Condition Monitoring of Cooling Towers
Cooling towers are systems that reject heat from fluids used in thermal processes.
This need is found in a wide range of applications, from room air conditioning to power generation.
Thus, cooling towers are employed in sugar and ethanol mills, thermoelectric and nuclear power plants, steel mills, chemical and petrochemical, pulp and paper, food industries, and others.
They are also found in commercial and industrial air-conditioning systems and in refrigeration plants.
For many of these applications, the temperature control process is indispensable and almost all the production must be stopped if this parameter exceeds its tolerance limits.
In these cases, cooling towers are considered critical machines for production continuity and for controlling the operating temperature of other critical equipments.
Therefore, ensuring the availability and reliability of these assets means avoiding damage to other machines, losses in process efficiency, production stoppages, and, consequently, financial losses.
Construction and Operation Principle
The constructive configurations and operating principles of cooling towers are diverse.
One of the most used types, because of its versatility and relative low cost, is the mechanical draught dry tower for cooling water, whose operation depends on electromechanical assemblies that require great attention from maintenance teams (see Figure 01).
These towers operate on the basis of cold air flowing through a fill.
Meanwhile, cold air comes into contact with droplets of the hot water, and basically two processes take place:
- exchange of sensible and latent heat from hot water to cold air;
- increased humidity in the cold air;
The air flow is forced or induced mechanically by means of a centrifugal or axial fan driven by an electric motor.
Since the static pressure losses through these towers are low, the fan operates at low rotational speeds, around 120 to 300 rpm.
To make this possible, the motor triggers the fan with the aid of a belt and a pulley or gearbox speed reducers, as shown in the Figure below.
In general, in axial fans, a diffuser is mounted with the functions of directing and increasing the air speed to the fan and protecting the blades.
In centrifugal fans, on the other hand, these functions are performed by the volute, as exemplified in the Figure below.
The operation of a cooling tower is controlled by measuring and controlling the output temperature of the humid air within pre-set limits.
This is done by regulating the air flow through the tower, whose electronic control switches the fan off when the lower temperature limit is reached and on when the upper limit is reached.
In these cycles, the electric motor is subjected to a non-continuous service regime, with several consecutive stops and starts.
Using a frequency inverter on the motor drive, the fan operates at different rotation speeds as a way to perform proportional capacity control.
Cooling Tower Defects
The electromechanical assembly formed by the motor, reducer, and fan is the biggest cause of defects that lead to maintenance stoppages in the cooling towers.
According to a study by Bristol-Myers Squibb (global biopharmaceutical industry) based on the monitoring of thirteen cooling towers (five with gear reduction and eight with belt reduction), the greatest recurrence of failures in this assembly is related to the electric motor, followed by the gear reducer.
Defects in gear motors and gear reducers are magnified by the fact that they are subjected to many starts, stops, and load variations in a short period of time.
In addition, the gearboxes are typically mounted inside the tower, where they are exposed to the humid environment with chemically treated water droplets.
Fan defects are less recurrent, yet the Pareto Principle applies: the least frequent cause is responsible for most of the effects, since most catastrophic failures in cooling towers are fan related.
Fan Defects and Failures
The main defects of fans are:
- Unbalancing of blades;
- Inadequate inclination or elevation of blades;
- Cracked, corroded or broken blades;
The unbalancing of blades can occur through many different mechanisms: contaminant accumulation, wear, cracks, breaks, corrosion, clogging of the condensate removal orifice, geometric imperfections, and even the incorrect positioning of corrective balancing masses.
This type of defect is known to cause high vibration amplitudes that propagate excessive dynamic stresses throughout the entire system.
Without the right tools to detect and monitor fan unbalance, its severity will evolve into defects in other components, such as shaft warping and defects in bearings and gears.
Due to the large displacements resulting from the unbalance, the greatest risk of operation under these conditions is the occurrence of repeated mechanical shocks between the unbalanced blade and the diffuser.
The stresses generated by these shocks are concentrated in the blade body, close to the connection with the fan hub, where cracks begin to develop until the blade breaks through fatigue.
A broken blade can damage other blades, the diffuser, or an entire cell, forcing reconditioning or replacement of various components, which also means long downtime for maintenance.
This same failure mode can be caused by fan or diffuser clearances, a warped shaft, or excessive resonance amplitudes when starting the machine.
All of these defects have the greatest potential to cause catastrophic failure if the assembly is poorly sized.
Two important vibration frequencies of the system are the motor rotation frequency (1xRPM) and the fan blade-passing frequency (1xPP).
Taking as an example a motor with rotation at 1700 rpm providing power to a 6-bladed fan rotating at 280 rpm, you have a blade-passing frequency of 1680 rpm. This represents a difference of only 20 rpm between 1xRPM and 1xPP.
When waves with close frequencies interact with each other, the phenomenon known as beating occurs, in which their amplitudes periodically add up.
That is, the vibration level is periodically amplified relative to what would be observed without the beating phenomenon.
With this, the already large magnitudes of vibration from defects such as unbalanced blades are also amplified, increasing the risk of catastrophic failure.
Defects of the Motor and Towers Reducers
In gear reducers, the biggest concerns are gear and bearing defects.
Reducers gearings are susceptible to excessive wear, cracks, and tooth breakage.
The main causes of these defects are gearbox overloading and misalignment between gears.
Gear reducers are frequently subjected to loads above those foreseen in the project, in order to increase production or to compensate for deficiencies in the dimensioning of the motoreducer assembly.
Misalignment between gears, on the other hand, is commonly introduced after maintenance procedures that require disassembly and reassembly of the components.
Bearings can suffer defects in their tracks, rolling elements, and cages, and the primary causes are related to lubrication failures: contamination, use of inadequate lubricant, insufficient lubrication, or lack of preventive maintenance and replacement.
These defects, when happening prematurely, can also be symptoms of other defects in the assembly, such as fan unbalance and misalignment relative to the motor shaft.
Misalignment, besides being one of the most recurrent defects in machines, presents a particularity in some cooling towers.
Commonly, the electric motor is mounted externally to the tower’s housing, to facilitate access for maintenance and to isolate it from the harsh interior environment.
A long floating shaft with flexible couplings at both ends is employed between the motor and the gearbox. Due to this configuration, the shaft acts as a lever of the dynamic stresses caused by misalignment.
These stresses act directly on the gearbox’s bearings, gears, and seals, beyond the design limits of these elements.
This induces track or rolling element bearing defects, cracks, breaks, and advanced gear tooth wear.
The seals, on the other hand, can wear down to the point where oil can leak out, exposing the gears to the risk of running without proper lubrication.
The Challenges of Cooling Towers Maintenance
This justifies the use of effective tools to avoid losses due to catastrophic failures in cooling towers.
One of these tools is the condition monitoring of the motor-gearbox-fan assembly, which allows the detection and follow-up of the evolution of these defects, giving the maintenance team time to plan their corrective actions.
However, the very nature of building and operating these towers poses challenges for condition-based maintenance. As we have listed in our blog, they are:
- the positioning of its components at height;
- components that are difficult to access or inaccessible during operation, in positions that pose a risk to the maintainer;
- lack of a security platform for access;
- high temperatures and moisture generation in the components;
- high levels of noise and vibration;
- Conventional models for measuring component vibration and temperature: costly and require the access of the maintainer to the components.
In periodic condition monitoring based on vibration measurement routes, where the analyst needs physical access to every measurement point on the machine, collecting signals at the fan and gearbox of a single cooling tower cell can take about 1.5 hours.
With a uniaxial transducer, at least 6 measurement points are required on these two components.
To measure the vibration at each point, it is necessary to stop the machine operation, place the transducer at the determined point, restart it, wait for it to reach the operating rotation and collect the vibration, repeating this process for each point while following safety procedures.
In this type of monitoring, the signals are usually collected under a single rotation and load regime, not providing enough information about all the different patterns of the assembly’s conditio.
Cooling Towers Monitoring
A suitable answer to address all these challenges is continuous condition monitoring using the new technologies available in the maintenance market.
Currently, it is possible to make use of sensors that are fixed at the collection points, continuously sampling the vibration signals in three directions and surface temperature, and communicating them remotely, without the use of cables, to a virtual platform for viewing and analyzing the collected data.
By using this methodology:
- The analyst’s exposure to risks related to excessive noise and difficult access to components at height and inside the tower is reduced;
- The temperature of the components is continuously measured, something that is important in a thermal system;
- It eliminates the need to stop the operation to measure the data;
- The time spent by the analyst on the measurement process is reduced;
- The condition pattern of the machine is known at all speeds and loads in which it operates;
- Understanding failure modes and the responsiveness of maintenance personnel to defects increases equipment reliability;
- Consequently, its availability is increased by decreasing the downtime for corrective and predictive maintenance;
Vibration and temperature sensors are commonly used to identify changes in machine operation and can assist in monitoring and identifying defects in the fan motor assemblies of cooling towers.
This is the value proposition of DynaPredict, a Bluetooth data logger with triaxial acceleration and temperature sensors and a three-year battery life that performs spectral analysis and allows data interpretation in the comfort of a room away from the factory floor.