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Spherical tanks are generally used for storing high-pressure liquefied gases and belong to Category 3 pressure vessels. The substances stored in them include flammable and explosive liquefied gases, as well as toxic and asphyxiating substances such as liquid ammonia and carbon dioxide. Therefore, stringent requirements are placed on the structural integrity of these tanks and their ability to remain leak-free under high pressure, requirements that exceed those for ordinary equipment. This is also the underlying premise for me writing this article. In the course of regular exchanges with relevant industry professionals, one particularly hot topic that often comes up regarding spherical tanks is: how exactly should the two safety valves on a spherical tank be configured and used? How to understand a backup safety valve? What is the role and function of the bypass line of a safety valve? What are the usage scenarios for the vent lines of spherical tanks? After going through all the standards and regulations, the aforementioned issues are either vaguely mentioned in passing or hardly addressed at all. However, these are indeed problems that exist in enterprises. Isn’t it evident that in some companies, the pipelines related to safety valves on top of spherical tanks are arranged in a very dense manner? If we fail to understand what purpose each of these numerous pipelines serves and in what situations they should be used, it is very likely that extremely serious accidents may occur during operations. In this article, I will analyze each of the aforementioned issues one by one for reference by industry professionals. (1) Configuration and backup of dual safety valves. Regarding the requirement that spherical storage tanks must be equipped with dual safety valves, the standard stipulates as follows: “SH/T 3007-2014 Design Code for Tank Farms in Petrochemical Storage and Transportation Systems”. Clause 6.4.2 states that the configuration of safety valves for pressure storage tanks shall comply with the following provisions: Pressure storage tanks must have an online standby safety valve as well as one auxiliary safety valve line. One full-bore shut-off valve should be installed before and after the safety valve respectively, and it should be marked as LO (lead-sealed open) on the design drawings. “GB 12337-2014 Steel Spherical Storage Tanks”. B.2.2 For spherical tanks used for storing flammable liquefied gases, at least two safety valves shall be installed. The discharge capacity of each safety valve must meet the requirements regarding the maximum discharge volume of the spherical tank under emergency conditions. According to “GB/T 37816-2019 Selection and Installation of Safety Relief Devices for Pressure Equipment”, in the following situations described in Section 5.1.4, it is necessary to install two or more safety valves in parallel: a) When it is essential to ensure the safe operation of pressure equipment, or when maintenance or replacement of safety valves must be carried out while the equipment continues to operate, two safety valves along with a rapid switching device for these valves should be installed. Additionally, each individual safety valve must be capable of meeting the required safety relief capacity for the pressure equipment. The primary purpose of installing dual safety valves is to guarantee the safe operation of spherical tanks; should one of the safety valves malfunction or need to be taken offline for periodic inspection, the other safety valve can still be used to maintain normal pressure levels within the tank. Therefore, this characteristic determines two important aspects in the design of safety valves. First, the discharge capacity and orifice size of each safety valve must meet the requirements under the condition of the maximum allowable discharge rate for the entire spherical tank. Second, the two safety valves serve as backups for each other, and both must remain in an open state at all times. This is different from the operation of some standby pumps and motors in the unit area (when one is running, the other remains shut down). In reality, for the two safety valves on a spherical tank, it is inappropriate to refer to one of them as the standby safety valve; there is no distinction between which one is the main valve and which one is the standby. Personally, I believe it’s more appropriate to call them dual safety valves. Should one of them fail or be taken out of service for inspection, the other one essentially functions as the main valve. (2) The function and purpose of the bypass line for safety valves: The bypass line for spherical tank safety valves is also referred to as a side path. This term applies to those safety valves whose discharge outlets lead to flare lines or other receiving facilities. For safety valves that discharge directly into the atmosphere, there is no such thing as a bypass or side path. In fact, for a sealed discharge system with a flare at the outlet of a safety valve, even if a secondary line or bypass is installed, it is inaccurate to refer to it as the safety valve’s secondary line. As we know, a secondary line for a valve is usually intended to allow fluid to flow directly through it in case the valve fails, needs to be taken out for maintenance, or does not meet the process requirements (often due to insufficient flow rate), thereby enabling normal material transfer or discharge upstream and downstream. This configuration is generally intended for various instrument control valves and emergency shut-off valves. However, this is not the case for safety valves. The bypass line of a safety valve is never designed to address faults in the valve itself; in fact, it can even be said that the bypass line has no connection whatsoever to the safety valve. Why? Because the problems they solve and the scenarios in which they are applied are vastly different. For the safety valve of a spherical tank, it is normally in a closed state; it opens only when the gas pressure inside the tank exceeds the set threshold pressure. Once the pressure is reduced back to normal levels, the safety valve returns to its closed position and remains so.