Page 77 - Reciprocating Compressors Operation Maintenance

64 Reciprocating Compressors: Operation and Maintenance

two pistons is connected to low-stage suction which acts to minimize the
differential pressure across the piston rings.
The applications best handled in truncated forged cylinders involve
small piston displacements for each stage and medium-to-high discharge
pressures.
The large-bore crank end is used for the low stage and the small-bore
head end for the high stage. By varying the bore sizes, an optimum
design is achieved to minimize rod loads. With this arrangement it is not
necessary to pack against second-stage discharge pressure.
Truncated cylinders of fabricated steel often find application at lower
pressures where it is desired to avoid the maintenance problems of the
tandem configuration. With the large-bore, low stage on the head end, it
is possible to remove the two-stage piston and rod assembly with a mini-
mum of disassembly.
Tail Rod. In this arrangement, the piston is in the center of the piston
rod, and the rod is the same diameter on both sides of the piston. This
prevents non-reversal loading due to equal areas and equal pressures on
both sides of the piston. Refer to Figure 2-24.
Cooled Piston Rod. This rod is drilled through the center axis to allow
pressure fed coolant, usually oil, to circulate up the rod core. Such circu-
lation helps to remove the heat of compression from rod and piston.

Piston Rod Materials

Compressor piston rods are made from various types of steel depend-
ing on stress levels and the composition of the gas handled.
Low carbon steels, such as AISI 1037, and low alloy steels, such as
AISI 4140, are commonly used. For certain corrosive gases, 410 stain-
less, K-Monel, or Hastalloy steels may be used.
The normal frictional forces acting between the piston rod packing
rings and the piston rod increase with pressure. Experience has shown
that piston rod packing wear becomes excessive with pressures above
1000 psi. Figure 2-25 shows the friction forces which act on a piston rod.
Therefore, a piston rod with a hardened surface in the packing area
will have less wear than will one not so hardened. This hardening may
take the form of carburizing, nitriding, induction hardening, plasma spray
with chrome oxide, tungsten carbide, or flame hardening. This also
implies that surface finish is important!