Tank choice

Feb. 1, 2006
IT'S NOT unusual for engineers at Brenner Tank LLC to receive inquiries about the legality of hauling acids in MC307 and DOT407 cargo tanks, said Brenner's

IT'S NOT unusual for engineers at Brenner Tank LLC to receive inquiries about the legality of hauling acids in MC307 and DOT407 cargo tanks, said Brenner's John Cannon.

“Prior to 1991, most acids were authorized for transportation in MC312 cargo tanks, but not for MC307 tanks,” he said at the National Tank Truck Carriers Maintenance Seminar held November 7-9 in Louisville, Kentucky.

Cannon also discussed determining the minimum thickness for cargo tanks.

Turning to acid handling, he said: “By late 1991, Docket HM181 became law, and much of the hazmat table was updated. Most acids were then authorized for transportation in MC307 and the coming DOT407 cargo tanks. However, some acids, such as fuming nitric acid, still require a DOT412 tank.”

The hazmat tables in 49 CFR list which acids can be hauled in a DOT407 tank.

There are advantages in using a DOT407 tank rather than a DOT412 in acid service, he pointed out. They include reduced inspection and repair costs and less tare weight.

He estimated an inspection fee for a DOT412 at $500, plus engineering time of a certified engineer. (The $500 is for an authorized inspector (AI) visit, which would only be applicable to ASME Certified tanks, like DOT 412s. Generally, this fee would not apply to DOT407 units, and so, the resultant AI visit cost would be $0.) Certain repairs will require a shop with an “R” stamp.

A DOT407 weighs less than a DOT412, and the heads on a DOT407 can be thinner, which also helps reduce weight, he said.

There also are other issues to be considered. “You have to ask if you want corrosive material in a tank that doesn't have much corrosion allowance,” Cannon said. “Your maintenance manager has to make the decision as to whether hauling acid in a MC307 or DOT407 is appropriate for your fleet.”

Minimum thickness

In discussing minimum thickness issues, Cannon noted that determining thickness often can be difficult, particularly in the MC300 series.

“Structural integrity is the factor driving minimum thickness requirements,” Cannon said.

Minimum shell thickness for DOT 400 series tanks is posted on the metal certification plate as required, but the MC300 series was not required to have the thickness listed.

When the thickness information is not immediately available, it can be obtained by contacting the tank trailer manufacturer, consulting a Truck Trailer Manufacturers Association publication on the subject, and/or hiring a design-certifying engineer.

All corroded or abraded areas outside the cargo tank wall must be thickness-tested. If the cargo tank is constructed of steel with steel reinforcing rings, the rings must be thickness-tested in four symmetrical positions around the circumference. If any reading varies from the average by more than 10%, an internal thickness test is required for the shell in the area covered by the ring.

Another factor, bending stress, controls the minimum thickness, especially on small-diameter tanks. Bending stress is an engineering term for the resultant effect of placing a large (60,000 pounds or more) load over a long (40-foot tank) beam. The stress must be substantially below the strength of the materials of construction. By and large, a thicker shell correlates with lower bending stress.

For example, a design-certifying engineer might find through calculations that a (minimum) shell thickness of .115 inch for a small-diameter tank results in unacceptably high bending stress. But by increasing the (minimum) shell thickness to .120 inch, the resultant bending stress might meet all applicable criteria.

“You also have to consider the ASME (American Society of Mechanical Engineers) code when considering minimum thickness,” Cannon said.

Turning to the MC307 cargo tank, he cautioned that the stated thickness may not necessarily be indicative of the minimum thickness of the tank.

Some older MC307 tanks have a different shell thickness on the bottom, versus the thickness on the side and top. “In this case, you need to be sure the entire tank is measured,” Cannon noted.

The stated thickness on an MC307 tank may be much greater than the regulatory minimum, he added.

There are options if the thickness does not meet requirements: Some tank trailers can be downgraded to lower specifications, or the vehicle can be restricted to non-code service.