Self cleaning valve

A fluid control valve having an actuator, a valve seat, a circular closure member cooperating with said valve seat to control fluid flow through the valve, a spindle turned by said actuator and provided with an externally threaded section and having said closure member at its lower end, a sleeve turned at its upper end by said actuator and rotating said spindle which extends into its lower end, a formation on the upper end of said spindle engaging the interior of said sleeve which is shaped to allow said formation to slide axially in said sleeve without relative rotation occurring therebetween, a housing provided with upper and lower tubular coaxially arranged and spaced casings one of which contains said sleeve and the other of which surrounds said threaded section of said spindle, an internally threaded collar screwed onto said threaded section of said spindle and having a part which engages with the surrounding casing in a way which constrains said collar to move axially but without rotation in the surrounding casing when the spindle is rotated by the sleeve, a flange extending radially outwards from said collar between said two casings with sufficient clearance to allow axial movement of said collar in a direction away from said valve seat, an adjustment nut screwed onto a threaded section of one of the surrounding casings, and a coil spring coaxially surrounding the casing carrying the nut and engaging said nut at one end and said flange at the other end to exert a resilient bias urging said spindle towards said valve seat while yielding resiliently to allow movement of said collar in a direction away from said valve seat.