Problems Occurred in Dedicated Valves

1.1 Valves containing corrosive media
According to data, the economic loss caused by replacing valves due to corrosion in a certain year in the United States was about 120 million US dollars, and the loss caused by boric acid corrosion was about 93 million US dollars. Corrosion of valves will pose a danger to the service life and safety performance of valves.
 
Corrosion of valves mainly includes electrochemical corrosion, pitting corrosion, stress corrosion and hydrogen embrittlement, of which electrochemical corrosion accounts for a large proportion. Many media are corrosive, and even the same media will corrode the valve at different rates and degrees under different temperatures, concentrations, and pressure. Hydrogen sulfide corrosion, carbon dioxide corrosion and chloride ion corrosion are the three major corrosions faced by underwater oil production equipment. In addition, in order to meet the requirements of wax removal and anti-scaling, various chemical reagents are added, and complex corrosion caused by the superposition of various factors is the main reason for the failure of underwater valves. The following can improve corrosion of valves:
(1) Select the appropriate valve material according to the working conditions such as the acidity, alkalinity, concentration and corrosiveness of the medium. For example, carbon steel valves will not corrode by concentrated sulfuric acid, but when the concentration of sulfuric acid is below 50%, the corrosion rate of carbon steel will be very fast. Stainless steel has good corrosion resistance, but severe corrosion will occur in concentrated nitric acid above 96%.
(2) Improve and optimize the parts which are easily corroded in the valve. Starting from the design, optimize the parts that are prone to corrosion in the valve, such as not using threaded connections as much as possible, avoiding concave structures, and setting up drainage holes as much as possible to avoid corrosion caused by the lack of flow of the medium.
(3) Apply surface treatment for the vavle. In view of the problems of high costs and difficult processing of materials with good corrosion resistance, surface treatment technology is one of the most widely used methods at present, that is, a layer of corrosion resistant material is attached to the inner surface of the valve to avoid direct contact between the valve metal and the medium, improving the corrosion resistance of the valve.
 
Fluorine is an excellent corrosion resistant material. At present, polytetrafluoroethylene (PTFE), polytetrafluoroethylene propylene (FEP) and polychlorotrifluoroethylene (PCTFE) are the most widely used. The specific physical properties are shown in table 1. In terms of atomic arrangement, fluorine atoms surround the carbon atom’s main chain structure, and the carbon-fluorine bond is one of the strongest bonds, which makes the fluorine structure stable. In addition, fluorine is non-metallic, and has the advantages of self-lubrication, acid and alkali resistance, and resistance to various organic solvents that metals do not have.
 
Table 1 A comparison of PTFE, FEP and PCTFE

 
Some scholars have improved the wear resistance, thermal stability and other properties of fluorine through modification technology to expand their application scopes. Sasikala and others used Mg2SiO4 as a filler to prepare a Mg2SiO4-PTFE composite material. It was found through experiments that Mg2SiO4 could effectively improve the hardness and thermal conductivity of the material and reduce the porosity of PTFE. Bo Jiang investigated the effects of glass fiber, halloysite, molybdenum disulfide, and talc powder on PTFE under different ratios. The results show that the filler can effectively improve the friction and wear, thermal expansion and mechanical properties of the material, and compared with the two-dimensional system, the elongation at the break of the material is increased by 40.0%, the tensile strength 2.3% and the bending strength 7.1%. The performance is improved significantly. Hao Yang and others research the impact of Al2O3 filling on friction performance of polytetrafluoroethylene. The research results show that with the increase of Al2O3 content, the friction coefficient increases and the wear volume decreases. The friction coefficient and wear volume of PTFE under different Al2O3 content are shown in Figure 1 and Figure 2.
 
 
Figure 1 The distribution of friction coefficient of PTFE under different Al2O3 content
 
 
Figure 2 The distribution of PTFE wear volume under different Al2O3 content
 
Spraying is also a kind of anti-corrosion technology that is widely used in surface treatment. It is mainly used in conditions where corrosion is not serious, such as atmosphere, salt water, sea water, etc. You Wang summarized the application of various nanostructured coatings on foreign warships. Among them, the nanostructured Al2O3 or TiO2 coating can significantly improve the service life of the plunger valve, valve stem, end shaft and other components, and save annual maintenance costs up to tens of billions of dollars. The comparison of performance between Al2O3/TiO2 nanocoatings and ordinary coatings is shown in Table 2.
 
2.2 Low-temperature and ultra-low temperature valves
Ultra-low temperature valves are mainly used to transport liquid low-temperature media, such as ethylene, liquefied natural gas (LNG), liquid oxygen, liquid nitrogen, liquid hydrogen and liquefied petroleum products. These types of media have temperatures lower than 150℃. The liquid expansion ratio is about 600:1, so the design of cryogenic and ultra-low temperature valves is significantly different from that of conventional valves. Low-temperature and ultra-low temperature valves have very high requirements on materials, and unqualified materials will reduce the overall performance of the housing and lead to leakages. Low-temperature and ultra-low temperature valves are required to have good sealing performance, and cryogenic treatment of the valve is required to make the metallographic structure of the material sufficiently stable and avoid brittle fracture caused by low temperatures.
 
Table 2 A comparison of performance of A12O3/TiO2 nano coatings and ordinary coatings

 

(1) Materials for cryogenic valves 

The material of low-temperature and ultra-low temperature valves needs to have good low-temperature resistance, compatibility with media, low thermal conductivity and good welding performance. Austenitic stainless steel is the most widely used low-temperature valve material because of its excellent low-temperature toughness.
 
Some scholars have compared valve materials and found that 316 stainless steel is more suitable for low-temperature valves than 304 stainless steel. The reason is that 316 stainless steel contains molybdenum, and molybdenum can effectively control the phase transformation. Because 304 stainless steel does not contain molybdenum, the significant martensitic transformation will lead to great local stress and deformation of the valve. The corresponding cryogenic treatment is required, and the cost is relatively high. It is more reasonable to choose 316 stainless steel. Qi Sun produced AISI 304 and Ni40, Ni60 cemented carbide by surface surfacing technology, analyzed the impact energy, fracture and intergranular structure, and proved that the material has good low-temperature performance.
 

(2) Cryogenic treatment

Austenitic stainless steel will undergo martensitic transformation in low temperatures and ultra-low temperatures, resulting in volume expansion, changes in shapes and sizes, which is not conducive to the valve's strength and sealing performance. Even 316 stainless steel does not undergo martensitic transformation, it will deform due to thermal stress caused by uneven cold shrinkage. In the actual processing process, the austenitic stainless steel will be cryogenically treated twice to ensure that the martensite is fully transformed and minimize the deformation of the valve under low temperatures and ultra-low temperatures.
 
Combined with the experiment, Weiwei Zhou used computer software to perform cryogenic treatment simulation, modify heat transfer coefficient, point out the main points of cryogenic treatment of austenitic stainless steel, and sum up the change rule of performance of austenitic stainless steel after comparing tests with different time and the number of cryogenic treatment. Xiao Liu and others studied the cryogenic treatment of 25 alloy steel and found that with the extension of cryogenic time and cryogenic times, the austenite structure was completely transformed into martensite structure, and the hardness and strength increased, that is, 52.6HRC and 1345MPa, the strain and linear expansion coefficient decreased, and the performance was stable at low temperatures. The distributions of hardness, elongation and yield strength of the specimens under different cryogenic treatments are shown in Figures 3 and 4.