Pressure-relief

DAVE GIRARD, Girard Equipment Inc, gave a summary of cargo tank pressure-relief venting, the regulations involved, and the testing required. He spoke at the National Tank Truck Carriers Cargo Tank Maintenance Seminar.

He pointed out that in order for tank trailers under the MC307 and DOT407 designation to continue qualifying for intended use, the venting system must conform to the Department of Transportation (DOT) code requirements. Before testing begins, the specification data plate should be checked to verify the cargo tank trailer code designation.

Every tank compartment must have one or more safety relief devices that limit the internal pressures of the tank to 130% of the tank's design pressure. An MC307 specification trailer employs two main devices to enable the tank to automatically adjust the internal pressures of the tank vessel.

The first is a pressure-actuated venting device that is set to open at no less than the tank design pressure. The venting capacity of this vent must be at least 12,000 cubic feet per hour, per compartment or 12,000 cubic feet per hour for each 350 square feet of exposed surface area.

Second is a fusible cap. The majority of MC307 tanks produced have compartments that need more venting capacity than a three-inch vent will allow, so additional venting capacity is needed. A three-inch fusible cap supplements the vent and is only needed when the tank vessel is under a considerable amount of pressure in a very short period of time as would occur in a fire or an overturn situation.

An external visual test is performed annually, or every six months on tank trailers with full opening heads. If the venting device appears corroded or damaged, it must be removed and a bench test performed. He noted the components should be kept clean and covered to prevent seal compromise.

“Do the test slowly for accuracy,” Girard said. He added that testing procedures should be documented and the records kept on hand for future reference.

There are different set pressures for when the vent opens on a MC307 vent and a DOT407 vent because the newer DOT407 code made allowances for product weight of the liquid in addition to the vapor pressure on the vent in case of an overturn situation.

Turning to a brief discussion on fusible caps, Girard said they are typically a nut and disc joined together with a fusible element. The number of fusible caps needed for a tank trailer depends on the size of the tank. Tank trailer manufacturers should have the information available relative to their product. (It should be noted that fusible caps can't be used on DOT-code tanks.)

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