Page 225 - Compression Machinery for Oil and Gas

214 SECTION II Types of Equipment

FIG. 5.32 Valve cap fixed volume clearance pocket. (Courtesy of Ariel Corporation.)

automatically thus allowing the compressor to match demand at all times
(Fig. 5.32).
A valve cap fixed-volume clearance pocket is a device installed over a com-
pressor valve. The added fixed volume is typically less than that provided by a
head-end pocket simply due to less space being available over a valve. A dis-
advantage of this device is that it requires a hole through the compressor valve
to provide a passage to the added clearance volume. This hole removes active
valve flow area reducing the efficiency of that valve.

Valve Clearance Spacer
A valve clearance spacer is a metallic ring that is installed between a compres-
sor valve and the cylinder body effectively moving the compressor valve farther
from the cylinder bore adding fixed clearance. The amount of added clearance is
small so this device is most often used to provide fine tune of the interstage pres-
sures in a multistage compressor. Of course, this device cannot be added or
removed while the compressor is running.
End deactivation, as shown in Fig. 5.33, refers to the situation where an end
of a double-acting cylinder, usually the head end, is deactivated effectively
reducing the capacity of that cylinder 50%. There are three devices commonly
used to deactivate an end:
– finger-type suction valve unloader;
– plug-type suction valve unloader; and
– head-end bypass.