Abstract: A valve casing is provided with a part-spherical interior sealing surface and with three valve ports each open from the outside of the casing and through the part-spherical sealing surface thereof. A part-spherical valve body or ball is mounted in concentric very slightly spaced relation to this sealing surface of the valve casing. The valve body carries a valve seal structure and is provided with a valve body passage extending through the valve body. The valve body is rotatable within the valve casing between: (1) a first position wherein said valve body passage provides a passageway between a first and a second of the ports and said valve seal structure is in sealing relation to the valve casing sealing surface around a third of said ports; and (2) a second position wherein said valve seal structure is in sealing relation around said first port and said valve body passage provides a passageway between said second and third ports.