Page 111 - Root Cause Failure Analysis

102 Root Cause Failure Analysis

Mechanical E@ciency
True mechanical efficiency (ME) is equal to the total input power divided by the total
output power.

Total Pressure

Total pressure (TP), in inches of water (gauge), is the sum of the velocity pressure and
static pressure.

Velocity Pressure
Velocity pressure (VP) is produced by the fan only when the air is moving. Air having
a velocity of 4,000 fpm exerts a pressure of 1 in. of water (gauge) on a stationary
object in its flow path.

Natural Frequency
General-purpose fans are designed to operate below their first natural frequency. In
most cases, the fan vendor will design the rotor-support system so that the rotating ele-
ment’s first critical speed is between 10 and 15 percent above the rated running speed.
While this practice is questionable, it is acceptable if the design speed and rotating-ele-
ment mass are maintained. However, if either of these two factors changes, there is a
high probability that serious damage or premature failure will result.

Inlet-air Conditions
As with centrifugal pumps, fans require stable inlet (suction) conditions. Ductwork
should be configured to ensure an adequate volume of clean air or gas, stable inlet
pressure, and laminar flow. If the supply air is extracted from the environment, it is
subject to variations in moisture, dirt content, barometric pressure, and density. How-
ever, these variables should be controlled as much as possible. As a minimum, inlet fil-
ters should be installed to minimize the amount of dirt and moisture that enters the fan.

Excessive moisture and particulates have an extremely negative impact on fan perfor-
mance and cause two major problems: abrasion or tip wear and plate out. High con-
centrations of particulate matter in the inlet air act as abrasives that accelerate fan-
rotor wear. In most cases, however, this wear is restricted to the high-velocity areas of
the rotor, such as the vane or blade tips, but it can affect the entire assembly.

Plate out is a much more serious problem. The combination of particulates and mois-
ture can form a “glue” that binds to the rotor assembly. As this contamination builds
up on the rotor, the assembly’s mass increases, which reduces its natural frequency. If
enough plate out occurs, the fan’s rotational speed may coincide with the rotor’s
reduced natural frequency. With a strong energy source like the running speed, excita-
tion of the rotor’s natural frequency can result in catastrophic fan failure. Even if cata-
strophic failure does not occur, the vibration energy generated by the fan may cause
bearing damage.