Page 213 - Reciprocating Compressors Operation Maintenance

Operation and Maintenance of Reciprocating Compressors 1 99

seat solidly. The unloader contains set screws or latches to facilitate hold-
ing of the assembly on position while the cover is being applied. Proceed
to apply and tighten the valve cover and then tighten the unloading valve
assembly on its seat and seat gasket by the aid of the jam screw and jam
screw lock nut.
CAUTION: Always remember that whenever valve covers (whether
unloading valves or otherwise) are removed, the jam screws and lock
nuts must be readjusted.
A stuffing box arrangement is usually provided in the jam screw to
prevent gas leakage along the diaphragm stem. The stuffing box may
contain several rings of packing with tension thereon applied by adjust-
ment of the stuffer.
Only sufficient tension should be applied to this packing to prevent
leakage along the stem. Excessive tightening of the packing will cause
binding of the stem and restrict proper action of the unloader. Suitable
packing is often placed between the jam screw locknut and the valve
cover to prevent gas leakage at that point.

START AND STOP CONTROL

In those installations where air requirements are intermittent, as in the
case of an air starting compressor for a diesel engine, start and stop con-
trol is desirable. The compressor in this instance shuts itself down when
the demand is low and automatically restarts on an increase of air
requirements.
Start-stop controls are composed of the same elements that make up
two-step bypass capacity control. These controls have, in addition, a
time-delay relay and an automatic water valve.
In this instance, the contacts of the pressure switch will be wired in the
pilot circuit of a magnetic starter as well as in the solenoid valve electri-
cal circuit. Thus, an increase in pressure will break the starter circuit,
stopping the compressor motor and, at the same time, causing the com-
pressor to unload as it slows to a halt.
Because the compressor is starting and stopping when the load is
extremely close to its peak, it is desirable to have the compressor com-
pletely unloaded at these instances. The incorporation of the pressure
switch starter and pressure switch solenoid circuit answers the unloading
problem for stopping, but the problem of starting still remains. For this
phase of the operation, the time-delay relay is often incorporated into the