mechanical (pump design)
Johnny Pellin
RE: Mechanical (pump design)
Before we all launch into possible root causes, is it possible to share some information that might help focus discussions:
- Where the pump is operating relative to BEP and NPSH margin
- What the vibration spectrum looks like
- Where the fracture is located (pump X sections and photos really help)
- What is the shaft material
Any info at all is really appreciatedRE: Mechanical (pump design)
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.)
RE: Mechanical (pump design)
RE: Mechanical (pump design)
"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
RE: Mechanical (pump design)
RE: Mechanical (pump design)
Can you provide pictures of the broken shafts?
RE: Mechanical (pump design)
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.)
RE: Mechanical (pump design)
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.)