Specialized valve selection considerations

The majority of fluid media applied in the process industry involve oil and gas streams of varying corrosivity levels. These flow conditions, including media streams, are considered to be either clean, fouled or abrasive (in the case of slurry conditions), the difference being mainly in the amount and type of suspended solids particles that lead to valve clogging or corrosion damage. In addition to this, media streams containing sulfur and other mixtures will contribute to a corrosive environment when combined with high temperatures. Such media streams require careful selection of materials to maintain adequate valve life. Corrosion engineers continue to research and develop materials to solve these problems.
　　First, the refining hydrocracking, coking unit special valve
　　Refining delayed coking unit is the decompression slag oil by deep thermal cracking to generate gas, light distillate and coke processing, is an important means of refineries to improve light oil yield and production of petroleum coke. The process is divided into two parts: coking and decoking. Coking is continuous production and decoking is intermittent production. The inlet and outlet of the heating furnace and coke tower are connected by four-way valves. The four-way valve is an important channel to switch the heating furnace into the coke tower. It belongs to the special valves, used in high temperature occasions, its quality directly affects the production capacity of the device, whether the domestic new design or the old device is mostly used in the imported four-way plug valve, but the price is expensive. Domestic four-way valves, on the other hand, are generally characterized by unreasonable structure, unstable quality and easy failure.
　　Refinery hydrocracking is one of the main crude oil refining processes. Since the hydrocracking unit operates at high temperature and high pressure, and the medium is flammable and explosive hydrogen and hydrocarbons, the working conditions are special, so the sealing must be reliable. Therefore, the design and structure of the valves put forward high requirements. At present, most of the domestic use of stainless steel wedge gate valve and DC globe valve.
　　Second, the oil and gas valve
　　In order to realize the control of oil and gas flow, oil and gas valve should have the following basic performance: sealing, pressure strength, safety, adjustability, fluid flow and switching flexibility. For high-pressure, flammable, explosive oil and gas media, first of all to solve the problem of sealing, to consider the special working conditions of oil and gas valve requirements.
　　① In the wet natural gas containing hydrogen sulfide and carbon dioxide gas, special requirements are put forward for the valve body material;
　　② in the wellhead device and gathering and transportation system in the presence of brine, residual acid and other corrosive media, the choice of valve body materials and anti-corrosion requirements;
　　③ Dust and solid particles accelerate the scouring and abrasion of the valve closure parts, making the sealing vice quickly fail;
　　④ outdoor in plateau, desert and high cold area, the valve material of low temperature embrittlement, bending deformation, etc.;
　　⑤ oil and gas special valves used in long-distance transportation pipelines, requiring the same life as the pipeline, decades without replacement.
　　All these show that oil and gas special valves are different from ordinary valves, in the harsh conditions to have high reliability, to meet the requirements of high strength and non-leakage.
　　Third, chlorine-containing conditions
　　Chlorine-containing conditions should refer to the selection of valves prepared by the American Chlorine Institute, “dry chlorine piping system”.
　　Chlorine or liquid chlorine containing conditions is highly corrosive conditions, especially in this case contains water. Chlorine mixed with water to form HCl (hydrochloric acid) will corrode the valve body and internal parts. Since chlorine has a high coefficient of thermal expansion, if liquid chlorine is enclosed in the valve cavity, it will lead to a high rate of pressure increase in the valve cavity. Valves used in this condition should have a reliable pressure relief function in the cavity.
　　Fourth, freezing (cryogenic) conditions
　　Although the valves for cryogenic conditions based on ASME B16.34 standards and API standards, but these valves also with other design features to ensure that it has a certain degree of reliable operation in cryogenic conditions. Such valves may also include bonnet extensions that extend the packing and operating mechanism away from the cryogenic fluid to allow operation of the stem packing at a higher temperature and to ensure that the valve operating mechanism does not freeze in service. mss sp-134 provides details of bonnet extensions that are included.
　　V. Hydrofluoric Acid Service
　　Valves for use in hydrofluoric acid service should be limited to only those types of valves that have been demonstrated in service or that have been tested to handle this service successfully. Valves that do not normally provide an opportunity for solids buildup are the valve type of choice. Hydrofluoric acid handling operations should be performed by certified technicians who have strict control over the listed valves. The design and material requirements and internal geometry details are very detailed for these valves (typically carbon steel valves with special monel internals or solid monel internals) and such valves should be designed to have a special construction for resistance to hydrofluoric acid corrosion. In hydrofluoric acid containing conditions, the valve inspection and testing should be higher than the standard used for typical process valves.
　　Six, hydrogen-containing conditions
　　This condition in the use of valves compared to conventional casting supplies often provide for its high casting quality. Because hydrogen is a very permeable fluid, pressure ratings greater than or equal to 600 pounds class of welded joint valves in use to reduce the potential source of leakage.
　　API 941 contains a range of hydrogen conditions in the selection and use of materials.
　　Seven, oxygen-containing conditions
　　Oxygen-containing conditions in the use of valves when applicable should follow the American Compressed Gas Association standard CGA G4.4-2003 “Oxygen Piping Systems”. Valves used in this service should be completely degreased, clean and installed under clean conditions and properly packaged and sealed, as oil and grease are highly flammable in the presence of oxygen. Guidance on this is given in CGA G4.1 Cleaning equipment for oxygen service. Proper handling and storage are necessary prior to installation.
　　Bronze or Monel valve body and internal materials suitable for oxygenated service are often used to prevent sparking and ignition due to energetic mechanical collisions. Specially formulated silicone-based greases are available for use in oxygenated service because standard hydrocarbon lubricants should not be used in the presence of oxygen.
　　VIII. Pulsating or unstable flow
　　For pulsating or unstable flow in the check valve, its selection needs to be given special consideration, for example, used in reciprocating compressors in the check valve, may be with the flow rate changes in the rapid opening and closing, which may lead to hammering and valve damage. On the pulsating and unstable flow in the use of the valve type there may be different opinions, but usually butterfly check valve, swash plate check valve and axial flow check valve is recommended for pulsating or unstable flow.
　　Nine, acid-containing conditions (wet H2S conditions)
　　Acid-containing conditions in the use of valve materials should be subject to NACE MR0103 standard. This standard for the downstream hydrocarbon processing industry limits the hardness of all steels; requires solution annealing of austenitic steels; prohibits the use of certain materials for pressurized parts (including valve stems); and imposes special requirements for bolted and welded valves.
　　Attention should be drawn to the user's responsibility in NACE MR0103, which states that the user shall specify in detail whether or not the bolts are to be exposed to H2S-containing environments. Unless specified by the user, bolts not inside the valve, such as bonnet joint bolts, tend to be subject to product standards, and sulfur-containing service conditions are not included in this standard. Body-bonnet bolting is not required to meet NACE requirements if the bolting material is not directly exposed to process fluids. If any sulfur leakage from a sulfur-containing oil cannot be eliminated or evaporated (e.g., by isolating the valve), then the bolted connection should be subject to the NACE standard.
　　Bolted connection materials should be given special attention if NACE-permitted materials are not considered necessary. This imposed hardness requirement will result in a reduction in strength. The strength of the bonnet connection bolts will be reduced and may not be suitable for the same design conditions as the standard bolting material.
　　X. Viscous or curing conditions
　　For viscous or solidified fluid conditions in the valve, such as liquid sulfur or heavy oil, in order to make the valve operable, often require steam tracing or steam casing to maintain adequate operating temperature. Special attention should be given to check valves because of the operational problems caused by hysteretic response.