These valves regulate flow by means of an adjustable orifice which is not pressure compensated, and flow through the valve is entirely dependent upon pressure drop at any particular setting of the orifice. Dual service-line models with an integral non-return valve around each control orifice provide for meter-in or meter-out control; single line versions of these are available. For flow restriction in P or T lines (where reverse free flow is not required) models without check valves are available. Adjustment options are either screw/locknut or handknob