The general valve selection is based on functions and characteristics. If the body material of the valve used for natural gas is not matched, it may cause leakages or internal leakages of the valve body and seal. The mechanical properties, strength and rigidity of the parts cannot meet the use requirements or they will even break, which will cause the natural gas medium to leak and cause accidents.
General selection characteristics of pigging valves are as the following:
1. Gate valves, butterfly valves and pipeline ball valves are used for fully opened and fully closed pigging valves. The wedge of the gate valve is flexible and has a pressure discharge hole. From the outlet side of the valve, as the pressure increases, the leakage increases. The butterfly valve can only be used in the pipeline of the low pressure system. Commonly used natural gas pipeline ball valves mainly include floating ball valves and trunnion mounted ball valves. Ball valves have short operating time and small medium resistance. They are usually used in normal temperature system pipelines.
2. Butterfly valves, globe valves and needle valves are mainly used to adjust the flow. Butterfly valves and globe valves are generally used for natural gas pipelines with large diameters and low pressure, which are easy to operate and suitable for small diameter pipelines where the pressure difference between before and after the valve is great. The needle valve is used when a small flow adjustment is required.
3. For pipelines with a nominal diameter less than 1.5 inches, lift check valves are usually used to prevent backflow. Swing check valves are adopted for pipelines with a nominal diameter greater than 2 inches. As for pipelines with larger diameters, a special split door check valve is used.
Since natural gas has the characteristics of being flammable, explosive and corrosive, the natural gas valve installed on the underground pipeline must meet the following requirements to ensure long-term safe operation:
1. Being easy to operate
Most of the underground pipeline valves are opened and closed manually, so the opening and closing torque of the valve is required to be small. The number of turns in the whole process cannot be too many, so that the gas source can be cut off as soon as possible after an accident.
2. Materials being corrosion resistant
The natural gas transported by the pipeline contains a large amount of hydrogen sulfide before desulfurization. Hydrogen sulfide is a toxic and highly corrosive gas, and it reacts with iron to form iron sulfide, which peels off by slices and corrodes mechanical equipment. Even if the natural gas is processed by desulfurization and other processes, there is still residual hydrogen sulfide. Therefore, the selection of the material for pipeline valves should be sulfur resistant and corrosion resistant.
3. Simple maintenance
The design of valve parts and components should consider structure of little maintenance or maintenance free to reduce the workload of maintenance as much as possible and the road closure due to valve maintenance, which affects traffic.
4. Reasonable structure
The buried gas valve should be of full bore design to reduce the flow resistance and facilitate the passage of pipeline cleaners or pipeline detectors. At the same time, save operating costs. Reduce the structure height as much as possible to save installation costs. The top of the valve should be equipped with a fully enclosed opening and closing indicator to make it easy for the operator to see the state of the valve at any time to avoid misoperation.
5. Good sealing
The leakage rate of natural gas valves is very strict. The CJ3055-95 Experiment and Inspection of Town Gas Valves stipulates that soft sealed valves are not allowed to have any detectable internal leakage under 1.1 times the nominal rated operating pressure, and allowable internal leakage for hard sealed valves under 1.1 times the nominal rated working pressure is less than 0.3DNmm3/s. As for external leakage, it is absolutely not allowed. Usually buried and more important valves all have welded valve bodies. In order to ensure the sealing performance of pipeline valves, the sealing pair is required to have excellent corrosion resistance, wear resistance, self lubricity and elasticity.
