Page 448 - Chemical process engineering design and economics

Design of Flow Systems 429

pipe. For laminar flow in a pipe the velocity profile is parabolic and a = 1/2. If
the velocity profile is flat, a = 1. For very rough pipes and turbulent flow a may
reach a value of 0.77 [7]. Unless the kinetic energy term in the mechanical energy
balance becomes large compared to the other terms, it suffices to let a = 1 for tur-
bulent flow, which occurs in many engineering applications.
The second term in the mechanical energy balance is the change in potential
energy. The third term is "pressure work," and its evaluation depends on whether
the fluid is compressible or incompressible. The last two terms are the work done
by the system, W, and the friction loss, E. For an incompressible fluid, the density
may be removed from the integral sign. Then, Equation 8.1 becomes

2
A(v /a) g Ap
——— + — Az+ — (8.2)
2 gc gc P
VALVE SIZING

Valve size is not necessarily the same size as the pipe to which it will be con-
nected. It is frequently less. The valve orifice size and the shape of the valve
plug, shown in Figure 8.11, determine the valve size. To size a valve, the flow rate

Vena Contracta

Orifice

Figure 8.11 Throttling valve plug and orifice.

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