Know your pipe. Materials, fittings, and joints. Episode nineteen of the Gas Service Animated series.

Four materials do most of the work in fuel gas piping. Schedule forty black steel pipe is the workhorse, threaded and rugged, at home almost anywhere in the system. Copper tubing is permitted only where the gas runs low in sulfur, no more than about three tenths of a grain of hydrogen sulfide per one hundred standard cubic feet. Corrugated stainless steel tubing is a listed system, installed strictly to its manufacturer's instructions and bonded. And polyethylene plastic serves outdoors underground, where the soil protects it.

Some materials are simply off the list. Cast iron pipe shall not be used for gas, it is brittle and its joints cannot be trusted. PVC and CPVC plastic shall not carry fuel gas at all, they are not rated for it. Aluminum tubing cannot run outdoors or underground; and where it contacts masonry, plaster, or insulation it must be coated to protect against corrosion — contact is allowed, bare metal is not. And used pipe and fittings cannot go back into service unless they are clean, free of foreign material, and proven adequate for gas.

Polyethylene has one home: outside, underground. Sunlight and physical damage are its enemies, so the plastic itself never runs above grade and never enters the building. To come up out of the ground it passes through an anodeless riser, a steel cased assembly that protects the plastic and hands the gas off to metal piping above grade. A tracer wire is buried along the plastic so the line can be located later. Below the soil, plastic. Above the grade line, metal. No exceptions in between.

Threaded joints have their own law. Metallic pipe threads are tapered, and every threaded joint gets a sealing material, a compound or tape that is nonhardening and resistant to the gas it will see. It goes on the male threads only, starting back from the open end of the pipe, so that nothing squeezes into the gas stream when the joint pulls tight. Sealant floating downstream can foul valves, regulators, and burner orifices. And threads that are stripped, chipped, or corroded never get reused. That pipe gets cut back and rethreaded, or replaced.

Copper and other nonferrous gas lines are never soft soldered. Gas joints in tubing are brazed, with a filler metal that melts above one thousand degrees Fahrenheit, so that heat near the joint cannot let it slip open. Low temperature solder, the kind used on water lines, is not permitted on fuel gas. Where the design allows it, a flared joint made with a proper flaring tool is an accepted mechanical option for nonferrous tubing.

Fittings follow the family rule. Steel pipe takes steel or malleable iron fittings. Copper takes copper alloy. Aluminum takes aluminum. Threaded fittings larger than four inches are not used, and cast iron fittings live under tight limits of their own. Corrugated stainless steel tubing accepts only the fittings in its manufacturer's listing. And when two material families must meet, they meet at a listed transition fitting, never at an improvised splice.

Old basements hide mystery pipe, so read before you touch. Steel pipe carries its standard rolled or stenciled into the surface. Copper is marked with its type and standard along the tube. Polyethylene is yellow and printed with the word gas and its plastic standard. Corrugated tubing wears its listing on the jacket. If a pipe carries no marking you can trust, treat it as unknown, and identify it before any wrench touches the system.

So on a service call, the piping itself gets inspected. Right material for the location: plastic buried, aluminum protected, nothing from the banned list. Joints made by the rules: sealant on male threads only, tubing brazed, never soft soldered. Transitions listed, risers proper, and corrugated tubing installed and bonded per its instructions. And anything unmarked or questionable gets identified and checked against the code, the manufacturer, and your company's procedures before the work begins.

Know your pipe. Four materials, each with a home turf. A banned list with no exceptions. Joints made by the rules, and families that meet only at listed transitions. Next episode: ranges and dryers.