How to maintain and upkeep the sealing surface of American standard gate valves?

2025-04-18 09:42:57

The maintenance and upkeep of the sealing surface of American standard gate valves (following ASME, API and other standards) should be based on their design characteristics (such as wedge gate, parallel double gate structure) and operating requirements (high temperature, high pressure, corrosion, etc.), with the core goal of ensuring the sealing surface fitting accuracy, surface integrity and resistance to medium erosion. The following are specific maintenance methods:

1、 Inspection and protection during daily operation

1. Leakage monitoring and preliminary treatment

Regular visual inspection: If minor leaks are found through auscultation (for hissing sounds), infrared temperature measurement (for sealing surface temperature difference), or soapy water testing (for low-pressure conditions), avoid tightening under pressure and handle it while the machine is stopped.

Medium cleaning control: Install a filter (accuracy ≤ 50 μ m) upstream of the valve to prevent particle impurities (such as welding slag and rust) from scratching the sealing surface; For media containing solid particles, priority should be given to using Gate valves with guide holes to reduce impurity retention.

2. Temperature/pressure monitoring

Avoid operating under excessive temperature and pressure (according to ASME B16.34 rated parameters), especially when the hardness of sealing surface materials (such as Stellite alloy) decreases with increasing temperature at high temperatures. It is necessary to monitor the operating parameters in real-time through DCS or on-site instruments.

2、 Regular maintenance (periodic shutdown inspection)

1. Preparation before disassembly

Marking installation position: For Wedge gate valves, mark the relative position of the gate and valve seat (such as stamping) to avoid misalignment of the sealing surface during reassembly; Parallel double gate plates need to record the spring preload or wedge position.

Safe pressure relief: After completely closing the valve, discharge the upstream and downstream pressure, confirm that there is no residual medium, and then disassemble.

2. Cleaning and inspection of sealing surface

Cleaning method:

Wipe the sealing surface with acetone or alcohol to remove oil stains and carbon deposits; Stubborn attachments (such as sulfide scaling) should be cleaned with a non-metallic scraper (such as a nylon knife), and the use of a wire brush is prohibited to avoid scratching the surface.

High pressure water flushing (pressure ≤ 10MPa) is suitable for large-sized valves, and it should be dried immediately after flushing to prevent rusting.

Defect detection:

Visual inspection: Observe for scratches (depth>0.1mm requires grinding), pits, cracks, or corrosion spots.

Dye penetrant testing (PT): For suspected cracks, ASTM E165 standard is used for testing, with a focus on inspecting the edges of the sealing surface and the fusion line of the weld overlay.

Flatness detection: Use a knife edge ruler or laser plane to ensure that the flatness tolerance of the sealing surface of the wedge gate valve is ≤ 0.01mm (according to API 6D requirements).

3. Minor defect repair (grinding treatment)

Grinding tools:

Manual grinding: Use a cast iron grinding disc (with a hardness lower than the sealing surface material), apply 120 #~800 # diamond grinding paste, and grind along an "8-shaped" trajectory with uniform pressure (about 0.3~0.5MPa).

Mechanical grinding: The electric grinding machine is equipped with a dedicated fixture, and the speed is controlled at 50-100rpm to avoid overheating and material annealing (especially for stainless steel sealing surfaces).

Grinding standard:

The roughness Ra of the sealing surface should be ≤ 0.8 μ m for hard seals, and smooth and scratch free for soft seals (such as rubber);

The sealing surface angle of wedge gate valve (such as 30 ° or 45 °) should be strictly aligned with the valve seat, with a deviation of ≤ 0.5 ° (according to API 600 standard).

4. Serious defect handling

Welding repair: If the wear depth is greater than 1mm or cracks appear, welding repair shall be carried out according to ASME BPVC Section IX

Remove defects to the base material, preheat (such as preheating 150-200 ℃ before Stellite welding), use TIG welding to weld cobalt based alloy, cool slowly after welding and perform 650 ℃ stress relief annealing.

After repair, it needs to be re ground and subjected to a sealing test (according to API 598, the shell test pressure is 1.5 times the rated pressure, and the sealing test is 1.1 times).

Component replacement: If the sealing surface cannot be repaired (such as thin-walled valve seat cracking), replace the valve seat or gate with the same material to ensure that the new component meets material standards such as ASTM A182.

5. Assembly and lubrication

Installation in alignment: Wedge gate valves require adjusting the wedge angle of the gate plate to fit the valve seat, and parallel double gate plates require calibrating the spring compression amount (according to the manufacturer's technical documents) to avoid excessive local stress on the sealing surface caused by unbalanced load.

Lubrication treatment:

Sealing surface (hard seal) must not be coated with grease to prevent coking at high temperatures; Soft seals can be coated with a small amount of food grade silicone grease to enhance the elasticity of the sealing surface.

Apply high-temperature lubricating grease (such as molybdenum disulfide based grease, temperature resistance ≥ 600 ℃) to the valve stem threads and guide parts to reduce indirect damage to the sealing surface caused by opening and closing friction.

3、 Long term maintenance strategy

1. Material compatibility management

Select sealing surface material based on the characteristics of the medium:

High temperature and high pressure steam: weld Stellite 6 (temperature resistance ≤ 650 ℃) or Stellite 21 (thermal shock resistance);

Medium containing H ₂ S/CO ₂: Inconel 625 overlay is selected to avoid stress corrosion;

Low temperature conditions (below -196 ℃): The sealing surface is coated with hard chromium (thickness 50-80 μ m) to prevent low-temperature brittle cracking.

2. Sealing surface protection technology

Surface hardening treatment: nitriding the sealing surface of low carbon steel (depth 0.3~0.5mm) to increase the surface hardness to HV ≥ 800;

Coating protection: Spray ceramic coating (such as Al ₂ O ∝/TiO ₂) with a thickness of 50-100 μ m to enhance wear resistance and corrosion resistance (suitable for chemical media).

3. Operation and maintenance records

Establish a "Valve Sealing Surface Maintenance File" to record the degree of wear, grinding frequency, welding position, and test results of each maintenance. When the same part is welded more than twice, it is recommended to replace the component (as recommended by API 600).

4、 Operation precautions

Opening and closing control:

Rotate the handwheel at a constant speed during manual operation to avoid collision damage to the sealing surface caused by impact force; Electric actuators need to set a torque upper limit (usually 1.2 times the rated torque) to prevent overloading and crushing of the sealing surface.

Long term disuse maintenance:

The valve is in the fully open or fully closed position, the internal medium is emptied, anti rust grease (such as Vaseline) is applied to the sealing surface, and a protective cover is added to prevent dust from entering;

Manually open and close once every quarter (half open full close full open) to test the sealing surface adhesion.

summarize

The maintenance core of American standard gate valve sealing surface is "prevention first, precise repair": prevent early damage through daily cleaning and condition monitoring, strictly inspect and grind according to standards during periodic maintenance, and select suitable materials and protective technologies for different media and temperatures. Pay special attention to the flatness and angle matching of the hard sealing surface to avoid sealing failure caused by installation deviation or improper operation, and ultimately achieve long-term reliable operation of the valve.