Case History No. 3

Industry & Location:

A utility station in the southwest

Problem:

Cracking in the center plate of two 5,000 horsepower F.D. fans

Source Detection:

Non-destructive testing during a planned outage

Cause of Problem:

The original structural design by the fan manufacturer resulted in excessive bending of the center plate and eventual fatigue failure. As shown here, a single hub support configuration often lends to excessive bending of the center plate.

Centerplate to Hub Connection

It is interesting that in this particular case, a stress analysis reveals low center plate stresses, but the natural frequency of the center plate corresponds very closely to the fan operating speed. Such a condition is undesirable because of its effect on fatigue life.

View of Repaired Centerplate

Solution:

Both F.D. fans and the intake for two primary air fans are positioned in a soundproof room enclosure as shown here.

Fan Room

Because of the restrictive size of the room, a twirling action occurs which prevents the gas from entering the fan in the correct angular direction and results in lower than expected operating efficiency. Installation of baffle plates and straightener vanes in the room attenuates the degree of turbulence in the room and improves fan efficiency. During a planned outage, the fan impellers were removed from the housings and completely rebuilt with a double hub configuration. This support concept minimizes the bending moment at the center plate and eliminates the likelihood of future center plate cracks. Any increase in efficiency also dampens the vibrational excitation of the center plate.

Benefit:

Reduced horsepower consumption and extended fan life.