Sensing at sea

Chris Hansford, Managing Director of Hansford Sensors, outlines some of the challenges faced when applying vibration monitoring to marine machinery and offers advice on how to successfully install and use accelerometers on marine vessels.

Vibration monitoring is a key tool used by marine engineers to manage the availability and maintenance of a wide range of equipment with rotating shafts, including pumps, motors, fans, gearboxes and engine systems.

Although 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, it is not without its challenges. 

For example, a major challenge for on-board monitoring is that ships do not have an earth facility, so special conditions have to be applied to the installation.  Similarly, it is important to consider the areas of classification; for example, on an oil tanker the cargo pumps may require intrinsically safe accelerometers connected via a barrier to the safe areas.

Since vibration is one of the main causes of failure in marine propulsion systems and auxiliary equipment, designers and engineers have invested a lot of time and energy in trying to minimise the problem.  This has naturally involved examining the key causes of vibration, such as poor alignment of rotating shafts in, for example, propulsion systems and turbochargers for main or auxiliary engines.  Poor alignment can be caused by inaccurate installation, wear, mechanical damage and even changes in temperature, and the resulting vibration increases friction and wear on other components, leading to premature failure. Consequently, the performance and service life of components is reduced, while operating and maintenance costs are significantly increased.  A considerable amount of bad publicity can be generated by severe failures, whether caused by the unscheduled docking for repairs of a container ship or the malfunction of ventilation fans in hotels on cruise ships.

The fact that so much is at stake for marine operators has driven the development of a range of tools and practices to prevent vibration and its consequences, such as alignment tools and automatic lubricators that can be applied during system construction, and routine maintenance procedures that enable engineers to minimise the development of faults.  Indeed, components themselves are continually being refined and upgraded to prevent, or offer greater resistance to, vibration.  However, to achieve a consistent and efficient management of essential machinery throughout its lifetime, the adoption of condition monitoring equipment is vital and, in marine applications, the measurement of vibration plays a major part in enabling operators and maintenance engineers to identify potential problems and act on them before they fulfil that potential and begin generating unnecessary costs.

Where marine machinery is concerned, especially propulsion and manoeuvring systems, engines and turbochargers, vibration monitoring is becoming a more widely adopted facet of condition monitoring that, alongside other powerful tool such as oil monitoring and thermal imaging, is protecting profits and enhancing performance.  To appreciate how vibration monitoring is bringing these benefits to marine applications, let’s look at the current technology.

The current array of sensors or accelerometers for vibration monitoring offered by market leaders 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.  These devices also offer robust and reliable service, thanks to stainless steel sensor housings that can prevent the ingress of moisture, dust, oils and other contaminants.

There are two main categories: AC accelerometers, which are used with a data collector for monitoring the condition of higher value assets such as wind turbines, and 4-20mA accelerometers, which are used with a PLC to measure lower value assets such as fans and pumps.  Both are capable of detecting imbalance, bearing condition and misalignment but AC accelerometers can also identify cavitation, looseness, gear defects and belt problems. 

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 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. 

Clearly, there is some powerful technology at work here and a wealth of experience and intelligence has gone into providing it but an accelerometer is only as good as the engineer who is responsible for it.  A poorly installed or maintained accelerometer will not offer the precision or longevity of which it is capable and in the marine sector, where operating conditions are subject to a wide range of variables, the need to maximise the effectiveness of these devices is great.  For example, readings taken in port will almost certainly be different from those taken when the vessel is at sea, and heavy weather will only amplify any such differences.  It is in everyone’s interest to specify the best accelerometers and apply the best practice in managing their performance, particularly as health and safety requirements in all industries continue to be refined.  The use of remote monitoring systems enables both performance and safety levels to be maximised, as engineers are no longer required to come into close contact with potentially hazardous equipment. 

To correctly specify an accelerometer, engineers need to consider the vibration level and frequency range that is to be measured, as well as environmental conditions, such as the temperature and whether there are any corrosive chemicals present.  A series of further considerations follow on from there, for example, is the atmosphere combustible? Are there weight constraints? In marine applications, there is no earth, which presents a further challenge but this and other difficulties have been addressed by the designers and engineers of accelerometers and the right research and training, or consultation with a market leader that has experience in a wide range of sectors, can swiftly enable the right decisions to be made.

To correctly install an accelerometer, engineers should mount the device directly onto the machine surface on a flat, smooth, unpainted surface that is larger than the base of the accelerometer.  The installer should ensure that the surface is free from grease and oil, as close as possible to the source of vibration and at right angles to the axis of rotation.  By following these guidelines, you will have already supported your accelerometer, and thus your maintenance regime, by enabling the device to give the most accurate measurements of vibration levels.

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, building a picture of machine condition and helping to create a future maintenance schedule.  With an efficient vibration monitoring system now in place, marine engineers find they have progressed to a new level of efficiency and, with systems well protected by accelerometers and associated machinery to monitor vibration, can move on to identify areas for further improvement in terms of machine performance, energy efficiency or output.  Vibration monitoring thus becomes an increasingly powerful tool and a vital one at a time when the effects of vibration in marine propulsion and auxiliary systems are potentially more costly than ever.

By adopting vibration monitoring as a key tool to ensure the availability and performance of moving equipment from hotel fans to propulsion systems and ensuring the correct installation and maintenance, vessel owners and maintenance engineers alike can reduce costs considerably and improve safety for crew and passengers.  Hansford Sensors offers AC and 4-20mA versions of the HS-100 and HS-420 Series which are intrinsically safe being ATEX and IEC Ex certified.  These industrial vibration sensors can be used to monitor vibration levels on pumps, motors, fans and all other types of rotating machinery found in marine applications.

Date: 05/03/2012