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LP Tanks now require overflow valves

Jan 1, 2003
From Ocean Navigator #122 May/June 2002

From Ocean Navigator #122 May/June 2002

Propane, at normal atmospheric pressure, is a gas; however, whencompressed, it becomes a liquid. This is why, when you shake a full or partially full LP (liquefied petroleum) cylinder, it sounds like it has liquid inside; it does. When the pressure in the tank drops, as is the case when you turn your LP stove or heater on, the liquid actually boils into vapor, which in turn raises the pressure again. This state of equilibrium continues until the tank is empty. It is for this reason that pressure gauges cannot be used to measure the quantity of LP gas within a tank. The pressure gauges are strictly for leak detection.

As of April, all LP tanks are required to be fitted with over-fill-prevention valves, which are intended to prevent unwanted propane leaks. The valves allow tanks to be filled only 80 percent of possible capacity, leaving room for expansion.
   Image Credit: Steve C D'Antonio

In order for this system to work, a vapor space must remain at the top of the inside of the cylinder (for horizontal cylinders, this space is opposite the side with the mounting feet), between the liquid and the tank. This space, about 20 percent of the tank capacity, allows the gas to properly boil off at the moment of demand. However, LP, just like diesel fuel, will expand when heated and contract when cooled. If you have your LP tank filled on a cool spring morning, and the operator tops off the tank, thinking he's actually doing you a favor, or just makes a mistake, he may actually be sowing the seeds of your demise. Once you place that cylinder aboard, and July arrives before you've needed to use the stove, the tank will do one of two things. It may vent LP gas through the pressure relief valve. If your LP locker has been properly constructed, this gas will harmlessly exhaust overboard. If the locker is not gas-tight, or if you've stored the tank in a sail locker or lazarette, it will fill these spaces and eventually settle in the bilge, with potentially explosive consequences — as detailed in an article by Gavin MacLaren in this year's Ocean Voyager issue.

As of April 1, 2002, the potential for this problem will have been nearly eliminated. As of that date, all LP tanks between 4 and 40 lbs are required in nearly all states in the United States to be equipped with what is known as an overfill prevention device (OPD). This will limit the amount of liquid that a tank can be filled with, ensuring a 20 percent vapor space. A float within the tank, which is attached to the valve, stops the filling process automatically.

There is no grandfathering of tanks now in service. If your tank is not equipped with one of these valves, it will not be eligible for a refill. The valve can be retrofit to some tanks (horizontal tanks are not retrofitable), but it's not worth doing on most ordinary steel tanks unless they are of an unusual size or configuration. Most aluminum tanks, because of their replacement cost, are worth retrofitting. Most LP service centers are able to perform this work, but unless you have a common tank size, be prepared to wait a few weeks for the valve to be ordered. The cost of a refit is about $25.

OPDs are identifiable by the triangular-shaped hand wheel, which is not removable. Both the hand wheel and the bronze valve body are stamped with the letters OPD. One final note, OPD valves will not allow gas to flow unless a hose is attached. This has caused some confusion, if the valve of an OPD-equipped tank is cracked to determine if there's anything in it, nothing will come out even if it's full. This is not a recommended practice, and it won't tell you how much gas is in the tank anyway; as previously mentioned, it may be nearly empty. Weighing is the preferred and safer method of determining fill state. The TW (tare weight), stamped on the tank, is the weight of the empty cylinder. Anything over that weight is gas.

Steve C. D'Antonio

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