Classification by Material Family
The first and most useful classification is by material, because it determines which specification applies and therefore which chemical, mechanical and heat treatment rules the fitting must satisfy.
| Family | Specification | Typical grades | Service |
|---|---|---|---|
| Carbon steel | ASTM A234 | WPB, WPC | Moderate and high temperature |
| Alloy steel | ASTM A234 | WP5, WP9, WP11, WP12, WP22, WP91 | Elevated temperature steam and process |
| Low temperature carbon and alloy steel | ASTM A420 | WPL6, WPL9, WPL3 | Cold service, impact tested |
| Austenitic stainless steel | ASTM A403 | WP304, WP304L, WP316, WP316L, WP321, WP347 | Corrosive and cryogenic service |
| Duplex and super duplex | ASTM A815 | UNS S31803, S32205, S32750 | Chloride bearing and offshore service |
| High yield carbon steel | ASTM A860 | WPHY 42, 46, 52, 60, 65, 70 | High pressure transmission lines |
| Copper, aluminium and plastics | Various | - | Utility and low pressure service |
Carbon steel remains the workhorse of the family: WPB is the most widely ordered fitting grade in the world, and it is normally paired with A106 Grade B pipe in the same size and schedule so that the ambient temperature strength is matched.
Classification by Manufacturing Route
The same material can be supplied as a forged fitting, a seamless fitting formed from pipe, a welded fabrication, a casting or a machined part. Forged and seamless formed fittings dominate pressure service because there is no weld in the fitting body. Fabricated fittings, built up from pipe and plate, are common for large diameter and non-standard configurations and are accepted in many codes provided the welding and heat treatment are qualified. Cast fittings are used where the shape cannot be produced economically by forming. The purchase order should always state the acceptable routes, because the price and lead time differ substantially between them.
Classification by Dimensions and Radius
Butt welding fitting dimensions are standardised in ASME B16.9, which covers elbows, tees, reducers, caps and stub ends in the common shapes and sizes. Radius is expressed as a multiple of the nominal diameter:
Long radius elbows have a centre to end radius equal to 1.5 times the nominal pipe diameter, written R = 1.5D, and are the default in most piping systems because they produce a lower pressure drop. Short radius elbows have a centre to end radius equal to 1.0D, written R = 1.0D, and are dimensioned to ASME B16.28; they are used where space is restricted and a higher pressure loss is acceptable. Elbows are produced in 45 degree, 90 degree and 180 degree forms, and the 180 degree return is available in both long and short radius patterns.
Classification by Connection and Pressure Class
Fittings are also classified by how they join to the pipe. Butt welding ends are the norm for pressure and high temperature service. Socket welding and threaded fittings for small sizes are dimensioned to ASME B16.11, and pipe threads follow ASME B1.20.1. Flanged connections use the flange dimensions and pressure temperature ratings of ASME B16.5 for sizes through NPS 24 and ASME B16.47 for larger sizes, with pressure classes from Class 150 through Class 2500. The pressure class is not a pressure value in the absolute sense: it is a rating that must be derated for the actual material group and service temperature before it can be used in a design calculation.
Reading a Fitting Description Correctly
A complete description contains five items: specification and grade, product form, size, wall thickness or schedule, and end type. For example, an A234 WPB 90 degree long radius elbow, NPS 8, Schedule 40, butt welding ends, is unambiguous, while "8 inch carbon steel elbow" is not. Adding the applicable dimensional standard, the required heat treatment or class, and the inspection certificate requirement to the purchase order prevents almost all of the disputes that otherwise arise at goods receipt.
Frequently Asked Questions
Q: What is the most common material for carbon steel pipe fittings?
A: ASTM A234 WPB. It is used for moderate and high temperature service, has a minimum yield strength of 35 ksi (240 MPa) and is widely available in every standard form and size.
Q: What is the difference between a long radius and a short radius elbow?
A: The centre to end radius. A long radius elbow has R = 1.5D against the nominal diameter and is dimensioned to ASME B16.9; a short radius elbow has R = 1.0D and is dimensioned to ASME B16.28, giving a tighter turn with a higher pressure drop.
Q: Which fitting specification applies at low temperature?
A: ASTM A420 covers wrought carbon and alloy steel fittings for low temperature service, with grades such as WPL6 that are impact tested. Carbon steel grades such as WPB are not intended for that duty.
Q: Are welded fabricated fittings acceptable for pressure service?
A: Yes, when the fabrication is welded and heat treated to a qualified procedure and accepted by the applicable code. The purchase order should state whether fabricated construction is permitted, because some specifications and line classes require a seamless or forged body.
Q: How are fittings traceable to their material?
A: Each fitting carries marking that includes the manufacturer, specification and grade, size and wall thickness, and the heat number. That marking links the fitting to the material test certificate for the heat, which is the basis of the traceability package.