The two major factors that dictate pipe sizing are 1.friction/metre run 2.velocity
The limits for above parameters are decided based on issues such as total pipe run,annual operating hours,operating cost and initial investment cost.A general guide for water services pipe would be a maximum of 5 m/100 metre run of pipe for the friction rate and a maximum flow velocity of 3 m/s.
Generally pipes of diamemetre lower than 6" are selected based on friction rate and the velocity automaticall y falls into the acceptable range.For the larger diameter pipes this does not hold good.The selection is often based on flow velocity.Large pipes as a result have very low friction drops as compared to smaller dia pipes.
An important factor in hot water heating & chilled water piping is velocity noise in occupied areas. The pressure drop and related velocity that can be easily tolerated in process areas, produces a background noise level in offices, apartments, hotel rooms, etc. will generate no end of complaints from the occupants of those spaces.
For noise concern we typically limit velocity to not exceed 6 FPS and pressure drop not to exceed 4 ft wg./100 ft of pipe. In mechanical rooms & where noise is not a concern we select piping for 6 to 8.6 FPS velocity. If operating hours would only be 2000 hours/yr or less, we can push piping up to 12 FPS providing noise will not be a concern. At this high velocity, pipe erosion can be a concern. If piping is old and corroded I would recommend not going over 6 FPS.
