(1) for the globe valve - 16 feet, (2) for the standard elbow - 2 feet, and
(3) for the angle valve - 8 feet. With a pipe length of 120 feet, the total
equivalent pipe length is 146 feet. This gives a total estimated pressure
drop of 312 psi. Since this is less than the estimated 570 psi (1420 psia
-850 psia), a lower inlet pressure can be used.
With the size of the pipe known and the pressure conditions approximated, it
is now possible to obtain a closer approximation by using Equation (1). If
an inlet pressure of 1250 psia is used, giving an effective system pressure
of 1050 psia P1 + P2, the following values for Equation (1) can be
calculated:
-----------
2
(0.01 - see Table 5-5)
p
[rho] = 0.71 lb/ft3 from ---------------------- = ----
14.7
1050
f = 0.026 (see Figure 5-1 for 3/4-inch, Schedule
160 pipe, turbulent flow)
L = 146 ft (see above)
V = 252 ft/sec.
(a)
Calculate compressed flow from Equation (3):
14.7
V2
---- = ----
and V2 = 31.1 ft 3 or
1050
2220
Q = 31.1 ft3/min.
(b)
Next calculate flow velocity:
Q
31.1
V = - or ------------------------
A
0.00206 (see Table 5-11)
V = 15,100 ft/min or 252 ft/sec
D = 0.512 ft (see Table 5-11
then using equation (1):
0.71 x 0.026 x 146 x 2522
[DELTA]P = ---------------------------
144 x 0.512 x 2 x 32.2
[DELTA]P = 360 psi
Since this is close to the permissible pressure drop of 400 psi, it can be
estimated that the inlet pressure can be close to 1250 psi.
3.
DESIGN FOR PRESSURE. The design of a piping system for pressure
capability requires that a design pressure P and a design temperature T be
established for the piping system. The design pressure must not be less than
the maximum difference in pressure across the pressure boundary for any
normal operating condition. The design temperature must not be less than the
actual metal temperature which exists for any specified normal operating
condition.
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