(b) Equivalent pipe length. The parts causing pressure drop
in a fluid system include not only the pipe, but also such components as
fittings and valves. Because the pressure drop across a valve or fitting can
be considered as being equal to the pressure drop through a length of pipe,
the L in Equation (1) can be considered as being made up of the length of the
actual pipe and an equivalent length of pipe representing the fittings and
valves. Figure 5-2 is a nomograph that can be used to approximate the
equivalent length of pipe in feed for common types and sizes of fittings and
valves.
(c) Weight density. Weight densities for liquids will have
values close to the weight density of water (62.2 pounds/feet3 at 80 deg.
F). Weight densities for gasses will vary significantly because pressures can
range from atmospheric to as high as 5000 psi. When approximating pipe sizes
for gasses, a calculation must be made to estimate the weight density of the
individual gas at an estimated effective pressure. (The U.S. Navy Diving-Gas
Manual, (Reference 16) contains extensively tabulated values of the densities
of breathing gas for typical hyperbaric chamber conditions.) The weight
density for gasses at any pressure is approximately equal to:
144 P
[rho] = -----
RT
where:
P = absolute pressure, psia (gage pressure + 14.7)
R = individual gas constant
T = absolute temperature in degrees Rankine (460 + F).
The weight densities for typical gasses at atmospheric pressure and 68 deg. F
are shown in Table 5-5. From Equation (2) and Table 5-5 it is apparent that
the weight density of breathing gasses at 68 deg. F can vary from about 0.01
per cubic foot for helium at atmospheric pressure to about 3.5 pounds per
cubic foot for oxygen at 5000 psi.
(d) Mean flow velocity. The fluid pressure drop varies as
the square of the mean fluid velocity. Velocity ranges for water and steam
are given in Tables 5-6 and 5-7. Use of these values will normally avoid
"excessive" pressure drop. Values considerably higher than those tabulated
have been used satisfactorily, however. Because an increase in pressure drop
may require an increase in source pressure, the effect of pressure on
fittings, valves, and pipe wall thickness must be considered. Table 5-8
identifies various pressure classes of fittings and valves with pipe schedule
numbers.
(3) Approach to Flow Approximation. The approximation approach
described in this manual is based on selective use of the above background
information and Tables 5-9, 5-10, and 5-11. For a required flow of liquids,
these tables can be used directly to find the liquid velocities and pressure
drops per 100 feet of pipe for common sizes of Schedule 40 pipe.
When a tentative pipe size has been selected, the equivalent pipe lengths for
the estimated valves and fittings can be estimated by using Figure 5-2.
These equivalent lengths are then added to the actual pipe length and the
total pressure drop is calculated using the psi drop per 100 feet given in
Table 5-9.
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