Valves are technically fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure. The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll.
The simplest, and very ancient, valve is simply a freely hinged flap which drops to obstruct fluid (gas or liquid) flow in one direction, but is pushed open by flow in the opposite direction.
Wikipedia about Valves
A valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways.
This is called a check valve, as it prevents or "checks" the flow in one direction.
Modern control valves may regulate pressure or flow downstream and operate on sophisticated automation systems.
Elbows - not only your body parts
An elbow is a pipe fitting installed between two lengths of pipe or tubing to allow a change of direction, usually a 90° or 45° angle, though 22.5° elbows are also made.
The ends may be machined for butt welding, threaded (usually female), or socketed, etc.
When the two ends differ in size, the fitting is called a reducing elbow or reducer elbow.
Elbows are categorized based on various design features as below:
Long Radius (LR) Elbows ? radius is 1.5 times the pipe diameter
Short Radius (SR) Elbows ? radius is 1.0 times the pipe diameter
90 Degree Elbow ? where change in direction required is 90°
60 Degree Elbow ? where change in direction required is 60°
45 Degree Elbow ? where change in direction required is 45°
A 90 degree elbow is also called a "90 bend" or "90 ell".
It is a fitting which is bent in such a way to produce 90 degree change in the direction of flow in the pipe.
It is used to change the direction in piping and is also sometimes called a "quarter bend". A 90 degree elbow attaches readily to plastic, copper, cast iron, steel and lead. It can also attach to rubber with stainless steel clamps.
It is available in many materials like silicone, rubber compounds, galvanized steel, etc.
The main application of an elbow (90 degree) is to connect hoses to valves, water pressure pumps, and deck drains.
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.
These elbows can be made from tough nylon material or NPT thread.Źródło: https://en.wikipedia.org/wiki/Piping_and_plumbing_fitting
Pipes made of steel - Wikipedia
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).
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.