A buyer’s guide to ASME B16.5 flanges: types, facings and pressure classes
Flange enquiries go wrong in a few predictable places: the wrong standard, the wrong facing or a bore that does not match the pipe. This guide covers what to specify.
Which standard?
ASME B16.5 covers steel pipe flanges and flanged fittings from NPS ½ to NPS 24. From NPS 26 to NPS 60, flanges are made to ASME B16.47, which has two series. Series A flanges are heavier and derive from the older MSS SP-44 design; Series B flanges are more compact and derive from API 605. The two series have different bolt circles and do not mate, so an enquiry for a large flange must state the series.
Flange types
| Type | How it attaches | Typical use |
|---|---|---|
| Weld neck | Butt-welded to the pipe through a tapered hub | High pressure, high temperature and cyclic loads |
| Slip-on | Slides over the pipe and is fillet-welded inside and out | Lower-pressure services where fit-up is easier |
| Socket weld | Pipe sits in a socket and is fillet-welded | Small-bore, high-pressure lines |
| Threaded | Screwed onto threaded pipe | Small-bore lines where welding is not allowed |
| Lap joint | Loose flange over a stub end welded to the pipe | Lines dismantled often, or needing easy bolt alignment |
| Blind | No bore | Closing pipe ends, valves and nozzles |
Facings
- Raised face (RF): the most common. The raised face is 1/16 in (1.6 mm) high on Class 150 and 300 flanges and 1/4 in (6.4 mm) on Class 400 and above.
- Flat face (FF): used against cast iron or non-metallic equipment, so the mating flange is not cracked when bolted up.
- Ring-type joint (RTJ): a machined groove that takes a metal ring gasket, used for high pressures and temperatures.
Pressure classes
B16.5 has seven classes: 150, 300, 400, 600, 900, 1500 and 2500. The class is not a pressure in bar or psi. The allowable pressure depends on the material group and the temperature, and falls as temperature rises. The table shows ratings for Group 1.1 materials, such as A105 forgings, at −29 to 38 °C.
| Class | Maximum working pressure, Group 1.1 at −29 to 38 °C |
|---|---|
| 150 | 19.6 bar (285 psi) |
| 300 | 51.1 bar (740 psi) |
| 400 | 68.1 bar (990 psi) |
| 600 | 102.1 bar (1,480 psi) |
| 900 | 153.2 bar (2,220 psi) |
| 1500 | 255.3 bar (3,705 psi) |
| 2500 | 425.5 bar (6,170 psi) |
These figures are for orientation. Always design to the current edition of ASME B16.5 for your material group and temperature.
Matching the bore
A weld neck flange is bored to match the inside diameter of the pipe it welds to, so the pipe schedule or wall thickness must be in the enquiry. A mismatched bore leaves a step at the weld that disturbs flow and complicates inspection.
Common materials
ASTM A105 for carbon steel, A350 LF2 for low-temperature service, A182 F304/F304L and F316/F316L for stainless steel, and A182 F51 for duplex.
Gaskets and bolting
A flange joint is only as good as its gasket and bolts. For raised face flanges, spiral wound gaskets to ASME B16.20 are the usual choice in hydrocarbon service. RTJ flanges take metal ring gaskets, also to ASME B16.20, in the R, RX or BX profile that matches the groove. Carbon steel flanges are normally bolted with ASTM A193 B7 stud bolts and A194 2H nuts, or B7M and 2HM where hardness limits apply for sour service.
Flange markings
Each B16.5 flange should be marked with the manufacturer’s name or trademark, the material grade, the pressure class, the size, the facing where required, the bore or schedule for weld neck flanges, and the heat number that links it to its certificate.
A complete flange description
Weld neck flange, ASME B16.5, NPS 6, Class 300, RF, bored to Sch 40, ASTM A105N, with EN 10204 3.1 certificate.
Frequently asked questions
Can a Class 150 flange bolt to a Class 300 flange?
No. They have different bolt circles and bolt sizes. Mating flanges must have the same size, class and standard.
When are RTJ flanges used?
Ring-type joints are common in the higher pressure classes and wherever the project specification calls for them. They need ring gaskets matched to the groove.
What does the N in A105N mean?
Normalized. Many oil and gas specifications require A105 flanges in the normalized condition.
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