Page 108 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

92 Applied Process Design for Chemical and Petrochemical Plants

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Figure 2-23. Pressure drop in liquid ....
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lines. By permission Crane Co., I -="'=i
Technical Paper #410, Engineering 0- E�
Div., 1957. Also see 1976 edition. I �� .I l 37
The friction loss or pressure drop, F0, is determined at where Pc = system end pressure = 22 + 15 = 37 psig (not
the design flow rate, Q 0, for the piping, valves, and fric- friction)
tion producing equipment (such as tubular heat exchang- Piping system pipe friction @ Q0 flow rate = 6 psi
ers, tubular furnaces/heaters), orifice or other meters, Heater, friction = 65 psi
and control valves. Because the system friction pressure Separator, friction = 1 psi
loss changes with flow rate through the system, recogni- Preheaters, 10 + 12 (friction) = 22 psi
tion must be given to the changes in flow rate (increase or Orifice, allow, friction = 2 psi
decrease) as it affects the pressure loss through the con- Total friction, excluding control valve, F0 = 96 psi
trol valves. For any design, the beginning and end points Assume pressure loss through control valve = 35 psi
of the system should be relatively constant for good
process operations. (2-59)
For good control by the valve, the pressure drop across
(or through) the valve must always be greater than the fric- 35 = (Ps - 37) - 96
tion losses of the system by perhaps 10% to 20% (see [9]).
Ps = 168 psi, at pump discharge, using assumed control
valve pressure drop of 35 psi
Example 2-3: Establishing Control Valve Estimated
Pressure Drop, using Connell's Method [9]. Note that P,. = 22 psig + 15 psi static Hd. = 37 psig
Assume that allowances must be made for a 10%
Refer Lo Figure 2-26 for an example to determine the increase in process flow rate, above design, Q 0. Pressure
pressure loss (drop) through the control valve. drop varies as the square of the flow rate.