The name comes from the wing-like structure and action of the valve disk which is arranged on the pipe diameter principle. The circular shaped disk turns about a diametrical axis within the cylindrical bore of the valve body and a quarter turn rotation of the disk opens or closes the valve. The basic simplicity of the design provides a compact and relatively low weight valve having few component parts. The quarter turn action offers quick opening or closing with ease of operation and the valve has good flow control characteristics. In the wide open position the only obstruction to flow is that due to the thickness of the disk, so the pressure drop across the valve is small. During the closing movement the rate of cut-off of the flow diminishes as the disk moves towards the closed position, making the valve well suited for flow regulation purposes. Butterfly valves are usually either resilient or metal to metal seated. Resilient seated valves may have the resilient seating arranged in the body or attached to the periphery of the disk. Metal seated valves will normally provide longer life than resilient seated designs but are more difficult to make completely leak-tight. They are better suited for higher temperature duties as they do not have the temperature restrictions imposed by resilient seatings. An extensive range of synthetic elastomer and plastic materials for use as resilient seatings has been developed as a result of close co-operation between valve designers and specialists in the metallurgical, rubber, and plastics industries. Positive shut-off with repeatability of performance is assured and the wide choice of seating materials provides the butterfly valve with an expansive range of service applications. If the valve is required for flow control purposes it is essential that there should be correct selection of size and type to give satisfactory and effective operation. The design concept of the butterfly valve is particularly well suited to large size manufacture. Sizes up to 10 metres diameter have been produced and even larger sizes are presently being considered for tidal and ocean thermal energy conversion schemes. In general industrial applications butterfly valves have found wide acceptance in the oil, gas, chemical, water treatment and process industries and are used in the condenser and circulating water systems of thermal power stations.