22-10-2012, 01:53 PM
Pumps & Hydraulic Turbines
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Pumps
The most common types of pumps used in gas processing
plants are centrifugal and positive displacement. Occasionally
regenerative turbine pumps, axial-flow pumps, and ejectors
are used.
Modern practice is to use centrifugal rather than positive
displacement pumps where possible because they are usually
less costly, require less maintenance, and less space. Conventional
centrifugal pumps operate at speeds between 1200 and
8000 rpm. Very high speed centrifugal pumps, which can operate
up to 23 000 rpm and higher, are used for low-capacity, high-head
applications. Most centrifugal pumps will operate with an approximately
constant head over a wide range of capacity.
Positive displacement pumps are either reciprocating or rotary.
Reciprocating pumps include piston, plunger, and diaphragm types.
Rotary pumps are: single lobe,multiple lobe, rotary vane, progressing
cavity, and gear types. Positive displacement pumps operate
with approximately constant capacities overwide variations in head,
hence they usually are installed for services which require high
heads at moderate capacities. A special application of small reciprocating
pumps in gas processing plants is for injection of fluids (e.g.
methanol and corrosion inhibitors) into process streams,where their
constant-capacity characteristics are desirable.
NET POSITIVE SUCTION HEAD
See NPSH definition in Fig. 12-1. There should be sufficient
net positive suction head available (NPSHA) for the pump to
work properly, without cavitation, throughout its expected capacity
range. Usually a safety margin of about 0.6 to 1 m of
NPSHA above NPSHR is adequate. Cavitation causes noise,
impeller damage, and impaired pump performance. Consideration
must also be given to any dissolved gases which may
affect vapor pressure. For a given pump, NPSHR increases
with increasing flow rate.
CENTRIFUGAL PUMPS
Figs. 12-6a through 12-6e are cross-sectional drawings
showing typical configurations for five types of centrifugal
pumps. A guide to selecting centrifugal pumps is shown in
Fig. 12-7. Horizontal centrifugal pumps are more common;
however, vertical pumps are often used because they are more
compact and, in cold climates, may need less winterizing than
horizontal pumps. The total installed cost of vertical pumps is
frequently lower than equivalent horizontal pumps because
they require smaller foundations and simpler piping systems
Series and Parallel Operation
Often pumps are installed in series or in parallel with other
pumps. In parallel, the capacities at any given head are
added; in series, the heads at any given capacity are added. A
multi-stage pump is in effect a series of single stage units.
Figs. 12-12 and 12-13 show series and parallel pumps curves,
a system curve, and the effect of operating one, two or three
pumps in a system. In both figures, the operating points for
both pumps "A" and "B" are the same only when one pump is
operating. For 2 or 3 pumps operating, the points are not the
same because of the pump curve shapes. Hence, due consideration
should be given to the pump curve shape when selecting
pumps for series or parallel operation.
Parallel operation is most effective with identical pumps;
however, they do not have to be identical, nor have the same
shut-off head or capacity to be paralleled. When pumps are operating
in parallel it is imperative that their performance
curves rise steadily to shut-off. A drooping curve gives two possible
points of operation, and the pump load may oscillate between
the two causing surging.