CPD Gas Piping Systems Design 2 — Questions and Answers
Question 1: Polyethylene (PE) gas piping installed underground must be installed at a minimum depth below finished grade to provide adequate protection. What is the typical minimum cover depth required?
- 6 inches
- 12 inches
- 18 inches (Correct answer)
- 24 inches
Correct answer: 18 inches
NFPA 54 and most codes require a minimum burial depth of 18 inches for plastic gas piping installed in private property to protect against accidental damage from digging and frost.
Question 2: Which code is the primary model code governing the installation of natural gas and LP-gas piping systems in the United States?
- NFPA 13
- NFPA 54 / ANSI Z223.1 (Correct answer)
- IPC Chapter 7
- ASME B31.1
Correct answer: NFPA 54 / ANSI Z223.1
NFPA 54, also known as ANSI Z223.1 National Fuel Gas Code, is the primary model standard governing fuel gas piping, equipment, and appliance installations in the US.
Question 3: Bonding of CSST (corrugated stainless steel tubing) gas piping is required primarily to protect against:
- Corrosion from soil contact
- Lightning-induced electrical surges that can perforate the tubing (Correct answer)
- Thermal expansion and contraction stresses
- High-pressure gas surges from the utility
Correct answer: Lightning-induced electrical surges that can perforate the tubing
CSST must be bonded to the electrical grounding system because lightning-induced transient electrical surges can arc through the thin stainless steel wall, causing pinhole perforations and potential gas leaks.
Question 4: An appliance requiring a gas supply pressure of 7 in. w.c. is connected to a medium-pressure branch line operating at 5 psi. Which component must be installed to reduce pressure to the appliance?
- A pressure relief valve
- A line pressure regulator (Correct answer)
- A solenoid shutoff valve
- An excess flow valve
Correct answer: A line pressure regulator
A line pressure regulator must be installed upstream of the appliance to reduce the medium distribution pressure (5 psi) to the low appliance inlet pressure (7 in. w.c.).
Question 5: When calculating the gas pipe size for a branch serving multiple appliances, the designer should use the:
- Sum of all connected appliance BTU/hr ratings with no reduction
- BTU/hr rating of only the single largest appliance on the branch
- Diversity-adjusted total load or the values from NFPA 54 sizing tables (Correct answer)
- Measured peak flow from an existing identical installation
Correct answer: Diversity-adjusted total load or the values from NFPA 54 sizing tables
Gas pipe branches are sized using either the diversity-adjusted connected load or the applicable sizing tables in NFPA 54, which account for pipe length, pressure drop, and specific gravity of the gas.
Question 6: LP-gas (propane) systems differ from natural gas systems in that LP-gas:
- Operates at lower pressures than natural gas at the appliance
- Is lighter than air and rises when leaked, reducing ceiling accumulation risk
- Has a higher specific gravity than air and accumulates in low areas when leaked (Correct answer)
- Requires larger diameter piping than natural gas for the same BTU load
Correct answer: Has a higher specific gravity than air and accumulates in low areas when leaked
LP-gas (propane) has a specific gravity greater than 1.0 relative to air, meaning it is heavier than air and will settle in pits, trenches, and low-lying areas when leaked, creating significant explosion hazards.
Question 7: Which type of valve is required by NFPA 54 to be installed on the gas supply to each appliance to allow for service and maintenance without shutting down the entire system?
- Ball cock valve
- Appliance shutoff valve (individual appliance valve) (Correct answer)
- Main service valve only
- Solenoid valve
Correct answer: Appliance shutoff valve (individual appliance valve)
NFPA 54 requires an individual appliance shutoff valve to be installed in the gas supply line to each appliance, accessible without removing permanent construction, to facilitate maintenance and emergency shutoff.
CPD Gas Piping Systems Design 2
Polyethylene (PE) gas piping installed underground must be installed at a minimum depth below finished grade to provide adequate protection.
What is the typical minimum cover depth required?