The present situation of gas valve in China and the selection of common gas valve

04 Dec

The present situation of gas valve in China and the selection of common gas valve

2019-12-04

With the strengthening of China’s environmental awareness and the change of energy structure, China’s urban gas industry has made great progress. The types of gas have developed from simple coal system to multiple gas sources. In the late 1990s, the continuous development, construction and utilization of China’s natural gas resources have promoted the development of China’s urban natural gas into a new era.

Contents:

The pressure of natural gas transmission and distribution is higher than that of man-made gas, so it is very important to store and transport natural gas safely in densely populated cities and towns. If the leakage and shutdown accidents caused by improper selection or poor quality of gas valves affect the normal life and production of the society, and the serious ones cause great losses to the life and property of the country and the people. Therefore, the author believes that the valve is an essential equipment for the safe operation, overhaul, transformation and development of gas transmission and distribution, storage system, and leaders and technicians engaged in the gas industry must pay enough attention to the selection of gas valves. In this paper, the material selection, structure modeling, working condition selection and relevant standards of gas valve are put forward for reference only in gas industry.

I. Current situation of gas valves in China

Before 1980s, there was no special valve for gas, only water valve. Since the reform and opening up, the development of economic construction has promoted the development of gas manufacturing industry. The valve manufacturers paid attention to the special valve for Yanwei gas. At that time, they mainly overcame the problem that there were more impurities in coal to gas, especially the “gas glue” formed by the mixture of tar, aromatic hydrocarbon and dust, which affected the valve seal or “bite” the valve stem. The development and application of natural gas have changed the nature of gas. Natural gas is cleaner and drier than coal-fired gas, but it contains sand dust, and the pressure is higher than coal-fired gas (the pressure of coal-fired gas is generally 0.05-0.3mpa, and the pressure of natural gas is generally 0.4-1.6mpa or higher). Under the high pressure, sand dust will form a strong scouring effect on the valve cavity. Under various gas source conditions, how to use it in urban areas, gate stations, storage equipment and underground pipes It is a new task for us to choose gas valve on the road.

II. Requirements for valves installed on underground pipe network

In order to have enough “strength” and “rigidity” — “strength” means that the valve should be able to bear the Zui high pressure of gas transmission and distribution in the pipeline, and more importantly, it must have enough “rigidity”, that is, it can keep the valve from deformation even under various external forces of the underground pipe network, which can guarantee the sealing performance of the valve:

  • 1. Reduce the structural height of the valve as much as possible (referring to the height from the center line of the valve flow section to the valve top) to reduce the buried pipe depth;
  • 2. The top of the valve shall be equipped with a fully closed opening and closing indicator, so that the operator can see the status of the valve at any time, which is an important condition for safe operation;
  • 3. Full diameter design, reduce the flow resistance, facilitate to pass through the pipeline cleaner or pipeline detector, and also meet the requirements of non-stop transmission of closed hole opener to open holes on the pipeline;
  • 4. Reliable sealing performance, cj3055-95 “test and inspection of urban gas valves” standard stipulates that: under 1.1 times of nominal rated service pressure, soft sealing valve is not allowed to have any perceived internal leakage, and under 1.1 times of nominal rated service pressure, hard sealing valve is allowed to have internal leakage less than 0.3dnmm3/s, which is relatively strict, such as: 500mm nominal diameter gas valve The internal leakage per minute shall not be more than 9 ml (the capacity of ordinary drinking cup is 200 ml) and the external leakage is not allowed;
  • 5. The shell of the gas valve used in the underground pipe network shall be corrosion-resistant. It is recommended to use different shell materials according to the transmission medium and service pressure of the pipeline;
  • 6. For the design of valve parts, the structure of less maintenance and no maintenance shall be considered, so as to reduce the workload of maintenance as much as possible, reduce the road closure due to valve maintenance and affect the traffic;
  • 7. Most of the underground pipeline valves are manually opened and closed. It is required that the opening and closing torque of the valve is small, and the number of turns in the whole process should not be too many, and the air source can be cut off as soon as possible after the accident;
  • 8. Various underground pipelines of urban roads in China are crowded and complex, so it is not suitable to excavate basement inspection wells, so direct buried valves are recommended.

III. Selection of valve body material of gas valve

According to the use pressure of the gas pipeline, the valve shell material is selected reasonably, which can not only meet the safe operation of the pipeline, but also reduce the cost and make the best use of everything.

  • 1. For the transmission and distribution pipeline under medium pressure class B (0.2MPa), it is recommended to use gray cast iron valve, which has the advantages of good corrosion resistance and low price, so it is very suitable for underground pipeline;
  • 2. For medium pressure class A (0.4MPa) or below, or even high pressure class B (0.8MPa), it is recommended to use ductile iron or cast steel for manufacturing, with emphasis on the selection of ductile iron. For example, compared with cast steel brand WCB, the mechanical property of the former is no less than that of the latter, while the corrosion resistance and casting process are better than that of the cast steel, and the price is also lower than that of the cast steel (generally about 70% of the cast steel). Therefore, the overall performance and price of the ductile iron valve are better than that of the cast steel valve. In the above pressure range, the ductile iron valve can be selected as much as possible, but especially Do not pay attention to the quality control of nodular cast iron;
  • 3. In places with high-pressure class A (1.6Mpa) or higher, it is recommended to select cast steel valves, and WCB brand is generally used to manufacture valves below 4MPa;
  • 4. In recent years, Zui gas companies began to use PE pipes, which are currently used below 0.4MPa pressure level.
  • PE ball valve has also come out. This series of ball valves can be directly buried and have good sealing performance. However, at present, the price of PE ball valve is relatively high, especially for ball valve larger than 100mm.

IV. Brief introduction of several gas valves currently used in China’s gas industry

1. Oil sealed plug valve
In the 1980s, China’s gas is mainly made of artificial coal. There are more impurities and “gas glue” in the artificial coal gas. The traditional single ram wedge type sluice valve can not meet the needs of the gas working condition more and more. This kind of valve generally has the problems of “not tightly closed” and the valve rod is blocked. The gas oil sealed plug valve solves the problems of not tightly closed and the valve rod is blocked. Its main technical features: hard sealing structure is adopted to make it have the function of scraping rubber; the sealing surface does not contact with gas impurities, there is no valve rod in the cavity, and there is no problem of valve rod seizing; the sealing grease and O-ring that are specially developed to resist the erosion of gas medium improve the performance of plug valve.
However, the oil sealed plug valve also has its weaknesses,

  • 1. It is not the same diameter as the pipeline, but about 75% of the pipeline flow area, which increases the flow resistance and eddy current, and can not pass through the pig;
  • 2. Before opening and closing the valve, grease shall be added to bring inconvenience to the user;
  • 3. The nominal diameter is generally below DN300. It is difficult to manufacture dn400, and users are not welcome if DN600 is too heavy;

20191203235107 51181 - The present situation of gas valve in China and the selection of common gas valve

2. Parallel double gate valve for gas
With the development of gas industry and the establishment of gas plants, special gas valves with large or even extra large nominal diameter are needed. The problem of artificial gas formation is solved one by one by using parallel double gate valve for gas, and the diameter of Zui is 1600mm.

  • 1. In the process of opening and closing, the parallel double gate valve can scrape off the “gas glue” on the sealing surface, which solves the big problem that the gas impurities adhere to the sealing surface and affect the valve sealing;
  • 2. The valve rod is equipped with a protective sleeve to prevent the valve rod from being eroded by “gas glue”;
  • 3. The lower part of the valve is provided with a drain hole side cover, which can clean the garbage falling on the bottom of the valve chamber;
  • IV. The design flow resistance of full diameter is small, and the pig can be connected;
  • V. the valve body adopts the specially designed “squirrel cage frame stiffener” to reduce the overall weight of the valve and enhance the strength and rigidity of the shell;
  • 6. RZ series gas gate valve shell is made of gray cast iron with low price;
  • 7. The gate valve of this series is equipped with a fully closed opening and closing indicator, so that the operator can clearly understand the status of the valve.

3. Butterfly valve
Compared with the above valves, butterfly valves are lighter, larger in diameter and lower in price. In the early 1980s, some gas companies once used butterfly valves. After about ten years of use, it is generally considered that the effect is not good.

  • 1. The isolation width of the sealing pair of butterfly valve is too narrow, which is easy to cause the valve disc to close excessively or not in place, affecting the sealing;
  • 2. The valve disc is in the middle of the valve cavity, which has large flow resistance and cannot pass through the cleaner;
  • 3. Because the middle part of the seal fitting is rubber or polytetrafluoroethylene, it is easy to be damaged or fall off during gas flushing, and the service life is short. It is proved that butterfly valve is not suitable for gas pipeline except for special position.

4. Flat gate valve
Flat gate valve is a general valve which has been used in gas pipeline in recent years. Its flow resistance is small when the valve is full bore, and the seal on both sides is formed by pressing PTFE or rubber O-ring on the gate. This kind of valve can’t be completely closed or fully opened without gas in the middle cavity, which is not convenient for users to judge whether the valve is closed tightly in emergency repair. If the valve is to be completely closed without gas in the middle cavity, the opening and closing torque will increase rapidly; some factories Home made reducer, such as DN200 caliber, with an inner diameter of only 150 mm. Users should pay special attention to it. Therefore, the flat gate valve is suitable for the occasion of lower pressure level system and smaller nominal diameter.
5. Gas gate valve with elastic seal
Elastic seal gas gate valve is different from flat gate valve. The features of the valve are similar to the elastic seat structure of the fixed ball valve. The sealing seat rings on both sides of the ram are of the elastic floating structure. The sealing faces are always close together during the opening and closing process, and can compensate the wear of the sealing pair, which is suitable for the occasion of high pressure system. When the valve is fully open and fully closed, there is no gas in the valve cavity, even if the remaining gas in the valve cavity is released There is still no pressure in the cavity, so users can safely construct the pipeline downstream of the valve, which greatly reduces the working time for gas stop preparation before construction.
There are two connection forms of elastic seal gas gate valve: flange type and welding type. Flange type is the traditional connection way. The welding type can directly weld the valve and the pipeline, saving the pipe laying cost; the two kinds of valves can be directly buried, avoiding unnecessary inspection wells, and bringing convenience to the construction.
When the elastic seal gas gate valve is fully opened, it looks like a section of pipeline with the same diameter of the pipeline, with minimal flow resistance; all sealing surfaces are hidden and do not contact with the medium. For the artificial coal to gas, the “gas glue” cannot adhere to the sealing surface, and for the natural gas, the dust and gravel can not wash the parts.
In a word, the elastic sealing gas gate valve is suitable for gas pipeline, especially for underground gas pipeline. From the situation of foreign investigation, many countries use the elastic flat gate valve on the natural gas pipeline. Compared with the nominal diameter elastic flat gate valve and the ball valve, they have the function of fire prevention, and the price is about 30% lower than the ball valve, so the use effect is the same. The disadvantage of Trz series elastic seal gas gate valve is that its structure height is large, and it has 4 nominal diameters. Because it is buried underground, the height above the central axis of the pipeline is the same as that of the common gate valve, and its disadvantage is not obvious, mainly that the height of the lower half is larger than that of the common gate valve, and the buried depth is larger during construction, but generally does not affect the pipeline laying;

20191203235324 28086 - The present situation of gas valve in China and the selection of common gas valve

V. Introduction of several special gas valves

1. Emergency shut-off valve for pressure regulator
The emergency cut-off valve for pressure regulator is an important safety protection device in gas transmission and distribution system. The emergency cut-off valve is to install the emergency trip mechanism on the upper or side position of the stop valve. It is also connected with the regulator body as a whole, and with the filter as a whole. Its function is to shut down the air source in an emergency after the regulator fails, causing overpressure or loss of pressure, so as to ensure safety. The large use of natural gas in the city has improved the pressure level system of the pipe network. The failure of the pressure regulator will bring disastrous consequences to the city. The domestic gas industry has paid more and more attention to this. The products selected in China have not formed a certain trend. Imported, joint venture and domestic products are hard to distinguish between the good and the bad, so the selection should be cautious. RAQ series gas emergency shut-off valve is connected with the filter as a whole, with novel and compact structure and high sensitivity. It can quickly cut off the gas source of the pressure regulator at the set overpressure (300 mm / H2O) and pressure loss (180 mm / H2O) to ensure the safety of users;
2. Gas emergency shut-off valve
In case of gas leakage or other disasters of gas boiler, gas air conditioner and gas equipment, the emergency shut-off valve shall cut off the gas source quickly to prevent the expansion of the disaster. At present, the gas emergency shut-off valve has not been popularized in China. With the strengthening of environmental protection awareness and safety awareness, as well as the development of gas industry, the gas emergency shut-off valve will certainly get people’s attention. The gas emergency shut-off valve is a valve to ensure safety. It is connected with the automatic monitoring equipment to execute the valve closing instruction. Therefore, the gas emergency shut-off valves are all electric valves. According to the existing data, there are two types of such valves, one is solenoid valve, the other is electric valve. Due to the high requirements of gas emergency shut-off valve, such as fast speed, sealing, small flow resistance, caliber specification, etc., the existing solenoid valve and electric valve have certain defects, which can not meet all the requirements. These valves are analyzed one by one as follows:

  • I. At present, most of the solenoid valves are imported. The speed of the solenoid valve is fast, but the flow resistance is large. The flow resistance Zui in the stop valve is large, the pressure loss is large, and the caliber is small. It is difficult to manufacture the large caliber solenoid valve. Generally, it can only be below DN150, and the price is expensive. If the large caliber solenoid valve is equipped to meet the requirements of the diameter, the technical and economic ratio index is poor. Some solenoid valves are opened by electromagnetic field. Once the power is lost, the valve will be closed even if the power is lost instantaneously, which is not in line with the actual working condition. Uncoupling solenoid valve, energized uncoupling and closing valve, has high sensitivity and fast speed, because with DC low voltage, a little interference or power supply “shaking” will cause misoperation, affecting normal life and production;
  • 2. Electric butterfly valve, with low price, is easy to be made into large caliber, and its opening and closing speed is faster than that of gate valve, and slower than that of solenoid valve; the flow anion is smaller than that of solenoid valve, and larger than that of gate valve; its sealing performance is worse than that of solenoid valve and gate valve, and because of the small sealing isolation width, the rapid opening and closing speed impact is easy to cause insufficient or excessive opening and closing;
  • 3. Electric gate valve, with higher price, small flow resistance and good sealing performance, but its speed is too slow to meet the requirements of emergency cut-off;
  • 4. The ball valve has small flow resistance and good sealing performance. However, the opening and closing speed of domestic or imported electric ball valve in the market is generally within 15-30 seconds, which can not meet the fast requirements. The user’s requirements for the opening and closing speed of emergency shut-off valve are generally within 10 seconds. The electric ball valve has many advantages. It can become an ideal emergency shut-off valve only by improving the opening and closing speed.

The emergency cut-off electric ball valve integrates the advantages of the ball valve, with the opening and closing speed of 3.75 seconds, which fills in the gaps at home and abroad. In addition, it has the function of anti-static and fire prevention in the structure design. The electrical equipment can be connected with the gas leakage alarm, disaster control equipment, fan and other auxiliary equipment. The overall design conforms to the price of dbj08-65-97 technical code for urban natural gas pipeline engineering It is much lower than similar imported products. Once it came out, it was accepted by gas users and received high praise.
3. Gas check valve
The compressor of gas storage and distribution station often needs to be equipped with check valve to prevent gas from flowing back into the compressor. The water check valve has been eliminated in the gas industry. The main disadvantage of the general gas check valve is that the valve plate vibrates with the reciprocating motion of the compressor, which makes a great noise when colliding with the valve body and easily damages the valve plate. The second is poor sealing.

Vi. Relevant standards for gas valves

The gas valve plays an important role in the gas industry, and the Ministry of construction attaches great importance to the normative construction of the gas valve. The industry standards of the gas valve include CJ3005-1992 general technical requirements for grey cast iron valves for urban gas, CJ3055-1995 test and inspection for valves for urban gas, CJ3056-1995 general technical regulations for ductile iron and cast steel valves for urban gas In June 2002, it also passed the industry standard of domestic manual gas valve. The formulation and promulgation of these standards standardize the valve industry, promote the progress and development of gas valves, and promote the development of gas industry; provide a reference for the research, production and use units; improve the understanding of the difference between gas valves and general valves, and ensure the safety margin of gas valves; they are welcomed by the gas industry.

VII. Conclusion of gas valve

According to the analysis of China’s national conditions and the status quo of gas valves, the following basic conclusions can be drawn.

  • 1. The underground pipe network is made of medium pressure class B and below, and it is recommended to use direct buried gray cast iron parallel double gate valve;
  • 2. The underground pipe network is of medium pressure class A and below, and it is recommended to use directly buried parallel double gate valve or elastic flat gate valve;
  • 3. For underground pipe network under high pressure class B or under high pressure class A, it is recommended to use directly buried elastic flat gate valve and fixed ball gas ball valve;
  • 4. The underground pipe network is of high pressure class A and above, and it is recommended to use fixed ball gas ball valve;
  • 5. For storage and distribution station or ground equipment with pressure below 0.4MPa and larger than DN500 caliber, gate valve shall be used as far as possible to reduce the price.
  • 6. In the pressure regulator or pressure regulating station, parallel double gate valve is recommended for low pressure level (below 0.4MPa), and floating type gas ball valve can be used for gas ball valve. If the pressure level system is lower than 1.6Mpa, fixed type gas ball valve and elastic flat gate valve can be used.
  • 7. In the pressure regulating box, if the butterfly valve must be selected due to the limited space conditions, the multi eccentric butterfly valve shall be selected, and the sealing material shall be polytetrafluoroethylene or hard seal, and it must be adjusted to the closed position accurately during commissioning.

China’s gas industry, especially the rapid development and popularization of natural gas, has driven the progress and development of gas valve manufacturing industry. With the introduction of West to east gas transmission, offshore natural gas, and Russian gas sources, new gas valves will come out to meet the needs of gas users.

Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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