Per AGA, 49 CFR 192, NFPA 54
Service line sizing methods, Weymouth and Spitzglass equations, fitting equivalent lengths
Gas service line sizing uses the Weymouth or Spitzglass formula to calculate the minimum pipe diameter that delivers the required gas flow at acceptable pressure drop. The design must ensure adequate pressure at the meter set for all connected appliances at peak demand conditions.
The Spitzglass formula calculates gas flow capacity in low-pressure distribution piping (below 1 psig) based on pipe diameter, length, pressure drop in inches of water column, and gas specific gravity. It is the standard method for residential and small commercial service line sizing.
Typical design pressure drop for residential service lines is 0.5 to 3 inches of water column for low-pressure systems and 10-20% of supply pressure for medium-pressure systems. The design must maintain minimum appliance inlet pressure at maximum demand.
Service line capacity depends on pipe diameter, length, inlet and outlet pressures, gas specific gravity, number and type of fittings (expressed as equivalent length), and elevation changes. Larger diameter and shorter length increase capacity, while more fittings and higher specific gravity reduce it.