Routine Operation Inspection and Abnormal Condition Handling of Fans

I.Routine Operation Inspection of Fans

1. No abnormal noise during fan and motor operation; no internal rubbing or jamming. The oil level in the oil station tank should be maintained within the 1/3 to 2/3 range. If abnormal drops or rises in oil level are found, the causes should be identified and addressed immediately.

2. Observe the oil quality in the oil tank through the oil sight glass; it should be transparent, free of emulsification and impurities, and free of moisture and water droplets on the sight glass.

3. Monitor the lubricating oil temperature to ensure it is normal. When the oil tank temperature exceeds 40°C, the electric heater should automatically cut out.

4. The moving blade (pitch) control oil pressure should be normally adjusted to 2.5–3.5 MPa, the bearing lubricating oil pressure to 0.3–0.35 MPa, and the differential pressure across the oil station strainer to less than 0.05 MPa. The bearing lubricating oil supply temperature should be adjusted to 30–40°C, and the lubricating oil flow rate should be greater than 8 L/min.

5. No leakage in the fan oil system; no leakage in the cooling water pipes of the oil cooler at the oil station; unobstructed cooling water flow. The cooling water pressure should be between 0.2–0.6 MPa, and the water temperature should be ≤38°C.

6. The normal operating point of the fan should be below the minimum stall line. The moving blade opening should be within the range of -30° to +13° (corresponding to a feedback indication of 0–100%). The remote and local opening indications should be consistent to ensure no system surge during fan operation and no overload of the forced draft (FD) fan motor.

7. The bearing temperatures of the fan and motor should normally be in the 50–70°C range. If the bearing temperature is found to exceed the normal range, and no abnormalities are found after inspection and adjustment of the oil system, the fan should be shut down early for inspection and treatment. If the motor bearing temperature exceeds 95°C, or the fan bearing temperature exceeds 90°C and the protection fails to actuate, the fan must be manually shut down.

8. The bearing vibration of the fan and motor during operation should be below 0.063 mm. If the vibration exceeds 0.10 mm, the fan should be shut down immediately.

9. The fan motor coil temperature should not exceed 115°C. There should be no signs of overheating, smoke, or fire in the fan motor and associated cables, and no burnt insulation odor on site. If any abnormality is found, the root cause should be identified and addressed immediately.

II. Fan Surge

Q-H Performance Curve of Axial Flow Fans

For the Q-H performance curve of an axial flow fan, if throttling is used to reduce the fan flow, the fan operates stably as long as the operating point is on the right side of Point K. When the fan flow Q < QK, the maximum pressure head generated by the fan will decrease accordingly and become less than the pressure in the pipeline. Because the duct system capacity is large, the pressure in the duct momentarily remains at HK. Therefore, the pressure in the duct is greater than the pressure head generated by the fan, causing the airflow to reverse direction and flow from the duct back into the fan. The operating point rapidly moves from Point K to Point C. However, the reverse flow reduces the air volume in the duct system, so the pressure in the duct drops rapidly, and the operating point rapidly descends along line CD to Point D where flow Q=0. At this time, the air volume supplied by the fan is zero.

Because the fan continues to run, when the pressure in the duct drops to the corresponding level at Point D, the fan starts to output flow again. To balance the pressure in the duct, the operating point jumps from Point D to the corresponding operating point F. As long as the externally required flow remains less than QK, the above process repeats. If the fan's operating state cycles continuously in the sequence F-K-C-D-F, and the frequency of this cycle resonates with the oscillation frequency of the fan system, resonance will occur, and the fan will experience surge.

III. Handling of Fan Surge

1. Symptoms

1. A "Fan Surge" alarm signal appears on the LCD.

2. Furnace pressure and air volume fluctuate significantly, and boiler combustion becomes unstable.

3. The current of the surging fan fluctuates drastically, and severe abnormal noise is detected during local inspection.

2. Causes

1. Severe ash accumulation on heating surfaces or in the air preheater, or accidental closure of flue gas system dampers, increases system resistance. This causes a mismatch between the fan moving blade opening and the incoming air/flue gas volume, driving the fan into the surge region.

2. Operating the fan moving blades with excessive adjustment amplitude pushes the fan into the surge region.

3. Deterioration of moving blade control characteristics causes two parallel-running fans to "fight for air" (compete for flow), or automatic control failure causes one of the fans to enter the surge region.

4. During high unit load, sootblowers are put into operation, or the supply air volume is too large.

3. Remedies

1. Immediately place the fan moving blade control in manual mode, close the moving blades of the other non-surging fan, appropriately close the moving blades of the surging fan, and coordinately adjust the induced draft (ID) and forced draft (FD) fans to maintain furnace negative pressure within the allowable range.

2. If surge occurs during parallel operation, stop the paralleling process, close the moving blades of the surging fan as soon as possible, and perform parallel operation again only after the cause is identified and eliminated.

3. If fan surge is caused by accidental closure of dampers in the air/flue gas system, open them immediately and adjust the moving blade opening. If the dampers are faulty, immediately reduce the boiler load and contact maintenance; if caused by sootblowing, stop it immediately.

4. If the surge disappears after the above handling, stabilize the operating conditions, further identify the cause, and take corresponding measures before gradually increasing the fan load. If the above handling is ineffective or seriously threatens equipment safety, shut down the fan immediately.

IV. Fan Stall

Formation of Rotating Stall in Moving Blades

Gas Flow Condition under Normal Fan Operation

Gas Flow Condition under Fan Stall

During normal fan operation, the blade angle of attack does not exceed the critical value, and the airflow passes smoothly along the convex surface of the blades. However, once the blade angle of attack exceeds the critical value (entering the saddle-shaped region), the airflow will separate from the convex surface of the blade, causing boundary layer separation and generating a large vortex region. At this time, the total pressure of the fan drops. This phenomenon is called fan stall.

V. Hazards and Handling of Fan Stall

Rotating stall poses a threat to the safe operation of axial flow fans. When rotating stall occurs within the cascade flow passage, it causes blockage of the passage, thereby changing the pressure before and after the blades. Under rotating stall conditions, the stall zone rotates and passes through each blade in sequence, subjecting each blade to an excitation force. When the frequency of rotating stall (i.e., the frequency of the excitation force) equals or approaches the natural vibration frequency of the blade, it will cause blade resonance. The alternating stress during resonance may reach a level sufficient to fracture the blades.

When fan stall occurs, timely adjustments should be made (closing the moving blades, opening/clearing the air duct) to bring the fan out of the unstable operating region.

2026 October 3rd Week FK Product Recommendation

Pillow Block:

A pillow block bearing (or plummer block) is a mounting used to support a rotating shaft with the use of bearings and various accessories. The assembly consists of a mounting block which houses a bearing. The block is mounted to a foundation, and a shaft is inserted, allowing the inner part of the bearing/shaft to rotate. The inside of the bearing is typically 0.025 millimetres (0.001 in) larger diameter than the shaft to ensure a tight fit. Set screws, locking collars, or set collars are commonly used to secure the shaft. Housing material for a pillow block is typically made of cast iron or cast steel.

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