Page 167 - Reciprocating Compressors Operation Maintenance

Operation and Maintenance of Reciprocating Compressors 1 53

Inertia may close it too far, even back to the seat before flow
forces cause it to open again. By the end of the piston stroke, it may
have oscillated several times.
Furthermore, at the end of the stroke, inertia may carry the valve
open instead of back toward the seat, and when backwash finally
closes the valve, it is with a slam.
Examination of the valve and seat may confirm fluttering. If the
stop plate shows no markings, it may indicate that the valve has not
fully opened. Normally the back of the valve plate itself will have
some sort of pattern from the springs. In a circular or plate valve, if
there is no definite pattern, and the plate appears to spin, then flutter-
ing is likely to cause failure.
The solution to fluttering is to lower the lift and/or to use lighter
springs.
• Resonance or pulsations can upset normal valve action. The ampli-
tude of pulsations is of considerably less consequence than the phase
relation of the pressure wave and crank angle, A pulsation can cause
late closing, thus slamming, as previously mentioned.
A series of high peak-to-peak pressure fluctuations surround the
valve, usually as the result of an improperly designed piping system.
As the sealing element tries to follow this pressure change, it flutters
erratically.
In some cases, the sealing elements will pound against the seat and
guard many times during a single piston stroke, causing breakage
from impact fatigue.
Pulsations of relatively high frequency, for example, between 50
and 100 Hz, are detrimental to compressor valve life.
Figure 3-19 depicts on a time basis what happens. The valve is not
only open at dead center, but the rate of pressure change is high. The
valve is slammed shut by back flow after dead center.
The problem can be solved by eliminating pulsation through pip-
ing changes. This is a case where the problem is outside the valve
designer's control and valve changes will not correct the problem.
• Flow pattern failures are infrequent, but they have happened. There
may be a disturbance within the cylinder passage leading to a valve
that causes some type of erratic valve action.
The cure is to change the lift and/or springing (depending on the
type of failure), or even to change the valve type.