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Accurately capturing critical asset health data with vibration sensors

Since the beginning of recorded history, humankind has seen the need for reliable sensors. With progress as the focus, he duly invented multiple varieties. In the second century AD The oldest vibration sensor known to man, a seismograph, was invented by Chinese astronomer Chang Heng. This device responded to distant disturbances by depositing a bronze ball from the mouth of one of eight dragons placed at intervals around a large urn. This indicated the direction of a distant earthquake and saved lives by enabling the government to send assistance to the affected area.

Methods have definitely changed but, the need for vibration sensing has not diminished; indeed, it has increased. Machinery has become more complex as has mankind’s demands for greater productivity from every mechanical system. This in turn, has led to a corresponding growth regarding the need for sophisticated vibration sensors that can maximise the performance of numerous engineering processes. Today’s designers and engineers have not only succeeded in providing devices of exceptional accuracy and reliability but, also in packaging that functionality in resilient enclosures to enable uses over a wide range of applications.

The efficiency of vibration sensors has propelled demand in a number of ways; as well as offering improved efficiency and increased performance; these devices also empower operators to satisfy ever- expanding regulations surrounding the health and safety of personnel. This has made the use of sensors in non-safe areas obligatory and as a result, vibration sensors have become an increasingly essential fixture in modern industry.

With a vast number of sensors in daily operation within modern engineering systems (in the paper industry, for example, there may be 450 to 650 sensors on each machine, primarily to monitor vibration on roller bearings) preserving high standards in the installation and maintenance of sensors has become a priority. This is important because, despite the fact that a vibration sensor offers accuracy and reliability, the instrument is only as good as its installer. For example, when mounting a sensor there are considerations that may determine a choice between drilling, tapping or glueing. Engineers need to be educated so as to understand how these mounting methods may affect their equipment.

To install and use vibration sensors to their maximum potential, engineers first need an understanding of how they function. Vibration sensors, which measure a magnitude of acceleration and are therefore a type of accelerometer, typically contain a piezoelectric crystal element bonded to a mass. When the accelerometer is subject to an accelerative force, the mass compresses the crystal, causing it to produce an electrical signal that is relative to the level of force applied. The signal is then amplified and conditioned using integral electronics that create an output signal, which is suitable for use by high level data acquisition or control systems. Output data from accelerometers mounted in strategic locations can either be read periodically using sophisticated hand-held data collectors, for immediate analysis or subsequent downloading to a PC, or routed via switch boxes to a centralised higher level system for continuous monitoring.

Most accelerometers can be divided into two categories: AC and 4-20mA. Although both operate under the same principle, there are differences between the two that engineers need to be aware of.

AC accelerometers are generally used with a data collector for inspecting the condition of higher value assets (such as wind turbines), while 4-20mA devices are effective when used with a PLC to offer an early warning of increasing vibration, (applications include fans and pumps). Both are capable of detecting imbalance, bearing condition and misalignment but AC accelerometers can also identify cavitation, looseness, gear defects and belt problems. AC accelerometers offer a wide frequency and dynamic range and are compatible with commercially available monitoring systems, while 4-20mA devices offer a cost-effective alternative when AC components are not required. Market leaders such as Hansford Sensors, offer a diverse range of accelerometers in both categories. Options include sensors that can; operate over a wide temperature range, measure both high and low frequencies, be packaged in stainless steel housings preventing the ingress of moisture, dust, oils and other contaminants. Hansford Sensors also offers the potential to gain a greater level of data acquisition from 4-20mA devices by introducing the HS-500 series modules into the circuit between the accelerometer and the data collector.

The resistance to contamination offered by today’s accelerometers is vital in divisions such as hydrocarbon engineering, where excessive vibration can cause anything from inflated energy consumption to catastrophic failure hence a realistic danger to humans. Monitoring vibration offers a crucial early warning to pending trouble, enabling engineers to take action, consequently any substantial damage that otherwise may have been caused. When assets such as fans and motors pass out of alignment, the subsequent vibration leads to excessive wear and untimely failure of parts. This will ultimately result in the knock on effect of a reduction to a power station’s generating capacity.

Gaining access to motors and fan units can be difficult and time consuming. However, by using a network of accelerometers positioned on machinery such as; fans, motors and gearboxes, a dedicated condition monitoring system will improve operational efficiency and can prevent problems before they occur. Alternatively, without a strong condition monitoring regime, companies will find themselves continually reacting to complications that have long since been created.

Once vibration data has been gathered it must be effectively analysed and interpreted. In some sectors, the most efficient means of data gathering is to relay the accelerometer output to a control room terminal. Once at the control room, the signal can be processed, analysed and reported via software to provide a real time display of all key operating parameters. Intelligent software such as this has achieved measurable results by allowing engineers to identify wear in key assets long before it becomes a problem. In turn this allows the planning of essential maintenance when production demands are minimal.

To choose vibration monitoring equipment correctly, engineers need to ask the proper questions from the outset. This may include consultation with a market leader, such as GVS Reliability Products, that has experience in a wide range of applications thus swiftly ensuring the right decisions can be made. From the initial questioning, such as, “What is the vibration level and frequency range that is to be measured?” engineers also need to consider atmospheric and environmental conditions (temperature, humidity, presence of corrosive chemicals or oils etc…). A series of further considerations need to follow as today’s vibrations sensors are available for a wide variety of industry including; offshore, petrochemical, utilities, paper, marine, metals, water, waste, aerospace, pharmaceutical, automotive and beyond.

We have come a long way since Chang Heng’s seismograph. Vibrations far smaller than earthquakes can now be detected instantly with accelerometers. Chang applied an earthly, scientific approach to the challenges presented by earthquakes, contradicting the orthodox opinion that viewed such disturbances as representations of the gods’ displeasure. While his elders awaited their fate, Chang preferred to take a more hands-on approach, and the intelligent engineers of today follow in his footsteps. By protecting plant and productivity using vibration monitoring techniques, the modern engineer channels the spirit of Chang Heng rather than crossing one’s fingers and hoping for the best.

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