pitting in the impeller (vertical pump)

[mjsamman]

Dear All,

I have an issue of an impeller of the following specification:
- [highlight #8AE234]pump type:[/highlight] vertical mixed flow pump
- [highlight #8AE234]rotation speed:[/highlight] 355 rpm
- [highlight #8AE234]impeller:[/highlight] (Dia. 1485 mm) , Material: Gr.1 A CD-4MCu , Standard: ASTM A890
- [highlight #8AE234]Design Point[/highlight] (Flow : 30800m'/h, Head : 15.5m)
- [highlight #8AE234]Nature of fluid:[/highlight] SEA-WATER
- [highlight #8AE234]maximum Temperature:[/highlight] 33·c
- [highlight #8AE234]Available NPSH at pump impeller axis[/highlight] 12.65 mFG
- [highlight #8AE234]Required NPSH at pump impeller axis[/highlight] 7.2(3%)mFG

Finding
- Some irregular shape pitting in the blade in many location (pictures attached )

Questioning
- the reason of the pitting?
- what kind of corrosion or erosion happened?
- Could it be material and casting problem or long term mis-operation problem

Request:
- recommended repair procedure

 

[LittleInch]

That looks to me like a materials issue and may be something to do with the heat treatment of the blades during manufacture.

you really need to get a good materials engineer involved here or look into the material / corrosion forums I think. It's not really a pump problem per se.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Strong]

The leading edge of the blades looks very sharp. Do someone grind the edge to make an "improvement"? Looks like Cavitation damage. It is in a unusual location. Perhaps from poor inlet flow conditions or air vortex.

Walt

 

[Artisi]

Certainly worth getting a metallurgist involved, but I wouldn't dismiss some hydraulic component in the damage you have shown - seems to be a similar location on each blade.
Pre-rotation, entrained air / vortex.
Have you been in contact with the manufacturer?

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.)

 

[LittleInch]

Can you say which side of the pump we're looking at?

Is it going clock wise from the view in your picture and we're looking at the higher pressure side or counter clockwise and we're looking at the low pressure side?

what's the reason for the bog holes in the root of the blade?

Any chance you can post a sectional view of the pump in its finished position?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

I agree, could me metallurgy or hydraulic. I have seen both do this type of thing.
This being a duplex stainless should have been solution annealed and water quenched after casting.
Though I would expect worse issues along where the blade meets the hub.
IT could be related to casting defects.
You really need to get the manufacturer involved.

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P.E. Metallurgy, Plymouth Tube