Feb 1, 2010 12:00 PM, By Rick Weber

NTTC Cargo Tank Maintenance Seminar...

• 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.

• 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.

• 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.

• 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.

• 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.

• Frame/suspension stress. This is fatigue damage to the frame/suspension caused by repetitive loading during operation of the cargo tank truck.

• 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.

• Preventive maintenance. Being proactive is the key. The most common schedule seemed to be one-month preventive maintenance. It takes that vehicle down for time, but the time to do preventative maintenance is less than the time for complete repair if something more drastic happened.   

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