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    Petroleum, chemical tank trailer venting important part of meeting EPA regulations

    Feb. 1, 2004
    VENTING equipment used on petroleum and chemical tank trailers to prevent vapors from escaping into the atmosphere is an important part of meeting Environmental

    VENTING equipment used on petroleum and chemical tank trailers to prevent vapors from escaping into the atmosphere is an important part of meeting Environmental Protection Agency (EPA) regulations. Just how the equipment should be handled was one the topics presented by Chad Betts of Betts Industries.

    He discussed the various specifications by type of tank trailer. Beginning with the MC306 tank trailer, he said the emergency vent has to have a set pressure not less than three psi. For the DOT406, MAWP can range from 2.65 up to four. The formula for set pressure is 1.1 to 1.38 times MAWP.

    Giving an example, he said 3.3 MAWP is the industry standard, so the set pressure ranges from 3.63 to 4.55. Four psi is the average.

    The emergency vent rating pressure of an MC306 is five psi and on a DOT406 it is 1.25 times the tank test pressure, but not greater than MAWP plus three psi. For typical DOT406 relief valves, the rating pressure is 6.25 psi.

    Standards for minimum flow rating for the MC306 should be at least 6,000 cubic feet of air per hour by a spring loaded device. For the DOT406, the minimum flow rating should also be 6,000 cubic feet per hour, but a primary device must be used regardless of how small the tank compartment may be.

    An MC306 can have fusible venting actuated at 250 degrees Fahrenheit or less, but fusible venting for the DOT406 is not allowed.

    Vacuum relief devices are required on both the MC306 and the DOT406. Typically a normal vent satisfies requirements for both cargo tanks.

    Shifting gears to chemical tank trailers, Betts said the emergency vent set pressure for the MC307 should not be less than the design pressure of the tank. The DOT407 requires a formula: 1.2 to 1.32 times tank MAWP.

    Betts discussed emergency vent rating pressure for the MC307, which is 1.3 times the design pressure of the tank, and the DOT407, which should be rated at not more than the tank test pressure.

    For minimum flow rating, the specifications call for the MC307 to have 12,000 cubic feet of air per hour for every 350 square feet of the tank. In no case should it be less than 12,000 cubic feet, he said. For the DOT407, the 12,000 cubic feet of air per hour applies for 350 feet of exposed tank.

    Taking a look at frangible venting, Betts said the MC307 for burst pressure is 1.3 to 1.5 times the design pressure of the tank. For the DOT407, fusibles are not allowed. A bursting disc can be put on the interior of a relief vent to keep product away from the valve.

    He said the MC307 has no surge requirement, but if the valve is changed or replaced, it has to be replaced with one that has one liter of maximum leakage. The DOT407 must have a vent that releases no more than one liter.

    On the subject of vacuum relief devices, he said:

    • Not required for the MC307, but recommended.

    • Required on DOT407 unless tank is rated for full vacuum. If not, requires vacuum valve rated for less than 80 percent of the designed vacuum of the tank.

    Turning to the MC312 and DOT412, Betts said:

    • MC312, if has air unloading lines, must have relief valve that can limit pressure to no more than 1.3 times the design pressure at the maximum inlet flow rate.

    • DOT412, 1.2 to 1.32 times MAWP for set pressure On the subject of minimum venting for the MC312 and DOT412, he said:

    • Requires 12,000 cubic feet of air per hour for 350 feet of the tank per hour of flow rating. If hauling corrosives with no secondary hazard, a formula can be used that reduces the minimum requirement.

    Fusibles:

    • MC312 allowed.

    • MC412 not allowed, frangibles are allowed in series with reclosing devices.

    Surge requirements:

    • MC312, less than one liter, if replacing relief valve.

    • DOT412, less than one liter leakage per device.

    Requirements for vacuum-relief valves:

    • MC312 not required, but highly recommended.

    • DOT412 required, unless rated for full vacuum, limited to 80 percent of the design vacuum.

    Information for vapor recovery systems:

    • Required in some areas, going to become more common with volatile organic compounds. Already required on MC306/DOT 406.

    • MC306 and DOT406 must be open before product transfer starts, three- to-four-inch valves, typically, usually cable operated.

    • Sequential-style topside vapor valves, receive air pressure and open sequentially, then valves at tank bottom are opened to begin unloading.

    • In interlock-style vapor valves, each vessel compartment is plumbed so air goes up to top vent, only when it is open can it go down to the bottom of the tank at the emergency valve.

    Vapor recovery for DOT407 and DOT412:

    • Vapor valves have to be open before starting product transfer, typical two- and three-inch, hydraulically operated.

    • Vapor line configuration, two-inch T-style hydraulically operated vapor valve located next to domelid in spill dam. Dual fitting for vapor line connections in place at top of tank and at ground level.

      Betts said the goals for vapor recovery are to supply sufficient vapor flow to prevent the emergency vents from opening and the cargo tank from being subjected to vacuum beyond its design limit so that the tank doesn't collapse.

    • Positively-, mechanically-opened vents don't react to actual tank pressure when the valve is opened before loading and unloading.

    • He noted that factors outside the vapor system will impact cargo tank during product transfer, such as the storage tank having an obstruction, but there are backup systems to protect the tank vessel if that happens.