These are Hayward WCV Series Wafer Check Valves. They are designed to fit between two standard flanges and are suitable for vertical or horizontal mounting. Normal sizes are 2” to 12” They are PVC bodied with EPDM seals and 316 stainless steel springs. All wafer check valves should be installed a minimum of 5 X pipe diameter downstream of pumps, valves or elbow. Spacers are not required with these to allow full disc opening. The pressure needed to open with spring for all sizes is 0.5 PSI.