How do I size the inlet and outlet headers for a softener or filter?
For simplex systems you should match the softener inlet / oulet piping size. For twin alternating systems you should match the softener inlet / oulet piping size. For triplex at N+1 size the header to match two of the units. So if you have a triplex with 3″ piping, size the header so that the cross sectional area is the same as two 3″ pipes at 7.07 in2 each or 14.14 in2 total. Calculating the minimum diameter gives D = Sqrt((4/PI)*14.14) = 4.24 inches. The veloicity of flow in a 4″ pipe at 350 gpm is 8.9 fps so this would not be an unreasonable choice. A 6 inch diameter pipe would assure full flow and can supply all three units making higher flows available. Answer: 4″ is adequate, 6″ provides for expansion.