Handbook of Turbomachinery Second Edition Revised - Ventech!

must be dealt with in attempting to optimize the sizing for a propellant
pump. When pump inlet Nss-values exceed approximately 8,000 (rpm-gpmft
units), an inducer is normally considered beneficial, and mandatory when
Nss exceeds approximately 10,000. Inducers are discussed in more detail in a
subsequent section. It can be shown that with normal attempts to optimize
engine and turbopump packaging, liquid rocket engine systems that operate
at chamber pressures in excess of approximately 300 psia cannot operate
without the benefit of the inducer’s added suction performance. In engine
systems that incorporate ‘‘low-pressure,’’ or ‘‘boost’’ pumps, a criterion for
sizing the head rise of the boost pump could be to avoid the use of an
inducer in the ‘‘high-pressure’’ pumping element. However, it should be
noted that the ‘‘low-pressure’’ pump itself is likely to be essentially a large
inducer. It is possible (or likely) that the final selection of propellant pump
specific speed will be driven by the maximum suction specific speed at which
the pump can operate (i.e., the level of inducer technology available to the
pump designer). It is often this constraint that results in the decision to
employ the ‘‘low-pressure’’ pumping element.
Referringagain toFig. 15,notethepresence andlocationofthe‘‘shaft
seal.’’ This seal serves two purposes—first, it prevents gross leakage of the
pumped fluid to maintain reasonable pump performance, and in some
installations, it (in conjunction with the ‘‘impeller rear seal,’’ also in Fig. 15)
provides a pressure drop at a specific radial location to set the pump’s axial
thrust load. In low to moderate chamber pressure engine systems, this seal is
often implemented as a spring-loaded face seal that engages a finely finished
surface on the rear of the impeller hub. Various materials (bronzes, coated
graphites, etc.) have served this seal’s purpose. The spring load ‘‘P’’ that is
required for effective sealing, together with the seal’s radial location (and
consequent rubbing velocity, ‘‘V’’) set an effective ‘‘power load’’ term for the
seal, denoted as ‘‘PV.’’ Depending on the seal material, leakage requirements,
and the pumped medium, this ‘‘PV’’-term is subject to various
constraints. The PV constraint for a nonreactive medium (such as Kerosene,
or RP-1) might be based on the resulting rate of seal wear, and the seal life
required. In a more aggressive medium (liquid oxygen, for example), the
maximum achievable PV may be based on avoiding ignition of the seal
component in the oxygen-rich atmosphere that exists in its vicinity. Upon
reaching a threshold of pump specific speed (the exact value depending on the
application), allowable PV values for spring-loaded face seals will be
exceeded, and other sealing systems, such as a labyrinth seal, will be required.
Whatever the particulars of a given situation, increasing pump specific
speed generally increases the difficulty of the shaft sealing problem and will
often present a fundamental constraint that must be dealt with during the
sizing and early design processes.
Copyright © 2003 Marcel Dekker, Inc.