Propane Two-Stage Regulator Capacity

Checked at the tank pressure that actually matters. The load converts to a volumetric demand at the fuel's energy content, and every stage has to pass all of it. THE SIZING TRAP IS INLET PRESSURE. A regulator's capacity is a function of the pressure across it, so a first stage that comfortably passes the load with a warm tank on a summer afternoon may not pass it with a cold tank on a January morning -- and tank pressure follows liquid temperature. First stages are sized at the MINIMUM expected tank pressure for exactly that reason, and this compares the same regulator at both so the fall is a number rather than a caution. The capacities themselves are entered from the manufacturer's table, because they depend on the spring, the orifice and the outlet setting and no generic relation covers a table the maker publishes. THE SECOND STAGE IS EASIER because its inlet is regulated at about 10 psig year-round, so its capacity does not move with the weather. Its constraint is LOCK-UP: every regulator lets outlet pressure rise slightly at zero flow, and if that pressure exceeds what the appliance or the downstream stage can take, the system is unsafe AT IDLE rather than at full fire -- the opposite of where anyone looks. WHY TWO STAGES EXIST is worth stating, because collapsing them looks like a simplification: carrying gas at 10 psig lets the interconnecting pipe be far smaller than carrying it at 11 inches of water column, so the first stage sits at the tank, the second at the building, and the long run between is small pipe. One regulator at the tank means the whole distance at 11 in wc and several pipe sizes more. AND THIS FAILS ON THE SAME MORNING AS THE TANK. Regulator capacity falls with tank pressure, tank pressure follows liquid temperature, and vaporization capacity falls with the same cold on the same drawn-down tank -- two causes that look identical at the appliance, separated by reading the tank pressure. This compares entered capacities against a computed demand. It does not read a capacity table or predict capacity at an unlisted inlet pressure, size the piping at either pressure (the gas pipe sizing calculation does that, and the two pressures size separately), predict the minimum tank pressure from the weather, select regulators or vent limiters, address regulator venting, which has its own location requirements, or determine what any standard requires. NFPA 58, the adopted fuel gas code, the regulator manufacturer's capacity tables, and the AHJ govern.

required flow = connected load / the fuel's energy content per cubic foot, compared against regulator capacities entered at each inlet pressure; lock-up is compared against the downstream rating.

Regulator capacities are ENTERED from the manufacturer's tables: capacity depends on the spring, the orifice and the outlet setting, and no generic relation covers a table the maker publishes.

One division and four comparisons.

Estimate. AHJ and licensed professional govern.

Field names used by the API: connected_load_btuh, btu_per_ft3, capacity_at_min_inlet_cfh, capacity_at_max_inlet_cfh, second_stage_capacity_cfh, lockup_psig, downstream_rating_psig, required_cfh, min_inlet_margin_cfh, min_inlet_pct, max_inlet_margin_cfh, capacity_fall_pct, second_stage_margin_cfh, lockup_margin_psig

• Capacities are entered read from the manufacturer's table at each inlet pressurethe regulator manufacturer
• The minimum tank pressure is entered it follows liquid temperature and is not predicted herethe design low ambient
• Piping is sized separately the two pressures size separatelythe gas pipe sizing calculation