Using technical assessment to identify root cause

A large pulp mill was shut down due to failure of the forced draft fan on its recovery boiler. Excessive vibration of the inboard bearing had broken the cooling water line inside the oil-filled bearing housing and flushed out the oil. The bearing journal had overheated, cracked and melted the Babbitt causing heavy scouring damage to the shaft surface.

When our corporate team arrived on site, the mill maintenance crew was busy hand-dressing the bearing surface on the shaft as a large air motor slowly turned the rotor. Meanwhile, the spare set of bearing sleeves for the fan had been packed up and shipped to a repair shop for re-Babbitting to a

Design changes affect fan performance

A boiler design change, which included upgraded black liquor firing guns, smaller air ports and air-to-fuel ratio changes (to minimize stack emissions) reduced fresh air demand by about 30%. Consequently, the fan was grossly oversized. It was originally sized for 300,000 CFM and 25 inches of static pressure using a 1,200 rpm, 1,500 hp motor. Boiler design changes had not included fan modifications. New requirements were for 225,000 CFM of air flow at 15 inches of static pressure, which equated to about 900 hp. The fan inlet dampers were heavily throttled.

Technical assessment reveals root cause

Our mission was to lead a technical assessment of the situation and recommend a permanent solution to the longstanding problem. We learned that the mill staff had been using a local vibrations consulting company to perform testing and balancing on a semi-annual basis. The vibration consultants identified the problem as a resonant frequency issue.

Original design data for the fan indicated that the first harmonic speed for the unit should be about 1,500 rpm — well above the 1,200 rpm operating speed. However, due to the reduced air flow, higher temperature conditions and a peculiar bearing pedestal sole plate design, the first harmonic speed was actually somewhere around 1,130 rpm.

Mill personnel confirmed that during boiler operation, a slight salt-cake carry-over from the air pre-heater was depositing on the fan rotor, and soon vibration would go from a good condition to a very rough condition. The harmonic resonance was then exacerbating the problem. Over the past two to three years, adding weights became the quick fix. We noted dozens of large weights on the rotor. Indications were that one had come off, which caused this catastrophic failure.

During the briefing meeting, the vibration consultant described the problem and recommendation a change of the harmonic speed. He described connecting heavy steel cables and turnbuckles to the bearing housings, which would be anchored to new concrete piers in the floor. His preliminary computer analysis data indicated that stiffening the foundation would raise the first harmonic speed to about 1,280-1,300 rpm.

The concept was different and initially sounded reasonable. However, our assessment identified an employee safety issue with cables and piers in the main maintenance aisle adjacent to the fan. Also, maintaining the proper cable tension could prove to be another ongoing challenge. The fan rotor was seven feet in diameter and six feet wide with a 12 inch shaft. The weight of the rotating assembly was 16,000 pounds. Stresses on the bearing housings were also identified as a potential issue that could introduce axial vibration.

From a technical perspective, we were concerned about inefficiency of the existing drive. Slowing the fan down to 900 rpm appeared to be a more practical solution. Power savings alone would be on the order of $400 per day. In addition, power factor and harmonic conditions would also be much improved by replacing the 1,500 hp motor.

Identifying solutions

During an extensive brainstorming session, a spare 900 rpm, 750 hp motor for the boiler induced draft fan was identified. Our evaluation also took into consideration long-term reliability and boiler capacity, safety concerns, future maintenance workload and cost alternatives. Starting the boiler back up with the existing 1,200 rpm drive motor was the agreed-upon interim direction.

The unit was started two days later with the re-Babbitted bearing sleeves and a clean fan rotor with only two balance weights. An extensive testing regimen was developed to verify air flows, static pressure and power requirements under various load conditions. Test data were analyzed and later confirmed that operating the fan at 900 rpm should work satisfactorily even without having to re-tip the fan blades. A month later, at the next available outage, the spare 900 rpm motor was installed and the 1,500 hp motor was sent out to be rewound for 750 hp at 900 rpm.

The rewound motor was reinstalled three months later, along with new fan bearings that did not have removable sole plates. Once the unit was performing satisfactorily, fan inlet louvers were reconfigured to match static pressure requirements. Fan vibration at full load on the boiler was down to 1/10 of the previous readings. Motor load was 595 hp — well below the 750 hp maximum. The boiler has continued to operate at or near full load with no fan problems.

Lessons to be learned

Be careful about running with the first solution offered by an outside expert. Generally, experienced, involved team members conducting problem-solving exercises as part of a technical assessment can be a very sound maintenance strategy.

A forced draft fan on a recovery boiler in a large pulp mill experienced excessive vibration resulting in bearing failure. A boiler design change reduced the speed and horsepower requirements of the fan motor. Matching the motor to the load solved the problem, improved fan performance and significantly reduced full-load fan vibration.

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