Page 260 - Root Cause Failure Analysis

248 Root Cause Failure Analysis

its critical speed, the point where the phenomenon referred to as resonance occurs.
This occurs because the additional mass affects the rotor’s natural frequency. Even if
the fan’s speed does not change, the change in natural frequency may cause its critical
speed (note that machines may have more than one) to coincide with the actual rotor
speed. If this occurs, the fan will resonate, or experience severe vibration, and may
fail catastrophically. The symptoms of plate out often are confused with those of
mechanical imbalance because both dramatically increase the vibration associated
with the fan’s running speed.

The problem of plate out can be resolved by regularly cleaning the fan’s rotating ele-
ment and internal components. Removal of buildup lowers the rotor’s mass and
returns its natural frequency to the initial, or design, point. In extremely dirty or dusty
environments, it may be advisable to install an automatic cleaning system that uses
high-pressure air or water to periodically remove any buildup that forms.

Speed Changes

In applications where a measurable fan-speed change can occur (Le., V-belt or vari-
able-speed drives), care must be taken to ensure that the selected speed does not coin-
cide with any of the fan’s critical speeds. For general-purpose fans, the actual running
speed is designed to be between 10 and 15 percent below the first critical speed of the
rotating element. If the sheave ratio of a V-belt drive or the actual running speed is
increased above the design value, it may coincide with a critical speed.

Some fans are designed to operate between critical speeds. In these applications, the
fan must make a transition through the first critical speed to reach its operating speed.
Such transitions must be made as quickly as possible to prevent damage. If the fan’s
speed remains at or near the critical speed for any extended period of time, 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 present in the vertical
direction. If support-structure flexing is found to be the root cause or a major contrib-
uting factor to the problem, it can be corrected by increasing the stiffness 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.

BLOWERS OR POSITIVE-DISPLACEMENT FANS
Blowers, or positive-displacement fans, have the same common failure modes as
rotary pumps and compressors. Table 20-2 (also see Tables 19-2 and 22-2), lists the
failure modes that most often affect blowers and fluidizers. In particular, blower fail-