a new practical and reliable approach to the vibration monitoring of cooling tower … ·...

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This tech note is provided by our reliability team and is for informational purposes and is not all-inclusive and cannot cover all unique situations. Machine Saver, Inc., makes no warranty of any kind with regard to this material, including, but not limited to, the implied warranties of merchantability and fitness for a particular purpose. Machine Saver, Inc., shall not be liable for errors, omissions, or inconsistencies which may be contained herein or for incidental or consequential damages in connection with the furnishing, performance, or use of this tech note.

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. The purpose

of this paper is to provide the benefits and advantages of an intelligent & smart vibration transmitter

technology that can trend and reliably protect a plant’s cooling tower fan assets. There are exhaustive

published documents on the advantages of the tri-axial vibration sensor which will not be the focus of

this document.

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, an intelligent transmitter

technology is now available that has its own dual sensor, 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.

VTB-COM



Remote monitoring portal details:

Monitor and observe trends of 3 axis overall vibration and temperature data: Acceleration,

Velocity, & Temperature

Machine vibration & temperature data is stored on a remote server indefinitely so that all

analytical and condition data can be found in the same place (from any computer that has

network access)

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 (Acceleration FFT, Velocity FFT)

Email and text messaging alerts upon alarms

Programmable High, Low and Band pass filters

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.

http://www.cti.org/



Cooling Tower Fan– Common Vibration and Temperature Faults

Machine Component/Fault

Frequency Order

Measurement Plane

Comments

Motor/Imbalance

1X, 2X Motor RPM

Radial

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.

Motor/Bent Shaft

1X, 2X Motor RPM

Axial

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

Radial (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

Radial

Rotor Bar Passing Frequency (FRBPF) = 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 Radial

Shaft/Coupling Misalignment may involve both Angular (Axial) and Parallel Offset (Radial) 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

Radial, 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.

Rolling Bearing Defects with Visible Damage to the Bearings

1X to 25X

25X to 100X

Radial

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. 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

Radial

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

Radial (Vertical)

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.

Gearbox/Worn or Broken

Gear Teeth

GMF X 3.25

Radial

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

Radial

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

Radial

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

Radial

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

Radial

Axial

Overheating in electric motor bearings is generally lubricant-related. 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

Mechanical Vibration Switches

PROS

Basic unit without hazardous approvals or start-up delays are inexpensive

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

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

CONS

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

Multi-Directional

Machine Monitoring

(Time Waveforms)

FFT

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.



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.

a new practical and reliable approach to the vibration monitoring of cooling tower … ·...

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