10.4 Gas Meters, Regulators, Pressure Levels & Overpressure Protection

10.4 Gas Meters, Regulators, Pressure Levels & Overpressure Protection

Quick Answer: Inside a building, IFGC 2018 Section 402.7 caps the maximum operating pressure at 5 psig unless one of seven listed conditions is met, and Section 416.1 requires overpressure protection devices wherever the serving gas supplier delivers gas at greater than 2 psi to piping serving appliances designed for 14 inches w.c. or less.

Gas Appliances and Piping is 40 of the 100 scored items on the Maryland Journey Plumber/Gas Fitter exam. Metering and pressure control sit right at the boundary between what the utility owns and what you are licensed to install, and PSI writes questions on exactly that boundary.

The Gas Service: Where the Utility Stops and Your Work Starts

Gas travels from a distribution main in the street, through a service line, to a service pressure regulator — a regulator installed by the serving gas supplier to reduce and limit service-line pressure to delivery pressure. Downstream of that regulator sits the meter set: the meter, its shutoff valve, the riser and the connection to the customer's house line.

IFGC 2018 Section 401.1 fixes the code's reach precisely: it applies from the point of delivery to the connections with the appliances. NFPA 54 defines the point of delivery as the outlet of the service meter assembly; where no meter is installed, the outlet of the service regulator or service shutoff valve; and for undiluted LP-gas, the outlet of the final pressure regulator. Everything upstream belongs to the utility or the propane supplier and is governed by federal pipeline rules (49 CFR Part 192), not by the IFGC.

Three meter-related code rules are heavily tested:

• IFGC 409.2 — Meter valve. Every meter shall be equipped with a shutoff valve located on the supply side of the meter.
• IFGC 401.6 — Interconnections. Where two or more meters are installed on the same premises but supply separate consumers, the piping systems shall not be interconnected on the outlet side of the meters.
• IFGC 401.7 — Piping meter identification. Piping from multiple-meter installations shall be permanently marked by the installer so the system supplied by each meter is readily identifiable. Section 409.3.3 adds a matching tag on each house-line shutoff valve.

Diaphragm Meters and How to Read the Index

A residential gas meter is a positive-displacement diaphragm meter. Two synthetic-rubber diaphragm chambers alternately fill and empty; slide valves switch the flow between them, and a tangent-and-crank linkage converts that reciprocating motion into rotation of the index. Every revolution therefore represents a fixed, known volume — which is why a meter can be used as a field measuring instrument, not just a billing device.

The index is the set of dials or the odometer wheel that records total consumption. On a dial index, the hands are typically marked 1,000, 10,000, 100,000 and 1,000,000 cubic feet, and adjacent dials rotate in opposite directions. Read left to right, and when a hand sits between two numbers, record the lower number. The utility converts cubic feet to CCF (hundreds of cubic feet) or therms for billing.

Below the index are the small test dials (also called proving or drive dials), typically 1/2, 1 or 2 cubic feet per revolution on residential meters and 5, 10 or 100 cubic feet on commercial meters. These are the dials you use to clock an appliance.

Worked Example — Clocking an Appliance

Shut off every other gas appliance including standing pilots, run the appliance being tested at high fire, and time one full revolution of a test dial.

CFH = (3,600 x dial size in cubic feet) / seconds per revolution

A furnace turns the 2-cubic-foot test dial once in 36 seconds on gas with a heating value of 1,030 Btu per cubic foot:

CFH = (3,600 x 2) / 36 = 7,200 / 36 = 200 CFH
Input = 200 CFH x 1,030 Btu/ft3 = 206,000 Btu/h

Compare that to the nameplate input. If the plate reads 200,000 Btu/h, the appliance is 3 percent over — acceptable. If it reads 150,000 Btu/h, the burner is badly overfired and the orifice or manifold pressure is wrong.

Elevated-pressure correction. A meter measures actual cubic feet at meter pressure. On a 2 psi meter you must multiply by a pressure factor:

Factor = (gauge pressure + atmospheric) / base pressure = (2 + 14.7) / 14.73 = 1.134
Corrected flow = 200 CFH x 1.134 = 227 CFH

Skipping that correction under-reads a 2 psi meter by roughly 13 percent. No correction is needed on an ordinary 7 in. w.c. meter.

Meter Location, Access and Protection

Meter siting comes mainly from NFPA 54 Section 5.7, which Maryland adopts at COMAR 09.20.01, plus each utility's own construction standard.

Indoor meters are permitted where the utility allows them, but they must still meet the ventilation, access and damage rules above, and any regulator serving them must be vented in accordance with IFGC 410.3.

Three Regulators, Three Different Jobs

Candidates lose points because they treat "regulator" as one device. There are three in a typical elevated-pressure system, plus two protective types.

IFGC 410.1 states the trigger: a line pressure regulator shall be installed where the appliance is designed to operate at a lower pressure than the supply pressure. Line gas pressure regulators shall be listed as complying with ANSI Z21.80/CSA 6.22, access shall be provided, they shall be protected from physical damage, and any regulator installed on the building exterior shall be approved for outdoor installation.

IFGC 409.4 requires a listed shutoff valve immediately ahead of each MP regulator — that is the "accessible shutoff valve upstream" rule, and it is separate from the appliance shutoff valve of Section 409.5.

The Seven MP Regulator Requirements (IFGC 410.2)

1. Approved and suitable for the inlet and outlet pressures of the application.
2. Maintains a reduced outlet pressure under lock-up (no-flow) conditions.
3. Capacity, per the manufacturer's published ratings, adequate to supply the appliances served.
4. Provided with access; where located indoors, vented to the outdoors or equipped with a leak-limiting device, either way complying with Section 410.3.
5. A capped or plugged tee fitting between the regulator and its upstream shutoff valve, positioned to take a pressure-measuring instrument and to serve as a sediment trap.
6. A capped or plugged tee fitting not less than 10 pipe diameters downstream of the regulator outlet for a pressure-measuring instrument — not required where the regulator serves an appliance with a pressure test port on the gas control inlet in the same room.
7. Where connected to rigid piping, a union within 1 foot of either side of the regulator.

Venting the Regulator

A regulator senses atmospheric pressure through its bonnet, so a diaphragm rupture can put fuel gas straight out of that opening. IFGC 410.3 therefore requires that pressure regulators requiring a vent be vented directly to the outdoors, with the vent designed to prevent the entry of insects, water and foreign objects. The exception is important and frequently tested: a vent to the outdoors is not required for regulators equipped with and labeled for use with an approved vent-limiting device installed per the manufacturer's instructions.

IFGC 410.3.1 governs the vent piping itself: it must be built of approved gas-piping materials, be not smaller than the vent connection on the device, and relief vents and combination relief/breather vents must run independently to the outdoors serving only a single device. Only breather vents may be manifolded, and only under an approved design that minimizes backpressure if a diaphragm ruptures. Vent length may not exceed the regulator manufacturer's stated limit. NFPA 54 adds that the vent shall terminate not less than 3 feet from a possible source of ignition, and that an antiflood-type breather vent fitting be used where the termination could be submerged by flood water or snow.

Exam Trap: A question asks how far a gas meter must be from a source of ignition and offers "3 feet." That clearance was deleted from the meter requirement in the 2009 edition of NFPA 54 and reassigned to the vent outlet of the line pressure regulator. The meter rules are about ventilation, access, damage, corrosion, vibration and temperature. The 3-foot ignition clearance belongs to the regulator vent termination.

The Pressure Ladder You Must Memorize

Because 1 psi equals 27.7 inches of water column, every pressure on a fuel gas job can be written either way.

IFGC 402.7 allows more than 5 psig inside a building only where one of seven conditions is met: welded or brazed joints; flanged joints with pipe-to-flange connections welded or brazed; piping in a ventilated chase or otherwise enclosed against accidental gas accumulation; areas used exclusively for industrial processing or heating, research, warehousing, or boiler and mechanical rooms; a temporary installation for a building under construction; piping serving agricultural appliances; or an LP-gas system operating above 20 psi that complies with NFPA 58.

Overpressure Protection (IFGC 2018 Section 416)

An overpressure protection device (OPD) is the second line of defense: it assumes the line regulator will eventually fail wide open and limits what reaches the appliance anyway.

• 416.1 Where required. Where the serving gas supplier delivers gas at a pressure greater than 2 psi for piping systems serving appliances designed to operate at 14 inches w.c. or less, overpressure protection devices shall be installed. Systems serving equipment designed for inlet pressures above 14 in. w.c. get overpressure protection as required by the appliance manufacturer's instructions.
• 416.2.1 Pressure limitation. Each device shall be adjusted to limit the gas pressure to each connected appliance to 2 psi or less upon failure of the line pressure regulator.
• 416.2.3 and 416.2.4. The device must achieve that limit independently of any other pressure control equipment, and the system must be designed so that failure of the primary pressure control device is detectable.
• 416.3 Devices. An OPD shall be one of four things: a pressure relief valve; a monitoring regulator; a series regulator upstream of the line regulator, continuously limiting the line regulator's inlet; or an automatic shutoff device in series with the line regulator that closes at the limit and remains closed until manually reset.
• 416.3.5 Vents. Discharge stacks, vents and outlet parts shall discharge safely to the outdoors, be protected from water, insects and blockage, and be not smaller than the outlet of the relieving device.

Devices may be integral to the service or line regulator or installed as separate units.

Worked Example — Sizing the Meter and Line Regulator

A Baltimore County rowhouse converting to gas has a 100,000 Btu/h furnace, a 40,000 Btu/h water heater, a 65,000 Btu/h range and a 30,000 Btu/h dryer.

Total connected load = 100,000 + 40,000 + 65,000 + 30,000 = 235,000 Btu/h
Demand in cubic feet per hour = 235,000 / 1,030 Btu per ft3 = 228 CFH

Round up to 230 CFH. A residential diaphragm meter in the 250 CFH class covers that at 7 in. w.c. If the utility instead runs a 2 psi service to a line regulator in the basement, IFGC 410.2 item 3 requires you to confirm from the manufacturer's published capacity table that the regulator passes at least 230 CFH at a 2 psi inlet and a 7 in. w.c. outlet setting. Never size a regulator by its pipe connection size — a 3/4-inch body may be rated anywhere from 200 to 1,500 CFH depending on spring, orifice and inlet pressure.

Identifying Elevated-Pressure Piping

IFGC 401.5 requires that, for other than steel pipe, exposed piping be identified by a yellow label marked "Gas" in black letters at intervals not exceeding 5 feet; the marking is not required on pipe in the same room as the appliance served. On multiple-meter buildings, 401.7 requires permanent identification of each meter's piping, and 409.3.3 requires an identification tag on each house-line shutoff valve. Serving utilities and local authorities routinely add their own marking for 2 psi service — a tag at the meter and at the line regulator — so the next technician does not connect a 14 in. w.c. appliance directly to an elevated-pressure line.