Fittings are the components that let a pipeline change direction, branch, change size, join and close off. This article shows the common fittings with photos for easy identification, together with the pros and cons of each connection type: butt weld, socket weld, threaded, flanged, hygienic clamp and grooved coupling. Click an image to enlarge it.

This is the second article in our piping series. Previous article: Industrial piping classification by material, standard and application.

1. Direction change – Elbows

45° Elbow – TNI — **45° Elbow**Changes flow direction by 45° with low pressure loss.

90° Elbow – TNI — **90° Elbow**The most common type; a right-angle change of direction.

180° Return Bend – TNI — **180° Return Bend**Reverses the flow; used in heat exchanger tubing and heating coils.

Elbows come in two radii: long radius (LR) elbows with a bend radius of 1.5 × the nominal size, low pressure loss and the default choice; and short radius (SR) elbows with a radius of 1 × the nominal size, used only where space is tight because the pressure loss is higher.

2. Branch fittings

Equal Tee – TNI — **Equal Tee**All three ends the same size; splits or combines flow.

Reducing Tee – TNI — **Reducing Tee**Branch smaller than the run pipe.

Lateral Tee – TNI — **Lateral Tee**45° branch; smoother flow than a standard tee.

Cross – TNI — **Cross**Four ends; rarely used in process piping.

Stub-in – TNI — **Stub-in**The branch pipe is welded directly into a hole cut in the run; cheapest option, may need a reinforcing pad.

Olets – branch outlet fittings

Olets are forged to match the curvature of the run pipe and welded onto it, replacing a tee when the branch is much smaller than the run. The olet itself provides the required opening reinforcement, so an additional pad is normally not needed.

Weldolet – TNI — **Weldolet**Butt-weld branch.

Sockolet – TNI — **Sockolet**Socket-weld branch for small bore.

Threadolet – TNI — **Threadolet**Threaded branch, typically for pressure gauges and vents.

Sweepolet – TNI — **Sweepolet**Contoured branch; smooth flow and good load resistance.

Elbolet – TNI — **Elbolet**Branch on the outside of an elbow, often for a thermowell.

Latrolet – TNI — **Latrolet**45° lateral branch.

Nipolet – TNI — **Nipolet**Olet with an integral nipple, for drain or vent valves.

3. Size change – Reducers

Concentric Reducer – TNI — **Concentric Reducer**Both ends on the same axis; for vertical lines and pump discharge.

Eccentric Reducer – TNI — **Eccentric Reducer**One side flat; for pump suction and pipes on racks.

Swage Nipple – TNI — **Swage Nipple**Connects a larger butt-weld pipe to a smaller threaded or socket-weld pipe.

Design tip: at pump suction, eccentric reducers are installed flat on top (FOT) so that no air pocket forms and causes cavitation. For pipes on racks, eccentric reducers are installed flat on bottom (FOB) to keep the same bottom-of-pipe elevation and avoid shimming the supports.

4. Joining and closure fittings

Threaded Coupling – TNI — **Threaded Coupling**Joins two threaded pipe ends.

Socket Weld Coupling – TNI — **Socket Weld Coupling**Joins two small-bore pipes by socket welding.

Half Coupling – TNI — **Half Coupling**Welded onto a pipe or vessel shell for a small connection.

Threaded Union – TNI — **Threaded Union**Removable joint, convenient for maintenance.

Socket Weld Union – TNI — **Socket Weld Union**Like a threaded union, but welded to the pipe.

Hex Nipple – TNI — **Hex Nipple**Short nipple threaded at both ends with a hex for tightening.

Pipe Nipple – TNI — **Pipe Nipple**Short length of pipe threaded at one or both ends.

Cap – TNI — **Cap**Closes off the end of a pipe.

5. Connection types – pros and cons

5.1 Butt weld (BW)

✔ Advantages

Completely leak-tight and the strongest joint
Suitable for high pressure and temperature
Smooth bore, low pressure loss
Most economical for large sizes

✘ Disadvantages

Requires bevelling and careful fit-up
Needs qualified welders and NDT
Cannot be dismantled
Weld root protrudes into the bore

Standards: ASME B16.9 (fittings), ASME B16.25 (butt-welding ends)
Use for: DN50 (2″) and larger, main lines, steam, chemicals, high pressure and temperature.

5.2 Socket weld (SW)

✔ Advantages

Easy alignment, faster than butt welding
No weld metal penetrates into the bore
Leak-tight with good pressure capacity for small bore

✘ Disadvantages

The gap between pipe and fitting invites crevice corrosion
Not for hygienic systems or highly corrosive fluids
Unsuitable for severe vibration

Standards: ASME B16.11
Use for: small bore (usually up to DN50), instrument branches, drains and vents. Installation note: withdraw the pipe about 1.5 mm (1/16″) before welding so the weld does not crack from thermal expansion.

5.3 Threaded connection

✔ Advantages

Fast field assembly
No welding; usable in no-hot-work areas
Can be dismantled

✘ Disadvantages

Hard to make completely leak-tight
Threading reduces the wall thickness
Not for vibration, high temperature, toxic or flammable fluids

Standards: NPT – ASME B1.20.1; tapered BSP – ISO 7 (R, Rc, Rp); parallel BSP – ISO 228 (G)
Use for: small bore (usually up to DN50): water, compressed air, galvanised pipe, pressure gauges. NPT (60°) and BSP (55°) threads are not interchangeable.

5.4 Flanged connection

✔ Advantages

Easy to dismantle and maintain
Connects to valves, pumps, tanks and equipment
Available for almost all pipe materials

✘ Disadvantages

Costly and heavy
Every flange pair is a potential gasket leak point
Lower bending-moment capacity than a weld

Standards: ASME B16.5 (Class 150, 300…), JIS B2220 (5K, 10K, 20K…), EN 1092-1 (PN10, PN16…)
Use for: valves, pumps, equipment and any point that must be dismantled. JIS 10K and ASME Class 150 flanges are not interchangeable.

Flange types by attachment to the pipe:

Weld Neck (WN) – TNI — **Weld Neck (WN)**Butt welded to the pipe, the strongest type; for high pressure, high temperature and vibration.

Slip-On (SO) – TNI — **Slip-On (SO)**The pipe slips in and is welded inside and outside; cheap and easy to align, lower fatigue strength than weld neck.

Socket Weld (SW) – TNI — **Socket Weld (SW)**Welded on one side; for small bore.

Threaded – TNI — **Threaded**No welding; for small bore and no-hot-work areas.

Lap Joint – TNI — **Lap Joint**Stub end in the pipe material (stainless), loose flange in cheaper steel; bolt holes are easy to align.

Blind – TNI — **Blind**Closes off a pipe end or spare nozzle and can be removed when needed.

Flange facings:

FF – Flat Face: for mating with cast iron, plastic or FRP equipment so the flange does not crack.
RF – Raised Face: the most common facing for steel flanges.
RTJ – Ring Type Joint: uses a metal ring gasket, for very high pressure and temperature.
Tongue & Groove, Male/Female: locate the gasket more securely.

Two mating flanges must have the same facing type.

5.5 Hygienic clamp (Tri-Clamp)

✔ Advantages

Very fast, tool-free assembly
Smooth surface without crevices, easy to clean (CIP/SIP)
Meets the hygiene requirements of food and pharma

✘ Disadvantages

Limited working pressure
Gaskets must be replaced periodically
More expensive than welding

Standards: ISO 2852, DIN 32676, ASME BPE
Use for: hygienic stainless piping, points that are frequently opened for cleaning, instrument and hygienic valve connections.

5.6 Grooved coupling

✔ Advantages

Fast installation without field welding
Accommodates small misalignment and expansion
Easy to dismantle

✘ Disadvantages

Expensive fittings
Rubber gaskets limit temperature and chemical compatibility
Pressure thrust must be considered

Standards: Manufacturer standard (usually with UL/FM approval for fire protection)
Use for: fire protection, cooling water, HVAC, large-bore water piping.

6. Quick reference table

Connection	Typical size range	Pressure / temperature	Dismantling	Typical applications
Butt weld	DN50 and larger	High	No	Main lines, steam, chemicals
Socket weld	Up to DN50	High (small bore)	No	Small branches, instruments, drains
Threaded	Up to DN50	Low – medium	Yes	Water, compressed air, galvanised pipe
Flanged	All sizes	Per rating (Class, K, PN)	Yes	Valves, pumps, tanks, equipment
Hygienic clamp	DN10 – DN100 (typical)	Low	Very fast	Food, pharmaceuticals
Grooved coupling	DN25 and larger	Medium	Fast	Fire protection, cooling water, HVAC

Table – Quick comparison of pipe connection types

Conclusion

Identifying fittings correctly and choosing the right connection type keeps piping tight, durable and easy to maintain, and avoids wasted materials. When designing in AutoCAD Plant 3D, TNI fixes the fitting and connection types in each project’s piping class, so the isometrics and MTOs always list exactly what needs to be purchased.

Need piping design or fittings?

TNI provides 3D piping design and supplies hygienic stainless steel pipe, fittings and valves, tube fittings and industrial fittings to ASME, JIS and DIN.

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References

ASME B16.5, B16.9, B16.11, B16.25 – Flanges, butt-welding and forged fittings, butt-welding ends.
ASME B1.20.1 – Pipe Threads (NPT); ISO 7, ISO 228 – Pipe threads (BSP).
JIS B2220; EN 1092-1 – Flanges.
ISO 2852, DIN 32676, ASME BPE – Hygienic clamp connections.
TNI internal training material “Pipe and Fitting” (2020). Some illustrations are credited to manufacturer catalogues.

Pipe Fittings & Connection Types: Identification, Pros and Cons

1. Direction change – Elbows