The important role of vibration sensors in the process industries.

Many years ago, vibration sensors, sometimes known as accelerometers, were used mainly in research and development projects studying the effects of vibration on new products such as consumer goods, aircraft, vehicles, satellites, etc.

This role for the sensors still continues, but a more important role has developed over the last twenty years in the field of plant condition monitoring. Here, many more vibration sensors are used to monitor the trends of vibration levels on vital process and production machinery, giving the maintenance engineer invaluable data to act upon in predictive and preventative maintenance programmes. Recent studies have shown that the typical financial loss for an unplanned plant shutdown is around £200,000 in a steelworks, and over £2.25 million on a semiconductor production line.

So, what is a vibration sensor?
The first ones appeared in the early 1900's and were large mechanical devices, devised to measure and to record tractive and brake effort in the automotive industry. They consisted of two tracing pens, one centred and one pendulum mounted, above a paper scroll. The scroll was housed on two clockwork driven drums operated by a winding handle. Before its invention, the only method of measuring acceleration was by observation.

Modern day vibration sensors are electronic devices, using either piezoelectric or piezoresistive technology. Most models used in plant monitoring applications are the piezoelectric types. Their construction consists of a crystal of piezoelectric material to which is attached a seismic mass. When the crystal is stressed in tension or compression, it generates an electrical charge which is proportional to the acceleration level it is experiencing. Internal circuitry converts this signal into a voltage or current (4-20mA) output for data collectors or process control loops. (see figure 1)
This robust device has no moving parts and offers long term stability and reliability. It has very wide frequency and dynamic ranges and signals can be integrated to give velocity and displacement values.

Piezoresistive accelerometers consist of a seismic mass which is attached to a cantilever beam. The beam is deflected whilst experiencing 'g' forces and this movement is converted to an electrical signal by resistance changes in a semiconductor sensing element. Internal electronic circuitry provides amplification of the signal and temperature compensation.

The frequency range of this device is lower than piezoelectric versions, but has the advantage of being able to monitor static or DC acceleration levels.

Over the last twenty five years, different versions of vibration sensors have been designed to suit a whole new range of environmental conditions in an ever increasing need to perform consistantly in differing industrial applications. These conditions have helped to create models which are primarily robust with an outer stainless steel construction. They will operate successfully in both high and low temperatures, typically over -55 to +140 degrees C. The stainless steel housing is welded to give protective sealing to IP68, or full liquid submersion in underwater applications. Radiation resistant versions are available, as are fully ATEX approved intrinsically safe models for operation in potentially explosive atmospheres. Cable choice for the different applications covers tough stainless steel braiding, submersible and high temperature PTFE options. Mechanical fittings are also important for the correct fixing of the sensor to ensure correct transfer of the vibration signal and come in a choice of differing threads, adaptors, quick-fit or glue mounting studs and magnets. Custom designed versions are also available, where a special sensor is created to exactly suit the conditions it will need to operate under.

So why are these vibration sensors used? Traditionally the maintenance of process and production plant was performed when a machine failed or at a time of reduced production activity, when the suspect item or component would be refurbished or completely replaced conveniently. These methods could result in unexpected costly shutdowns with delivery schedules slips and expensive part replacement, when it was probably not necessary.
Vibration sensors with a variety of outputs including temperature and AC or 4-20 mA offer a range of packages to monitor the vibration of critical machinery. In machinery such as motors, fans, gearboxes, centrifuges, pumps and vibratory conveyors, the bearing is one of the key parts. Over time these can deteriorate to the point where they can completely fail. As this deterioration occurs, the vibration signature changes and can be analysed to establish its level of performance.

This is where the vibration sensor comes into its own. When mounted on the bearing, it picks up the vibration levels which can be captured and analysed by the maintenance engineer. The analysis will indicate what state the part is in and what corrective procedures need to be actioned. This important information allows the engineer to plan when corrective or replacement work can be carried out at the most convenient time so as to minimise plant down time and the cost of the resulting production loss.

Vibration analysis can be done in several ways. As an excellent entry-level instrument, portable vibration meters with probes can be used from bearing to bearing and from the vibration level digital reading, indicate any problem.

A more detailed approach can be achieved with Fast Fourier Transform (FFT) analysis by relating the frequency of the vibration to the characteristic signature of each failure mode, vibration analyses help engineers to identify the cause of the problem. Bearing problems causes can be many, including excessive or inadequate lubrication; contaminated grease; angular or parallel mis-alignments; excessive drive belt tensions; loose fitting bearings; excessive loads or axial thrusts and weak motor foundations.

More and more industries are becoming aware of the importance of vibration condition monitoring of their process or production plant.

The water industry now monitors its many water processing pumps from those situated in remote pumping stations to those in large scale sophisticated water treatment and de-salination sites.

The food processing industry needs to monitors its many rotating machinery parts in vibratory conveyor systems, cooling fans, food pumps and mixers as a production line shutdown of Britain's favorite food, the chip, can cause cost losses of many thousands of pounds.

Building services is another massive area where the performance of motors, pumps and fans is critical in public buildings such as hospitals, airports, railway stations, office blocks, hotels, etc.
Processing plant for petrochemicals, oil, gas chemicals use vast amounts of motors, pumps and fans, all of which have a critical role and need monitoring to warn of increasing vibration levels.
Another topical area where vibration sensors are being used is with the latest construction of new wind power farms where the rotor of the blades need to be supervised for optimum and continuous performance.

In fact, everywhere you look rotation in bearings, whether rotary or linear is taking place and is potentially a vibration monitoring point.
The vibration sensors are available with a choice of signal outputs. Direct AC signals or 4-20mA, created with integral electronics, compatible with direct PLC usage and eliminating the need for an external electronics conditioning unit. The sensor signal may be used with a choice of associated systems depending on the requirement of the plant engineer.

The entry level portable, battery operated Vibration Meter, complete with a vibration sensor probe and protective carrying case, ideal for regular manual monitoring of point vibration levels around the factory. Typical vibration meter kits cost under £600.

Multichannel switches boxes are robust industrialised cabinet units with a manual selector switch for selection over a number of vibration sensors at permanent monitoring points, with the resultant single signal being monitored by other data capture and analysis equipment.

Multichannel rack systems with built-in vibration sensor conditioning and pre-set alarms with operating relays offering continuous, simultaneous vibration signals from a group of permanently sited sensors.

What of the future? Conditioning monitoring is so obvious in its benefits and advantages for plant efficiency, greatly reducing potential losses financially in lost production and energy, together with the enhancement of the company's image with improved deliveries, lower product cost and improved product quality. The role of the vibration sensor in the monitoring of the process and other industries will undoubtably increase into the future.

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July 2006

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