Introduce the sealing principle of high-temperature and high-pressure gate valves

2025-04-18 09:20:05

The sealing principle of high-temperature and high-pressure Gate valves is based on the design of the sealing pair structure, material performance matching, and load transfer mechanism. By responding to thermal deformation at high temperatures, sealing pressure requirements at high pressures, and medium characteristics, reliable and leak free sealing is achieved. The following is an analysis from four aspects: core sealing mechanism, structural design, material selection, and adaptability to working conditions:

1、 Core Sealing Mechanism: Precise Coupling and Load Transmission of Sealing Pair

1. Sealing secondary structure - wedge seal is dominant

Wedge angle design (3 °~6 ° tilt angle):

The sealing surface between the gate and the valve seat is wedge-shaped, and the axial force applied by the handwheel or actuator when closed（

F) Convert to radial sealing pressure through a wedge-shaped inclined surface. Under high-pressure conditions, the medium pressure (P) also assists in generating additional forces (such as the combined force of valve stem thrust and medium pressure), resulting in a total seal pressure much higher than the medium pressure (usually ≥ 1.5P).

Elastic compensation structure:

Elastic wedge gate: slotted in the middle or designed with elastic arms (such as API 600 standard gate valves), allowing for slight deformation of the gate at high temperatures, adapting to the unevenness of the valve body/seat caused by thermal expansion (tolerance ≤ 0.05mm), and avoiding leakage caused by local stress concentration.

Floating valve seat: The valve seat is floated by a spring or medium pressure, and when the pressure is high, the valve seat moves towards the gate, dynamically compensating for the gap between the sealing surfaces (such as Z61Y gate valves used in power plants).

2. Bi directional sealing and self sealing effect

Positive sealing (with medium pressure and closing force in the same direction): The gate is pushed by the medium pressure and adheres to the downstream valve seat. The sealing pressure is provided by the combination of closing force and medium force, resulting in high sealing reliability.

Reverse sealing (reverse of medium pressure and closing force): relying on the wedge-shaped locking of the gate and upstream valve seat, sufficient pre tightening force (handwheel or actuator torque) is required to overcome the medium thrust and ensure that the sealing surface is in contact (some structures are designed with auxiliary sealing lips or pressure balance holes).

2、 Material selection: Hard sealing system resistant to high temperature, high pressure, and wear

1. Matching of sealing surface materials

Valve seat and gate:

Welding hard alloy or nickel based alloy, utilizing the high hardness (erosion resistance) and low thermal expansion coefficient (reducing high-temperature deformation difference) of the material, ensures that the sealing surface maintains the bonding accuracy (surface roughness Ra ≤ 0.8 μ m) under working conditions above 500 ℃ and PN ≥ 16MPa.

The valve seat base is made of high-temperature alloy steel, which has undergone solution treatment to improve its creep resistance and avoid deformation of the sealing surface base material under long-term high pressure.

Valve body/valve cover:

Using cast steel or forged steel, it can withstand high pressure through thick walled design, while controlling the heat conduction rate to reduce temperature difference deformation in the sealing surface area.

2. Auxiliary sealing materials

Valve stem packing: Flexible graphite packing (temperature resistance -200~650 ℃, compressive strength ≥ 300MPa) is used under high temperature and high pressure, and a ring seal is formed by pre tightening the cover to prevent the medium from leaking out along the valve stem; Partial structures are equipped with stuffing box heat dissipation grooves to prevent graphite from failing due to high-temperature oxidation.

Gasket: The connection between the valve body and the valve cover uses metal wrapped gaskets or octagonal ring gaskets, which rely on bolt pre tightening force to form an initial seal. Under high pressure, the gasket undergoes plastic deformation to fill the micro gaps.

3、 Structural Design: Key Measures for Dealing with High Temperature and High Pressure

1. Thermal expansion compensation design

Flexible connection between gate and valve stem: Through pin or T-shaped groove hinge (non rigid fixation), the gate is allowed to move slightly in the axial direction, compensating for the length difference between the valve body and gate due to material thermal expansion coefficient differences (such as valve body carbon steel vs gate stainless steel).

Valve cover extension design: Under high temperature conditions (such as steam pipelines ≥ 540 ℃), the neck of the valve cover is extended (length ≥ 2 times nominal diameter) to reduce the temperature in the packing box area (temperature difference ≤ 100 ℃) and avoid the failure of the packing due to overheating.

2. Sealing surface load strengthening

Torque calculation optimization: According to API 606 or ASME B16.5 standards, the torque of the handwheel or electric actuator should include the sealing force to overcome the medium pressure, ensuring that the sealing pressure meets the standard when closing (such as metal hard seals requiring ≥ 60MPa).

Diversion hole and pressure balance: High pressure large-diameter gate valves (DN ≥ 300) are equipped with gate diversion holes to balance upstream and downstream pressures before opening, reducing opening torque; When closed, the diversion hole is misaligned, forcing all medium pressure to act on the sealing surface, enhancing the sealing effect.

3. Surface treatment and precision control

Sealing surface grinding: CNC grinding machine is used for processing, with a flatness error of ≤ 0.002mm/m (equivalent to level 1 accuracy). The matching sealing surface ensures a bonding rate of ≥ 95% through grinding, eliminating micron level gaps (such as completely blocking leakage channels below 5 μ m).

Hardened layer depth: The thickness of the hard alloy layer deposited by welding is ≥ 3mm, ensuring that the effective sealing surface thickness (remaining ≥ 1.5mm) is maintained even after long-term erosion and wear (such as 1000 opening and closing cycles).

4、 Adaptability to working conditions: Failure prevention under high temperature and high pressure

1. Thermal stress resistance design

Avoid sharp angles or abrupt cross-sections in the sealing surface area, use large rounded corners for transition, and reduce thermal stress concentration at high temperatures;

The valve body is equipped with heat dissipation ribs or guide grooves to balance temperature distribution and prevent sealing surface deviation caused by thermal deformation.

2. High pressure seal reliability verification

Conduct shell and seal tests according to API 598, and additional testing is required for reverse sealing;

Some gate valves used in nuclear power plants need to pass high-temperature and high-pressure life tests.

Summary: Core sealing principle of High temperature and high pressure gate valve

Through the geometric load amplification effect of wedge-shaped sealing pairs, combined with high-temperature and high-pressure resistant hard sealing materials and elastic compensation structures, the following is achieved during the opening and closing process:

The combined action of mechanical preload force and medium pressure forms a sufficient sealing pressure ratio;

Material thermal stability and structural flexibility, compensating for high temperature deformation and working condition fluctuations;

High precision machining and surface treatment eliminate micro leakage channels.

Finally, under harsh working conditions (such as temperature of 500 ℃ and pressure of 25MPa), ensure that the sealing surface fits tightly and the leakage rate is controlled within industry standards (such as ISO 5208A).