Case History No. 17

Industry & Location:

A pulp and paper mill in the southeast

Problem:

Excessive vibrations and structural cracking of recovery boiler stack.

Source of Detection:

Obvious structural failures of stack and associated ductwork

Cause of Problem:

This particular recovery boiler is equipped with two I.D. fans which are operated in series. The series operation is necessary to develop an adequate amount of static pressure to operate the scrubber system. The scrubber is used to remove both particulate and gas contaminants from the recovery boiler stack gases.

Recovery boilers are shut down on a routine basis for cleaning of the furnace area and exhaust duct system. It was observed during the operation of this boiler that moderate vibrations occurred throughout the stack and duct system during the period of time immediately after the cleaning process. However, after a month of operation, it was found that the duct system began to vibrate violently to the point that serious fatigue failures were occurring at the stack and ductwork. The vibrations became more serious with elapsed time after the cleaning process.

Solution:

Measures were taken to structurally stiffen the stack and the duct system. This structural stiffening offered little relief of the cracking problems.

An investigation of the flow rate determined that the volumetric flow rate from the boiler and through the two I.D. fans was reduced as the boiler experienced pluggage problems after the cleaning process. The seriousness of the vibrations corresponded with the reduction in volumetric flow rate.

It became obvious that the vibrations were a direct result of poor fan efficiency which occurs when operating a fan to the left of the peak of the curve (reduced volume). Since this condition could not be prevented, each I.D. fan rotor was equipped with shroud blade extensions.

Fan with Shroud Extensions

On a radial tip fan impeller of this type, such blade extensions maintain a circulation effect in the corners of the scroll during reduced volumetric flow operation. Maintaining a reasonable flow pattern on the outside corners of the scroll prevents vortex shedding and collapsing of the flow at the fan discharge which resulted in the elimination of the vibration problems entirely. The fan performance curve is shown altered by these extensions.

"A" Curve is without Shroud Extensions
"B" Curve is with Shroud Extensions

Benefit:

Elimination of the need to replace a structurally cracked stack and duct system. Rather, the system could be repaired instead of replaced.