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Stainless Steel Pipe Fittings
Stainless steel piping systems can be joined by butt welding and, in the heavier 40S and 80S schedules, by threaded connections.
The fabrication of piping systems is made possible by the availability of complementary fittings and flanges. These are specified by the same DN or NPS designator and schedule numbering system as for pipe and enable joining to each other and to equipment such as vessels and valves.
Butt welding pipe fittings
A piping system using butt welding fittings has many inherent advantages over other forms.
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Welding a fitting to the pipe means that it is permanently leakproof.
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The continuous metal structure formed between pipe and fitting adds strength to the system.
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Smooth inner surface and gradual direction changes reduce pressure losses and turbulence and minimise the action of corrosion and erosion.
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A welded system utilises a minimum of space.
Atlas Specialty Metals supply butt welding fittings in the following forms.
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Elbows 45˚ and 90˚ – long radius and short radius.
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Return bends 180˚ – long radius and short radius.
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Reducers – concentric and eccentric.
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Tees – equal and reducing.
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Caps.
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Stub ends – Type B.
Butt welding fittings can be supplied in either seamless or welded construction and are covered by specification ASTM A403M (or ASTM A815M for Duplex grades).
Screwed and socket weld fittings
Piping systems can also be connected using screwed fittings – BSP threaded ("150lb") low pressure fittings and NPT class 3000 high pressure fittings in Grade 316.
Socket weld fittings are also used in high pressure piping systems and are available in grade 316L to suit schedule 80S wall thickness piping.
Manufacturing specification:
ASTM A182M (3000lb NPT and socket weld).
ASTM A351 (BSP castings)
ANSI B16.3 (BSP dimensions)
ISO 4144 (BSP dimensions)
ASTM A182M (3000lb NPT and socket weld).
ASTM A351 (BSP castings)
ANSI B16.3 (BSP dimensions)
ISO 4144 (BSP dimensions)
Pipe flanges
A flange is a ring of steel (forged, cut from plate, or rolled) designed to connect sections of pipe, or to join pipe to a pressure vessel, valve, pump or other integral flanged assembly. Flanges are joined to each other by bolting, and are joined to the piping system by welding or threading (or loose when stub ends are used).
The basic types of flanges are:
| • slip-on | • weld neck |
| • socket weld | • orifice |
| • blind | • threaded |
| • lap joint |
Forged stainless steel flanges are designed to the following common pressure ratings:
Classes 150, 300, 600, 900, 1500 and 2500. Standard sealing face is Raised Face (RF).
Classes 150, 300, 600, 900, 1500 and 2500. Standard sealing face is Raised Face (RF).
Manufacturing specification: ASTM A182M
Plate stainless steel flanges are cut and machined from plate – Table ‘D’, Table ‘E’, etc.
Manufacturing specification: AS2129.
Types and applications of flanges
| Slip-on flanges | The flange is slipped over the pipe and then welded both inside and outside to provide sufficient strength and prevent leakage. Slip-on flanges are also used as loose back-up flanges when stub ends are used. |
| Blind flanges | This is a flange without a centre bore, used to shut off a piping system or vessel opening. |
| Weld neck flanges | Designed to be joined to a piping system by butt welding.They are relatively expensive due to the weld neck, but are preferred for high-stress applications. |
| Lap joint flanges | This is again similar to a slip-on flange, but has a radius at the intersection of the centre bore and the flange face to accommodate a lap stub end. |
