Home Hot Wash THMG159 – Hot Wash: Propane, Part I

THMG159 – Hot Wash: Propane, Part I


In this hot wash episode, Bob and Mike break down a listener-submitted propane incident and discuss how to handle multiple issues at once.

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Complete Show Notes

5:55 Background on the Incident

  • Hazmat team was called to the scene of a fire in a garage in a residential area – involved multiple cylinders
  • Encountered a two family house with a driveway separating the structures
  • The garages were in the back of the property behind the house, and the fire was located almost in the middle of the block
  • Cylinders were outside, but in a confined area between a house and a garage with fences on both sides (probably 400 square feet total)

6:40 First Steps When Arriving On the Scene

  • NFPA 472: “Survey the hazardous materials/WMD incident to identify special containers involved, to identify or classify unknown materials, and to verify the presence and concentrations of hazardous materials through the use of monitoring equipment.”
  • First task is to assess the structure of the cylinder(s), as this can give us a number of valuable clues
  • These clues include markings on valves, stickers, and high vs. low pressure data
  • In this case, the listener observed that the cylinders were short, had a welded seam and foot ring, and that most had valves with OPD
  • Came to the conclusion that they were BBQ propane tanks

11:30 Using Meters to Identify and Classify Unknown Materials

  • NFPA 472: “Collect and interpret hazard and response information from printed and technical resources, computer databases, and monitoring equipment.”
  • Never assume anything, even if you’re almost positive you’re dealing with a specific substance (like propane)
  • In this situation, the hazmat team proceeded as if they were dealing with propane
  • We’d use three meters to help confirm its physical and chemical properties – LEL, PID, and metal oxide
  • If we got close enough to a leaking valve, we got a reading on the LEL meter but nothing on the PID (as expected)
  • Metal oxide meter would pick up small concentrations of combustible gas
  • Propane coming out of the valve that’s right side up will probably be under pressure – applying soapy water helps us prove that (if we see bubbles)
  • May sound unimportant, but a flammable liquid with a high vapor pressure can give us the same readings without producing rapid bubbles
  • Use the metal oxide meter to determine that the vapors are heavier than air – makes it easier to classify the substance as a flammable gas (propane)
  • Hazmat team was picking up higher concentrations on the floor than they were an inch or two above the ground, so there was a concern about vapor buildup
  • Every one of the nine tanks were leaking (we knew this because they were producing bubbles), which was also cause for concern

22:55 Visual Examination of Cylinders for Damage

  • NFPA 472: Describe the type and extent of damage to containers.”
  • There was nothing to indicate that the propane had started the fire, but there was also no evidence to the contrary
  • There was some blackening and soot on all of the containers, but none of them appeared to have been in high heat or had any flame impinging
  • There also wasn’t any type of deep discoloration in the paint – all of the white was still there
  • None of the metal was showing signs of major damage – in fact, the worst fire damage was the burned off stickers
  • All of the valving and welds were intact, and the foot rings were on and seemed to be unaffected
  • Used a TIC to inspect the tank within the electromagnetic spectrum of 700 nanometers to 1 millimeter

27:15 Cooling Down the Burning Cylinders

  • When compressed liquified gas turns into a vapor, it auto-refrigerates to its boiling point (which is -41 degrees Fahrenheit for propane)
  • The water coming out of the hose the team used to spray down the tanks was probably around 60 degrees Fahrenheit
  • This means the hazmat team was actually heating the cylinders up, rather than cooling them down – essentially enabling the leak by adding energy in the form of heat to the cooling liquid inside the tank
  • However, they were doing the right thing – water spray had nothing to do with keeping temperature and everything to do with dispersing vapor

31:40 Using Soapy Water on the Cylinders

  • We like to use soapy water on all of the welded areas, as well as the neck and the valve
  • When propane tanks are involved with fire, tanks usually become damaged at two main places – the weld points and the threads connecting the valve stem
  • Damage at the weld points is much less common than damage to the valve stem threads
  • Threads and valve stems are often made from two different metals that expand at different rates when heated
  • This causes the threading to become loose – doesn’t go back to the same tolerance it had before when it cools, which leads to leaks
  • In this situation, every one of the nine tanks had a leak at this critical point

33:50 Basic Deductions and Predictions for the Future

  • NFPA 472: “Predict the likely behavior of released materials and their containers when multiple materials are involved.”
  • We didn’t feel there was any danger of the tanks failing since they seemed to be in good shape
  • We know we’re dealing with a product that’s much heavier than air, so we predicted it would come out and hug the ground
  • Our major concern was the leak at each end of the tanks at the threads
  • Product could potentially collect on the ground and flow to a source of ignition – particularly dangerous in a residential area
  • Manholes in the street can also act as sources of ignition – they lead to most major utilities, so there are switches, transformers, etc. right nearby
  • A third source of ignition was the smoldering guard that was only ten feet away from the cylinder staging area – kept in mind that anything that was still hot could be a source

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