Handbook of Turbomachinery Second Edition Revised - Ventech!

Although shrouded centrifugal impellers have been most widely used,
the open-faced design bears mention. As its name implies, this type of
impeller contains no front shroud, with the result that the entire impeller
blade arrangement can be seen. The impellers in the propellant pumps for
the Titan launch vehicle have used the open-faced design, as have several
experimental high-pressure liquid hydrogen pumps built under government
sponsorship. This configuration has the obvious advantage that the entire
impeller blade array can be easily accessed, which makes manufacturing
relatively simple, especially with the advent of numerically controlled
machining. Open-face impellers do, however, have some drawbacks.
Perhaps the most serious is the precise control that is required of the
clearances between the impeller blades and the front of the pump housing.
Pump developed head can fall off rapidly as this clearance increases, due to
the increased energy dissipated by recirculation in the area of the gap. This
situation can make open-faced impellers impractical in a turbopump where
axial shaft motion is required. Incidentally, there have been impellers
designed that were both open-faced and using back-face antivortex ribs for
axial thrust control. These types of impellers can be particularly troublesome
by virtue of the fact that two axial clearances must be closely
controlled for the pump to function correctly. The reliable history of pumps
of this type bears testimony to the craftsmanship of their builders. The other
potential drawback to open-faced impellers is that without a front shroud,
the impeller generates a different set of axial thrust forces than a shrouded
impeller. Whether this is a problem or not depends, of course, on the design
of the turbomachine for which the impeller is intended. It has, however,
caused some difficulties in the past.
Before we proceed to some examples of turbopump design configurations,
it would be desireable to summarize the axial thrust control devices
that are in common use. As stated above, impeller seals or wear rings fall
into this category, as do balance ribs or antivortex ribs on the back face of a
centrifugal impeller. Rolling element bearings of several types are commonly
used in conjunction with seals and ribs and are used to react any residual
rotor thrust that may exist. Ball bearings of various kinds, including deep
groove balls, angular contact balls, and split inner race balls, have all been
used successfully as thrust bearings in production-type machinery. To this
author’s knowledge, tapered roller bearings have not been used in this
capacity. However, current trends in rocket turbomachinery design are (and
have been for some time) resulting in machinery that is both physically
smaller and operating at higher pressures. This combination has been
consistently generating resultant rotor axial forces that are beyond the
capability of rolling element bearings to react to for any length of time. To
react to these high forces, a device known as a series flow balance piston has
Copyright © 2003 Marcel Dekker, Inc.