Case History No. 5

Industry & Location:

A pulp and paper mill in the southeast

Problem:

Excessive vibrations on a boiler penthouse seal air blower

Source of Detection:

Frequent bearing replacement and continuous vibration problems

Cause of Problem:

Penthouse seal blowers are standardized fan designs of commercial grade quality. An operating speed of 3600 RPM is not uncommon since this choice of speed results in minimum capital cost. Seal air blowers are considered incidental auxiliary equipment on large precipitators which customarily cost over a million dollars. Because the blowers receive little attention during the engineering stage, they are primarily purchased on an initial cost basis and their static pressure and volumetric capacity are not specified properly from a fan engineering standpoint. As a result, they generally develop more pressure than is needed.

Because of the excessive pressure capability of a 3600 RPM blower, seal air blowers are generally operated with an outlet butterfly damper in a throttled position. The throttled position of the outlet damper is required to prevent overloading of the fan motor which has not been properly selected from a fan engineering standpoint. A 3600 RPM blower operating with the outlet damper in an open position and blowing into a relatively unrestricted chamber results in excessive amperage drawn at the fan motor.

Solution:

In this particular case history, it was determined that the pressure drop across the butterfly outlet damper represented over 80 percent of the pressure developed by the fan. By using the fan laws, the fan performance at the next lower synchronous speed is found more than adequate. The reduction in static pressure because of the speed reduction is less than the restrictive pressure loss of the outlet damper. Even with the speed reduction, the volumetric flow from the standard pressure blower with the outlet damper in an open position exceeds the volumetric flow in the prior condition with the outlet damper in a throttled position. Adequate volumetric flow is provided to the penthouse chamber and the problem is solved.

Before and After, Penthouse Seal Sir Blower

Benefits:

Reduced horsepower consumption and elimination of the continuous maintenance problems.