The difference between NPT, BSPP and BSPT seals
Thread: noun. a helical ridge on the outside of a screw, bolt, etc. or on the inside of a cylindrical hole, to allow two parts to be screwed together.
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.
Pressure systems in the process industry use different ways of sealing depending on the geographical region, size of the pressure system and the environment. There are regional differences in sealing types as well as industry preferences. For example, many pressure systems onboard ships use BSPP adapters while many applications in the oil and gas industry use NPT fittings. While one sealing style is not inherently better than another each style has its application.
NPT (National Pipe Thread) seals are the most popular type of seal for pressure calibration systems in the U.S. and Canada. NPT male adapters have a taper thread that wedges into the female NPT adapter. 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 and damage to the threads. Thread sealant is needed to seal but only 2 turns of thread sealant is required. Any more than that and the seal can leak around the thread sealant. Ralston Instruments has a wide variety of male NPT adapters , male Quick-disconnect NPT adapters, male NPT calibrator adapters, female NPT adapters, female NPT gauge adapters and female Quick-disconnect NPT gauge adapters.
BSPP (British Standard Parallel Pipe) is most popular in the UK, Europe, Asia, Australia, New Zealand and South Africa. It is a parallel thread fitting that uses a bonded seal ring to do the sealing. This bonded ring seal 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. No thread sealant is needed to form a seal.
BSPT (British Standard Pipe Thread) is similar to NPT except for one important difference. The angle across the flanks of threads (if you sliced the fitting in half long-ways and measured the angle from root to crest to root) is 55 degrees instead of 60 degrees as it is for NPT. Thus an NPT male will fit into a BSPT fitting or vice versa but they will not seal. This is a popular fitting in China and Japan but is very rarely used in North America unless the equipment to which it is attached was imported. Thread sealant is needed to seal the male and female fitting together. Ralston Instruments has adapters to both male BSPT adapters and female BSPT adapters.