Page 156 - Know and Understand Centrifugal Pumps

Shaft Deflection

The coefficient of thermal expansion of 316 stainless steel is 9.7 x 10-6
in/in per degree Fahrenheit. The metric equivalent is 17.5 x 10-6
mm/mm. per degree Centigrade. See the next Table and note the
expansion on a pump whose centerline is 10 inch above its base.

ATEMPERATURE ATEMPERATURE THERMAL EXPANSION
F" c" Inches Millimeters
100 "F 55 "C 0.0097 0.245
200 "F 110 "C 0.01 90 0.490
300 "F 165 "C 0.0291 0.735
400 "F 220 "C 0.0388 0.900
500 "F 275 "C 0.0485 1.230
600 "F 330 "C 0.0582 1.470

As you can see, the pump casing will grow against its shaft almost 0.030
inches with an increase of 300°F. There are many tolerances in a pump
that are tighter than 0.030 inches. This means a rotary element will
scrape and rub a stationary element.
You may even see the same evidence of gouges and wear around the
circumference of strict tolerance rotary elements, leaving a corres-
ponding spot on the stationary elements at any other point around the
volute circle of the pump.

This condition is probably misalignment, indicating a maintenance
problem. The mechanic should be trained to correct this. Follow
correct alignment procedures, as well as correct bolt torque procedures.

Inspect gasket surfaces for knots and irregularities. Look for bent dowel
pins and misaligned jack bolts, dirt and any other factor that might lead
to misalignment.
Next we'll discuss evidence marks and prints that arc different, but to
the untrained eye, they may appear the same. You may see a spot or arc
of wear and gouging on the rotary elements, and a circumferential wear
circle on the bore of the close tolerance stationary elements. This is a
maintenance-induced problem. This is the sign of a physically bent
shaft, or a shaft that is not round, or a dynamic imbalance in the shaft-
sleeve-impeller assembly. The solution is to put the shaft on a lathe or
dynamic balancer, verify its condition, and correct before the next
installation.
The next condition and physical evidence we'll mention is rare, but we
need to cover it in case you should ever see it. You might see scratch
and gouge marks all around the circumference of strict tolerance rotary
element ODs, and stationary element bores alike. This condition and
marks is evidence of a 'Lack of Control'. It could be from any of the

139