Wagons carrying iron ore or coal are susceptible to rapid deterioration of the body due to corrosion and abrasion. Repairs to carbon steel bodies may be needed after as little as four years, and they rarely last beyond 16 to 20 years6. In South Africa, Spoornet’s coal wagons operating on the Richards Bay coal export line, originally plated in Symbol 8 mild steel and Corten A steel, had to be replated after only 8 years.
Corrosion, especially rusting of surfaces in contact with the iron ore, occurs due to a combination of moisture, oxygen, weak acids, high stresses, dissolved salts and contaminant particles of different composition from the ore car plates. Abrasion occurs mainly during loading and unloading. Metal loss from the floors and walls of BHPIO carbon steel wagons has been measured as 0·25 to 0·60mm/year. This is similar to the average wall thickness loss of 0·19 to 0·57mm/year found on Spoornet’s Corten coal car bodies.
Expert system
The car maintainer must decide when to patch, replate, or purchase a new body. If this is done too early money is wasted. Too late, and safety may be compromised through structural failure.
To answer these questions, Lynx Engineering developed an expert system for body maintenance management. Car Care* has enabled BHPIO to reduce the cost of fleet body maintenance by 20%, and increase the operational life of the bodies by 10%. This has been achieved by allowing each car to operate until just before its critical fatigue stage is reached.
Every car is monitored by measuring plate thicknesses at critical areas, recording the number of trips, recording the location of visible cracks and other plate damage, and calculating the fatigue life consumed as a result of loading and cyclical stresses. This makes fatigue life prediction as accurate as possible.
As this process can be costly and time consuming, there is a tendency to carry out a structural assessment, using finite element analysis, on the generic design rather than on individual cars. This approach is fraught with danger because it assumes that cars experience similar conditions.
In the real world, even cars in captive fleets experience different loadings and frequency of use, as well as displaying individual structural characteristics as a result of either manufacture (varying initial plate thicknesses) or maintenance. For these reasons it is best to monitor cars individually.
Car Care monitors the structural integrity of individual wagons by keeping a database on the history of each car which is updated every time plate thickness measurements are performed. This happens when the car enters the shop for routine maintenance - about once every 15 months. Critical measurement areas are identified by bar codes so that an ultrasonic thickness gauge can feed data directly into the program.
Weighbridge data, linked to the individual wagon by an Amtech identification tag, determines the weight and distribution of the product carried and establishes frequency of use for each car.
The ’engine house’ behind Car Care is an expert system, based on finite element analysis, which re-assesses the structural condition of the body every time it is checked. This ensures that whenever new information on plate thickness, loading, defects and such are entered, the program performs a complete re-evaluation of structural integrity. Each car receives an updated fatigue profile and an accurate forecast of its remaining life.
Now the car maintainer knows exactly how long the body can continue to operate safely. He can focus on any area of potential concern on the body and apply a localised repair. A user-friendly graphical interface allows him to view the car structure from every possible angle, and defects and repairs are entered by ’pointing and clicking’ the area where they have occurred.
*Car Care is being marketed under the brand name Fleet Expert by Lynx Engineering of Perth, WA.
