The structure and size of the valve's end connection are critical to the installation and use of the valve. Improper selection may result in mismatched dimensions or connection structures during installation, leading to installation difficulties. Therefore, special attention should be paid when selecting a valve to ensure the correct and suitable connection end face structure is chosen, thus ensuring smooth installation of the valve.
Valve end face connection forms are generally divided into three types: welded end connections, threaded end connections, and flanged end connections, with flanged end connections being the most commonly used structure.
1. Flange Connection Structure
It is achieved by using flanges, gaskets, and bolts to connect valves, and it is very convenient to install and remove flange-connected valves. It is suitable for a wide range of nominal diameters and pressure ranges. Flanges connected to valves can be divided into flat welding flanges, raised face flanges, socket welding flanges, and integral flanges, etc.; according to the sealing surface, they can be further divided into flat face flanges, raised face flanges, concave-convex flanges, tongue and groove flanges, and ring joint flanges. The choice of various flange structural forms and sealing surface forms depends on different operating conditions. Generally speaking, flat welding flanges, due to their low structural strength and rigidity, are mostly used in operating conditions that are relatively gentle, such as valves used for low-pressure circulating water or generally low-pressure compressed air, etc. Its advantages are low price and easy installation. Raised face flanges have a better structure, strength, and rigidity due to their neck and conical transition structure. They can be non-destructively tested when welded to pipes, so they can be used in more demanding conditions, such as oil and gas and chemical media. Socket welding structures are commonly used in pipelines with PN ≤ 10.0 MPa and DN ≤ 40. The choice of valve sealing surface is also based on its operating conditions. The flat face structure is often used with flat welding structural forms in relatively gentle conditions, raised face structures are often used with raised face and socket welding structures, which are more widely used. The concave-convex surface is often used with raised face and socket welding forms, and the tongue and groove surface is similar to the concave-convex surface structure in terms of use, while the ring joint surface structure is often used with raised face structural forms.
2. Threaded Connection
Threaded connections are detachable connections and are commonly used in situations where welding is not suitable or disassembly is required. Threaded connections are divided into internal and external threaded connections. Internal threaded connections usually have valve body ends machined into tapered or straight tube female threads, which are connected to pipes that are machined with tapered or straight tube male threads. External threaded connections have valve body ends machined with male threads for easy installation and removal of threaded-end valves. Compared with welding connections, valves with threaded connections have lower strength but stronger sealing. Therefore, when applied in petrochemical plants, they are often subject to the following conditions:
Mostly used in pipelines with DN ≤ 40;
Threaded connections of pipe fittings should use taper thread;
Valves with threaded connections should not be used for managing highly toxic media;
Valves with threaded connections are not recommended for temperature operating conditions above 200℃ and below -45℃;
Valves with threaded connections are not recommended for parts that may experience stress corrosion, clearance corrosion, or variable loads due to vibration, pressure pulsation, and temperature changes;
When valves with threaded connections are used on pipelines carrying flammable gases, seal welding is recommended for sealing.
