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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