Protección Automatizada

Los fabricantes tienen un gran potencial  para reducir costos y aumentar la productividad mediante la automatización. Sin embargo, para mejorar y aumentar la producción y maximizar lel beneficio, las líneas deben funcionar sin problemas. Para ello, un sistema de control de vibraciones es un apoyo esencial.

The Oxford English Dictionary describes automation as, “Automatic control of a manufacturing or other process through a number of successive stages”, and also “the use of automatic devices to save mental and manual labour”.  Manufacturers might add that automation is a powerful means by which machinery can carry out the work of a human being to achieve a significant economic advantage.  However, for many companies in the manufacturing or processing sectors, gaining that economic advantage has not always been possible, as the costs of designing, specifying, purchasing, maintaining and running automated systems has been prohibitive. 

The advance of technology and a reduction in purchasing costs has, in recent years, enabled more operators to capitalise on automation, allowing a broader range of manufacturers to take advantage of the many options available.  As a result, more and more manufacturers are looking to vibration monitoring to support their investment.

Design developments in vibration sensor technology have been driven by the demands of different industries.  This is because vibration is a common problem in machines across applications, sometimes resulting from misalignment of rotating equipment due to poor installation, sometimes the consequence of natural wear and tear.  However, it is increasingly possible to reliably identify sources of wear with the use of vibration monitoring equipment. 

For example, in a wind farm, where the nature of the machinery, the location and the elements all present their own challenges, the ability to condition monitor turbines remotely is vital to maintain turbine availability and avoid costly downtime.  Vibration sensors, also known as accelerometers, can be mounted on a turbine’s main shaft bearings, drive train gearbox, and generator enabling the collection, analysis, and compilation of operational data that helps eliminate recurring failures.  In the vibration monitoring of cranes, where accurate condition monitoring has been hard to achieve because of the inherent difficulties in interpreting the data from slow-moving equipment, a series of sensors can be placed around the bearing to take separate readings for a series of repeating part-revolutions made by the crane operator during the test, enabling the test operator to isolate the required fault signals from the background noise.  And in the monitoring of water and wastewater pumps, vibration sensors can be mounted within compact stainless steel sensor housings that can withstand high levels of moisture and contamination.  These examples of how vibration sensors are used illustrate the continuing development of vibration monitoring technology and how it can be applied across a range of industries.  

Vibration sensors can be mounted in a number of key positions on mechanical equipment, offering the potential for continuous monitoring and analysis, an inexpensive option when balanced against the potential cost of a shutdown, and when condition monitoring measures are in place to detect factors such as vibration, machine downtime can virtually be eliminated.  The continuing enhancements in vibration monitoring sensors and measuring instrumentation have given engineers some powerful options and, when used as part of a planned predictive maintenance programme, they can make a major improvement to plant performance and profitability.

The accelerometers used to measure levels of vibration are typically easy to install and use; however, an accelerometer is only as good as the engineer who is responsible for it.  A poorly installed or maintained accelerometer will not offer the precision or longevity of which it is capable and so it is in everyone’s interest to specify the best accelerometers and apply the best practice in managing their performance.  In order to achieve this, it is worth spending a little time considering what an accelerometer is and how to effectively install one.

Accelerometers contain a piezoelectric crystal element, which is bonded to a mass.  When subjected to an accelerative force, the mass compresses the crystal, and this causes the crystal to produce an electrical signal that is proportional to the force applied.  This output is then amplified and conditioned by inbuilt electronics to produce a signal that can be used by higher level data acquisition or control systems either ‘online’ or ‘offline’.  An online system is one that measures and analyses the output from sensors that interface directly with a PLC.  An offline system is created by mounting sensors onto machinery and connecting them to a switch box; engineers can then use a hand-held data collector to collect readings.

The first thing to consider when specifying accelerometers is that there are two main categories: AC accelerometers and 4-20mA accelerometers.  AC accelerometers are typically used with data collectors for monitoring the condition of higher value assets such as turbines, while 4-20mA components are commonly used with PLCs to measure lower value assets, such as motors, fans and pumps.  Both AC and 4-20mA accelerometers can identify misalignment, bearing condition and imbalance, while AC versions offer the additional capability to detect gear defects, belt problems, looseness and cavitation.  Hansford Sensors offers AC and 4-20mA accelerometers that are intrinsically safe, being ATEX and IEC Ex certified, and can be used to monitor vibration levels on pumps, motors, fans and all other types of rotating machinery.

Care must be taken during the installation of vibration sensors to ensure the maximum level of performance.  Condition monitoring depends on stability; a poorly mounted accelerometer may give readings that relate not only to a change in conditions but also to the instability of the sensor itself.  Accelerometers should be mounted as close as possible to the source of vibration onto a surface than has been made free from grease and oil.  The surface should be smooth, unpainted and larger than the base of the accelerometer itself.  It should also be flat and this may require the creation of a flat surface using spot facing tools to eliminate instability.  A good spot facing kit will give you all the necessary tools needed to accurately mount a vibration sensor onto the rotating machine, including a tapping drill, taps, tap wrench and a spot facing tool.  These kits are now available to allow for different mounting threads; ¼, M6 and M8.  Correct mounting of the sensor is vital to ensure true readings and, where possible, mounting a sensor via a drilled and tapped hole directly to the machine housing will give the best results.  However, if the housing is not flat, a spot facing installation kit allows creation of a flat surface. 

For rotating machinery, vibration analysis has proved a convenient and highly effective method of measurement with which to assess machine condition.  Accelerometers can be easily mounted on casings to measure the vibrations of the casing and/or the radial and axial vibration of rotating shafts.  A typical technique in vibration monitoring has been to examine the individual frequencies within the signal that correspond to certain mechanical components or types of malfunction, such as shaft imbalance or misalignment, so that analysis of this data can identify the location and nature of a given problem.  A typical example would be a rolling-element bearing that exhibits increasing vibration signals at specific frequencies as wear increases.  

A range of sensor and detection technologies are now available with which to maximise machine uptime by extending operating life beyond recommended maintenance intervals and, at the opposite end of the scale, identifying rapid increases in vibration that could lead to a catastrophic failure.  The efficiency of vibration sensors has driven demand in a number of ways; as well as offering enhanced efficiency and increased performance, these devices also enable operators to satisfy the ever-more robust regulations imposed regarding health and safety, which have made the use of sensors in non-safe areas a prime requisite.  As a result, vibration sensors have become an increasingly essential fixture in modern engineering.

Training modules that cover the installation and function of vibration sensing equipment are now offered by BINDT (British Institute of Non-Destructive Testing) as part of a wider study course that looks at the philosophy and application of condition monitoring techniques.  This is important because, despite the fact that a vibration sensor offers high levels of reliability, its performance depends on its installation.  As mentioned earlier, when mounting a sensor there may be a choice between drilling, tapping or glueing but engineers need to understand and consider how these methods may affect the warranties on their equipment.  

To achieve the best specification it is advisable to work closely with a supplier that has a depth of industry experience and knowledge.  Careful consideration must be given to issues such as the vibration level and frequency range to be measured, while environmental conditions, such as the temperature and presence of corrosive chemicals, will affect the specification.  Once the most appropriate sensors have been selected it is important that advice is followed and care is taken during installation to ensure the maximum level of performance.  For example, accelerometers should be located as close as possible to the source of vibration.  Also, devices should be mounted onto a flat, smooth, unpainted surface, larger than the base of the accelerometer itself and this surface should be made free from grease and oil.  Condition monitoring depends on stability and readings from a poorly mounted accelerometer may relate not only to a change in conditions but also to the instability of the sensor itself. 

Once you have specified the right equipment and installed carefully in order to yield the most repeatable and consistent measurements, machine reliability data can easily be analysed to predict potential problems before they occur.  Increases in vibration indicate deteriorating operating conditions, such as wear or misalignment, and vibration sensors, as we have seen, can identify these changes swiftly and with exceptional reliability.  The massive potential for these tools to benefit the engineering industry has dramatically increased demand, which, in turn, has driven the manufacturers of vibration monitoring devices to enhance and adapt their products to suit a broadening range of industries and specifications, resulting in accelerometers that are increasingly easy to install and use. 

Far from being an expensive option, the use of vibration monitoring can enable companies to operate with enhanced performance and increased flexibility, both vital attributes at a time when industry is coming under increasing pressure to boost productivity and cut operating costs.

Automation has the potential to boost plant productivity and efficiency, not to mention product quality, but it can only do so if the engineering system is adequately monitored and protected.  Effective maintenance is essential to protect plant from the failure of automated equipment and the many benefits it brings to a manufacturing operation.

Date: 03/07/2012