An adjustable non-return ball valve for connection in a pipeline includes an elongate unobstructed main housing (14) arranged (16, 18) to be connected at each end to flow pipes (10, 12). There is an auxiliary housing (20) located intermediate the ends of the main housing (14) which has a longitudinal axis obtuse to the longitudinal axis of the main housing (14). There is a removable circular valve seat (38) extending across the main housing (14) at an obtuse angle relative thereto. The main housing (14) is formed in two sections (14a, 14b) which are joined together at the plane of the valve seat (38). A ball (24) is positioned to close upon the valve seat (38) and is loosely entrapped in the auxiliary housing (20) to allow flow in only one direction through the main housing (14), in which event, and in use, the ball (24) automatically rides up in the auxiliary housing (20) and away from the valve seat (38). The ball (24) and the auxiliary housing (20) having diameters greater than the diameter of the main housing (14), thereby preventing the ball (24) from moving through the main housing (14). The junction of the auxiliary housing (20) and the main housing (14) forms a ramp (34) to assist in moving the ball (24) into the auxiliary housing (20) in response to flow through the main housing (14). There is an adjustable, mechanically operable closure device (30) for the valve which fits at an upper end of the auxiliary housing (20) and is arranged to urge and hold the ball (24) downwards toward the valve seat (38) to throttle or close off the valve according to the setting of the closure device (30).