A UPVC double union foot valve is a specialized type of check valve, typically installed at the inlet of a pump's suction line, to prevent backflow and maintain the pump's prime. The "double union" design allows for easy installation, disassembly, and maintenance without cutting the pipe. 

Features and how it works

• UPVC construction: The valve is made from unplasticized polyvinyl chloride (UPVC), a durable, lightweight, and corrosion-resistant plastic.
• Strainer: A mesh screen at the valve's inlet prevents large debris and particles from entering and damaging the pump.
• Double union: Union connectors are located on both ends of the valve. These consist of a union nut that screws onto the valve body, connecting it to the pipe. This allows the valve to be removed for maintenance by simply unscrewing the nuts, which is a major advantage over permanently glued valves.
• Check valve mechanism: Inside the valve, a disc or a ball (depending on the design) sits on a seat.
  • Flowing: When the pump is operating, the suction pressure forces the disc or ball off its seat, allowing fluid to flow in one direction.
  • Preventing backflow: When the pump is turned off, the spring-assisted mechanism or the force of gravity pushes the disc or ball back against the seat, forming a tight seal that prevents fluid from flowing backward.
• Full bore flow: In some double union ball valve designs, the internal mechanism allows for a full bore flow when open, meaning there are no obstructions to catch debris and clog the valve. 
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Applications

UPVC double union foot valves are suitable for a wide range of applications that involve pumping liquids from a lower source, including: 

• Water supply systems: Wells, boreholes, and reservoirs.
• Agricultural irrigation: Maintaining pump prime for consistent water delivery to crops.
• Wastewater treatment: Controlling the flow of sewage and other wastewater.
• Industrial and chemical processing: Handling various liquids, including concentrated acids and chemicals.
• Aquaculture and pisciculture: Water systems for fish farming.
• Swimming pool filtration: Ensuring proper water circulation. 
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Installation tips

• Install the valve at the submerged end of the suction pipe, fully below the water line.
• Use the union connectors to your advantage. If using threaded connections, apply Teflon tape to the male threads.
• If using solvent cement connections, follow the correct procedure:
  • Apply primer and cement to both the pipe and the valve socket.
  • To prevent cement from dripping into the operating parts of the valve, install it upright or remove the union ends and glue them to the pipe separately.
  • Hand-tighten union nuts and avoid using tools.
• Always check the flow direction arrow on the valve body and install it correctly to prevent internal parts from blowing out under pressure.