Gas Supply, Transmission, and Distribution Systems
This domain tests pressure management and physical behavior across transmission lines, distribution mains, services, and meters: MAOP, pressure regulation, class location, odorization, pipe sizing, and line pack.
Core rules
- MAOP is the maximum allowable operating pressure; every component, flange, valve, and regulator must be rated for it. Design factors fall as population density rises under class-location rules.
- Transmission pipelines normally use high pressure and large diameter; distribution systems step pressure down through city gate, district regulators, and service regulators.
- Pressure-relief and overpressure-protection devices must be sized for the largest credible supply failure, not just normal demand.
- Odorization is required in distribution so leaks are detectable at the LEL; odorant intensity must survive adsorption and distance.
- Pipe sizing balances allowable pressure drop against velocity, noise, and future load. Equivalent length includes fittings and valves.
Worked example
A 10 km, 0.50 m inside-diameter pipeline holds gas at 60 bar gauge. Physical volume V = πD²/4 × L = 0.7854 × 0.25 × 10,000 = 1,963 m³. Convert to standard volume at 1.013 bar and ignore temperature change: Vstd ≈ 1,963 × (61/1.013) = 118,200 m³ if Z = 1. At Z = 0.90, Vstd ≈ 131,300 m³. That line pack is why a transmission line can supply demand briefly after upstream isolation, and why depressurization takes time.
Traps
Mixing gauge and absolute pressure is the most common error; add atmospheric pressure before using gas laws. Ignoring compressibility Z overstates line pack at high pressure. Do not size a regulator only on flow: check inlet pressure, droop, lockup, and relief capacity.