Page 224 - Chemical engineering design

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PIPING AND INSTRUMENTATION
axial above 7000. Doolin (1977) states that below a specific speed of 1000 the efficiency
of single-stage centrifugal pumps is low and multi-stage pumps should be considered.
For a detailed discussion of the factors governing the selection of the best centrifugal
pump for a given duty the reader should refer to the articles by De Santis (1976), Neerkin
(1974), Jacobs (1965) or Walas (1983).
Positive displacement, reciprocating, pumps are normally used where a high head is
required at a low flow-rate. Holland and Chapman (1966) review the various types of
positive displacement pumps available and discuss their applications.
A general guide to the selection, installation and operation of pumps for the processes
industries is given by Davidson and von Bertele (1999) and Jandiel (2000).
The selection of the pump cannot be separated from the design of the complete piping
system. The total head required will be the sum of the dynamic head due to friction
losses in the piping, fittings, valves and process equipment, and any static head due to
differences in elevation.
The pressure drop required across a control valve will be a function of the valve
design. Sufficient pressure drop must be allowed for when sizing the pump to ensure that
the control valve operates satisfactorily over the full range of flow required. If possible,
the control valve and pump should be sized together, as a unit, to ensure that the optimum
size is selected for both. As a rough guide, if the characteristics are not specified, the
control valve pressure drop should be taken as at least 30 per cent of the total dynamic
pressure drop through the system, with a minimum value of 50 kPa (7 psi). The valve
should be sized for a maximum flow rate 30 per cent above the normal stream flow-rate.
Some of the pressure drop across the valve will be recovered downstream, the amount
depending on the type of valve used.
Methods for the calculation of pressure drop through pipes and fittings are given in
Section 5.4.2 and Volume 1, Chapter 3. It is important that a proper analysis is made of
the system and the use of a calculation form (work sheet) to standardize pump-head calcu-
lations is recommended. A standard calculation form ensures that a systematic method
of calculation is used, and provides a check list to ensure that all the usual factors have
been considered. It is also a permanent record of the calculation. Example 5.8 has been
set out to illustrate the use of a typical calculation form. The calculation should include
a check on the net positive suction head (NPSH) available; see section 5.4.3.
Kern (1975) discusses the practical design of pump suction piping, in a series of
articles on the practical aspects of piping system design published in the journal Chemical
Engineering from December 1973 through to November 1975. A detailed presentation
of pipe-sizing techniques is also given by Simpson (1968), who covers liquid, gas and
two-phase systems. Line sizing and pump selection is also covered in a comprehensive
article by Ludwig (1960).

5.4.2. Pressure drop in pipelines
The pressure drop in a pipe, due to friction, is a function of the fluid flow-rate, fluid
density and viscosity, pipe diameter, pipe surface roughness and the length of the pipe.
It can be calculated using the following equation:
u 2
P f D 8f L/d i 5.3
2