Page 452 - Chemical process engineering design and economics

Design of Flow Systems 433

For adequate process control, the pressure drop across the valve for a linear
valve is

(Ap)v
—————— = 0.33
AH' + (Ap) v

(Ap) v = (0.33 / 0.67) (15) = 7.388 psi (0.510 bar)

Substituting into Equation 8.7, the valve size is
( 1.2 V' 2
C v = 65 I ——— I = 26.20
I 7.388 )

or, after rounding, C v = 26. Now, a valve can be selected from a manufacturer's
catalog.

PIPE SIZING

Pipe sizing consists of determining the diameter, length, and wall thickness.

Pipe Length

Determining pipe length for a flow system is a simple problem. After locating all
equipment, the length of pipe is automatically determined. Piping is nearly always
connected from one process unit to another by making ninety-degree turns. Occa-
sionally, a forty-five degree turn is needed.

Pipe Diameter

Threaded piping is available in 12 in (30.5 cm) or smaller, but is usually used in
sizes 2 in (5.08 cm) and smaller because fabrication costs increase rapidly above 2
in (5.08 cm) [1]. Threaded piping is used mostly for utilities and welded piping for
process piping [31].The inside diameter of a pipe could be calculated by optimiz-
ing pumping and piping costs. As the inside diameter of the pipe increases, the
liquid velocity decreases, and the cost of pumping decreases. This occurs because
the factional pressure loss decreases with a decrease in liquid velocity. On the
other hand, as the pipe diameter increases, its weight increases, and the installed
cost of the piping increases. As illustrated in Figure 8.13, the pipe diameter se-
lected is at the total minimum cost. For most purposes, such as rough or prelimi-
nary designs, and for small installations, this calculation is not necessary. Rules-
of-thumb are sufficient. Ludwig [13] lists velocities for several liquids and pipe

Copyright © 2003 by Taylor & Francis Group LLC