Internal leak detection and backflow prevention in a flow control arrangement

An apparatus for reducing backflow and detecting leaks in an interconnected pressurized liquid delivery system, the apparatus comprises:

• a first supply inlet adapted to be connected to a first supply source for a first liquid;
• a first supply line connecting the first supply inlet to a point of use outlet, the first supply line comprising a first valve, a first vessel, and a second valve in that order from the first supply inlet to the point of use outlet;
• a second supply inlet adapted to be connected to a second supply source for a second liquid;
• a second supply line connecting the second supply inlet to the point of use outlet, the second supply line comprising a third valve, a second vessel, and a fourth valve in that order from the second supply inlet to the point of use outlet;
• a sweep gas inlet adapted to be connected to a sweep gas source, the sweep gas being inert relative to the first liquid and the second liquid and relative to processes served by the liquids;
• a vent line connecting the sweep gas inlet to a vent outlet, the sweep gas flowing at a constant flow rate from the sweep gas inlet to the vent outlet via the vent line;
• a first vent valve connecting the vent line with the first vessel;
• a second vent valve connecting the vent line with the second vessel;
• a flow switch disposed on the vent line between said two vent valves and the vent outlet for sensing fluid that flows through the vent line into the vent outlet, the flow switch configured to actuate responding to fluid passing through the vent line into the vent outlet exceeding the constant flow rate, wherein the fluid comprises the sweep gas and any leaked liquid;
• and
• a control system that:
• in a first liquid supply state, opens the first valve, the second valve, and the second vent valve, and closes the third valve, the fourth valve, and the first vent valve, the liquid flowing from the first supply inlet to the point of use outlet via the first valve, the first vessel, and the second valve, in that order
• in a second liquid supply state, opens the third valve, the fourth valve, and the first vent valve, and closes the first valve, the second valve, and the second vent valve, the liquid flowing from the second supply inlet to the point of use outlet via the third valve, the second vessel, and the fourth valve, in that order, and
• in a stop state, closes the first valve, the second valve, the third valve, the fourth valve, and opens the first vent valve and the second vent valve such that the flow of sweep gas from the sweep gas inlet to the vent outlet sweeps out liquid in the first vessel and in the second vessel via the vent outlet.