Case History No. 7

Industry & Location:

A utility station in the south

Problem:

Excessive fan housing pulsations

Source of Detection:

The dislodging of thermal insulation because of excessive housing vibrations.

Cause of Problems:

This case history is very similar to Case History No. 6 except that scrubbers are used to remove sulfur dioxide fumes instead of hydrogen fluoride fumes. Coal fired utility plants are customarily equipped with gas desulfurization systems for this purpose. At this particular station, two 4,000 horsepower I.D. fans are used to induce flue gases from the boiler and a gas scrubber system. A scrubber bypass damper is used to blend the hot flue gases with the gases exiting the scrubber system. Such a bypass procedure is not uncommon since continuous stack gas monitors allow the operator to accurately control the amount of bypass gases.

Because the scrubber system has proven to be more efficient than originally anticipated, the bypass damper is operated in a more open position which results in a lower pressure drop requirement across the scrubber and less pressure requirement from the individual I.D. fans. Because of the reduced pressure requirement of the I.D. fans, the individual inlet box dampers are operated in a throttled position and are less than 50% open even at maximum megawatt capacity. Operating a centrifugal fan with throttled inlet box dampers results in reduced efficiency and excessive pulsations of the fan housing.

Original Inlet Box Dampers

Solution:

The inlet box dampers had to be replaced with ones which are more aerodynamically efficient to eliminate inlet turbulence. Also, the fans had to be modified so that they developed less pressure; thus, eliminating the need to operate with the inlet damper throttled. The existing radial tip fan impellers have been replaced with airfoil impellers which are considerably more efficient. Because of the high collection efficiency of upstream precipitators, little concern from maintenance engineers was expressed about the use of airfoil fan impellers on an induced draft application. The replacement of the fan impellers in conjunction with inlet box dampers has resulted in considerable horsepower savings and has totally eliminated the housing pulsation problem. The cost to replace impellers and inlet box dampers is approximately one-fourth the cost of a total fan replacement project. For that reason, the payback of this project was measured in months and not years.

Benefit:

A lowering of horsepower consumption at each I.D.fan by 1300 horsepower at a power generation of 250 megawatts.