Handbook of Turbomachinery Second Edition Revised - Ventech!

tional to clearance distance) with diffuser dynamic loads (which are inversely
proportional to clearance distance). Pump manufacturers have generally
determined the appropriate range of clearance by experiment, and the criteria
thus developed are difficult to defend on an analytical basis. However,
clearance values of 6 to 10% of the impeller diameter have given good results.
There are also cases where vane rows that carry the name ‘‘diffuser’’
have been incorporated into a centrifugal stage and perform little (if any)
diffusion. In very high-pressure rocket propellant pumps, the outboard edge
of the discharge volute (which resembles the letter C in cross section) can be
subjected to very large bending stresses due to the high pressure immediately
outboard of the impeller. The incorporation of a vane row at this location
provides a very convenient set of ‘‘struts,’’ which react the aforementioned
energy as tension in the vane row. Using a ‘‘diffuser’’ in this way is another
method of reducing volute (and thus pump) weight.
When the pumped fluid exits the diffuser (if any), it enters the
discharge volute. Seen from the outside, the volute resembles a spiral-shaped
horn, or scroll. Internally, the volute is a flow passage that is nearly spiralshaped
and with a flow area that gradually increases in the circumferential
direction. Figure 15 shows two cross sections of atypical discharge volute.
Discharge volutes are typically designed to one of two possible criteria.
Up to the late 1950s, the distribution of flow area in the volute was typically
tailored to maintain constant average velocity in the volute. Up to this
point, liquid rocket engine systems were typically fixed-thrust systems, and
the problems unique to a large, pump-fed variable thrust engines were not
yet fully appreciated. As a part of various investigations into pumps that
could operate across wider flow ranges, Report 936 of the NACA (by Brown
and Bradshaw) showed analytically the advantages of designing a volute
with an area distribution designed to conserve angular momentum.
It can be shown that when a volute is designed to conserve average
velocity, the angular momentum distribution is nonuniform and monotonically
decreasing as we proceed around the volute toward the pump
discharge. By applying the moment of momentum equation to the volute
bulk flow, and by making an assumption of a free-vortex distribution in the
radial direction, an impeller exit pressure distribution can be calculated.
When integrated in the circumferential direction, it can be shown that a
nonzero net radial load exists on the impeller. This load must be reacted by
the shaft-bearing system. This net radial load generally increases at higherthan-design
flow coefficients, and numerous component tests of propellant
pumps showed bearing distress due to this load during testing.
When the above constant-velocity area distribution is modified so as
to conserve angular momentum (i.e., rCy ¼ constant), the volute becomes a
diffusion device at and below the design flow coefficient. Moreover, the
Copyright © 2003 Marcel Dekker, Inc.