Even when cooling tower fans are not considered a critical piece of equipment, they are quite simple machine arrangements and highly spared. Evaporative cooling towers are a critical component in many power generation and chemical plants, and other process facilities. Catastrophic equipment failure can result in safety hazards and expensive repairs, even simple mechanical unexpected problems can result in unplanned downtime and cause a plant’s lowered production.
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.
In the past, vibration monitoring was a technical challenge due to slow rotational speeds, a variety of support structures, and wet corrosive environments. Mechanical vibration cutoff switches have been the most economical and preferred choice for over 60 years, used to shut down machinery when vibration levels became excessive. The switch is typically mounted onto the mechanical equipment support and connected to the motor starter circuit. These switches have proven to be unreliable, and in many instances allowed extensive machinery damage before motors were disabled. Furthermore, switches did not allow for advance warning of problems.
Vibration switches are made of mechanical components, including springs, magnets, and lever arms, which can freeze and become inactive. Additionally, calibration of mechanical vibration switches is imprecise. They can be set too high to catch significant vibratory events, or so low that ambient conditions continually trip the switch. In this case, the switch is usually disarmed or bypassed, essentially eliminating any protection at all.
Many customers have tried to complement the basic protection provided by the switches, implementing more proactive solutions, such extend their Walkaround data collection programs to cover the Cooling Tower Fans, but they have also been found ineffective for measuring fan and gearbox degradation, since they are inaccessible during fan operation, some others wanted to go a step further have installed permanent sensors on the gearboxes, that allows having a snapshot of the condition in that moment, but still relies on the inaccurate protection provided by the switch.
Today’s options for Cooling Tower Monitoring
We don’t want to put Mechanical Vibration switch manufacturers on the spot, this has been a profitable market niche for a long time, but there is not a single reason to stick with mechanical switches other than the simplicity and the low cost, in fact, most of the same manufacturers have already electronic solutions, I think this comes more from the OEM side, which has the tendency to avoid changes and to get thru the customer’s requirements with the less trouble and also to reduce buyout costs. If you are in the process of writing a new Cooling Tower specification or need to retrofit or upgrade one at your plant, please consider the new options available.
You probably don’t need to go as far as an API670 rack-based solution with condition monitoring software, but there are at least 2 or 3 intermediate solutions that could make your life and the plant maintenance crew easier.
We are listing some options from less to top, and only considering vibration protection options since personal injuries can be caused if a loose blade flies through the fan stack guard, harming personnel on the top platform level, or falling to the ground.
Option I: Electronic Vibration Switch
These switches have important advantages over mechanical switches. The electronic versions utilize an accelerometer with an integration circuit to convert the reading into velocity units, which is recommended for these low-speed machines and is more accurate to measure the amount of vibration than their mechanic counterparts. In many electronic vibration switches, the transducer may be integral to the switch housing or external and connected to the switch by a cable. Even though this switch is hermetically sealed and cased in stainless steel, it can withstand harsh cooling tower conditions, but for cooling towers is recommended to use a remote sensor on the gearbox and the electronic unit installed outside the cell to facilitate accessibility with the unit in service. Some switches generate an overall, proportional signal (4-20 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.
Note: An important option for an electronic switch is an alarm 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 start-ups, to be overcome.
Option II: Multichannel Electronic Vibration Switch
From here, you can add more channels to extend coverage to other components of the machine train, the recommendation would be to add channels to have 2 or 4 channels depending on fans critically, your budget, and economic justification, so instead of using a one-channel vibration switch, use one of the multichannel monitoring modules available in the market, the priority order would be:
1. Existing already considered Option I Gearbox accelerometer next to the output shaft, this sensor will focus mostly on the fan and will catch imminent failure of internal components, this channel will allow you through frequency analysis to trend vibration and catch specific components problems (gears and bearings).
2. Motor accelerometer coupling side (DE), this sensor will catch vibration in the motor, the coupling side is most prone to fail first, as the motor is usually not too big, a single sensor will also catch vibration from the NDE side too, also electrical and rotor-related vibration problems.
3. Having an additional accelerometer on the pinion or high-speed side of the Gearbox will allow the detection of internal component problems at an earlier stage.
4. Having a 2nd accelerometer on the motor NDE will complete the vibration channels of the main components.
To have channels 1 and 2 is a big improvement from a mechanical switch upgrade and most people related to this field would say it is sufficient from the protection and operator monitoring standpoint to know the status of both, the gearbox and motor, but if the vibration level reaches an alarm, you will need to go next to the fan with a portable analyzer for diagnostics.
From this point, having an electronic switch (1-4 channels), you have solved some major issues not covered with mechanical switches:
1. Accurate measurement
2. Dual levels of protection; alarm and trip,
3. Trip/Alarm delay for transient events (in some mechanical switches you can only have a manual or PLC electrically activated holding-resetting relay device)
4. 4-20mA output, which can be used for operators for trending, this fact allows you to move from pure reactive maintenance, to proactive and scheduled.
5. A sensor is already installed on the machine, so you have a raw signal for analysis.
I would like at this point to discuss some other possible options, such as using a PLC and transmitters for control instead of switches, but the major issue for cooling towers is the lack of instrumentation and most of the control is still based on relays. The corrosive environment requires frequent instrumentation maintenance. There are some other critical parameters besides vibration data on the fans to prevent failures. Gearbox oil levels, and oil temperature or pressure (it depends on the gearbox), motor bearing temperatures, etc. If the number of channels reaches an important quantity it would be good to consider other ways such as Modbus communications instead 4-20mA analog signals to send the channel values to the DCS console for the operators.
Option III: Complementing protection with an online Condition Monitoring System
The effort that most organizations place in condition monitoring is usually determined by asset criticality. If the asset is highly critical, more in-depth monitoring is justified. This is why large, valuable rotating equipment, like turbines and compressors, are usually equipped with expensive continuous condition monitoring systems, these systems are typically handled by a dedicated group that could be local or remote inside the organization, sometimes a third-party company. However, even less critical assets can benefit from extensive condition monitoring. In fact, benchmark data indicates that the deeper throughout the asset base that condition monitoring is employed, the lower the operating costs and the higher reliability the site will enjoy.
In addition, knowledge of asset condition can help reduce the likelihood of those unexpected equipment failures that invariably happen on nights or weekends.
Using a route-based strategy to access additional condition monitoring points is labor intensive because valuable technician time must be spent on non-productive travel time to get to the field. Additionally, many plants are experiencing shortages of skilled labor due to an aging workforce. Rather than consuming technician time making trips to the field, time is better served in analyzing the information derived from condition monitoring sources in the market, or expanding the system you could have for your critical assets.
Rototec-RMS can help you to define your next step when you are considering migrating from Mechanical Switches to better and more reliable options that suit your organization's needs.
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#ConditionMonitoring #VibrationMonitoring #MachineProtection #MachineryReliability #API670 #MachineryMonitoringSystem
Luis Casas
Engineering Manager
Rototec-RMS
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References:
· Wilcoxon Sensing Technologies
4-20 mA vibration sensors outperform vibration switches
· IMI Sensors a PCB Division
State of the Art Technologies for Protection of Industrial Cooling Towers
· SPX Cooling Technologies
Vibration Management for Cooling Tower Components
· Metrix Instruments
Vibration Monitoring of Cooling Towers
· Emerson
Instrumentation – It’s Not Just for Process Control Anymor
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