Page 199 - Centrifugal Pumps Design and Application

174 Centrifugal Pumps: Design and Application

motors in a world built around 50 and 60 Hz electric systems, or 3,000
and 3,600 RPM speed limits with two pole motors. This introduces the
need for speed-increasing gear systems, which must be justified in ex-
change for high-speed pump advantages. NPSHR increases with increas-
ing speed, placing limits on speed for a given NPSHA. Need arises to
improve suction performance as much as possible to extend speed limits.
Material capabilities must be recognized to keep stress levels within pru-
dent design limits. Modern seal technology is required to meet the de-
mands of combined high speeds and high pressures. Bearing design so-
phistication is frequently required to ensure reliable operation, and
recognition of the influence of bearings on shaft dynamics is often neces-
sary. Increased noise generation can occur with high-speed equipment
due to high power densities and lightweight construction.
Industrial acceptance of high-speed pump technology is illustrated by
Karassik, who has indicated that the introduction of high-speed boiler
feed pumps in 1954 was followed by the steadily increasing use of these
machines, culminating in total abandonment of the older 3,550 RPM
equipment by 1971.
This chapter will deal primarily with an unconventional pump type
particularly suited for operation at high to very high speeds to produce
typically very high heads at low to moderate flow rates. Although this
pump type has enjoyed wide acceptance in industry, comparatively little
on this design has appeared in the literature.
An early commercial application of this design began in 1959 for an
aircraft service. The pump was used in the Boeing 707 for takeoff thrust
augmentation in jet engines, at a time when engine power was relatively
low and the world had not caught up with the generally longer runstrip
requirements for jet aviation. This pump rotated at 11,000 RPM and de-
livered 80 GPM of water to the combustors at 400 psi (well above com-
bustor pressure to allow atomization), to increase the engine mass flow
rate and increase thrust by 15% for takeoff. The unit weighed only 8 ¥2
pounds, including step-up gearing from the 6,500 RPM power takeoff
pad to pump speed. Some 250 units were produced for this service.
In 1962, an industrial version of the new pump type rated to 100 HP
and 6,000 feet of head in a single stage was introduced to the petrochemi-
cal industry. In the past two decades, this concept has grown into a family
of products ranging from 1 to 2,500 HP, direct drive to 25,000 RPM and
heads to 12,000 feet. Most commonly these products consist of a single
high-speed stage, but as required, employ two or three stages to satisfy
need for extreme heads or the combination of high head and low
NPSHA. Over the past two decades many thousand machines of this type
have been placed in service around the world.