What are the special sealing methods for manual gate valves?

2025-04-18 08:56:23

The sealing performance of Manual gate valves directly affects their reliability under harsh working conditions such as high pressure, high temperature, corrosion, or granular media. In addition to conventional soft seals (such as rubber and PTFE) and hard seals (metal to metal), various special sealing treatment methods have been developed for special working conditions, mainly divided into three categories: material modification, structural optimization, and surface strengthening. The specific details are as follows:

1、 Special treatment method for valve seat sealing

1. Combination sealing structure (soft hard composite seal)

Principle: Combining the high sealing performance of soft seals with the high temperature and high pressure resistance of hard seals, the valve seat or gate adopts a composite design of "metal matrix+flexible sealing layer".

The inner side is a metal hard sealing surface (such as stainless steel overlay hard alloy), which can withstand high temperature, high pressure, and particle erosion;

The outer layer is covered with flexible materials (such as expanded graphite and flexible ceramic fibers) to fill small gaps and achieve dual sealing.

Applicable scenarios: High temperature steam (≤ 550 ℃), granular media (such as coal chemical ash water), balancing sealing accuracy and wear resistance.

Advantages: The leakage level can reach API 598 Class VI (zero leakage), and the temperature and pressure resistance performance is better than that of pure soft seals.

2. Metal bellows seal (self tightening seal)

Principle: A metal bellows (made of 316L, Inconel, etc.) is installed at the connection between the gate and the valve stem to compensate for temperature/pressure fluctuations through elastic deformation of the bellows, while isolating the medium from the external environment.

Structural features:

One end of the corrugated pipe is welded to the gate plate, and the other end is fixed to the valve stem, forming a fully enclosed sealed chamber;

No need for packing sealing, eliminating the risk of leakage caused by packing wear (especially suitable for toxic, flammable and explosive media).

Applicable scenarios: high-pressure hydrogen pipelines (10~50MPa), low-temperature liquefied gases (-196 ℃ liquid nitrogen), radioactive media, and working conditions with extremely high sealing reliability requirements.

3. Pressure self tightening valve seat (self sealing valve seat)

Principle: The valve seat is designed as a floating type, relying on the pressure of the medium to push the valve seat to press against the sealing surface of the gate plate. The higher the pressure, the stronger the sealing force (reverse sealing characteristic).

Structure:

The back of the valve seat is equipped with a spring or wedge-shaped inclined surface, which provides initial sealing force under low pressure;

When under high pressure, the medium enters the back chamber of the valve seat, pushing the valve seat towards the gate direction, forming a dynamic sealing force.

Advantages: No additional torque sealing required, suitable for high-pressure reciprocating fluctuation conditions (such as compressor outlet pipelines), reducing gate wear.

4. Surface strengthening treatment (hard seal upgrade)

Supersonic flame spraying (HVOF):

Spray ceramic coatings such as tungsten carbide (WC) and chromium carbide (Cr3C2) on the surface of the valve seat/gate, with a hardness of up to HV1200~1500 and a particle erosion resistance increased by more than 5 times.

Applicable scenarios: media containing sand particles and catalyst powders (such as petrochemical catalytic cracking units).

Laser Cladding:

Localized fusion of nickel based alloys (such as Inconel 625) or cobalt based alloys (Stellite) forms a dense metallurgical bonding layer, with a temperature resistance of up to 700 ℃ and resistance to high temperature oxidation and intergranular corrosion.

Applicable scenarios: High temperature steam pipelines of supercritical boilers (570 ℃, 25MPa).

2、 Special treatment method for packing seal

1. Multi layer waveform filler (metal+flexible material composite)

Structure: Thin metal corrugated sheets (stainless steel, Monel) are alternately stacked with flexible graphite layers, and then molded into a stuffing box.

Advantages:

The metal skeleton provides structural support to prevent the filler from being squeezed out by high-pressure media;

The flexible graphite layer absorbs valve stem vibration and thermal expansion (compensation amount ± 0.5mm), reduces valve stem wear, and is suitable for high temperature (≤ 650 ℃) and high-frequency opening and closing conditions (such as power plant bypass valves).

2. Fat injection dynamic seal (auxiliary sealing system)

Principle: Install a sealing grease injection hole at the bottom of the packing box, and inject specialized sealing grease (such as high-temperature silicone grease, molybdenum disulfide lubricating grease) through a high-pressure grease gun to form an "active sealing ring".

Structural features:

The grease injection pressure should be 0.5-1MPa higher than the medium pressure to ensure that the sealing grease penetrates into the gap of the packing;

Equipped with pressure monitoring device, real-time display of packing sealing status, supporting online maintenance.

Applicable scenarios: High pressure natural gas pipelines (PN100~250), high viscosity media (such as heavy oil), can repair minor leaks without stopping the vehicle.

3. Magnetic fluid seal (non-contact seal)

Innovative technology: Utilizing the viscous properties of magnetic fluid (nanoscale magnetic particle suspension) under the action of a magnetic field to form a sealing ring around the valve stem.

Advantages:

Without contact friction, the wear rate of the valve stem is reduced by 90%, and the opening and closing torque is reduced by 30%;

Suitable for vacuum systems (10 ⁻⁶ Pa) and ultra clean media (such as pharmaceutical pure water) to avoid contamination of filler fibers.

Limitations: Currently in the experimental stage and not yet commercially available on a large scale.

3、 Customized sealing design for special working conditions

1. Bidirectional sealing structure (double blocking and discharge, DBB)

Principle: Both sides of the gate are equipped with independent sealing surfaces, which can simultaneously seal the upstream and downstream media, and release the valve chamber pressure through the middle discharge port.

Structure:

Double seat design, with double wedge or parallel double gate plates for the gate plate;

The valve chamber is equipped with a pressure relief hole to prevent abnormal pressure rise from damaging the sealing surface (in accordance with ASME B16.34 standard).

Applicable scenarios: pipelines that require online maintenance (such as dual isolation conditions), multi-directional flow systems (such as four-way valves on offshore platforms).

2. Low temperature cryogenic seal (anti shrinkage design)

Handling method:

The valve seat and gate adopt a negative tolerance fit (interference fit of 0.05~0.1mm) to offset material shrinkage at low temperatures (such as a carbon steel shrinkage rate of 0.3% at -196 ℃);

The filler uses polytetrafluoroethylene (PTFE)+glass fiber reinforced material to avoid low-temperature hardening and brittle cracking;

The sealing surface is plated with hard chromium (thickness 50 μ m) to improve surface hardness and cold welding resistance.

Applicable scenarios: LNG receiving stations, low-temperature pipelines for air separation equipment.

3. Anti corrosion sealing (fully lined/wrapped treatment)

method:

The interior of the valve body, gate plate, and valve seat are coated with corrosion-resistant materials (such as perfluoroalkoxy (FEP) and polyvinylidene fluoride (PVDF)), with a thickness of 2-5mm;

The sealing surface adopts a "coating layer+metal backing" structure to ensure a balance between strength and corrosion resistance.

Applicable scenarios: Strong corrosive media such as concentrated hydrochloric acid (31% concentration) and hydrofluoric acid (HF), with a lifespan more than three times longer than ordinary stainless steel valves.

4、 Sealing performance testing and standards

Special sealing treatment needs to be verified through rigorous testing:

High pressure gas seal test (such as API 598 Annex F): leakage rate ≤ 0.1 × DN mm ³/s (DN is the nominal diameter);

Hot cycle sealing test (GB/T 26480): Cycle 5 times in the range of -20 ℃~450 ℃ without leakage;

Particle erosion test (ISO 15848-1): Introduce a medium containing 100 μ m quartz sand and run it for 100 times, with a sealing surface wear of ≤ 0.02mm.

summarize

The core of special sealing treatment is to solve the problems of leakage, wear, thermal expansion and contraction of conventional seals under extreme working conditions through material innovation (such as metal bellows, nano coatings), structural optimization (self tightening valve seat, bidirectional sealing), and dynamic compensation (grease injection sealing, waveform packing). When selecting, it is necessary to consider the characteristics of the medium (temperature, pressure, corrosiveness, particle size), opening and closing frequency, and maintenance requirements. For example, high-pressure hydrogen should be sealed with corrugated pipes, granular media should be sprayed with HVOF, and low-temperature working conditions should be designed with anti cold shrinkage to achieve precise sealing of "one working condition, one solution".