Hansford Sensors Ltd. - PRODUCT NEWS






Common sense

Vibration sensing has become an increasingly common tool in the process industries, enabling engineers to detect exactly which components are due to fail and when, and thus facilitating the efficient replacement of parts. How the use of vibration monitoring solutions keeps production lines running smoothly in a range of applications.

The process and manufacturing industries use a broad range of moving mechanical equipment to carry out their operations; all have pumps, motors, fans and conveyors, and the continued effectiveness of each component must be ensured to optimise process control and uptime. In the current economic climate, with the challenges faced by manufacturers in 2013 looking set to continue for the coming year – especially the pressures on operating costs – an effective maintenance strategy to support these components is critical.

As machinery has become more complex and business managers have demanded greater productivity from each mechanical system, there has been a corresponding growth in the need for sophisticated monitoring equipment that can help maximise the performance and availability of production and process plant.

Although there are a range of monitoring techniques available to plant and maintenance engineers, vibration sensing is one of the most important. Vibration is an excellent indicator of wear or imbalance within a system, is relatively easy to detect and can provide a extremely accurate early warning of possible problems. Correctly deployed, vibration monitoring can be a valuable tool for extending the operating life of components and systems beyond recommended maintenance intervals, while enhancing system performance, energy efficiency and uptime; the bottom line is that an effective vibration monitoring regime will cut operating costs.

Vibration detection is typically used with rotating shafts, pumps, motors, fans, gearboxes and engine systems, using accelerometers (vibration sensors) to detect subtle changes in operating conditions, with the sensor output either being fed to a hand held instrument or transmitted to a central monitoring point.

Accelerometers, particularly those supplied by the market leaders, are easy to install and use, operate over a wide temperature range, and measure high and low frequencies, with low hysteresis characteristics and excellent levels of accuracy. These devices are also robust and reliable, offering excellent resistance to the ingress of moisture, dust, oils and other contaminants; sensors are also available for use in hazardous and submerged applications.

Given the range of problems and potential vibration monitoring solutions, it is well worth pausing to consider what an accelerometer is, in order to understand how it works and ultimately achieve the best possible specification. 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. Most leading manufacturers offer AC and 4-20mA accelerometers that are intrinsically safe, being ATEX and IEC Ex certified, for monitoring vibration levels on pumps, motors, fans and all other types of rotating machinery.

With the wide range of vibration sensors now available it is possible to cater for the specific requirements of many industries that depend on the smooth running of their production lines. Take, for example, the food and drink processing industry. A broad range of equipment is utilised during the manufacturing process; chillers, ovens and cooling towers all have pumps, motors, fans and conveyors, and the continued effectiveness of each component must be ensured to enable precision process control. Whether the highest standards of performance are consistently achieved depends on efficient condition monitoring and the right vibration sensor specification.

A standard 100mV/g AC sensor provides an ideal solution for the majority of food and beverage manufacturing equipment. 100mV/g AC sensors can be supplied as either top or side entry components, and are also available with stainless steel enclosures that meet the strict hygiene standards imposed within many food and drink process industries. However, while a 100mV/g AC sensor frequently provides the right monitoring solution it is sometimes preferable to install 4-20mA sensors interfaced directly to a PLC so that alarms can be pre-set to shut the machinery down immediately if the ‘safe’ vibration levels are exceeded. For example, where large fans are used in drying or cooling processes, a build-up of debris on the impeller can cause imbalance. Eventually, the accumulation of debris can become so heavy that small lumps may fly off, causing additional imbalances in the motor shafts and, consequently, greater vibration. Under such conditions, there is a danger that impellers may break off and cause damage. However, by installing 4-20mA sensors, either interfaced directly to an existing PLC or similar system, machinery can be shut down before such damage occurs.

Different applications require different solutions. For example, components of a vibration monitoring system specified to protect cooling tower systems must also be able to detect vibration in large area fans at both extremely slow rotational speeds and at relatively fast speeds at periods of maximum cooling. Sensors are now available to accommodate both ends of the operating spectrum, with low and high levels of sensitivity; typically, they would be mounted on the input and output ends of a drive shaft.

Similarly, recent product introductions include triaxial accelerometers, which enable three axes to be read simultaneously, to reduce installation and measurement times. These devices are typically protected by a stainless steel casing, with an operating temperature range of -55 to +140ºC and sealing to give an ingress protection level of IP67. Also available are submersible IP68 rated accelerometers, which have been developed to measure vibration levels on rotating machinery with partial or complete immersion in water at depths of up to 100 metres, equipped with either an integrated waterproof silicon cable or an integrated PUR cable, which provides additional resistance to oil.

With such a range of tools to choose from, it is important to consider the specification and this applies not only to accelerometers but also to switch boxes. In common with the accelerometers themselves, switch boxes must be suited to the location, for example, being sheathed in stainless steel for food processing plant, with perhaps dual output options available for switching between vibration and temperature accelerometers.

Since vibration is one of the main causes of failure in moving machinery, designers and engineers have invested a lot of time and energy in trying to minimise the problem. The fact that so much is at stake as a consequence of inefficiency or failure has driven the development of a range of tools and practices to prevent vibration and its consequences, and routine maintenance procedures that enable engineers to minimise the development of faults. Indeed, components themselves are continually being refined and upgraded to prevent, or offer greater resistance to, vibration. To achieve a consistent and efficient management of essential machinery throughout its lifetime, the adoption of vibration monitoring equipment is vital and, in most process and manufacturing applications, the measurement of vibration plays a major part in enabling operators and maintenance engineers to identify potential problems and act on them before they fulfil that potential and begin generating unnecessary costs.



July 2013

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