Protecting Cooling Towers with Vibration Sensing
The Importance of Cooling Tower Availability
The sight of cooling towers is becoming more and more common throughout a wide range of industries. Regardless of the size, type, or configuration of cooling towers, there is a single common denominator: they need to be readily available. This requirement demands that the most effective condition monitoring and system maintenance procedures are put in place.
At the heart of each cooling tower is a large fan system providing forced air to the heat exchangers. This system typically comprises one or more rotating fans, each often several metres in diameter, together with drive motors, gearboxes, and associated linkages and control mechanisms.
Almost all of these components have moving parts that must be installed and aligned correctly to ensure continued reliable operation. Over time, they are subject to normal wear and tear and exposure to a wide range of environmental conditions that can significantly increase the risk of failure.
Monitoring the condition of such critical systems is essential if availability and uptime are to be maintained. This is where vibration monitoring of all rotating parts plays a major role.
Monitoring Options for Cooling Towers
Monitoring options are extremely dependent on the criticality of the application and the budget available. Installing a greater number of sensors can provide additional information, but the key components remain the motor and gearbox assemblies.
Vibration sensors can be installed for all of these applications, and selecting the correct sensor is of the highest importance.
Within the Hansford Sensors range of vibration sensors, specific models have been designed to withstand the potentially harsh conditions present within cooling towers. These sensors also offer the required frequency range to suit the application. Some applications are designated as intrinsically safe, and certified products are available for these environments.
Mounting Locations for Vibration Sensors
The most common positions to monitor are:
Motor drive end and non drive end, where the sensor is mounted radially to monitor motor bearing condition measured in g.
Gearbox input and output shafts, where the sensor is mounted radially. This enables monitoring of bearing condition measured in g and fan out of balance measured in velocity. An optional axial accelerometer on the input shaft provides a good indication of thrust on the shaft.
Offline Vibration Monitoring
In an ideal situation, online monitoring is always preferred. However, where budget constraints exist, offline monitoring techniques are widely used on cooling towers.
With sensors installed as described, and depending on accessibility, a local junction enclosure can be installed close to the motor. Multicore cable using screened twisted pair is then provided to connect the junction enclosure to a switch or connection enclosure. In some applications, the sensor cable is connected directly to the switch enclosure.
The switch enclosure is an industrial enclosure designed to withstand harsh conditions. It is available in various materials depending on environmental requirements, including mild steel, stainless steel, and polycarbonate.
A commercially available data collector can then be connected to the switch enclosure, the sensors powered, and vibration data collected.
Online Vibration Monitoring
Using either the offline configuration described above or, in some low cost installations, 4 to 20 milliamp sensors, outputs can be connected to protection, monitoring, or PLC systems.
In the simplest configuration, four or five 4 to 20 milliamp accelerometers, scaled in g or velocity to suit the required parameters, are installed as previously detailed.
Each sensor is connected to a local junction enclosure, with multicore screened twisted pair cable connecting the enclosure to the PLC system for trending and alarming.
In applications requiring more detailed analysis, vibration modules with specific filters matched to cooling tower characteristics can be installed. These systems use AC accelerometers, junction enclosures, and multicore cable, terminating in an industrial enclosure housing vibration modules.
The vibration modules condition the accelerometer signals to provide bearing condition or velocity outputs as required.
The system also allows a data collector to be connected for more detailed analysis, with outputs connected to a PLC for trending and monitoring.
In applications where a PLC is not available, the configuration can be expanded to include local displays and trip functions to protect and shut down cooling towers if required.
