Handbook of Turbomachinery Second Edition Revised - Ventech!

In addition to shaft seals, shaft bearing systems (also shown in Fig. 15)
can present problems and constraints in regard to basic specific speed sizing.
In particular, rolling element bearings (i.e., ball and roller bearings) are
prone to difficulties in the environment presented by a liquid rocket engine
system. Attempts to optimize pump specific speed by raising rotating speed
typically raise the rotational speed of a rotating element bearing to a point
where the internal bearing loading and heat generation become troublesome.
In a liquid rocket engine system environment, suitable lubricant fluids
are usually absent, and a ball or roller bearing is simply cooled using a
metered amount of the pumped fluid. In all cases of which this author is
aware, these fluids lack the viscosity required for formation of the typical
EHD film that is considered necessary for ‘‘lubrication’’ in the normal sense.
In addition, many of these fluids (liquid oxygen, for example) are chemically
aggressive. For industrial-grade ball bearings, DN-values in excess of
500,000 (bearing ID in mm multiplied by operating speed in rev/min)
degrade life noticeably. When proper lubrication is absent, prediction of
results before the fact becomes virtually impossible. Bearing DN-values of
1.6 million have been achieved using liquid oxygen as a coolant. In these
cases, bearing lifetimes have proven spurious, unrepeatable, unpredictable,
and sometimes less than a single required engine firing.
The dynamics of an operating turbopump rotor may also limit the
achievable pump specific speed. Although a detailed discussion of rotor
dynamics is beyond the scope of this chapter, it should suffice to say that
successful operation of a pump rotor at speeds substantially in excess of 1 or
more critical speeds is highly dependent on the availability of adequate
damping at bearings and seals to suppress resonances or oscillations at
subsynchronous frequencies. This is often a problem in liquid hydrogen
turbopumps due to the extremely high head rises involved.
A relatively conservative design philosophy might set a propellant
pump’s specific speed at 1,500 Ns units as a ‘‘first cut’’ for further evaluation.
At this N s-value, an isentropic efficiency close to the maximum is usually
attainable. In addition, suction performance, sealing, and fabrication
difficulties are kept to a minimum. In the event that pump weight exceeds
that allowable, Ns can be increased in an incremental fashion until the
component’s weight requirement is met, or until another fundamental
constraint becomes evident.
Inlet Sizing
When a workable Ns sizing has been achieved, sizing of the pump inlet is
normally the next order of business. A centrifugal pump stage is normally
sized to one of two criteria. The first is inlet sizing for maximum efficiency.
Copyright © 2003 Marcel Dekker, Inc.