How to understand \"valves dedicated for chlorine equipment and pipelines\"?

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The \"Technical Specifications for Chlorine Safety in Chemical Enterprises\" (GB 11984-2024), issued on November 28, 2024, states in section 4.3.3 that specialized valves should be used for chlorine-related equipment and pipelines, and gaskets with resistance to chlorine, pressure, and temperature should be employed. ”This provision was introduced following the upgrade of this standard. Article 5.9 of GB11984-89 stipulates: \"Valves shall be subjected to pressure testing one by one.\" ”Clause 4.8 of GB11984-2008 stipulates: \"Valves shall be subjected to pressure testing one by one, and a regular replacement system shall be established for important pipelines and valves.\" ”As can be seen from the above, drawing on the lessons learned from safety accidents in the chlorine industry, the revised standards specify requirements for the valves used in chlorine-related equipment and pipelines (that is, dedicated valves). Such provisions appear for the first time in China’s national, industry, and group standards related to chlorine, and they are based on the principle of greater safety in order to prevent accidents such as chlorine leaks and poisoning resulting from valve leaks or failures. What are “special valves for chlorine equipment and pipelines”? By consulting standards such as GB11984-2024 and those related to valve terminology, no definitions are provided in these standards for the terms relating to valves specifically used for chlorine equipment and pipelines. Nor is there any clear explanation of such valves in the document \"Technical Specifications for Chlorine Safety in Chemical Enterprises – Standard Promotion.\" This presentation only specifies that a dual-sealing system consisting of valve packing sealing and bellows sealing is used to prevent any \"zero leakage\" of chlorine gas. Article 12.4.2 of the \"Safety Design Standards for Caustic Soda Plants\" (T/HGJ 10600-2019) states: \"The selection of valves shall meet the following requirements: 1. For valves used with liquid chlorine, bellows-sealed globe valves are preferred.\" 2 For valves used in dry chlorine gas environments, bellows-sealed gate valves or globe valves are preferred; when ball valves or butterfly valves are used, they must have a reliable sealing design. ”This group standard recommends the use of valves with bellows seals, but this requirement is not mandatory; it is merely a recommendation ； The product standard for \"Bellows Globe Valves for Chlorine\" (JB/T 14315-2022), issued in 2022, specifies the structure, inspection and testing, as well as inspection rules for such valves. Does this mean that the valves used in pipelines for chlorine-related equipment must be of the bellows type? What is a “special-purpose valve”? By consulting relevant materials to explain the definition of \"industrial specialty valves\", it can be understood from various perspectives: due to the requirements of industrial production processes, such valves are manufactured using special materials, based on a structure similar to that of conventional valves, in order to meet the specific needs of industrial production. Similarly, in response to the specific requirements of industrial production, special designs are applied to the structure of the valve body, resulting in new types of valves that differ from conventional valves; such valves are also known as industry-specific valves. ”According to this definition: The special valves used in chlorine pipeline equipment must not only meet the requirements for valves themselves (the valve body material must satisfy strength and corrosion resistance requirements, the sealing system must ensure zero leakage, and the structural design must meet the requirements of the processing operations), as well as the process-related requirements (applicable temperature and pressure); this explanation is consistent with the requirements outlined in the relevant guidelines. Dry chlorine (dry chlorine: liquid or gaseous chlorine containing dissolved water in a solution. At 10°C, chlorine is considered dry chlorine when its water content is 30 ppm. At the same water content, if it is at -20°C, it is considered wet. At 5°C, chlorine is considered dry chlorine if its water content does not exceed 100 ppm. ) The commonly used valves in systems include globe valves, ball valves, plug valves, and butterfly valves. Different types of valves have different requirements: for example, in the case of globe valves, attention must be paid to the design of the packing box, as even a temporary leak in the valve stem can lead to a reaction between chlorine and moist air, resulting in corrosion of the valve stem. To this end, selecting a bellows seal or a long stuffing box can extend the service life. For bellows seals or long stuffing boxes, PTFE or flexible graphite packing should be used, along with Haslelloy C-276 (UNS N10276) and C22 (UNS N06022) valve stems. Ball valve: The structure of the ball valve must ensure that, when the valve is closed, any excess pressure is discharged toward the side of the pipeline with higher pressure. This excess pressure results from the expansion of liquid chlorine trapped within the ball chamber or the valve body cavity when the valve is closed. Pressure can be released because pressure relief holes are provided on the ball; if pressure relief is directional (in the form of openings or channels), it is necessary to provide the valve body with a reliable indication of the direction of pressure sealing. A passage is left on the high-pressure side of the valve body, or an automatic pressure relief seat is used in the valve chamber. Plug valve: A plug valve normally has a constriction; similar to a ball valve, it can also hold liquid chlorine when closed. Therefore, the valve stem and chamber must also be provided with pressure relief holes to allow drainage to the side of higher pressure. Soft-seat “high-performance” butterfly valves: Most soft-seat high-performance butterfly valves use standard adjustable packing – a mechanism for wetting the valve shaft/disc assembly along with a single soft-edge seal ring. The soft-seat butterfly valve closes tightly; just a 1/4 turn is sufficient, and its stem sealing mechanism offers greater reliability compared to that of lift-type plug valves. When selecting valves for chlorine systems, the following factors should be given priority consideration. 1. Stem sealing (when the valve is used frequently and subject to large temperature fluctuations, those selecting the valve should consider using alternative stem sealing methods such as bellows sealing or load-type packing to prevent leaks at the stem seal). 2. Directional shutdown (if the valve needs to be closed in both directions, a one-way valve cannot be used). 3. Ambient temperature (ambient temperature and process temperature should be considered when selecting materials). 4. Potential for wet chlorine (under normal conditions, valves are used in environments with dry chlorine, but dry chlorine can become wet if exposed to water; therefore, the material selection should take into account the need to be suitable for both dry and wet conditions). Except for Category 1 applications, where moisture intrusion may occur, lined valves with non-forged iron valve bodies are more suitable). 5. Throttling (Throttle valves may be exposed to extremely low temperatures due to large pressure differences; the valve selected should meet the expected minimum temperature rating). 6. Chloride stress corrosion (stress is generated when valve components become wet with chlorine or are exposed to a chlorine-rich environment as a result of leaks; such components cannot be made of 300 series stainless steel). In summary: According to the \"Technical Specifications for Chlorine Safety in Chemical Enterprises\" (GB 11984-2024), section 4.3.3 states that specialized valves should be used for chlorine-related equipment and pipelines, and gaskets with resistance to chlorine, pressure, and temperature should be employed. ”The specialized valves mentioned can be understood as follows: regardless of the type of valve chosen, its structure must be capable of ensuring zero leakage of chlorine-containing media; the valve body and sealing materials must be able to resist chloride-induced stress corrosion. Meanwhile, the appropriate grade and type of valve should be selected based on the operational conditions of the equipment and pipelines. Meeting these conditions can be understood as specialized valves for chlorine equipment and pipelines.