This Pentair air relief valve is made of nylon. They are used on the top of the pool filter tank (and often are referred to as the pressure-relief valve). This valve helps to relieve pressure that builds up inside the filter. By opening up, these types of valves are designed to allow excess pressure to escape. At a normal pressure level the valve should close again.
Replacement part for Pentair SM and SMBW 2000 Filters.