Material Optimization and Repair Technology for Offshore High-Pressure Gate Valves

Abstract: High-pressure gate valves, which prevent fluid flow through the tight seal between the gate and the valve seat, provide excellent sealing performance and are easy to operate and maintain, making them widely used in the oil and gas exploration industry. In offshore oil and gas exploration, high-pressure gate valves are primarily used to connect pipelines in high-pressure systems such as wellheads, well control devices, and well testing equipment. They play a critical role in controlling fluid pressure, regulating flow direction, and preventing accidents. However, due to prolonged exposure to high-salinity spray environments and high-pressure fluid pipelines, these valves are susceptible to body wear and seal failure. Therefore, this study investigates the materials, operating mechanisms, and repair technologies of high-pressure gate valves. A set of repair processes specifically designed for marine environments is proposed to enhance their reliability and safety.

A high-pressure gate valve is a type of globe valve. Its main components include the valve body, bonnet, core shaft, valve stem, gate, valve seat, and sealing packing. During opening and closing, the gate—driven by the valve stem—moves vertically along the valve seat's sealing surface, thereby opening or closing the passage and allowing the medium in the pipeline to flow or be isolated. With continuous advancements in technology, materials, and manufacturing processes, gate valves have become increasingly diverse in both structure and composition. They are classified by stem structure as wedge-type or parallel-type, by actuation mode as hydraulic, pneumatic, or electric, and by body material as metallic or non-metallic. The metallic and non-metallic categories include carbon steel, stainless steel, and other alloy materials. Currently, high-pressure gate valves used in offshore oil and gas exploration equipment are typically available in 2-inch, 3-inch, and 4-inch sizes. They are designed and manufactured in accordance with the American Petroleum Institute (API) 6A standard, with pressure ratings of 34.5 MPa, 69 MPa, and 103.4 MPa. These valves are manufactured in Material Grades DD or EE, comply with PSL-3G production standards, and meet PR2 performance requirements. They also offer resistance to oil-based drilling muds, high temperatures, and high pressures.

Non-metallic seals play a crucial role in maintaining the overall sealing performance of high-pressure gate valves. Due to the relatively high operating pressures and medium temperatures in offshore oil and gas applications, the choice of non-metallic materials is limited. Currently, the most commonly used high-performance non-metallic materials are polytetrafluoroethylene (PTFE) and polyetheretherketone (PEEK). PTFE, also known as Teflon, is a synthetic polymer in which all hydrogen atoms in polyethylene are replaced by fluorine. It exhibits excellent chemical stability, corrosion resistance, and high-temperature tolerance, allowing long-term operation at temperatures of up to 250 °C. PEEK is a high-performance specialty engineering plastic with numerous advantages, including high-temperature tolerance up to 260 °C, excellent mechanical properties, and superior chemical resistance. In recent years, ongoing domestic innovation has broken the technological monopoly of foreign manufacturers, establishing PEEK as a widely recognized thermoplastic with comprehensive performance suitable for harsh operating environments. In high-pressure gate valves, non-metallic seals are generally categorized as stem seals and seat seals. As shown in Figures 1 and 2, the seals are installed within grooves. When pressure is applied, the lip of the seal housing presses against the groove, generating a constant spring force that compensates for wear, deformation, and misalignment of the mating components. Furthermore, as system pressure rises, compressive stress is exerted on the housing, ensuring effective sealing under both high- and low-pressure conditions.

Figure 1 Stem Seal Diagram

Figure 2 Seat Seal Diagram

Considering the service environments of both valve types, the stem seal is designed for applications with large axial dimensions to prevent substantial contaminants from entering the valve cavity. It features a three-arc lip structure comprising a carbon-fiber-filled special PTFE plug seal, a carbon-fiber-filled special PTFE V-ring, a PEEK V-shaped seat, and a T-stem. The seat seal primarily ensures precise positioning while preventing the entry of contaminants such as mud and sand. It comprises a PEEK shim seal and a glass-fiber-filled PTFE retaining ring, which together ensure the sealing performance of the shim seal.

Considering the operating conditions of high-pressure gate valves, and in accordance with the temperature, material grade, specification level, and performance requirements for offshore oil and gas applications, the latest edition of the API 6A standard provides specific guidelines for selecting metallic materials. These requirements for high-pressure gate valves are summarized in Table 1.

Table 1. Metallic Material Requirements for High-Pressure Gate Valves

Note: CRA refers to corrosion-resistant alloys, which are carbon or low-alloy steels alloyed with specific elements to achieve high corrosion resistance. Applicable standards include API 6A, ISO 15156, and NACE MR0175.

Table 1 indicates that, based on current domestic industrial technology, the metallic materials most commonly used for high-pressure valves include low-alloy steel 4130, martensitic stainless steel 410, and INCONEL 725, a nickel–chromium–molybdenum–niobium alloy. Research has shown that:

Low-alloy steel 4130 is a high-strength carburized steel widely used for forged pressure-bearing and pressure-control components in pressure vessels.

Martensitic stainless steel 410 is primarily used for components requiring high plasticity, toughness, and resistance to impact loads. It is also suitable for equipment exposed to corrosive media at ambient temperatures.

INCONEL 725, a nickel–chromium–molybdenum–niobium alloy, exhibits excellent toughness and corrosion resistance and is commonly used in downhole equipment operating in corrosive or acidic environments.

Based on studies of adjustable performance requirements for high-pressure gate valves in offshore oil and gas exploration, and considering advancements in domestic industrial technology, the following metallic material selection and surface treatment requirements are recommended for high-pressure gate valves:

Body: AISI 4130; seat ring bore and seal ring groove hardfaced with INCONEL 625.

Bonnet: AISI 4130; stuffing box and seal ring groove overlay welded with INCONEL 625.

Gate: AISI 410; surface treated with HVOF supersonic spraying of WC.
Valve Seat: AISI 410; surface treated with HVOF supersonic spraying of WC.
Valve Stem: INCONEL 725; thread surfaces treated with Teflon.

High-pressure gate valves used in offshore oil and gas exploration operate under extreme temperature and pressure conditions. The primary failure modes during service include seal failure, body and gate erosion, vibration fatigue, environmental corrosion, and component wear, each affecting different parts of the valve. The following representative failure cases are analyzed to demonstrate repair processes designed to enhance performance and extend service life. 

The valve body and bonnet are critical pressure-bearing components of high-pressure gate valves. The valve body is made of metal and sealed with gaskets. The main failure modes include erosion and indentation in the gasket groove, as well as rust and wear in the flow channel. Body defects are primarily small, non-penetrating flaws, such as sand holes and air pockets in the body end flanges, center flanges, neck, and bottom. Other defects include flaws in flange and threaded holes, as well as material loss caused by machining. Sealing surface defects primarily involve dents and impact damage. Surface rust (flow channel pitting) should be partially polished to a bright finish. Rust in the seat ring bore should be removed by polishing, followed by grinding. Erosion and indentation in the gasket groove must be assessed; if they exceed the allowable limits, the groove should be enlarged and welded. Minor wear in the stuffing box can be corrected by grinding and polishing, whereas severe erosion requires enlargement and welding. For defects in the valve body and bonnet, the affected areas should first be cleaned, and then inspected to confirm their weldability. Local preheating should be applied before welding. After welding, slow cooling should be carried out using insulated cotton wraps. Appropriate welding materials must be selected, with preheating applied before welding and tempering performed afterward to ensure that the base material meets the required 75K strength grade according to ISO 10423, as well as the -60 °C impact and hardness requirements, while maintaining its mechanical properties. Hole-sinking defects in valve bodies and bonnets are rare but usually result in scrapping. In such cases, a thread strength analysis is performed, with a minimum strength requirement of 36 K. To avoid unnecessary heat treatment, low-grade welding consumables are preferred. The repair process generally involves enlarging the hole, performing penetrant testing, and, once confirmed to be defect-free, carrying out repair welding using gas-shielded welding. After welding, slow cooling is applied, followed by machining and finishing. Surface defects on valve bodies and bonnets require different treatments. Strength calculations must first verify whether the thinnest section meets the required strength criteria. If it does not, the valve is scrapped. If the thinnest section meets the requirements, repair welding is carried out using cladding. Commonly used materials include E316L-16, ER316L, ENiCrMo-3, and ERNiCrMo-3, which provide enhanced corrosion resistance.

The gate and valve seat are critical pressure-control components in high-pressure valves, subject to erosion, extrusion, and friction during operation. In offshore oil and gas exploration and production, gate and valve seat failures typicallIn offshore oil and gas exploration and production, gate and valve seat failures typically manifest as corrosion of the sealing surfaces, wear, and delamination of spray coatings. They typically manifest as corrosion of the sealing surfaces, wear, and delamination of spray coatings. To repair gate surface defects, the original coating must be removed, the component sandblasted, and then resurfaced using advanced domestic supersonic thermal spraying technology. After spraying, the surface is ground and polished to achieve the required dimensions. Currently, thermal spraying is commonly used for components exposed to corrosive media. However, due to complex surface geometries, thermal spraying may not achieve complete coverage. In such cases, multiple passes at different angles must be applied to ensure full coverage and restore corrosion resistance. Repair of valve stem surface defects follows a similar process. Valve stems typically experience wear, erosion, and thread damage during service, for which direct replacement using a repair kit is generally recommended. Repairs should balance quality, cost, and timing considerations. Currently, valve stems are commonly treated with salt bath nitriding (QPQ) after grinding. This treatment significantly enhances surface wear resistance, corrosion resistance, and fatigue strength. QPQ is a modern surface hardening and modification technology that provides overall superior performance compared to conventional coating processes.

After a gate valve has been repaired, all components should be assembled in the correct sequence. Upon completion of assembly, a pressure test must be conducted to verify both operational functionality and sealing performance. During assembly, particular attention must be paid to the alignment of the gate and valve seat. When installing the valve stem, packing, and seals, all components and contact surfaces must be thoroughly cleaned. A thin, even layer of grease should then be applied to provide lubrication, protection, and proper sealing. Bolt tightening and torque values must be applied in strict accordance with the mechanics manual. Gate valve testing primarily consists of pressure testing, starting with a body strength test. All hydrostatic and functional tests must be completed before the air-tightness test, followed by a bore (diameter) inspection. The specific test scope and test pressures are defined in the applicable specifications. For PSL2, PSL3, PSL3G, and PSL4 valves, three hydrostatic seat tests at the rated working pressure shall be performed on the same side. After the pressure-holding period, an opening test shall be conducted at full differential pressure. For manually operated gate valves, the opening torque shall be measured and recorded. The hydrostatic test must verify that no visible leakage occurs throughout the specified pressure-holding period. During the air-tightness test, no visible leakage (bubbling) shall be observed in the water tank for the duration of the pressure hold.

Drawing on the standard requirements for gate valves set out in relevant specifications, this study examines the structural mechanisms, material selection, failure modes, and repair processes of high-pressure gate valves. The study further explores advanced domestic welding and thermal spraying techniques. Drawing on these findings, a comprehensive set of technical requirements and repair procedures has been established. A set of technical requirements and repair procedures for gate valves used in offshore oil and gas exploration is proposed. Implementing these measures can significantly reduce failures and extend service life, providing practical guidance for selecting and maintaining high-pressure gate valves.