A 20 pound portable propane cylinder has been overfilled to more than 80%. The owner has reconnected the cylinder to the barbecue grill and improperly stored the grill and cylinder in a heated garage which is attached to a home. The owner smelled a strong odor of gas, called 911 for emergency assistance, and evacuated the home.
When you arrive on the scene, you meet the owner, who informs you that everyone has safely evacuated the house. The owner advises you that the only potential source of gas in the home is the propane grill; however, the grill has been placed in storage and was not being used at the time the gas odor was smelled.
NOTE: The cylinder has been unsafely stored indoors.
Portable 20-pound propane cylinders are widely used for barbecue grills and space heaters. They are usually refilled at cylinder filling stations and transported home and hooked up by the user. The 20-pound class cylinder is usually of two- or three-piece welded steel construction.
Cylinders generally have one fitting welded in the service end of the cylinder. The fitting is threaded to a female normal pipe thread (NPT) and is raised above the surface of the cylinder. One combination service valve and pressure relief valve (PRV) is installed in the fitting.
Using a two person team in proper protective clothing and personal protective equipment (PPE), a quick walk around of the exterior of the building should be made to confirm that there are no other sources of either natural gas or propane supplying the building. If other sources of gas (propane or natural gas) are discovered, they should be isolated by closing the valve(s).
A two-person entry team in PPE should gain access to the building and use a combustible gas indicator (CGI) to determine the level of flammable vapors inside the structure. If CGI readings exceed 10% of the lower limit, the entry team should withdraw and ventilate the building using positive pressure ventilation. Ventilating the interior space will lower the level of flammable vapors inside the structure.
When CGI readings indicate levels lower than 10% of the lower limit, the entry team can search the interior of the building to locate the barbecue grill and 20 pound cylinder.
The barbecue grill and cylinder should be moved outside and secured in a safe area. Once the overfilled cylinder is exposed to a cooler ambient temperature, the internal pressure in the cylinder will drop and the cylinder's pressure relief valve should remain closed.
Options for rendering the overfilled cylinder safe include:
Note: Laying the cylinder on its side will release liquid as opposed to vapor, providing the needed room in the vapor space for expansion. The local propane dealer can usually assist emergency responders in dealing with overfilled containers and should be notified early in the incident.
An overfilled propane container is a common type of propane emergency and is the source of numerous emergency calls to fire departments every year.
Overfilling Prevention Devices (OPDs) are being installed on new tanks and a retrofit program initiative by the propane industry will soon eliminate most overfill problems. However, there are an estimated 50 million 20 pound propane cylinders in service in the United States, and emergency responders need to be aware of the potential for overfill emergencies.
The classic propane tank overfill emergency occurs when a propane cylinder is overfilled at a propane filling station and then placed in a warm area like the trunk of a car or inside of a building. A propane tank must be filled to no more than 80% of its capacity. This allows for expansion with normal temperature rises that may occur. However, if the container was filled to more than 80% of its capacity and the owner places the container in a warmer enviornment, the propane liquid inside the container will be warmed by ambient heat, the liquid propane will expand, the internal pressure will increase, and the container may reach a hydrostatic condition.
To understand this problem, consider the fact that the normal boiling point of propane at 14.7 psi is -43.7° F. This means that liquid propane can exist in an open container or as a cloud of liquid droplets in air only if its temperature is -43.7° F or lower.
At a temperature of 70° F, propane's vapor pressure is 124 psi. Consequently, it can only be maintained as a liquid at room temperature in a pressure vessel designed to withstand these changes in temperature and pressure.
At a temperature setting of 162° F, propane's vapor pressure is 390 psi, which is above the normal setting of 375 psi for pressure relief valves on propane cylinders. So a properly working propane container that has been correctly filled to 80% of its capacity can be heated to 162° F before its pressure relief valve will open.
When 375 psi is reached, the cylinder's pressure relief valve will function as designed and relieve the pressure inside the container. When the internal pressure drops below the pressure relief valve setting, the valve closes.
Note: PRV settings for ASME tanks are 250 psi and for motor fuel tanks are 312.5 psi. The typical container involved in an overfill situation may involve a propane space heater inside a building under construction, or a propane tank on a recreational vehicle which has been parked inside a heated garage. However, the most common container involved in an overfill is a 20-pound cylinder used for the home barbecue grill.
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.
This is a top-perspective view of the incident area. Its purpose is only to get the general idea of the incident and surroundings.
In this section, we present an animated version of the incident.