ASME Vessels. Generally refers to pressure vessels and alloy high-pressure vessels, the design of which conforms to specifications, materials, and requirements of pressure vessel code adopted by the American Society of Mechanical Engineers. A pressure vessel is a closed container designed to hold gases or liquids at a pressure that is different from the ambient pressure. These vessels, usually cylindrical, have a body with end caps, called heads, fitted to the ends of the body. Pressure vessels are designed to operate safely at a specific pressure and temperature, referred to as the "Design Pressure" and "Design Temperature".