Dosing pump unit

A metering pump assembly for admixing into an exhaust gas flow a required demand for a fluid reduction agent, comprising:

• a metering pump which on an entry side is connected to a connection for connecting to a reduction agent tank, and at an exit side is connected to a pressure conduit as well as a return conduit to the reduction agent tank, the metering pump comprising an associated drive, by way of a control of the drive a quantity of the reducing agent delivered by the metering pump is set exactly to the required demand,
• a return valve biased against the fluid pressure produced by the metering pump, is arranged in the pressure conduit, the return conduit is connected to the pressure conduit in front of the return valve in the flow direction of the reduction agent, wherein a biasing force which keeps the return valve closed, is larger than a force acting on the return valve by way of a fluid pressure of an opened return conduit, and with a closed return conduit, the force acting on the return valve by way of the fluid pressure exceeds the biasing force of the return valve,
• a first shut-off valve for the selective closing of the return conduit, is arranged in the return conduit,
• a premixing device arranged downstream of the metering pump that mixes the reduction agent delivered by the metering pump with a pressurized gas, and
• a second shut-off valve to selectively interrupt the supply of pressurized gas to the premixing device is arranged on an entry side of the premixing device in a pressurized gas conduit leading to the premixing device, the second shut-off valve in the pressurized gas conduit is coupled to the first shut-off valve in the return conduit to close the return conduit when the pressurized gas conduit is open,
• wherein the first shut-off valve may be actuated by pressurized gas, and the pressurized gas conduit leading to the premixing device is connected behind the second shut-off valve in the flow direction to an actuation connection of the first shut-off valve,
• wherein the metering pump assembly is adapted to perform a starting operation during which the reduction agent is delivered by the metering pump through the return conduit back into the tank until the metering pump and the pressure conduit are completely filled with reduction agent and after complete filling, the return conduit is closed by the first shut-off valve, so that the reduction agent may no longer flow back into the reduction agent tank, and
• wherein the first shut-off valve is completely closed preventing the reduction agent delivered by the metering pump from returning to the reduction agent tank at all times when each of (i) the pressurized gas supply is connected to the entry end of the pressurized gas conduit, and (ii) the pressurized gas pressurizes the pressurized gas conduit and (iii) the reduction agent is being delivered to the premixing device simultaneously occur.