Page 122 - Root Cause Failure Analysis

110 Root Cause Failure Analysis
lined blades and linkage arranged to move adjacent blades in opposite directions for
even throttling.

Air-flow controllers must be inspected frequently to ensure that they are fully opera-
ble and operate in unison with each other. They also must close tightly. Ensure that
the control indicators show the precise position of the vanes in all operational condi-
tions. The “open” and “closed” positions should be permanently marked and visible at
all times. Periodic lubrication of linkages is required.

Turn-buckle screws on the linkages for adjusting flow rates should never be moved
without first measuring the distance between the set-point markers on each screw.
This is important if the adjustments do not produce the desired effect and you wish to
return to the original settings.

Operating Methods
Because fans are designed for stable, steady-state operation, variations in speed or
load may have an adverse effect on their operating dynamics. The primary operating
method that should be understood is output control. No methods can be used to con-
trol fan output: dampers and fan speed.

Dampers
Dampers can be used to control the output of centrifugal fans within the effective con-
trol limits. Centrifugal fans have a finite acceptable control range, typically about 15
percent above and below its design point. Control variations outside this range
severely affect the reliability and useful life of the fan.

The recommended practice is to use an inlet damper rather than a discharge damper
for this control function whenever possible. Restricting the inlet with suction dampers
can effectively control the fan’s output. When using dampers to control fan perfor-
mance, however, caution should be exercised to ensure that any changes remain
within the fan’s effective control range.
Fan Speed

Varying fan speed is an effective means of controlling a fan’s performance. As defined
by the fan laws (discussed earlier), both volume and pressure can be controlled
directly by changing the rotating speed of the fan. However, caution must be used
when changing fan speed. All rotating elements, including fan rotors, have one or
more critical speeds. When the fan’s speed coincides with one of the critical speeds,
the rotor assembly becomes extremely unstable and could fail catastrophically.

In most general purpose applications, fans are designed to operate between 10 and 15
percent below their first critical speed. If speed is increased on these fans, there is a
good potential for a critical-speed problem. Other applications have fans designed to