Tips presented on weight, thickness, overfilling

Feb. 1, 2008
Optimizing payloads was just one of several pieces of advice presented at the National Tank Truck Carriers Cargo Tank Maintenance Seminar in St Louis,

Optimizing payloads was just one of several pieces of advice presented at the National Tank Truck Carriers Cargo Tank Maintenance Seminar in St Louis, Missouri. The conference took place November 12-14, 2007.

Discussing weight issues was Peter Weis of Polar Tank Trailer LLC. He was joined by Nick Paulick of Brenner Tank LLC, who discussed minimum thickness for cargo tanks, and Bob Koeninger of Dixon Bayco, who presented basics for overfill protection equipment.

“A poorly scaling trailer can greatly reduce your payload,” said Weis. “Many states allow over 80,000 pounds on state roads. Take advantage of available overweight permits if your trailer is rated for the load.”

At the same time, he reminded carriers to avoid illegal axle loads that result in citations, which can bring fines that cut into the bottom line. He pointed out that there is increasing public scrutiny of the damage done to roads by trucks that are overloaded. In addition, law enforcement officials have more and more sophisticated enforcement techniques for checking inner axle groups.

States generally use the expanded bridge formula table, with special limitations. The formula, W=500 (LN/(N-1) + 12N + 36) is defined as “W” equals the maximum weight in pounds that can be carried on a group of two or more axles to the nearest 500 pounds; L equals the distance in feet between the outer axles of any two or more consecutive axles; and N equals the number of axles being considered.

Weis pointed out that some states apply calculations differently from the Bridge Formula — such as certain New England, western, and central states, as well as Michigan.

(More information can be accessed on the Bridge Formula online at the Federal Highway Administration Web site at Just search Bridge Formula.)

When carriers are considering buying new tank trailers, he recommended they address scaling issues before production; approve scale drawings for all anticipated tractors; provide accurate tractor information; be sure to understand the trailer drawings; and be sure the required payload will be achieved.

Weis noted that trailer slope makes no significant difference in the rear axle weight, but moving the upper coupler can generally equate to 70 pounds per inch.

Cargo tank thickness

Turning to thickness considerations for tank trailers, Paulick defined minimum thickness as the greatest of:

  • 90% of the minimum manufactured thickness from Tables I and II of the applicable specification(s).

  • The thickness required to comply with structural integrity requirements.

  • The thickness required to comply with the formulae in the applicable specification(s).

  • The thickness required to comply with the American Society of Mechanical Engineers (ASME) Code (if applicable).

He noted that many MC300-series cargo tanks were built with actual manufactured thickness above that required by these criteria, which provides corrosion allowance.

Paulick added that minimum thickness is not the 90% reduction factor for in-service minimum thickness [from 180.407(i)(5)] and is only applicable to the minimum manufactured thickness tables. When people calculate the thickness, they cannot “simply“ take 90% of the “manufactured thickness” listed on the dataplate to determine in-service minimum thickness, he said.

To determine the correct in-service minimum thickness (per Truck Trailer Manufacturers Association (TTMA) TB No 116) the data plate should be checked for the declaration of minimum thickness. If it's not listed there, then obtain a statement of minimum thickness from the cargo tank manufacturer. If that also is not possible, then a design certifying engineer will have to be consulted. The engineer will require detailed measurements of tank geometry and dataplate information.

There also are other factors to consider:

Sides, top, and bottom sheet thicknesses may differ.

Head thicknesses on multi-compartment tanks may differ.

He added that a registered inspector must exercise good judgment regarding structural condition of the tank, and if unsure, should consult with the manufacturer.

If the tank is below the minimum thickness, there are certain solutions. MC312 tanks may be upgraded to DOT407 specification, which requires certification by the engineer, and will require new dataplates. Changes also may require adjustments to venting and accident damage protection. Some tanks may pass after the pressure rating or maximum payload are downgraded.

More information is available from the TTMA Technical Bulletin No 116, Determination of MC300 Series Cargo Tank Minimum Thickness.

Discussing overfill protection equipment for tank trailers, Koeninger said it may be the “product we love to hate,“ but it is specified by law and is used to prevent a condition that could cause a spill. He offered advice on ways to optimize its use and advised carriers to “keep it simple and standardize equipment throughout your fleet.“

Overfill protection devices installed on the loading rack connect to a plug and socket connection on the loading vehicle, which is connected to an overfill protection probe. Sensors are on the vehicle and some will not only warn of overfill problems, but will detect heel left in the tank.

Tips for getting the best performance from the equipment include specifying 18-gauge, tin-coated copper wire and 150-volt minimum insulation. He recommended filling all wire crimp connections with silicone sealant to protection against moisture and avoiding the use of razor knives to strip insulation. “Use a clear dielectric grease on sockets and monitor connections,” he said.

The types of equipment used vary by regions and may require two-wire or five-wire systems. He reminded carriers “do your homework,” consult OEMs for advice, ask about warranty claims, check for warranty periods, and specify manufacturer — all before ordering equipment.

When an equipment supplier is chosen, it's a good idea to get installation and trouble shooting information, sign up for seminars, and obtain posters with information about the equipment that can be put up in the shop.