Page 274 - Root Cause Failure Analysis

262 Root Cause Failure Analysis

pressure will be transmitted to any piping or machine connected to the discharge pip-
ing and located within 28 ft of the compressor. Note that, for a single-acting cylinder,
the wavelength will be twice as long.

Imbalance
Compressor inertial forces may have two effects on the operating dynamics of a recipro-
cating compressor, affecting its balance characteristics. The first is a force in the direc-
tion of the piston movement, which is displayed as impacts in a vibration profile as the
piston reaches top and bottom dead-center of its stroke. The second effect is a couple, or
moment, caused by an offset between the axes of two or more pistons on a common
crankshaft. The interrelationship and magnitude of these two effects depend on such
factors as number of cranks, longitudinal and angular arrangement, cylinder arrange-
ment, and amount of counterbalancing possible. Two significant vibration periods
result, the primary at the compressor’s rotation speed (X) and the secondary at 2X.

Although the forces developed are sinusoidal, only the maximum (Le., the amplitude)
is considered in the analysis. Figure 22-1 shows relative values of the inertial forces
for various compressor arrangements.