High Gas Molecular Weight
It is always recommended
to use a gas analysis for the highest accuracy for predicted performance.
For the
normal natural gas mixtures with .56 to .80 specific gravity a specific
gravity entered for performance predictions is acceptable. If
the gas contains gas constituents outside the normal pipeline gas mixtures
(ie., carbon dioxide, nitrogen, hydrogen sulfide, hydrogen...) a gas analysis
will provide more accurate performance results.
When gas mixtures are lighter
than .35 specific gravity (10 mole weight) or heavier than 1.35 specific
gravity (39 mole weight) it is recommended to contact Ariel Applications
Engineering. The
VMG gas method should be used for
gasses outside the use of natural gas.
When only a specific gravity is
specified, the gas properties (K value and compressibility) are based
upon a generalized hydrocarbon mixture. This could lead to inaccurate
compressor performance predictions when other gas constituents are present
outside the normal natural gas mixture. Air
should never be entered by a specific gravity.
High pseudo-Q
values are common for high gas molecular weight applications. These
high pseudo-Q values can be remedied with slower operating speeds.
Often times these are also wet gas or low suction pressure applications.
See the Packager's Standards Section
6 for information on applying the appropriate cylinder lubricant and rates
for high gas molecular weight applications.
Ariel
Corporation Application Manual
20 April 2004