First, pipe fittings include elbows, tees, four-way, reducers, expansion joints, valves, flanges, gaskets, bolts, nuts, instrument components and so on. Everything in a pipeline is called a pipe fittings (pipe element), except for the pipe. It should be said that components other than pipes on pipelines can be called fittings, which is a common statement. Therefore, for the pipe professional, it is believed that the usual pipe fittings are mainly tees, four-way, elbows, large and small heads, pipe caps, pipe joints, union joints, pipe tables, etc. It can be seen as a conventional understanding of our industry. Flanges, flanges, gaskets, bolts can be classified as pipe fittings, but the pressure gauge nozzle belongs to the instrument element, which is professionally counted by the instrument, and installed by the process pipeline when installing.

Second, in a broad sense, if the pipe components are considered to be pipe fittings, flanges are pipe fittings. However, the pipe fittings mentioned in the usual project management links do not include flanges. The complete pipeline (excluding anti-corrosion and thermal insulation) is composed of pipes (pipes), pipe fittings, flanges, fasteners, valves, and instrument components. Pipeline only refers to the pipeline installed on site (if the finished part with the pipe is not within the scope of the pipeline, such as the straight pipe section of the flow orifice strip); Pipe fittings refer to tees, elbows, bosses, nozzles, filters, special parts, etc.; Instrument components only refer to instruments, such as pressure gauges, thermometers, etc. There is also a clear classification in budget quotas.

Third, according to the flange standard, the flange does not belong to the pipe fittings. It and gaskets and fasteners are collectively called flange joints. From the specification definition: flange does not belong to pipe fittings, flanges belong to pipe components or pipe components. GB20801-2006 "Pressure Pipeline Specification: Industrial Pipeline" terminology explanation: pipe fittings include elbows, tees, reducers, flanged short knots, caps and unions. TSG2009 pressure pipeline safety supervision regulations stipulate that pressure pipeline components include pipeline components and pipe supports. The explanation of terms in GB50235 and GB20801 has a comment: pipeline elements include flanges and valves connecting pipelines.