Piping glossary
45 common piping, flange and materials terms, explained briefly.
- ANSI / ASME B16.5
- The standard for pipe flanges and flanged fittings NPS ½ through 24, covering dimensions, materials and pressure–temperature ratings.
- ASME B16.11
- Standard for forged socket weld and threaded fittings, in classes 2000 to 9000.
- ASME B16.47
- Standard for large-diameter flanges NPS 26 to 60, in Series A and Series B.
- ASME B16.9
- Standard for factory-made wrought buttweld fittings: elbows, tees, reducers, caps and stub ends.
- ASME B31.3
- The Process Piping code, used to design piping in chemical plants, refineries and similar facilities.
- ASME B36.10M
- Standard for welded and seamless wrought carbon and alloy steel pipe dimensions.
- ASME B36.19M
- Standard for stainless steel pipe dimensions; introduces the S schedules (5S, 10S, 40S, 80S).
- Blind flange
- A solid flange with no bore, used to close the end of a pipe, valve or nozzle.
- Bolt circle (PCD)
- The diameter of the circle through the centres of a flange's bolt holes; also called pitch circle diameter.
- Butt weld
- A weld joining two parts end to end, usually with bevelled edges, giving full-penetration strength.
- Charpy impact test
- A notched-bar test measuring toughness, required for low-temperature materials such as A350 LF2 and A333 Gr 6.
- Class (pressure class)
- A pressure–temperature rating designation for flanges and fittings, e.g. Class 150, 300, 600.
- Concentric reducer
- A reducer whose two ends share the same centreline.
- DN (diamètre nominal)
- The metric nominal size of a pipe, e.g. DN 100 corresponds to NPS 4.
- Duplex stainless steel
- Stainless steel with a mixed austenite–ferrite structure, giving high strength and chloride resistance; e.g. 2205.
- Eccentric reducer
- A reducer with one side flat, used at pump suctions and on pipe racks.
- ERW
- Electric resistance welded pipe, formed from coil and welded without filler metal.
- Gasket
- A compressible seal placed between flange faces to prevent leakage.
- Heat number
- The identification of the steel melt a product was made from, traced on the mill test certificate.
- IBR
- Indian Boiler Regulations — rules governing materials and inspection for boilers and steam piping in India.
- ID (inside diameter)
- The bore of a pipe: OD minus twice the wall thickness.
- Lap joint flange
- A loose flange that slides over a stub end, allowing rotation and the use of cheaper backing material.
- LR elbow
- Long-radius elbow with a centreline radius of 1.5 × the nominal pipe size.
- MTC (mill test certificate)
- A document from the manufacturer certifying chemical composition and mechanical properties of a heat; often to EN 10204 3.1 or 3.2.
- NACE MR0175
- Standard (ISO 15156) setting material requirements, such as hardness limits, for sour H₂S service in oil and gas.
- NB (nominal bore)
- Indian and British term for nominal pipe size, e.g. 100 NB = NPS 4.
- Normalizing
- Heat treatment above the critical temperature followed by air cooling, refining grain structure; the N in A105N.
- NPS (nominal pipe size)
- North American pipe size designation in inches. Above NPS 14 it equals the outside diameter.
- OD (outside diameter)
- The outside diameter of a pipe, which is fixed for a given NPS regardless of schedule.
- PMI
- Positive material identification — a non-destructive check of alloy composition, usually by XRF or OES.
- PN
- European pressure rating for flanges (EN 1092-1), roughly equal to ambient pressure in bar.
- PREN
- Pitting resistance equivalent number = %Cr + 3.3 × %Mo + 16 × %N; higher means better resistance to chloride pitting.
- Raised face (RF)
- A flange facing with a raised ring that concentrates gasket stress; the most common facing type.
- RTJ (ring type joint)
- A flange facing with a groove for a solid metal ring gasket, used for high pressure.
- Schedule (Sch)
- A designation of pipe wall thickness, such as Sch 40 or Sch 80; higher numbers mean thicker walls.
- Seamless pipe
- Pipe made by piercing a solid billet, with no weld seam.
- Slip-on flange
- A flange the pipe slides into, fillet-welded inside and outside.
- Socket weld
- A joint where the pipe is inserted into a socket and fillet-welded on the outside, used for small bore.
- SR elbow
- Short-radius elbow with a centreline radius equal to the nominal pipe size.
- STD / XS / XXS
- Weight classes of pipe: standard, extra strong and double extra strong.
- Stub end
- A short pipe length with a flared lap, used with a lap joint flange.
- Super duplex
- Higher-alloy duplex stainless steel such as 2507 (UNS S32750) with PREN above 40.
- TPI (third party inspection)
- Inspection by an independent agency on behalf of the buyer before dispatch.
- UNS number
- Unified Numbering System designation for an alloy, e.g. S31603 for 316L, independent of product form.
- Weld neck flange
- A flange with a long tapered hub butt-welded to the pipe; the strongest flange type.
Last updated 2026-11-02