Here is something else that adds at little more detail with regard to forklifts.
"A propane cylinder contains liquid and vapour. We only fill cylinders with liquid to 80% to allow room for the propane to grow if the temperature increases. This is done because we don't want liquid to be in contact with the relief valve, which is built into the service valve. If the relief valve were to vent due to an overpressure situation, it will vent vapour. Not necessarily something we want to happen, but if it does, we have a bit of propane vapour in the air. One of the properties of propane that you must be aware of, is that propane expands 277 times when it changes state from a liquid to a vapour! So if the cylinder is on it's side and vents liquid, that little bit of liquid just made a VERY LARGE amount of propane vapour!
Some cylinders ARE designed to lie horizontally for RV's and forklifts. If you look at these cylinders closely, you will see that the relief valve is mounted in the upper portion of the vapour space so that it will not vent liquid. The service valves on these tanks has a tube inside that curves upward to the top of the vapour space for the same reason.
There are specific types of torches and burners for liquid service. They use cylinders with a specific type of connection. These cylinders stand upright so the relief valve is again, in communication with the vapour portion of the cylinder, and a tube on the service valve, which you cannot see, goes down to the bottom of the cylinder to draw out liquid.
Forklift cylinders are different again! They have a locator pin on the forklift that must go into a hole on the cylinder when you put it in place. The relief valve is placed in those cylinders so that whether it is vertical or horizontal, the service valve is still in the vapour space. They too have a tube on the service valve that will either point up or down depending on whether the cylinder is meant for liquid or vapour service...
Too make a long story short...Any appliance like a BBQ or stove that might be commected to a 20LB cylinder should have the cylinder always in an upright position!"