Discussion on double-valve piping issues
Thread Content
Section 2.4.7 of the Chemical Pipeline Design Manual explains that dual valves should be installed on pipelines carrying hazardous materials, with a vent valve placed in between. I didn’t understand the instructions regarding the operating states of these dual valves and the vent valve: during production, the dual valves are closed while the vent valve is open; when switching to another mode of operation, the vent valve is closed and the dual valves are opened. Please discuss why this is the case.
Opening the vent valve is a measure to prevent internal leakage in the dual valves. But since it is a hazardous medium, it should not be released directly; if either of the two valves leaks, it will lead to the leakage of the medium, which may cause an accident. Now, it is required to install blind flanges between the two valves, and these blind flanges are swapped during process changes.