Weymouth
q = 28 * d^2.667 * {[(P1^2 - P2^2)/(SG * L) * (520/T)]}^0.5
q is the flow rate, scfh
d is the ID, inches
P1 is the upstream pressure, psia
P2 is the downstream pressure, psia
SG is the specific gravity of the gas
Lm is the length of pipe in miles
T is the temperature, deg F
Panhandle
q = 36.8 * E * d^2.6182 * [(P1^2 -P2^2)/Lm]^0.5394
E is the flow efficiency as a decimal
Both are empircal correlations