Page 119 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Fluid Flow 103

2
q\ = 40,700Yd [(L\P) (P' 1 )/(KT 1 Sg)J 112 (2-80) 5. Determine expansion and contraction losses, fittings
same units as Equation 2-79 above and at vessel connections.
6. Determine pressure drops through orifices and con-
where Y = net expansion factor for compressible flow through trol valves.
orifices, nozzles, or pipe 7. Total system pressure drop
K = resistence coefficient, ft
P' = pressure, lbs/sq in. absolute L'i.P Total= (L + Leq) (L'i.P/100) + Item 5 + Item 6 (2-57)
w, = flow rate, lbs/sec.
8. If pressure drop is too large, re-estimate line size and
Isothermal conditions, usually Jong pipe lines [3]: repeat calculations (see paragraph (A) above) and
also examine pressure drop assumptions for orifices
I and control valves.
I 2
" � 144 gA C. Air
v,
v ( fL For quick estimates for air line pressure drop, see
i D
Tables 2-12A and 2-l 2B.
lbs/sec (2- 3)
D. Babcock Empirical Formula for Steam
plus the conditions listed. The equation is based on steady
flow, perfect gas laws, average velocity at a cross section, Comparison of results between the various empirical
constant friction factor, and the pipe is straight and hori- steam flow formulas suggests the Babcock equation as a
zontal between end points. good average for most design purposes at pressure 500
psia and below. For lines smaller than 4 inches, this rela-
D = pipe ID, ft tion may be 0-40 percent high [56].
L = pipe length, ft
A = cross-sectional area for flow for pipe, sq ft
w 2 �
p 1 - Ps = L'i.P = 0.000131 (1 + 3.6/d) (2-82)
B. Alternate Vapor/Gas Flow Methods pd"
L'i.P/100 feet= w2 F/p (2-83)
Note that all specialized or alternate methods for solv-
ing are convenient simplifications or empirical proce- Figure 2-32 is a convenient chart for handling most in-
dures of the fundamental techniques presented earlier. plant steam line problems. For long transmission Jines
They are not presented as better approaches to solving over 200 feet, the line should be calculated in sections in
the specific problem. order to re-establish the steam specific density. Normally
Figure 2-31 is useful in solving the usual steam or any an estimated average p should be selected for each line
vapor flow problem for turbulent flow based on the mod- increment to obtain good results.
ified Darcy relation with fixed friction factors. At low Table 2-13 for "F" is convenient to use in conjunction
vapor velocities the results may be low; then use Figure 2- with the equations.
30. For steel pipe the limitations listed in (A) above apply.
Darcy Rational Relation for Compressible Vapors and
1. Determine C 1 and C 2 from Figure 2-31 and Table 2-11 Gases
for the steam flow rate and assumed pipe size respec-
tively. Use Table 2-4 or Table 2-8 to select steam veloc- 1. Determine first estimate of line size by using sug-
ity for line size estimate. gesLed velocity from Table 2-4.
2. Read the specific volume of steam at conditions, 2. Calculate Reynolds number Re and determine fric-
from steam tables. tion factor, f, using Figure 2-3 or Figure 2-33 (for
3. Calculate pressure drop (Figure 2-31) per 100 feet of steel pipe).
pipe from 3. Determine total straight pipe length, L.
4. Determine equivalent pipe length for fittings, valves,
Lcq·
LiP/100feet = C C V (2- 81)
2
1
5. Determine or assume losses through orifice plates,
4. From Figure 2-20 or 2-21 determine the equivalent control valves, equipment, contraction and expan-
lengths of all fittings, valves, etc. sion, etc.