Propane (LP) Gas Systems and Appliances
LP questions test the physical properties of propane, containers and regulators, pressure testing, appliance operation, and combustion safety. This is the domain where a wrong answer is also a real-world explosion risk, so expect specific numbers.
Properties and containers
- Commercial propane boils near minus 44 degrees F and is about 1.5 times as heavy as air, so it pools in low spaces.
- A 20 lb cylinder holds about 4.7 gallons. The 80 percent fill rule leaves vapour space for expansion, and the OPD stops filling at that point.
- DOT cylinders require periodic requalification; ASME tanks are fixed to the unit. Cylinders travel upright with the valve closed and never inside the living space.
- The relief valve protects against overpressure. Never plug, paint over, or cap it.
Regulation and distribution
- Two-stage regulation: the first stage drops container pressure to about 10 psi, the second delivers 11 inches of water column, roughly 0.4 psi, to the manifold.
- Manometer testing at 11 in. WC with appliance valves closed; a falling column indicates a leak. Soap solution confirms the location.
- Excess flow valves, pigtails, and ACME or POL fittings. Use LP-rated sealant on male threads only, leaving the first two threads bare.
Appliances
- Absorption refrigerators, furnaces, water heaters, ranges, and catalytic heaters each have an orifice sized for manifold pressure. Derate roughly 4 percent per 1,000 ft above sea level.
- Combustion air and venting are part of the appliance. A blocked flue or a missing vent cap spills CO into the coach.
Worked example
A 20 lb cylinder filled to the 80 percent limit holds about 16 lb of propane. At 4.24 lb per gallon that is 3.8 gallons, and at 91,500 BTU per gallon about 345,000 BTU. A 30,000 BTU/h furnace plus a 6,000 BTU/h water heater draw 36,000 BTU/h, so continuous operation lasts about 9.5 hours. That arithmetic explains the cold-weather call about a furnace that will not stay lit.
Traps
- Testing with a flame instead of a manometer or soap solution.
- Filling beyond 80 percent, which removes the expansion space.
- Mounting a propane detector high on the wall. Propane sinks, so detectors go low; CO detectors go at sleeping height.
- Wrapping PTFE tape on flare fittings. The flare seals on its cone, and tape can crack it.
- Accepting a regulator that reads correct pressure at lockup without checking flowing pressure.