Home Podcasts THMG084 – MC 331 Compressed Gas Cargo Tanker Overview

THMG084 – MC 331 Compressed Gas Cargo Tanker Overview


In this episode, Bob and Mike talk about the MC-331 pressurized container.

Complete Show Notes

3:40 Differences Between Tankers

  • We distinguish between different kinds of tankers based on what they hold
  • MC-331 is usually a compressed liquefied gas tanker carrying things like LPG or anhydrous ammonia
  • Each tank is specifically designed to meet the physical-chemical specifications of the product they’re carrying
  • The 49 CFR (DOT Code of Federal Regulations) details all of these specifications

4:25 Overview of MC-331 LPG Tankers

  • Tubular in shape with round cross sections at the ends – however, some have square backs to protect gauges, shut-offs, etc.
  • Single compartment, unlike most gasoline and bulk liquid tankers that have baffles or walls
  • Usually made of steel with a single manway at the top near one end of the tank -look for a flange disc with a bunch of bolts around it
  • Color scheme
    • Lower third of the tank can be painted however the owner chooses
    • Upper two-thirds must be painted white or some other reflective color – this keeps heat from being absorbed into the liquid
  • Unlike DOT 406 and 407, these tankers have no frame – tank itself is so sturdy that it makes up the structure and is mounted directly to the chassis of the truck
  • Size dictates function
    • Smaller tankers used for residential drop-offs because the streets are tighter with smaller turning radiuses
    • Tractor-trailer type tankers hold up to 11,500 gallons and are used for larger applications or when there’s more space
  • ASME (American Society of Mechanical Engineers) has to certify every one of these tanks to make sure they’re pressurized properly – maximum allowable workable pressure (MAWP) must between 100 and 500 psig
  • LPG tankers are usually between 250 and 265 psig – same pressure as the propane for your gas grill, but at much higher volumes

9:20 Basic Features of MC-331 Tankers

  • You’ll find these characteristics regardless of the size or volume of the container
  • Common traits include:
    • Pressure relief devices – referred to as pressure relief devices or safety relief valve and are designed to keep the tank from over-pressurizing or creating a BLEVE in emergency situations
    • Spring loaded valves – these self-close whenever the optimal pressure is reached and have to be located somewhere in the vapor space
    • Tankers must discharge upwards and away from the tank itself – we want flames as far away from the tanker as possible
    • Must be set to open at no more than 110% of the tank’s design pressure and must be able to keep the tank from exceeding 120% of the design pressure – helps ensure the tank won’t BLEVE from excessive pressure
    • Reinforced recessed skid-protected gauge wells – gauges for temperature, pressure, liquid level, etc. have to be protected in case of rollover
    • Pipe system can be installed however the owner would like it to be – up/down, left/right, back/front, etc.
    • Pipe system is directly mounted to the belly of larger tanks and surrounded by a protective cage to keep people from driving into them

17:10 More on MC-331 Piping and Valving Features

  • All piping connections must be permanently marked to indicate whether they communicate with the liquid or vapor space when the tank is full
  • Connections include:
    • Liquid fill line
      • Connected to a spray bar inside the tank
      • Spray bar lets the tank fill more quickly
      • Labeled “liquid” or “spray fill”
      • Pipe will have a quarter turn ball valve and an inlet backflow preventer or check valve
    • Liquid discharge lines
      • Used to offload liquid propane via a PTO- or hydraulically-driven pump
      • This pump is rated for the flammable liquid moving through it
      • Excess flow valve – closes the opening when liquid flows above a certain rate
    • Vapor equalization line
      • Connected directly to the vapor space
      • Similar to DOT 406 and DOT 407
      • Designed to allow equalization among the vapors
      • Also has a ball or gate valve – keeps pressure at the top so things stay cool
    • Internal self-closing stop valves
      • In line with each pipe and recessed into the tank
      • Internal and cannot be seen from the outside of the tank
      • Activated via emergency shutoffs and pressure differentials
    • Hydrostatic relief valves
      • Found in the liquid lines
      • Designed to protect the piping and equipment from pressures from liquid expanding between the closed valves
      • Spring loaded and self-resetting – keeps liquid in lines from heating and over-pressurizing
    • Pressure bleed pipes
      • Allow users to manually bleed off pressure from the lines after the valving is closed
      • Works similarly to hydrostatic relief valves, but in the hose lines between tank and tanker

22:55 Common MC-331 Markings

  • Required markings include:
    • Data or spec plate – includes DOT design code
    • Manufacturer name and tanker serial number
    • Tank material specifications
    • Water capacity in pounds and gallons
    • Designed working pressure
    • Original test date
    • Proper shipping name of the commodity – proper names must appear on both sides and ends
    • Proper placards affixed to both sides and both ends of the tank
    • Set of initials (either QT or NQT) – designates quenched and tempered or not quenched and tempered, which relates to how the tank was built

25:10 Things to Remember

  • Propane is propane – same rules apply regardless of the size of the vessel
  • A full tank is never liquid full – liquid sometimes indicates an overflow condition
  • Liquid expands or contracts, which prevents it from blowing up
  • The SRV should have a weather cap, but crud can still get in – this causes the cap to stick open and leads to leaks
  • Hooks or loops on top don’t mean the tank should or can be lifted – MC-331s can’t be turned upright when they’re full because they’re incredibly sturdy and heavy

Have a question? Send an email to feedback@thehazmatguys.com or leave a message on our Haz Mat Guys comment hotline: 843-628-1484

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