Considerations for Selecting Special Valves

　　Most fluid media used in the processing industry involve oil and gas flows with varying levels of corrosiveness. These flow conditions, including media flow, are considered either clean, dirty, or abrasive (in slurry conditions), with the distinction mainly based on the quantity and type of solid suspended particles that lead to valve blockage or corrosive damage. Additionally, media flows containing sulfur and other mixtures, when combined with high temperatures, contribute to the formation of corrosive environments. For such media flows, careful material selection is required to maintain sufficient valve lifespan. Corrosion engineers continue to research and develop materials to address these issues.
　　1. Special valves for hydrogenation cracking and coking units
　　The delayed coking unit in refining processes is a method that processes vacuum residue oil through deep thermal cracking to produce gas, light distillate oil, and petroleum coke. It is an important means for refineries to increase light oil yield and produce petroleum coke. The process is divided into coking and decoking. Coking is a continuous production process, while decoking is a batch production process. The inlet and outlet of the heating furnace and the coke tower are connected by a four-way valve. The four-way valve is an important passage for switching the heating furnace to the coke tower. It is a special valve used in high-temperature situations, and its quality directly affects the production capacity of the unit. Most new designs and old units in the country use imported four-way plug valves, which are expensive. Domestic four-way valves generally have issues with unreasonable structure, unstable quality, and susceptibility to failure.
　　Hydrogenation cracking in refineries is one of the main crude oil refining processes. Due to the operation of hydrogenation cracking units under high temperature and pressure, with flammable and explosive hydrogen and hydrocarbons as the medium, the working conditions are special, so sealing must be reliable. Therefore, higher requirements are placed on the design and structure of valves. Currently, most domestic selections are stainless steel wedge gate valves and straight-through stop valves.
　　2. Special valves for oil and gas
　　To achieve control over oil and gas flows, special valves for oil and gas should possess the following basic performance characteristics: sealing, pressure resistance, safety, adjustability, fluid flow capability, and flexibility in switching. For high-pressure, flammable, and explosive oil and gas media, sealing must be prioritized, and the special working condition requirements for oil and gas valves must be considered:
　　1. Special requirements for valve body materials in wet natural gas containing hydrogen sulfide and carbon dioxide;
　　2. The presence of brine, residual acid, and other corrosive media in wellhead devices and gathering systems requires careful selection of valve body materials and anti-corrosion measures;
　　3. Dust and solid particles accelerate the erosion and wear of valve closing components, leading to rapid failure of the sealing pair;
　　4. In outdoor environments in plateaus, deserts, and cold regions, issues such as low-temperature brittleness and bending deformation of valve materials;
　　5. Special valves for oil and gas used in long-distance transportation pipelines are required to have the same lifespan as the pipeline, lasting for decades without replacement.
　　All of these indicate that special valves for oil and gas differ from ordinary valves, requiring high reliability under harsh conditions to meet high strength and leak-proof requirements.
　　3. Chlorine-containing conditions
　　The selection of valves for chlorine-containing conditions should refer to the "Dry Chlorine Gas Pipeline Systems" compiled by the American Chlorine Institute.
　　Chlorine gas or liquid chlorine conditions are highly corrosive, especially when water is present in these conditions. The HCl (hydrochloric acid) formed by the mixture of chlorine and water will corrode the valve body and internal components. Due to chlorine's high thermal expansion coefficient, if liquid chlorine is trapped in the valve cavity, it will cause a rapid increase in pressure within the valve cavity. Valves used in such conditions should have a reliable cavity pressure relief function.
　　4. Cryogenic (low-temperature) conditions
　　Although valves for low-temperature conditions are based on ASME B16.34 standards and API standards, these valves also incorporate other design features to ensure reliable operation in low-temperature conditions. Such valves may also include extended valve cover designs, which extend the distance between the packing and operating mechanism and the low-temperature fluid, allowing for operation of the valve stem packing at a higher temperature and ensuring that the valve operating device does not freeze during use. MSS SP-134 provides details on some designs that include extended valve covers.
　　5. Hydrofluoric acid-containing conditions
　　Valves used in hydrofluoric acid conditions should be limited to those types that have been proven in use or can successfully handle such conditions in testing. Valves that do not provide opportunities for solid material accumulation are preferred. Hydrofluoric acid handling operations should be conducted by qualified technicians who strictly control the valves on the market. The design and material requirements, as well as the internal geometry details for these valves (typically carbon steel valves with special Monel internals or solid Monel internals), are very detailed, and these valves should be designed with a special structure resistant to hydrofluoric acid corrosion. In hydrofluoric acid conditions, the inspection and testing of valves should exceed the standards used for typical process valves.
　　6. Hydrogen-containing conditions
　　Valves used in these conditions often require a higher casting quality compared to conventional cast products. Because hydrogen is a highly permeable fluid, welded connection valves with pressure ratings of 600 pounds or greater reduce potential leakage sources during use.
　　API 941 includes material selection and usage ranges for hydrogen-containing conditions.
　　7. Oxygen-containing conditions
　　Valves used in oxygen-containing conditions should, when applicable, follow the American Compressed Gas Association standard CGA G4.4-2003 "Oxygen Pipeline Systems." Valves for such conditions should be completely degreased, clean, installed under clean conditions, and properly packaged and sealed, as oils and fats are highly flammable in the presence of oxygen. Relevant guidelines are provided in CGA G4.1 for cleaning equipment in oxygen conditions. Proper handling and storage are necessary before installation.
　　Bronze or Monel valve body and internal component materials suitable for oxygen-containing conditions are often used to prevent sparks and fires caused by high-energy mechanical impacts. Specially formulated silicone-based lubricants are used in oxygen-containing conditions, as standard hydrocarbon lubricating oils should not be used in the presence of oxygen.
　　8. Pulsating or unstable flow
　　Check valves used in pulsating or unstable flow require special consideration. For example, check valves in reciprocating compressors may open and close rapidly with changes in flow, potentially leading to hammering and damage to the valve. There may be differing opinions on the types of valves used in pulsating and unstable flow, but generally, butterfly check valves, tilting disc check valves, and axial flow check valves are recommended for such conditions.
　　9. Acidic service conditions (wet H2S conditions)
　　In acidic service conditions, the materials used for valves should comply with NACE MR0103 standards. This standard, which applies to downstream hydrocarbon processing industries, restricts the hardness of all steels; requires austenitic steel to be solution annealed; prohibits certain materials for pressure-containing parts (including valve stems); and imposes special requirements for bolted connections and welded valves.
　　It should be noted that the user's responsibilities in NACE MR0103 require users to specify whether bolts will be exposed to H2S environments. Unless specified by the user, bolts not inside the valve, such as cover connection bolts, often comply with product standards, and sulfur-containing conditions are not included in this standard. If the materials used for bolted connections do not directly bear the process fluid, then the valve body-cover bolting does not need to meet NACE requirements. If any sulfur leakage from sulfur-containing products cannot be eliminated or evaporated (e.g., in isolation valves), then the bolted connections should comply with NACE standards.
　　If materials allowed by NACE are deemed unnecessary, special attention should be given to bolted connection materials. This imposed hardness requirement will lead to a reduction in strength. The strength of cover connection bolts may decrease and may not be suitable for the same design conditions as standard bolting materials.
　　10. Viscous or solidifying conditions
　　Valves used in viscous or solidifying fluid conditions, such as liquid sulfur or heavy oil, often require steam tracing or steam jackets to maintain sufficient operating temperatures for operability. Due to the lagging response of check valves, special attention should be given to them to avoid operational issues.