A flange enquiry that says only "6 inch flange" cannot be priced, let alone supplied. Type, pressure class, facing, material and bore all change the part, and getting one of them wrong is usually discovered at bolt-up. This article explains the six flange types covered by ASME B16.5, what distinguishes them structurally, how the pressure classes translate into working pressure at temperature, and the minimum information a procurement engineer should put on a requisition.
Where ASME B16.5 applies
ASME B16.5 covers pipe flanges and flanged fittings from NPS 1/2 to NPS 24 in pressure classes 150, 300, 400, 600, 900, 1500 and 2500. It fixes dimensions, tolerances, facing details, bolting, marking and the pressure-temperature ratings for each listed material group. Flanges larger than NPS 24 fall under ASME B16.47, which has two dimensional series, A and B, that are not interchangeable with each other. European EN 1092-1 (PN-rated) flanges are a different system with different bolt circles, so the standard must always be stated.
Carbon steel flanges are most commonly forged to ASTM A105, which is the standard partner for A53 and A106 Grade B pipe, with A350 LF2 for low-temperature service. Stainless flanges are forged to A182 in grades such as F304, F304L, F316 and F316L. Material and class together determine the rating, as the table further down shows.
The six flange types compared
| Type | Attachment to pipe | Relative strength and fatigue | Typical use | Avoid when |
|---|---|---|---|---|
| Weld-neck (WN) | Single full-penetration butt weld on a long tapered hub; bore matches pipe | Reference case: highest static strength and fatigue life | High pressure, high or sub-zero temperature, cyclic or vibrating lines, critical process | Cost matters more than integrity on low-pressure utility lines |
| Slip-on (SO) | Pipe slides in; two fillet welds, inside and outside | About two-thirds of weld-neck static strength; about one-third of its fatigue life | Low-pressure water, air, HVAC and general plant piping | High pressure or temperature, fatigue or vibration |
| Socket-weld (SW) | Pipe enters a socket; one external fillet weld, with a gap of about 1.6 mm (1/16 in) left at the bottom | Static strength as slip-on; fatigue strength about 50 % higher than slip-on | Small-bore high-pressure piping, NPS 2 and below | Corrosive fluids, where the socket gap acts as a crevice |
| Lap-joint (LJ) | Not welded; backs up a stub end that is butt-welded to the pipe | Pressure capacity little better than slip-on; fatigue life about one-tenth of weld-neck | Stainless or alloy lines with carbon steel backing flanges; systems dismantled often; alignment of bolt holes | Severe cyclic service |
| Threaded | Screwed onto threaded pipe, sometimes seal-welded | Limited by threads; thin walls unsuitable | Small sizes, typically to NPS 4; locations where welding is prohibited | Thin-wall pipe, high temperature steam, large sizes |
| Blind | No bore; bolted to close a line, valve or nozzle | Highly stressed in bending at large sizes | Line ends, future connections, pressure testing, isolation | Not applicable; select class and thickness by rating |
Weld-neck, slip-on and socket-weld: what the weld does
The three welded types differ mainly in the weld. A butt weld develops at least the strength of the parent pipe; a fillet weld develops roughly one-third of that. A weld-neck flange therefore behaves as a continuation of the pipe, with the tapered hub spreading bending stress into the wall. That is why specifications call for weld-neck flanges on steam, thermal oil, process and any cyclic or vibrating line, and why the bore must be ordered to match the pipe schedule.
A slip-on flange is cheaper to buy and easier to fit, because the pipe can be cut short and the flange adjusted for alignment before welding. The penalty is the two fillet welds: about two-thirds of the pressure strength and about one-third of the fatigue life of a weld-neck. For chilled water, condenser water, compressed air and fire mains at Class 150 that is normally acceptable, which is why slip-on flanges dominate building services work.
Socket-weld flanges were developed for small-bore, high-pressure lines where butt welding small pipe is slow. The pipe is inserted, withdrawn about 1.6 mm to leave an expansion gap, and fillet-welded outside. The gap is the weakness: it traps liquid and starts crevice corrosion, which is why many stainless and corrosive-service specifications prohibit socket-weld joints and call for butt-weld fittings instead.
Lap-joint and threaded: the no-weld options
A lap-joint flange is a loose ring with a radiused bore that sits behind a stub end. The stub end, made to ASME B16.9 or in the lighter MSS SP-43 pattern, is butt-welded to the pipe and provides the sealing face; the flange never touches the fluid. This allows a carbon steel A105 backing flange on a 316L stainless line, a real saving in large sizes, and lets the flange rotate to align bolt holes. The price is fatigue life of about a tenth of a weld-neck, so lap joints belong on low-pressure, non-critical and frequently dismantled lines.
Threaded flanges need no welding, which makes them the default where hot work is prohibited and in galvanised systems where welding would destroy the coating. Their limits are set by the thread: on the smallest sizes threading removes more than half of the pipe wall, and ASME B31.3 requires that threaded steel pipe for steam above 250 psi, or for water above 100 psi at temperatures above 220 °F, be seamless and at least Schedule 80. In practice threaded flanges are used to NPS 4 and rarely above Class 300.
Pressure classes and the pressure-temperature table
The class number is not a working pressure. It is an index into a table that gives the maximum working pressure for each material group at each temperature, and the pressure falls as temperature rises. The table below is the ASME B16.5 rating for Material Group 1.1, which includes A105 forgings, A350 LF2, A216 WCB castings and A516 Grade 70 plate.
| Temperature | Class 150 | Class 300 | Class 400 | Class 600 | Class 900 | Class 1500 | Class 2500 |
|---|---|---|---|---|---|---|---|
| minus 29 to 38 °C | 19.6 bar | 51.1 bar | 68.1 bar | 102.1 bar | 153.2 bar | 255.3 bar | 425.5 bar |
| 100 °C | 17.7 bar | 46.6 bar | 62.1 bar | 93.2 bar | 139.8 bar | 233.0 bar | 388.3 bar |
| 200 °C | 13.8 bar | 43.8 bar | 58.4 bar | 87.6 bar | 131.4 bar | 219.0 bar | 365.0 bar |
| 300 °C | 10.2 bar | 39.8 bar | 53.1 bar | 79.6 bar | 119.5 bar | 199.1 bar | 331.8 bar |
| 400 °C | 6.5 bar | 34.7 bar | 46.3 bar | 69.4 bar | 104.2 bar | 173.6 bar | 289.3 bar |
Three things to read from the table. Class 150 loses rating quickly with temperature: 19.6 bar at ambient becomes 13.8 bar at 200 °C and 6.5 bar at 400 °C, so a Class 150 flange is not a 20 bar flange on a hot line. Class 300 holds its rating far better and is the usual step up for steam and thermal oil. Below minus 29 °C the rating may be no higher than the minus 29 °C value, and prolonged service above 425 °C is not recommended for these carbon steels because of graphitisation. Stainless and alloy flanges have their own material groups; refer to the standard for those. Every component across a joint (flange, gasket, bolts) must be rated for the class and temperature of the line.
Facings and what to write on an enquiry
All B16.5 flange types except the lap-joint are supplied with a raised face (RF) by default: approximately 1.6 mm (1/16 in) high for Class 150 and 300 and approximately 6.4 mm (1/4 in) for Class 400 and above. Flat face (FF) is used against cast iron or non-metallic mating flanges that would crack under a raised-face bolt load. Ring-type joint (RTJ) facings, with a machined groove and a metal ring gasket, are used at high pressure and temperature, generally from Class 600 upwards.
A complete flange line on a requisition reads, for example: "Flange, weld-neck, NPS 6, Class 300, RF, ASME B16.5, ASTM A105, bore to match Sch 40 (7.11 mm wall)". Type decides how it joins the pipe; size and class decide the bolt pattern and rating; facing decides the gasket; standard decides the dimensions; material decides the rating table; and bore decides whether a weld-neck or socket-weld joint can be welded without mismatch. For lap-joint flanges add the stub end specification; for threaded flanges state the thread form.
Key takeaways
- ASME B16.5 covers NPS 1/2 to 24 in classes 150 to 2500; larger sizes are B16.47 Series A or B, and EN PN flanges are a different, non-interchangeable system.
- Weld-neck flanges are the reference for strength and fatigue; slip-on flanges offer about two-thirds of the strength and one-third of the fatigue life and suit low-pressure utility service.
- Socket-weld flanges suit small-bore high-pressure lines but create a crevice; lap-joint flanges save alloy cost but have about a tenth of the weld-neck fatigue life.
- The class is an index, not a pressure: a Class 150 A105 flange is rated 19.6 bar at ambient but only 6.5 bar at 400 °C.
- Specify type, size, class, facing, standard, material and bore on every enquiry line.
Frequently asked questions
Can a slip-on flange be used on a steam line?
Most steam specifications call for weld-neck flanges because of thermal cycling and the lower fatigue life of the double fillet weld. For low-pressure heating steam within a building, the line class may permit slip-on at Class 150; check it before ordering rather than assume.
What is the difference between Class 150 and PN 16?
They are ratings from two different standards, ASME B16.5 and EN 1092-1, with different bolt circles and thicknesses in most sizes. A Class 150 A105 flange is rated 19.6 bar at ambient; a PN 16 flange is nominally 16 bar. They are not interchangeable.
Why does the weld-neck flange bore have to match the pipe schedule?
Because the flange is butt-welded to the pipe end. A bore mismatch leaves a step at the weld root that weakens the joint and disturbs flow, so always state the pipe schedule or wall thickness when ordering.
How Al Safsaaf can help
Al Safsaaf Trading & Contracting Company supplies flanges in carbon steel and stainless steel for MEP and industrial projects across Saudi Arabia, alongside the pipe and fittings they connect. Send the type, size, class, facing and material from your drawing through our contact page and we will confirm the matching items.
Sources
- Types of flanges: welding neck, slip-on, socket weld, lap joint, threaded, blind (Wermac) — construction and use of each type; slip-on two-thirds strength and one-third fatigue life; lap joint one-tenth fatigue life; socket weld 1/16 in gap and 50 % higher fatigue than slip-on; fillet weld one-third of butt weld; threading removes about 55 % of wall; B31.3 threaded steam and hot water rule; stub ends to B16.9 and MSS SP-43
- ASME B16.5 flange types (Projectmaterials) — B16.5 up to NPS 24, B16.47 Series A and B above NPS 24, classes 150 to 2500, threaded typically to 4 in, socket weld 1.6 mm gap, materials A105, A350, A182
- Pressure-temperature ratings ASTM Group 2-1.1 (Wermac) — ASME B16.5 ratings in bar for Class 150 to 2500 from minus 29 to 400 C; Group 1.1 materials; 425 C graphitisation note; below minus 29 C rule
- Raised face height (Wermac) — 1.6 mm (1/16 in) for Class 150 and 300; 6.4 mm (1/4 in) for Class 400 to 2500
- ASTM A106 seamless pipe (Wermac) — A105 flanges paired with A106 Grade A, B and C pipe
