In Mechanical Engineering, Why should acetylene gas cylinders used for gas welding be erected in upright position?

very simple. Acetylene gas used for gas welding is stored in  cylinders of liquid acetone contained in porous material. Liquid acetone can dissolve large amount of acetylene gas under high pressure without changing the nature of gas. As acetylene gas is highly explosive in the presence of oxygen so acetylene cylinders are not kept horizontally, It is just because we have to avoid the leakage of liquid acetone from the cylinder that can readily dissolve acetylene gas which could explode in the air.

Acetylene gas is commonly used for gas welding because of its simplicity in production and transportation and its ability to achieve high temperature in combustion (e.g. around5,000oF).

Acetylene is highly unstable and flammable and would explode in elevated pressure when reacting with oxygen in air. Storing acetylene gas in cylinders under pressure is very dangerous. Gas acetylene used for welding purposes is stored in cylinders of liquid acetone contained in porous material (like firebrick). This is for cooling purpose in the event of thermal decomposition and to ensure that there is no free space left for acetylene gas. It also prevents the formation of high-pressure air pockets inside the cylinder. Dissolved acetylene in acetone will no longer be in contact with oxygen and is not subject to decomposition. Acetone is used because it is capable of dissolving large amount of acetylene gas under pressure without changing the nature of the gas.

The cylinders for gas welding i.e. oxygen cylinders and acetylene cylinders, when not in use should be stored separately because any mixture of these gases resulting from accidental leakage can be highly explosive. When in use, acetylene cylinders should always be kept in upright position. Otherwise, acetone liquid will be drawn from the cylinders with the gas if they are kept horizontally, resulting in significant leakage of acetone liquid will result.