Page 163 - Reciprocating Compressors Operation Maintenance

Operation and Maintenance of Reciprocating Compressors 149

«The guard or stop plate, frequently the most misunderstood part of a
valve assembly, has a threefold purpose:
1. Provides a guide for motion of the valve sealing element
2. Controls the lift of the valve
3. Retains the valve return spring
Because guards are not exposed to differential pressures, they need
only to resist the impact forces of the sealing elements making contact.
* Valve sealing elements function by alternately moving to open the valve
when it is exposed to differential pressures (allowing gas to pass
through) and then moving in the reverse direction (blocking gas flow),
• Valve springs close the valve at the end of the piston stroke when the
differential pressure across the valve approaches zero. Each spring is
designed for the particular environment it will encounter. The substi-
tution of springs with different characteristics may introduce serious
reliability and safety risks.

VALVE LIFT

Valve lift is defined as the distance the valve sealing elements move
from closed to fully open. The life of any valve depends upon the amount
of lift, which varies according to compressor speed, valve diameter, pres-
sures, and molecular weight of the gas.
One authority stated that the first question to be answered when
designing or selecting a valve for a given set of operating conditions is
"What is the proper valve lift?" In any valve application to a compressor,
lift affects both efficiency and durability.
Too high a lift will cause premature valve failure due to impact
fatigue. Too low a lift will result in excessive gas velocity through the
valve, high losses, and consequently inefficiency. These facts must be
reconciled in a sound commercial design.
Rotative speed is the most important factor influencing valve lift. This,
of course, involves the speed with which the valve must open and close.
Experience has shown that a good valve design has good durability with
a 0.200-inch lift at 300 rpm; whereas, the lift must be reduced to 0.040
inch when running at 1800 rpm. In any event, maintaining valve lift is
critically important and proper maintenance and repair techniques are
essential. For example, worn seats or guards can often be refinished, but
the original lift may be altered if refinishing is incorrectly done.