Fuel-Gas Pipe Capacity (Spitzglass)
The most gas a bore carries within an allowable pressure drop.
Example
You enter
- Allowable pressure drop (in w.c.) 0.5
- Pipe inside diameter (in, actual bore) 1.049
- Pipe length (ft) 100
- Gas specific gravity 0.6
You get
- Max flow 172.9
- Velocity at that flow 480 fpm
Checked against published worked examples from: Spitzglass low-pressure gas-flow equation (public), solved for flow.
Details, formula, and sources
ScopeQ = 3550 sqrt(dH D^5 / (SG L K')), K' = 1 + 3.6/D + 0.03 D. A 1 in bore (ID 1.049), 100 ft, 0.6 SG natural gas at a 0.5 in w.c. allowable drop carries 173 CFH at 480 fpm. A longhand alternative to the NFPA 54 / IFGC capacity tables; the actual bore, not nominal, and NFPA 54 governs. Flags a drop above the ~1.5 psi low-pressure range.
Formula or tableSpitzglass low-pressure solved for the flow: Q = 3550 * sqrt((dH * D^5) / (SG * L * K')), K' = 1 + 3.6/D + 0.03*D. dH (in w.c.), D (in actual bore), L (ft), SG. Velocity from Q and bore area.
Source and editionPublished Spitzglass low-pressure gas-flow equation (public engineering formula); the inverse of the gas-pipe pressure-drop tile and a longhand alternative to the NFPA 54 / IFGC capacity tables, by name. Spitzglass reads 12-20% under the IFGC Eq. 4-1 tables at house sizes (3/4 in, 50 ft, 0.5 in w.c.: 122 CFH against the table's 151), so it errs toward a larger pipe; the tables govern.
Free accessNFPA 54 free read-only at nfpa.org/freeaccess and codes.iccsafe.org; NFPA 54 governs the installation.
What governsEstimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
API field namesdrop_inwc, id_in, length_ft, sg, flow_cfh, velocity_fpm
- Inside diameter actual bore, not nominal pipe sizeSpitzglass equation
- Low-pressure regime valid to ~1.5 psi (41.5 in w.c.); a larger allowable drop is flaggedSpitzglass equation