Page 134 - Reciprocating Compressors Operation Maintenance

Operation and Maintenance of Reciprocating Compressors 121

by gravity or a pressurized system. If new oil is used, it may come from
an overhead tank or through a pressurized system from a bulk tank.
The divider block takes oil from the pump and precisely meters it to as
many lines as required. This is accomplished through a stack of "piston
sections" or "feeder sections," each of which has a moving piston of a
given displacement.
Piston displacements may vary, and the number of sections may vary,
so the actual block displacement is different from one to the next. All pis-
tons move hydraulically, actuated only by the flow into the block. Each
piston moves over and back in turn, once for every block cycle.
The principle is this: each piston movement delivers a positive dis-
placement down one line. When one piston moves, that line gets its shot.
But, in addition, reduced diameters in the piston center section force the
inlet oil to move the next piston in the stack. When all have moved over,
a reversing passage sets up the pistons to start back down the other side,
Clearly then, each line must take its shot of oil, or that piston will not
valve the flow to the next piston. This is how each line can be protected
against interrupted flow,

Pumps for Divider Block Systems

There are three basic types of pumps used for these systems. All pres-
surized inlet pumps must have an adequate supply of pressure and vol-
ume to handle the required flow rates. The typical range of inlet pressure
varies from mere inches of water column to 30 psi.
The first type of pump is the box lubricator converted to pressurized
inlet. The sight glass and drip tube are replaced by a cap with inlet fit-
tings for the supply lines. The suction tube is removed or plugged. Typi-
cally, two pumps are manifolded together for each lubrication zone—one
active with capacity for the entire system and the other an installed spare.
The second type of pump is the injector pump, which employs a
diesel-fuel-type piston and barrel assembly. Typically, one pump head is
used for each adjustable zone of lubrication. The pumps are larger than
the box lubricator type and are mounted in a heavy duty cast reservoir.
The third type is the Gerhardt Pump, which is a similar barrel and pis-
ton design with inlet ports and discharge check valve. The primary differ-
ence is that the flow rate is made by rotating the piston, which has a heli-
cal groove in relation to the inlet ports on the barrel.