(1) threaded connection valve. This connection is usually the valve into the end into a tapered pipe or straight pipe thread, it can be connected to the tapered pipe nipple or pipe. Since such a connection may present a larger leak channel, these channels can be plugged with sealants, sealing tapes, or fillers. If the material of the valve body is weldable but the coefficient of expansion varies widely or the range of variation in operating temperature is large, the threaded connections must be nectar welded. Threaded valves are mainly nominal pass through the valve below 50mm. If the path size is too large, the connection part of the installation and sealing is very difficult. In order to facilitate the installation and removal of threaded valves, pipe fittings are available in place on the piping system. Nominal diameter of 50mm below the valve can use the pipe joints as a pipe joint, pipe joints thread connecting the two parts together. (2) flange connection valve. Flanged valves, the installation and removal are more convenient. But the valve than the screw connection bulky, the corresponding price is high. So it is suitable for a variety of diameter and pressure pipe connections. However, when the temperature exceeds 350 ° C, the load on the bolts is significantly reduced due to a loosened bolt, gasket, and flange, and leakage to the flanged flange subjected to high stress may occur. (3) welding connection valve. This connection is suitable for all kinds of pressure and temperature, more load than the conditions of use, more reliable than the flange connection. But the welded connection of the valve removal and reinstallation are more difficult, so its use is limited to usually long-term and reliable operation, or conditions of load moment, the temperature is high occasions. Such as thermal power stations, nuclear power projects, ethylene projects on the pipeline. Welded valves with nominal diameters up to 50 mm usually have weld sockets to receive the pipe at the load plane end. As the socket welding and sockets in the formation of gaps between the pipe, which may make the gap by some media corrosion, and pipeline vibration will fatigue joints, so the use of socket welding is subject to certain restrictions. In the larger nominal diameter, the conditions of load carved, the temperature is high, the valve body often used to weld the groove, at the same time, butt welds have the original grid requirements, must use skilled welding workmanship to complete this work.