Page 157 - Root Cause Failure Analysis

Compressors 145

Cool inlet air is desirable. For every 5°F of ambient air temperature reduction, the vol-
ume of compressed air generated increases by 1 percent with the same power con-
sumption. This increase in performance is due to the greater density of the intake air.

It is preferable for the intake air to be taken from outdoors. This reduces heating and
air conditioning costs and, if properly designed, has fewer contaminants. However,
the intake piping should be a minimum of 6 ft above the ground and screened or, pref-
erably, filtered. An air inlet must be free of steam and engine exhaust. The inlet should
be hooded or turned down to prevent the entry of rain or snow. It should be above the
building eaves and several feet from the building.

Discharge Discharge piping should be the full size of the compressor’s discharge
connection. The pipe size should not be reduced until the point along the pipeline is
reached where the flow has become steady and nonpulsating. With a reciprocating
compressor, this generally is beyond the aftercooler or the receiver. Pipes to handle
nonpulsating flow are sized by normal methods, and long-radius bends are recom-
mended. All discharge piping must be designed to allow adequate expansion loops or
bends to prevent undue stress at the compressor.

Drainage Before piping is installed, the layout should be analyzed to eliminate low
points where liquid could collect and to provide drains where low points cannot be
eliminated. A regular part of the operating procedure must be the periodic drainage of
low points in the piping and separators, as well as inspection of automatic drain traps.

Pressure-Relief Valves All reciprocating compressors must be fitted with pressure-
relief devices to limit the discharge or interstage pressures to a safe maximum for the
equipment served. Always install a relief valve capable of bypassing the full-load
capacity of the compressor between its discharge port and the first isolation valve. The
safety valves should be set to open at a pressure slightly higher than the normal dis-
charge-pressure rating of the compressor. For standard 100 to 115 psig two-stage air
compressors, safety valves normally are set at 125 psig.

The pressure-relief safety valve normally is situated on top of the air reservoir, and
there must be no restriction on its operation. The valve usually is of the “huddling
chamber” design, in which the static pressure acting on its disk area causes it to open.
Figure 10-15 illustrates how such a valve functions. As the valve pops, the air space
within the huddling chamber between the seat and blowdown ring fills with pressur-
ized air and builds up more pressure on the roof of the disk holder. This temporary
pressure increases the upward thrust against the spring, causing the disk and its holder
to fully pop open.

Once a predetermined pressure drop (Le., blowdown) occurs, the valve closes with a
positive action by trapping pressurized air on top of the disk holder. The pressure-
drop setpoint is adjusted by raising or lowering the blowdown ring. Raising the ring
increases the pressure-drop setting, while lowering it decreases the setting.