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On-line condition monitoring and performance assessment: London, UK, 17 March 1988

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Page 1: On-line condition monitoring and performance assessment: London, UK, 17 March 1988

= M e e t i n g rep On-line condition monitoring and performance assessment London, UK, 17 March 1988

To open this meeting, M. J. Neale (Michael Neale & Associates, Farnham) briefly described the impetus behind condition-based monitoring systems, namely that over-all their cost is lower than maintenance schedules which rely on regular over-haul periods or, worse still, run until breakdown.

Three types of system were inlroduced: wear debris monitoring, an on-line method used to detect surface distress; vibration monitoring, used to detect changes in the dynamics of moving parts and requiring computerized data acquisition and control techniques: and performance monitoring, in which the integrity or effectiveness of particular equipment is checked against the purpose for which it was designed. Performance monitoring is more specific to the application and is used less frequently than wear debris and vibration monitoring.

The first presentation was entitled "Development of an on-line oil debris monitor for large plain bearings'. O. Lloyd (CEGB, Marchwood) discussed the background to the monitor development. He explained how the filtration principle had been adapted for on-line monitoring of large plain bearings such as those in turbine generators. Up to 1 kg of debris per week is removed from turbine generators. Only 1% of this is wear debris, typically consisting of white metal and bronze from the plain bearings, abrasive and adhesive rubbing wear and perhaps fragments spalled from bearings and gears. The debris is collected in a tilter membrane, causing a pressure drop across it. At a pre- determined level of change the membrane is automatically replaced and the old one is analysed. Lloyd explained that regeneration costs alone could be £2 million per week for high-rated plant in the event of failure, so there is good incentive to prevent breakdown.

In the second part of this presentation R.F. Griffin (Muirhead Vactric Components, Beckenham) described the production version of the monitoring system, the SDM 100. This microprocessor-based instrument automatically samples the oil and monitors the number of pump strokes by the filtering meter required to build up a certain pressure across the filter membrane. The process is repeated, say, twice per day and the number of pump counts needed to build up a certain pressure drop is compared, Any difference in the two readings can be related to the amount of debris present.

'On-line fault detection by acoustic emission' was the title of the next paper, given by W.A. Stewart (British Steel Corporation, Teeside). The author explained the need for a system to detect the onset of stress corrosion cracking (SCC) in the stove domes of blast furnaces. Oxides of nitrogen combine with steam inside the domes to produce acids which corrode the dome welds. A high level of stresses adjacent to the weld lines renders the welds susceptible to SCC. This process is signalled by the generation of high-frequency stress waves which can be detected using suitably placed arrays of acoustic emission (AE) transducers.

A vast amount of data is generated; advanced data handling software automatically processes the information collected to provide accurate indications of areas in which SCC is occurring. Excellent results have been obtained at Redcar blast furnace, where ultrasonic examination has been used to confirm the AE measurements. The technique is also being used to monitor the shell of the blast furnace and the booms of onloaders on cranes used to load ships, as fatigue cracking is occurring in the booms. Monitoring this type of structure would be too

expensive using ultrasonic methods.

Another non-destructive examination technique was described by L. Jackson (British Gas On-Line Inspection Centre, Cramlington) in his talk 'On-line condition monitoring of high- pressure pipelines'. £ 5 billion worth of pipe steel lies in the ground in the UK. Any failure would be catastrophic, so periodic assessment is required to detect and put right damage that otherwise might lead to failure. Of most concern is metal loss, in the form of mechanical interference, corrosion and spalling. For example, pipelines may be gouged during excavation work, resulting in corrosion and the formation of corrosion pits.

Two methods of detecting damage exist. The first involves filling a pipe with water, leaving it for 24 h and observing any leakage. This is very expensive. The second technique is to send 'intelligent pigs' into the pipelines. These two- module magnetic inspection devices are carried along by the flow of product through the pipe. Hundreds of sensors scan the walls every few mm, generating many data: analysis of the data is the most expensive part of the process. Information on the position and size of the defect is obtained. Instrument design problems arise because pipe diameters vary between about 0.15 and 1.10 m and pressures can be as much as 13.8 MPa so pigs have to be of rugged construction. Bends in pipelines create many problems.

The final presentation of the morning by C. Smallman (British Gas R & D Division, Newcastle- upon-Tyne) was entitled ' Intelligent condition monitoring of gas compressors'. The approach has been to combine traditional performance monitoring with intelligent knowledge-based systems ('expert systems'). Attempts have been made to monitor RB211-

NDT International August 1988 295

Page 2: On-line condition monitoring and performance assessment: London, UK, 17 March 1988

-Meeting top based compressors used by British Gas, but difficulties have restricted the system to the compressor wash cycles. A compressor in need of a wash is costing money: automatic indication of this is therefore highly desirable. Smallman estimated potential savings of £400 per compressor per day. A prototype system is currently under test.

The afternoon session opened with a talk on 'Recent advances in machinery health management ' by K. Pipe (Stewart Hughes Ltd). Emphasis was placed on achieving minimization of operational and maintenance costs, for which adequate prognostic and data management techniques are required. Mr Pipe described how pattern recognition techniques such as representational modelling could yield information on impending failure. A library of failure modes is stored. If the pattern in the data is that of a faulty machine this is a sufficient indicator of ill-health: it is not necessary to assess the pcrformance of a machine and compare it with the usual performance.

N.D. Burns (Unilever Research, Port Sunlight) then described the +Design of fault-monitoring systems for packaging machinery' . He explained that in the consumer goods industry, just-in-time manufacturing and minimal downtime were of paramount importance. The monitoring systems most compatible with these requirements link the functions of quality monitoring, condition monitoring, fault monitoring and

production logging. Expcrt systems are being used to provide reliable fault indications from sensors on the production linc.

The third paper of the afternoon session was given by N. Welsh ( IRD Mechanalysis Ltd) on "Industrial applications for spike energy measurement systems'. Conventional vibration detection, analysis and correction systems rely on measurements of velocity and displacement, but these parameters were found to have limitations when examining the deterioration of rolling element bearings. As an alternative, spike energy circuits have been designed to sense the amplitude of the microsecond range pulses caused by impacts between bearing elements with microscopic flaws. The spike energy circuit actually measures a parameter termed Gsj, comprising the broad- band energy, the spike height and the repetition rate. Welsh spoke of a survey of 148 Canadian companies which revealed that spike energy measurement was also being used to detect air leaks in valves, lack of lubrication and leaks in boilers. He suggested that a future on-line system may combine the technique with acoustic methods as the two complement each other.

Finally, .1. Brind (Marathon Oil UK Lid) gave a talk entitled 'A condition monitoring package for offshore installations', in which he described Marathon Oils" unique approach to monitoring the Brae B platform which both satislies production requirements and

legislation and optilmzc> labour material rcquirctnenl,. I,ikelv modes of equipment failure a:ele assessed, as x~cre the parameters or funclions o[ parameters that could deicer the ~,ymptoms of each failure. Maintcnancc ~ork is onl\ undertaken when one of lhcsc parameters indicatcs that. if Icft alone, it could lead to failure or would result in unacceptabtc pcrl\~rtnancc. The maintenance work is restricted to thai which is necessary at the time.

The Brae B platform weighs 40 000 t topside and contains 64 parent equipments and 650 sub- equipments that requirc monitoring. Thc svstcm uscs the latest electronic data collectors. interfaced to a host Ihat stores data and only alerts maintcnancc staff whcn an unacceptable trend starts. It was decided lhnl transducers would only be added when the condition could not bc inferred reliably from existing instrumentation: high accuracy is not required when looking for trends. Accepting that there are types of failure that cannot bc anticipated allowed the number o1" on-linc parameters to be minimized.

In summary, this meeting brought together researchers and managers concerned with condition monitoring and provided them with a variety of approaches being used or under development m a wide range of industries.

-1. Burrows

-Book Reviews Non-destructive testing B. Hull and V. John

Macmillan Education, UK, 1988 ( 144 pp, £9.95)

The authors of this book are quite correct in stating that N D T does not have a good textbook for use in higher and further education. With this book, the authors aim to fill this gap and to stimulate the

teaching of NDT methods.

Chapters on the main techniques are included: penetrant testing, magnetic particle testing, eddy currents, ultrasonics and

radiography. Optical inspection. neutron radiography, laser-induced ultrasonics, time of flight diffraction, acoustic emission and thermography are dealt with briefly in a separate chapter.

296 NDT International August 1988