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A Guide to Vibration Sensing

Plant engineers must understand the need for accurate data capture as part of a predictive maintenance plan in modern manufacturing. This article offers advice on how to specify vibration sensors to carry out condition monitoring in different applications.

This article originally appeared in IMPO's May issue

Since the beginning of recorded history, mankind has had need of reliable sensors and duly invented many varieties in the cause of progress.  The oldest vibration sensor known to man was a seismograph invented by Chinese astronomer Chang Heng in the second century AD.  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, thus indicating the direction of a distant earthquake and potentially saving lives by enabling the government to send assistance to the affected area.

Methods may have changed but the need for vibration sensing has not diminished; indeed, it has increased.  As machinery has become more complex and mankind has demanded greater productivity from each mechanical system, there has been a corresponding growth in the need for sophisticated vibration sensors that can maximize the performance of many 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 a variety of resilient enclosures to enable its use within a wide range of applications. 

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. 

With so many sensors in daily operation within modern engineering systems (in the paper industry, for example, there may be 500 to 600 sensors on each machine, primarily to monitor the vibration levels on the roller bearings) maintaining high standards in the installation and maintenance of sensors has become a priority, which is why training in the use of condition monitoring components such as vibration sensors is now provided.  This is important because, despite the fact that a vibration sensor offers high levels of accuracy and reliability, the component is only as good as its installer.  For example, when mounting a sensor there may be a choice between drilling, tapping or gluing but engineers need to understand and consider how these methods may affect the warranties on their equipment. 

To install and use vibration sensors to their fullest potential, engineers first need to understand how they operate.  Vibration sensors, which measure a quantity 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 proportional to the level of force applied.  The signal is then amplified and conditioned using inbuilt electronics that create an output signal, which is suitable for use by higher level data acquisition or control systems.  Output data from accelerometers mounted in key 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 centralized or higher level system for continuous monitoring.

These accelerometers can be divided into two categories: AC and 4-20mA.  Although both operate under the same principle there are subtle differences between the two that engineers need to be aware of when specifying systems.  AC accelerometers are typically used with a data collector for monitoring the condition of higher value assets such as wind turbines, while 4-20mA devices are used with a PLC to measure lower value assets, such as 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 variety of accelerometers in both categories that can operate over a wide temperature range, measuring both high and low frequencies, packaged in stainless steel sensor housings that can prevent 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 inserting HS-500 modules into the circuit between the accelerometer and the data collector. 

The resistance to contamination offered by today’s accelerometers is vital in sectors such as hydrocarbon engineering, where excessive vibration can cause anything from inflated energy consumption to catastrophic failure.  Vibration monitoring offers a vital early warning, enabling engineers to take action before any substantial damage is caused.  When machinery such as fans and motors run out of alignment, the resulting vibration leads to excessive wear and premature failure of parts and, ultimately, a resulting reduction in the power station’s generating capacity.  Gaining access to motors and fan units can be difficult and time consuming and without a strong condition monitoring regime engineers will find themselves continually reacting to problems that have already been created.

However, by using a network of accelerometers positioned on machinery, such as fans, motors and gearboxes, a dedicated condition monitoring system to improve operational efficiency can prevent problems before they occur. 

Once data has been gathered it must be effectively analyzed and interpreted and, in some sectors, the most efficient means of doing so is to relay outputs from these accelerometers to a control room terminal that runs monitoring, analysis and reporting software to provide a real time display of all key operating parameters.

To specify vibration monitoring equipment correctly, engineers need to ask the right questions from the outset and consultation with a market leader that has experience in a wide range of sectors can swiftly enable the right decisions to be made.  From the initial questions, such as, “What is the vibration level and frequency range that is to be measured? Engineers need to consider environmental conditions, such as the temperature, humidity, and whether there are any corrosive chemicals present.  A series of further considerations follow on from there because today’s vibrations sensors are available for a wide range of sectors, including offshore, petrochemical, utilities, paper, marine, metals, water and waste treatment, aerospace, pharmaceutical, automotive and beyond.

We have come a long way since Chang Heng’s ingenious seismograph and vibrations far smaller than earthquakes can now be detected instantly with accelerometers.  Chang applied an earthly, scientific attitude 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 today’s engineers can follow in his footsteps by protecting their plant and productivity using vibration monitoring techniques, rather than crossing their fingers and hoping for the best.

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