Handbook of Turbomachinery Second Edition Revised - Ventech!

is a parasitic device, whose flow requirements have to be added to the pump
flow capacity. If the balance piston is included in the initial design, this need
not present a problem. However, if the device needs to be added as a retrofit,
its flow requirement can penalize both the efficiency and the suction
performance of the pump to some degree. There have been past cases where
unanticipated high rotor thrust has necessitated the addition of a balance
piston. The addition of this device was both difficult and costly. As stated
previously, the accurate prediction of axial thrust during design is of the
utmost importance.
Using the techniques and references outlined in the preceding
paragraphs, it should be possible to compute representative axial thrust
forces for any desired turbopump component. The next important design
consideration concerns how these components are mechanically integrated
to form a complete turbopump.
Axial thrust considerations affect the mechanical design integration of
the turbopump in a number of ways. How are the turbine and pump
components to be coupled, and what will be their physical arrangement?
What types of axial thrust control devices are necessary, and how will they
fit into the overall design? If any axial thrust issues remain, can they be
resolved by some relatively simple mechanical design change(s)? If not, aero/
hydrodynamic modifications to turbine and/or pump elements may prove
necessary.
In many instances, the basic coupling and arrangement of turbopump
components will be dictated by considerations other than axial thrust, and
the designer will have little or no freedom to alter them. For example, in the
turbopumps for the Thor and Atlas missiles, the independent shafts for the
turbine and the twin pumps were made necessary by the large angular
velocity difference between the two. In this case, the turbine axial thrust was
reacted to by a suitably sized ball bearing on the turbine shaft. In the case of
the pumps, it worked out that the fuel (RP-1) and oxidizer (LOX) pumps
exerted axial thrust loads that were similar. In this event, the pumps could
be placed in a back-to-back arrangement on the same driveshaft such that
the pump thrusts were, for the most part, offset. The unbalanced thrust
again was reacted to by a suitable ball bearing. It should be noted here that
the ability to react to the steady-state unbalanced thrust with a rolling
element bearing was facilitated by the relatively low shaft angular velocity
(approximately 6,000 rpm) and the availability of an excellent bearing
lubricant in the form of the RP-1 fuel mixed with an extreme-pressure
additive. Few contemporary machines will enjoy this luxury.
A simple example of a direct-drive turbopump with thrust reacted to
by a rolling element bearing is shown in Fig. 28. In this example, the
pumping element is a large (12-in. diameter) inducer, which imparts a head
Copyright © 2003 Marcel Dekker, Inc.