We are always looking for an easier way to do things. One example is trying to provide water pipe-sizing tables in the codes. While this seems easy enough, sizing tables do not always provide an accurate pipe size.
Usually this type of method will leave out one of the many necessary items needed to ensure accurate water pipe sizing. It would be very convenient to have this type of table and I hope that one day we can come up with an accurate method.
Instead, you must follow several steps to accurately size pipe.
First, call the water utility (or perhaps the fire department) to get both the static and residual pressures for the water main. The static is pressure at no flow and the residual is pressure at a 1 gal. per minute flow rate.
Once you know the static and residual pressure, you can plot the pressure of the system at the anticipated flow rate calculated for the building. When you have that information, you can begin your pressure loss calculations.
The pressure loss calculations use the system pressure available and subtract all known pressure losses for the water supply system. Among the first things to subtract from the available system pressure are the static or height losses to raise the water to the uppermost level of the building distribution system.
In order to convert from feet of height to pounds per square inch pressure you need to multiply the height times 0.433. This is a direct conversion derived from 1 psi being equal to 2.31 ft. of water column.
You must also know the losses for the fixture with the greatest demand at the farthest point in the system. Be aware that this may not necessarily be the highest fixture but the fixture with the greatest demand pressure. For instance, a lavatory or sink may be the highest fixture but a water closet located on a lower floor or closer to the main building riser would have a higher operating pressure.
Operating pressures of low-flush fixtures vary greatly according to type. A gravity-type water closet may require less pressure to operate than does a pressure-assisted model or even a flush-valve model. It is not uncommon for these types of fixtures to require a minimum of 25 psi in order to function properly.
Consider losses through the water meter assembly. This is usually obtainable from the water utility company or from the manufacturer of the water meter. You need to know the demand of the building and the size of the water meter being installed to determine the pressure loss through the meter.
Find out whether a backflow prevention device is required on the water service. Be careful on what type of backflow prevention device is installed and its pressure vs. flow characteristics.
Water softeners, filters and other conditioning equipment will also affect system pressure. Moreover, you may be required to install an additional backflow preventer on the inlet to the water softener or other equipment. Sometimes this double loss through two backflow prevention devices may mean you need to install a pressure booster pumping system.
Once you have subtracted all of the known pressure losses, the remainder of the pressure can be used to overcome the friction loss in the piping system. Find the longest run of pipe and divide by the available pressure. That will give you the amount of friction loss per foot of piping to be used to calculate the pipe size.
Sometimes you can avoid an expensive pressure booster system if you increase the pipe diameter, thereby lowering the friction loss.