As a piece of machinery operates, it vibrates. If measured and analyzed, the frequency and amplitude of this vibration can provide insight into the machine’s condition. With the proper tools, these vibration signals can alert personnel or — in worst case scenarios — even shut down the equipment when deteriorating conditions or events occur. Vibration-monitoring equipment is available to aid in maintenance forecasting as well as to help prevent further damage to machinery and components.
The proper level of vibration management adequate for protecting the cooling tower and supported processes must be determined by the owner. The five options identified in this article can help monitor and protect cooling tower components from the potentially destructive effects of vibration.
A basic understanding of components and vibration terminology is helpful in choosing an appropriate vibration-monitoring option.
- A vibration analyst can identify the source of vibration based on its frequency (the rate at which it occurs) and determine its severity based on the amplitude (the amount of vibration).
- A transducer is a device that converts one form of energy into another.
- An accelerometer is a type of transducer commonly used in vibration measurement that generates an electric charge proportional to the acceleration force applied to it. Accelerometers can be very sensitive and provide output signals for simultaneous occurrences.
- A velocimeter is a type of accelerometer that has on-board electronics to integrate the data into a velocity unit.
- A signal conditioner/transmitter is a solid-state electronic device that can include an integral transducer or connect to a separate transducer. Its purpose is to convert the raw signal into an amplitude measurement proportional to the overall condition. It is useful for simplifying the signals for basic trending, alarm or shutdown. It also is useful if long cable runs are required to connect the transducers to an auxiliary system, such as a plant supervisory control and data acquisition (SCADA) system or a distributed control system (DCS).
- Fast Fourier Transform (FFT) converts a vibration waveform from the time domain (time vs. amplitude) to the frequency domain (frequency vs. amplitude). The results of this process substantially simplify the identification of the vibration frequencies and the associated amplitudes of each. A graphical example of the time waveform plot and the transformed spectrum data is shown in figure 1.
- Vibration analysis is the process by which an analyst uses measured vibration responses to evaluate the condition of a system and make appropriate recommendations.
Vibration-Management Options for Monitoring Cooling Tower Operation
Mechanical Vibration Switch. A mechanical vibration switch (figure 2) is the simplest way to manage vibration. The switch is typically mounted onto the mechanical equipment support and connected to the motor starter circuit. If it realizes a vibratory event that exceeds the setpoint (typically 1g), it will trip. These switches may be set up so that the trip initiates an alarm to alert the system operators and automatically shut down the equipment.
These mechanical inertial switches sense acceleration in one direction. They are a good first step to potentially mitigating damage caused by excessive vibration, but they are not monitoring devices. They do not measure vibration or collect data; rather, they react to a specified level of acceleration. In many instances, it may be too late to prevent failure. In such cases, the switch is intended to notify and prevent further damage.
It is important to note that during cooling tower startup, it is common for vibration levels to be temporarily elevated. An important option for mechanical vibration switches is the remote reset function, which is an electric coil used to reset the switch to the normal condition or hold it as such during startup. Another benefit is that the trip setpoint may be set at a level appropriate for normal operating conditions rather than for the relatively higher startup level. This results in a significant improvement in protection as well as convenience.
Electronic Vibration Switch. These switches have important advantages over mechanical switches. The upgraded solid-state, electronic versions utilize an accelerometer or velocimeter that measures the amount of vibration. The transducer may be integral to the switch housing or external and connected to the switch by a cable. Some switches output an overall, proportional signal (4 to 20 mA) for plant management connectivity to a DCS or SCADA system. Additionally, a BNC terminal allows the transducer’s raw signal to be transmitted to an analyzer.
An important option for an electronic switch is a trip delay function. This feature has an adjustable time-delay counter that will initiate once the preset vibration level is exceeded. If the vibration still exceeds the limit at the end of the delay, the trip engages. This allows for transient vibration events such as startup to be overcome.
Accelerometer or Velocimeter With or Without Signal Conditioner/Transmitter. To gather data from a cooling tower gearbox, the cell must be shut down to mount the transducer, then re-energized for data to be collected. Then, the process must be repeated in order to retrieve the transducer. To avoid this, it is possible to permanently mount transducers within the cooling tower cell. This also saves money by avoiding costly shutdowns that reduce cooling tower capacity and tie up manpower. To simplify the process, a switch or junction box that serves as a central connection point may be installed for the vibration analyst, allowing him to gather data from multiple accelerometers via one switch. Data can be gathered more quickly and more often for additional reliability. This option is recommended when the analyst wants to collect data at regular time intervals.
These transducers may be powered and equipped with a signal conditioner for transmitting the overall, proportional signal to a separate system. Additionally, permanent transducers may be used in conjunction with vibration switches to add analysis capability to the basic protection system.
Surveillance System. Surveillance (monitoring) systems are a good option when frequent data collection is warranted and the collection location is difficult to reach. These systems regularly and automatically poll the transducers for overall amplitude values several times a day. Amplitude changes may signal conditions that require investigation by the preventive maintenance staff.
The data gathered by these systems typically is stored in a database for trending. Some surveillance systems can automatically shut down equipment or set off alarms if vibration exceeds levels specified by the operator.
The system may include automatic notification features that send alerts through email or mobile apps. Wireless systems are growing in popularity but may not poll and transmit information as rapidly as conventional wired systems.
Protection System. Advances in computing power make protection systems a cost-effective upgrade from surveillance systems. They have the added benefit of collecting data almost continuously across all transducers installed on the equipment. When system failure carries severe consequences, a protection system can be a good investment.
In summary, vibration switches provide a good first step toward risk mitigation by alarming or shutting down equipment when excessive vibration occurs. More sophisticated devices provide the data needed to analyze maintenance requirements and reduce downtime. Surveillance and protection instrumentation provide additional risk reduction and maintenance scheduling tools for critical systems.
Regardless of the vibration-monitoring system, vibration response data must be actively managed in order for it to be useful. The data collected shows the transducer’s reaction to vibration rather than the machine’s actual motion. Consequently, it is critical that the transducer be mounted, connected and calibrated properly. Finally, no level of automation is a substitute for a trained analyst. Understanding of cooling tower system components and vibration-monitoring devices is required to install and connect monitoring tools, then gather, maintain and interpret the vibration data. Only then will data management promote increased component reliability and reduced downtime.
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