A New Practical and Reliable Approach to the Vibration Monitoring of Cooling Tower Fans Abstract Modern vibration data collectors are typically equipped with uni-axial and tri-axial sensors. With all the published advantages of the tri-axial vibration sensor, the purpose of this paper is to move forward and to provide the benefits of an intelligent & smart vibration transmitter technology that can trend and reliably protect a plant s cooling tower fan assets. Background Cooling towers lower the temperature of process water by using either direct or indirect contact with cooling air. While there are cooling towers fans that are turned by a belt driven sprocket/motor system, most cooling towers use a right angle gear drive/motor system; or, in some few new applications, a directly coupled motor is used to turn the cooling fans. The newest and most modern method uses direct drive permanent magnet motors. This eliminates the need for a gearbox, jack shaft, pillow block bearings and couplings which removes the alignment of mechanical components, reduces maintenance costs, and provides improved reliability. Cooling towers are an important system component of production in many industries. Their failure can cause expensive repairs and reduced loads during peak demand. For many plants, losing the cooling process leads to a costly process slow down or even shut down.
Cooling Tower Fan - Vibration Monitoring In the last decade, operators typically used hand held vibration data collectors coupled to a tri-axial sensor for their route based machine monitoring program. Located in the tower cell, the gearbox was inaccessible, so the vibration readings were taken from the external motor. For a cooling tower, this procedure is less likely to be able to detect impeding gearbox and cooling fan failures. Further, around the country, operators performing route based vibration measurements experience difficulty in monitoring the cooling fan processes because there are too many machines; or they are located in remote, hazardous, and unpleasant areas. Leveraging from the proven operating performance of the tri-axial sensor, new vibration sensor technology is now available that has its own dual accelerometer, signal processor (DSP), microcontroller, and special algorithms to detect, calculate, process and protect all cooling tower cells. Traditional Vibration Devices- Mechanical Switch and Loop Powered Transmitters Many plants still use less sophisticated vibration shutdown devices that were provided by the manufacturer of the cooling tower cell. With their increasing significance, cooling towers require more reliable protection than provided by mechanical vibration switches. Similar to the mechanical vibration switch, the two-wire vibration transmitter does not have any analysis capabilities for a deteriorating machine. In the end, it is the owners of the cooling tower cells who have to bear the plant s diminished output and revenue losses when their cooling processes are not properly protected against the excessive and destructive machine forces Intelligent Vibration Transmitter (IVT) Continuous Protection The IVT is compact transmitter that integrates a dual 3-axis vibration & temperature module to the condition of the cooling tower fan and other drive components. The improved resolution for low and high frequencies, the IVT can anticipate machine rotational faults and bearing faults by providing timely condition machine updates and alerting the maintenance team in charge of optimizing the cooling process- 24/7. Figure 1., indicates the X,Y, and Z vibration measurement planes detected by the IVT.
When all aspects of reliability and maintainability are considered for a cooling process, the IVT is a dependable alternative to the mechanical vibration switch and the 4-20mA vibration transmitter. For plant owners, it guarantees the continuity of their cooling processes to optimize their production output. For the manufacturers of the cooling tower cells, it provides a reliable and value-added feature to their product offering. The condition monitoring side to IVT was created as a smart and compact system that monitors a range of cooling tower performance parameters and sends the vibration and temperature data to the cloud for analysis.
IVT Designed to Avoid Unexpected and Costly Cooling Process Shutdowns With a combination of best practice techniques, correct setting of vibration alarm settings, and interpretation of vibration spectra, a cooling tower can be protected against damaging forces such as imbalance, misalignment, and bent shaft. Developing problems like defective rolling element bearings and gearbox defects can be detected early enough to allow plant management the time to plan, schedule, and make repairs to minimize cooling process downtime.
How IVT is Connected to Provide Flexibility and 24/7 Protection Key Benefits and Flexibility of the IVT The IVT and integrated cable assembly was designed for wet, submerged, and corrosive environments and can be used to detect and monitor the vibration levels of the common right angle gear drive or in some new applications- the direct drive permanent magnet motors. By measuring vibration continuously, machine degradation can be monitored and impending failures can be prevented to avoid unscheduled shutdowns.
Online overall vibration can be obtained at any time from any location, thereby minimizing machine shutdown. Trended overall vibration levels can be kept on the cloud for analysis for future reference. Early alarms can be set-up to provide sufficient time for management to plan for the scheduling and purchasing of parts thereby minimizing cooling process downtime. Universal mounting, any orientation Low power sensor that detects and measures both vibration and temperature parameters Long distance and reliable Modbus RS485 RTU digital communications Special algorithms that provide improved resolution at lower and higher frequencies Measuring a motor, speed reducer, or a fan s bearing vibration & temperature provides an advance indication of possible bearing load or faults in the cooling process system or problems with a bearing s lubrication Excessive vibration is an early indication of bearing misalignment or packing issues Rising machine temperature provides advance warning of component wearing problems Overall vibration & temperature levels can be coupled to a PLC, DCS, or SCADA control system for machine protection The more machine vibration & temperature data that can be reviewed, the greater the results will be for reliability, quality and best practices for baseline machine levels Less wire or Wi-Fi via Ethernet Modbus or cellular gateway Permanent installation allows safe access to cooling tower components that are located in remote, hazardous, and dirty environments Provides unique machine vibration signature that can be compared to similarly located group of machines for baseline levels Reduces unplanned shutdowns and minimizes cooling process downtime Provides 24/7 overall vibration and temperature levels for remote portal Remote Monitoring Portal
The system operates as a fully customizable portal that enables users of the IVT system to monitor overall vibration data & machine temperature from all the IVTs remotely from a smart cellphone, tablet or laptop. Remote monitoring portal details: Monitor and observe trends of overall vibration 3 axis and temperature data: Acceleration, Velocity, & Temperature Machine vibration & temperature data is stored on the cloud Monitor status of all sensors connected to RS485 field bus network or save to the cloud for vibration analyst to review the vibration data Vibration & Temperature data can be easily exported Time waveform and spectrum data available with third party condition monitoring software
Option for alarm alert text messaging (SMS/4G) Recommended Vibration Levels On Cooling Tower Fans The Cooling Technology Institute (CTI) offers manuals, white papers, and vibration standards for use on cooling tower cells. Copies of these resourceful documents can be obtained by contacting the Cooling Technology Institute directly at 281-583-4087 or on the web at www.cti.org. These CTI vibration standards are given only for the three primary vibration frequencies present at cooling towers: fan speed, motor speed & blade-pass frequency. No overall vibration level standards are currently offered by CTI. Vibration levels at the motor speed are to be measured at the motor inboard bearing in the horizontal & axial directions. Vibration levels at the fan speed are to be measured at the fan bearings or gearbox output in the horizontal direction. Vibration levels at the blade-pass frequency are to be measured radially at the fan shroud or stack at the blade elevation level. CTI vibration standards are given for each type of tower construction: concrete, metal, wood & fiberglass. All CTI standards are expressed in units of displacement (mils-pk-pk). The IVT is a case mounted transmitter that detects and measures absolute vibration levels. For that reason, as long as the speed of the motor, fan, or blade pass frequency are known, the mils pk-pk levels acquired from the CTI vibration standards can be converted to velocity (in/sec, pk or in/sec, rms). For example: If you obtain 6 mils pk-pk at 600 CPM (10 Hz) from the CTI diagnostic chart, you can simply convert mils pk-pk to in/sec, pk by using the following conversion formula- V = (CPM/19,100) D => V= (600)/19,100) 6.0 => V= (0.03141) 6.0 => V = 0.1885 in/sec pk To convert the velocity measurand from pk to rms, multiply 0.1885 X 0.7070 = 0.1333 in/sec rms Cooling Tower Fan Common Vibration and Temperature Faults Since excessive vibration is a machine s reaction to internal and external forces, vibration and temperature monitoring and analysis can be utilized as an indication of a tower s mechanical condition. Certain machine faults, e.g., imbalance, occur at certain frequencies. By determining the frequency of the machine fault and by measuring the amplitude of the vibration signal, we can find out what internal or external forces are causing the vibration. The expected frequency faults in the vibration spectra are called frequency orders (1X, 2X, 70X). They coincide at or around the normal running speed (1/4X, 1/3X, 1/2X, 1X, 2X) or can extend to many times the speed of the running machine (X10, X25, X50).Imbalance or misalignment, for example, occurs at the speed of the rotating shaft (X1) and overheating in motors and gearboxes can be caused by increased bearing loads due to machine imbalance or misalignment.
The damaging forces on a tower are listed in the table below by each system component. The format is machine component/fault, frequency order, typical measurement plane, and comments. Cooling Tower Fan Common Vibration and Temperature Faults Machine Component/Fault Frequency Order Measurement Plane Comments Motor/Imbalance 1X, 2X Motor RPM Motor/Bent Shaft 1X, 2X Motor RPM Axial Small amplitudes of axial vibration can occur. Imbalance can be intensified by mechanical resonance. 1X Motor RPM vibration can also be caused by Soft Foot. Improper end clearance on jack shaft can also cause excessive axial vibration. Bent shaft can cause roller bearings misalignment. Motor/Mechanical Looseness 1/2X,1/3X,1/4X,1X,2X, Motor RPM (Vertical) There may be some vibration levels on the horizontal plane, but, the amplitudes will be highest near the mechanical fault. Excessive coupling wear can lead to looseness. Motor/Rotor Bar and Stator Defects 1X,2X,3XMotor RPM 2X Line Frequency Rotor Bar Passing Frequency (F RBPF) = Motor RPM X No. of Rotor Bars. Broken rotor bars are common faults that cause electrical imbalance. Small amplitudes of axial vibration can occur. Motor/Coupling Misalignment 1X,2X,3X 4X,5X,6X, Low Level Harmonics Axial and/or Shaft/Coupling Misalignment may involve both Angular (Axial) and Parallel Offset () Misalignment. Misalignment can occur under the following conditions: 1. Machine alignment and installations errors; 2. worn roller bearings; 3. settling of bases, foundations, and tower structure; 4. shift of relative position of machines after installation. Motor/Resonance Less Than, Equal to, or Greater Than Motor RPM, Axial Resonance appears when a source frequency coincides with the natural frequency of the support structure, base foundation, piping, or mechanical component, e.g., rotor, gearbox, or belt driven systems. If the fans are driven by variable frequency drives, the fan speeds may cause resonant frequency vibrations that can affect the tower structure. Resonance can be confirmed by verifying that a small change in speed causes the 1X Motor RPM vibration level to change greatly.
The vibration frequencies begin to manifest themselves in the 5 KHz to 15 KHz range. As the roller bearing wear increases and approaches failure, there will be an increase in overall vibration levels in the 500 Hz to 2500 Hz range. Rolling Bearing Defects with Visible Damage to the Bearings 1X to 25X 25X to 100X For bearing defects within 1X to 50X Machine RPM, schedule a machine repair as soon as possible and inspect the roller bearings. If required, replace the roller bearings and find the fault(s) causing the bearing defects, e.g., imbalance, misalignment, improper bearing loads, excessive bearing temperature, contaminated lubrication, or, insufficient bearing lubrication. Gearbox/Imbalance 1X Fan RPM Axial Fan imbalance due to fan blade breakage or a detached blade can cause serious damage to the tower structure and danger to operators. Imbalance can also be due to: 1. Sand, dirt, or ice build-up, or corrosion on fan blades; 2. Water that has seeped inside the hollow fan blades; 3. Incorrect fan blade pitch or improper mounting of blades during fan assembly. Gearbox/Mechanical Looseness 1X,2X Fan RPM (Vertical) Gearbox/Worn or Broken Gear Teeth GMF X 3.25 There may be some vibration levels on the horizontal plane, but, the amplitudes will be highest near the mechanical fault. If a tower structure is shaking, check for excessive looseness in the gearbox output shaft which can be due to worn rolling bearings. GMF X 3.25 is the typical gear analysis frequency range. Shaft misalignment can cause high loads on the input gear, which causes misaligned gears and can lead to worn or broken gear teeth. Gearbox/Pinion and Gear Misalignment GMF X 3.25 Misaligned gear sets generally produce higher GMF X 2 amplitudes. As in most gearbox faults, the problem is often displayed by the spacing of the sidebands around the GMF harmonics. Misalignment can cause stress inside the shaft that can damage couplings and incorrectly load the roller bearings, Gearbox/Aerodynamics Blade Pass Frequency (BPF) Fan RPM X Number of Blades Strong winds can affect the aerodynamic performance of the fan blades by creating low frequency forces that can be damaging to a tower structure. Another example applies to an older wooden tower that underwent vibration pulsations when the low rotating blades (BPF) obtained a lift as they passed over the jackshaft. Belt Drive Pulley System/Worn or Improper Belt tensions 1X,2X,3X,4X RPM of Belt Small amplitudes of axial vibration can occur. Belt Drive Pulley System/Misaligned Pulley 2X RPM of Belt Axial Excessive pulley or extreme sheave wear may appear as imbalance.
Motor and Gearbox/Roller Element Bearings/Overheating 1X Motor RPM 1X Fan RPM Axial Overheating in electric motor bearings is generally lubricantrelated. The ball bearings used in most electric motors are pre-greased, shielded ball bearings. Normal motor bearing operating temperatures range from 140 F (60 C) to 160 F (71 C). Normal motor roller bearing operating temperatures range from 140 F (60 C) to 160 F (71 C). Roller bearings in gear drives normally operate at 160 (71 C)-180 F (82 C) Overheating in motors and gearboxes can be caused by increased bearing loads due to machine imbalance or misalignment. Overheating can also be caused by improper bearing assembly, component wearing problems, or balls skidding within the bearing. Contamination of the roller bearings lubricant by solid particles, water, and other fluids can reduce the life of the bearings. Lack of or improper lubrication generally causes overheating or excessive wear in the roller bearings. These conditions can result from insufficient or excessive lubrication, improper lubricants, e.g., viscosity is the load bearing component of the lubricant. Too thin, then the bearings cannot properly carry the load; and too thick, then the amount of friction will generate heat. Packing the space around the roller bearings with grease can also cause excessive heat. Avoid the use high pressure grease guns since they may rupture the bearing seals. Summary of Traditional Vibration Devices and IVT PROS Basic unit without hazardous approvals or start-up delays are inexpensive Mechanical Vibration Switches CONS Limited frequency response- typically 0 to 100 Hz By their design, these shock devices are sensitive to Acceleration (G) only Acceleration is not best vibration detection measurand for the low RPMs encountered on large cooling fans No trending capabilities or analysis capabilities Acceleration sensitive switches can provide advance warning about deteriorating conditions of the machine- especially on bearings and gear mesh which are high frequency. They do a good job here, but, are weak on low frequencies such as fan rpm. There are many non-functional mechanical switches in the field and your process equipment investments may not be properly protected against excessive and destructive
machine forces. Sensitive to one axis only 2-Wire Loop Powered Vibration Transmitters PROS CONS Industrial grade steel casing with electronics potted with epoxy The 4-20mA can be run over long distances with minimal signal losses compared to voltage type signals Saves on cable wire because it only needs 2 wires to function Better frequency response than a mechanical vibration switch, typically 2 Hz to 1500 Hz for Velocity and 10 Hz to 1500 Hz for Acceleration Optional built-in temperature sensors 4-20mA signal is highly susceptibility to indirect and direct two way radio interference There are no field accessible calibration potentiometers to adjust, so, this electronic device is simply a pass/fail and disposable unit There are no fault protocols for problem transmitters Sensitive to one axis only Once installed the performance of this device can be verified with a portable vibration shaker system, but, in the field or back at the factory, they cannot be recalibrated. There are no calibration potentiometers to adjust, so, this electronic device is simply a pass/fail and disposable unit. Intelligent Vibration Transmitter PROS Proven tri-axial analog sensor design; DSP for calculations Smart addressable microcontroller for onboard signal conditioning Reliable Modbus RS485 RTU Digital Communications Permanent Installation and not a route based sensor technology Certified CSA, CL I Div. 2 Groups A,B,C,D Dual tri-axial sensors and one temperature sensor Wide frequency and temperature range Acceleration or Velocity vibration measurand Firmware configurable band-pass filters Programmable start-up and trip delay Automatic sensor self-test diagnostics and dual sensor verification Multi-color and SMART LED status Universal mounting, any orientation Utilizes less wiring or Wi-Fi communications 24/7 protection and condition machine monitoring Unique 3-Axis vibration signature can be viewed with free software (see example of machine signatures below). Time waveforms and FFT are available. The plots below are for reference only.
Multi-Directional Machine Monitoring (Time Waveforms) FFT Conclusion Cooling tower fans represent a unique application in rotating machinery. The prime problem for most of these machines is fan imbalance or coupling misalignment. In addition, rolling element bearing
defects and speed reducer problems account for many cooling fan failures which can be monitored with low cost case mounted sensors such as the IVT. Continuous on-line vibration monitoring has a tremendous advantage over periodic vibration monitoring. A key advantage is the ability for an automatic machine shut-down when the process is faced with an impending catastrophic failure, e.g., a broken fan blade that is causing machine and structure imbalance. To simplify the setting of the alarm/shut-down set-points, an operator just needs to input the same vibration set-points for all three axes (X, Y, and Z). Without a hand-held vibration data logger, this takes the guess work out of trying to find the greatest source of machine vibration. Further, with on-line vibration system, there is no need to place operators in peril for machines that are located in remote, hazardous, or unpleasant locations. The safety issues alone can justify the cost to purchase, install and maintain a continuous vibration monitoring system. The vibration diagnostic data obtained from the IVT enables reliability and maintenance teams to trend and review the vibration spectra generated by the tri-axial IVT. The unique vibration signature acquired for each IVT can forecast potential machinery problems and pinpoint their cause. A database can be developed to record performance, establish machine histories, assist maintenance diagnostics and extend machinery reliability. In essence, the IVT is a proven technology that provides continuous, reliable, and maintainable vibration data that enables practical problem solving to prevent machinery failure and to aid in reducing process risks and downtime losses.