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Requirements for valves installed on the underground piplines

  • Sep 03, 2019
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With the strengthening of China's environmental awareness and the change of energy structure, China's urban gas industry has made a great progress. The fuel gas types have evolved from simple coal-made to multiple gas sources. In the late 1990s, the continuous development, construction and utilization of China's natural gas resources prompted China's urban natural gas development to enter a new era.

The pressure of natural gas transmission and distribution is higher than that of artificial gas, and it is extremely important to store and transport safely in densely populated towns. Due to improper selection or poor quality of gas valves, accidents such as leakage or suspension of production will occur and affect the normal life and production of the society, and severely, the heavy losses will bring significant losses to the country and people's lives and property.  Therefore, we believe that the valve is an indispensable important equipment for the safe operation, maintenance, modification and development of gas transmission, distribution and storage systems.  Leaders and technicians engaged in the gas industry must pay sufficient attention to the selection of gas valves. This article will give a view on the material selection, structural modeling, working condition selection and relevant standards of gas valves, and is only for the gas industry engineering selection reference. 

First, the status quo of China's gas valves
Before the 1980s, there were no gas-specific valves, only water valves. Since the reform and opening up, the development of economic construction has promoted the development of the gas-fired manufacturing industry.

Valve manufacturers have paid attention to the development of gas-specific valves. At that time, they mainly tried to overcome the problem that the the coal-containing gas contains more impurities, especially the "gas glue" formed by the mixture of tar, aromatic hydrocarbons and dust affects the valve sealing or "biting" valve stem, from the structure design. The development and application of natural gas has changed the nature of the gas.  Natural gas is clean and dry compared with coal gas, but contains sand dust, and the pressure is higher than that of coal gas (coal gas pressure is generally 0.05-0.3 MPa, while the pressure of natural gas is generally 0.4～1.6MPa or even higher, the sand dust will form a strong flushing effect on the valve cavity under the action of high pressure. Under a variety of gas source conditions, how to choose gas valves in urban areas, gate stations, storage facilities, underground pipelines is a new topic before us.
 
Second, the requirements for valves installed on the underground pipe network should be met.
There must be sufficient strength and rigidity - "strength" is the highest pressure that the valve should be able to withstand the gas in the pipeline. More importantly, it must have sufficient "stiffness" to survive the various external forces of the underground pipe network. It can keep the valve from deforming and ensure the sealing performance of the valve:

1. Reduce the structural height of the valve as much as possible (refer to the height of the centerline of the valve flow section to the top of the valve) to reduce the depth of the buried pipe.

2. The top of the valve should be equipped with a fully enclosed opening and closing indicator, which is convenient for the operator to see the state of the valve at any time, which is an important condition for safe operation.

3. Full-path design will reduce the flow resistance, make it easy for the pipeline cleaner or detector to pass through the pipeline, and allow the non-stop closed-type opening machine to open holes in the pipeline.

4. Reliable sealing performance. According to CJ3055-95, the experiment and inspection of urban gas valve standard, the soft seal valve does not allow any perceived internal leakage at 1.1 times the nominal rated pressure, while the allowable internal leakage of the hard-sealed valve at 1.1 times the nominal rated service pressure is less than 0.3 DNmm3/s. This requirement is strict. For example, a gas valve with a nominal diameter of 500 mm cannot leak more than 9 ml per minute. (The capacity of an ordinary drinking cup is 200 ml). As for external leakage, it is not allowed.

5. The housing of the gas valve used in the underground pipe network should be corrosion resistant. It is recommended to use different casing materials according to the conveying medium and the pressure of the pipeline.

6. For the valve component design, we should consider the use of less maintenance and maintenance-free structure. Minimize the workload of maintenance and repair. Reduce the closure of roads due to valve maintenance and repair, or else it will affect traffic.

7. Most of the underground pipeline valves are manpower opening and closing. 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. The air source can be cut off as soon as possible after the accident occurs.

8. The underground pipelines of China's urban roads are crowded and intricate. It is not advisable to excavate the basement wells. Therefore, direct buried valves are recommended.