Page 115 - Reciprocating Compressors Operation Maintenance

f O2 Reciprocating Compressors: Operation and Maintenance

ed through the ground from compressor unbalance can appear as a vibra-
tion of the piping, but this is unusual.
Pulsation can build up due to resonance and force piping to vibrate.
Coincidence of the pulsation frequency can result in violent vibration.
Therefore, it is logical and proper to detune the system by reducing the
exciting pulsation or stiffening the piping to reduce the vibration or a
combination of both.
Piping supports must be spaced in a way that the natural frequency of
the piping span or configuration does not coincide with any exciting pul-
sation frequency being generated by the compressor.
Certain piping configurations should be used with precautions, or
avoided if at all possible. Expansion loops, "Z"- or "L"-shaped piping
overhead, cross-overs, and long sweeping bends are all susceptible to
vibration from small amounts of pulsation.

EFFECT OF PULSATION ON PERFORMANCE

It is well documented that the performance of a compressor can be
materially affected by the piping. More or less gas can be compressed, at
the expense of horsepower, because of the effects of pulsation and the
interaction with the piping.
Some of the effects pulsation can have on the indicator card are shown
in Figure 2-58. Good pulsation control minimizes these effects. Nozzle
diameter and length between the cylinder and bottle have a great effect
on the indicator card. A good rale to follow is to make the nozzle as big
in diameter as possible and as short as possible.

Earlier in this chapter, we alerted the reader to a special non-lubricated
reciprocating compressor configuration, the labyrinth piston, or "Laby"
machine. Although available since the mid-1940s, labyrinth compressors
are not as well known in the United States as they are in other parts of
the world.
In labyrinth piston compressors, an extremely large number of throt-
tling points provide the sealing effect around pistons and piston rods. No
contact seals are used.
Whereas plastic sealing rings depend on permanent mechanical fric-
tion for efficient performance, the labyrinth principle embodies an
extremely small clearance between sealing element and counterpart. This