Page 133 - HVAC Pump Handbook

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Physical Description of HVAC Pumps

130 HVAC Pumps and Their Performance

less than 70 percent or greater than 120 percent of the flow at the
BEP? Unfortunately, this is not the case in actual practice. Many
pumps are selected at 40 to 60 percent of BEP flow with the result-
ing wear on bearings and case rings. Many HVAC pumps require
continuous maintenance due to improper selection that results in
high radial thrusts in the pumps. A volute type pump operating near
its best efficiency point should run for years without maintenance.
The author has personal experience with variable speed, volute
pumps that have run for over 25 years without any evidence of wear
in the pump. There was no replacement of bearings, shaft sleeves, or
wear rings; mechanical seals were replaced periodically due to water
conditions.

5.7.2 Leakage control in centrifugal pumps
The unwanted flows in centrifugal pumps are from the packing or
mechanical seal on the pump shaft and from the impeller discharge
back to the impeller suction. Controlling these flows is an integral
part of the pump designer’s work. Both of these flows can affect the
performance of a pump. The packing or mechanical seal must limit
shaft leakage without affecting greatly the torque required to turn
the pump. Leakage from the impeller discharge to the suction reduces
the pump efficiency.
The leakage is a most undesirable flow in a centrifugal pump. The
basic method of controlling this flow is through the use of a ring on
the pump casing as shown in Fig. 5.20a. This ring is called flat, as it is
a simple ring with constant internal and external diameters. Large
pumps may have L-shaped rings to control bypassing. This ring is
also called a case wear ring, but there should be no wear on such
rings with proper control of radial thrust that is described elsewhere
in this book.
It is important that the clearance between the casing ring and the
impeller be controlled. This is a key part of volute pump design, as
the pump designer must design the rigidity of the pump shaft so that
the shaft does not touch the case ring during maximum allowable
radial thrust. Opening the radial clearance between the shaft and the
case ring will cause greater leakage and lower pump efficiency.
There has been a tendency to cheapen HVAC pumps by eliminating
the case ring as shown in Fig. 5.20b. This results in the bronze
impeller running against cast iron, and it diminishes the ability to
control the actual radial clearance. If the cast iron rusts or is worn
away, the entire volute must be replaced, while only the case ring
needs to be replaced when it is provided in the pump.

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