The performance and selection of natural gas exhaust valve?

Because natural gas is flammable, explosive and corrosive, natural gas valves installed in underground pipelines must meet the following requirements in order to ensure long-term safe operation:

1.1 Corrosion Resistance of Natural Gas Valve Materials Performance and Selection Strategy of Natural Gas Valve

The natural gas transported by pipelines contains a large amount of hydrogen sulfide before desulfurization (this is a poisonous and highly corrosive gas, which reacts with iron to form iron sulfide, which flakes off and corrodes machinery and equipment). Even if natural gas is treated by desulfurization process, there is still residual hydrogen sulfide. Therefore, the selection of pipeline valves should choose sulfur-resistant and corrosion-resistant materials.

1.2 Rationality of Natural Gas Valve Structure

Buried gas valves should be designed for full diameter, reduce flow resistance, facilitate the passage of pipeline sweepers or pipeline detectors, and save operating costs; reduce the structural height as much as possible to save installation costs; the top of the valves should be equipped with fully closed opening and closing indicators, so that operators can see the status of the valves at any time, so as to avoid misoperation.

1.3天然气阀门密封性好坏  

The leakage requirement of natural gas valves is very strict. The standard CJ3055-95 "Test and Inspection of Urban Gas Valves" stipulates that soft-sealed valves are not allowed to have any noticeable internal leakage under 1.1 times nominal rated service pressure, and hard-sealed valves are allowed to have less than 0.3D Nmm3/s under 1.1 times nominal rated service pressure. As for leakage, it is absolutely not allowed. Usually buried and more important valves are all welded body structure. In order to ensure the sealing performance of pipeline valves, excellent corrosion resistance, wear resistance, self-wettability and elasticity are required.

1.4 Natural Gas Valve Easy to Operate

Most of the underground pipeline valves are opened and closed manually, so it is required that the opening and closing torque of the valves should be small, and the number of turns should not be too many, so that the gas source can be cut off as soon as possible after the accident occurs.

1.5 Natural Gas Valve Maintenance Simple

Valve parts design should consider the use of less maintenance, maintenance-free structure, as far as possible to reduce the workload of maintenance, to reduce the valve maintenance and closed roads, affecting traffic.

2.1 Comparison and Analysis of Several Natural Gas Valves in Common Use

At present, the gas valves used in China's natural gas industry can be divided into three main types, namely gate valves, ball valves, butterfly valves and so on. The following three aspects are analyzed and compared for these valves:

2.2 Comparison of Working Principles and Structural Characteristics of Natural Gas Valves

Gate valve opens and closes the valve by moving the gate up and down, so as to realize the "full open and full close" control of a certain part of the pipeline system, and to meet the medium pressure drop requirement which only produces small pressure drop. Gate valves are usually suitable for working conditions that do not require frequent opening and closing, and keep the gate fully open or closed. It is not suitable for regulating or throttling. Gate valves are generally designed with full diameter and low flow resistance. They can be passed through sweeping balls and pipeline detectors. The gate valve has a high structural height (generally 3-5 times the diameter of the pipe), which is suitable for the case of deep buried pipeline.

The ball valve is opened and closed by rotating the sphere (only 90 degrees of rotation is required for opening and closing). Ball valve switch is light, small, can be made into a large diameter, reliable sealing, sealing surface and spherical surface are often in closed state, not easy to be eroded by media, widely used in various industries. The utility model has the advantages of simple structure, convenient maintenance, full-path design, small flow resistance, and can be passed through sweeping balls and pipeline detectors.

The butterfly valve is designed according to the principle of pipe pull. Its flow control element is an inclined disc. The disc is fixed on the mandrel, and the opening and closing are controlled by rotating mandrel. The seat is fixed on the body wall. Its valve body is pancake type, suitable for regulating medium flow. Butterfly valves are small in size, light in weight and easy to operate, but they have large flow resistance and can not pass through sweeping balls and pipeline detectors.

From the above structural characteristics and working principles, gate valves and ball valves are more suitable for natural gas pipelines.

2.3 Economic Comparison of Natural Gas Valves

By comparing the cost of installing a valve with rated pressure of 4 kg and nominal diameter of DN200, we find that the cost of using ball valve is the highest, which is about three times the cost of gate valve and butterfly valve. Although the price of gate valve is much higher than butterfly valve (about 4 times of butterfly valve), because the gate valve can be buried directly, it saves a lot of installation costs, so that the overall cost of gate valve and butterfly valve is similar. The performance and service life of the gate valve are much better than that of the butterfly valve. So from this point of view, gate valve should be the preferred valve.

Safety comparison of 2.4 natural gas valves

With the continuous improvement of technical level, the safety of various gate valves has been improved. Parallel double gate valves are equipped with stem protection sleeves to protect the valve stem from erosion by medium; the shell adopts a special design of "squirrel cage frame stiffener", which reduces the overall weight of the valve and enhances the strength and stiffness of the shell; the elastic sealing gate valve adopts elastic hard seal, and when the valve is fully opened or fully closed, the sealing pair completely isolates the medium from the valve chamber to make the valve fire-resistant, fire-resistant, and stiffness. High temperature and corrosion resistance. Gate valves are equipped with fully enclosed opening and closing indicators to enable operators to clearly understand the status of the valve.

Ball valves also have the characteristics of fire resistance and high temperature resistance. After burning PTFE material on the sealing seat at high temperature in fire, the metal sealing seat and all sealing parts can form metal-to-metal sealing structure, which can prevent gas medium from diffusing and prevent the disaster from expanding. In addition, it also has anti-static structure, which can make the static electricity formed during the opening and closing process of the ball valve into the underground, avoid electrostatic accumulation of ignition medium, and ensure the safety of the equipment. Limited position locking mechanism can prevent operator from misoperation or illegal operation.

The sealing pair of butterfly valve has too narrow isolation width, which can easily lead to excessive or inadequate valve disc closure and affect the sealing. In addition, because the middle part of the sealing pair is rubber or PTFE, it is easy to damage or fall off in gas scouring, and it is easy to damage in case of high temperature and short service life.

From the above analysis results, we can see that butterfly valves are not suitable for buried gas pipelines from any point of view. However, due to space constraints, butterfly valves can only be used. When choosing butterfly valve, multi-eccentricity and high quality butterfly valve should be selected. Polytetrafluoroethylene (PTFE) or hard seal should be used as sealing material. When debugging, it must be adjusted to the closing position accurately.

Gate valves and ball valves are suitable for Buried Natural Gas Pipelines in terms of structural characteristics and safety. But they also have their own shortcomings. Ball valves need high technical level from design to manufacture, so their cost is high. When the gate valve is opened and closed, it needs to rotate many turns, which takes a long time to open and close. Therefore, when selecting valves, we should synthesize all kinds of factors, and try our best to save cost under the condition of ensuring safety and reliability. From this point of view, we should select valves reasonably according to the gas characteristics and the pressure of pipelines, which can not only meet the safe operation of pipelines, but also achieve the goal of reducing cost and making the best use of goods.