NATURAL GAS. 
15 
Failure to appreciate that the absolute pressure, rather than merely 
the gage pressure, must be used when computing the effect of pres- 
sure on gas volume, or heating value content, has been 
responsible for most of the misunderstanding re- f^%£ssv/?£ 
garding the effect of variation in gage pressure on \ 
gas quality and gas service. 
DIFFERENTIAL PRESSURE. 
This is simply the difference between the pressure 
at the inlet and outlet of a gas line. Thus, if the 
inlet gage pressure of a gas line were 50 pounds and 
the outlet gage pressure 10 pounds the differential 
pressure would be 40 pounds. In gas transmission it 
is necessary to have a differential pressure in order 
to secure driving power to force the gas through the 
line. That is, the differential pressure is the pressure 
that is lost in overcoming the friction of the gas mov- 
ing through the line. 
EFFECT OF PRESSURE ON GAS VOLUME, KNOWN AS BOYLE’S LAW. 
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There is a definite relationship existing between 
the volume and pressure of natural gas. That is, 
when the gas is compressed or allowed to expand, it 
approximately follows Boyle’s law. This law may 
be stated as follows : “ The volume of a gas at con- 
stant temperature varies inversely as the absolute 
pressure to which the gas is subjected; or, what is 
the same thing, the product of the absolute pressure 
and the volume of a given quantity of gas remains 
constant.” 
Thus, if the volume is doubled, or one-half of the 
gas is removed from a fixed reservoir, the absolute 
pressure will be reduced one-half. Conversely, if the 
absolute rock pressure in a fixed reservoir is reduced 
to one-half, the volume of gas remaining compressed 
in that reservoir will be reduced to one-half. 
It has been the universal custom of the natural 
gas industry to disregard the small deviation of 
natural gas from Boyle’s law and in measuring com- 
putations to assume that the gas follows the law ex- 
actly. Tests made on the West Virginia gas indicate 
that the per cent of deviation increases with the pressure. That is, 
while there is no perceptible deviation at pressures under 15 pounds, 
at 150 pounds the deviation would be about 6 per cent. That is, the 
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FIG. 2. DIAGRAM 
SHOWING RELA- 
TION OF ATMOS- 
PHERIC AND GAGE 
PRESSURE. 
