Threaded Fittings

NPT versus BSP Threads

NPT and BSP threads are not interchangeable, and mixing them creates a joint that leaks. NPT, or National Pipe Thread, is the American standard defined in ASME B1.20.1, with a 60 degree thread angle and a taper of 1 in 16. BSP comes in two forms: BSPT, the tapered form, with a 55 degree thread angle and a taper of 1 in 16, and BSPP, the parallel form, with a 55 degree thread angle and no taper, which seals on a gasket or an O ring at the face rather than on the thread. Because the thread angles differ, an NPT male will engage a BSPT female only partially and will leak under pressure even when tightened hard. Always confirm which thread standard the project uses, and specify it explicitly on the order, since the two look similar to the eye.

When to Use Threaded Fittings

Threaded fittings are the practical choice where welding is impractical or undesirable: for small bore instrument and utility lines, for connections that must be assembled and disassembled in the field, for maintenance tie-ins, and for lines in hazardous areas where hot work is not permitted. Their limitation is the thread itself, which is the sealing element and the weakest point of the joint. Threaded joints are therefore not used above the size and pressure limits set by the applicable code, and are generally restricted by ASME B31.3 to sizes of 2 inch and smaller for severe cyclic service. In sour service, threaded joints are additionally restricted because the thread root acts as a stress concentration, and the number and size of threaded joints is often limited by the project specification.

Threaded fittings: where hot work is not allowed

Threaded fittings are used where a line has to be assembled without welding, where it is small bore and utility, or where the joint may have to be broken later. The thread standard and the pressure class are separate decisions, and the thread has to be stated, because NPT and BSP are not interchangeable.

Grades supplied and minimum properties

The grade fixes the minimum strength and the chemistry. The table below gives the requirements that separate the forged fitting grades ordered most often; the full composition and mechanical table for each grade is on its own page.

The complete chemistry and mechanical table for each grade, with the notes that qualify it, is on the individual grade page – for example ASTM A105 flange. The stainless grade has the higher tensile requirement but the lower minimum yield strength, so the two are not interchangeable on the strength table alone. The class sets the wall, so the grade only has to be compatible with the pipe it is screwed onto.

Source: specified minimum yield and tensile strength for each grade, per ASTM A105 / ASME SA105 and ASTM A182 / ASME SA182. The standard named against each grade is the governing document.

• A105
• A350 LF2
• F304
• F304L
• F316
• F316L
• F321

Pipe Nipples in NPT and BSP Threads

A pipe nipple is a short length of pipe threaded at both ends, used to join two fittings, valves or lines without a weld. The thread is most of the specification: nipples are threaded to ASME B1.20.1 (NPT) or to BS 21 (BSPT and BSPP), and the two systems are not interchangeable. The body is seamless or welded pipe, normally carbon steel to ASTM A106 or A53, or stainless to ASTM A312 TP304 and TP316; the governing product specification for steel nipples is ASTM A733.

Three shapes cover most orders:

• Hex nipple – a hexagon at the middle so it can be tightened with a wrench, used between two female threads.
• Barrel nipple – a plain barrel with no hex, for close assemblies where there is no room for a wrench.
• Reducing nipple – a different thread size at each end, to step between two line sizes.

Overall length is stated as close, short barrel or long barrel, and the finish is normally black or hot dip galvanized, with stainless pickled or as machined. Sizes run from NPS 1/8 to NPS 4. A reducing coupling is the same joint made with a fitting rather than a length of pipe, and is supplied in the same thread standards.

Send the thread standard, both end sizes, the overall length and the material, and we quote availability and price with the mill test certificate. Where nipples are cut from a pipe run, the heat number of that run carries over to the certificate.

Download Threaded Fittings data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.

What decides the price

There is no list price for fittings: the same drawing can be quoted at different numbers depending on seven things. Send them and the quotation comes back within one business day.

Reply within one business day. Send the seven items above with the quantity and the destination port. Send the specification or WhatsApp the specification.