By: Simon Fridlyand
Monitoring the health of plant machinery by checking vibration can provide effective control of ignition sources in hazardous environments.
The principles of monitoring the health of plant machinery by checking vibration are straightforward. All operating machines vibrate. Since an increase in vibration almost always accompanies deterioration in running conditions, it is possible to gain information about a machine’s condition by monitoring vibration levels.
The overall level of vibration indicates the general condition of the machine; vibration analysis can be used to determine the cause of vibration, including such factors as unbalance, misalignment or bearing defects.
Bearing vibration and overheating become extremely important issues if the process involves flammable vapours or gases, or combustible powders such as sugar, flour or wood. Overheated bearings present a source of ignition. There are numerous examples where overheated bearings have caused fires and explosions in industrial plants.
To combat these dangers, the National Fire Prevention Association (NFPA) in the U.S. has published several standards related to the topic. These include: NFPA-654, NFPA-664, NFPA-61, and others that outline requirements for hot surfaces and bearings in particular.
Machinery vibration monitoring programs are effective in reducing fires and explosions in industrial plants where flammable/combustible atmospheres exist, and as well help curb the overall operating costs of industrial plants.
Vibrations produced by industrial machinery are vital indicators of machinery health. Machinery monitoring programs record a machine’s vibration history. Monitoring vibration levels over time allows the plant engineer to predict problems before serious damage occurs. Machinery damage and costly production delays caused by unforeseen machinery failure can be prevented.
Ideally, this information could be integrated, via a computerized maintenance management system (CMMS) or an enterprise asset management system, with other asset health data for an overall picture of the total plant and operations equipment condition.
With expanded use of the Internet and access to wireless technology, remote monitoring of machinery and data transmission is getting easier and quicker. All segments of an organization, including operations and management, can share information and access data. This works to ensure that assets are used in the best manner possible, with minimum costs, to provide continuous improvement in plant operations and maintenance functions.
Critical to vibration monitoring and analysis is the machine-mounted sensor. Three parameters representing motion detected by vibration monitors are displacement, velocity and acceleration. These parameters are mathematically related and can be derived from a variety of motion sensors. Selection of a sensor proportional to displacement, velocity or acceleration depends on the frequencies of interest and the signal levels involved.
Displacement sensors: Displacement sensors are used to measure shaft motion and internal clearances. Monitors have used non-contact proximity sensors such as eddy probes to sense shaft vibration relative to bearings or some other support structure. These sensors are best suited for measuring low-frequency and low-amplitude displacements typically found in sleeve bearing machine designs.
Piezoelectric displacement transducers (doubly integrated accelerometers) have been developed to overcome problems associated with mounting non-contact probes and are more suitable for rolling element bearing machine designs. Piezoelectric sensors yield an output proportional to the absolute motion of a structure, rather than relative motion between the proximity sensor mounting point and target surface, such as a shaft.
Velocity sensors: Velocity sensors are used for low- to medium-frequency measurements. They are useful for vibration monitoring and balancing operations on rotating machinery. As compared to accelerometers, velocity sensors have a lower sensitivity to high-frequency vibrations. Thus, they are less susceptible to amplifier overloads. Overloads can compromise the fidelity of low- amplitude, low-frequency signals.
Traditional velocity sensors use an electromagnetic (coil and magnet) system to generate the velocity signal. Now, hardier piezoelectric velocity sensors (internally integrated accelerometers) are gaining in popularity due to their improved capabilities.
Accelerometers: Accelerometers are the preferred motion sensors for most vibration monitoring applications. They are useful for measuring low frequencies to very high frequencies and are available in a wide variety of general-purpose and application-specific designs.
The piezoelectric accelerometer is unmatched for frequency and amplitude range. The piezoelectric sensor is versatile, reliable and the most popular vibration sensor for machinery monitoring.
Piezoelectric sensors: The rugged, solid-state construction of industrial piezoelectric sensors enables them to operate under most harsh environmental conditions. They are unaffected by dirt, oil, and most chemical atmospheres. They perform well over a wide temperature range and resist damage due to severe shocks and vibrations.
The piezoelectric element in the sensor produces a signal proportional to acceleration. This small acceleration signal can be amplified for acceleration measurements or converted (electronically integrated) within the sensor into a velocity or displacement signal.
The piezoelectric velocity sensor is more rugged than a coil and magnet sensor, has a wider frequency range, and can perform accurate phase measurements.
Choosing an industrial sensor
When selecting a piezoelectric industrial vibration sensor (acceleration, velocity or displacement), many factors should be considered so that the best sensor is chosen for the application. The user who addresses application-specific questions will become more familiar with sensor requirements. Typical questions include:
* What is the vibration level?
* What is the frequency range of interest?
* What is the temperature range required?
* Are any corrosive chemicals present?
* Is the atmosphere combustible?
* Are intense acoustic or electromagnetic fields present?
* Is there significant electrostatic discharge (ESD) present in the area?
* Is the machinery grounded?
* Are there sensor size and weight constraints?
Other questions must be answered about the connector, cable and associated electronics:
* What cable lengths are required?
* Is armoured cable required?
* To what temperatures will the cable be exposed?
* Does the sensor require a splash-proof connector?
* What other instrumentation will be used?
* What are the power supply requirements?
Vibration sensors certified as being intrinsically safe should be used in areas subjected to hazardous concentrations of flammable gas, vapour, mist or combustible dust in suspension.
Intrinsic safety requirements for electrical equipment limit the electrical and thermal energy to levels that are insufficient to ignite an explosive atmosphere under normal or abnormal conditions.
Even if the fuel-to-air mixture in a hazardous environment is in its most volatile concentration, intrinsically safe vibration sensors are incapable of causing ignition. This greatly reduces the risk of explosions in environments where vibration sensors are needed.
The monitoring of the health of plant machinery by checking vibration can provide effective control of ignition sources in hazardous environments.
Simon Fridlyand, P.Eng., is president of S.A.F.E. Engineering, a Toronto-based company specializing in industrial health and safety issues and PSR compliance. For more information, email us.