Valves are essential equipment for gas transmission and distribution, storage system safety operation, maintenance, renovation and development. If the gas valve is improperly selected or of poor quality, it may cause accidents such as leakage and production stoppage. Once an accident occurs, it will affect the normal life and production of the society, and it will bring significant losses to the country and people's lives and property. Therefore, the selection of gas valves must be cautious.
1. Status of gas valve application
With the development of the gas industry, the demand for gas-specific valves is increasing. Shanghai Ba valve manufacturers continue to introduce new products to meet the needs of market competition. At present, China's buried gas valves are mainly composed of gate valves, ball valves and butterfly valves. The transmission mode mainly includes manual, worm gear transmission, electric, pneumatic, gas-liquid linkage and so on. There are two types of installation methods: silo well and direct burial. According to relevant information, the most widely used in urban coal gas transmission and distribution systems is the manual gate valve. Taking Tianjin as an example, there are more than 120 kilometers of medium-pressure pipelines in the three districts of Hebei, Hongqiao and Beichen, and more than 300 valves, of which more than 80% are gate valves, followed by butterfly valves and ball valves. Before the implementation of the gas source conversion project, artificial gas is operated in the gas pipelines of the above three districts. The artificial gas contains more impurities, especially the "gas glue" formed by the mixture of tar, aromatic hydrocarbon and dust, which often affects the valve seal and even "kills" the valve stem. Therefore, in the selection of the valve, we mainly choose those valves that can solve this problem from the structural characteristics, so as to ensure flexible opening and closing of the valve and no leakage. After years of practical exploration and data analysis, we found that gate valves (including parallel double gate gate valves and elastic seal single gate gate valves) are more suitable. However, with the end of the Tiandai coal project, the nature of the gas has changed. Natural gas is cleaner and dry than coal gas, but it contains sand dust and pressure, which is higher than coal gas. Under high pressure, sand dust will form a strong flushing effect on the valve cavity and the gas contains extremely corrosive hydrogen sulfide. Therefore, how to choose natural gas valves in urban underground pipelines is a new topic before us.
2. Analysis of natural gas valve selection
2.1 Requirements for buried natural gas valves
Natural gas has the characteristics of flammable, explosive and corrosive, so the natural gas valve installed on the underground pipe network should meet the following requirements:
2.1.1 Material corrosion resistance
The natural gas transported by the pipeline contains a large amount of hydrogen sulfide before desulfurization (this is a toxic and corrosive gas, which reacts with iron to form iron sulfide, which is exfoliated and corrodes mechanical equipment). Even natural gas treated by a process such as desulfurization still has residual hydrogen sulfide. Therefore, the selection of pipeline valves should be selected from sulfur-resistant corrosion-resistant materials.
2.1.2 Reasonable structure
Buried gas valves should be designed for full bore diameter, reduce flow resistance, facilitate pipeline cleaners or pipeline detectors, and save operating costs; reduce structural height as much as possible to save installation costs; the top of the valve should be fully enclosed and opened and closed The indicator makes it easy for the operator to see the state of the valve at any time to avoid misuse.
2.1.3 good sealing
The leakage requirements of natural gas valves are very strict. The standard of CJ3055-95 "Experiment and inspection of urban gas valves" stipulates that soft-sealed valves are not allowed to have any perceived internal leakage at 1.1 times the nominal rated working pressure, and the hard-sealed valves are 1.1 times. The allowable internal leakage at a nominal rated service pressure is less than 0.3 DNmm3/s. As for the external leakage is absolutely not allowed. Usually buried and more important valves are all welded body structure. In order to ensure the sealing performance of the pipeline valve, the sealing pair is required to have excellent corrosion resistance, wear resistance, self-lubricity and elasticity.
2.1.4 Easy to operate
Most of the underground pipeline valves are manpower opening and closing. Therefore, the opening and closing torque of the valve is required to be small, and the number of revolutions in the whole process cannot be too much, so that the gas source can be cut off as soon as possible after the accident occurs.
2.1.5 Simple maintenance
Valve parts design should consider the use of less maintenance, maintenance-free structure, minimize the workload of maintenance and repair, reduce the closure of roads due to valve maintenance, affect traffic conditions.
2.2 Comparative analysis of several common valves
At present, there are three main types of gas valves used in China's natural gas industry, namely gate valves, ball valves, butterfly valves, etc. The following analysis and comparison of these valves from three aspects:
2.2.1 Comparison of working principle and structural characteristics
The gate valve is opened and closed by the up and down movement of the gate to realize the "full opening, full closing" control of a certain part of the pipeline, and it only meets the requirements of the medium pressure to produce a slight pressure drop. Gate valves are generally suitable for applications that do not require frequent opening and closing, but also keep the shutter fully open or fully closed. Not suitable for use as an adjustment or throttling. The gate valve is generally designed for full bore diameter and has low flow resistance and can be used to clean the ball and pipe detectors. The height of the gate valve structure is high (usually 3-5 times of the pipe diameter), which is suitable for the deep buried depth of the pipeline.
The ball valve is to rotate the ball to open and close the valve (open and close only need to rotate 90 °). The ball valve switch is light and small in size, can be made into a large diameter, and the sealing is reliable. The sealing surface and the spherical surface are often closed, and it is not easily eroded by the medium, and is widely used in various industries. The utility model has the advantages of simple structure, convenient maintenance, full-path design, small circulation resistance, and can pass the cleaning ball and the pipeline detector.
The butterfly valve is designed according to the principle of the pipe retaining plate. The flow control component is a disc with an angle of inclination. The disc is fixed on the mandrel and is controlled by a rotating mandrel. The valve seat is fixed on the wall of the valve body. The valve body is a pancake type and is suitable for regulating the flow of the medium. The butterfly valve is small in size, light in weight and easy to operate, but has large flow resistance and cannot pass through the ball and pipe detector.
From the above structural characteristics and working principle, the gate valve and ball valve are more suitable for natural gas pipelines.
2.2.2 Economic comparison
We compare the cost of installing a valve with a rated pressure of 4 kg and a nominal diameter of DN 200. The ball valve is the most expensive, about three times the cost of the gate valve and butterfly valve. Although the price of the gate valve is much higher than that of the butterfly valve (about 4 times that of the butterfly valve), since the gate valve can be directly buried, a large amount of installation cost is saved, so that the overall cost of the gate valve and the butterfly valve is similar. From the results of many years of use, the performance and service life of the gate valve is far superior to the butterfly valve. Therefore, from this point of view, the gate valve should be the preferred valve.
2.2.3 Security comparison
With the continuous improvement of the technical level, the safety of various gate valves has been continuously improved. The parallel double gate valve is equipped with a valve stem cover to protect the valve stem from the media. The shell is specially designed with a "squirrel cage frame rib" to reduce the overall weight of the valve and enhance the strength and rigidity of the casing. The elastic sealing gate valve adopts elastic hard seal. When the valve is fully open or fully closed, the sealing pair completely separates the medium from the valve inner cavity to make the gate valve have the characteristics of fire resistance, high temperature resistance and corrosion resistance. The gate valve has a fully enclosed opening and closing indicator that allows the operator to clearly understand the condition of the valve.
Ball valves are also fire resistant and resistant to high temperatures. After the high temperature of the fire burns down the PTFE material on the sealing seat, the metal sealing seat and each sealing part can form a metal-to-metal sealing structure, prevent the gas medium from spreading and prevent the disaster from expanding, and it also has an anti-static structure. The static electricity formed by the ball valve during the opening and closing process is introduced into the ground to avoid static accumulation of the igniting medium and ensure the safety of the equipment; the ball valve limited position locking mechanism can prevent the operator from operating or illegally operating.
The isolation width of the sealing pair of the butterfly valve is too narrow, which may cause the valve flap to be over- or under-position during the closing process, which may affect the sealing. In addition, since the middle part of the sealing pair is rubber or Teflon, it is easily damaged or peeled off during gas flushing, and In case of fire, it is easy to damage due to high temperature and has a short service life.
From the above analysis results, we can see that the butterfly valve is not suitable for buried gas pipelines from any angle. However, due to space constraints, only butterfly valves can be used. When selecting the butterfly valve, the multi-eccentric high quality butterfly valve should be used. The sealing material should be made of Teflon or hard seal. It must be accurately adjusted to the closed position during commissioning.
Gate valves and ball valves are more suitable for buried natural gas pipelines in terms of structural characteristics and safety. But they also have their own shortcomings. Ball valves require a high level of technology from design to manufacture, so their cost is high. When the gate valve is opened and closed, it needs to rotate a lot of turns, and the opening and closing time is longer. Therefore, we should combine various factors when selecting valves, and save costs as much as possible while ensuring safety and reliability. From this point of view, we should reasonably select valves according to gas characteristics and pipeline use pressure, which can meet pipelines. Safe operation can achieve the purpose of reducing the cost and making the best use of it.