Page 243 - Chemical engineering design

PIPING AND INSTRUMENTATION
Pump discharge
Pump suction 0.06d C 0.4m/s 219
0.02d C 0.1m/s
Steam or vapour 0.2d m/s
where d is the internal diameter in mm.
Simpson (1968) gives values for the optimum velocity in terms of the fluid density.
His values, converted to SI units and rounded, are:
Fluid density kg/m 3 Velocity m/s
1600 2.4
800 3.0
160 4.9
16 9.4
0.16 18.0
0.016 34.0
The maximum velocity should be kept below that at which erosion is likely to occur.
For gases and vapours the velocity cannot exceed the critical velocity (sonic velocity)
(see Volume 1, Chapter 4) and would normally be limited to 30 per cent of the critical
velocity.

Economic pipe diameter
The capital cost of a pipe run increases with diameter, whereas the pumping costs
decrease with increasing diameter. The most economic pipe diameter will be the one
which gives the lowest annual operating cost. Several authors have published formulae
and nomographs for the estimation of the economic pipe diameter, Genereaux (1937),
Peters and Timmerhaus (1968) (1991), Nolte (1978) and Capps (1995). Most apply to
American practice and costs, but the method used by Peters and Timmerhaus has been
modified to take account of UK prices (Anon, 1971).
The formulae developed in this section are presented as an illustration of a simple
optimisation problem in design, and to provide an estimate of economic pipe diameter
that is based on UK costs and in SI units. The method used is essentially that first
published by Genereaux (1937).
The cost equations can be developed by considering a 1 metre length of pipe.
The purchase cost will be roughly proportional to the diameter raised to some power.
n
Purchase cost D Bd £/m
The value of the constant B and the index n depend on the pipe material and schedule.
The installed cost can be calculated by using the factorial method of costing discussed
in Chapter 6.
n
Installed cost D Bd 1 C F
where the factor F includes the cost of valves, fittings and erection, for a typical run of
the pipe.
The capital cost can be included in the operating cost as an annual capital charge. There
will also be an annual charge for maintenance, based on the capital cost.
n
Cp D Bd 1 C F a C b 5.10