A LOT OF central county residents who use propane gas to fuel their outdoor cooking grills regularly stop by places such as Clement Hardware in Old Severna Park to have their empty cylinders or tanks refilled from the store's propane gas dispenser.

But the Clement brothers - Gordon, 38, and Jeff, 54, who bought the Severna Park landmark in 1996 from their parents, David and Edith Clement - have a word to the wise: If you bought your propane tank before 1998, don't expect to refill it right before you put the steaks on. It may take more time and considerably more money than you expected.

Four years ago, the National Fire Protection Association wrote a provision into its code requiring all small propane tanks (small is anything with a capacity of 40 pounds or fewer; the average grill owner uses a 20-pound tank) to be equipped with an overfilling prevention device. An OPD is mounted inside the tank beneath the handle; when the propane reaches the float at the end of the device, the refilling process automatically stops.

Why is an OPD necessary? Changes in temperature cause propane gas to expand, which means adequate space must be left when a tank is filled. If it isn't, the gas can leak from the tank or can enter the piping system, putting dangerous pressure on the appliance being used. Both situations can result in fire.

To reduce that danger, the 1998 code required that all small tanks eventually be equipped with OPDs. The code was adopted by the Maryland Fire Prevention Commission and went into effect in April, making all small propane tanks without OPDs illegal.

So if your tank is covered with cobwebs and looks like it has been through a few too many grilling wars, it probably precedes the OPD era. A tank equipped with an OPD has a handle shaped a little like a firefighter's badge with the letters OPD engraved on it.

The Clement brothers, who have always enjoyed schmoozing with customers, say they are beginning to dread seeing empty tanks in a customer's hand.

First, customers have to be told that if their tank is more than 4 years old and doesn't have an OPD, it cannot be refilled - a new tank is $32, and the gas is $12. Adding insult to injury, the cost to have the store dispose of the old tank is $5 more.

Some people take the tanks home to get rid of them, Gordon says. But from the dozens of abandoned tanks piling up in the service area behind the store, it appears that many more customers are happy to let the store deal with the problem.

Another difficulty, though, is a severe shortage of legal tanks - at this point, the Clements' customers are putting their names on a waiting list.

In the store this week to have his old tank refilled, Reg Reese of Severna Park got the bad news. "I'll just wait," says Reese, whose name tops the list. "I'd rather do business with my local hardware store."

Gordon points out that the area has several exchange locations at which customers have been able to take an empty tank and pick up a filled one. These locations include the Food Lion on Ritchie Highway, the Amoco station at Macy's Corner, and the Shell station on Benfield Road.