Material Selection and Analysis of Ball Valves for Oxygen Pipelines

Abstract

This paper presents the material selection criteria for ball valves used in oxygen pipelines, explores their structural design, and describes critical considerations such as antistatic structures, oil-free requirements, degreasing treatments, and relevant inspection and testing methods.

1. Overview

Oxygen is frequently used in pipeline systems across the metallurgical, petrochemical, coal chemical, medical, and aerospace industries. As a powerful oxidizer, oxygen accelerates combustion, and when it interacts with combustible materials, it can ignite or even cause explosions. Therefore, oxygen is classified as a hazardous medium due to its combustibility. To prevent explosive incidents related to ball valves in oxygen pipelines, strict standards are applied to material selection, structural design, oil-free requirements, degreasing treatments, as well as inspection and testing, to ensure the safe and reliable operation of oxygen pipeline systems.

2. Material Selection

The Asian Industrial Gas Association (AIGA) standard AIGA 021/05 defines guidelines for selecting metallic and non-metallic materials used in oxygen pipeline systems. The materials prohibited in AIGA 021/05 include brass alloys, cobalt alloys, copper alloys, copper-nickel alloys, cast and forged austenitic stainless steel, nickel-based alloys, and tin bronze. In comparison with the JB/T 12955 standard, AIGA 021/05 provides more comprehensive specifications and serves as an authoritative reference for selecting ball valve materials in oxygen pipelines.

2.1 Main Materials

The selection of primary materials for ball valves in oxygen pipelines should consider factors such as operating pressure, temperature, material flame resistance, oxygen purity, and flow rate. The preferred materials are non-oxidizing, non-corrosive, and low-carbon, as high-pressure oxygen can generate heat through friction with carbon in steel, posing a significant fire risk in pure oxygen environments. Nickel-based alloys generally contain over 50% nickel. Both nickel and copper demonstrate exceptional flame resistance, and their combination further improves this property. As a result, Monel 400 and Monel K500, containing approximately 63% nickel and 30% copper, offer superior flame resistance compared to Inconel 600 and Inconel 625. The aluminum content of bronze alloys should not exceed 2.5%. For low-pressure oxygen pipeline ball valves, typically selected materials include austenitic stainless steel and copper alloys. Austenitic stainless steels typically belong to the 304, 316, 304L, and 316L series, while copper alloys often include 16-4 silicon brass, specifically the casting grade GB/T 12225 ZCuZn16Si4. High-pressure oxygen pipeline ball valves are predominantly made from Inconel 600, Inconel 625, Monel 400, and Monel K500.

The pressure and flow rate limits for the ball valve are designed to minimize turbulence caused by oxygen and reduce the effects of particulate impurities within the pipeline. When selecting materials, it is essential to consider factors such as flow velocity within the valve, friction, static electricity, potential ignition sources from non-metallic materials, and the possibility of contaminants, such as rust on the surface of carbon steel. Material selection must adhere to strict standards to ensure safety and performance. The flow velocity within the ball valve body must not exceed the maximum permissible oxygen flow rate for the pipeline. The oxygen flow rate limits for various pipeline materials are provided in Table 1. If the flow velocity in the ball valve channel exceeds the specified limits, the primary material should be selected from the permitted materials based on the corresponding operating pressure. The pressure and thickness requirements for permitted materials are specified in Appendix D of AIGA 021/05. Exempt pressure refers to the maximum allowable pressure for a specific alloy material in an oxygen-rich environment where particle impact is possible, without considering oxygen flow rate limitations. Exempt materials are engineering alloys that are not subject to oxygen flow rate restrictions under specific pressure, thickness, and purity. Flame retardancy refers to a material's property, either inherently or after treatment, that substantially retards the spread of flames. The flame retardancy of austenitic stainless steel lies between that of low-carbon alloy steel and nickel- or copper-based alloys, with nickel-copper alloys showing superior flame resistance.

Table 1: Carbon Steel and Austenitic Stainless Steel Pipe Oxygen Flow Rate Limits

2.2 Sealing Gaskets

Sealing gaskets must be selected based on the operating pressure of the oxygen system. For pressures up to 3 MPa, PTFE (polytetrafluoroethylene) or grease-free graphite composite gaskets can be used, subject to temperature requirements.For pressures above 3 MPa, stainless steel spiral-wound gaskets or gaskets made from non-ferrous alloys, such as Monel or Inconel, should be used.

2.3 Packing

Packing materials must be selected based on the operating pressure of the oxygen system and the potential for adiabatic compression due to high-pressure fluctuations. Suitable materials include PTFE, grease-free flexible graphite, or a combination of non-ferrous alloy wire and grease-free flexible graphite. For high-pressure oxygen applications (typically above 3 MPa), combination packing is highly recommended. The top and bottom layers should be made of non-ferrous alloy wire and grease-free braided graphite, while the middle layer should contain non-ferrous alloy wire and grease-free flexible graphite.

2.4 O-Ring

O-rings are elastomeric sealing components widely used in high-pressure oxygen applications. Common materials, such as Kalrez, Viton, and Fluorel, are suitable for oxygen applications due to their excellent chemical resistance and high thermal stability. O-rings rated for explosion-proof applications must be used for design pressures exceeding 10 MPa.

2.5 Sealing Pair

The seat sealing ring of a soft-seated ball valve must be made from materials with high oxygen compatibility and resistance to combustion, such as PTFE, FEP, and PCTFE. For high-pressure oxygen applications, a metal-to-metal sealing design is required. To prevent galling between the ball and valve seat sealing surfaces, a sufficient hardness differential must be maintained. The ball sealing surface must be harder than the valve seat sealing surface, with a minimum hardness difference of 50 HBW. As per JB/T12955 standard, the metal sealing surface can be overlaid with cobalt-based or nickel-based alloys. Following machining, the welded layer must have a minimum thickness of 2 mm. Cobalt-based and nickel-based alloys exhibit comparable flame-retardant properties. The surface roughness of the sealing interface is critical to ensuring optimal sealing performance. For soft-seated ball valves, the ideal ball sealing surface roughness is Ra ≤ 0.4 μm. For metal-seated ball valves, the ideal surface roughness for both the ball and valve seat sealing surfaces is Ra ≤ 0.1 μm.

2.6 Grease

The lubricant must be a fluorinated grease that is resistant to both high and low temperatures and exhibits low flammability. If halogenated chlorotrifluoroethylene (CTFE) liquid grease is used, it must be restricted to dry gas applications, as it allows moisture to penetrate the oil film, potentially causing severe corrosion.

3. Structural Design

The internal flow paths and passageways of the ball valve used in oxygen pipelines must be smooth, free of sharp edges, and devoid of burrs. Component edges must be rounded, and surface irregularities, such as depressions and protrusions, should be minimized to mitigate the risk of ignition caused by oxygen flow dynamics.

3.1 Avoiding Adiabatic Compression

A ball valve functions by rotating the ball 90° around the valve stem axis to control the flow. During this operation, the ball remains in contact with the valve seat, leading to friction. Rapid opening or closing can cause adiabatic compression, potentially leading to dangerous conditions. To mitigate the risk of adiabatic compression, the following measures are recommended:

• The ball valve must not be equipped with a fast-opening mechanism. A worm gear reducer must be installed to ensure gradual operation, thereby preventing adiabatic compression caused by abrupt changes in oxygen flow rate.
• A pressure-balancing hole must be incorporated into the ball, connecting the internal valve passage to the central cavity. This equalizes the pressure between the passage and the cavity, preventing excessive flow rates and temperature increases that could lead to adiabatic compression.
• A bypass valve must be installed. The bypass valve must be opened prior to the main valve to equalize pressure between the upstream pipeline, valve cavity, and downstream pipeline. This ensures pressure consistency when the main valve is opened, thereby preventing adiabatic compression.

3.2 Structure

Ball valves for oxygen pipelines are classified into floating ball valves (Figure 1) and trunnion ball valves (Figure 2). Floating ball valves are typically used in small-diameter, low-pressure applications. Trunnion ball valves are commonly used in medium- and large-diameter applications and are suitable for use across a wide range of pressure levels. To prevent dust and oil ingress into the packing area, both floating and trunnion ball valves should be equipped with dust covers between the packing and the operating handle or actuator, which act as seals. A sealing gasket must be placed between the dust cover and the bracket to further prevent dust and oil contamination in the packing area.

Figure 1 Floating ball valves

Figure 2 Trunnion ball valves

3.3 Sphere

The welded interior of a hollow sphere leads to concentrated welding stresses and thin wall sections, which pose significant safety risks. As a result, hollow spheres are prohibited for use in oxygen service; only solid spheres are allowed. The sphere's shape and positional tolerances, including roundness, coaxiality, and circular runout, must conform to the requirements of GB/T 26147. For spheres used in high-pressure oxygen service, finite element analysis (FEA) of the stress field must be performed to prevent deformation and optimize structural integrity.

3.4 Anti-Static Function

Internal Anti-Static Protection Mechanism

To eliminate the risk of static electricity buildup caused by friction between materials during valve operation, an anti-static device must be integrated. This is achieved by incorporating a small spring and a steel ball into the valve stem. The preloaded spring ensures continuous contact between the steel ball, the sphere, and the valve body, facilitating the effective dissipation of static electricity.

External Anti-Static Protection Mechanism

Conductive grounding bolts must be installed on the valve’s inlet and outlet flanges, with bolt sizes ranging from M6 to M10, to ensure proper grounding.

4. Special Treatment

4.1 Degreasing

The contact of oxygen with grease can result in combustion and pose an explosion risk. Therefore, all parts, tools, and components of ball valves for oxygen pipelines must undergo degreasing. The degreasing method must adhere to HG20202 standards. Metals should be degreased using solvents such as triethylene trichloride, ethylene dichloride, trichloroethylene, acetone, alcohol, or other non-flammable inorganic cleaning agents, or by ultrasonic degreasing. Metal parts should be degreased through heating, with the cleaning solution temperature maintained between 60°C and 900°C. Steel parts should not be heated below 80°C, while non-ferrous metals should be heated between 70°C and 80°C. The solution should be stirred frequently, with an immersion time of 1 hour. White, non-cotton cloths should be used for wiping. PTFE non-metallic components (such as valve seat seals, packing, and mid-way gaskets) should be degreased at room temperature by immersing them in a degreaser for 1.5 to 2 hours. After removal, the components should be dried until the odor of the degreaser is no longer detectable.

4.2 Oil-Free

To prevent external grease from contaminating internal components such as packing, ball valves for oxygen pipelines must be provided with oil-free devices. The primary method involves placing a dust cover between the packing and the operating handle or drive mechanism, and permanently marking the dust cover with the words "oil-free" in red.

5. Inspection and Testing

5.1 Degreasing Test

The common methods for degreasing tests include the camphor test, ultraviolet irradiation test, and direct test. The camphor and ultraviolet irradiation tests comply with Appendix A of JB/T12955. The direct test involves wiping the surface of valve parts with clean, dry, white filter paper to ensure no grease residue remains.

5.2 Pressure Test

Shell strength and sealing tests for ball valves in oxygen pipelines must follow the methods specified in GB/T26480.

Test Pressure: The high-pressure gas shell and sealing test pressures are both 1.1 times the maximum allowable working pressure of the valve at 38°C. The low-pressure gas sealing test pressure should be between 0.4 and 0.7 MPa.

Pressure Test Duration: The minimum duration for maintaining the test pressure must adhere to the requirements in Table 2.

Table 2 Duration of test pressure maintenance

5.3 Antistatic Test

After successfully completing the valve pressure test, the ball valve must be dried, cycled open and closed at least five times, and have a ball, stem, and body resistance of less than 10 Ω. The power supply voltage during testing must not exceed 12 V.

6. Conclusion

Oxygen is a hazardous medium, so ball valves for oxygen pipelines must prioritize safety in their design and meet specific requirements for material selection, structural design, antistatic functionality, oil prevention, degreasing, inspection, and testing. While in operation, attention should be paid to oil prevention, dust control, fire safety, and antistatic measures. The ball valve must operate slowly, and regular monitoring and maintenance should be conducted to ensure the valve's reliability and safety.