Page 447 - Chemical process engineering design and economics

428 Chapter 8

reaches a preset value, the safety valve opens, dumping the reactor contents into a
holding tank. Also, control valves can be designed to fail wide open if the air sup-
ply fails so that the cooling-water flow rate is a maximum to prevent the reactor
from overheating.

MATERIALS OF CONSTRUCTION

Selecting materials of construction is an important aspect of designing flow sys-
tems. The process engineer, more than any other engineer, must handle corrosive
as well as dangerous fluids. We will not discuss corrosion here. The interested
reader can refer to Fontana and Greene [4] for further details.
The designer, in order to increase the reliability of his design, should criti-
cally examine all parts of his flow system to determine what parts contact the
fluid. This is particularly true of pumps and valves where critical parts may be
overlooked, for example, seals. The designer should also be aware that some or-
ganic solvents attack polymeric materials, such as rubber and plastics. Thus, in
addition to selecting metals to avoid corrosion, the designer checks the compatibil-
ity of polymeric materials with solvents. Erosion of piping and fittings by the
process fluid must also be considered. Solids suspended in fluids may cause ex-
cessive wear of piping, pumps, and valves. Even for a pure liquid, as the velocity
approaches 10 ft/s (3.05 m/s) [31], erosion will occur. Corrosion data for a given
fluid may be obtained from Craig and Anderson [5] or by consulting equipment
manufacturers. The Chemical Engineering Handbook [1] also contains some data
on corrosion.

MACROSCOPIC MECHANICAL ENERGY BALANCE

The most important relationship in designing flow systems is the macroscopic
mechanical-energy balance, or Bernoulli's equation. Not only is it required for
calculating the pump work, but it is also used to derive formulas for sizing valves
and flow meters. Bird, et al. [6] derived this equation by integrating the micro-
scopic mechanical-energy balance over the volume of the system. The balance is
given by

2
A(v /a) g f 2 dp
_
——— + Az+ | — + W + E = 0 (8.1)
2g c gc Ji P
The units of each term are ft-lb F/lb M, where pound force is lb F, and pound
mass is lb. The conversion factor, g, equals 32.2 lb-ft/s2"lbF. In the first term,
M c M
the kinetic energy term, the factor a corrects for the velocity profile across the

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