Aquascape®️ 29098 3″ Dual Union Check Valve Assembly. The Dual Union 3″ CVA Check Valve effectively prevents backflow in ponds and water features while providing a professional pump and plumbing connection inside filters and waterfall vaults. The heavy-duty check valve flapper is built to last and maximizes water flow through the valve when open while preventing the backflow of water when the pump is shut off or during a power outage. Union connections on both sides of the valve make pump and filter maintenance quick and easy without the use of tools.