Why must acetylene cylinders be kept in an upright position?

Acetylene cylinders need to be kept in an upright position primarily to prevent acetone from being carried out with the gas. Acetylene is dissolved in acetone within the cylinder to stabilize the gas and control the pressure. If the cylinder is tilted or laid down, the acetone can escape along with the gas during usage, which can lead to improper gas flow and potential safety hazards. Maintaining the cylinder upright ensures that only the gas is withdrawn, allowing for safer and more effective usage.