Condition Monitoring of Cooling Tower Fans
The Cooling Tower Fan (CTF) is the evaporative “heat sink” of an industrial cooling system. Its operational performance is essential to an End User’s continuous process. Since the CTF is the open end of a cooling system, it requires attention to design detail to ensure it functions to specification and is sustainable. The loss of the CTF at peak heat load can shut down a process plant causing huge losses, thus the CTF is categorised as critical. Usually cooling tower fans are constructed well above ground level with motors, gearbox and fan drives mounted on a structure that is susceptible to flexing. This movement can result in misalignment, unbalance and gear drive problems. An effective vibration monitoring system will gives advanced warning of stressed machinery. Cooling Tower Fans account for a major share of electricity used and it is a well recorded fact that an out of balance fan will consume or waste up to 15% more energy
Cooling Tower fans have certain aspects that make them stand apart from other rotating equipment. Designing a vibration monitoring system must take into account the following:
- Combination of both slow turning & normal running speeds.
- Harsh environment, high humidity, >95% non condensing
- Structural rigidity and foundation problems.
- Drive shaft alignment
- Fan imbalance & eccentricity
- Gear mesh defects.
- Motor, gearbox & fan bearings
- Motor electric faults, cracked rotor bars etc.
- VERTICAL on drive end of the motor. Measures Peak Velocity mm/sec to detect bearing deterioration looseness & unbalance
- AXIAL on motor. Measures Peak Velocity in mm/sec to detect jack or carden shaft alignment and axial thrust.
- VERTICAL on gear box in main bearing area. Measures Peak Velocity in mm/sec to detect gear mesh, bearing condition and structural looseness.
- HORIZONTAL nearest to fan drive. Measures Displacement microns (Pk-Pk) to detect fan unbalance and looseness
VIBRATION DIAGNOSTIC CHART - FAULTS COMMON TO COOLING TOWER FANS
|1||Unbalance||Proportional to Unbalance in Radial Direction||1xRPM||Single Steady Reference Mark|
Most common cause of vibration. Correct by balancing each rotating parts before assembly then after assembly.
|2||Misalignment of couplings, bearings or bent shaft|
Large in axial direction 50% or more of radial vibration
|1xRPM usual 2x or 3xRPM|
Single double or triple reference mark
|Best identified by appearance of large axial vibration. Use dial indicators or other method for positive diagnosis. If sleeve bearing machine with no coupling misalignment balance the rotor|
Unsteady - use velocity measurements
|High frequency 35K to 55K CPM not related to RPM.||Erratic or fanning reference mark||Sensor proximity to a defective bearing is a clear indicator. Noise analysis may be useful if frequency is very high.|
|Often highest in vertical direction||2xRPM||1 or 2 reference marks erratic||Usually accompanied by unbalance and/or misalignment|
|5||Aerodynamic Forces||Axial readings may be higher than normal|
1xRPM or number of blades on fan or impellor x RPM
Steady if one blade damaged - like unbalance
Likely to occur where blade is bent or out of track. Use noise analysis for high frequencies.