The difference between pipes and tubes is a matter of sizing. For instance, PVC pipe for plumbing applications and galvanized steel pipe are measured in iron pipe size (IPS). Copper tube, CPVC, PeX and other tubing is measured nominally, basically an average diameter. These sizing schemes allow for universal adaptation of transitional fittings. For instance, 1/2″ PeX tubing is the same size as 1/2″ copper tubing. 1/2″ PVC on the other hand is not the same size as 1/2″ tubing, and therefore requires either a threaded male or female adapter to connect them. When used in agricultural irrigation, the singular form “pipe” is often used as a plural.
Pipe is available in rigid joints, which come in various lengths depending on the material. Tubing, in particular copper, comes in rigid hard tempered joints or soft tempered (annealed) rolls. PeX and CPVC tubing also comes in rigid joints or flexible rolls. The temper of the copper, whether it is a rigid joint or flexible roll, does not affect the sizing.
The thicknesses of the water pipe and tube walls can vary. Because piping and tubing are commodities, having a greater wall thickness implies higher initial cost. Thicker walled pipe generally implies greater durability and higher-pressure tolerances. Pipe wall thickness is denoted by various schedules or for large bore polyethylene pipe in the UK by the Standard Dimension Ratio (SDR), defined as the ratio of the pipe diameter to its wall thickness. Pipe wall thickness increases with schedule, and is available in schedules 20, 40, 80, and higher in special cases. The schedule is largely determined by the operating pressure of the system, with higher pressures commanding greater thickness. Copper tubing is available in four wall thicknesses: type DWV (thinnest wall; only allowed as drain pipe per UPC), type ‘M’ (thin; typically only allowed as drain pipe by IPC code), type ‘L’ (thicker, standard duty for water lines and water service), and type ‘K’ (thickest, typically used underground between the main and the meter).
Wall thickness does not affect pipe or tubing size.[28] 1/2″ L copper has the same outer diameter as 1/2″ K or M copper. The same applies to pipe schedules. As a result, a slight increase in pressure losses is realized due to a decrease in flow path as wall thickness is increased. In other words, 1 foot of 1/2″ L copper has slightly less volume than 1-foot of 1/2 M copper.
Materials
Water systems of ancient times relied on gravity for the supply of water, using pipes or channels usually made of clay, lead, bamboo, wood, or stone. Hollowed wooden logs wrapped in steel banding were used for plumbing pipes, particularly water mains. Logs were used for water distribution in England close to 500 years ago. US cities began using hollowed logs in the late 1700s through the 1800s. Today, most plumbing supply pipe is made out of steel, copper, and plastic; most waste (also known as “soil”) out of steel, copper, plastic, and cast iron.
The straight sections of plumbing systems are called “pipes” or “tubes”. A pipe is typically formed via casting or welding, whereas a tube is made through extrusion. Pipe normally has thicker walls and may be threaded or welded, while tubing is thinner-walled and requires special joining techniques such as brazing, compression fitting, crimping, or for plastics, solvent welding. These joining techniques are discussed in more detail in the piping and plumbing fittings article.
Galvanized steel potable water supply and distribution pipes are commonly found with nominal pipe sizes from 3⁄8 inch (9.5 mm) to 2 inches (51 mm). It is rarely used today for new construction residential plumbing. Steel pipe has National Pipe Thread (NPT) standard tapered male threads, which connect with female tapered threads on elbows, tees, couplers, valves, and other fittings. Galvanized steel (often known simply as “galv” or “iron” in the plumbing trade) is relatively expensive, and difficult to work with due to weight and requirement of a pipe threader. It remains in common use for repair of existing “galv” systems and to satisfy building code non-combustibility requirements typically found in hotels, apartment buildings and other commercial applications. It is also extremely durable and resistant to mechanical abuse. Black lacquered steel pipe is the most widely used pipe material for fire sprinklers and natural gas.
Most typical single family home systems won’t require supply piping larger than 3⁄4 inch (19 mm) due to expense as well as steel piping’s tendency to become obstructed from internal rusting and mineral deposits forming on the inside of the pipe over time once the internal galvanizing zinc coating has degraded. In potable water distribution service, galvanized steel pipe has a service life of about 30 to 50 years, although it is not uncommon for it to be less in geographic areas with corrosive water contaminants.