The total pressure drop is then compared to the given source or delivery
pressure to determine pipe size adequacy. This relatively quick process can
be repeated for a different pipe size if the pressure drop is found to be too
high or if a smaller pipe size can be used.
CAUTION:
If higher pressures require a stronger pipe than
Schedule 40, the calculation shown on Table 5-10
must be used in estimating pressure drop.
To estimate the pipe size for a required gas flow, more calculations are
needed than for a liquid. Estimation of weight density for gasses was in the
discussion of Darcy's formula, above in paragraph (2). An effective gas
pressure is needed for this estimate. An effective pressure is also needed
to calculate the gas flow velocity because, to deliver a given amount of gas
expressed in cubic feet at standard atmospheric pressure (SCFM), an increase
in system pressure will reduce the required gas velocity. The following
guidelines are recommended for selecting the effective gas pressure:
(a) If the system does not require a high flow rate or if the
number and types of fittings are not accurately known, a pressure drop equal
to about 10 percent of the inlet pressure (P1) should be selected. For
this condition, either P1 or the outlet pressure (P2) can be used as the
effective system pressure.
(b) If the system requires a high flow rate and if the number
and types of, fittings are accurately known, a pressure drop equal to from 20
percent to 40 percent of P1 can be selected. Since higher gas velocities
may result in sonic flow across a valve, values close to 40 percent of P1
should be used with caution. For pressure drops greater than 10 percent of
the P1, the effective system pressure should be made equal to P1 + P2 /
2. With the effective gas pressure estimated by one of the two procedures
described above, the gas weight density can be calculated by using the weight
density given in Table 5-5 and making the weight density proportional to the
absolute pressure. (The tables in Reference 16 can be used for more accurate
values.) Having been given the required gas flow in scfm, the required flow
at the effective gas pressure should be estimated by the relationship:
P1
V2
(3)
---- = ----
P2
V1
where:
P1
=
atmospheric pressure, 14.7 psia
P2
=
effective gas pressure, psia
V1
=
required flow scfm
V2
=
flow at effective pressure, cfm
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