is used the flow line can be connected by means of a conduit 12 to a pressure regulator 14 the pressure regulator is connected by means of a conduit 15 to a valve 16 the valve and the injection pump are connected by means of a conduit 18 in the preferred embodiment the injection pump has a pumping mechanism that comprises a piston 22 and an injection or pumping plunger 24 the power or downward stroke of the pumping mechanism results from intermittent application of a fluid pressure from the valve 16 into a piston chamber 28 the return or upward stroke of the pumping mechanism is caused by spring 26 when the pressure in the piston chamber is vented the cycling of the pumping mechanism draws additive fluid into the injection pump through conduit 46 from supply reservoir 48 and forces it out of the injection pump through conduit 44 into main process flow line 10 the upward movement of the pumping mechanism is limited by an adjustable restriction device 50 referring to fig2 the adjustable restriction device comprises a stroke adjuster 51 in the preferred embodiment a threaded stud screwed into the piston chamber 28 the movement of the pumping mechanism can be further restricted in addition to that depicted by screwing the stroke adjuster.