Journeyman Plumber ExamGas PipingHard
A plumber is installing a natural gas line for a new commercial kitchen. The gas meter is located 80 feet from the kitchen, and the total load for all appliances is 350,000 BTU/hr. The gas supply pressure is 0.5 psi (14 inches water column). Using a pressure drop of 0.5 inches water column, what is the minimum nominal pipe size required for black iron pipe if the specific gravity of the gas is 0.60?
- A1-1/2 inch
- B2-1/2 inch
- C2 inch
- D1-1/4 inch
Show answer & explanationAnswer & explanation
Correct answer: C. 2 inch
To size this pipe, you would refer to a gas pipe sizing table (e.g., from NFPA 54/IFGC) for 0.5 psi supply pressure, a 0.5 inch water column pressure drop, and a specific gravity of 0.60. For 350,000 BTU/hr over 80 feet, a 2-inch pipe is required. A 1-1/2 inch pipe typically handles less than 300,000 BTU/hr at this distance and pressure drop, while a 2-inch pipe can handle well over 350,000 BTU/hr.
Why the other options are wrong
- A. This size is too small for the given BTU load and distance, likely resulting in excessive pressure drop.
- B. While this size would work, it is larger than the minimum required and thus less cost-effective.
- D. This size is too small for the given BTU load, distance, and pressure drop.
Gas Pipe Sizing (Medium Pressure)
Sizing gas piping for medium pressure (0.5 psi to 5 psi) systems involves using specific tables that account for supply pressure, pressure drop, specific gravity, BTU load, and pipe length.
- Tables are different for low (less than 0.5 psi) and medium (0.5-5 psi) pressure systems.
- Pressure drop is a critical factor in sizing.
- Specific gravity of the gas affects flow capacity.
Memory trick: Pressure, Flow, Length, and Gas – Don't forget the Gravity, alas!