Discussion on setting double valves and the root valve for safety valve settings?

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Chapter 34: Process Design of Safety Relief Devices, Volume 2, 5th Edition of the Handbook of Chemical Process Design, p.81: (4) Isolation valves for safety valves – Isolation valves shall not be installed at either the inlet or outlet of safety valves. If it is necessary for maintenance purposes (e.g., when the discharged medium contains solid particles, or when a safety valve cannot be closed after opening and needs to be disassembled for repair; or when it is used to discharge viscous or corrosive media), a shut-off valve may be installed, provided that the following requirements are met. ① The shut-off valves installed at the inlet and outlet of the safety valve must be sealed in the open position. ② The bypass valve of the safety valve must be sealed in a closed position. ③ The aforementioned isolation valves should be full-bore gate valves or ball valves, and must have the same diameter as the pipeline they are connected to. ④ The valve stem of a gate valve must be installed horizontally or downward. Chapter 36: Process Design of Typical Pipe Fittings, Volume 2 of the 5th Edition of the Handbook of Chemical Process Design, p.118: 1.2.3 Double Valves ① For liquefied petroleum gas, as well as other flammable, toxic, or valuable liquids, highly corrosive fluids such as concentrated acids and caustic soda, and fluids with special requirements (such as those with unpleasant odors that cause severe environmental pollution), two valves in series should be installed on the pipes leading from the bottom of the tank to other equipment. Whether there are valves nearby the other equipment or not, these double valves should be installed; one of them should be positioned right next to the tank’s connection port. When the storage tank has a large capacity or is located at a great distance, it is advisable to use a remotely controlled valve for this purpose. To reduce the number of valves, several pipes are combined and connected to a single pipe fitting whenever operation permits. The drain valve of the container containing the aforementioned medium should also be a double-valve, as shown in Figure 36-3. The sampling valve and drain valve on the aforementioned medium pipeline should have dual valves installed or not, depending on the frequency of operation and other conditions. ② When the unit is in operation and it is necessary to shut down the equipment for maintenance, cleaning, or regeneration, dual valves should be installed, with a check valve placed between them. When the device is disconnected from the system, both valves close and the check valve opens. Other measures can be taken to replace the dual valves. For the backup reboiler, due to its larger valve diameter and strict requirements regarding pressure drop, a single valve (usually a stem-type gate valve) can be installed along with an \"8\"-shaped blind plate; each side of the reboiler should be equipped with its own drain valve. For devices that need to switch to regeneration mode, since the regeneration temperature is often much higher than the operating temperature, installing a reversible elbow in this case allows for a safe transition while also preventing significant thermal stress, as shown in Figure 36-4. ③ The utility material pipes for purging should, as much as possible, not be permanently connected to the process material pipes; instead, they should be connected via quick-connect couplings at hose stations. When a direct connection is required for operations, it should be made using two valves, with an inspection valve in between. The inspection valve should be opened when feeding is stopped, or it should be opened after applying a lead seal (CSO). A check valve is added in situations where pressure may fluctuate, as shown in Figure 36-5. If the pressure gauge for the common fluid is located far away from this valve assembly, a pressure gauge can be installed between these two valves to allow for on-site monitoring of the pressure of that common fluid during use. This connection method is also suitable for situations where auxiliary materials such as oxygen and hydrogen are frequently fed into the process system. To prevent liquid materials from contaminating the water system, when water needs to be added frequently, the water pipe should be connected to the gas space of the equipment; in such cases, it is also not necessary to use a double valve. ④ When designing high-pressure waste heat boilers and steam systems, it is possible to refer to the relevant regulations issued by the General Administration for Electric Power Construction under the Ministry of Electric Power (these regulations may change; therefore, the applicable standards at the time of use should be consulted). The \"Technical Specifications for the Design of Steam and Water Pipelines in Thermal Power Plants\" (DL/T5054—1996) stipulates as follows. Clause 8.2.5.1: For pipes with PN ≥ 4 MPa (table), two stop valves shall be installed in series for drainage and water release. Article 8.2.6: For the venting devices of pipes with PN ≥ 4 MPa (table), two stop valves shall be installed in series. ⑤ For the upstream sections at the junctions where hydrocarbons and toxic/harmful chemical agents, etc., are connected to other process materials, as well as on vent and drain pipes, double valves should be installed; reference can be made to Table 36-1. What are everyone’s opinions regarding the installation of a root valve for safety valves? Without a root valve, how is it possible to perform safety valve testing when there’s no opportunity to shut down the system? Where must double valves be installed? Is it permissible to install single valves? How can energy isolation be ensured when single valves are used?