Introduction
Ordering pipe fittings correctly requires understanding several key parameters: nominal size, wall thickness (schedule), pressure rating, end connections, material grade, and applicable standard. Incorrect ordering leads to mismatched flanges, wasted time, and project delays. This FAQ explains how to properly specify and order pipe fittings.
1. Nominal Pipe Size (NPS / DN)
Pipe size is specified by Nominal Pipe Size (NPS) in inches or Diametre Nominal (DN) in millimeters. Important: NPS does not equal the actual outside diameter for smaller sizes. For example, NPS 2 pipe has an outside diameter of 2.375 inches, not 2.0 inches.
Common NPS-DN conversions: NPS 1/2 = DN15, NPS 1 = DN25, NPS 2 = DN50, NPS 4 = DN100, NPS 8 = DN200, NPS 12 = DN300, NPS 24 = DN600.
How to measure: Measure the outside diameter of the existing pipe with a caliper. For pipes NPS 12 and smaller, the OD is fixed by the standard regardless of wall thickness. For NPS 14 and larger, NPS equals the OD in inches.
2. Wall Thickness (Schedule)
The pipe wall thickness is specified by the Schedule number. Common schedules include SCH40 (standard), SCH80 (extra strong), SCH160 (double extra strong), and XXS. The schedule determines the pressure-carrying capacity of the pipe and fittings.
How to determine: Check the existing pipe specification, or measure the wall thickness with a caliper and compare to a schedule table. The fitting must have the same wall thickness (or matching pressure class) as the mating pipe.
3. End Connection Type
Specify how the fitting connects to the pipe:
Butt weld (BW): Fitting ends are beveled for welding to the pipe. Most common for larger sizes and high-pressure services.
Threaded (NPT/BSPT): Female or male threads for screwing onto threaded pipe. Common for small-bore and low-pressure applications.
Socket weld (SW): Pipe slips into a socket and is fillet-welded. Common for small-bore high-pressure lines.
Flanged: The fitting comes with an integral flange connection. Specify flange facing (RF, RTJ, etc.) and pressure rating.
4. Flange-Specific Parameters
When ordering flanges, specify:
Flange type: WN, SO, BL, LJ, SW, TH
Size (NPS) and pressure rating (Class or PN)
Standard: ASME B16.5, DIN, EN, JIS, GOST
Facing: RF, MFM, RTJ, TF
Material grade: A105, 304/304L, 316/316L, etc.
5. Material Grade
Specify the exact material standard and grade. Examples: ASTM A234 WPB (carbon steel butt weld fittings), ASTM A105 (carbon steel flanges), ASTM A312 TP304/304L (stainless steel pipe/fittings), A860 WPHY (high-yield pipeline fittings). Always include the material standard number, not just "carbon steel" or "stainless steel."
6. Standard and Compliance
Specify the manufacturing standard. For buttweld fittings: ASME B16.9, MSS SP-75, EN 10253, DIN, or GOST. For flanges: ASME B16.5, B16.47, EN 1092-1, DIN, JIS, or GOST. Also specify any additional requirements such as 3.1 material certificates, third-party inspection, or special testing.
Sample Order Specification
A complete order specification might look like:
"10 pieces - 90° Long Radius Elbow, NPS 4, SCH40, ASTM A234 WPB, ASME B16.9, butt weld ends, black painted, with 3.1 MTC."
"2 pieces - Weld Neck Flange, NPS 2, Class 150 RF, ASTM A105, ASME B16.5, with 3.1 MTC."
Conclusion
Correctly ordering pipe fittings requires specifying size, wall thickness, end connection, material, standard, and pressure rating. Double-check all parameters against the existing piping or project drawings, and always request material certificates for traceability. When in doubt, provide the manufacturer with as much detail as possible for confirmation.
References
1. ASME B36.10M, Welded and Seamless Wrought Steel Pipe.
2. ASME B16.9/B16.5, Fittings and Flanges Standards.
