Low-Pressure Fuel-Gas Pressure Drop (Spitzglass)
Pressure drop (in. w.c.) and velocity for low-pressure fuel gas via the Spitzglass equation.
Example
You enter
- Gas flow (CFH) 1000
- Pipe inside diameter (in, actual bore) 1.049
- Pipe length (ft) 100
- Gas specific gravity 0.6
You get
- Pressure drop 16.73
- Velocity 2777 fpm
Checked against published worked examples from: Spitzglass low-pressure gas-flow equation (public).
Details, formula, and sources
ScopeWith a flag when the drop exceeds the low-pressure validity range.
Formula or tableSpitzglass low-pressure: Q = 3550 * K * sqrt((dH * D^5) / (SG * L)), K = 1/sqrt(1 + 3.6/D + 0.03*D). Solved for dH given Q (CFH), 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 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 namesflow_cfh, id_in, length_ft, sg, drop_inwc, velocity_fpm
- Inside diameter actual bore, not nominal pipe sizeSpitzglass equation