The suction valve unloader is a small piston and
cylinder assembly installed over the suction valve
(see Figure 1.1A). The small piston is connected to
a rod that extends into the suction valve assembly.
The unloader piston is depressed when gas
pressurized to 40 psi or more above suction
pressure is injected into the unloader cylinder.
In the depressed position, the piston pushes the rod
against the suction valve disk to hold the suction
valve in an open position. In this position, no
compression will occur, even though the pistons are
still moving up and down.
The unloaders are controlled by devices which
control the flow of gas into and out of the unloader
cylinder. The different types of unloader controls
may be summarized as follows: