Code Stories · why the rule exists
Gas: Sediment Traps (Drip Legs)
The rule in Massachusetts today
No local verbatim NFPA 54 text (the inspectapedia file is the front matter only). 248 CMR 5.09 was read in full and amends 9.1.1, 9.1.1.1, 9.1.1.3, 9.1.1.4, 9.1.19.1, 9.1.22, 9.1.23, 9.2.1, 9.6.1.4, 9.6.3.5 and 9.6.8, not 9.6.7. Rule as tested in the bank (q17): a sediment trap is required ahead of the appliance, as close to the inlet as practical, unless it is incorporated in the appliance; illuminating appliances, ranges, clothes dryers, decorative vented appliances, gas fireplaces and outdoor grills are exempt.
Why it exists
Gas piping can carry pipe scale, cutting chips, thread-sealant bits, dirt and sometimes moisture. At the tee, the gas turns 90 degrees while heavier particles keep going straight down into the capped leg, because of their momentum and gravity. That keeps grit out of gas valve seats, regulators and burner orifices, where it can cause leaks or poor combustion. The older idea behind 'drips' comes from manufactured coal gas, which carried a lot of moisture that condensed and ran to low points in the piping.
How it came to be
- pre-1950s (manufactured gas era) — Drip pots at low points collected condensed moisture from manufactured gas (as described in a 1981 patent). patents.google.com
- 2012 — The National Fuel Gas Code still treats the two together under one section heading. inspectapedia.com
Make it stick
Mnemonic: 'Gas turns the corner; dirt goes straight down.' The trap only works if the gas has to change direction at the tee. A plain elbow with a nipple hanging off it does not.
On the exam
Expect: correct configuration (tee with gas entering from above and turning horizontally, capped nipple below), location (downstream of the appliance shutoff valve, as close to the appliance inlet as practical, where one is not built into the appliance; NFPA 54-2012 9.6.7, not amended by 248 CMR 5.09), and the exempt appliances (illuminating appliances, ranges, clothes dryers, decorative vented appliances for fireplaces, gas fireplaces and outdoor cooking appliances).
What people get wrong
- Any capped nipple on the gas line is a drip leg. Without the change of direction at a tee, debris is not separated.
- Natural gas is clean, so sediment traps are obsolete. Construction debris and scale still get into piping. Codes still require them at most appliances.
Try it: 2 real practice questions
Q1. Under NFPA 54-2012 as adopted in Massachusetts, which appliance is NOT required to have a sediment trap at its inlet?
NFPA 54 requires a sediment trap ahead of an appliance, as close to its inlet as practical, unless one is built into the appliance. It exempts illuminating appliances, ranges, clothes dryers, decorative vented appliances, gas fireplaces and outdoor grills. Furnaces, boilers and water heaters are not exempt, because debris could foul their controls.
NFPA 54-2012 9.6.7Q2. The purpose of a sediment trap (drip leg) at an appliance is to:
At the tee the gas turns, but heavier moisture, scale and pipe dope keep going straight down into the capped nipple by momentum and gravity. That keeps debris out of the gas valve and burner orifices, where it could make the valve stick or burners misfire; the drip leg has no effect on pressure.
248 CMR 4.00These come from the PressureTest bank. Start 7 days free for the full bank, timed exam simulations and drawn isometric problems.