Gas Piping Layout, Cutting, and Threading

Gas Piping Layout, Cutting, and Threading

Black steel pipe is the standard material for natural gas and propane distribution piping inside buildings. Unlike galvanized pipe, black steel has no zinc coating that can flake off and clog gas valves or orifices. Every joint must be mechanically sound and sealed against leakage, because even a small gas leak creates a fire and explosion hazard. This lab teaches you to cut, ream, and thread black steel pipe and to assemble a leak-free subassembly using approved fittings and sealants.

Safety First

Before picking up any tool, review these non-negotiable rules:

• Eye protection: Safety glasses are required throughout the lab. Cutting oil, metal chips, and thread filings are ejected at speed.
• Hand protection: Leather gloves protect against sharp burrs and hot pipe. Remove gloves only when operating the power threader's trigger or chuck controls — gloves can catch in rotating parts.
• Lockout/tagout (LOTO): If the power threader must be adjusted, a die changed, or a jam cleared, the machine must be de-energized and locked out before any contact with the chuck or carriage. Follow your facility's LOTO procedure every time.
• Housekeeping: Cutting oil and chips on the floor create slip and puncture hazards. Clean around the threader continuously.
• No open flames: This is a gas piping class. Although pipe in this lab carries no gas, build the habit now — never allow ignition sources near gas piping work.

Tools and Their Use

Rotary pipe cutter: A hardened cutting wheel is tightened against the pipe and the tool is rotated around the pipe while the feed screw is advanced slightly with each revolution. Advancing too fast crushes the pipe wall; too slow wastes time. The goal is a square, clean cut with no deformation of the pipe end.

Reamer: Cutting leaves an inward burr (a ridge of metal rolled into the bore). A reamer — either a spiral-fluted hand reamer or the built-in reamer on many pipe cutters — removes this burr. An unreamed pipe restricts flow and can trap debris. Ream every cut end before threading.

Manual and power pipe threader: A pipe threader cuts tapered National Pipe Taper (NPT) threads. The taper is standardized: threads are cut at an 1/16-inch taper per inch of thread length, allowing joints to draw tight as they are made up. A power threader does the same job faster. Keep the cutting head flooded with threading oil throughout the cut to lubricate the dies, cool the metal, and flush chips away from the cutting edges. Run the machine in reverse briefly to back the die off cleanly.

Thread ring gauge: After threading, check thread length and taper with a ring gauge. Proper engagement means the pipe end face sits flush with — or within one turn of — the face of the gauge. Threads that are too short will not seal; threads that are too long waste material and can bottom out in fittings.

Measuring and Cutting to Length

Before cutting, determine the end-to-end measurement of each pipe section from the drawing. Then calculate the cut length by accounting for the amount of thread that will engage inside each fitting — called thread engagement or make-up. A simple method:

1. Look up the center-to-face dimension of each fitting from a fitting dimension table.

2. Subtract one make-up allowance (thread engagement depth) for each threaded end.

3. The result is the cut length of the pipe.

Mark the pipe clearly with a soapstone marker before cutting. Double-check measurements — gas piping must fit the layout drawing.

Applying Sealant

Only sealants rated for gas service may be used on gas piping joints. Two approved options:

• Gas-rated pipe joint compound (pipe dope): Apply a thin, even coat to the male threads only, starting one thread back from the pipe end to prevent compound from entering the pipe bore. Use the brush built into the compound cap.
• Gas-grade PTFE tape: Wrap the male threads in the direction of the thread (clockwise when looking at the pipe end), starting one thread back, overlapping half the tape width with each pass. Two full wraps is a common minimum; follow the manufacturer's direction. Use only tape specifically labeled for gas service — standard white plumber's tape is not rated for gas.

Never apply sealant to female threads. Never use both pipe dope and tape on the same joint unless the manufacturer explicitly permits it.

Making Up Threaded Joints

Thread joints by hand first to confirm threads are aligned — cross-threading is immediately obvious when you cannot turn the fitting smoothly by hand. Once hand-tight, use a pipe wrench to advance the fitting the number of turns recommended for the pipe size. Over-tightening can crack fittings or strip threads; under-tightening leaves the joint loose and prone to leakage. A correctly made-up joint will show two to three threads still exposed on the pipe after wrench tightening.

T-Connection Assembly

Assembling a T-connection from a drawing requires you to maintain alignment — all outlets of the tee must point in the correct plane — and to achieve proper make-up depth on every leg. Assemble the run (straight-through) connections first, then bring in the branch. Check alignment against the drawing before final tightening. Log thread condition, make-up turns, and any defects in your lab log for instructor review.

Quality and Inspection

Your instructor will inspect four criteria: correct thread length, absence of cross-threading, no excess sealant inside the pipe bore, and proper joint make-up. Failing any criterion or committing a safety violation means the assembly must be redone. Attention to quality here directly supports the leak-free systems required for licensure and for the NCCER credentialing process.