What Is a Half Coupling?
A half coupling is a forged pipe fitting that provides a small outlet on a larger run pipe. It looks like a short piece of tube - essentially a full coupling cut down to roughly half its length - and its bore matches the nominal size of the branch that will be connected. One end is welded to the header or run pipe, and the other end accepts a branch pipe either by socket welding or by a tapered thread. Because it projects only a little from the surface of the header, it is the most economical way to create an instrument, drain, vent or small process connection where a shaped outlet fitting is not required.
Types and How They Are Supplied
Half couplings are supplied in two connection forms. The socket-weld type has a plain bore with a defined socket depth so that the branch pipe is inserted and secured with a fillet weld. The threaded type has a tapered pipe thread, normally to ASME B1.20.1, and may be supplied with a seal weld if the service demands it. Both forms are produced as straight and reducing items, so a half coupling can carry a branch one or more sizes smaller than the outlet itself. Related fittings include full couplings, reducers, weldolets and socket outlets; the half coupling is normally preferred where the branch is small, the pressure class is moderate and the run pipe wall is thick enough to accept the attachment weld without local reinforcement.
Dimensions and Pressure Classes to ASME B16.11
The dimensions, tolerances, marking and material requirements for forged socket-welding and threaded fittings are given in ASME B16.11, which covers sizes from NPS 1/2 through NPS 4. Fittings are grouped by pressure class, and the class is tied to the wall schedule of the pipe they are intended to match:
| Connection | Class | Corresponding pipe schedule |
|---|---|---|
| Socket welding | 3000 lb | Schedule 80 |
| Socket welding | 6000 lb | Schedule 160 |
| Socket welding | 9000 lb | Double extra strong |
| Threaded | 2000 lb | Schedule 80 |
| Threaded | 3000 lb | Schedule 160 |
| Threaded | 6000 lb | Double extra strong |
In Chinese petrochemical practice the equivalent fitting standard is SH/T 3410, which lists half couplings up to DN80; installations below DN40 account for the great majority of demand, and above that size a reducing tee or a shaped outlet is usually more economical. Each fitting must be marked with the material grade, pressure class, size and the manufacturer's identification.
Materials and Selection
Half couplings are forged, so the usual grades are ASTM A105 for carbon steel, ASTM A350 LF2 for low-temperature service, ASTM A182 F304, F304L, F316, F316L and F321 for stainless steel, and ASTM A182 F11, F22 and F91 for chromium-molybdenum service. The material of the half coupling should match or be compatible with the run pipe, and it must be suitable for the design temperature of the joint. Because the pressure rating of a forged fitting is derived from its class and material group, a stainless steel half coupling and a carbon steel half coupling of the same class do not have the same allowable pressure at temperature; the rating tables of the governing code must be consulted for the material actually supplied.
Installation, Welding and Inspection Points
The typical sequence is to mark the position on the run pipe, cut a hole no larger than the fitting bore plus the specified tolerance, fit the half coupling perpendicular to the pipe axis and weld it with a full fillet. For socket-weld fittings a small gap of about 1.6 mm (1/16 in.) should be left at the bottom of the socket before welding, because the pipe expands on heating and a tightly bottomed branch can crack the weld root. Weld size and preparation follow the applicable code, and for threaded fittings an approved thread compound is used, with seal welding added where specified. On thin-wall or high-pressure headers the designer must confirm that the opening does not require reinforcement. Inspection normally includes dimensional and orientation checks, visual examination of the fillet, magnetic particle or dye penetrant examination where required, and the system hydrostatic test after installation.
FAQ
Q: When should a half coupling be used instead of a tee?
When the branch is small relative to the run pipe, typically below DN50, a half coupling is a simpler and cheaper solution than a reducing tee. Above that size, or where the branch carries significant flow, a tee or a shaped outlet fitting gives better flow and structural behaviour.
Q: Can a half coupling be welded onto a thin-wall pipe?
Yes, but the opening reduces the available ligament of the run pipe, so the wall thickness and the need for reinforcing pad or a thicker header must be evaluated against the design code before welding.
Q: What is the difference between a half coupling and a full coupling?
A full coupling is essentially a short length of tube threaded or socketed at both ends and used to join two pipes in line. A half coupling is about half as long and is welded to the surface of a run pipe to create an outlet.
Q: Are half couplings available in stainless steel and alloy steel?
Yes. They are forged in carbon steel, low-temperature carbon steel, austenitic stainless steel and chromium-molybdenum grades, and the grade should be selected to match the run pipe and the design temperature.
Q: How is a threaded half coupling sealed?
The tapered thread carries most of the seal, assisted by an approved thread compound. Where leakage is unacceptable the joint is seal welded, and the weld procedure must allow for the material and for any post-weld heat treatment the code requires.
