Most fluid media used in the process industry involve oil streams of varying levels of corrosiveness.These flow conditions, including media flow, are considered either clean, dirty, or abrasive (in the case of slurries), the difference being the amount and amount of suspended solids that can cause valve blockage or corrosion damage. type.In addition to this, media streams containing sulfur and other compounds will contribute to the creation of a corrosive environment when combined with high temperatures.For such media flows it is necessary to maintain an adequate service life of the valve by careful selection of materials.Corrosion engineers continue to research and develop materials to address these issues.
(1) Special valves for refinery hydrocracking and coking units
Refinery delayed coking unit is a process of deep thermal cracking of vacuum residue to generate gas, light distillate oil and coke. It is an important means for refineries to improve the yield of light oil and produce petroleum coke. The process is divided into two parts: coking and decoking. Coking is continuous production, and decoking is discontinuous production. The inlet and outlet of the heating furnace and the coke drum are connected with a four-way valve. The four-way valve is an important channel for switching the heating furnace into the coke drum. It is a special valve and is used in high temperature occasions. Its quality directly affects the production capacity of the device. Both new designs and old devices in China mostly use imported four-way plug valves, but they are expensive. The domestic four-way valve generally has the problems of unreasonable structure, unstable quality and easy failure.
Refinery hydrocracking is one of the main crude oil refining processes. Because the hydrocracking unit operates under high temperature and high pressure, the medium is flammable and explosive hydrogen and hydrocarbons, and the working conditions are special, so the sealing must be reliable. Therefore, higher requirements are placed on the design and structure of the valve. At present, most of the domestic stainless steel wedge gate valves and DC globe valves are used.
(2) Oil and gas special valve
In order to control the flow of oil and gas, the special valve for oil and gas should have the following basic properties: sealing, compressive strength, safety, adjustability, fluid flow and switching flexibility. For high-pressure, flammable, and explosive oil and gas media, the seal must first be solved, and the special working conditions of oil and gas special valves must be considered:
① In the wet natural gas containing hydrogen sulfide and carbon dioxide gas, special requirements are put forward for the material of the valve body;
② There are brine, residual acid and other corrosive media in the wellhead device and gathering and transportation system, so the selection of valve body materials and anti-corrosion requirements;
③ Dust and solid particles accelerate the erosion and wear of valve closing parts. Make the sealing pair fail quickly;
④ In the outdoors on plateaus, deserts and alpine regions, low temperature embrittlement of valve materials, bending deformation, etc.;
⑤ Special valves for oil and gas used in long-distance transportation pipelines require the same service life as the pipeline and will not be replaced for decades.
All of these show that oil and gas special valves are different from ordinary valves, and have high reliability under harsh conditions, and meet the requirements of high strength and non-leakage.
(3) Chlorine-containing conditions
The selection of valves for chlorine-containing conditions should refer to the “Dry Chlorine Gas Pipeline System” compiled by the American Chlorine Society.
The service with chlorine gas or liquid chlorine is a highly corrosive service, especially if the service contains water. HCI (hydrochloric acid) formed by mixing chlorine with water will corrode the valve body and trim. Due to the high thermal expansion coefficient of chlorine, if liquid chlorine is sealed in the valve cavity, the pressure in the valve cavity will increase rapidly. Valves used in this condition should have a reliable mid-cavity pressure relief function.
⑷ Freezing (low temperature) working condition
Although the valves for cryogenic service are based on ASME B16.34 and API standards, these valves also have other design features to ensure some reliable operation in cryogenic service. Such valves may also incorporate a bonnet extension design that extends the distance of the packing and operating mechanism from cryogenic fluids, thereby allowing operation of the stem packing at a higher temperature and ensuring that the valve operating mechanism does not freeze during service . MISS SP-134 provides some details including the bonnet extension design.
⑸ Condition of hydrofluoric acid
Valves used in hydrofluoric acid service should be limited to valve types that have been demonstrated in service or have been tested to successfully handle this service. Valves that generally do not provide an opportunity for accumulation of solid matter are the preferred valve type. Hydrofluoric acid handling operations should be performed by qualified technicians who strictly control the valves on the market. The design and material requirements and details of the internal geometry for these (typical carbon steel valves with special monel trims or solid monel trims) are very detailed and the valve should be designed to be resistant to hydrofluoric acid Corroded special structure. In HF-containing service, the inspection and testing of the valve should be higher than that used for typical process valves.
⑹ Hydrogen condition
Valves used in this service are often specified to be of high casting quality compared to conventional casting supplies. Because hydrogen is an extremely permeable fluid, weld-connected valves with pressure ratings greater than or equal to 600 psi reduce potential sources of leakage in service. API941 covers the selection and use of materials in hydrogen service.
⑺ Oxygen condition
Valves used in oxygen-containing service should follow the American Compressed Gas Association Standard CGA G4.4-2003 “Oxygen Piping System” when applicable. Valves for this service should be fully degreased, clean and installed in clean conditions and properly packaged and sealed, as oils and greases are highly flammable in the presence of oxygen. Guidance is given in CGA G4.1 Cleaning Equipment for Oxygen Service. Proper handling and storage are necessary prior to installation.
Bronze or Monel body and trim materials suitable for oxygen-containing service are often used to prevent sparks and ignition from high-energy mechanical impacts. There are specially formulated silicone-based greases for use in oxygen-containing service because standard hydrocarbon oils should not be used in the presence of oxygen.
⑻ pulsating or unstable flow
Check valves used in pulsating or unstable flows, which require special consideration, such as those used in reciprocating compressors, may be opened and closed rapidly with changes in flow, which may cause Hammering and damage to the valve. There may be differing opinions on the types of valves used in pulsating and unstable flows, but generally butterfly check valves, swash plate check valves, and axial flow check valves are recommended for pulsating or unstable flows.
⑼ Acid-containing condition (wet H2S condition)
The use of valve materials in acid-containing conditions should comply with the NACEMR0103 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 pressure-containing parts (including valve stems); and proposes special requirements for bolted connections, welded valves, etc. Require.
Attention should be paid to the responsibility of the user in NACE MR0103, which states that the user shall specify whether the bolts are exposed to H2S containing environments. Unless specified by the user, bolts that are not inside the valve, such as bonnet connecting bolts, are often subject to product standards, and sulfur-containing conditions are not included in such standards. Body-bonnet bolting does not need to meet NACE requirements if the material used for the bolting does not directly withstand the process fluid. If any sulfur leakage from sulfur-containing oils cannot be excluded or evaporated (such as block valves), then the bolted connection should comply with NACE standards.
If a NACE permitted material is deemed unnecessary, then special attention should be paid to the bolted joint material. This imposed hardness requirement will result in a reduction in strength. The strength of the bonnet attachment bolts will be reduced and may not be suitable for the same design conditions as standard bolting materials.
⑽ Viscous or curing conditions
Valves used in viscous or curing service, such as liquid sulfur or heavy oil, often require steam tracing or steam jackets to maintain adequate operating temperatures for valve operability. Special attention should be paid to check valves because their hysteretic response can cause operational problems.