fitting
A (generally small) component or part of a device or machine, especially one that is standardized and replaceable; specifically, a standardized part of a piping system used to connect sections of pipe together, such as a coupling.
fitting: a standardized connector that joins pipes and channels
A fitting is a standardized component designed to join, redirect, or terminate sections of pipe, tubing, or conduit in fluid and gas systems. Unlike a valve or pump, a fitting has no moving parts; it simply creates a secure connection between two or more segments. Fittings are manufactured to precise specifications so that a fitting of a given size and thread type will connect reliably to any compatible pipe without custom fabrication or modification.
The most common fitting types are elbows (which change flow direction, typically at 90 or 45 degrees), tees (which split flow into two branches), couplings (which join two pipes of the same diameter end-to-end), and reducers (which connect pipes of different diameters). Each type comes in multiple materials: brass, stainless steel, carbon steel, PVC, and polyethylene are standard choices depending on the fluid, pressure, temperature, and cost constraints of the system. Connection methods include threaded (NPT, BSPP), compression, push-to-connect, and soldered joints.
The size of a fitting is expressed by its nominal pipe size (NPS) in inches for most industrial systems, though metric sizes exist in many European and Asian applications. A 1-inch elbow, for example, is designed to fit 1-inch nominal pipe. The wall thickness and pressure rating of the fitting must match or exceed that of the pipe it joins; undersizing a fitting is a common cause of rupture and leakage. Pressure ratings are stamped on fittings and are measured in PSIG (pounds per square inch gauge) or bar.
Why fittings fail and how to prevent it
Fittings leak or fail when threads are cross-threaded, when thread sealant (PTFE tape or pipe dope) is omitted or applied incorrectly, when a fitting rated for lower pressure than the system receives is installed, or when corrosion weakens the material. Threads corrode when incompatible metals are joined (brass and steel in wet environments) or when the fluid is chemically aggressive. Compression fittings fail if the ferrule (a soft copper or plastic ring) is not properly seated or is reused after being removed.
The term fitting comes from its function: it "fits" pipes together. In British English, the word sometimes refers to a household appliance (a "fitted kitchen"), but in mechanical engineering and plumbing trade, fitting always means a connector. The fitting sits between the pipe selection and valve selection in a system design: a system is only as strong as its weakest fitting, so selection demands attention to material compatibility, pressure rating, temperature rating, and the nature of the fluid being conveyed.