What is the best sealing treatment method for manual gate valves?

2025-04-18 09:26:11

The sealing effect of Manual gate valves depends on the working conditions (pressure, temperature, medium characteristics) and sealing requirements. There is no "absolute best" method, and it needs to be selected based on specific scenarios. The following are mainstream sealing treatment methods and their applicability analysis to help determine the optimal solution under different working conditions:

1、 Comparison of Treatment Methods for Core Sealing Pair (Gate Plate+Valve Seat)

1. Hard seal (metal to metal seal)

Principle: The metal sealing surface is directly adhered through high-precision machining, and the gap is sealed by relying on the sealing pressure ratio (mechanical preload force+medium pressure).

Typical method:

Material reinforcement:

The sealing surface is welded with hard alloy (such as cobalt based Stellite, nickel based Inconel), with a hardness of HRC ≥ 55, a temperature resistance of 500 ℃ or above, and resistance to erosion and wear (suitable for granular media or high temperature and high pressure scenarios, such as power plants and petrochemicals).

The valve seat/gate plate is made of forged steel (such as A105, F316), surface quenched (hardness HRC35~45), with lower cost than welding, suitable for medium pressure (PN ≤ 16MPa) and medium temperature (≤ 425 ℃) working conditions (such as ordinary industrial pipelines).

Structural optimization:

Elastic wedge gate: slotted in the middle or designed with elastic arms (such as API 600 standard structure), allowing for small deformations to compensate for valve body thermal expansion or machining errors (parallelism tolerance ≤ 0.05mm), improving fitting reliability (leakage rate ≤ 0.1 × DN mm ³/s).

Floating valve seat: The valve seat is floated by spring or medium pressure, and automatically adheres to the gate plate under high pressure, dynamically compensating for the gap between the sealing surfaces (especially suitable for reverse sealing conditions).

Advantages: High temperature resistance (up to 700 ℃), high pressure (PN ≥ 42MPa), corrosion resistance and particle erosion resistance, long service life (≥ 1000 opening and closing without leakage).

Disadvantages: The processing accuracy requirements are extremely high (surface roughness Ra ≤ 0.8 μ m, flatness ≤ 0.002mm/m), the cost is high, and regular grinding and maintenance are required.

Best Applicable: High temperature and high pressure (such as steam, oil and gas), containing solid particles, strong corrosive media (matching with corrosion-resistant alloys).

2. Soft seal (metal+non-metal seal)

Principle: Soft sealing materials (such as rubber, polytetrafluoroethylene (PTFE), nylon) are embedded on the surface of the valve seat or gate plate, and the material's elastic deformation is used to fill the micro gaps.

Typical method:

Selection of sealing material:

Rubber products (nitrile rubber NBR, ethylene propylene rubber EPDM): temperature resistance ≤ 120 ℃, low pressure (PN ≤ 10MPa), suitable for clean media such as water and air, excellent sealing performance (leakage rate close to zero), low cost.

Plastic materials (PTFE, RPTFE): Temperature resistance -200~260 ℃, corrosion resistance (strong acid and alkali), but low compressive strength (≤ 10MPa), easily scratched by particles.

Structural design:

The soft sealing ring adopts dovetail groove embedding or molding to prevent detachment under high pressure; The gate is reinforced with a metal backing to prevent excessive compression failure of soft materials.

Advantages: Low sealing ratio (only 5-10MPa to achieve zero leakage), small opening and closing torque, no need for high-precision grinding, low cost.

Disadvantages: Limited temperature and pressure resistance, susceptible to medium erosion (such as rubber swelling when in contact with oil, PTFE high-temperature creep), and particle media can easily scratch the sealing surface and cause leakage.

Best Applicable: Low pressure and room temperature (≤ 16MPa, ≤ 200 ℃), cleaning media (water, gas, non particulate liquid), such as civil water supply and drainage, air conditioning systems.

3. Combination seal (hard seal+elastic compensation)

Principle: Combining the high temperature and high pressure resistance characteristics of hard seals with the deformation compensation ability of elastic structures.

Typical method:

Metal sealing surface+elastic loading: The valve seat is preloaded by a spring or bellows to ensure that the sealing surface fits well at low temperatures and low pressures, and the medium pressure assists in enhancing the seal at high pressures (such as Z61Y Gate valves used in nuclear power plants).

Double sealing surface design: The main seal is made of hard alloy overlay welding, and the auxiliary seal is a flexible graphite ring, which balances rigid sealing and emergency compensation at high temperatures (used in high safety situations).

Advantages: Suitable for a wide range of working conditions (temperature -50~650 ℃, pressure 1.6~42MPa), high reliability, and leakage rate that meets strict standards (such as API 598 Class VI soft seal grade or Class IV hard seal grade).

Disadvantages: Complex structure, highest cost, requiring precise control of the pre tightening force of elastic components.

Best Applicable: Scenarios with large fluctuations in working conditions and high safety requirements (such as petrochemicals, nuclear power, high-pressure steam pipelines).

2、 Auxiliary sealing treatment (valve stem, flange connection)

1. Valve stem packing seal

Flexible graphite packing: temperature resistance -200~650 ℃, compressive strength ≥ 300MPa, forming a ring seal through pre tightening of the gland, suitable for high temperature and high pressure (such as power station gate valves), requiring regular adjustment of the gland torque (recommended torque: DN50 about 50N · m, increasing by 30N · m for every 50mm increase).

PTFE V-shaped packing: resistant to low temperature and corrosion, suitable for low pressure and room temperature (≤ 150 ℃), particle free media, low friction coefficient (10%~20% less opening and closing torque), but prone to hardening and failure at high temperatures.

2. Flange connection sealing

Metal winding pad (graphite+stainless steel): universal type, temperature resistance -200~650 ℃, pressure ≤ 42MPa, bolt pre tightening force needs to be controlled according to ASME B16.5 standard (e.g. DN100 PN16 requires 8 M16 bolts, total pre tightening force ≥ 50kN).

Octagonal ring gasket (RTJ sealing surface): High pressure special (PN ≥ 16MPa), relying on metal ring plastic deformation to fill the gap, requiring high-precision flange sealing surface (roughness Ra ≤ 1.6 μ m).

3、 Surface treatment and machining accuracy

Grinding of sealing surface:

Hard sealing requires paired grinding (adhesion rate ≥ 95%), surface roughness Ra ≤ 0.8 μ m (soft sealing can be relaxed to Ra ≤ 3.2 μ m), flatness error ≤ 0.002mm/m (equivalent to level 1 accuracy), which directly affects the leakage rate (such as reducing Ra from 1.6 μ m to 0.8 μ m, leakage can be reduced by 50%).

Coating strengthening:

Ultrasonic spraying ceramic coatings (such as Al ₂ O ∝, Cr ₂ O ∝) with a hardness HV ≥ 1200, a thickness of 50-100 μ m, and a temperature resistance of over 800 ℃, suitable for extreme wear-resistant conditions (such as slurry and coal powder pipelines).

4、 Summary: The core principle of "optimal" sealing treatment

Priority of working conditions:

High pressure and high temperature/harsh medium: choose hard sealing (hard alloy welding)+elastic wedge structure+high-precision grinding, sacrificing cost for reliability (such as power plants and petrochemicals must meet standards).

Low pressure room temperature/cleaning medium: * * Soft seal (rubber/PTFE inlay) * * The most cost-effective, meeting zero leakage requirements (such as civil use, water supply and drainage).

Collaboration between Structure and Materials:

Hard seals require elastic design (such as floating valve seats and elastic gate plates) to compensate for processing errors and thermal deformation, while soft seals require a metal backing to prevent sealing material compression failure.

Processing and installation accuracy:

Regardless of whether the seal is soft or hard, the roughness and flatness of the sealing surface, as well as the pre tightening force of the bolts, are the key factors determining the effectiveness (especially for hard seals, where an error exceeding 0.05mm may result in leakage).

Final conclusion: Under harsh industrial conditions such as high temperature, high pressure, and granular/corrosive media, the elastic wedge hard seal structure of hard alloy surfacing combined with high-precision grinding and flexible graphite fillers is the most effective comprehensive solution; Under normal working conditions, soft seals are preferred for their low cost and ease of maintenance. In practical applications, it is necessary to make comprehensive decisions based on the characteristics of the medium, pressure and temperature parameters, and cost-effectiveness. If necessary, the effectiveness can be verified through sealing tests (such as API 598).