MISSISSIPPI STATE -- People trying to refill their propane tanks for summer grilling may encounter an extra expense as they find they must buy a new safety valve.

As of April 1, all four- to 40-pound propane tanks must be equipped with an Overfill Protection Device. This is a new valve that replaces the one on existing tanks. The new valves are marked with "OPD" to designate their compliance with the state law and have three-lobed valve handles, rather than the five-lobe type found on older propane tanks.

"The device will not allow the tank to be filled more than 80 percent full, leaving some room for the gas to expand," said Herb Willcutt, safety specialist with Mississippi State University's Extension Service.

A typical grill's 20-pound cylinder holds five gallons of propane. The propane is filled in the bottle as a liquid, and the new valves have a float that rises as the bottle is filled to lock off the opening once the tank has reached 80 percent full.

Willcutt said when a propane tank gets hot, the pressure rises dramatically. Both old and new valves have a relief valve that allows gas to escape if too much pressure builds up. At a temperature of 70 degrees, an 80 percent-full propane tank has a vapor pressure of 124 pounds per square inch.

"This is a safe, workable pressure," Willcutt said. "Overfilling contributes to much higher pressure, especially when the tank warms. When pressure rises beyond about 325 psi, the tanks are designed to release gas into the atmosphere. In an enclosed area, this creates an explosion and fire potential."

If a propane tank overheats as it is venting, Willcutt said to get it out of enclosed areas and away from sources of fire, lay it on its side and cool it with a water hose. Even if the tank is burning from the relief valve, cooling with a water hose can reduce the risk of fire to surrounding structures and allow the relief valve to shut itself off when the tank reaches a safe internal pressure.

"Laying a non-burning tank on its side allows the liquid to escape faster and relieves pressure more quickly. The water hose cooling it will reduce the pressure enough that it will quit relieving itself," Willcutt said.

The risk of a propane tank exploding is usually only found when the tank is engulfed in a fire, Willcutt said.

Owners of propane tanks with the old valve can replace these at most locations that refill tanks. Either exchange the tank for a new one with the correct valve or place the new valve on a structurally sound existing tank.

"If you're replacing the tank and don't want the old one, the best option is to leave it with the dealer and let them make sure it is properly disposed of," Willcutt said.

State law does not allow propane tanks to be transported in the passenger compartment of a vehicle. This means they can't be carried in sport utility vehicles, hatchbacks or similar passenger vehicles. Transport them upright in the trunk of a car or bed of a truck and secured so they can't roll around.

Never leave a propane cylinder in an enclosed vehicle, home or garage where it might become overheated.