HBD Condition Monitoring Devices at the center of Ohio Derailment
Yet again maintenance proves itself to be crucial, this time in the railroad industry. On Friday February 3rd a train derailed on the Ohio-Pennsylvania border in East Palestine, Ohio.
This derailment made national news as the city had to be evacuated while the vinyl chloride from the cars was burned. Since this was such a large incident multiple news outlets have broadcasted the story, but they haven’t dived into what really caused this accident.
While the National Transportation Safety Board is still investigating, the preliminary investigation shows that condition monitoring could have prevented this accident…or could it have?
What is a Hot Box?
Across the nation there are devices called “Wayside Hot-Box Detectors” or HBDs on all railroads. Approximately 6,000 HBDs are stationed on the railroad network in the United States, and they are spread 15 – 30 miles apart.
These HBDs are the most utilized bearing condition monitoring system in North America. The detectors use infrared sensors to record the temperature of railroad bearings, axles, and brakes. The detector can determine if a bearing is exceeding optimal operating temperature by concluding that the difference between the infrared temperature of the bearing and the ambient temperature exceeds a predetermined threshold.
Two examples of this threshold are:
- A Bearing is operating 170°F above the ambient temperature.
- A Bearing is operating at 95°F above the temperature of the opposite bearing on the same axle or hotter.
If an HBD determines a bearing is operating at too high of a temperature an alert will be sent via radio to the engineer on board. The train is expected to remove the bearings that triggered the alarm and send them in for inspection.
This was the case in Ohio. The engineer and conductor received an alert from an HBD, and they immediately pulled the emergency brake. Unfortunately, as they were slowing down the trail derailed. Multiple witnesses reported seeing a wheel on fire as the train was rolling through town.
From this information it appears the three rail workers on board the train took the appropriate measure when the sensor alerted them a bearing/axel was too hot. However, upon conducting further research it appears the system of HBDs in North America are largely ineffective.
How accurate are HBDs?
A study published in the International Journal of Rail Transportation indicates the largest bearing condition monitoring system in the country may not be as accurate as it appears. From 2001 – 2007 nearly 40% of bearings flagged by HBDs and removed to be inspected were classified as ‘non-verified’ meaning there was no indication of failure or defect on the bearing. From 2009 – 2018 HBDs failed to detect 151 bearing failures in North America all of which led to disastrous derailments.
Their inaccuracy is largely in part due to where the infrared temperature measurement is taken. For more accurate results the temperature should be read closer to the inboard raceway and at a vertical scan angle, however because of the different classes of bearings used on different trains, the devices cannot be configured to accurately read all the bearings on all the trains that use the railways.
It is also important to note HBDs are considered reactive condition monitoring systems, meaning their intended purpose is to detect heat radiating from a bearing shortly before failure occurs. HBDs are typically spaced 15-30 miles apart. While this sounds frequent, bearings can increase more than 1472°F in twenty-five minutes and the Federal Railroad Administration reports a bearing can burn off in just 1 to 3 minutes. It has even been reported that train derailments have occurred within 96 seconds of passing an HBD that did not alert the crew.
Some efforts have been made to convert the HBD system to more of a predictive maintenance tool by connecting the readings of HBDs so train operators can be alerted that a bearings temperature is increasing between readings even if it has not exceeded the predetermined threshold. However, this has not been widely implemented in North America.
What causes a bearing to overheat?
Bob Hagan, a former state representative who worked in the railroad industry for 50 years stated that trains used to only be about 100 cars long which is approximately 1 mile. Now, due to staffing shortages and other factors trains sometimes span 2-3 miles. This prevents the most basic condition monitoring tool from being used…your senses. On a mile long train, you can turn your neck while going around a bend and see if a bearing is on fire. Visual inspections are also much easier when you only have to walk down a mile long train.
The train that derailed in Ohio was carrying 141 loaded cars, 9 empty cars, and 3 locomotives for a grand total of 153 cars.
The study above indicated there is a linear increase in temperature as a function of speed and increasing the load from an empty railcar (17%) to a fully loaded railcar (100%) raises the bearing temperature by about 23°F. It is important to note that healthy bearings have similar operating temperatures despite an increase in speed or load.
What’s being done about this?
Multiple studies have indicated HBDs are not as effective as they appear, and these researchers are attempting to come up with a solution. After collecting data from the field and laboratory it has been concluded that HBDs cannot distinguish between a healthy and defective bearing. The researchers concluded that HBD systems cannot be corrected with proper IR sensor alignment, optimized calibration, or several IR sensors on one device because temperature alone is not a good indicator of bearing health.
Based on this conclusion researchers are working to develop a system that utilizes temperature, load, and vibration sensors mounted directly on the bearings for continuous condition monitoring. They anticipate this development to transform bearing condition monitoring.