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THMG022 – DOT 407


In this episode, Mike and Bob discuss another common over-the road tanker that’s often seen during our responses – the DOT 407. They discuss the specifications of these tanks and their thoughts on the remediation of a DOT 407-related possible emergency.

Complete Show Notes

4:20 Overview of DOT 407

  • Cargo bulk tank that primarily carries liquids and gasses and is usually called a chemical hauler
  • It has a circular cross section (for up to 40 psi) and a double shell construction with insulation
  • Can have one or two compartments
  • Mostly carries flammable and combustible liquids, oxidizers, corrosives, and poisons

9:50 Differences Between Various Tanks

  • Spec tanks – CFR 49 defines the requirements that dictate tank construction and which materials can be carried
  • Non-Spec tanks – these are exempted from DOT requirements and usually haul asphalt, sulfur, and any type of pneumatic trailers
  • 400 series tanks have thicker shells, reinforced rollover protection, and manholes that can withstand a minimum static pressure of more than 36 psi
  • Also, any non-reclosing devices must be in series with a reclosing device

12:15 Construction of DOT 407 Tanks

  • Can be made of:
    • Aluminum alloy
    • Mild steel
    • Stainless steel (90% or greater DOT 407 tanks in service are stainless steel)
    • Carbon steel
    • Carbon steel with liner
  • Typically hold between 5,000 and 7,000 gallons
  • Can be insulated or uninsulated
  • Can contain between 1-4 compartments, but some have up to 6
  • May or may not have a frame – sometimes the shell serves as the frame for the trailer
  • If the tank thickness is less than 3/8″ thick, the tank must have circumferential reinforcement
    • This reinforcement can be in the form of bulkheads, baffles, ring stiffeners, or any combination of the three
    • It’s uncommon to find baffles in the 407, making cleaning difficult
    • There must be no more than 60″ between the reinforcements

16:30 DOT 407 Insulation and Leaks

  • If the tank is insulated, you may not be able to see the ribs
  • Insulation can be up to 5″ thick
  • These tanks may also have coils to keep the product hot – this is controlled by the vehicle’s thermostat
  • These trucks are also good for vacuuming things up
  • Leaks most commonly occur in the manway or safety relief device

21:35 What 407s Haul and How They’re Loaded/Unloaded

  • They can haul every hazard class other than Class 7 Radiological
  • They can typically be loaded or offloaded through two methods:
    • Through the manway on top (splash loading)
    • Through the bottom valve (either in the rear or the midpoint)
  • More and more trucks have integrated vapor recovery methods
  • Differences between offloading a 406 vs. a 407
    • 407s have less protection and most drops are between the rails
    • 406s shear off most easily near the belly of the truck
    • A Bettes valve is used to close the valve on a 407 if there’s a problem

26:30 Tank Pressure Requirements

  • The MAWP (maximum allowable working pressure) for a 407 is 25 psig
  • If the pressure necessary is over 35 psig, the entire tank must be built to the ASME (American Society of Mechanical Engineers) spec code
  • This is in contrast with the MC307, which has an ASME spec code of 50 psig
  • If the tan is unloaded by a vacuum, it must meet ASME standards and its external design pressure must be 15 psig; MAWP for these vacuum tanks are 25 psig (minimum)
  • Note: psig is pounds per square inch gauge, which is the difference between atmospheric pressure and pressure being measured (calculation)
  • There is also absolute pressure in which the user must subtract atmospheric pressure (14.7 psi) from the reading to get the pressure measurement

31:05 Safety Relief Valves and Vents

  • These must be mounted to prevent water accumulation of water – freezing of safeties is bad
  • Must allow total venting of the compartment to 130% of the tank’s design
  • These have spring-loaded or pressure activated vents called the “Christmas Tree”
  • This tree includes the air pressure gauge and the valve – used for purging or pressurizing with an inert gas to unload the tank

32:30 More on DOT 407 Tanks

  • Fusible plugs or caps must melt at 250 degrees Fahrenheit if the tanker is carrying flammable liquids
  • Frangible discs must release between 130-150% of the design pressure
  • Manway openings must be 15″ or greater and have to withstand 150% of the tank’s design pressure
  • Tankers are usually offloaded under non-emergency conditions via gravity – may go to an external pump at a facility if pumping and pressure is required
  • In emergency conditions, we’ll generally offload via a Wilden pump and top load the truck

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