Staying abreast

Feb. 1, 2006
VENTING violations often found during roadside inspections can be prevented by staying abreast of the regulations and following proper testing and maintenance

VENTING violations often found during roadside inspections can be prevented by staying abreast of the regulations and following proper testing and maintenance procedures.

That was the message from Ted Turner of the Federal Motor Carrier Safety Administration at the National Tank Truck Carriers Cargo Tank Maintenance Seminar held November 7-9 in Louisville, Kentucky.

He pointed out that venting devices limit the pressure inside the tank. They are required to communicate with the vapor space and shall be mounted, shielded, or drained to eliminate the accumulation of water.

Turner noted that roadside inspections often reveal that the markings on venting devices are illegible or even non-existent. Fusible devices on older tanks often are cracked and/or leaking.

Other venting violations uncovered at roadside inspections include non-reclosing devices on DOT407 tank trailers and devices extending above the overturn protection equipment.

Some tank trailers may have improper venting devices, while others have been found to have no vent at all, or to have inadequate emergency venting capacity, he said.

Cargo tanks also are found that have venting devices that have been improperly modified.

Turner noted that there are specific federal requirements for the MC300 and DOT400 cargo tank series. Emergency venting capacity is calculated based on a cargo tank's exposed area. The venting capacity of all venting devices installed on a tank are added together to obtain the emergency venting capacity.

He reminded the audience that basic venting categories include emergency venting, loading/unloading venting, and normal venting. Emergency venting involves pressure actuated by tank vapor pressure, and it protects the tank from overpressurization that can be caused by a fire or by overfilling.

Capacity is rated in standard cubic feet of air per hour (SCFH) at a specific test pressure. Airflow capacity requirements are based on tank exposed area.


During loading and unloading, venting allows the air in the tank to equalize, preventing tank collapse or rupture.

Typically, the venting is not pressure-actuated, but mechanically operated by air, hydraulics, or push rods. Depending on product and locality, vapors may be captured to prevent escape to the atmosphere.

Under normal circumstances, the vent is actuated when pressure in the tank reaches a preset point. Combination pressure/vacuum vents are designed to compensate for vapor pressure changes due to heating/cooling of lading.

Normal vents are typically small, low-capacity devices and are used on MC306 cargo tanks. They only are allowed on DOT406 cargo tanks if vapor pressure is 1.0 psig at 115° F [CFR173.33(c)(1)(iii)] or if hauling gasoline per special provision B33[172.102(c)(3)].

DOT400 series tanks must have venting devices marked with the manufacturer's name, model number, set pressure in psig, and flow rating capacity in standard cubic feet per hour.

Testing requirements

When testing vents, a suitable device (bench test) must be used; pressure should be increased slowly; and a record kept of opening/reseating pressure.

If there is vent failure for a DOT400 series tank, the vent must be repaired or replaced. Failure in a MC300 series requires repair or replacement with a device capable of reseating to a leak-tight condition after a pressure surge and must leak no more than one liter.

Replacing a vent in the MC300 series requires each pressure relief system to be designed to withstand dynamic pressure surges in excess of the design set pressure as specified in the regulations. The vent must be able to withstand the dynamic pressure surge reaching 30 psig above the design set pressure and sustained above the design set pressure for at least 60 milliseconds with no more than one liter of leakage before reclosing to a leak tight condition. This requirement must be met regardless of vehicle orientation.