Case casting shrinkage


Q  -  We built a 12-stage horizontal, axial-split pump in duplex stainless steel
casing material. It will be used in an offshore water injection service.

When we first assembled the pump, its rotor did not fit properly into the casing.
The shaft was machined exactly per the drawing but appeared to be 1/4" longer
than the casing. One outermost impeller was slightly off-center from the volute
centerline.

To fix the problem we stripped the rotor, machined a new slightly-shortened shaft,
and built a  new rotor. In my 5 years of working with the company, we have built
few 12-stage pumps and this was our first in duplex stainless steel casing. What
went wrong, and how can we avoid this mistake in the future?



A  -  Castings in duplex stainless steel (and other special materials) shrink more
than those of carbon steel, or cast iron. This can be a problem in a multi- stage
pump where the over-all length of the case casting is relatively long.

This will not be a problem if several case patterns are available for different
casting materials because the necessary casting shrinkage adjustment can be
built into the pattern. But for economic reasons, manufacturers typically use the
same pattern equipment for different casting materials.

One way of preventing this type of problem from happening is to get a full-size
mylar print of the casing machining drawing. The transparency print is laid over
the casting to check for shrinkage and casting irregularities. Corrective actions
can be taken early on in the manufacturing cycle, such as making adjustment in
the shaft length to compensate for casting shrinkage, if needed.


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