FAILURE MODES AND EFFECTS ANALYSIS
4.15 Variable-Speed Drive
Hydraulic couplings are subject to instability, which results in fan speed surging.
4.16 Controls
Problems with controls subsystems include the following:
• Frozen air lines
• Vanes that opened too quickly
• Lack of bearing-temperature sensors
• Vibration sensors mounted vertically rather than horizontally
• Proximity vibration sensors that are found unreliable
• Failure of damper torque operator 4.17 Ductwork
Problems with ductwork include the following:
• Insufficient bracing
• Uneven distribution of fly ash and/or gas to the fans
• Excessive system resistance
• Insufficient bracing of stationary vanes
• Ductwork misalignment with housing
• Corrosion
• Expansion joint failure
5
TROUBLESHOOTING
Troubleshooting and industry experience help identify probable causes and provide suitable corrective actions. Tables 5-1 through 5-7 provide potential problems, possible causes, and corrective actions. Many of these problems apply to both centrifugal and axial flow fans.
Problems that are unique to one type of fan have been identified, but virtually all of the potential problems apply to both FD and ID fans.
Table 5-1
Fan Troubleshooting
Problem Possible Cause Corrective Action
Deposits on blades Clean the entire rotor.
Cracked blade welds Repair the welds (see Appendix B).
Missing balance weights Rebalance the fan.
Ash inside hollow blades Cut openings in blades, remove ash, and repair holes.
Loose bearing or pedestal bolts Check the torque of bolts.
Loose foundation bolts Check the torque of bolts.
Cracked pedestal welds Inspect and repair the welds.
Coupling/shaft misalignment (typical symptom is a vibration that is 2x running speed)
Check alignment.
Damaged bearing(s) Check oil for contamination, water, and particles; inspect bearings;
check bearing alignment; and check bearing temperatures.
Damaged thrust collars Inspect thrust collars and alignment.
Fan wheel moving on shaft Inspect hub bolts and hub-to-shaft fit.
High vibration
Foundation problems Inspect the foundation for cracks;
inspect grout.
Table 5-1
Fan Troubleshooting (continued)
Problem Possible Cause Corrective Action
Fan wheel distortion Verify whether the fan has experienced extreme temperature transient; inspect the wheel for distortion.
High vibration (cont.)
Imbalance in inlet vanes Verify that the inlet vanes at both inlets are synchronized.
Motor problem Check motor vibration, check motor bearings, verify that the motor is running within its magnetic center, and check motor bearing
temperatures.
Seal wear (seals that have excessive wear permit air/gas stream leakage)
Unknown Balance the fan.
Air/gas distribution: ducts,
Reinstall the seal according to the proper procedure.
Improper operation Verify that the variable-speed drive has been tuned to deliver the ordered input.
Electric motor alignment (increases the hp required)
Incorrect readings Recalibrate sensors.
Fan wheel clearance
Check and realign the motor.
Shaft Excessive clearance between the inlet ring of the fan wheel and the housing inlet cone
Check clearances between the fan and the housing.
Bent, undersized Verify and replace the shaft.
Fan size or type not the best for the application
Check the fan type and design.
Low density Verify air or gas temperature.
Determine the actual fan operating point.
Horsepower too high
Point of operation on curve
Check fan assembly and adjustment,
Table 5-1
Fan Troubleshooting (continued)
Problem Possible Cause Corrective Action
Horsepower too high (cont.) Incorrect motor selection Verify motor selection/size per application.
Blown fuses Inspect fuses.
Impeller touching housing Inspect and verify access.
Fan will not start
Wrong voltage Verify voltage for application.
Wrong fan rotation Check blade angles in relation to fan rotation.
Air volume too small
Inlet or outlet obstructions Remove obstructions.
Table 5-2
Bearing Troubleshooting
Problem Possible Cause Correction Action
Verify water flow and temperature (self-contained bearing cooling).
Inadequate cooling
Verify oil flow and temperature (circulating lube oil system); verify that pumps are operating and that filters are not blocked.
Erroneous data Verify that thermocouple/RTO is operating correctly; a portable thermometer may often be used to check the bearing operating temperature against fixed sensor.
Lube oil contaminated Verify that the lube oil is clean and of sufficient amount.
Oil ring damage Verify that the oil ring is operating correctly.
Heat flingers damaged Confirm that heat flingers (if
applicable) are not damaged or dirty.
Bearing alignment Check bearing alignment.
High bearing temperature
Bearing damage Open and inspect the bearing:
1. Check for excessive or insufficient clearance.
2. Wipe the bearing surfaces.
Table 5-2
Bearing Troubleshooting (continued)
Problem Possible Cause Correction Action
Fan shaft alignment Verify that the fan shaft is not distorted or misaligned.
Cooling fan problem (axial flow fan)
Verify that the cooling fan is operating properly.
External heat source Verify that no externally applied heat source is present, such as high ambient temperature.
High bearing temperature (cont.)
High vibration See guidance on high vibration previously in this table.
Table 5-3
Lubrication System Troubleshooting
Problem Possible Cause Corrective Action
Low oil level Add oil.
Pumps not operating Repair pumps.
Dirty filter Clean or replace the filter.
Incorrect system lineup Review the valve lineup.
No lube oil flow
Heat exchanger pluggage Inspect the heat exchanger.
Dirty filter Clean or replace the filter.
Pump capacity Examine pumps.
Incorrect line Verify proper valve positions.
Oil viscosity Check oil temperature and type.
Pressure switches Verify set points and operation.
Relief valve Verify set points and position.
Low oil pressure
Flow orifice Verify size of the flow orifice.
Pump operation Check the pump alignment for damage.
Relief valve Verify position of the relief valve.
Cooling fan Examine operation of the cooling fan.
Noise/vibration
Oil viscosity Check temperature and oil type.
Table 5-3
Lubrication System Troubleshooting (continued)
Problem Possible Cause Corrective Action Cooling water leaks Examine and test heat exchangers.
Filter Verify the integrity of filters.
Oil contamination
Storage tank Verify that the oil source is clean.
Heat exchanger Inspect heat exchangers.
Cooling fan (air cooled) Verify fan operation.
Cooling water Check temperature and flow rate of cooling water
Heater Check set point and operating of heaters.
High oil temperature
High bearing temperature Inspect the bearing and bearing clearance to identify heat generation.
Table 5-4
Hydraulic System Troubleshooting
Problem Possible Cause Corrective Action Leakage in rotating union Replace the union.
Leakage in oil line Replace the line.
Hydraulic oil leakage in the fan diffuser
section (axial flow fan)
Leakage in hydraulic cylinder Replace the cylinder.
Cooling air fan stopped Start the fan.
Hydraulic oil leakage in intermediate section (axial
flow fan) Cooling air duct blocked Clear the duct.
Table 5-5
Troubleshooting Noise Level
Problem Possible Cause Corrective Action Insufficient bearing clearance Adjust or replace the bearing.
Damaged bearing surface Replace the bearing or liner.
Inadequate lubrication Add oil or check the lube oil system.
Worn or damage coupling Replace the coupling.
Coupling alignment; poor wheel balance
Realign the coupling; balance the wheel.
Damper or inlet vanes failed close Inspect and repair the dampers or vanes.
Stall Review the fan operating point.
Fan wheel rub Inspect clearance between the wheel and housing.
Motor noise See Electric Motor Predictive and Preventive Maintenance Guide [3]
Impeller hitting inlet Realign the impeller.
Loose motor bolts Tighten the bolts.
Wrong pulley size Replace the pulley.
Defective bearings Replace the bearings.
Noise
Bent or undersized shaft Replace the shaft.
Table 5-6
Troubleshooting Fluid Drive
Problem Possible Cause Corrective Action High fluid drive oil
temperature
Inadequate cooling 1. Verify that the ambient
temperature of the cooling water is within design limits.
2. Verify that there are no restrictions in the water lines: check drain points to confirm water supply to various points.
3. Verify that the water side of the heat exchanger is clean and unobstructed.
4. Vent the cooler of any air pockets that may have formed if the system was disassembled for maintenance.
5. Verify that cooling system is properly aligned; confirm that all valves are functioning properly.
6. Check system strainers/filters for dirt or debris buildup.
7. Vent system lines to remove any air pockets.
8. Conduct a visual inspection of the quality of hydraulic oil; check for water or dirt contamination.
9. Inspect the cooling water system for proper pressure.
Table 5-6
Troubleshooting Fluid Drive (continued)
Problem Possible Cause Corrective Action Poor oil circulation 1. Check filters: shift or clean as
required.
2. Verify that the hydraulic oil system is aligned for operation.
3. Inspect all piping, valves, and filter assemblies for leakage.
4. Verify that all designated valves are opened correctly, and inspect oil side of the heat exchanger for clogs or buildup of dirt. Restrictions at the pump suction point may indicate a buildup of sludge in the oil reservoir.
5. Inspect pumps for proper operation; low discharge pressure may indicate a worn impeller. Check for dirty suction strainers.
6. Verify that relief valves are properly set and not opening prematurely.
7. Vent oil side to remove any air pockets that may have formed if the system was opened.
Incorrect oil viscosity or contamination
Verify that the proper oil is used, and analyze oil quality.
Oil heaters operating Shut off heaters.
Low oil level Verify oil level.
High fluid drive oil temperature (cont.)
Bearing damage or wear Inspect bearings.
High oil temperature See corrective actions for high oil temperature in Table 5-3.
Plugged strainers Clean strainers.
Damaged oil pumps Inspect the pump for damage or internal debris.
Low fluid drive oil pressure
Relief valves open Verify that relief valves are functioning properly.
Table 5-6
Troubleshooting Fluid Drive (continued)
Problem Possible Cause Corrective Action
Loose bolts Verify that all foundation and machinery hold-down bolts are properly tightened; inspect for cracked bolt heads. Ensure that fasteners on the steel housing are properly tightened. Inspect all peripheral connections and equipment for damaged or loose fittings.
Alignment Check the alignment between the motor and fluid drive and between the fluid drive and fan.
High fluid drive vibration
Bearing damage Inspect fluid drive bearings.
Labyrinth seal Inspect seals for damage.
Piping and valves Check for leaks.
Fluid drive oil leaks
Input/output shafts Check for scoring on the shafts.
Table 5-7
Fan Performance Troubleshooting
Problem Possible Cause Corrective Action
Inlet vanes Verify that inlet vanes are responding to the controls. Verify that the inlet vanes on double-inlet fans are synchronized.
Fan speed control Verify that speed control (if equipped) is responding properly.
Uneven air flow For double-inlet fans, verify that the flow is even on both inlets (within 5%).
System conditions Evaluate that the actual fan performance correct for actual conditions.
Wheel to housing clearance Verify that the clearance between the wheel and housing is within
recommended limits.
Poor fan performance
Fan damage Inspect fan blades for damage.
Table 5-7
Fan Performance Troubleshooting (continued)
Problem Possible Cause Corrective Action
Turning gear high oil temperature
Oil contaminated Analyze oil, and check viscosity.
Alignment Check alignment between fan and turning gear.
Balance Check the balance of the coupling and clutch.
Turning gear high vibration
Loose bolts Check torque of hold-down bolts.
Fan surge If fan is operating with inlet vanes less than 30% open, the fan may be in an unstable range. Changes in the system or operation may be the easiest solution. Fans with inlet box dampers may become unstable when the dampers are less than 50%
open.
Parallel operation Multiple fans operating in parallel can be unstable, depending on the shape of the system curve and the fan curves.
Pressure pulsations (centrifugal)
Operating point Measure the fan flow and head, and plot the operating point to verify that the fan is not operating in the unstable region.
Pressure pulsations (axial) Stall Verify the actual operating point of the fan versus the stall line.