“As you go about inspections, you want to make sure that the people who do these inspections know what they're looking for,” said Darrell Bowman of the Virginia Tech Transportation Institute (VTTI). “Then have in place some sort of quality control so the person can go back and certify that procedure. These are lessons learned from the industry itself. We consolidated that into a document that will be good for new fleets coming on board. Also, as you bring new people into the operation, you can give this to them and will give them overview.”
There were two purposes: to identify factors that affect the service life of cargo tanks; and develop the document.
Researchers conducted interviews at 19 sites throughout the United States including eight fleets, two test and inspection facilities, two facilities that perform Wet Fluorescent Magnetic Particle Testing (WFMPT), three re-chassis facilities, two facilities that repair insulated tanks, and two mobile test and inspection facilities.
“A lot of places we went to did multiple types of operations, so we were able to capture even more information,” Bowman said. “We spent several days at some sites. We sat down with the head of maintenance or the administrator of the fleet or facility to understand operational factors that affect the bottom line. Then we did a group interview with the maintenance staff. We were able to hear some excellent examples of things that go on in shops or over the road that affect the service life of cargo tanks. We asked for written procedures, and some required a nondisclosure agreement. It allowed us to understand the industry as whole. No competitive information is in here. We tried to leave out anything that would cause a competitive disadvantage to the facilities.”
The data analysis included reviewing interview and focus group audio files to identify and document key sections (over 30 hours of audio recordings were reviewed), transcribing key sections of audio files (594 pages of transcripts were created), and organizing transcripts by source (company), with keyword searches for comments of interest.
The collected data was placed into eight factor categories:
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Tank distortion/bulkhead reversal. He said it's caused by either excessive positive or negative pressure inside the cargo tank and typically is the result of loading and unloading procedures and valve failure. It could be because of improper modifications to a valve, overloading by volume over time, or inadequate operator training. Overloading is pumping with too much product or pumping out too quickly and overpowering the capacity of the vent. An operator may load the tank more quickly than the vent system can handle. The result is local straining of the shell or bulkhead at the juncture of the shell and bulkhead beyond the yield strength of the material.
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NTTC Cargo Tank Maintenance Seminar...
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Tank rupture. This refers to the catastrophic failure of the tank itself. It's a lower probability. It's when a single pressure load exerted on the tank causes the hoop stress to exceed a certain value. One value often used to determine burst pressure is the flow-stress, the average of yield strength and ultimate tensile strength of the tank material.
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Tank/trailer impact damage. This refers to impact loading to the tank wall or to a structure attached to the tank wall. Resulting damage can be categorized as breaching of the pressure boundary or plastic deformation of the tank. It occurs a lot in home delivery, where drivers have to back around trees. Drivers tend to hit things that are hard. Damage is dependent on momentum of the cargo tank and colliding object, the material strength of CT, CT lading density, and stored energy due to pressure. Causes are road hazards, emergency maneuvers, improper driving, load issues (basically being too heavy, or shifting of a load that results in rollover) or product solidification.
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Tank component failure. He said each specification tank is built to a specific design requirement. There are many functional appurtenances and plumbing within and on the tank required for full service. A single valve could take the tank out of service, so things attached to the tank are as important as the tank itself. A common factor is weld quality (welding without a pad, undercutting, over-grinding the weld). Other causes are improper repairs, construction, design, and modification and inspection.
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Tank out of spec condition. This focuses on failure to design, construct, repair, or even modify the cargo tank in accordance with 49 CFR that will result in inspection failure for the cargo tank. Factors include not being designed correctly by the manufacturer, not built correctly, modified correctly, or inspected correctly.
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Fatigue damage to the tank. Fatigue damage occurs with operation over time. Factors include cyclic loading of the tank itself. As it goes down road, cyclic twisting occurs on the tank that could weaken the tank. Damage is cumulative. It's caused by cyclic pressurization, which is equivalent to taking it just above design range with no deformation. The process is repeated over time. It leads to fatigue damage of the tank. Sometimes fatigue damage is caused just by the poor condition of the road. Damage can also occur in off-road applications.
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Frame/suspension stress. This is fatigue damage to the frame/suspension caused by repetitive loading during operation of the cargo tank truck.
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Material thickness reduction. This was the biggest point discussed during the study. Exterior tank corrosion can be caused by salts from roadway, especially in northern-tier states or in southern states just from the atmosphere and product overflow. Galvanic corrosion can happen when an aluminum tank is mounted on a steel chassis, and there's no separation.
Cargo tank fleet-facility operations include:
Lading management. Give an overview of what the issue is and related regulations and recommendations from the industry on how to improve cargo management. The key is to just know that product you are putting into a tank, because that summarily affects cargo tank life.
Record keeping/tracking key measures. This is the process of trying to figure out what's causing tanks to have a shortened life. Fleets don't want to scrap cargo tanks before their time. It affects the bottom line.
Fleet/facility investment. It's about investing in your equipment and trying to get safety-related technology, but also about investing in your drivers. What you want is people who care about what they do. That will go a long way for you to extend the life of a cargo tank, because they're the ones that are using it day in and out.
Driver training.
Driver inspection. Use drivers and technicians to identify problems early. Drivers see that tank every day. Have them do the inspections.
