Visual inspection and routine maintenance are the keys to ensuring the longevity and safe operation of steel vacuum tanks. When the thickness of a steel tank is in doubt, ultrasonic testing can provide a definitive measurement of the remaining steel, at a given point, or across a broader spectrum of locations.

An electronic ultrasonic thickness gauge can test the wall thickness of materials including steel and plastic, and determine the effects of corrosion or erosion on any tank structure where access is limited to the outside.

The technology works by sending a pulse of high-frequency soundwaves into a solid material to analyze its properties. Echo patterns from the pulse are analyzed to measure material thickness or detect any flaws. The first hand-held thickness gauges were developed in the 1970s. The development of digital and increasingly miniaturized components has led to the creation of an array of sophisticated battery-powered, hand-held devices.

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Bruce Heck, owner of Alberta Tank Truck & Supply Ltd. in Drayton Valley, Alberta, has been inspecting truck tanks for the better part of a decade. While his business concentrates primarily on tank trucks serving the gas and oil industry, the testing procedures and principles are identical to those used for trucks that transport sewage or water.

Heck says ultrasonic testing is generally employed as part of a broader routine truck inspection, or a full safety inspection to certify a truck to transport dangerous goods.

“If the owner suspects there’s a problem with a tank because of corrosion on the outside or if the tank has been scraped, we occasionally provide just an ultrasonic inspection to help them determine whether the tank should be repaired or scrapped,” he says.

Heck uses an Olympus ultrasonic tester that provides a reading of the thickness of both the paint on the tank and the tank material itself. That allows him to test tank thickness from the outside of the tank, rather than the inside. The tank does not need to be emptied to perform a successful test, since the device can differentiate between steel and whatever the tank may be carrying.

“However, the thickness of the paint needs to be taken into consideration in determining the thickness of the steel,” he says. “The tester will give me separate readings on the thickness of the paint and the thickness of the tank material, but it won’t tell me how thick they both are together.”

Heck uses an ultrasonic gel applied to a smooth surface of the tank to ensure the tester probe makes sufficient contact with the material of the tank. The thickness readings are supplied immediately.

“We start by checking the tank in the spots where we believe it’s likely to become thinner,” he says. “Generally, we test the top third of the tank where vapor will corrode the interior surface, because liquids don’t often corrode the inside of a tank below liquid level. Vacuum trucks are also typically eroded where gritty material enters the tank. If we find a spot that’s too thin, we’ll do more tests at other spots to help determine if that tank should be repaired or discarded.”

The cost of the ultrasonic inspection is charged out at shop labor rates, usually bringing the final cost to less than $100, Heck says.

Cost and suppliers
Contractors can perform ultrasonic testing themselves, using hand-held testers provided by a wide range of suppliers.

“We produce a basic model that’s suitable for truck tank inspections that’s designed to be simple to use and ready to go out of the box,” says Scott Bate, technical sales associate with DeFelsko Corporation, a manufacturer and supplier of ultrasonic testers based in Ogdensburg, N.Y. “The default material setting for the tester is for steel and it works on material to a thickness of five inches.”

To perform an accurate test, the tank surface should first be cleaned of dirt, rust or scale.

Bate notes that the pattern of application of the ultrasonic gel can allow the operator to perform either a spot thickness reading or a reading across a broader expanse of the tank’s surface.

“If you spread the gel across the range of the surface you’re interested in and then set the tester to scan mode, you can pass the probe across the length of that gel,” he says. “The scan takes 20 readings per second and provides an average reading of the tank thickness across that range, in addition to the thickest and thinnest points in that part of the tank.”

Whether a contractor chooses to invest in an ultrasonic tester largely depends on the number of trucks in the fleet and frequency of inspections. The basic hand-held model from DeFelsko—the PosiTector UTGC1—currently sells for $995.

Says Bate: “It’s definitely something that can be used accurately by a truck contractor without any specialized training.”