Page 145 - Root Cause Failure Analysis

Compressors 133

drives, capacity variations can be obtained with a proportionate reduction in speed. A
50 percent speed reduction is the maximum permissible control range.

Helical-lobe compressors are not designed for frequent or constant cycles between
load and no-load operation. Each time the compressor unloads, the rotors tend to
thrust axially. Even though the rotors have a substantial thrust bearing and, in some
cases, a balancing piston to counteract axial thrust, the axial clearance increases each
time the compressor unloads. Over time, this clearance will increase enough to permit
a dramatic rise in the impact energy created by axial thrust during the transition from
loaded to unloaded conditions. In extreme cases, the energy can be enough to physi-
cally push the rotor assembly through the compressor housing.

The compression ratio and maximum inlet temperature determine the maximum dis-
charge temperature of these compressors. Discharge temperatures must be limited to
prevent excessive distortion between the inlet and discharge ends of the casing and
rotor expansion. High-pressure units are water-jacketed to obtain uniform casing tem-
perature. Rotors also may be cooled to permit a higher operating temperature.

Either casing distortion or rotor expansion can cause the clearance between the rotat-
ing parts to decrease and allow metal-to-metal contact. Since the rotors typically
rotate at speeds between 3,600 and 10,OOO rpm, metal-to-metal contact normally
results in instantaneous, catastrophic compressor failure.

Changes in differential pressures can be caused by variations in either inlet or dis-
charge conditions (Le., temperature, volume, or pressure). Such changes can cause the
rotors to become unstable and change the load zones in the shaft-support bearings.
The result is premature wear or failure of the bearings.

Always install a relief valve capable of bypassing the full-load capacity of the com-
pressor between its discharge port and the first isolation valve. Since helical-lobe
compressors are less tolerant to overpressure operation, safety valves usually are set
within 10 percent of absolute discharge pressure, or 5 psi, whichever is lower.

Liquid-Seal Ring
The liquid-ring, or liquid-piston, compressor is shown in Figure lG8. It has a rotor
with multiple forward-turned blades rotating about a central cone that contains inlet
and discharge ports. Liquid is trapped between adjacent blades, which drive the liquid
around the inside of an elliptical casing. As the rotor turns, the liquid face moves in
and out of this space due to the casing shape, creating a liquid piston. Porting in the
central cone is built in and fixed, and there are no valves.

Compression occurs within the pockets or chambers between the blades before the
discharge port is uncovered. Since the port location must be designed and built for a
specific compression ratio, it tends to operate above or below the design pressure
(refer back to Figure 1M).