Cooling Towers: Condition monitoring to avoid failures

Cooling towers are systems designed to transfer the residual heat from a thermal process to the atmosphere. They use a stream of water or other fluid at a lower temperature. For this reason, the system is used in a wide range of applications, from air conditioning to generating energy.

In this way, towers can be found in plants (sugar, ethanol, thermoelectric and nuclear), steel mills, chemical, petrochemical, pulp and paper industries, etc.

For many of these applications, controlling process temperatures is indispensable. Almost all production is stopped if the temperature limit is exceeded.

In these cases, cooling towers are critical machines. Ensuring the availability and reliability of these assets therefore means avoiding efficiency drops in processes or production stoppages and, consequently, financial losses.

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.

Learn more about cooling towers and the main maintenance challenges for these assets.

Construction and Principle of Operation

The construction configurations and principles of operation of cooling towers are diverse. One of the most widely used types is the mechanical draft tower for cooling water, which operates using electromechanical assemblies.

These towers operate on the basis of cold air flowing through a filler. In this medium, the cold air comes into contact with droplets of 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;

Important Components

The air flow is forced or induced mechanically by means of a centrifugal or axial fan driven by an electric motor.

As static pressure losses are low, the fan operates at low rotational speeds (around 120 to 300 rpm). For this reason, the motor drives the fan using belt and pulley speed reducers or gearboxes.

In general, axial fans are fitted with a diffuser to direct the air. This increases the air speed while preserving the blades.

In centrifugal fans, the volute performs these functions, as shown in the figure below.

The operation of a cooling tower is controlled by measuring and adjusting the humid air outlet temperature within pre-established limits.

This control is achieved by regulating the air flow through the tower. In this way, the 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 stops and starts in succession. Using a frequency inverter in the motor drive, the fan operates at different rotation speeds for proportional capacity control.

Faults in Cooling Towers

The electromechanical assembly made up of the motor, gearbox and fan is the biggest cause of defects that lead to maintenance stoppages in cooling towers.

According to a study by Bristol-Myers Squibb (a global biopharmaceutical company), the greatest number of faults is related to the electric motor, followed by the gear reducer.

Faults in gear motors and gear reducers are aggravated by the fact that they are subject to many starts, stops and load variations in a short space of time.

In addition, gearboxes are assembled inside the tower. That is, where they are exposed to the aggression of the humid environment with droplets of chemically treated water.

Fan faults are less recurrent, but 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 fan faults are related to the blades, including: unbalance, improper inclination or elevation; cracked, corroded or broken blades.

Blade unbalance can occur through many different mechanisms. These include the accumulation of contaminants, wear, cracks, breaks, corrosion, clogging of the condensate removal orifice, geometric imperfections and even the incorrect positioning of corrective balancing masses.

This type of problem is known for causing high vibration amplitudes that propagate excessive dynamic stresses throughout the system. As the issue progresses, faults will occur in other components, such as shaft warping and problems with bearings and gears.

The greatest risk of operating under these conditions is the occurrence of mechanical shocks between the unbalanced blade and the diffuser. This tension, concentrated in the body of the blade near the connection with the fan hub, begins to show cracks which can cause the blade to break due to fatigue.

A broken blade can damage the others, the diffuser or an entire cell, forcing several components to be reconditioned or replaced.

The cause of the failure mode can be looseness in the fan or diffuser, a warped shaft or excessive resonance amplitudes when starting the machine. All these faults therefore have a greater potential to cause catastrophic failure if the assembly is poorly dimensioned.

Motor rotation frequency

Two important vibration frequencies in the system are the motor’s rotation frequency (1xRPM) and the fan’s blade passage frequency (1xPP).

Taking the example of a motor rotating at 1700 rpm supplying power to a 6-bladed fan rotating at 280 rpm, you get a blade pass 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.

In other words, the vibration level is periodically amplified in relation to the state without the beating phenomenon.

As a result, the already large magnitudes of vibration from faults such as unbalanced blades are also amplified, increasing the risk of catastrophic failures.

Defects of the Motor and Towers Reducers

In gear reducers, the biggest concerns are gear and bearing defects. Gearboxes are susceptible to excessive wear, cracks and broken teeth. Thus, the main causes of these defects are overloading of the gearbox and misalignment between gears.

Gearboxes are often subjected to loads in excess of those provided for in the design, in order to increase production or compensate for deficiencies in the sizing of the gearmotor assembly.

Misalignment between gears occurs after maintenance procedures that require the components to be disassembled and reassembled.

Bearings can have defects in their raceways, rolling elements and cages. This can be caused by faults, contamination or inadequate lubrication. Defects can also be symptoms of other defects in the assembly, such as unbalancing of the fan and misalignment in relation to the motor shaft.

In addition to being one of the most common defects, misalignment is a particular problem in some cooling towers.

Read more: Shaft misalignment and its contribution to mechanical failures (dynamox.net)

Between the motor and the gearbox there is a long floating shaft with flexible couplings at both ends. Because of this configuration, the shaft acts as a lever for the dynamic stresses caused by misalignment.

These stresses act directly on the bearings, gears and seals of the gearbox, beyond the design limits of these elements.

This leads to defects in the raceways or rolling elements of the bearings, cracks, breaks and advanced wear of gear teeth.

Seals can wear to the point of allowing oil to leak out, exposing the gears to the risk of working without proper lubrication.

The Challenges of Cooling Towers Maintenance

The very nature of the construction and operation of these towers poses challenges for condition-based maintenance. Traditional offline vibration and temperature monitoring requires stopping the machine’s operation.

That is, place the transducer at the given point, turn it back on, wait for it to reach operating speed and collect the data. This process is repeated for each point while following safety procedures, which can take more than an hour.

In this type of monitoring, signals are collected under a single speed and load regime. This does not provide sufficient information on all the different patterns of the condition of the assembly.

Read more: Cooling towers and their maintenance challenges (dynamox.net)

Efficient monitoring of cooling towers

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;
• There is no need to stop the operation to measure the data;
• The time spent by the analyst on the measurement process is reduced;
• The machine’s condition pattern is known at all speeds and loads at which it operates;
• It increases the understanding of failure modes and the responsiveness of the maintenance team to defects, increasing the reliability of the equipment;
• Consequently, their availability is increased by reducing downtime for corrective and predictive maintenance;

In this way, vibration and temperature sensors help to identify changes in machine operation and help to monitor and identify defects.

Do you want to bring this solution to your industry? Talk to our specialists!