Page 210 - Reciprocating Compressors Operation Maintenance

I 96 Reciprocating Compressors: Operation and Maintenance

BYPASS CONTROL

The simplest, and therefore, the most widely used agent of regulation
is suction valve unloading of which the bypass control is typical.
The term bypass control, when used in a discussion of compressor
capacity control is somewhat misleading. The implication of the term is
that the gas flow is diverted around the compressor so that it "bypasses"
the compressor cylinder. This is not true, of course, for the actual process
involves drawing the gas into the cylinder on the discharge stroke
through the same valves used for entry. If the term were not so unwieldy,
this process could be referred to more accurately as a "free flow, non-
compressing" control, which is actually what does take place. Gas is per-
mitted to move freely in and out of the cylinder, with no compression
taking place.
At a preset pressure limit called the cut-out point, a pressure sensing
element pilot device causes air to be admitted to the suction valve
unloaders. Pressure sensing elements typically include trigger switches,
pressure switches, and instrument pilots.
Figure 3-53 shows a typical pressure switch. Fitted with bellows and
spring combinations, these elementary switches will usually give good
service in their intended operating ranges.
Two typical suction valve unloaders are shown in Figure 3-54.
The unloader mechanism, attached directly to the suction valve, con-
sists of valve fingers operating in the unloader cylinder directly on the
valve strips. The spring-loaded fingers are forced to the unloading posi-
tion either by the movement of the diaphragm stem through a piston and
power spring in the unloader cylinder or directly by the stem.
This mechanism, when actuated, holds the suction valve strips in the
open position allowing the air or gas to pass freely in and out of the suc-
tion valves without compression in the cylinder, thus reducing the deliv-
ered capacity of the cylinder and unit.
Air pressure applied to the diaphragm through action of the control
unit, causes the diaphragm stem to move downward forcing the valve
fingers, through the unloader mechanism, to open the valve strips and
hold them solidly against the valve guard.
When the control air is released from the diaphragm, the diaphragm
spring retracts the diaphragm stem from the unloader mechanism returning
it to its original position. This removes the force from the unloader fingers