2021 IFGC Chapter 4
Lesson 12 covered what fuel gas piping is made of and how it goes in. This lesson answers the two questions an inspector, a plans reviewer, or an estimator hears on every gas job: is the pipe big enough, and was it proven tight before anyone lit a burner? Sizing lives in 2021 IFGC Section 402 and testing and purging in Section 406. The figure below is the layout we will size step by step, a small community building on a low-pressure natural gas system: a meter, a main run in three sections, and two branches. Texas cities choose their own fuel gas and mechanical code, so confirm the edition your jurisdiction has adopted (Lesson 1) before you quote any number from this lesson.
Section 402.1 states the goal: the piping must carry the maximum demand and still deliver gas to each appliance inlet at or above the minimum pressure that appliance needs (402.6 repeats it as a pressure rule). Under 402.2 the demand is the sum of the maximum inputs of every appliance served, all assumed to run at once, unless a diversity of load can be established. Flow is also adjusted above 2,000 feet of elevation. Section 401.8 sizes pipe for the full number of outlets intended, so a plan showing a future range gets pipe sized for it now.
Nameplates list Btu per hour, but the tables use cubic feet per hour (cfh). Divide by the gas heating value; natural gas is commonly taken as about 1,000 Btu per cubic foot. The 2021 Chapter 4 text does not state that conversion, so use the supplier's value and check the adopted code.
Section 402.3 allows three routes: the tables or equations in 402.4 or 402.5, the tables in a listed piping system's installation instructions, or engineering methods. Tables 402.4(1) through (37) cover every material except noncorrugated stainless steel tubing, which 402.5 sends to the sizing equations. Schedule 40 metallic pipe comes first (Tables (1) through (7)), then semirigid copper, CSST, and polyethylene. Whatever table you use, one of three length methods sets the length.
A table's header is the fine print that decides whether it applies. Table 402.4(1) is for Schedule 40 metallic pipe, natural gas, inlet pressure under 2 psi, pressure drop 0.3 inch water column, specific gravity 0.60. Tables 402.4(2) through (4) use drops of 0.5, 3.0, and 6.0 inches, and the last two are intended for initial supply pressures of 8.0 and 11.0 inches water column or greater. Tables 402.4(5) through (7) cover 2, 3, and 5 psi inlets. A table that does not match the supply pressure and allowed drop gives capacities that are not true for the system.
Read down to the row for your length and across to the pipe size; the entry is capacity in cfh. Two trends catch a misread: capacity rises with pipe size and falls with length.
| Length | 1/2 in. | 3/4 in. | 1 in. | 1-1/4 in. | 1-1/2 in. |
|---|---|---|---|---|---|
| 30 ft | 72 | 151 | 284 | 583 | 873 |
| 50 ft | 55 | 114 | 215 | 442 | 662 |
| 60 ft | 50 | 104 | 195 | 400 | 600 |
| 100 ft | 38 | 79 | 148 | 304 | 455 |
The longest length method (402.4.1) finds the longest run from the point of delivery to the most remote outlet and uses that one length, one table row, for every section; each section's own load picks the pipe size. It is simple and conservative. The branch length method (402.4.2) sizes the longest run the same way, but each other branch uses the length from the point of delivery to the most remote outlet on that branch, so a branch near the meter can drop a size. The hybrid pressure method (402.4.3) covers a higher-pressure system feeding a line pressure regulator: upstream piping uses the length to the most remote regulator, and piping after it uses the length from the regulator to its most remote outlet. No method uses just the length of the short piece in front of you.
The meter feeds a main run of three 20 foot sections (A, B, C), a 10 foot branch D to a water heater, and a 10 foot branch E to a range. Inputs of 65,000, 150,000, and 200,000 Btu per hour (water heater, range, unit heater) become 65, 150, and 200 cfh at about 1,000 Btu per cubic foot.
Worked example. The longest run is 20 + 20 + 20 = 60 feet. Section loads: A carries 65 + 150 + 200 = 415 cfh; B carries 150 + 200 = 350; C carries 200; D carries 65; E carries 150. Pick the smallest pipe whose capacity in the right row is at least the load.
| Section | Load (cfh) | Longest length method (60 ft row) | Branch length method |
|---|---|---|---|
| A (main) | 415 | 1-1/2 in. (1-1/4 in. carries only 400) | 1-1/2 in., 60 ft |
| B (main) | 350 | 1-1/4 in. (400) | 1-1/4 in., 60 ft |
| C (main) | 200 | 1-1/4 in. (1 in. carries only 195) | 1-1/4 in., 60 ft |
| D (branch) | 65 | 3/4 in. (1/2 in. carries only 50) | 1/2 in., 30 ft row (72) |
| E (branch) | 150 | 1 in. (3/4 in. carries only 104) | 1 in., 50 ft row (215) |
By the branch length method D is 20 feet of main plus its own 10, so the 30 foot row applies, and 1/2 inch carries 72 cfh, enough for 65. E is 20 + 20 + 10 = 50 feet; 3/4 inch carries 114 there, short of 150, so E stays 1 inch. The close calls on the main (415 against 400, 200 against 195) are why you run the numbers instead of eyeballing. Ask which method the designer used, since it changes the row you check. These lengths land on table rows; the 2021 Chapter 4 text states no rule for a length between two rows, so check the adopted code.
Section 406.1 requires visual inspection and a pressure test before acceptance and initial operation, and 404.20 says the test happens before the piping is put in service or concealed. That is why the inspection is called before the drywall or trench closes: joints stay exposed (406.3). The pipe is cleared of foreign material first (406.1.6). The test medium is air, nitrogen, carbon dioxide, or another inert gas; oxygen shall not be used (406.2).
A system may be tested whole or in sections, but a valve shall not be the bulkhead between gas and test medium unless a double block and bleed system is installed (406.1.4). An appliance designed for less than the test pressure is disconnected and its outlet capped (406.3.3); one rated at or above it is isolated by closing its own shutoff (406.3.4). A new branch to a new appliance needs only its own test, with the tie-in checked with leak-detecting fluid (406.1.3).
The test pressure is at least 1-1/2 times the maximum working pressure and never less than 3 psig (406.4.1). A low-pressure system still gets 3 psig; a 5 psig system gets 7.5 psig. Pressure is read with a manometer or a device calibrated to show loss from leakage, the pressure source is isolated before the test, and a mechanical gauge's top of scale must not exceed five times the test pressure (406.4), so a 0 to 100 psi gauge cannot show a small leak in a 3 psig test. Duration is at least 1/2 hour for each 500 cubic feet of pipe volume or fraction; under 10 cubic feet, or in a single-family dwelling, 10 minutes is the minimum; no test need run beyond 24 hours (406.4.2).
Worked example. 300 feet of 4 inch Schedule 40 pipe (inside diameter 4.026 inches, from Table 402.4(1)) has a cross-section of 3.1416 / 4 x (4.026 / 12) x (4.026 / 12) = 0.0884 square feet, so its volume is 0.0884 x 300 = 26.5 cubic feet. That is over 10 and under 500, so one period applies: 30 minutes. A campus system of 1,100 cubic feet gives 1,100 / 500 = 2.2, the fraction counts as a whole unit, so 3 x 1/2 hour = 1-1/2 hours.
Any drop on the gauge is treated as a leak unless readily attributed to another cause, such as a temperature change (406.5). Leaks are found with an approved detector or noncorrosive fluid, then repaired and retested (406.5.1, 406.5.2).
Before gas goes on, check for open fittings or ends and confirm unused outlet valves are closed and plugged or capped (406.6.2). Right after gas goes on, check for leakage and shut the supply off if any is indicated (406.6.3). Appliances stay off until the leak check is done, the piping is purged, and the appliance connections are checked (406.6.4); appliances are then purged before use (406.7.3).
Purging has two tiers. Section 406.7.1 requires the outdoor procedure where design pressure is over 2 psig or the piping has a section meeting Table 406.7.1.1: 2-1/2 up to under 3 inches longer than 50 feet, 3 up to under 4 inches longer than 30 feet, 4 up to under 6 inches longer than 15 feet, 6 up to under 8 inches longer than 10 feet, or 8 inches and larger at any length. For Table 406.7.1.1 piping, air is displaced with inert gas first, then the inert gas with fuel gas (406.7.1.2). The open end discharges outdoors through a shutoff valve, at least 10 feet from ignition sources and building openings and 25 feet from mechanical air intakes, attended with a listed combustible gas indicator reading 0 to 100 percent. Fuel gas stops at 90 percent by volume in the pipe, and people not involved stay 10 feet back (406.7.1.3, 406.7.1.4). Small systems at 2 psig or less, built entirely of pipe below those criteria, may be purged indoors or outdoors (406.7.2), for example with a listed combustible gas detector at the discharge.
In the field
In a 24-unit apartment rehab, the plumber runs a new 3 inch gas main 60 feet to a boiler room and plans to purge it by cracking a union inside the room. Table 406.7.1.1 puts a 3 inch line over 30 feet in the outdoor procedure, so the inspector asks for the inert gas step, a valve-controlled discharge outdoors at the required distances, an attended combustible gas indicator, and a stop at 90 percent gas.
Meters serving separate consumers must not be interconnected on the outlet side (401.6), and piping from multiple meters is permanently marked (401.7). Every meter has a shutoff on its supply side (409.2); one meter serving several buildings or tenants needs a shutoff for each (409.3). A line pressure regulator is required where an appliance is designed for a lower pressure than the supply, with access and protection from damage (410.1), and regulator vents go outdoors unless a labeled vent-limiting device is used (410.3). Inside buildings the maximum operating pressure is 5 psig unless an exception applies, such as welded or brazed joints or a boiler or mechanical room (402.7).
1. Under IFGC 402.4.1, the longest length method sizes each section of a gas piping system using:
2. A designer will supply natural gas to a small commercial building at an inlet pressure of less than 2 psi and will allow a pressure drop of 0.3 inch water column in Schedule 40 metallic pipe. Which table applies?
3. Using Table 402.4(1), a 1 inch Schedule 40 pipe on a 50 foot length carries about how many cubic feet of gas per hour?
4. A section of Schedule 40 pipe in a low-pressure natural gas system (Table 402.4(1)) carries 300 cfh and the longest length used for sizing is 100 feet. What is the smallest size that works?
5. Under the branch length method (IFGC 402.4.2), a branch that is not on the longest run is sized using:
6. A section of a low-pressure natural gas system carries 500 cfh, and the longest length is 60 feet. From Table 402.4(1), the smallest pipe that meets the load is:
7. Under IFGC 402.2, unless a diversity of load can be established, the demand used to size a gas piping section is:
8. Which of the following may be used as the pressure test medium for new fuel gas piping under IFGC 406.2?
9. A new gas system in a fellowship hall has a maximum working pressure of 5 psig. The minimum test pressure under IFGC 406.4.1 is:
10. A church hall gas system operates at a low pressure of about one-half psig. The inspector reviews the test procedure. What is the lowest test pressure the code allows?
11. A commercial gas system has 25 cubic feet of pipe volume and is not in a single-family dwelling. What is the minimum test duration under IFGC 406.4.2?
12. A plumber tests a new gas system in a rehab building at 3 psig and installs a mechanical gauge that reads 0 to 100 psi. What should the inspector do?
13. A new gas main serves a boiler room at 7 inches water column. Which piping section would require the outdoor purging procedure of IFGC 406.7.1 because of Table 406.7.1.1?
14. While purging new 3 inch gas piping outdoors with fuel gas under IFGC 406.7.1.3, the operator must stop introducing fuel gas when the indicator shows: