Vibration is not unusual on a ship not least because of the amount of reciprocating and rotating equipment attached to a metal body. Usually the need is to reduce vibration by way of flexible mountings for machinery, the use of vibration compensators and better hull propeller designs that reduce cavitation.
However, although temperature and pressure are the main parameter checks for engines, sometimes more information can be gleaned from measuring vibration on engines and in other items of machinery.
Much effort has gone into reducing vibrations inside machines but it can never be eliminated entirely. Vibrations will change both as the machinery operating conditions change and due to wear between components. Several engineering specialists and some equipment and component makers such as SKF produce vibration sensors and measuring equipment. Making use of non-invasive vibration analysis to measure factors such as bearing surface condition, along with associated techniques such as ultrasonic inspection to check for potential cracks in the bearings, vibration sensor technology can identify gradual developing problems as well as sudden changes.
Measuring vibration with sensors
The sensor technology used to measure levels of vibration, together with the instruments and software for gathering and analysing data, is typically easy to install and use. However, it is not without its challenges. For example, ships do not have an earth facility, so special conditions have to be applied to the installation of on-board monitoring equipment. Similarly, it is important to consider the areas of classification; on an oil tanker, for instance, the cargo pumps may require intrinsically safe accelerometers connected via a barrier to the safe areas.
Sensors and accelerometers for vibration monitoring offered by companies such as Hansford Sensors can operate over a wide temperature range, measuring both high and low frequencies with low hysteresis characteristics and excellent levels of accuracy. When it comes to the often aggressive environments presented by life at sea, stainless steel sensor housings enable these devices to operate for long periods without failure by preventing contaminants such as moisture, dust and oil.
To correctly install an accelerometer, engineers should mount the device directly onto the machine on a flat, smooth, unpainted surface that is larger than the base of the accelerometer. The area must be free from grease and oil and as close as possible to the source of vibration. Once data has been collected in the most appropriate and efficient manner, machine reliability data must be analysed and interpreted, either by on-board engineers or by a remote monitoring centre.
Ultrasound technology only has a relatively small penetration into the marine maintenance sphere but it does have great possibilities especially as the equipment can be used for multiple purposes. Usually ultrasound testing is done by surveyors and specialists but there is no reason why it cannot be used by crew and there are products available from the likes of SDT that will permit ultrasound data to be included in with other data for maintenance and condition testing.
Ultrasound can be used in conjunction with many other sensing technologies and can help pinpoint problems that may be hinted at by trend data but difficult to identify. For example leaks from pressurised systems may be a cause for investigation rather than a fault with the pump or compressor even though that option is suggested by other data. Feeding that information into the accumulated database may in future be used to prompt a check for leaks before overhauling a compressor.