Cooling Tower Condition Monitoring
Cooling towers are a critical component of production in multiple industries today. They are used in power plants, chemical plants, refineries, food processing units and large buildings (HVAC). Cooling towers mostly act as heat exchangers to cool water from hot temperature to required normal temperature and indirect cooling equipment to remove heat from water discharge.
Cooling Towers are made up of a motor, gearbox, fan and connecting jackshaft. The gearbox translates the motors 1500/1800 RPM to a lower RPM for the fan. Fan speed varies from less than 100 RPM to a few hundred RPM depending on diameter. Rolling element bearings are standard and should be monitored using properly placed Industrial accelerometers. Care must be taken to waterproof all accelerometer connectors by flooding with Silicone Grease. For monitoring system, the LF (Low Frequency) option should be selected for the low-speed gearbox shaft.
Only “X” or Horizontal Accelerometers marked in Red are recommended for Full-Time Vibration Monitoring (Protection) System. Walk Around Data Collector or PdM Programs usually require “X” Horizontal (Red), “Y” Vertical (Green) and “Z” Axial (Yellow) Sensors for analysis. When economic decisions must be made to reduce sensor count due to the size or critical of the motor driven.
Pump “X” or horizontal sensors should be installed first as they are closer to the load zone of the bearing and in direction of freedom of movement.
Cooling Tower Vibration Diagnostic
Vibration monitoring offers a vital early warning, enabling engineers to take action before any substantial damage is caused. Access to continuous data and on-line vibration measurement allows you to detect problems before they lead to failure, providing insight into the health of cooling tower gearboxes and motors. Vibration alerts warn of pending problems, enabling the further investigation to detect and diagnose the fault so maintenance can be scheduled. Vibration and temperature measurement would lead to the identification of a number of mechanical faults with predictive diagnostics such as:
- Misaligned gears
- Bent shaft
- Damaged couplings
- Lubrication problems
- Structural and mechanical looseness
- Water intrusion
- Misaligned drive shaft
- Rolling element bearing failure
- Chipped, broken, or worn gear teeth
Cooling Tower Vibration Measurement
Cooling tower fans operate under different load conditions. They often only run at peak load for short periods. With differing stresses over a prolonged period, the mechanical components can develop degraded performance, which leads to failure. As with any rotating equipment, bearing and gear failures, misaligned drive shafts, and excessive vibration are common.
On the other hand, the structure of a cooling tower makes collecting vibration data on the gearbox difficult and dangerous without permanently installed sensors i.e. while the motor is often accessible, the gearbox and fans are usually located inside the cooling tower cell, making these components inaccessible while the fan is in operation. Because the gearbox is a typical failure point, lack of feedback on the machine’s health puts you at risk for unexpected failure.
The classic legacy solution involved the use of earthquake mechanical switches. These devices utilized spring and magnet concept-based mechanical switches to shut down the fan in case of high vibration amplitudes. Reliability becomes an issue with mechanical switches due to harsh cooling tower environments, especially in critical applications. In fact like every other application cooling towers have their own application issues which may be summarized as:
- Effect of the flexibility of the tower on vibration amplitudes
- High vibration values during startup
- Usage of complex speed-reducing gearboxes (1500/120 RPM)
- Effect of neighboring cells on the measurement i.e. starting cell 2 may shut down cell 1
- Speeds are slowly based on the linear velocity of the tip of the blade leading to the necessity of a vibration analyzer capable of processing low speeds and frequencies.
- Water build up in blades and corrosion from bad pH and acidic environments
- Reversing fans in cold climates
- The often have large distances to control room
- Lack of ease of access to gearboxes and fan bearings for vibration analysis with portable data collectors