Fittings and branch connections — how a pipe changes direction

Pipe comes in straight lengths. Plants are not straight. Everything between the two is a fitting — and like flanges, fittings are dimensionally standardised so that a designer can set out a route long before anybody orders steel.

This page follows on from pipe sizes and schedules, and it reuses the most important idea there: the outside diameter is fixed by NPS. That is what makes a standard catalogue of fittings possible at all.

The four you will meet constantly

Elbow — changes direction, 90° or 45°.

Tee — takes a branch off a run, at 90°.

Reducer — changes size, either concentric or eccentric.

Cap — closes the end of a pipe.

Between them these cover the overwhelming majority of a spool. Everything else is a special case.

A catalogue of butt-weld fittings drawn at one pipe size and grouped by purpose. Changing direction — a long radius 90 degree elbow, a tighter short radius 90 degree elbow, a 45 degree elbow, and a 180 degree return. Branching — an equal tee, a reducing tee with a smaller branch, a cross with two opposed branches, and a 45 degree lateral. Changing size and ending a line — a concentric reducer with both ends on one centre-line, an eccentric reducer with one side flat, a cap closing a pipe end, and a stub end with a flared lip. — Every one of these is drawn at the same pipe size, which is the whole point — the outside diameter is fixed by NPS, so each shape is a catalogue item with published dimensions. Compare the two 90° elbows: same pipe, different turning radius.

A pallet of new carbon steel butt-weld fittings standing in a fabrication yard. Elbows, concentric reducers and tees are grouped together, their bodies covered in light mill rust and stencilled markings, while the machined weld bevels around every open end show as bright rings. Pipe racks and a plant stand behind. — The same shapes as steel. Note the bright ring around every opening — that is the machined weld preparation, and it is the only part of a new fitting that is not left as it came from the forge. The markings on the bodies carry the size, the schedule and the heat number.

Long radius and short radius

An elbow’s centre-line radius is quoted as a multiple of the nominal size:

LR — long radius, R = 1.5D. The default. Gentler turn, less pressure drop, less erosion, easier to inspect.
SR — short radius, R = 1.0D. Only where space genuinely will not allow an LR elbow.

For very large bores and gentle routes you will also see bends — pipe bent to a radius of 3D or 5D rather than a fitting welded in. A bend has no weld at the turn at all, which is why pipelines and pig-able lines use them.

The dimensions are fixed, and that matters

Butt-weld fitting dimensionsSource: ASME B16.9 — dimensions are centre-to-end, in mm

18 rows


1/2	15	38.1	16	25	25
3/4	20	38.1	16	29	25
1	25	38.1	22	38	38
1 1/4	32	47.6	25	48	38
1 1/2	40	57.2	28	57	38
2	50	76.2	35	64	38
2 1/2	65	95.2	44	76	51
3	80	114.3	51	86	51
4	100	152.4	64	105	64
6	150	228.6	95	143	89
8	200	304.8	127	178	102
10	250	381	159	216	127
12	300	457.2	190	254	152
14	350	533.4	222	279	165
16	400	609.6	254	305	178
18	450	685.8	286	343	203
20	500	762	318	381	229
24	600	914.4	381	432	267

These dimensions are fixed by the standard and do not change with wall thickness — a SCH 80 elbow is the same length as a SCH 40 elbow of the same size. That is what lets a designer set out a route before the wall thickness is finalised. Always confirm against the current edition of B16.9 before ordering or fabricating.

Look at what this table does not have: a column for schedule. A NPS 6 long radius elbow is 228.6 mm centre-to-end whether the wall is 3.4 mm or 18.26 mm. ASME B16.9 fixes the geometry; only the wall thickness changes, and it changes on the inside.

This is why a piping designer can lay out a route, calculate lengths and issue an isometric before the stress engineer has finalised the wall thickness. The geometry will not move.

Concentric or eccentric — a reducer decision that matters

A concentric reducer keeps both ends on the same centre-line. An eccentric reducer keeps one side flat and shifts the centre-line.

In a vertical line, use concentric; there is no top or bottom to worry about.

In a horizontal line the choice is a real engineering decision:

Flat side up (FOT, flat on top) — the top of the pipe stays level, so gas cannot collect in a pocket. This is what you use on a pump suction line: a vapour pocket drawn into a pump causes cavitation, which destroys impellers.
Flat side down (FOB, flat on bottom) — the bottom stays level, so liquid drains. Used where a line must drain completely, such as flare headers and hygienic piping.

Two horizontal pump suction lines compared. With a concentric reducer the top of the pipe steps down, creating a high point where vapour collects. With an eccentric reducer fitted flat side up, the crown of the pipe runs level into the pump and gas cannot gather. — The same reduction, two orientations. Only one of them avoids creating a vapour pocket immediately upstream of a pump.

Branch connections: the part beginners underestimate

Taking a small line off a big one seems trivial. It is the most mechanically interesting joint in the system.

When you cut a hole in a pipe wall, you remove metal that was carrying load. The hoop stress that metal was taking has to go somewhere, and it concentrates around the opening. ASME B31.3 therefore requires an area replacement check: the cross-sectional area of metal removed must be replaced within a defined zone around the opening.

That replacement can come from three places:

The fitting itself. A tee is forged with extra metal at the crotch. A weldolet has a heavy body. These are inherently reinforced — no calculation needed, no pad.
Spare wall thickness in the header. If the header was specified thicker than the pressure required, the surplus may be enough. This has to be proven by calculation.
A reinforcing pad. A ring of plate welded around the opening, adding back the missing area.

Four branch connections in section on a header pipe: an equal tee with extra forged metal at the crotch, a weldolet with a heavy fitting body, a plain stub-in with metal simply removed, and a stub-in with a reinforcing pad and a tell-tale vent hole. — Where the replacement metal comes from. The first two bring their own; the third relies on spare header wall and must be calculated; the fourth adds a pad.

Branch connection types and when each is usedSource: General piping practice; reinforcement to ASME B31.3 para. 304.3


Equal tee	1 : 1	Inherent — none needed	Branch is the same size as the header	Nothing particular. The simplest and strongest option.
Reducing tee	1 : 1 down to about 1 : 2	Inherent — none needed	Branch is one or two sizes below the header	Long lead time on uncommon size combinations
Weldolet	About 1 : 2 and below	Inherent — the fitting body is the reinforcement	Small butt-welded branch off a larger header	Needs a contoured cut in the header and a skilled fit-up
Sockolet	Small bore, NPS 2 and below	Inherent	Small socket-welded branch — vents, drains, instrument tappings	Socket crevice; often barred in high-pressure or sour service
Thredolet	Small bore, NPS 2 and below	Inherent	Threaded branch on utility or low-criticality service	Leak path through the thread; rarely permitted on hydrocarbons
Elbolet / latrolet	Small bore	Inherent	Tapping into an elbow, or a 45° branch off a header	Specify the header size and elbow radius when ordering
Stub-in, unreinforced	Any, where the calculation allows	None — must be proven by calculation	Header wall has enough spare thickness to take the opening	Only acceptable when the B31.3 area replacement check passes
Stub-in with reinforcing pad	About 1 : 2 and above	Added pad around the opening	Cheap large branch where the header alone is not strong enough	Pad must have a tell-tale vent hole, or trapped gas bursts it during PWHT

The branch-to-header size ratio drives most of this choice. As a rough guide, a branch of the same size or one size down takes a tee; below roughly half the header size, a branch fitting is normally cheaper and lighter than a reducing tee.

Reading the “olet” family

The names are trade names from a single manufacturer that became generic, covered dimensionally by MSS SP-97. They decode logically once you see the pattern — the first part is the branch end type, -olet is the branch fitting:

Weldolet — butt-welded branch
Sockolet — socket-welded branch
Thredolet — threaded branch
Elbolet — sits on an elbow
Latrolet — 45° lateral branch
Nippolet — an integral short nipple

Each is ordered by both sizes: the header it sits on and the branch it carries. A weldolet for a NPS 2 branch on a NPS 12 header is a different part from the same branch on a NPS 8 header, because the base is contoured to the header’s curvature.

Socket weld and threaded fittings

Small-bore fittings to ASME B16.11 are forged, not butt-welded:

Socket weld — the pipe drops into a recess and is fillet welded on the outside. A deliberate 1.5 mm gap is left at the bottom of the socket so the pipe can expand without loading the weld in a shrink-fit. Bottoming the pipe out is a rejectable defect.
Threaded — screwed, no welding. Restricted to small bore, low pressure and non-hazardous service, and often seal-welded afterwards.

Both create a crevice where liquid sits and corrodes, and both are far weaker in fatigue than a butt weld. Many hydrocarbon line classes ban them outright above a modest pressure — the same reasoning covered in flanges and pressure classes.

What the line class decides for you

As with materials, you will rarely choose freely. The piping material specification states, for each class, which fittings are permitted, which branch connection to use at each size combination, and when a pad is acceptable. Many projects include a branch table — a grid with header size down one axis and branch size across the other, and the required connection type in each cell.

Read it before designing a branch. It has already made the decision, and it is the document the fabricator will be checked against.

What to take away

Fitting geometry is fixed by ASME B16.9 and does not change with schedule — that is what lets a route be set out before the wall is finalised.
LR (1.5D) is the default elbow; SR (1.0D) costs pressure drop and erosion life.
Eccentric reducer, flat side up, on horizontal pump suction lines. Flat side down where the line must drain.
Cutting a branch removes load-carrying metal. It must be replaced — by the fitting, by spare header wall, or by a pad.
A reinforcing pad always has a tell-tale vent hole. Never plug it.
The line class and its branch table have already chosen. Read them first.