What is a Vibration Monitoring System?

Unscheduled downtime, or time spent performing maintenance on a machine that has unexpectedly failed, can cost companies enormous amounts of money, in terms of time, process losses and human resources. A vibration monitoring system is a tool that looks for irregularities and other anomalies in a machine’s health by measuring its vibration levels. It helps to protect the facility, the process and the people. This system can identify anything from unbalance, to misalignment, to a loose part and helps prevent downtime on machines. A vibration monitoring system, even for a single channel, is one of the most effective ways to predict a potential machine problem and provide early warning of machine failure. It is vital to many industries for safety and keeping the process running.

How Does a Vibration Monitoring System Work?

Vibration monitoring systems work by using a method known as vibration trend analysis. Vibration trend analysis is a process that monitors for irregularities in the vibration signature of a machine. When the vibration amplitude goes up or down significantly, under steady state conditions, it is usually an indication that something is changing inside the machine. The overall amplitude is the main indication of a machine malfunction, and is perfectly suited for a 4-20 mA output to a Control System (PLC, SCADA, RTU etc.). The overall amplitude includes the various individual frequency and amplitudes generated by the machine components, its rotation or reciprocating motion, and or process issues. Some monitoring systems record vibration waveforms and identify potential machine issues by revealing abnormalities in the recorded vibrations.

The Characteristics of Vibration

The main characteristics of vibration are amplitude, frequency, phase, form, and when using a proximity sensor, position. Amplitude refers to the severity of the vibration in question. Frequency helps pinpoint the source of the vibration. Changes in vibration frequency, not associated with machine speed changes, can also indicate a problem. Attention to frequency can identify specific problems quickly. Phase helps with determining when and where the vibration is occurring. When studying the waveform of a given vibration, the varying amplitudes and frequencies can reveal a great deal of information about the machine in question. From the time the machine begins operations to the specific moment when a part may have a problem or issue, the amplitude measured by a vibration monitoring system can show differences in movements. With a proximity sensor, the position of the rotor in the bearing clearance, or the rotors axial movement, may be observed and important in assessing the machine’s condition. In most situations, the overall amplitude is used by the vibration monitoring system to compare to vibration limits, or alarm values.

How is Vibration Measured?

While vibration is recorded using frequency and amplitude, it is measured in terms of acceleration, velocity and displacement. These characteristics can be monitored with sensors and vibration analysis equipment. This equipment can vary regarding the elements it measures, as well as how it is measured.

For example, machines with Rolling Element Bearings (REB) are usually monitored with seismic vibration sensors, either velocity sensors or accelerometers. REB machines that operate less than 60 Hz usually use velocity sensors. Machines with Fluid Film Bearings (Journal Bearings or Sleeve Bearings) are usually monitored with proximity probe sensors. 

Accelerometers usually use piezoelectric crystals (PZT sensors), which generate an electrical charge when any change in acceleration, pressure, or force is identified. Accelerometers can also be designed with MEMS (microelectromechanical systems) technology to measure acceleration. Velocity sensors may take the amplified accelerometer output (piezoelectric or MEMS), integrate the signal, and output a velocity signal. In high temperature applications a moving coil velocity sensor is often used to measure vibration using velocity.  Proximity probes are used to measure relative motion between a rotor surface and bearing, usually with a Fluid Film Bearing. Proximity Probes can measure vibration, axial position and rotor speed. The vibration signals from the sensors can be digitized and recorded for trend analysis.

Popular Methods of Vibration Monitoring

1. Walk-Around/Periodic Vibration Collection

This type of monitoring usually uses an accelerometer to record vibration levels at each radial bearing and axial bearing of the machine on a periodic basis, usually monthly or quarterly. It does not use transient data (start up and shutdown data) and is best accompanied by a vibration switch (electrical or mechanical), which helps prevent machine failure between each periodic data run.

2. Walk Around Vibration Data Collection (with waveform or transient data)

This is most often performed after a problem has been identified during the walk-around data collection process, or if a vibration switch shut the machine down.

3. Continuous Vibration Monitoring (without waveform or transient data)

These monitoring systems are often equipped with alarm and shutdown capabilities to help prevent machine failures. This type of monitoring uses transmitters that provide a 4 to 20 milliamp (mA) signal connected directly to the control system to monitor the vibration data.

4. Continuous Vibration Monitoring (with process data correlation, without waveform or transient data)

This monitoring concept incorporates alarms and shutdown capabilities that help prevent machine failures and identify abnormal trends. Data is sent using 4 to 20 mA transmitters connected directly to the control system that has been connected to the ‘Plant Information Network’ for correlation with the process information (load, flow, level, pressure, temperature, etc.). The vibration data along with the process data provides a higher degree of understanding how the machine is actually behaving.

5. Continuous Vibration Monitoring (with process data correlation)

Single channel or multichannel monitoring equipment with alarm and shutdown capabilities that help prevent machine failure and easily identify vibration trends. This type of monitoring also allows you to perform diagnostics on your machine when you have the correct transducer suite. Vibration, speed, thrust position, and impact data are usually incorporated into the data suite.

6. Continuous Vibration Monitoring (with both process data correlation and waveform and transient data)

This type of monitoring is equipped with the same features as above, which allow you to identify abnormal trends given the normal processing conditions and prevent machine failures. You can also run machine diagnostics on-demand and use sensors that are integrated with the plant process data. This is the highest form of machinery vibration monitoring.

Common Applications of Vibration Monitoring Systems

Vibration monitoring systems can be used for a variety of different applications. Here are just a few of the most common industrial uses:

It is important to note that the number of vibration sensors used on a rotating or reciprocating machine depends on the criticality of the equipment. Critical equipment, that if shutdown for just a day could cost over $100,000, should have XY vibration sensors at each radial bearing, thrust sensors, and a phase trigger at a minimum. If the machine is less critical, <$10,000 per day impact, perhaps just a single vibration sensor at the load side bearing closest to the coupling would be adequate. In between these extremes, judgement must be used as to the best way to monitor the machine so that the plant can receive a return on their investment (ROI), which may be one or two sensors with switch capabilities (Alert/Shutdown).

1. Rotating Machinery

Rotating equipment and machinery such as motors, fans, turbines, and gearboxes are some of the most common places you’ll find a vibration monitoring system. Due to the multiple different pieces rotating simultaneously and creating their vibrational patterns, vibration monitoring can be crucial to ensure all components are working properly. With rotating equipment, one issue can lead to damage and catastrophic failure of the equipment. Vibration monitoring can help identify misalignment, unbalance, rubs, bearing faults or even resonance issues in rotating machinery.

2. Bearings and Gears

In the bearings and gears of a machine, various vibrations happen at varying frequencies, which is why vibration monitoring systems are so commonly utilized. With gears, multiple different vibration patterns are created as the equipment performs its desired task. These varying vibrations are too difficult for humans to monitor alone, and employing a monitoring system for early warning is essential to a machine’s continued functionality.

3. Reciprocating Compressors, Engines, and Pumps

Equipment like reciprocating compressors, engines, and positive displacement pumps all have multiple, actively vibrating components within. Reciprocating machinery is special in that its parts move in both rotary and linear patterns. Monitoring vibrations on these distinct parts can help identify potential failures before they can cause damage to the machine.

4. Bridges

Vibration monitoring is crucial when it comes to making sure bridges are structurally sound. Vibration trends can help develop a baseline for the behavior of a bridge that is undamaged and healthy, then detect when it may be experiencing structural issues. If a bridge starts to show abnormal vibrational behavior, maintenance crews can perform the required inspection to determine the source of problem.

5. Pipes

A vibration monitoring system is essential for pipes that may be prone to vibration-induced problems. A monitoring system not only helps to alert when issues begin to occur, but can also help to find the source of problem areas. This way, high-risk areas on the pipe can be monitored for potential problems moving forward.

Installing a Vibration Monitoring System

Taking steps to ensure machinery and equipment are working correctly and able to last as long as possible is essential to any business. With a vibration monitoring system to measure any irregularities and alert you of potential problems, you’ll be able to help your machinery run more effectively for a longer period without significant damage.

Here at Metrix, we have been creating and installing industrial vibration monitoring systems for years. We’re one of the most-preferred vibration monitoring companies and suppliers around the globe and offer our services in more than 40 countries. Whether you’re looking for vibration sensors, installation, or advice, we've got you covered. For more information about our products and their applications, or to request information about a specific product, contact Metrix today.

By Everett Jesse, Vice President of Sales and Marketing