Page 25 - Reciprocating Compressors Operation Maintenance

1 2 Reciprocating Compressors: Operation and Maintenance

Note: There is frequent confusion in transmission of pressure data. It is
recommended that specific notation be made after each pressure as to
whether it is gauge or absolute. Use the symbol psig or psia. If psig is
given, be sure the barometric pressure is also specified.
Also, because a column of a material of a specified height will have a
weight proportional to its height, the height can be used as a force mea-
sure. It is reduced to a unit area basis automatically, since the total weight
and the area are proportional. For example:

Feet of water = (ft H 2O)
Inches of water = (in. H 2O)
Inches of mercury = (in. Hg)
Millimeters of mercury = (mm Hg)

With the exception of barometric pressure, when pressures are
expressed in the above terms, they are gauge pressures unless specifical-
ly noted as absolute values.
Atmospheric pressure is measured by a barometer. It is designed to
read the height of a column of mercury. The upper end of the tube con-
taining the mercury is closed and is at zero absolute pressure. The lower
end of the tube is submerged in a pot of mercury, the surface of which is
open to the atmosphere. The weight of this column of mercury exactly
balances the weight of a similar column of atmospheric air.
Although this gauge really measures a differential pressure, by design
one of those pressures is zero, and the actual reading is true absolute or
total pressure of the atmosphere. 14.696 psia sea level measure is equal
to 29.92 in. Hg.

PRESSURE DEFINITIONS ASSOCIATED WITH COMPRESSORS

Inlet or suction pressure is the total pressure measure at the compres-
sor cylinder inlet flange. Normally expressed as gauge pressure but may
be expressed as absolute pressure, which is gauge pressure plus atmos-
pheric pressure (14.696). It is expressed in pounds per square inch, psig
(gauge) or psia (absolute).
Discharge pressure is the total pressure measured at the discharge
flange of the compressor. It is expressed the same as the suction pressure,
psig or psia.