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valve is a control component in the fluid conveying system, which has the functions of diversion, cutoff, regulation, stabilization, prevention of reverse flow or overflow and pressure relief. As early as 4000 years ago, humans have tried to use valves. In the process of making salt in ancient China, people used to put wooden plugs in bamboo pipes to draw brine from the salt well. This is the embryonic form of a valve. It was not until Watt invented the steam engine that valves were officially used in large numbers in industrial production.
Throughout the history of the development of valves, the materials used to make valves are diverse, including brass, forged steel, stainless steel, cast steel and cast iron. In the early days, China's valve companies mainly focused on independent research and development of structures and self-preparation of materials. With the urgent need for advanced valve technology, individual companies introduced some advanced valve production technologies from abroad, which caused breakthroughs in the domestic industry. This not only makes the valve's quality significantly improve, but also greatly extends the service life of the
valve. Valves have a wide range of uses and are indispensable in economic construction, production and people's lives. Valves play a decisive role in the process of mining, processing and transportation of natural gas, ore and oil, production process of medicine, food, chemical products, thermal power, hydropower, nuclear power as well as in the mechanical fluid system of aircraft, automobiles and ships, which are indispensable control devices in various fluid devices. In addition, various valves with special performance also play an important role in new technology fields such as national defense, military projects and aerospace.
The main technical parameters of a
valve include nominal pressure and nominal diameter. Valves with nominal pressure of PN less than and equal to l .6Mpa are low-pressure valves. Valves with nominal pressure of PN between 2.5 and 8Mpa are medium-pressure valves. Valves with nominal pressure of PN from 10.0Mpa to 80.0Mpa are high-pressure valves. Valves with nominal pressure of PN 2100Mpa are ultrahigh pressure valves. Valves with a nominal diameter of DN less than and equal to 40 mm are valves of small diameters. Valves with a nominal diameter of DN between 50 and 300 mm are valves of medium diameters. Valves with a nominal diameter of DN from 350 to 1200 mm are valves of large diameters. Valves whose diameters are greater than 1400 mm are valves with extra large diameters. The most widely used valves are gate valves, globe valves, ball valves, butterfly valves, check valves, safety valves, etc.
A gate valve is a widely used valve. The wedge connects and stops the medium through lifting and putting down the valve stem. A gate valve has the advantages of low fluid resistance, no erosion of the sealing surface when fully opened, no requirement for the flow direction of the pipeline's medium, no directions, long service life and being strong. At the same time, a gate valve has the disadvantages of great length, long closing and opening time, and inconvenient maintenance.
A globe valve is also a widely used valve, and it usually has a diameter lower than 100 mm. Its working principle is similar to that of a gate valve. The medium is connected or stopped by the up and down movement of the valve disc along the center line of the valve seat. It has the advantages of simple manufacture, convenient maintenance, being strong and durable. At the same time, it also has the shortcomings of only allowing one-way direction of the medium, great fluid resistance, and poor sealing.
Compared with a gate valve or a
globe valve, a ball valve is a new type of valve that is gradually being widely used. The sphere is controlled by the valve stem to rotate 90° and make the valve closed or opened, and it has an opening and closing effect in the pipeline. A ball valve has the advantages of small fluid resistance, no erosion of the sealing surface when fully opened, small sizes, good sealing, easy operation, short opening and closing time, which is widely used in petrochemical, electric power, nuclear energy, aviation, aerospace and other sectors. The main disadvantage of ball valves is that their maintenance is relatively difficult. There are two types of ball valves, namely the floating ball type and trunnion type.
A butterfly valve is also a widely used valve at present. The clack of the butterfly valve is a disc. Through the rotation of the valve stem, the disc rotates 90 degrees within the range of the valve seat to realize the opening and closing of the valve. A butterfly valve plays the role of opening, closing and regulating in the pipeline. It has the advantages of simple structure, light weights, convenient operation, and good sealing performance. Its disadvantage is that the valve disc is easy to be scoured by the medium when it is fully opened.
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check valve is also called a valve with a one-way flow. It is an auxiliary valve that relies on the strength of the fluid and the weight of its disc to automatically open and close. Its function is to prevent the medium from flowing back. It is generally installed at the outlet of the water pump to prevent damage to the water pump caused by the water hammer. The most commonly used check valves are swing check valves and lift check valves.
When the pressure of the medium exceeds the set value, a safety valve can automatically open and release the pressure. When the pressure is normal, it can automatically close to ensure the normal operation of the system. According to the structure, a safety valve can be divided into the spring type, lever type and pulse type.
Valves with a working temperature less than -101℃ are called ultra-low temperature valves based on the working temperature. Valves whose working temperatures are between -101℃and -29℃ are called cryogenic valves. Valves which have working temperatures from -29℃to 120℃ are normal temperature valves. Valves that have working temperatures between 120℃ and 425℃ are called medium temperature valves. A valve with a working temperature greater than 425℃ is a high-temperature valve.
