Maximising the efficiency and reliability of drive trains
21 November 2012
Two prominent bearing manufacturers provide insights into the design and maintenance of power transmission systems: the effects of external drive components on motor driven system efficiency, and the proper management of complex online condition monitoring systems.
The goal of motor efficiency has captured the collective imagination of engineers. But not everything is as it seems, writes Phil Burge. There has been an extremely successful campaign to promote energy efficient motors, and while highly laudable, this has perversely created a situation where the focus for energy savings is almost exclusively on the motor, ignoring what goes on elsewhere in the system.
Energy efficient motors have the capacity to offer major improvements in many applications and should always be fitted in new builds. However, this is not always the case in system refurbishment or repair. In manufacturing and process equipment, motors are rarely used in isolation; there will at least be a gearbox and some form of transmission mechanism, both of which will consume energy.
It may be in this configuration that most energy is being wasted from the motor, in which case changing to an energy efficient version will pay dividends. It is not uncommon, however, to find that energy efficient motors have been installed in production systems, only for the end user to discover that modest energy savings of just a few per cent have been realised. This is generally due to a number of reasons:
- energy wastage was already greater elsewhere in the system, and changes should have been made here first;
- introducing a new motor impacted on the efficiency of other system components, with a detrimental effect on energy consumption;
- the new motor has been incorrectly specified; even energy efficient motors must be matched to the demands of the system if they are to operate optimally, and
- the new motor may only be marginally more efficient than the unit it replaced, so savings are small. This is especially true in applications with intermittent duty or where systems cycle on and off with extended periods of motor idling, where the savings made are outweighed by the cost of the new motor.
The bottom line is that before making changes you must firstly consider the system as a whole and understand where the greatest inefficiencies are to be made. Bear in mind that every moving or rotating component, down to the motor, pump or fan bearings themselves, will have some degree of inefficiency, however small. These inefficiencies have to be calculated and combined to produce an overall figure for the system.
This holistic approach can often be far more cost effective. For example, simply adjusting, re-balancing or realigning drive mechanisms can significantly reduce vibration and heat losses, and thus energy consumption. It may also enhance reliability and cut your maintenance costs – it is worth noting that there is a direct correlation between unreliability and energy inefficiency, as an unreliable system will almost certainly waste energy, while high energy consumption can frequently be an excellent indicator of a system that is wasting energy.
In practice, although the latest generation of energy efficient motors represents a step forward, motors have been reasonably efficient for many years. Problems tend to occur because they have been incorrectly specified (often oversized for the duty required), badly installed or poorly maintained.
Assuming that an existing motor has been correctly sized and fitted, the chances are it will already be operating at around 80 to 85 percent efficiency. Switching to a new energy efficient version may improve this to 90 percent or greater, but if inefficiencies remain elsewhere in the system, then any gain will immediately be reduced or even cancelled out entirely.
For example, worm or spur type gearboxes are widely used yet are inherently inefficient – worm gears are at best between 65 and 80 percent efficient in terms of energy losses as they have high levels of internal sliding friction. Similarly, if belt drive transmission mechanisms are used then again energy losses through friction, slippage, noise and heat can potentially be high. V-belts are most commonly found in industry and can operate with efficiencies of around 95 percent. Problems occur, however, if they are incorrectly maintained, with efficiency levels rapidly falling by 10 percent or more.
Making simple adjustments in the system before investing in a full motor replacement can be beneficial. A word of caution: be aware that it is possible that rectifying existing faults may increase your energy consumption. As an example, consider a belt driven agitator: if the belt is slipping then the logical step is to re-tension it; depending on the degree of original slippage, re-tensioning may mean that the agitator turns faster, with greater resistance in the material being mixed with a corresponding increase in energy required.
The lesson is: it is important to take account of the often complex interdependencies between all parts of a production system.
Who monitors the monitoring system?
Condition monitoring (CM) has been used in the marine industry for many years, typically using data collectors operated by ship's staff, writes Dirk Schulzer. However, with the advent of larger, more complex machinery, particularly on vessels such as large LNG carriers, automated online CM systems have become more popular.
The use of online CM systems has a major advantage in that remote monitoring and advice can be readily provided by fleet technical management, OEMs or shore-based CM specialists. Higher workloads and the reducing number of staff on modern ships also make remote monitoring more attractive. In the event of an alarm condition, an automated monitoring system will typically provide alarm text for the duty engineer in the control room and an automatic notification to the remote monitoring facility. Trend and analysis data would also be transferred ashore for diagnostic purposes.
So far so good, but what happens if data is not received when it should be? It is not as easy as one might imagine to check that all the necessary data is being received. When focusing on technical data analysis alone it is easy to overlook missing data. The worst-case scenario is that a major failure occurs with no alarm from the monitoring system. Investigation after the event often reveals that the system had not been online for some time and nobody had noticed the lack of data, or had assumed that the unit was down for maintenance.
My company has ongoing CM contracts with clients, ranging from industrial plants through to wind turbines, offshore and marine applications, and to date, the marine sector has proven to be the most challenging of these. We have found that the support effort required to ensure that marine CM systems remain online actually outweighs the analytical requirement for fault diagnosis.
For example, consider a system that monitors the main engine turbocharger on a modern diesel electric cruise ship, which may have six engines and twelve turbochargers. The monitoring devices will pick up a tachometer signal from each turbocharger to track the speed and synchronise the analysis. This strategy ensures that ‘empty’ data is not collected when the turbocharger is idle. On the cruise ship, the individual engines are stopped and started on a daily basis, depending on power requirements at the time for propulsion and hotel services. Additionally, an engine might be stopped for maintenance, or a particular engine may be used in port only on low sulphur fuel to avoid fuel type changes.
Now consider how the onshore monitoring facility would receive this data. Perhaps it might receive data from the ‘A’ bank turbocharger on a particular engine, but none from the ‘B’ bank. This would suggest a possible failure of the ship’s tachometer and the monitoring engineer would then contact the ship to request that it checks this, and state that this machine will not be monitored until the tachometer probe is replaced. There are many examples of this type of situation and new administrative systems are now being introduced that monitor the monitoring systems.
The CM system’s health is monitored, starting from the transducer taking measurements through to the receipt of data at the monitoring centre. If a transducer or its cable is damaged, for example, the next data set sent from the ship can contain an error message indicating this fault. The monitoring computer should be able to send a daily message to the local monitoring boxes in the engine room to check that communications are intact. The monitoring computer should also send a daily ‘still alive’ message to the monitoring centre.
It is only by attention to detail in these areas that remote monitoring systems can achieve the reliability that is expected of them.
Phil Burge is with SKF; Dirk Schulzer is with Schaeffler’s marine aftermarket