Protecting plant with vibration monitoring

Article at how maintenance engineers can maximise machine uptime with modern vibration monitoring tools.

Automation has the potential to boost plant productivity, quality and economy but these benefits come at a price; as more and more companies become increasingly dependent on the constant availability of automatic machinery, the spectre of costly downtime looms over the operators of engineering firms.  Effective maintenance is essential to protect plant from the failure of automated equipment and the challenge for engineering operators, particularly in the current economic climate, is to identify the most cost-effective and efficient method of providing that maintenance.

Vibration monitoring is one of the key tools for plant and equipment maintenance in the modern engineering environment.  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.

Providing the capability to detect fractional changes in the operation of rotating components via vibration monitoring is one of the most effective preventative maintenance measures an engineer can take.  Increases in vibration indicate deteriorating operating conditions, such as wear or misalignment, and vibration sensors, also known as accelerometers, can identify these changes swiftly and with exceptional accuracy.  Accelerometers can be used as either in-line or hand-held instruments and are increasingly easy to install and use; this is because 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.  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.

Accelerometers contain a piezoelectric crystal element bonded to a mass.  When the sensor is subject to an accelerative force, the mass compresses the crystal, causing it 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. 

Vibration monitoring offers the option of measuring output 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.

To appreciate just how all this works in practice, let’s consider a hypothetical scenario.  Take, for example, the failure of an air handling unit in a pharmaceutical manufacturing facility.  This could result in a change in conditions that may compromise product quality, leading not only to downtime but waste, delays, dissatisfied customers and all the complications that follow.  However, if it is not possible to access the air handling unit because of the risk to health and safety, how can it be regularly and satisfactorily checked and maintained?  The answer is to employ vibration monitoring and attach sensors to the motor and journal bearings.  For example, in a belt drive, the placement of a sensor on the drive end and non-drive end, with one sensor on each journal bearing, could provide a cost-effective vibration monitoring solution using just four sensors.

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.  This may require the creation of a flat surface using spot facing tools; 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.  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.

Of course, 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.  Hansford Sensors was recently asked for advice on vibration monitoring by an overseas customer that had identified high levels of wear in a conveyor pulley bearing over a short time span.  The maintenance team purchased our HS-423 sensor to gain dual output - 4-20mA acceleration into the PLC 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.  Despite the concurring readings, the management chose to ignore the warnings.  The result was a catastrophic bearing failure, causing a fire that damaged equipment and, worse still, put lives at risk.

In this case, the production department ignored the warnings 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. The situation is similar to that experienced when switching on the ignition in your car and seeing 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 factory story serves not only as a warning to operators who do not see the value of preventative maintenance but also underlines the power of accelerometers, which, if used correctly, can prevent such costly and dangerous plant failures. 

Date: 21/03/2012