Page 263 - Chemical process engineering design and economics

Compressors, Pumps, and Turbines 243

of 67 % at a rotational speed of 31,000 rpm. The recovered power can be used
to drive a pump or compressor.
The energy available in a high-pressure liquid or gas process stream must
be balanced by the energy required by a compressor or pump. Thus, the power
delivered by an energy-recovery turbine must be absorbed at the same rate by
the compressor or pump. Also, a gas turbine, turboexpander, and hydraulic tur-
bine require an electric motor or steam turbine for starting the driven machinery.
Figure 5.22 illustrates a system for starting an axial compressor which is driven
by an expander. At startup, the energy from the expander is not available so that
the electric motor drives the compressor. When the processes approaches steady
operation, the expander supplies some of the energy to operate the compressor.
Eventually, the process reaches steady state, and the motor may continue
to supply power to the compressor if there is insufficient power delivered by the
expander. If the power delivered by the expander exceeds the power needed by
the compressor, then the excess power will be absorbed by the generator and
delivered to the plant's electrical-distribution system.

PUMPS

Like compressors, pumps are divided into two main categories according to their
principle of operation, positive-displacement or dynamic. In positive-
displacement pumps, pressure is developed by trapping a quantity of liquid in a
chamber and then compressing it to the discharge pressure. In a dynamic pump,
the fluid first acquires kinetic energy which is then converted to pressure. The
classification of pumps according to this scheme is shown in Figure 5.24, and
some characteristics of selected pumps are given in Table 5.13. Examples of
these pumps are shown in Figure 5.25 For a more detailed discussion of pumps
than will be given here, the reader should refer to Holland and Chapman [45].

POSITIVE-DISPLACEMENT PUMPS
The characteristic feature of positive-displacement pumps is that ideally they will
deliver the same volume of liquid at every stroke regardless of the discharge pres-
sure. In practice, the flow rate will decrease with increasing pressure because of
increasing leakage pass the seals. This is shown by the characteristic curve in Fig-
ure 5.26. The characteristic curve, which is supplied by the pump manufacturer, is
a plot of pressure or head against the flow rate of water. Head is the height of a
column of liquid that exerts a pressure equal to a given pressure. The difference
between the ideal and actual flow rate is called slip. A very high pressure will be
developed if the discharge line of a positive-displacement pump becomes blocked.
Thus, in order to prevent damage to the pump and piping, a pressure relief valve
must be installed across the pump. As soon as the design pressure is exceeded,
the relief valve automatically opens and discharges liquid into the pump inlet. Be-

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