What is a Metal-Sealed Gate Valve?
Basic Structure
Valve Body and Gate: Typically made from cast steel, stainless steel, or other metals. The gate can have a wedge or parallel structure, operating through vertical movement to open or close the valve.
Sealing Surface: The valve seat and gate contact surfaces are constructed with hard materials such as hardfacing alloys (e.g., stainless steel welded with Stellite) or ceramics, offering high wear and temperature resistance.
Stem: Commonly designed for rising movement, actuated manually, electrically, or pneumatically.
Working Principle
Opening and Closing Mechanism: By rotating the handwheel or actuator, the stem drives the gate vertically. When fully lifted, the passage is unobstructed; when lowered, the gate seals tightly against the seat, cutting off flow.
Hard-Sealing Mechanism: Sealing relies on precision-machined metal-to-metal contact, typically requiring significant closing force to ensure sealing integrity.
Core Features
Resistance to Extreme Conditions: Suitable for high temperatures (up to 500°C and above), high pressures (PN16–PN420), aggressive media (acidic, alkaline), and particulate-laden fluids.
Long Service Life: Hard materials offer excellent resistance to erosion and wear, significantly outlasting soft-seal valves.
Low Leakage Rate: Although not achieving zero leakage like soft seals, precision lapping or elastic seat designs can meet high sealing standards (such as ANSI Class VI).
Typical Application Scenarios
Energy and Power Industry
① Thermal Power Plants
High-Temperature Steam Lines: Main and reheater steam systems operate at 540°C–600°C and 20–30 MPa. Hard-sealed gate valves (e.g., Cr-Mo steel with Stellite overlays) withstand thermal stress without deformation.
Boiler Feedwater Systems: Controls the flow of high-temperature, high-pressure water, preventing leakage caused by soft-seal material softening.
Flue Gas Desulfurization (FGD) Systems: Treating corrosive gases containing SO₂ and Cl⁻ with nickel-based alloy (e.g., Inconel 625) sealing surfaces for superior acid resistance.
② Nuclear Power Plants
Coolant Systems: Used in primary reactor loops to control borated water or high-pressure water flow, requiring radiation- and high-temperature-resistant materials (e.g., titanium alloys, special coatings).
Auxiliary Systems: In pipelines handling radioactive waste, valves must prevent leakage and resist corrosion rigorously.
Oil and Petrochemical Industry
① Oil and Gas Extraction and Transmission
Sandy Crude Pipelines: Extraction fluids contain sand and wax; hard-seal metal surfaces resist particle erosion and prevent puncturing that soft seals would suffer.
Long-Distance Gas Pipelines: In high-pressure (10–25 MPa) and low-temperature (-40°C) conditions, double-gate hard-sealed designs with elastic compensation ensure reliable sealing.
② Refining Units
High-Temperature Oil Control: In catalytic cracking units operating above 400°C, hard-sealed gate valves with tungsten carbide coatings are used.
Corrosive Media: Units like sulfur recovery facilities handling acidic gases (H₂S, SO₂) require Monel or Hastelloy alloy sealing surfaces.
③ Coal Chemical Industry
Coal Slurry and Slag Handling: Coal gasification systems with high particulate content utilize hard-sealed gate valves with ceramic coatings for extended service life.
Synthesis Gas Pipelines: Handle high-temperature (300°C) and high-pressure (6–10 MPa) synthesis gases (CO, H₂) requiring explosion-proof designs and hydrogen-resistant materials.
Metallurgical and Steel Industry
① Blast Furnaces and Converters
Pulverized Coal Injection Systems: Control gas-solid flows containing coal powder with wear-resistant hard-sealed gate valves.
High-Temperature Gas Dedusting: Water-cooled valve bodies combined with hard-alloy seals are used in basic oxygen furnace (BOF) gas recovery systems operating at 800–1000°C.
② Continuous Casting and Rolling Mills
Cooling Water Systems: Hard-sealed gate valves withstand water hammer and abrasive impurities in secondary cooling zones.
Rolling Oil Control: Valves endure high-temperature (150–200°C) lubricants and iron oxide scale particles on hot rolling lines.
Special Industrial Applications
① Pulp and Fiber Industries
Corrosive Liquor Control: For handling black liquor (NaOH, Na₂S) or acidic media (e.g., sulfuric acid), composite hard seals with 316L stainless steel and fluoroplastic linings are employed.
② Mining and Mineral Processing
Slurry Pipelines: Valves designed with full bore to reduce clogging risks; sealing surfaces use ultra-hard silicon carbide (SiC).
③ Environmental Protection and Wastewater Treatment
High-Temperature Incineration Systems: Hard-sealed gate valves regulate dusty and acidic gas flows from hazardous waste incinerators (400–600°C).
Special Operating Conditions
① Extreme Temperatures and Pressures
Cryogenic Applications: LNG pipelines (-162°C) use valves treated with cryogenic processes to prevent embrittlement.
Ultra-High-Pressure Systems: Specialized forged-body hard-sealed valves are used in polyethylene production at 100–300 MPa.
② Two-Phase Flow and Solid-Laden Media
Gas-Solid Mixtures: Streamlined flow channels reduce deposition, with hard-wearing sealing surfaces for systems like pneumatic ash transport.
Slurry Control: Alkaline slurry handling (pH>12) in alumina production requires valves resistant to strong alkalis and particle erosion.
Irreplaceability of Hard-Sealed Gate Valves
Hard-sealed gate valves are the preferred—and often the only—choice in scenarios such as:
Media containing hard particles (e.g., coal slurry, ore slurry) where soft seals are easily punctured.
Ultra-high temperatures or frequent thermal cycling (e.g., power station steam systems) that degrade soft seals.
Highly corrosive environments (e.g., acidic oil and gas, chemical media) needing corrosion-resistant alloys.
High differential pressure conditions (e.g., long-distance natural gas pipelines) requiring strong metal-to-metal sealing.
Valve selection must consider media composition, temperature-pressure profiles, and particle content to match appropriate sealing materials and structural designs (e.g., elastic or double-disc gates) for reliable long-term performance.
Advantages and Limitations
Advantages:
Exceptional stability under extreme environments; resistant to aging and deformation.
Long maintenance cycles, reducing downtime.
Suitable for media containing solid particles, avoiding puncture failures common with soft seals.
Limitations:
High precision required for sealing surface machining, leading to higher costs.
Sealing performance at low pressure and low temperature may be inferior to soft seals.
Higher operating torque demands stronger actuators.
Key Considerations for Selection
① Media Characteristics Analysis
Corrosiveness:
Acidic media (e.g., H₂S, sulfuric acid): Choose Hastelloy C276, titanium alloy (Ti Gr2), or nickel-based alloys (Inconel 625).
Alkaline media (e.g., NaOH): Prefer Monel 400 or 316L stainless steel.
Particle Content:
Hard particles (e.g., coal dust, mineral sands): Use sealing surfaces coated with tungsten carbide (WC) or silicon carbide (SiC).
Soft particles (e.g., fibers, slurry): Opt for full-bore designs to prevent clogging.
Viscosity and Phase State:
High-viscosity fluids (e.g., heavy oil): Enlarge valve passage to reduce pressure drop.
Gas-liquid two-phase flow: Use dual-gate elastic sealing structures to accommodate phase transition impacts.
② Working Condition Parameters Matching
Temperature Range:
High Temperature (>300°C): Valve body made from chrome-molybdenum steel (e.g., A217 WC9) and sealing surfaces overlaid with Stellite 6.
Low Temperature (<-50°C): Cryogenically treated austenitic stainless steels (e.g., 304L) to avoid brittleness.
Pressure Rating:
High Pressure (PN100 and above): Forged body valves (e.g., ASTM A182 F22) with precision-ground sealing surfaces.
Vacuum or Low Pressure: Elastic seat designs to compensate for low sealing contact force.
Pressure Fluctuations and Water Hammer:
Frequent opening/closing or water hammer conditions: Prefer wedge-type gates equipped with slow-closing devices.
③ Material Selection Essentials
Valve Body and Gate Materials:
General Service: Cast steel (WCB), stainless steel (CF8M).
Severe Corrosion: Duplex stainless steel (2205), super austenitic stainless steel (254SMO).
Sealing Surface Materials:
Wear Resistance: Tungsten carbide (HV≥1200), plasma-sprayed ceramics (Al₂O₃-TiO₂).
High-Temperature Corrosion Resistance: Stellite 21, Inconel 718.
Stem and Packing:
Stem: 17-4PH precipitation-hardened stainless steel for erosion resistance.
Packing: Flexible graphite (for high temperature) or PTFE (for corrosion resistance).
④ Structural Design Key Points
Gate Types:
Elastic Wedge Gate: Suitable for applications with large temperature fluctuations (e.g., steam systems) to compensate thermal deformation.
Double Gate Design: Used in high-pressure differential or particulate media, employing spring preload for enhanced sealing.
Seat Design:
Fixed Seat: Simple structure, ideal for stable conditions.
Elastic Seat: Equipped with disc springs to adapt to pressure fluctuations and micro-deformations.
Flow Passage Optimization:
Full Bore Design: Reduces pressure loss, suitable for slurry or viscous media.
Reduced Bore Design: Cost-saving for clean media.
⑤ Sealing Grade and Standards
Leakage Class:
ANSI Class IV (allowing slight leakage): General industrial applications.
ANSI Class VI (zero bubble leakage): Strict conditions (e.g., nuclear power).
Standards Certification:
API 600: Standard for petroleum and natural gas industries requiring minimum valve body wall thickness and material testing.
ISO 10434: General specifications for high-temperature, high-pressure gate valves.
Fire-Safe Certification (API 607/6FA): Essential for flammable and explosive environments.
⑥ Actuation Methods
Manual Operation:
Small diameter (DN≤150), low-pressure conditions, cost-effective.
Electric Actuators:
Large diameter (DN≥200), high-pressure, or remote control applications; require torque matching (e.g., over 1000 N·m).
Pneumatic/Hydraulic Actuation:
Fast open/close (e.g., emergency shut-off valves), response time <5 seconds.
Gearbox Reduction:
High torque applications (e.g., DN600 hard-sealed gate valves), to reduce manual operation force.
⑦ Maintenance and Service Life Considerations
Maintainability:
Top-entry design preferred for easy seal replacement without dismantling pipelines.
Service Life Prediction:
Based on medium wear rates (e.g., sand content in g/m³), maintenance schedules should be planned accordingly.
Spare Parts Compatibility:
Modular sealing surfaces and valve body designs allowing easy replacement (e.g., split-seat structures).
⑧ Economic Evaluation
Initial Cost:
Hard-sealed gate valves generally cost 2–5 times more than soft-sealed ones but offer better long-term savings.
Life Cycle Cost (LCC):
Includes downtime losses, spare parts replacement frequency, etc. Hard-sealed valves are more cost-effective under high-temperature and high-pressure conditions.
⑨ Special Requirements Adaptation
Explosion-Proof Requirements:
ATEX/IECEx-certified valves for explosive environments (e.g., chemical plants).
Sanitary Design:
Polished surfaces (Ra≤0.8μm) and dead-angle-free structures for food and pharmaceutical industries.
Customized Design:
Extended bonnets (for cryogenic applications) and heat sinks (for high-temperature applications).
⑩ Testing and Acceptance
Pressure Testing:
Sealing test at 1.5 times nominal pressure, holding pressure for ≥3 minutes.
Material Inspection:
Hardness testing (e.g., HRC≥55) and spectral analysis for material composition verification.
Operational Simulation Testing:
Abrasion resistance tests for particulate media according to standards such as ISO 21003.
