Home Hot Wash THMG183 – Leaking MC331 Scenario, Part I

THMG183 – Leaking MC331 Scenario, Part I


In this episode, Bob and Mike review MC 331 and explore an overturned tanker situation.

Thank to our sponsors, CavCom, Inc., First Line Technology, and All Safe Industries.

This was originally going to be a one episode hot wash, but it expanded – much like propane does! In this episode, Bob and Mike explore how propane works and get into the weeds on tanks and operations.

Complete Show Notes

5:15 MC 331 Basics

  • Only tanker that’s designed to take a gas, compress it, and transport it in a compressed liquid form
  • Very sturdy since its transporting substances that are under a lot of pressure
  • Robust construction – stout, cylindrical shape and round heads
  • Round shapes holds more pressure and increases surface area – holds all the pressure uniformly because there are no corners or weak spots
  • Generally made from thick, welded steel

11:45 Background on the Scenario

  • First due units arrive on-scene to find a 331 laying on the passenger side – driver was able to get out and is on the side of the road with no injuries
  • Very obvious the vehicle flipped over while in motion, skidded along the road, went onto an embankment, and came back onto the road
  • Vehicle was carrying propane
  • There were transfer valves in both the liquid and vapor phases on the belly of the tank
  • There were also relief valves at the top of the tank
  • Since it was flipped, the vapor space was on the top – that means the PRV may have been under liquid
  • Additionally, being flipped means all of the lines may have been pushing out liquid
  • With all compressed gasses, if you see a vapor cloud, you’re dealing with a liquid – if not, you’ve got a vapor

27:40 What Would We Do If the Tank Was Leaking?

  • According to Bob, this isn’t a big deal – we do need to know if it’s a vapor or a liquid, though
  • We’re going to stick with liquid, since that’s the worst case scenario (and the most typical)
  • Too much potential for UEL/LEL problems if we flip the tank over – could be a spark source
  • Liquid leak will stop on its own when levels drop below the liquid leaking point – becomes a gas at that point
  • Auto-refrigeration has to take place inside the tank – this can’t happen since liquid is being pushed out at the vapor space pressure
  • Until it hits the vapor space, there is no boiling – pressure in headspace is okay, although it is getting a little bigger
  • Depending on the size of your leak, there could be an auto-refrigeration incident happening outside the tank

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