Incident Overview

A security officer on patrol on the ground level of a shopping mall parking garage smells a strong odor of gas. He investigates further and finds that the odor is strongest near the center of the garage. Several shoppers going to their cars have reported smelling the same odor. The security officer calls 911 and requests that the fire department respond.

The fire department dispatches one engine company to investigate. Upon arrival at the shopping mall, the company officer positions the fire apparatus on the south side of the garage and meets the security officer. The security officer draws a sketch for the company officer indicating where the odor is the strongest in the garage.

It is a hot and humid day and there is very little air moving. The garage is open to the outside on all four sides.

The company officer sends two firefighters into the garage with a Combustible Gas Indicator (CGI) to investigate the source of the odor. The firefighters enter at the ground level on the south side and read 5% of the Lower Flammable Limit on the CGI. This information is relayed to the company officer, who requests that the security officer block off the entrance to the parking garage. He directs the firefighters to continue their investigation.

Using the CGI to survey the area, the firefighters discover that the highest reading is obtained near a white automobile. The car has a small black and white diamond shaped label on the rear of the vehicle with the word "Propane" on it. This information is relayed to the company officer.





Summary of Cylinder Construction Features

Motor vehicle propane fuel tanks are designed for installation in cars, vans, pickup trucks and buses. These tanks are built in a variety of shapes and sizes to accommodate different vehicle designs and space requirements. Their capacities range from 25 to 65 gallons water capacity.

All new motor vehicle fuel tanks for new vehicles must have a maximum working pressure of 312.5 psig. Pressure Relief Valves (PRV) must be vented to the outside of the vehicle. All fittings must be vented to the outside of the vehicle if the tank is installed within a confined space within the vehicle, such as in the trunk or in a camper shell.

Valve Fittings

Most motor vehicle fuel tanks usually have a maximum of 5 threaded openings. The number, size and location of valve fittings in mobile tanks will vary. These fittings usually include a pressure relief valve, a liquid and vapor fitting, a tank gauge (which reads full, half full, or empty) and a fixed Maximum Liquid Level Gauge. Most valve fittings are threaded and the sizes of the openings are limited to 1/4-inch, 3/4-inch, and 1-inch female NPT.

Valve Protection

Mobile tanks have special hardware to protect tank valves. Typically, mobile tanks are equipped with a protective ring or box which consists of a metal plate that partially surrounds the fittings and valves on the tank. This protective ring may be welded directly to the tank or it may be bolted to brackets that are welded to the tank.

Mounting Hardware

ASME tanks used in mobile service must be securely attached to the vehicle. All tank manufacturers install special hardware for mounting the tank to the vehicle. The location of the mounting hardware varies according to the type of vehicle on which the tank is being installed. Steel strips or brackets are usually drilled so that they can be bolted to the vehicle.


Vehicles powered by propane are usually identified with a diamond shaped label, although this type of marking is not universal. The label should be located on an exterior surface on the lower right rear of the vehicle. The diamond shaped marking consists of a border and the letters "PROPANE" of silver or white reflective luminous material on a black background.





Incident Action Plan

Tactical Objectives

  1. Isolate people from the problem, establish Hazard Control Zones, and control ignition sources.

  2. Identify the source of the leaking propane and stop the leak.

Methods of Control

Locating the source of a gas leak in a large parking garage can be a frustrating and time consuming job. There are many different types of vehicles in a parking garage and the source of the odor could be a range of possible flammable liquid or gases, including a small 20 pound propane cylinder improperly stored inside of a van or car trunk or a natural gas or propane powered vehicle fuel tank.

Both LPG and natural gas have odorants to make it easier to detect a leak. The most effective way to identify the source of the leak is to use a Combustible Gas Indicator (CGI) to systematically survey the area and map out the concentrations of the flammable gas.

The company officer should draw a sketch of the scene and record the CGI readings and note the time and location of each reading. This information will help identify safe, unsafe, and dangerous areas and may help narrow the search area to three or four vehicles.

Once the area has been surveyed using the CGI and the search area has been narrowed, individual vehicles should be inspected to determine the source of the leak. If the vehicle is an automobile, the fuel tank will usually be located inside the trunk. The trunk will need to be opened to gain access to the tanks valves and fittings. Special consideration should be given to the potential risk of ignition of flammable gas that has accumulated inside the automobile’s trunk. The contact switch for the trunk light is a potential source of ignition. The risk to emergency responders can be reduced by opening the hood and disconnecting the 12 volt car battery before opening the trunk.Once access to the trunk has been gained, a CGI reading should be taken to determine if the atmosphere is within safe limits.

The protective ring or box containing the fittings should be inspected to see if there is frost on the box. The lid to the ring or box must be removed to access the valves and fittings.

The most likely source of the leak will be the vapor or liquid valves or the Fixed Maximum Liquid Level Gauge. These valves should be checked for leaks and secured. Motor fuel tank shutoff valves are right-to-tight, left-to-loosen. Make sure you are turning the valve off, not on. Close valves hand tight.

If there is no frost on fittings or valves, a soap and water solution can be applied to the connections to determine if there is a small leak. The gauge on the fuel tank should be examined to determine if the tank is full or empty.

If the source of the leak is not in the trunk, the engine compartment should be checked. Leaks in engine compartments can be controlled by shutting off the supply valve on the fuel tank in the trunk.

Remember, even though the supply valve has been shut off, the fuel line still contains propane. It will take a period of time for the leak to stop, depending on the size of the fuel line and leak.

Note: Fires in engine compartments can be controlled in the same manner. First gain access to the trunk, and then close the supply valve.





Additional Factors

Additional Factors to Consider for this Operation Include:

  1. Propane is odorized for safety and early detection of a leak. The most commonly used odorant for propane is ethyl mercaptan. The odor threshold of ethyl mercaptan is about 0.1 ppb.

    NOTE: The odor threshold is the lowest concentration of a material's vapor in air that is detectable by human smell.

  2. Propane can be smelled at a very low level. It is possible to initially over-react to the situation before a complete size-up has been made. Evacuating large areas (in this case a shopping mall) based on odor complaints from citizens can create additional safety problems. Using a Combustible Gas Indicator is the most effective way to evaluate the hazards and risks.

  3. The Lower Flammable Limit for propane is 2.15% (21,500 ppm) and the upper flammable limit is 9.60% (96,000 ppm). Concentrations of 10% (2,150 ppm) of the Lower Flammable Limit should be considered unsafe. Atmospheres in the flammable range (2.15 to 9.60%) are dangerous.

  4. At 10% LFL precautionary steps should be taken to control ignition sources and access of emergency response personnel and citizens to the area.

    [2.15% LFL x 10,000 = 21,500 ppm x 10% of the LFL = 2,150 ppm]

    Using 10% of the LFL for propane to determine unsafe areas provides a very good margin for firefighter safety. Personnel working in these areas must use full protective clothing and self-contained breathing apparatus.

  5. Contact the local propane marketer for technical assistance.





 Combustible Gas Meter

This is an interactive combustible gas meter. Move your mouse or finger (touch device) over the leak area to view the Gas-in-Air percentage change.


 Incident Map

This is a top-perspective view of the incident area. Its purpose is only to get the general idea of the incident and surroundings.

 Incident Video

In this section, we present an animated version of the incident.