Page 237 - Intro Predictive Maintenance

228 An Introduction to Predictive Maintenance

fan’s speed remains at or near the critical speed for any extended period, serious
damage can occur.

Lateral Flexibility
By design, the structural support of most general-purpose fans lacks the mass
and rigidity needed to prevent flexing of the fan’s housing and rotating assembly.
This problem is more pronounced in the horizontal plane, but also is present in the
vertical direction. If support-structure flexing is found to be the root-cause or a major
contributing factor to the problem, it can be corrected by increasing the stiffness
and/or mass of the structure; however, do not fill the structure with concrete. As it
dries, concrete pulls away from the structure and does little to improve its rigidity.

10.2.2 Blowers or Positive-Displacement Fans
Blowers, or positive-displacement fans, have the same common failure modes as
rotary pumps and compressors. Table 10–5 (see also Tables 10–2 and 10–9) lists the
failure modes that most often affect blowers and fluidizers. In particular, blower fail-
ures occur because of process instability, caused by start/stop operation and demand
variations, and mechanical failures caused by close tolerances.

Process Instability
Blowers are very sensitive to variations in their operating envelope. As little as a one
psig change in downstream pressure can cause the blower to become extremely unsta-
ble. The probability of catastrophic failure or severe damage to blower components
increases in direct proportion to the amount and speed of the variation in demand or
downstream pressure.

Start/Stop Operation. The transients caused by frequent start/stop operation also have
a negative effect on blower reliability. Conversely, blowers that operate constantly in
a stable environment rarely exhibit problems. The major reason is the severe axial
thrusting caused by the frequent variations in suction or discharge pressure caused by
the start/stop operation.

Demand Variations. Variations in pressure and volume demands have a serious im-
pact on blower reliability. Because blowers are positive-displacement devices, they
generate a constant volume and a variable pressure that depends on the downstream
system’s back-pressure. If demand decreases, the blower’s discharge pressure contin-
ues to increase until (1) a downstream component fails and reduces the back-pressure,
or (2) the brake horsepower required to drive the blower is greater than the motor’s
locked rotor rating. Either of these outcomes will result in failure of the blower system.
The former may result in a reportable release, whereas the latter will cause the motor
to trip or burn out.

Frequent variations in demand greatly accelerate the wear rate of the thrust bearings
in the blower. This can be directly attributed to the constant, instantaneous axial