Ultrasound Detection and Condition Indicators in PdM
Predictive maintenance (PdM) is about performing measurements and other observations on plant and equipment in order to develop a sensitivity to changes in the condition which might be indicators of future failure. This task is either continuous (online) or periodic (offline). The aim is to warn of a malfunction in good time in order to avoid production stopping prematurely. It is therefore based on the regular collection of objective, relevant information on the health of the equipment: one term used to cover all such measurements is “condition indicators”.
The condition indicator concept
A condition indicator is the result of processing data from a measurement. Its development or transformation indicates to the inspector that an equipment malfunction has occurred or is about to occur. This information should then trigger a suitable reaction. There is a wide range of indicators available, from very simple to highly sophisticated. The rate of production rejects, the machine's output, the temperature of a bearing, the concentration of metal particles in the lubricant and vibration are just some examples.
In ultrasound, four Condition Indicators are used for screening
Summary screen of the SDT270 and SDT200 ultrasound detectors showing the 4 condition indicators
The SDT ultrasound inspection method applies up to four Condition Indicators to each measurement taken: RMS, Max RMS, Peak and Crest Factor. Selecting and combining these indicators depending on the application, will alert the inspector in good time of a developing problem so that the appropriate response such as reducing the time interval between checks, additional measurements or scheduling a repair.
- The RMS or effective value characterizes the energy in the signal. As it is very stable and repeatable it is ideal for working out trend curves. On the other hand it is not very sensitive to the detection of impact. It is used in applications for valve leaks and checking steam traps and lubrication.
RMS level of healthy bearing being lubricated: the RMS level is decreasing when applying lubricant
- The Max RMS is the highest of the sub RMSs calculated every 250ms. Compared with the RMS it is used to find out if the signal is stable or if it varies but it is also not very sensitive to the detection of impact. It is particularly useful when testing steam traps.
RMS and Max RMS levels of healthy steam trap: on a healthy steam trap the difference between RMS and Max RMS is significant
- The Peak value is the maximum instantaneous value recorded during the measurement cycle. It is extremely sensitive to any abrupt variation but can therefore be unstable. It is used primarily for impacts produced by early stages of failure in bearings and gears. It is also useful for modulating steam traps.
The Peak-value increase, when RMS-value remains stable, indicates the bearing early stage failure.
- The Crest Factor is the ratio of the Peak value to the RMS value. It can therefore also be unstable.The Crest Factor is used to estimate the failure severity for bearing and gear. During the early stage of the failure the CF is increasing because the RMS remains low as the peak is increasing. During the advanced stage of the failure the CF is decreasing because the peak remains unchanged as the RMS is increasing.
The Crest Factor increase indicates the failure worsening. The decrease reports the final stage before breakdown.
Conclusion
By using the four Condition Indicators correctly, the ultrasound inspector will be able to provide the first line of defence against unexpected equipment failure. Untimely production stoppages will be avoided and as soon as an alarm is triggered a suitable action can be taken.
Monitoring by ultrasound is both quick and easy, it therefore makes both economic and logistic sense - essential factors for a winning condition-based maintenance programme.
Jean-Paul EMMANUELE
Thomas J. Murphy
SDT Training Managers
