10.5 Gas Leak Detection, Odorization & Emergency Response

10.5 Gas Leak Detection, Odorization & Emergency Response

Quick Answer: Fuel gas must carry a distinctive odor detectable at one-fifth of the lower explosive limit — about 1.0 percent gas in air for natural gas and 0.42 percent for propane. IFGC 2018 Section 406.5.1 permits leaks to be located only with an approved gas detector, a noncorrosive leak-detection fluid, or other approved methods — never a flame.

Odorization: The One-Fifth Rule

Natural gas and propane leave the ground colorless, tasteless and effectively odorless. The rotten-egg smell is an added odorant, usually a mercaptan such as ethyl mercaptan or tertiary butyl mercaptan.

• 49 CFR 192.625(a) requires that a combustible gas in a distribution line contain a natural odorant or be odorized so that at a concentration in air of one-fifth of the lower explosive limit, the gas is readily detectable by a person with a normal sense of smell.
• NFPA 58 Section 4.2.1 applies the same standard to LP-gas: all LP-gases shall be odorized with a warning agent making them detectable by a distinct odor at a concentration in air of not over one-fifth the lower limit of flammability. Since the 2017 edition, NFPA 58 also requires the presence of odorant to be verified by sniff test or other means and documented before final delivery to the end user.

The arithmetic is worth doing once so the number sticks:

Natural gas (methane) LEL = 5.0 percent, so 0.20 x 5.0 = 1.0 percent gas in air
Propane LEL = 2.1 percent, so 0.20 x 2.1 = 0.42 percent gas in air

In other words, a customer should smell gas at roughly one-fifth of the way to an ignitable mixture — a designed-in safety margin, not a coincidence.

Odorant Fade

Odorant is not permanent, and every gas fitter is expected to know why.

• Adsorption. Mercaptan sticks to the bare interior surface of new steel pipe and new propane tank shells. This is the classic failure: a new or long-idle tank strips the odorant out of the vapor space, and a real leak produces no smell. NIOSH investigated a 2019 propane explosion that killed a firefighter in which odor fade was a key contributing factor.
• Oxidation. Rust, mill scale and moisture chemically consume the odorant.
• Migration. Gas leaking underground and passing through soil, concrete, drywall or plywood loses its odorant along the way — the reason an outdoor service-line leak can enter a house smelling of nothing.
• Human factors. Olfactory fatigue deadens the nose after a few minutes of exposure, and anosmia, hyposmia, colds, allergies, smoking, age and some medications blunt it entirely.

The remedy on new work is conditioning: saturating the new pipe or new container with odorized gas before it is put into service, and never relying on smell alone as the leak-detection method.

LEL, UEL and the Band You Never Stand In

A reading anywhere between LEL and UEL means an explosive atmosphere exists right now — evacuate, do not ventilate from inside, and do not operate anything electrical. A reading above the UEL is not safe either: opening a door dilutes that mixture straight back down through the flammable band.

Vapor density drives where you take the reading. Natural gas collects at ceilings, in attics and in the top of joist bays. Propane sinks into basements, crawl spaces, floor drains, sumps, elevator pits and window wells — a clean reading at chest height in a basement proves nothing about the floor.

Approved Leak-Detection Methods — and the One That Kills

IFGC 2018 Section 406.5.1 is short and absolute: leakage shall be located by means of an approved gas detector, a noncorrosive leak detection fluid, or other approved leak detection methods. Note the two adjectives that carry the weight.

• Approved gas detector. A handheld electronic combustible gas detector or combustible gas indicator (CGI) reading in percent LEL or percent gas by volume. For purging work, IFGC 406.7.1.4 requires a listed CGI that displays a volume scale from 0 to 100 percent in 1-percent or smaller increments.
• Noncorrosive leak-detection fluid. Use a listed leak-detection solution. Ordinary ammonia-bearing household cleaners are corrosive and can produce stress-corrosion cracking in brass fittings and attack the jacket of corrugated stainless steel tubing. Bubble solutions also stop working below freezing unless they are winter-rated.
• Other approved methods. Ultrasonic detection on high-pressure systems, or infrared imaging, where the authority having jurisdiction approves it.

No code anywhere lists a flame as a detection method. NFPA 54 Section 4.3.1 goes further and states that where work is being performed on piping that contains or has contained gas: smoking, open flames, lanterns, welding or other sources of ignition shall not be permitted; artificial illumination shall be restricted to listed safety-type flashlights and safety lamps; and electric switches shall not be turned on or off. A match, a lighter, a candle or a standard flashlight in a gas atmosphere is the ignition source, not the instrument.

Leak Check Versus Pressure Test — Two Different Operations

Candidates routinely answer a leak-check question with pressure-test numbers. They are separate procedures with separate media, pressures and timing.

Two related rules complete the picture. IFGC 406.1.2 allows minor repairs and additions made after the pressure test to skip a new pressure test, provided the work is inspected and the connections are tested with a noncorrosive leak-detecting fluid or other approved method. IFGC 406.1.3 requires that where a new branch is run to a new appliance, only the newly installed branch be pressure tested, with the tie-in connections to existing piping tested by leak-detection fluid. And IFGC 406.6.4 is the gate on the whole job: appliances shall not be placed in operation until the piping has been leak checked under 406.6.3, purged under 406.7, and the appliance connections themselves checked for leakage.

Exam Trap: A question asks what a gas fitter must do immediately after turning the gas on to a system restored after an interruption of service, and one option reads "test at 3 psig with air for 10 minutes." That is the pressure test of Sections 406.4.1 and 406.4.2, performed with an inert medium before gas is admitted. The required action after gas is on is a leak check with fuel gas at operating pressure, and if leakage is indicated the gas supply must be shut off until repairs are made.

Purging: Into Service and Out of Service

Purging replaces one atmosphere inside the pipe with another. Get it wrong and you manufacture a flammable mixture inside a pipe or inside a building.

IFGC 406.7.1 sends the job outdoors where either condition is met: the design operating gas pressure is greater than 2 psig, or the piping being purged contains one or more sections meeting the size and length criteria of Table 406.7.1.1.

A CSST EHD size of 62 is treated as equivalent to nominal 2-inch pipe.

• 406.7.1.1 Removal from service. Where existing gas piping is opened, the opened section is isolated and the line pressure vented per 406.7.1.3. Where piping meeting the table is taken out of service, the residual fuel gas shall be displaced with an inert gas — nitrogen, not air.
• 406.7.1.2 Placing in operation. Where piping containing air and meeting the table is placed in operation, the air shall first be displaced with an inert gas, and the inert gas then displaced with fuel gas. That inert "slug" is what keeps fuel and air from ever meeting in a large-volume pipe.
• 406.7.1.3 Outdoor discharge. The open end shall discharge directly to an outdoor location, and all five of these apply: the point of discharge is controlled with a shutoff valve; it is not less than 10 feet from sources of ignition, not less than 10 feet from building openings and not less than 25 feet from mechanical air intake openings; it is continuously attended and monitored with a combustible gas indicator; purging with fuel gas stops when 90 percent fuel gas by volume is detected in the pipe; and everyone not involved is evacuated from within 10 feet of the discharge.

IFGC 406.7.2 permits an indoor or outdoor purge only where both conditions hold: the design operating pressure is 2 psig or less and no part of the piping meets Table 406.7.1.1. Section 406.7.2.1 then allows five methods — discharge to the outdoors; discharge through an appliance burner that is not located in a combustion chamber and that has a continuous source of ignition; discharge through a burner designed for the purpose with a continuous source of ignition; discharge monitored with a listed combustible gas detector, stopping the moment fuel gas is detected; or purging by the gas supplier under written procedures.

Read that second method carefully: the burner must be outside the combustion chamber. Purging into a combustion chamber is prohibited, because unburned gas accumulating in a firebox and then meeting the igniter produces a delayed ignition or a flashback. Never purge into a confined space, crawl space, attic or duct either.

Finally, IFGC 406.7.3: after the piping system is placed in operation, appliances and equipment shall themselves be purged before being placed into operation. Purging is complete only when you have bled each appliance's manifold and pilot line, relit every pilot, and verified that every burner lights promptly and burns with a clean blue flame.

When You Find a Leak

1. Create no ignition source. Do not operate a light switch, thermostat, garage-door opener, doorbell, cell phone, standard flashlight or vehicle inside or beside the building.
2. Shut the gas off at the right valve. For an appliance leak, use the appliance shutoff valve — required within 6 feet of the appliance, in the same room (IFGC 409.5.1). For a house-line leak, use the house-line valve (409.3) or the meter valve on the supply side of the meter (409.2).
3. Evacuate before you investigate if the odor is strong, the source is unknown, or a detector reads inside the flammable band. Leave the door open behind you and move upwind.
4. Ventilate from outside by opening doors and windows. Remember that natural gas rises and propane pools low.
5. Call from a safe distance outside: 911 first, then the utility. In Maryland that is BGE at 1-800-685-0123 or Washington Gas at 844-WASHGAS (844-927-4427), option 1. For excavation damage, Miss Utility at 811. Do not re-enter to make the call.
6. Do not relight anything until the defect is repaired, the affected piping is retested, the system is purged and each appliance is checked.

Red-tagging. Where an appliance or system is unsafe — a leaking gas valve, a cracked heat exchanger, a blocked or disconnected vent, flue-gas spillage, an unapproved connector — the qualified technician shuts it off, applies a red condemnation tag stating the defect and the date, and notifies the owner and the serving utility. A licensed Maryland journey plumber and gas fitter may not leave a known hazardous condition energized, and removing another technician's red tag without correcting the defect transfers that liability squarely onto you.

Carbon Monoxide

Carbon monoxide (CO) is the product of incomplete combustion: colorless, odorless, non-irritating, and it binds to hemoglobin roughly 200 times more readily than oxygen does. Its symptoms — headache, fatigue, nausea, dizziness, confusion, then collapse — mimic the flu. The diagnostic clue is that symptoms improve when the occupants leave the building and return when they go back in.

Those thresholds tell you something practical: a UL 2034 residential alarm is a death-prevention device, not a health device. A furnace producing a steady 25 ppm in the living space will never set one off, which is why a combustion analyzer or a low-level monitor belongs in your bag. On a CO call, evacuate, call 911, ventilate, shut off the suspect appliance, get medical evaluation for anyone symptomatic, and find the actual cause — blocked vent, cracked heat exchanger, insufficient combustion air, or building depressurization — before restoring service.