Page 262 - Root Cause Failure Analysis

250 Root Cause Failure Analysis

Demand Variations
Variations in pressure and volume demands have a serious impact on blower reliabil-
ity. Since blowers are positive-displacement devices, they generate a constant volume
and a variable pressure that is dependent on the downstream system’s back pressure.
If demand decreases, the blower’s discharge pressure continues to increase until (1) a
downstream component fails and reduces the back pressure or (2) the brake horse-
power required to drive the blower is greater than the motor’s locked rotor rating.
Either of these result in failure of the blower system. The former may result in a
reportable release, while the latter will cause the motor to trip or bum out.

Frequent variations in demand greatly accelerate the wear on the thrust bearings in the
blower. This can be directly attributed to the constant, instantaneous axial thrusting
caused by variations in the discharge pressure required by the downstream system.

Mechanical Failures

Because of the extremely close clearances that must exist within the blower, the
potential for serious mechanical damage or catastrophic failure is higher than with
other rotating machinery. The primary failure points include thrust bearing, timing
gears, and rotor assemblies.

In many cases, these mechanical failures are caused by the instability discussed in the
preceding sections, but poor maintenance practices are another major cause. See the
troubleshooting guide Table 22-2 for rotary-type, positive-displacement compressors
for more information.