Page 246 - Centrifugal Pumps Design and Application

Double-Case Pumps 219

so that the thermal expansion is away from the coupling. This maintains
the axial gap at the pump-to-driver coupling.

Design Features for Pumping Hot Oil with Abrasives

One of the most difficult double-case pump applications is the pumping
of hot oil (500°F and above) with substantial quantities (2% or more) of
entrained abrasive solids.

Surface Coating. The key to prolonged periods of operation without
maintenance is the application of a hard surface coating, which may ex-
tend service life by a factor of 4 or more. The coating should have a mini-
mum hardness of 60 Rockwell C. It should be applied to all wear sur-
faces, to all accessible hydraulic passages in impellers and inner cases,
and to the outside of the impeller shrouds. The coating is typically ap-
plied with a high-velocity spray process that produces a strong mechani-
cal bond. High coating density and proper coating thickness are critical,

Impeller and Case Wear Rings. Impellers designed with extra stock on
the integral wear ring surfaces are generally preferable to replaceable im-
peller wear rings. Worn impeller wear ring surfaces can be re-coated and
ground to size. They run against case wear rings that are coated in the
bores and on the ends, and are sized to match the impeller running sur-
faces.

Key ways. Unless special design precautions are observed, rapid erosion
occurs in keyways that are subjected to more than one stage of differen-
tial pressure. The two center stage impellers of opposed-impeller type
pumps should be welded together at the hubs, and the key (or keys) ter-
minated blind in a relief. A shrink-fit land is provided between the impel-
ler bore and the shaft at the high-pressure end to seal against leakage and
prevent erosion. Similar construction should be used for the sleeve under
the throttle bushing of an opposed-impeller pump, or under the balancing
drum of a pump with inline impellers.

Double-Case Pump Rotordynamic Analysis

The rotordynamic analysis requirements for a double-case pump de-
pend on the size, rotational speed, and horsepower of the specific pump.
Dry and wet critical speed analyses are adequate for small and medium-
size pumps running at 5,500 rpm and below. This type of analysis is de-
scribed in Chapter 19. Most current specifications only require critical
speed analyses, and a full scale test with specified vibration limits.