Valve assembly

A method of assembling a valve sub-assembly, comprising the steps of:

• providing an inner valve member comprising an inner concentric valve seat;
• providing an outer valve member comprising an outer concentric valve seat;
• arranging the inner valve member so as to position the inner concentric valve seat radially inward of the outer concentric valve seat to provide an annular flow opening therebetween;
• radially aligning the inner concentric valve seat relative to the outer concentric valve seat to maintain concentricity thereof;
• axially aligning the inner concentric valve seat relative to the outer concentric valve seat along an axis, the axis being substantially orthogonal to a plane of the concentric valve seats;
• providing a radial alignment means of the assembly, to maintain concentricity of the inner concentric valve seat relative to the outer concentric valve seat;
• providing an axial alignment means of the assembly, to maintain axial alignment of the inner concentric valve seat relative to the outer concentric valve seat along an axis;
• such that the inner valve member is a separate component from the outer valve member and is mounted in fixed relation to the outer valve member via mounting means, comprising the axial and radial alignment means;
• providing a moveable plunger, being moveable between a closed position and an open position to vary a flow restriction at the valve seat, and arranged to simultaneously open and close the inner concentric valve seat and the outer concentric valve seat when moving between the closed position and the open position;
• and
• aligning the moveable plunger relative to the concentric valve seats such that the moveable plunger simultaneously seals the inner concentric valve seat and the outer concentric valve seat when in the closed position.