Page 245 - Chemical engineering design

PIPING AND INSTRUMENTATION
The equation can be simplified by substituting typical values for the constants.
A The normal attainment for a chemical process plant will be between 221
90 95%, so take the operating hours per year as 8000.
E Pump and compressor efficiencies will be between 50 to 70%, so take 0.6.
p Use the current cost of power, 0.055 £/kWh (mid-1992).
F This is the most difficult factor to estimate. Other authors have used
values ranging from 1.5 (Peters and Timmerhaus (1968)) to 6.75 (Nolte
(1978)). It is best taken as a function of the pipe diameter; as has been
done to derive the simplified equations given below.
B, n Can be estimated from the current cost of piping.
a Will depend on the current cost of capital, around 10% in mid-1992.
b A typical figure for process plant will be 5%, see Chapter 6.

F, B,and n have been estimated from cost data published by the Institution of Chemical
Engineers, IChemE (1987), updated to mid-1992. This includes the cost of fittings, instal-
lation and testing. A log-log plot of the data gives the following expressions for the
installed cost:
Carbon steel, 15 to 350 mm 27 d 0.55 £/m
Stainless steel, 15 to 350 mm 31 d 0.62 £/m
Substitution in equation 5.12 gives, for carbon steel:
0.53 0.03 0.37
d, optimum D 366 G
Because the exponent of the viscosity term is small, its value will change very little
over a wide range of viscosity
2
D 10 5 Nm s 0.01 cp , 0.03 D 0.71
at 2 2 0.03
D 10 Nm s 10 cp , D 0.88
Taking a mean value of 0.8, gives the following equations for the optimum diameter,
for turbulent flow:

Carbon steel pipe:

d, optimum D 293 G 0.53 0.37 5.14
Stainless steel pipe:
d, optimum D 260 G 0.52 0.37 5.15

Equations 5.14 and 5.15 can be used to make an approximate estimate of the economic
pipe diameter for normal pipe runs. For a more accurate estimate, or if the fluid or pipe
run is unusual, the method used to develop equation 5.13 can be used, taking into account
the special features of the particular pipe run.
The optimum diameter obtained from equations 5.14 and 5.15 should remain valid
with time. The cost of piping depends on the cost power and the two costs appear in the
equation as a ratio raised to a small fractional exponent.
Equations for the optimum pipe diameter with laminar flow can be developed by using
a suitable equation for pressure drop in the equation for pumping costs.