Sizing a gas distribution system, whether it is a low pressure or elevated pressure system, follows the same basic procedure. The procedure outlined in this section follows the recommendations in Appendix C of the National Fuel Gas Code. This method is the one most commonly used for sizing gas piping. However, it does not attempt to optimize tube size to minimize material cost. This method is simply a recommendation. More rigorous engineered sizing methods may be used, provided they are acceptable to the authority having jurisdiction.
First, a sketch of the planned system should be prepared showing the distance between connections and the individual gas load at each appliance. Gas demand, or input rating information, can be determined from the appliance rating plate or the appliance manufacturer. In cases where specific appliance load data are not available, consult the National Fuel Gas Code for generic appliance input load ratings. If Table 3 through Table 6 of this document are to be used to size the system, gas demand values should be calculated in terms of cubic feet per hour (CFH).
The design pressure of the system, allowable pressure drop and specific gravity of the gas supply should be determined. Also, the length of the piping should be measured from the point of delivery (the meter in a low pressure system or the elevated to low pressure (line) regulator upstream of the low pressure portion of an elevated pressure system) to the most remote outlet in the building. This "longest" length will be the only length used in determining the size of any section of the gas piping in this portion of the system.
In the appropriate capacity table ( Table 3 and 4 for low pressure portions of systems and Table 5 and Table 6 for elevated pressure portions), the row showing this "longest" length (or the row showing the next longer length if the table does not list the exact length) should be selected. If it was determined that the gas supplied has a specific gravity other than 0.60, on which the tables were based, it will be necessary to multiply the values in the selected row by the appropriate factor from Table 7.
The adjusted values in the selected row should be used to locate all gas demand values for the particular portion of the piping system. Then, starting at the most remote outlet or connection, find in the selected horizontal capacity row the gas demand required at that point. If the exact gas demand value is not shown, the next larger value in the same row (to the right) should be chosen. Above this demand value (at the top of the table) is the correct size of tube to be used in supplying this capacity. This method should be used to size each outlet and section of piping. For each section of piping, the total gas demand supplied by that section should be used to determine the tube size.
In the case of the elevated pressure system, this procedure should then be used to size the portion of the piping from the meter to the elevated to low pressure (line) regulator. The "longest" length used to determine the size of the piping in this section should be taken as the distance from the meter to the most remote elevated to low pressure regulator.