Page 100 - An Introduction To Predictive Maintenance

90 An Introduction to Predictive Maintenance

complete revolutions (720 degrees) are required for all cylinders to complete a full
cycle.

Piston orientations. Crankshafts on positive-displacement reciprocating compressors
have offsets from the shaft centerline that provide the stroke length for each piston.
The orientation of the offsets has a direct effect on the dynamics and vibration ampli-
tudes of the compressor. In an opposed-piston compressor where pistons are 180
degrees apart, the impact forces as the pistons change directions are reduced. As one
piston reaches top dead center, the opposing piston also is at top dead center. The
impact forces, which are 180 degrees out-of-phase, tend to cancel out or balance each
other as the two pistons change directions.

Another configuration, called an unbalanced design, has piston orientations that are
neither in-phase nor 180 degrees out-of-phase. In these configurations, the impact
forces generated as each piston changes direction are not balanced by an equal and
opposite force. As a result, the impact energy and the vibration amplitude are greatly
increased.

Horizontal reciprocating compressors (see Figure 5–9) should have X-Y data points
on both the inboard and outboard main crankshaft bearings, if possible, to monitor the
connecting rod or plunger frequencies and forces.

Screw. Screw compressors have two rotors with interlocking lobes and act as posi-
tive-displacement compressors (see Figure 5–10). This type of compressor is designed
for baseload, or steady-state, operation and is subject to extreme instability if either
the inlet or discharge conditions change. Two helical gears mounted on the outboard
ends of the male and female shafts synchronize the two rotor lobes.

Analysis parameters should be established to monitor the key indices of the com-
pressor’s dynamics and failure modes. These indices should include bearings, gear
mesh, rotor passing frequencies, and running speed; however, because of its sensitiv-
ity to process instability and the normal tendency to thrust, the most critical monitor-
ing parameter is axial movement of the male and female rotors.

Bearings. Screw compressors use both Babbitt and rolling-element bearings. Because
of the thrust created by process instability and the normal dynamics of the two rotors,
all screw compressors use heavy-duty thrust bearings. In most cases, they are located
on the outboard end of the two rotors, but some designs place them on the inboard
end. The actual location of the thrust bearings must be known and used as a primary
measurement-point location.

Gear mesh. The helical timing gears generate a meshing frequency equal to the
number of teeth on the male shaft multiplied by the actual shaft speed. A narrowband
window should be created to monitor the actual gear mesh and its modulations. The
limits of the window should be broad enough to compensate for a variation in speed
between full load and no load.