Regarding the choice between G threads and NPT threads

G thread is a straight pipe thread, while NPT is a threaded seal. Compared to the latter, the NPT has a stronger sealing performance than G. G is generally used at the inlets and outlets of air storage tanks, as well as at the drain ports. The NPT is mainly used in high-pressure equipment and in applications where high sealing requirements are needed. Both types of threads are recommended for applications where disassembly is not required over the long term. Accessories such as the oil filler cap that need to remain open for an extended period can experience leaks at the two types of threads due to repeated disassembly. Generally, for pipe openings that need to remain open for a long time to allow lubricant to be added, it is recommended to use M standard threads, in accordance with the standard GB/T2878. This approach works very well in compressor systems, as repeated opening and closing of the threads does not result in any leaks.

Got it. It’s suitable for applications where disassembly is not frequent. Then, does the type of medium to be sealed (lubricating oil, process gas) and the operating temperature affect its selection? Some of the drain ports and oil in/out ports of the compressor use M threads and G threads, but it seems that these ports are simply flattened; they do not have the design specified in 2878.1. In such cases, won’t there be any leaks during the airtightness tests (especially helium tests)?

G thread is a straight pipe thread; the connection thread does not provide sealing by itself (unless used in combination with ZG thread, in which case it provides some degree of thread sealing). For the male and female G-thread connections to provide a seal, it is necessary that their end faces be flat and of sufficient width, and gaskets must be used to achieve sealing; these gaskets can be made of metal or non-metal materials. As a result, the range of sealing pressures and temperatures is quite wide. The NPT is sealed by threads, and its auxiliary sealing material is linseed oil paint or separate PTFE tape; due to material limitations, the allowable temperature range is not wide.

Whether using thread sealing or gasket sealing, the purpose is ultimately to prevent leakage at the interface. If the operating conditions of G threads (sealing pressure and operating temperature) can cover those of NPT threads, then can NPT threads be eliminated? Moreover, for assembly workers, the installation conditions for NPT threads are relatively more complex compared to G threads~

G and NPT thread seals are just the tip of the iceberg. At present, in China there are no fewer than 7 or 8 types of pipe threads, including those from domestic and international sources, as well as those based on imperial and metric systems and old and new standards; it will take a long time to study them in detail and distinguish between them. That’s enough to write a paper. The Warring States era of pipe threads.

G-thread is a straight pipe thread that does not have any sealing function of its own; sealing must be achieved through washers at the top of the bolt or on its bearing surface； NPT is a tapered pipe thread, a connection method that allows for pressure-sealing with little or no sealing material required ； Also being female threads, NPT can be connected directly to pipes, whereas G threads usually require fittings to connect to pipes ； Due to different stress conditions, in most cases the pressure resistance of tapered pipe threads is lower than that of straight pipe threads; however, on thick-walled equipment, tapered pipe threads can achieve better results than straight pipe threads ； .