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Abstract
The main material selection and process control of high-pressure slab gate valves for high-sulfur natural gas gathering and transportation projects are introduced. The harsh working conditions and corrosion forms of natural gas containing high hydrogen sulfide raw materials are analyzed, and the sealing structure and functional design methods of the high-pressure slad gate valve are given.
 
Overview
With the continuous growth of natural gas energy consumption, clean extraction and purification of high-sulfur natural gas have become one of the leading directions for energy development. In the gas field surface gathering and transportation project, the working medium of the valve is raw natural gas or sour gas, fuel gas or acid gas. The H2S content is between 12.31% to 17.05%; The CO2 content is between 7.89% to 10.53%; The Cl content is from 28g/l to 65g/l (reference value) and there is also H2O. According to the regulations of gas reservoir classification, natural gas reservoirs are classified as high-sulfur gas reservoirs when the H2S concentration is between 2% to 10%.

H2S is a colorless, irritating and highly corrosive gas. In view of the complex and harsh corrosive environment, the slab gate valve used on the surface gathering and transportation project should have good resistance to high sulfur, high acid and good sealing performance. It is a must that the medium does not leak and spread to meet the requirement for zero leakage. At the same time, the valve should have sufficient strength and corrosion resistance, which can meet requirements for the continuous and safe operation during the design life and working conditions.
 
For raw natural gas with high H2S content, when the medium temperature is between -35 to 60℃ and the pressure rating is higher or equal to 900, the crack propagation caused by sulfide stress cracking and stress corrosion cracking is fast under low temperature and high pressure sour gas environment, which is easy to cause a major safety accident. Firstly, the sulfide stress cracking (SSC) and stress corrosion cracking problem should be solved by the material selection and manufacture of the main body of the high-pressure slab gate valve. Secondly, material failure due to hydrogen-induced cracking (HIC) and electrochemical weightless corrosion must still be considered. SSC refers to that when the medium contains H2S and water, the hydrogen atoms generated in the wet hydrogen sulfide environment penetrate into the steel and dissolve in the crystal lattice, increasing the brittleness of the steel and forming cracks under the action of the extra tensile stress or residual stress, namely sulfide stress corrosion cracking. HIC refers to that when the medium contains H2S and water, H2S reacts with metal, and hydrogen is separated out during electrochemical corrosion, forming hydrogen-containing blisters and hydrogen embrittlement. And hydrogen induces sulfide stress corrosion cracking, namely hydrogen-induced cracking.
 
Main body materials and process control
Under high-sulfur natural gas conditions, the main materials of the body, bonnet and wedge of the high-pressure slab gate valve should meet the requirements for NACE MR0175/ISO15156 Standards. Even if the corrosion allowance is increased based on the wall thickness of the valve body, it cannot solve the problem of high-sulfur corrosion. The problem of high-sulfur corrosion can be solved by correct selection of materials. For high-pressure slab gate valves used in gathering and transportation projects under high-sulfur natural gas conditions, the valve body and bonnet which are made of specially smelted high-quality high-sulfur-resistant carbon steel WCB, WCC, LCC or nickel-based alloy Incoloy 825 should be adopted. The body of the high sulfur resistant high-pressure slab gate valve shall be of integral casting or integral forging structure.
 
The valve body made of WCB, WCC or LCC which has resistance to high sulfur corrosion must be smelted in an electric arc furnace. The low carbon, ultra low sulfur, low phosphorus fine grained, and pure steel are refined by the refining furnace. Non-metallic inclusions must be controlled during smelting and casting to minimize the possibility of non-metallic inclusions in the metallographic structure of the casting. The sulfide inclusions in the steel must be spheroidized and there should not be obvious columnar segregation and banded structure. The Si-Ca alloy is used as deoxidizing material, and the final deoxidation is modified with Al or Ti. The material of the valve body is limited by sulfur resistance. The material of the high-pressure slab gate valve guarantees S of forgings being lower than or equal to 0.008%, P being lower than or equal to 0.015%, castings S being lower than or equal to 0.012% and P being lower than or equal to 0.02%. The hardness of any part of the valve body does not exceed 200HBW. Castings are treated by annealing or normalizing and high temperature tempering, and forgings are treated by normalizing or normalizing plus tempering. The raw materials of high-pressure slab gate valve body castings should meet high-sulfur corrosion resistance and pass the SSC (standard and simulated working conditions) and HIC (standard) sulfur resistance evaluation tests. Therefore, the internal control standards and chemical composition for sulfur-resistant steel electric arc furnace smelting process and ultra-low sulfur and high-purity steel are more stricter. Among them, S is a key element to increase the hot cracking resistance of high-sulfur steel. S reacts with Mn in the steel to form banded MnS inclusions, which will increase the susceptibility to hydrogen-induced cracking (HIC). Therefore, the S content must be controlled within the ultra-low sulfur range.  At the same time, C is a key element that increases the strength and hardness of high-sulfur resistant steel and the cracking tendency of steel, so it is controlled within the standard range of enterprise internal control.
 
The valve body material of Incoloy 825 for high-sulfur corrosion resistance adopts vacuum smelting or other alternative processes plus the electroslag remelting process. The formation of inclusions is controlled during the production process to minimize non-metallic inclusions. Use electromagnetic stirring to prevent segregation. The effective forging ratio of the valve body forgings is not less than 3 to ensure that the finished forgings have a uniform forging structure. Studies have shown that Incoloy 825 alloy valves have excellent resistance to hydrogen sulfide and chloride ion corrosion, and can be widely used in gathering and transportation projects for the exploitation of high-sulfur natural gas. The nickel-based alloy adopts solid solution treatment. Incoloy 825 nickel-based alloy parts need special fixtures and tools for processing, and cannot be directly in contact with Fe-based metals and Cu during manufacturing, packaging and transportation to avoid metal pollution.

 

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