Abstract: A liquid injection arrangement for inserting a foaming agent into water for a fire fighting application, or a similar use, requires that metered quantities of a liquid chemical foamant or other agent be injected into a conduit assembly conveying a water stream. A desired concentration of foamant requires varying the foamant flow as a function of water flow. A tank holds a quantity of liquid chemical foamant, and at least one pump has an inlet port coupled to the tank and an outlet port coupled to the conduit assembly. A hydraulic motor drives the pump and a valve is coupled with the hydraulic motor and a source of pressurized hydraulic fluid. A sensor is placed adjacent to the outlet port of the pump to feed back a signal representing the pressure of the water flowing through the conduit assembly, and a control system is responsive to the signal and to a flow rate of water flowing through the conduit assembly, producing a signal for controlling a speed at which the hydraulic motor drives the foamant pump.