The requirements for thickness tests are set forth in 49 CFR 180.407(i). Additional requirements are found in §178.320(a), §178.345-2, §178.346-2, §178.347-2, and §178.348-2. The shell and head thickness of all unlined CTs used for the transport of materials corrosive to the tank must be 76 measured at least once every 2 years, except CTs measuring less than the sum of the minimum prescribed thickness plus one-fifth of the original corrosion allowance, which must test annually.
Measurements must be made using a device capable of accurately measuring thickness to within ± 0.002 inches (0.0508 mm). Any person performing thickness testing must be trained in the proper use of the thickness testing device used in accordance with the manufacturer’s instruction. A Design Certifying Engineer (DCE) must certify that the CT design and thickness are appropriate for the reduced loading conditions by issuance of a revised manufacturer’s certificate, and the CTMV’s nameplate must reflect the revised service limits. An owner of a CT that no longer conforms to the minimum thickness prescribed for the specification may not return the CT to HM service. The tank’s specification plate must be removed, obliterated, or covered in a secure manner.
Thickness testing must be performed in the following areas of the CT wall, as a minimum:
- Areas of the tank shell and heads, and shell and head area around any piping that retains lading.
- Areas of high shell stress such as the bottom center of the tank.
- Areas near openings.
- Areas around joint welds.
- Areas around shell reinforcements.
- Areas around appurtenance attachments.
- Areas near suspension system attachments and connecting structures.
- Known thin areas in the tank shell and nominal liquid level lines.
- Connecting structures joining multiple CTs of carbon steel in a self-supporting CTMV.
Minimum thicknesses for MC 300, MC 302, MC 303, MC 304, MC 305, MC 306, MC 307, MC 310, MC 311, and MC 312 CTs are determined based on the definition of minimum thickness found in 49 CFR 178.320(a), which states that minimum thickness means the minimum required shell and head (and baffle and bulkhead when used as a tank reinforcement) thickness needed to meet the specification. The minimum thickness is the greatest of the following values:
- For MC 330, MC 331 and MC 338 CTs, the specified minimum thickness found in the applicable specification(s).
- The thickness necessary to meet with structural integrity and accident damage requirements of the applicable specification(s).
- The thickness as computed per the ASME Code requirements (if applicable).
For MC 331 CTs constructed before October 1, 2003, minimum thickness shall be determined by the thickness indicated on the UIA forms minus any corrosion allowance. For MC 331 CTs constructed after October 1, 2003, the minimum thickness will be the value indicated on the specification plate. If no corrosion allowance is indicated on the UIA form, then the thickness of the tank shall be the thickness of the material of construction indicated on the UIA form with no corrosion allowance. In-Service minimum thickness for MC 300, MC 301, MC 302, MC 303, MC 304, MC 305, MC 306, MC 307, MC 310, MC 311, and MC 312 CTs is based on 90 percent of the manufactured thickness specified in the DOT specification, rounded to three places. Minimum thickness values for DOT 400 series CTs can be found in Tables I and II of §178.346- 77 2, §178.347-2, and §178.348-2.
