Page 179 - Compression Machinery for Oil and Gas

168 SECTION II Types of Equipment

Compressors With a Crosshead
With this type of compressor the piston connects to a piston rod, to crosshead
and then back to the crankshaft via a connecting rod. The crosshead coverts the
rotary motion of the crankshaft to purely linear motion in the piston rod. This
means that the piston can be double acting, compressing on both outbound and
inbound stroke. Additional sealing of the gas at the piston rod packing is facil-
itated allowing gas compression to take place with zero leakage to the atmo-
sphere. These compressors are widely used in the size range of 50kW up to
about 20MW and pressures from atmospheric to 340MPa. They are available
in different configurations from 1 throw up to 10 throws. Single-throw compres-
sors are usually small (<100kW) and may have one or two cylinders (tandem),
and they can be horizontal throw or vertical.
The most common arrangement is horizontal balance opposed with two
crank throws between each pair of bearings. So opposing throws are at 180
degrees crank angle and will balance each other except for a couple due to
the axial distance along the crankshaft for each throw. This configuration is
offered in a two-, four-, and six-throw configuration. The two- and four-throw
configuration will have a small unbalance couple and the six throw is typically
fully balanced if all throws are equal reciprocating weight. If an odd number of
cylinders are required then a dummy balance weight is used on the throw with-
out a cylinder. This arrangement is used for both high speed (>600rpm) and low
speed (200–600rpm).
Another common arrangement is a single throw between crankshaft bear-
ings. This is also called the variable crank arrangement. This highly flexible
arrangement is used on many slow-speed compressors and allows odd no crank
throws from 1 to 10 throws in a horizontal configuration and up to six throws
with a vertical cylinder configuration, cylinders are usually opposed but not
always, it even allows throws to be different stroke although this is unusual.
Balance can be equal to the balance opposed geometry above, but as this con-
figuration is exclusively used for slow-speed applications the inertia forces are
lower and so balancing of opposing throws is less critical.

Other Types of Reciprocating Compressors
Other much less common types of reciprocating compressors have the throws
oriented in “W,” “V,” “L,” and radial configurations. These have been used his-
torically in both with and without crosshead variants, mostly in air and natural
gas compression. Such configurations are very rare today.
The vertical reciprocating compressor is available for process and natural
gas applications in both lubricated and nonlubricated labyrinth piston to cylin-
der bore sealing design. The piston has no piston rings or wearbands and relies
on a labyrinth to provide sealing between the head-end and crank-end compres-
sion chambers. Of course, the cylinder bore has to be oriented vertically for this
to work. Vertical compressors are typically used on clean dry gas applications