Page 148 - Root Cause Failure Analysis

136 Root Cause Failure Analysis

The primary operating control inputs for rotary positive-displacement compressors
are discharge pressure, pressure fluctuation, and unloading frequency.

Discharge Pressure This type of compressor will continue to compress the air vol-
ume in the downstream system until (1) some component in the system fails, (2) the
brake horsepower exceeds the driver’s capacity, or (3) a safety valve opens. Therefore,
the operator’s primary control input should be the compressor’s discharge pressure. A
discharge pressure below the design point is a clear indicator that the total down-
stream demand is greater than the unit’s capacity. If the discharge pressure is too high,
the demand is too low and excessive unloading will be required to prevent failure.

Pressure Fluctuation Fluctuations in the inlet and discharge pressures indicate
potential system problems that may adversely affect performance and reliability. Pres-
sure fluctuation generally is caused by changes in the ambient environment, turbulent
flow, or restrictions due to partially blocked inlet filters. Any of these problems will
result in performance and reliability problems if not corrected.

Unloading Frequency The unloading function in rotary positive-displacement com-
pressors is automatic and not under operator control. Generally, a set of limit
switches, one monitoring internal temperature and one monitoring discharge pressure,
is used to trigger the unloading process. By design, the limit switch that monitors the
compressor’s internal temperature is the primary control. The secondary control, or
discharge-pressure switch, is a fail-safe design to prevent overloading the compressor.

Depending on design, rotary positive-displacement compressors have an internal
mechanism designed to minimize the axial thrust caused by the instantaneous change
from fully loaded to unloaded operating conditions. In some designs, a balancing pis-
ton is used to absorb the rotor’s thrust during this transition. In others, oversized thrust
bearings are used.

Regardless of the mechanism used, none provides complete protection from the dam-
age imparted by the transition from load to no-load conditions. However, as long as
the unloading frequency is within design limits, this damage will not adversely affect
the compressor’s useful operating life or reliability. However, an unloading frequency
greater than that accommodated in the design will reduce the useful life of the com-
pressor and may lead to premature, catastrophic failure.

Operating practices should minimize, as much as possible, the unloading frequency of
these compressors. Installation of a receiver tank and modification of user-demand
practices are the most effective solutions to this type of problem.

Reciprocating
Reciprocating compressors are widely used by industry and are offered in a wide
range of sizes and types. They vary from units requiring less than 1 hp to more than