Automatic improvements

The potential of automation as a force that can enhance productivity, quality and economy in process industries depends on its reliability.  That reliability can be strongly supported by vibration monitoring, says Chris Hansford, Managing Director of Hansford Sensors.

Vibration monitoring is a key component of predictive maintenance, allowing parameters of condition in machinery to be regularly checked so that any significant change that may signal a developing failure can be acted upon.  As with other methods in the broader field of condition monitoring to which it belongs, vibration monitoring enables maintenance to be scheduled cost-effectively so that repairs or replacements can be promptly provided to avoid the costly consequences of machine failure.  

Condition monitoring has proved to be so cost-effective across industry because machines that have begun to exhibit defects are at greater risk of failure than those without defects, and are therefore more likely to generate unwelcome downtime costs.  This may sound rather obvious but it is also true to say that historically - and even currently, despite the range of condition monitoring options available - there has been a widespread belief that it is cheaper to continue running worn equipment rather than replace it and, so the (flawed) thinking follows, wring more life out of the soon-to-be replaced parts.  However, hard experience in the industrial environment shows that this is often proved dramatically wrong. 

One such instance occurred when we were asked to advise an overseas customer who was concerned about high levels of wear in a conveyor pulley bearing over a short time span.  The maintenance team purchased our HS-423 sensor, which gives a dual output: 4-20mA acceleration into a PLC (Programmable Logic Controller) and AC output via a data collector.  The HS-423 sensor triggered a preset alarm level in the PLC, alerting engineers that acceleration levels had increased significantly and the results were corroborated by acceleration readings from a hand-held data collector.  At this point, it would appear that the vibration monitoring solution had solved the problem; technically, it had but vibration monitoring is only as good as the respect that the engineers or production manager has for it.  Despite the concurring readings from both the online and the offline monitoring equipment, the management chose to ignore the warnings.  What the management hoped for was to continue production and save time by postponing a scheduled maintenance period and addressing the problem; what resulted was a catastrophic bearing failure, causing a fire that damaged equipment and, worse still, put lives at risk.

Vibration monitoring can provide a vital signal of a defect and then measure the deterioration of the condition with successive signals but it is up to engineers and plant managers to take action on the evidence of this data and, as we have seen with the example of the conveyor pulley bearing, intervention is often much more cost-effective than risking machine failure.  Our overseas customer ignored the warnings triggered by the vibration monitoring system in order to continue production but the eventual costs of unplanned downtime and major repairs far exceeded those that would have been caused by a short, planned period of preventative maintenance.  This customer was posed a similar question to that which is posed to the driver who switches on the ignition and sees an engine warning light:  Do you perform simple maintenance there and then, or ignore the warning and risk a more costly failure further down the line?

The story of the conveyor pulling bearing incident is an instructive one but it is important to appreciate that the purpose of vibration monitoring is not only to prevent negative outcomes; it is also to generate positive ones.  Vibration monitoring can not only prevent costly and dangerous plant failures, it can maximise profit and efficiency by protecting and enhancing the performance of equipment, and by extending intervals between maintenance periods. 

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.  

So how important is vibration monitoring when it comes to automation?  Primarily in the sense that while automation has the potential to boost plant productivity and efficiency, not to mention product quality, it can only do so if the engineering system is adequately monitored and protected.  Automatic machinery brings many benefits but only when downtime is kept at bay.  Effective maintenance is essential to protect plant from the failure of automated equipment, particularly in the current economic climate, and vibration monitoring is a highly efficient method of providing that maintenance. 

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 in the use of condition monitoring components such as vibration sensors is now provided by organisations such as BINDT (British Institute of Non-Destructive Testing).  This is important because, despite the fact that a vibration sensor offers high levels of reliability, its performance depends on its installation.  For example, 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 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 centralised or higher level system for continuous monitoring. 

Handheld data collectors and analysers are now commonplace, providing a line of defence when permanent online vibration instrumentation cannot be economically justified, or supplying a ‘second opinion’ when online systems are in place but, for example, the decision to stop the process is critical.  The more critical the machinery in terms of safety and/or plant profitability, the more likely it is that a permanent online system will be employed.

The best of today’s accelerometers are robust and reliable devices, with stainless steel sensor housings that prevent the ingress of moisture, dust, oils and other contaminants, and the capability to operate over a wide temperature range, measuring high and low frequencies, with low hysteresis characteristics and excellent levels of accuracy.

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. 

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.  It is therefore important to eliminate instability using spot mounting. 

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. 

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. 

As illustrated at the beginning of this article with the story of the catastrophic bearing failure, any engineering system is only as good as those who manage it and even the exceptional capability and repeatability of vibration monitoring systems cannot save a plant if the warnings are ignored.  However, providing the capability to detect fractional changes in the operation of rotating components via vibration monitoring remains one of the most effective preventative maintenance measures an engineer can take.  

Date: 15/05/2012