25.3 Steel Pipe, Plastic Pipe, Threading & Oxy-Acetylene

25.3 Steel Pipe, Plastic Pipe, Threading & Oxy-Acetylene

Trade Area D still has steel pipes, threading, oxy-acetylene cutting, welding with filler rods, and plastic pipes and fittings (outline item 86). Copper ACR from 25.1–25.2 does not cover a 4-inch steel discharge, a threaded king valve, or the helper who glued PVC into a suction line. The CBT stack is FBC Mechanical 2023 Chapter 11 plus Table 305.4, Pipefitters Handbook, 3rd Edition (1967) (R82) for thread engagement, offsets, and torch work, and OSHA 29 CFR 1926 (July 1, 2025 edition on the 2026 list) Subpart J for welding and cutting.

Quick Answer: Steel refrigerant pipe is ASTM A53 or A106, joints threaded, welded, mechanical, or press-connect (1108.8). Schedule 80 for Group A2/A3/B2/B3 sizes 1½ inches and under (1107.4.1). NPT compound goes on male threads only (1108.3.4). Oxy-acetylene cuts and welds steel; acetylene ≤15 psig in the hose. Plastic is not refrigerant pipe.

Steel pipe on a refrigeration circuit

Table 1107.4 lists ASTM A53 and ASTM A106 steel pipe, and ASTM A254 / A334 steel tube. Note b: ASTM A53 Type F (furnace-butt-welded) shall not be used for refrigerant lines with an operating temperature less than −20°F. Low-temperature suction on a freezer is not a Type F hardware-store special.

1107.4.1 raises the wall for flammable and toxic groups: the minimum weight of steel pipe for Group A2, A3, B2, and B3 refrigerants shall be Schedule 80 for sizes 1½ inches or less. A2L (R-32, R-454B) is a subclass of Class 2 in ASHRAE 34; when the question names A2/A3/B2/B3 and 1-1/4-inch steel, Schedule 80 is the row. Group A1 (R-410A, R-134a, R-404A) commonly uses Schedule 40 where the listed pressure rating is enough — still not Type F on a −30°F blast freezer.

Ammonia remains 1101.1.2 / IIAR, typically steel, never copper. Class A is the statutory home for large ammonia plants; Class B candidates still must know why copper fails on NH3.

1108.8: steel pipe joints are mechanical, threaded, press-connect, or welded. 1108.9: steel tube joints are flared, mechanical, press-connect, or welded — not a plumbing 50/50. 1108.1.1: dissimilar metals use an approved adapter and a dielectric fitting or union meeting ASSE 1079 when the metals would otherwise galvanically eat each other. A copper suction bolted to a steel rack header without a listed transition is a leak and a corrosion cell.

Stop valves on steel (and copper) follow 1109.8: systems with more than 6.6 pounds of refrigerant get stop valves at the compressor/condensing-unit suction inlet and discharge outlet and at the liquid-receiver outlet. More than 100 pounds adds receiver inlets and condenser inlets/outlets when condensers run in parallel. 1109.8.3: do not let 1-inch OD and larger copper or aluminum tubing support the valve — the valve needs its own hanger (Section 25.4).

Threading — NPT, engagement, compound

FBC Mechanical 1108.3.4: threads shall conform to ASME B1.1, B1.13M, B1.20.1, or B1.20.3. Thread lubricant, pipe-joint compound, or thread tape shall be applied on the external threads only and shall be approved for the piping material. That is male-only dope. Compound on the female thread is how you pack the first inch of a TXV or a compressor port with shredded PTFE.

NPT (tapered pipe thread, ASME B1.20.1) seals on the taper, not on a gasket. The Pipefitters Handbook is the CBT table for threads per inch, engagement length, and cut length after takeoff. Typical NPT: 1/2-inch and 3/4-inch are 14 TPI; 1-inch through 2-inch are 11.5 TPI. Cut square, ream, die to the handbook thread length, and stop when the fitting is wrench-tight on the taper — not when the helper has buried every thread plus a backup wrench on the valve body.

Worked thread example. A 1-inch Schedule 40 nipple into a steel receiver port: male threads get listed refrigeration-compatible compound or PTFE (not yellow gas tape used as a license to run new interior fuel lines — F.S. 489.105 still bars new interior LP/natural-gas piping except appliance changeout disconnect/reconnect). Leave the leading thread bare so shreds do not enter the circuit. Back the valve with a second wrench on the valve body. Then pressure-test (25.4); a soapy “good enough” on an untested 400-psi discharge thread is not a 60-minute 1110.5 test.

Do not thread-sealant a flare, a press-connect, a brazed cup, or a gasketed flange. Chapter 18 already parked compound on hydronic NPT; this section parks it on steel refrigerant NPT only.

Oxy-acetylene cutting and filler-rod welding

The CILB oxy-acetylene cutting item is steel. You preheat a spot to kindling temperature with the mixed flame, then squeeze the oxygen cutting lever so a jet oxidizes and blows the slag out of the kerf. That is how you drop a hanger, a Unistrut, or a steel main — not how you “cut” ACR copper (copper is cut with a tube cutter, then reamed).

Welding with filler rods (1108.3.5 plus the handbook): clean mill scale, bevel heavy wall if the procedure says so, and add approved filler so the weld metal fills the groove. A neutral flame (equal acetylene and oxygen, sharp inner cone) is the usual welding flame. An oxidizing flame (excess oxygen) burns the steel. A carburizing flame (excess acetylene, feathery acetylene feather) adds carbon. Brazing copper on the same cart still uses a neutral to slightly reducing flame and the nitrogen purge from 25.2 — the cutting oxygen lever is not a purge.

Acetylene is unstable above 15 psig as a free gas. Keep the acetylene regulator at or below 15 psig. Cylinders stay upright. OSHA 1926.350: oxygen and fuel-gas cylinders in storage are separated by 20 feet or a 5-foot 1/2-hour fire wall; caps on; no oil on oxygen regulators. Use reverse-flow check valves and flashback arrestors. Crack oxygen slowly; a slammed oxygen valve with oil in the regulator is a shop fire.

Never pressure-test a refrigerant circuit with the oxygen cylinder (Section 25.4 / 1110.3). Oxygen plus compressor oil is an explosion, not a leak test.

Plastic pipes and fittings — what they are for

Plastic is not in Table 1107.4. PVC, CPVC, ABS, and PEX are not refrigerant metals. The CILB plastic pipes/fittings item is a negative on the refrigerant side and a positive on condensate and listed hydronic:

Table 305.4 horizontal spacing, for when plastic is the right pipe: PVC 4 feet, CPVC 1 inch and smaller 3 feet, PEX 1 inch and smaller 2-2/3 feet (32 inches), with mid-story guides on the vertical rows that carry note c. Hanging PVC every 4 feet does not convert it into ACR.

F.S. 489.105 still does not give Class A or Class B potable water, sanitary sewer, or pool piping. Plastic condensate to an existing safe waste is inside the grant. A new sanitary tap is not. Plastic refrigerant “because it doesn’t corrode in the salt air” is not a coastal exception.

Florida HVAC scenario

A Class A crew in Duval County hangs a 2-inch Schedule 40 A53 discharge on an A1 rooftop rack (over 25 tons — Class A). They thread the king valves with compound on the male NPT, weld a steel offset with a filler rod and a neutral oxy-acetylene flame, and cut a hanger from angle with the cutting lever. The helper then wants to run Schedule 40 PVC for the last 20 feet of suction “because plastic is on the outline.” Wrong column: outline item 86 tests that you know plastic is not refrigerant pipe. On the same roof, a Class B bidder cannot take the 40-ton rack at all; the Class B 3-ton change-out in the parking-lot office still uses ACR, still brazes with nitrogen, and still runs PVC only on the 3/4-inch condensate, sloped 1/8 inch per foot, not into a sanitary wye.

Traps: (1) Type F steel on a −30°F freezer. (2) Schedule 40 on a 1-inch A2/A3/B2/B3 line that 1107.4.1 wants Schedule 80. (3) Dope on female threads or on a flare. (4) Oxygen as a “test gas.” (5) PVC suction.