【Common Issues in Haichuan Pipeline Design】When should dual valves be installed?

Thread Content

When should a dual valve be installed? In processes, dual valves are often used, such as double-hand valves or double check valves. So, in what situations are dual valves necessary? According to SH3012, double valves must be installed for the venting or drainage of pipelines carrying extremely hazardous media, and the discharge must take place into a closed recovery system ； For pipelines carrying highly hazardous media, dual valves should be installed for venting or draining; when a single valve is used, a blind flange or cover should be applied ； Double valves should be installed for the venting or draining of high-pressure pipelines; when a single valve is used, a blind flange or cover should be added ； For the continuous drainage at the lowest point of combustible gas pipelines, a dual-valve system should be installed, and the liquid discharged should be sent to a closed system ； Combustible gas pipelines should be drained only during startup and shutdown; a valve can be installed, along with a plug, pipe cap, or flange cover. When a shut-off valve is installed for draining on pipelines carrying flammable liquids and vapors, a plug, pipe cap, or flange cover should be placed at the end. The above are the regulations set by SH3012 regarding the dual-valve setup for the venting and drainage of regular materials. Double valves should also be installed on the nitrogen pipelines at the utility stations; this is also specified in 9.4.1 of SH3012. The reason for this is that nitrogen poses a risk of asphyxiation, which is why special regulations have been established for it. According to SH3007, storage tank drain pipes shall be equipped with dual valves. This regulation is the \"Design Code for Tank Areas in Petrochemical Storage and Transportation Systems\"; it applies primarily to petrochemical storage tanks, which are generally considered major hazard sources. In the event of a leak, the consequences can be severe. Therefore, as a precaution, a dual-valve design is required for the drain pipes. Double valves should be installed upstream of the connection points where materials such as hydrocarbons and toxic, hazardous chemicals meet other process materials. The dual valves mentioned above refer to cut-off valves; there is also another option, which are dual check valves. Based on the author’s experience, dual check valves are generally installed at the plant boundaries or in other situations where preventing backflow is crucial and check valves are prone to failure. These two check valves are designed in different ways or come from different manufacturers, so that if one fails, the other is likely to still function, thereby meeting the process requirements.