Flame sprayed hard seal

I. Core function of ball valve

1. Intercepting and regulating fluid
   • Cut-off function: when the through hole on the sphere is aligned with the pipeline, the fluid can flow freely; After rotating 90, the sealing surface of the ball is closely attached to the valve seat, blocking the flow of medium, and the sealing level can reach Class VI (zero leakage) of API 6D.
   • Adjustment function: some V-shaped notched spheres (such as V-shaped ball valves) can control the flow through the rotation angle, and the streamlined design of the notch edge can reduce the fluid turbulence, which is suitable for scenes with high adjustment accuracy (such as chemical reaction feed control).
2. Ensure sealing reliability
   • The matching accuracy (surface roughness Ra < 0.8 μ m) and geometric shape (roundness error < ＜0.01mm) of spherical sealing surface and valve seat are the key to sealing. For example, the soft sealing ball valve adopts PTFE valve seat, and the surface of the ball is plated with hard chromium (thickness 50~80μm), which can realize bubble-level seal; Metal hard-sealed ball valves (such as high-temperature working conditions) can resist the leakage caused by high-temperature deformation by grinding the ball and the conical surface of the valve seat (sealing specific pressure > 6 MPa).
3. Adapt to the requirements of complex working conditions
   • Anti-erosion: in the medium containing particles (such as pulp and residual oil), the spherical material should be wear-resistant (such as tungsten carbide coating), and the surface hardening treatment (HRC≥60) can reduce erosion wear;
   • Corrosion resistance: in chemical media (such as sulfuric acid and seawater), Hastelloy C-276 or duplex stainless steel 2507 is used for the ball to avoid pitting or stress corrosion;
   • High temperature resistance: Inconel 718 ball is used in high temperature steam (500℃+) scene, and its surface is aluminized to enhance oxidation resistance.

II. Structure Type and Working Principle of Sphere

1. Classification by structural form

2. According to the sealing surface position classification

• Floating ball: the ball floats under the pressure of medium, squeezing the downstream valve seat to realize sealing, which is suitable for low pressure and small caliber (DN≤150mm), such as household gas ball valve;
• Fixed ball: the ball is fixed by the upper and lower shafts, and the medium pressure is transferred to the valve body through the valve seat, which requires additional sealing force (such as spring loading). It is suitable for high pressure and large diameter (DN≥300mm), such as the main valve of gas pipeline.

III. Matching of Spherical Materials with Industrial Applications

1. General industrial scene

• Water/steam system: 304 stainless steel ball (oxidation resistance, low cost), chrome-plated surface to prevent scale deposition;
• Natural gas pipeline: 2Cr13 martensitic stainless steel sphere (strength σ b > 700 MPa) with Stellite alloy surfacing (abrasion resistance) to resist HS corrosion in natural gas;
• Food and medicine: 316L stainless steel ball (electro-polished, Ra < 0.4μ m), conforming to FDA certification, resistant to CIP cleaning (steam at 134℃).

2. Special materials for extreme working conditions

• Ultra-low temperature (-196℃): LNG pipeline is cryogenic treated with 316L+liquid nitrogen to eliminate stress concentration in austenite structure and avoid low-temperature embrittlement;
• Strong corrosion (concentrated nitric acid): titanium alloy TA2 sphere, the passivation film is resistant to oxidative acid corrosion, and the corrosion rate in 68% nitric acid is less than ＜0.05mm/year;
• High temperature and high pressure (600℃, 30 MPa): Inconel 625 sphere, after solution+aging treatment, the high temperature creep strength (10000 hours @ 600℃) > 100 MPa.

IV. Key technical points of sphere design

1. Geometric optimization of sealing surface
   • The ball sealing surface is usually designed as a double eccentric or triple eccentric structure (such as butterfly valve ball). The eccentricity (0.5~2mm) can realize the line contact of the sealing surface, reduce the friction torque (30% less than concentric structure) and compensate the sealing failure caused by temperature deformation.
2. Wear-resistant and erosion-resistant technology
   • In the medium containing solid particles, the ball adopts the combination of “tungsten carbide coating+ceramic valve seat”. For example, the surface of the ball of a slurry ball valve is sprayed with WC-Co coating (thickness 0.3~0.5mm), and the hardness HV is ≥ 1200, and the wear life can reach 5~10 times of that of ordinary stainless steel.
3. Anti-static and fire protection design
   • In the petrochemical scene, a conductive spring (resistance < 10Ω) should be installed between the ball and the valve stem to prevent the friction from generating electrostatic sparks; The fireproof structure requires that the ball and the valve seat can still be sealed in case of fire (800℃), and the double protection of metal sealing surface and graphite sealing ring is usually adopted.

V. Typical industrial application cases

• Shale gas mining wellhead: a fixed ball valve is used. The ball is made of 2507 duplex stainless steel certified by NACE MR0175, which is resistant to HS (partial pressure > ＞0.3MPa) and Cl (content > ＞5000ppm). The surface of the ball is coated with high-speed flame spraying (HVOF)WC to resist the erosion of quartz sand in fracturing fluid.
• Main loop of nuclear power plant: The ball of nuclear grade ball valve is made of 316LN stainless steel (nitrogen control is 0.08~0.12%). After 100% ultrasonic flaw detection (UT) and magnetic particle flaw detection (MT), Inconel 625 alloy is surfacing on the sealing surface, which meets the requirements of ASME BPVC Section III nuclear grade equipment and can run for a long time in high temperature and high pressure water environment of 343℃ and 15.5MPa.
• LNG marine loading and unloading system: The ball of ultra-low temperature ball valve is treated with 0Cr18Ni9+cryogenic treatment (kept at -196℃ for 24 hours), and its structure is transformed into stable austenite, with impact energy (-196℃)≥34J. It is matched with PTFE)+ nickel alloy valve seat to prevent cold brittle failure caused by LNG leakage.

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