As the name suggests, pipe thread refers to the thread used on a pipe. Here, the pipe refers to a nominal pipe. Since this type of pipe is called a nominal pipe, the pipe thread is actually a nominal thread. Pipe threads, as a form of pipeline connection, are widely used for the connection and sealing of small and medium-sized pipelines transporting liquids and gases. There are three common types of pipe threads. They are: NPT thread, BSPT thread, and BSPP thread.
The main differences between the three types of threads:
| Pipe Thread | Angle | Taper/Parellel | Top & Bottom | Sealing Form | Standard |
| NPT | 60° | Tapered | Flat top, flat bottom | Filler | ASME B1.20.1 |
| BSPT | 55° | Tapered | Round top,round t bottom | Filler | ISO7-1 |
| BSPP | 55° | Parellel | Round top,round t bottom | Gasket | ISO228-1 |
Sealing principles and sealing methods of three types of pipe threads
Whether it is a 55 ° sealed pipe thread (BSPT) or a 60 ° sealed pipe thread (NPT), the sealing pair of the thread must be filled with medium during screwing. Generally, PTFE sealing tape is used to wrap the external thread, and the number of wraps varies from 4 to 10 depending on the thickness of the PTFE sealing tape. When the gap between the top and bottom of the tooth is aligned, it tightens with the tightening of the pipe thread. The inner and outer threads are pressed against each other, first eliminating the gap between the pressed sides. Then, as the tightening force increases, the tooth top gradually becomes sharper, the tooth bottom gradually becomes dull, and the gap between the tooth top and tooth bottom gradually disappears, achieving the purpose of preventing leakage. When there is a transition or interference fit between the top and bottom of the tooth, they first press against each other, causing the top of the tooth to gradually become dull and the bottom of the tooth to gradually become sharper, and then the tooth flank contacts and gradually eliminates the gap. Thus achieving the sealing function of the pipe thread.
The interference 55 ° non sealed pipe thread (BSPP) itself does not have a sealing function, and the thread only serves a connecting function. Therefore, a sealing gasket is needed for end face sealing. There are two forms of end face sealing: one is to use a flat gasket on the end face of the male thread, and the other is to use a combination gasket (elastic gasket sintered on the inner side of the metal ring) on the end face of the female thread.
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