A pump apparatus including: a first pumping element comprising a first housing having a first material inlet, and a first discharge outlet to a first delivery line; a second pumping element comprising a second housing having a second material inlet, and a second discharge outlet to a second delivery line; a first venturi associated with the first housing; a second venturi associated with the second housing; a controller controlling actuators operating a first inlet valve on the first material inlet, a second inlet valve on the second material inlet, a first discharge valve on the first discharge outlet, a second discharge valve on the second discharge outlet, wherein the controller is configured to control the first inlet valve, the second inlet valve, the first discharge valve, and the second discharge valve such that when the first pumping element and the second pumping element are both in operation, the first pumping element and the second pumping element each alternate between a load phase and a discharge phase, wherein alternation between the load phase and the discharge phase by the first pumping element and the second pumping element is out of phase; and a compressed air supply delivering cyclically to the first venturi to reduce the first housing pressure for charging and to the first housing for pressure discharge, and to the second venturi to reduce the second housing pressure for charging and to the second housing for pressure discharge; characterised in that an air cycling valve is interposed in each venturi, each venturi being connected to an expansion conduit which in turn is connected to the respective delivery line (23, 24). a first pumping element comprising a first housing having a first material inlet, and a first discharge outlet to a first delivery line; a second pumping element comprising a second housing having a second material inlet, and a second discharge outlet to a second delivery line; a first venturi associated with the first housing; a second venturi associated with the second housing; a controller controlling actuators operating a first inlet valve on the first material inlet, a second inlet valve on the second material inlet, a first discharge valve on the first discharge outlet, a second discharge valve on the second discharge outlet, wherein the controller is configured to control the first inlet valve, the second inlet valve, the first discharge valve, and the second discharge valve such that when the first pumping element and the second pumping element are both in operation, the first pumping element and the second pumping element each alternate between a load phase and a discharge phase, wherein alternation between the load phase and the discharge phase by the first pumping element and the second pumping element is out of phase; and a compressed air supply delivering cyclically to the first venturi to reduce the first housing pressure for charging and to the first housing for pressure discharge, and to the second venturi to reduce the second housing pressure for charging and to the second housing for pressure discharge; characterised in that an air cycling valve is interposed in each venturi, each venturi being connected to an expansion conduit which in turn is connected to the respective delivery line (23, 24).