Cannon says calculating minimum tank shell thickness not as simple as looking at a table or a dataplate

JOHN Cannon of Walker Group Holdings believes that determining minimum thickness of a cargo tank is an “often misunderstood topic.”

Speaking during the National Tank Truck Carriers' 2010 Cargo Tank Maintenance Seminar, Cannon said he has, on numerous occasions, heard of a scenario similar to this:

“We have a 7,000-gallon MC307 with a 10-gauge shell, and the lowest value on our latest thickness test is below 90% of the original measurement. What should we do?”

He said the best answer to that question is not to remove the tank from hazmat service, redo the thickness test, re-rate the cargo tank to a lower service limit, or replace the corroded shell areas.

The best answer: Make sure you know the real minimum thickness.

John Cannon

So how do you go about making sure you know the real minimum thickness? He provided some history to frame the issue.

He said that the enactment of HM-183 in 1985 meant that “for the first time in a very structured way, it was going to compel continuing maintenance of cargo tanks, and that would have a direct bearing on minimum thickness. In 1991, mandatory thickness testing began.”

From the Code of Federal Regulations: Title 49:

  • MC300 Series Cargo Tanks: 178.340 (structural integrity of 306s, 307s, or 312s), 178.341 (minimum thickness tables for 306s), 178.342 (for 307s), and 178.343 (for 312).

  • DOT400 Series Cargo Tanks: 178.345-3 (structural integrity), 178.345-8 (accident damage protection). “It's often overlooked in the industry,” Cannon said. “The shell has to be able to absorb the impact prescribed in the HMR.” Also, 178.346-2, 178.347-2, and 178.348-2 (which have tables).

  • All cargo tanks: 180.407(i)

He said that while many regulations imply formulas to determine thickness, others have typical-thickness tables, such as 178.347-2 (shell) and 178.347-2 (heads). He said the most important regulation is 180.407(i).

“It requires a thickness test every two years for most unlined vessels hauling materials corrosive to the tank,” he said. “That's subject to a number of interpretations, but when in doubt, do the thickness test. It specifies 10-plus locations where thickness readings must be made. At a bare minimum, if you have a thickness test map from the tank manufacturer, use that as a guide.

“The part most frequently missed is the sentence that refers back to 178.320(a) for the definition of minimum thickness, which is the greater of the thickness: in the specification; in applicable tables; to meet structural integrity; to sustain accident loadings; to meet ASME (as applicable); or to comply with a ‘B’ note.”

He said 180.407(i) offers in-service minimum thickness tables for when tables in the applicable table control.

“It suggests options for tanks that do not meet the in-service minimum thickness requirements,” Cannon said. “Thus, the determination of minimum thickness is not so simple as looking up a value in a table.”

He gave the correct processes for:

  • DOT400 Series cargo tank:

    1. Look at the “nameplate” required by 178.345-14.

    2. Stop.

  • MC300 Series cargo tank:

    1. Look at the “metal certification plate,” if provided. “The problem is that the plates weren't even required until 1985. After '91, many manufacturers started putting DOT minimum thickness on a specific plate. It's possible that the manufacturer has the minimum thickness for you on the plate.”

    2. Contact the OEM for a statement of minimum thickness. “The problem is that some companies that made 300 Series tanks in the '80s are no longer in business.”

    3. Read TTMA TB No. 116.

    4. Hire a design certifying engineer.

What if a cargo tank fails?

  • Retest. “Several times, we've been made aware of cases where UT was not properly calibrated,” Cannon said.

  • Recertify.

  • Downgrade. “It's available for some MC312 tanks as far as changing product density. The thing you can't do is take a 312 and recertify as a 307.”

  • Section. “If there is localized corrosion.”

  • Rebarrel.

  • Remove from hazmat service. “And call your friendly tank manufacturer.”

Cannon said many regulations affect the minimum thickness of a cargo tank.

“Don't fall into the trap of thinking it's just as simple as looking at a table — it's not,” he said. “Several factors contribute to the determination of minimum thickness, including accident loadings. Finding the minimum thickness of DOT400 series cargo tanks is easy. But be careful if you have an MC300 series tank you inherit or come across in your shop. Finding the minimum thickness of an MC300 series cargo tank can be tricky.”  ♦

 

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