Valves have various connection types, including flanges, wafers, butt welds, threads, ferrules, and clamps. These connections represent the connection between the valve and the pipeline or equipment. The choice of valve connection type is influenced by many factors and cannot be chosen arbitrarily. This issue cannot be ignored, as the majority of valve leaks, bubbling, dripping, and leakage occur here.
Flanges are the most common connection type used in valves and can be categorized by the shape of the mating surfaces as follows:
1. Smooth: Used for valves with low pressures and relatively easy to machine.
2. Concave-convex: Used for higher operating pressures and suitable for medium-hard gaskets.
3. Tongue-and-groove: Used for gaskets with greater plastic deformation, widely used in corrosive media, and offers excellent sealing performance.
4. Trapezoidal groove: Uses an elliptical metal ring as a gasket and is used for valves with operating pressures ≥ 64 kg/cm² or high-temperature valves.
5. Lens: The gasket is lens-shaped and made of metal. Used for high-pressure valves with operating pressures ≥ 100 kg/cm² or high-temperature valves.
6. O-ring: This is a newer flange connection type, developed with the advent of various rubber O-rings. It offers a more reliable sealing effect than standard flat gaskets.
Threaded connections are a simple connection method, commonly used for small valves, but are categorized into two types:
1. Direct sealing: The internal and external threads directly act as sealants. To ensure a leak-proof joint, lead grease, hemp, and PTFE tape are often used. PTFE tape is becoming increasingly popular. This material offers excellent corrosion resistance, a superior sealing effect, and is easy to use and store. It can be removed intact during disassembly because it forms a non-sticky film, significantly superior to lead grease and hemp.
2. Indirect sealing: The force from tightening the threads is transferred to the gasket between the two surfaces, allowing the gasket to perform the sealing function.
The connection and sealing principle of a ferrule connection is that when the nut is tightened, pressure is applied to the ferrule, causing its cutting edge to bite into the outer wall of the pipe. The outer tapered surface of the ferrule then seals tightly against the inner tapered surface of the fitting under pressure, thus reliably preventing leaks.
The advantages of this connection are:
1) Small size, light weight, simple structure, and easy assembly and disassembly;
2) Strong connection strength, wide application range, and resistance to high pressure (1000 kg/cm2), high temperature (650°C), and shock and vibration;
3) A variety of materials can be used, making it suitable for corrosion resistance;
4) Low machining precision requirements;
5) Convenient for high-altitude installation.
Another common connection is the wafer connection, which directly clamps the valve and two pipe ends together using bolts. There are many other valve connection options, such as small valves that do not need to be disassembled and are welded to the pipe; and some non-metallic valves use socket-and-spigot joints.