Page 258 - Root Cause Failure Analysis

20

FANS, BLOWERS, AND FLUIDIZERS

Tables 20-1 and 20-2 list the common failure modes for fans, blowers, and fluidizers.
vpical problems with these devices include output below rating, vibration and noise,
and overloaded driver bearings.

CENTRIFUGAL FANS

Centrifugal fans are extremely sensitive to variations in either suction or discharge
conditions. In addition to variations in ambient conditions (Le., temperature, humid-
ity, etc.), control variables can have a direct effect on fan performance and reliability.

Most problems that limit fan performance and reliability are caused, either directly or
indirectly, by improper application, installation, operation, or maintenance. However,
the majority are caused by misapplication or poor operating practices. Table 20-1 lists
failure modes of centrifugal fans and their causes. Some of the more common failures
are aerodynamic instability, plate-out, speed changes, and lateral flexibility.

Aerodynamic Instability
Generally, the control range of centrifugal fans is about 15 percent above and below
its BEP. Fans operated outside this range tend to become progressively more unstable,
which causes the fan’s rotor assembly and shaft to deflect from their true centerline.
This deflection increases the vibration energy of the fan and accelerates the wear on
bearings and other drive-train components.

Plate Out

Dirt, moisture, and other contaminates tend to adhere to the fan’s rotating element.
This buildup, called plate our, increases the mass of the rotor assembly and decreases

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