Page 110 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

96 Applied Process Design for Chemical and Petrochemical Plants

or, from [ 111, for gases or vapors: AP = (L + ZL,,) (@/loo’ from Table 2-10)
+ Item (4) + Item (5) (2-57)
Flow, SCFH *t
If this pressure drop is too large (or too small), recheck
the steps using larger or smaller pipe as may be indicated.
Table 2-22 [53] or Figure 2-24 are convenient to use,
although they give much more conservative results (about
Flow, SCFH (temperature corrected) i twice unit head loss) than the method and figures just ref-
963 C: J(P, - P, )(PI + P, ) erenced. When using Figure 2-24 the results agree accept-
Jscr
s’h = (2 - 67A) ably well with tests on 15-20-year-old steel pipe.

where S, = specific gravity relative to air = 1.0 Example 2-5: Water Flow in Pipe System
PI = inlet pressure (14.7 + psig)
P2 = outlet pressure (14.7 + psig) The system of Figure 2-27 consists of 125 feet of
qh = flow rate, standard cu ft./hr (SCFH) unknown size schedule 40 steel pipe on the discharge side
T = flowing temperature, “R abs, (“F + 460) of a centrifugal pump. The flow rate is 500 gallons per
C’v = valve coefficient of flow, full open (from manufac- minute at 75°F. Although the tank is located above the
turer’s tables) pump, note that this elevation difference does not enter
into the pipe size-friction drop calculations. However it
*The effect of flowing temperatures on gas flow can be will become a part of selection of the pump for the service
disregarded for temperatures between 30°F and 150°F. (see Chapter 3). For quick estimate follo~ these steps:
Corrections should apply to other temperatures above or
below [ll]. 1. From Table 2-4, select 6 fps as a reasonable and usu-
tWhen outlet pressure P2 is less than M inlet pressure PI ally economical water rate.
the square root term becomes 0.87 PI [ 1 I].
From Table 2-10, a 6-inch pipe has a velocity of 5.55
Friction Loss For Water Flow fps at 500 gpm and a head loss of 0.720 psi/IOO ft.
The 5-inch pipe has a velocity of 8.02 fps and might
Table 2-10 is quite convenient for reading friction loss
in standard schedule 40 pipe. It is based upon Darcy’s be considered; however 5-inch pipe is not common-
rational analysis (equivalent to Fanning). ly stocked in many plants, and the velocity is above
usual economical pumping velocities. Use the 6-inch
Suggested procedure:
pipe (rough estimate).
1. Using known flow rate in gallons/minute, and a
suggested velocity from Tables 2-4,2-5,2-6,2-7 and 2- 2. Linear feet of straight pipe, L = 125 feet.
8 select an approximate line size. 3. From Figure 2-20, the equivalent length of fitting is:
2. Estimate (or use actual drawing or measured tabula- 6 inch-90” ell E 14 feet straight pipe (using medium
tion) total linear feet of pipe, L. sweep elbow to represent a welding ell). Note that
3. Estimate (or use actual tabulation) number of this is given as 6.5 feet from Figure 2-21. This illus-
elbows, tees, crosses, globe valves, gate valves and trates the area of difference in attempting to obtain
other fittings in system. Convert these to equivalent close or exact values.
straight pipe using Figure 2-20 or 2-21, Leq, or to
head loss using Figures 2-12 through 2-16. Note pre- 3 90” ells = 3 (14) = Le, = 42 ft (conservative)
ferred pipe size/type for charts. 1 tee = 1 (12) = Le, = 12 (Run of std. tee)
4. Determine expansion and contraction losses (if any) 1 6” open Gate Valve = (1) (3.5) = Le, = 3.5
from Figures 2-12, 2-15, and 2-16. Convert units: 1 sudden enlargement in tank @ d/d‘ = 0; = lo’,
head loss in feet times 0.4331 = psi. (This term can Figure 2-21
usually be neglected for most liquids at reasonable
velocities < lO’/sec.) Total Le, = 6’7.5 feet
5. Estimate pressure drop through orifices, control 4. Neglect expansion loss at entrance to tank, since it
valves and other items that may be in system, per will be so small.
prior discussion. 5. No orifices or control valves in system.
6. Total pressure drop. 6. From Table 2-10, at 500 gpm, loss = 0.72 psi/lOO eq ft.