CPD Gas Piping Systems Design 1 — Questions and Answers
Question 1: According to NFPA 54, what is the minimum required distance between an above-grade outdoor gas meter and a source of ignition such as an electrical panel?
- 3 feet (Correct answer)
- 5 feet
- 10 feet
- 15 feet
Correct answer: 3 feet
NFPA 54 requires gas meters to be located at least 3 feet from sources of ignition, including electrical panels and switches.
Question 2: A sediment trap (drip leg) installed in a gas supply line should be located:
- Downstream of the appliance shut-off valve, immediately upstream of the appliance inlet (Correct answer)
- Upstream of the gas meter at the service entrance
- At every change in pipe direction throughout the distribution system
- Only on high-pressure gas lines above 2 psi
Correct answer: Downstream of the appliance shut-off valve, immediately upstream of the appliance inlet
A sediment trap must be installed immediately upstream of the appliance inlet connection, downstream of the individual appliance shut-off valve, to catch debris before it enters the appliance.
Question 3: When sizing a natural gas piping system, the designer must account for the total connected load in BTU/hr. Which method is commonly used when diversity of gas usage is expected in a large system?
- Maximum demand method using 100% of all connected appliances
- Diversity factor method applying a usage factor less than 1.0 (Correct answer)
- Pressure drop method based only on peak single-appliance load
- Static pressure method ignoring flow velocity
Correct answer: Diversity factor method applying a usage factor less than 1.0
The diversity factor method applies a usage factor less than 1.0 to total connected load because not all appliances operate simultaneously in large systems, resulting in a more economical pipe sizing.
Question 4: What is the maximum allowable pressure drop permitted in a low-pressure natural gas distribution system (under 2 psi) supplying residential and light commercial appliances, per NFPA 54?
- 0.5 in. w.c.
- 1.0 in. w.c. (Correct answer)
- 3.0 in. w.c.
- 6.0 in. w.c.
Correct answer: 1.0 in. w.c.
NFPA 54 limits pressure drop to 0.5 in. w.c. for the supply piping system and an additional 0.5 in. w.c. for the longest run, totaling no more than 1.0 in. w.c. from the meter to the appliance for low-pressure systems.
Question 5: Which gas pipe material is prohibited for use inside a building above ground, per most model codes?
- CSST (corrugated stainless steel tubing)
- Black steel pipe
- Copper tubing (Type K or L)
- Cast iron pipe (Correct answer)
Correct answer: Cast iron pipe
Cast iron pipe is not approved for gas piping systems in any location; approved materials include steel, wrought iron, CSST, copper, and certain PE plastics (for underground only).
Question 6: Per NFPA 54, a gas appliance flexible connector used to connect a range to the gas supply line must not exceed:
- 18 inches in length
- 3 feet in length
- 6 feet in length (Correct answer)
- 10 feet in length
Correct answer: 6 feet in length
NFPA 54 limits appliance flexible connectors to a maximum length of 6 feet, and they must not be concealed within or extend through walls, floors, or ceilings.
Question 7: When pressure-testing a new gas piping system before placing it in service, what is the minimum test pressure and duration required by NFPA 54 for systems operating at 14 in. w.c. or less?
- 3 psi for 10 minutes (Correct answer)
- 1.5 times operating pressure for 15 minutes
- 6 in. w.c. for 5 minutes
- 10 psi for 30 minutes
Correct answer: 3 psi for 10 minutes
NFPA 54 requires a minimum test pressure of 3 psi (or 1.5 times the maximum operating pressure, whichever is greater) held for not less than 10 minutes for low-pressure systems.
CPD Gas Piping Systems Design 1
According to NFPA 54, what is the minimum required distance between an above-grade outdoor gas meter and a source of ignition such as an electrical panel?