Pipe (fluid conveyance)

A pipe is a hollow, cylindrical section used to move substances that can flow. These substances are called fluids, which include both liquids and gases.

Beyond moving fluids, pipes can also transport slurries, powders, and small solids. Pipes are also useful in construction for structural applications. A hollow pipe is much stiffer per unit of weight than a solid rod. This makes them efficient for building supports or handrails.

In engineering, there is a specific difference between a pipe and a tube. A pipe is usually defined by a nominal diameter and a "schedule." The schedule tells you how thick the pipe walls are. A tube is often specified by its outside diameter (OD) and wall thickness. Tubes are often made to custom sizes with very precise tolerances. While the terms are often used interchangeably in daily life, they mean different things to professionals. In the United States, the word "tube" is used more often, while "pipe" is more common globally.

Metal pipes are manufactured using several distinct processes. One method is centrifugal casting, where hot, liquid metal is spun to form a shape. Another method creates seamless pipe, also known as SMLS. This is made through rotary piercing, where a solid billet is drawn over a piercing rod. Because seamless pipes have no joints, they are often seen as stronger and more reliable for high pressure.

Other pipes are made by welding. This involves rolling a metal plate and joining the edges together. One common method is Electric Resistance Welding (ERW). In ERW, an electric current passes through the edges to create heat. This heat forms pools of molten metal that merge the pieces into one solid seam.

Large-diameter pipes, those 16 inches or larger, use different technologies. Some are made using Submerged Arc Welding (SAW). There are two main types of SAW pipes: longitudinal (LSAW) and spiral (SSAW). LSAW pipes are made by bending and welding wide steel plates. They are often used in the oil and gas industry but are very expensive. SSAW pipes are made by welding steel coil in a spiral or helicoidal pattern. Because they use coils instead of plates, SSAW pipes are often cheaper. These large pipes are essential for moving massive amounts of resources across the world.

Humanity has used pipes for thousands of years. The earliest known use was in Ancient Egypt. The Pyramid of Sahure, built around the 25th century BC, featured a temple with copper drainage pipes.

In 1817, Henry Osborne developed a process for making iron musket barrels. By 1820, he began making iron gas tubes for pioneers like Samuel Clegg. In the 19th century, steel pipes were often held together with rivets or H-shaped bars. It was not until the early 1930s that welding became the standard method for making steel pipes.

Many materials are used to build pipes depending on the job. Metallic pipes are often made of steel, iron, or copper.

Copper is very popular for domestic water systems because it is great at transferring heat. Aluminum is used when weight is a concern. In the past, people used wood or lead for plumbing. The word "plumbing" actually comes from the Latin word for lead, *plumbum*. However, lead is toxic and is no longer allowed for new drinking water systems. In the US, it is estimated that 6.5 million lead service lines from before the 1930s are still in use.

Plastic is now a very common material for pipes. It is lightweight, does not rust, and is easy to connect. Common types include PVC (polyvinyl chloride) and polyethylene. Plastic is often used for buried municipal water lines. For low-pressure needs like sewage or storm water, engineers often use concrete or ceramic. These materials are heavy and durable. By choosing the right material and manufacturing method, engineers can ensure that water, gas, and even food powders move safely from one place to another.