Made to measure maintenance strategies

14 July, 2008

Over the past year, much has been written about the benefits of creating condition-based predictive maintenance strategies around the intelligence garnered from and through vibration monitoring and analysis: and this was in fact the theme of a presentation Monitran gave at the recent MAINTEC 2008 exhibition.

Fundamentally, your condition-based monitoring, and the analysis of any vibration levels, will only be as good as the sensor (i.e. its output characteristics, build quality and so on) and the choice of how and where you install or use it.

A good match

Vibration sensors must be matched to the application otherwise they may provide misleading information, may not last long in their intended application or could even contravene industry regulations for use in hazardous areas. But with so many sensors available, from different manufacturers, where does one start?

There are essentially two types of vibration sensor: those with AC outputs and those with DC. The former are best suited to analysis, as the AC output can be put through an FFT analyser to yield information on the amplitude and structure of the vibration. The latter are best suited to ‘machine protection’ as they produce an output proportional to velocity or acceleration, in the range 4-20mA, so are suitable for interfacing with monitoring and shut-down circuitry.

Temporary or permanent?

The nature of the installation, whether it be permanent or temporary, will influence your mounting and wiring decisions and guide you towards either connectors or integral cables. If the installation is to be temporary, employing magnetic mounts, consider using ID tags on the sensors and marking clearly on each machine where the corresponding sensor should be positioned.

The most important consideration is the operating environment, which includes temperature, water, humidity and hazardous atmospheres. These considerations will steer you towards different sensor characteristics such as intrinsically safe (ATEX, IECEx...), sealing (IP67, 68...) and operating temperatures.

Space issues

Also, the environment, and specifically the space available for the sensor and its cable, will govern whether the sensor’s cable joins the body at the top or side. Space may also influence the decision whether to use integral cable versus connector.

The above considerations will certainly help you narrow down your choice of sensor and, for most seemingly ‘novel’ applications, you will probably be surprised how many standard products fit the bill. If not, then most manufacturers offer ‘special purpose’ sensors.

These special purpose sensors may, for example, be able to operate at higher temperatures, up to a few hundred degrees Centigrade, or provide higher sensitivity than normal. Other options available include charge output accelerometers, dual output devices for combined analysis and monitoring, devices which measure acceleration and temperature, tri-axial sensors and seismic accelerometers.

Protecting equipment

Have no doubt, vibration sensors certainly need to be fit-for-purpose, as they are often used to protect equipment worth thousands, or even hundreds of thousands of pounds. In addition, a well-planned predictive maintenance regime, which includes the use of vibration monitoring and analysis, will also reduce the need for unscheduled plant shutdown and the risk of personal injury to staff.

Further, when standard and even ‘special purpose’ sensors don’t seem to quite fit the bill it is worth remembering that a number of sensor OEMs offer a full custom design service.