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Monitoring the condition of machinery is a practical means of protecting plant and equipment against unexpected failure, and of all conditions that might be monitored vibration levels and temperatures (or rather changes in one or both) often provide the earliest indication that maintenance is necessary. Furthermore, monitoring conditions continuously not only protects plant that needs to run 24/7 but it can also be the foundation of your Condition-Based Predictive Maintenance (CBPM) strategy

Temperature and vibration levels can of course be measured at regular intervals, using handheld instruments, and results recorded. However, it can be difficult to determine what the inspection intervals should be because component wear depends on usage and load profiles; which will typically vary from machine to machine.

Then there is the matter of deciding how best to respond when a condition does change. For example, if an asset’s vibration levels are found to be higher than usual following a series of ‘normal’ readings you have no way of estimating when the levels started to rise. And without an understanding of how quickly the levels rose, it is impossible to estimate the rate at which further deterioration will occur. Also worthy of note is that failures can still occur, even between close inspections.

Accordingly, continuous condition monitoring is highly recommended, particularly on machinery which, if it fails, could halt production and may even damage other equipment.

As most readers will be aware, there are some very advanced systems on the market which use complex algorithms to analyse vibration signatures and temperature changes in order to identify failing components automatically. Such systems tend to require considerable expenditure. In most instances, though, all you are really after is:

• An indication that levels have changed;
• Visibility of the rate at which they changed; and
• A means of automatically raising an alarm if predetermined levels are exceeded.

Thankfully, all three objectives are achievable with relative ease and within a typical engineering/maintenance budget.

Common Sense

To monitor temperature and vibration, at the same time and location, dual output devices are available, which will simplify wiring.

Each sensor will need to be ‘sized’ against the machine it is to protect, and we recommend that any sensor’s sensitivity should be such that its output (or outputs if dual) will be about 80% of maximum when the machine on which it is mounted is operating under normal conditions. As for gauging what ‘normal’ is, we recommend the use of a handheld measurement device, such as a vibration meter (see figure 1), to establish not only the levels but also where best to mount the sensors.

If the machinery is in a hazardous environment, intrinsically safe sensors must be used. For instance, Monitran and an engineering partner company recently developed a bespoke condition monitoring system to protect the rail load-out system of a new biomass power station in the north of England. In this instance, vibration and temperature levels are being measured using 39 MTN/2285IT-4P dual output sensors (see figure 1); permanently mounted on assets which include motors, gearboxes, fan assemblies and bearing housings, all of which are in an ATEX II zone.

In most cases the sensors are wired into local junction boxes, typically one per asset, which in turn connect to a wall-mounted microprocessor-based condition monitoring unit, located in the site’s Motor Control Centre (MCC). The condition monitoring unit was built and pre-calibrated by Monitran and whilst the MCC is not an ATEX II zone consideration had to be given to the fact that the unit is electrically connected to sensors which are in a hazardous environment. Zener barriers were employed in the unit to supress any voltages that could potentially cause a spark.

The condition monitoring unit has a 5-inch 800 x 480 pixel LCD screen, which updates at five-second intervals and displays readings for the 78 channels (i.e. temperature and vibration for each of the 39 dual output sensors), and connects to the site’s SCADA via Modbus TCP Ethernet. The alarm level for each channel was set in the SCADA, on which a ‘look-back’ history can also be accessed to determine at what rate any of the conditions being monitored might be changing.

Protecting chip production

A close derivative of the above condition monitoring unit is also being used in a system to protect chip production, for one of the UK’s leading producers. Chip production runs 24/7 and requires a variety of machines with rotating and moving parts, as well as flowing water, hot oils and pneumatics. The servicing and repair of equipment needs to be carefully scheduled to have minimum impact on production.

In this application, 161 accelerometers are used. Of these, 136 are MTN/2200 general purpose, top-entry accelerometers with AC outputs. Sealed to IP67, these sensors have a frequency range of 2Hz to 10kHz and an operating temperature range of -55^oC to 140^oC. The remaining 25 sensors are MTN/1105 high temperature accelerometers, which can be mounted on surfaces at up to 250^oC and, in this case, are mounted on a number of hot oil pumps.

The sensors connect to five condition monitoring units which connect via Ethernet to two PCs, one in the engineering office and one in the production office. Both PCs run a simple but effective software program, developed by Monitran, which uses a ‘traffic light’ colouring system to reflect each asset’s condition as either ‘Healthy’, ‘Warning’ or ‘Alarm’. The software also allows engineers to view historical data and see how quickly the warning or alarm condition arose, enabling them to make an informed decision on what actions should be taken, and how quickly.

Historical data can also be viewed on the factory floor, and authorised maintenance engineers can open the condition monitoring units and access data via a touch-screen display. The engineers can also connect a spectrum analyser or oscilloscope to the signal conditioning unit corresponding to the channel of interest to view the raw signal.

Attention

The above bespoke condition monitoring systems (in the biomass power plant and in the chip production factory) both provide their operators with a ‘first line of defence’; a system that flags if conditions have changed and provides a record of the rate at which they changed

Moreover, much of the hardware and software developed for the condition monitoring units is now at the heart of Monitran’s MTN/5000 (see figure 3); a turnkey solution that does not require users to be condition monitoring experts in order to get the system installed, programmed and protecting assets. That said, the unit is available with Modbus TCP/IP for networking purposes or to enable multiple MTN/5000s to operate together within a larger monitoring or control system.

In summary, continuous condition monitoring is easy to implement and, once running, can provide the earliest warning that maintenance may be required. Also, the data being monitored can contribute greatly towards a larger condition-based predictive maintenance strategy; one that helps you walk that fine line between servicing machinery on a time-based basis (whether the machinery needs it or not) and ‘running to fail’.

Monitran

www.monitran.com

+44 (0)1494 816569