NPT, BSPP and BSPT Thread Differences
The difference between NPT, BSPP and BSPT threads
Pressure and fluid systems in the automotive industry use different ways of sealing depending on the geographical region, size of the system, and environmental circumstances. Regional differences and industry preferences are also determining factors in the type of seals used. For example, many pressure systems on ships use BSPP adapters, while applications in the oil and gas industry tend to use NPT fittings. Following is a brief overview of each type of fitting, along with an explanation of the regions and applications where they’re best suited.
NPT (National Pipe Thread) seals are the most popular type of seal used in pressure 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 when you connect stainless steel to stainless steel, over-tightening or poor lubrication can cause galling 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.
BSPT (British Standard Pipe Taper) is similar to NPT but there are a few important differences. 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. Another important difference is that for many BSPT pipe sizes, the thread pitch differs from NPT. Thus an NPT male will sometimes 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.
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 ring seal to do the sealing. This seal is sandwiched between a shoulder on the male fitting and the face of the female fitting, and is squeezed in place. BSPP threads 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.