Hayward S311SX ProSeries Plus Sand Filter, 31-Inch, Side-Mount Top diffuser ensures even distribution of water over the top of the sand media bed All internal piping is 2-Inch to give smooth, free-flowing performance Manual air relief purges any trapped air during operation of the filter system Unitized, corrosion-proof, spherical filter tank molded of rugged, colorfast reinforced thermoplastic for maximum strength and longer life Optional valve and coupling selections for plumbing ease and versatility