The capacity of a low pressure natural gas (less than 1 psi, 6.9 kPa) pipe line with a small pressure drop can be calculated with the Spitzglass formula
q = 3550 k ( h / l SG)1/2 (1)
q = natural gas flow capacity (cfh)
h = pressure drop (in Water Column)
l = length of pipe (ft)
k = [d5/(1 + 3.6/d + 0.03 d)]1/2
d = inside diameter pipe (in)
SG = specific gravity
For natural gas the nominal BTU/cf varies from about 900 to 1100 BTU/cf. In general it is common to set
The specific gravity of natural gas varies from 0.55 to 1.0.
The downstream pressure in a houseline after the meter/regulator is in general in the range of 7 to 11 inches Water Column, or about 1/4 psi.
The capacity of a 100 ft natural gas pipe with a nominal diameter 0.5 inches (actual ID 0.622 in) and 0.5 inches WC pressure drop can be calculated as
k = [(0.622 (in))5/(1 + 3.6 / 0.622 (in) + 0.03 0.622 (in))]1/2
q = 3550 0.117 (0.5 (in) / 100 (ft) 0.60 )1/2
= 37.9 (cfh)
Specific gravity of natural gas is set to 0.60.
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