As most industrial plants require more than one equipment to be connected for facility to be able to deliver value, we need a way to somehow connect these and that’s why joints are integral part of any process plant. There are a number of different options when it comes to piping joints: flanges, hubs, screwed joints, compression fittings, etc. This article will cover screwed joints (threaded connections).
Pipe threads are either straight (parallel) or tapered, while both options have a couple of design options themselves - threads may be according to national piping standard or british standard; be right-handed (RH) or left-handed (LH). Neither NPT, nor BSP is superior to the other one and the selection is driven by a number of factors, including geographical location, equipment size and industry. Pressure systems on board of ships use BSPP adapters, while Oil & Gas industry prefers NPT fittings; while NPT threads are common in the North and South America, BSP threads are widely used in most European, Asian, African countries and Australia.
Most threads are based on trade size (which is approximately equal to the thread's major diameter), rather than actual diameter, which can lead to some confusion when measuring ports. For example, 1" NPT pipe thread has an outside diameter (OD) of 1.315". On the contrary, in the modern version of standards for metric pipe thread, nominal size is simply a size number, where listed diameter size is the major outer diameter of the external thread.
Straight (parallel) threads are meant only to mechanically join parts and to hold a fitting in place. That’s why some type of seal, either an O-ring (elastomer), hard metal or a soft seat seal is required. Straight pipe threads include NPSM (National Pipe Straight Mechanical), NPSH (National Pipe Straight Hose) and BSPP (British Standard Pipe Parallel). Frequent applications of NPSH and NPSM thread we can see in our daily lives are the garden hose (GHT) and fire hose (NST).
Tapered threads have a slight taper. When mated together and tightened, the threads compress and form a seal, however usually a thread sealant is required, as sealing on a threaded joint only takes place on two to three threads within the joint once fully tightened. For a leak-free seal, we recommend using a sealant compound (Permabond A131, ROCOL Oilseal, ROCOL Foliac Manganese, Loctite 572, Loctite 577, Loctite 270, Swagelok SWAK, Furmanite FCE, etc.) or Polytetrafluoroethylene (PTFE) tape, which will assist by filling any surface imperfections such as scratches on the threaded faces. Another option may be a seal weld, but obviously, it may not be considered if the joint may require to be undone.
They seal due to the "out of roundness principle" which means that the male stretches the female fitting until there is so much force that the connection can hold pressure. One of the challenges with this design is that if you connect stainless steel to stainless steel then over-tightening or poor lubrication can cause gauling / galling (a form of wear caused by adhesion between sliding surfaces) and damage to the threads. Sometimes MPT and FPT are used in the literature instead of MNPT and FNPT, respectively.
From historical point of view, in 1864, William Sellers, being the president of Franklin Institute, presented a standard for nuts, bolts and screws, which became the National Pipe Tapered (NPT) thread.
NPT Size Table
Pipe thread sizes are standard and for any NPT pipe, NPT thread dimensions can be found using the table below.
These values are for reference, for more detailed information please refer to the relevant standard. NPT, NPSC, NPTR, NPSM and NPSL are covered under ASME B1.20.1.
BSP is divided into two types
BSPT (British Standard Pipe Taper), aka G or Rp
BSPP (British Standard Pipe Parallel), aka R or Rc
BSPT is very similar to NPT fitting with slight difference on the included angle (60 vs 55 in the case of NPT), while BSPP employs parallel threads and provides a seal via a bonded seal ring, which is sandwiched in-between a shoulder on the male fitting and the face of the female fitting and is squeezed in place. BSPP pressure gauges have a longer male thread and use a copper crush washer that is squeezed in between the bottom of the male fitting and the bottom of the female BSPP hole forming a pressure tight seal.
BSP Size Table
Just like in the case of NPT connection, there is a table you can use for BSPP and BSPT thread.
These values are for reference, for more detailed information please refer to the relevant standard:
ISO 228: for joint of external and internal parallel threads (G)
ISO 7 or EN 10226: for external taper (R), internal parallel (Rp) and internal taper (Rc)
Difference between NPT and BSP
There are a number of differences:
BSPP is totally different from both BSPP and NPT, as it employs parallel threads.
Threads of BSPT and BSPP are at 55° angle, while it is 60° for NPT.
NPT threads are flattened at the peaks and valleys (aka Sellers form), while BSPT/BSPP threads are rounded (aka Whitworth form).
BSPP and BSPT (in most cases) have different threads per inch (TPI) (and consequently pitch), as opposed to NPT.
Because of aforementioned differences, mostly it is not recommended to substitute BSPT and BSPP threads for NPT threads, with a couple of minor exceptions:
Both 1/2" NPT and 1/2"BSP have 14 threads per inch so they match up quite well. The primary difference is in the angle of the thread, which is 60 vs 55, but usage of a PTFE tape can provide sufficient seal for low pressure applications.
The same thing is applicable for 3/4" NPT and 3/4" BSP, both of which have 14 threads per inch.
So, unless you are working with low pressures and are totally out of options, it is not advisable to use NPT with BSP. Results may be devastating! Most probably, it won't go any further than 2-3 turns, a slack fitter will say it's good enough and leave it. Please bear in mind that only 1 thread will be sealing the pressure off and the "unexpected" will happen sooner rather than later, if pressures are sufficiently high. Although it is one of the most frequent thread mismatches, it does not mean that it's safe to do so!