How to deal with common internal leakage faults of high temperature and high pressure gate valves?

2025-04-18 10:29:00

Internal leakage failure of high temperature and high pressure Gate valves (leakage of medium between valve disc and valve seat sealing surface) is a common and urgent problem in industrial pipeline systems, and its core cause is often related to sealing surface failure. The following is a systematic solution based on the cause of the malfunction:

1、 Analysis of the core causes of internal leakage

Sealing surface damage or failure

Mechanical damage: The hardness of the material decreases at high temperatures, and the erosion of medium particles (such as welding slag and oxide scale) leads to scratches and pits on the sealing surface; Frequent switching or excessive pressure difference can cause impact wear on the sealing surface (detachment of hard alloy overlay).

Thermal deformation and stress: The difference in thermal expansion coefficient between valve body/disc materials (such as stainless steel and carbon steel) leads to misalignment of the sealing surface; The sealing surface undergoes creep or plastic deformation under long-term high temperature (such as age hardening of cobalt based alloys above 650 ℃).

Corrosion and scaling: Electrochemical corrosion (such as stress corrosion cracking) occurs on the sealing surface under high temperature and high pressure media (such as sulfur-containing and water vapor); Scale and chemical medium crystals wedged into the sealing surface gap inside the steam valve.

Valve not fully closed or stuck

Insufficient valve stem travel: Improper limit of valve stem nut during installation, or thread wear leading to incomplete seating of the valve stem; Deviation in stroke adjustment of electric/pneumatic actuators (such as uncalibrated limit switches).

Skewness or jamming of the gate: Wear of the guide sleeve, bending of the valve stem, or deformation of the valve body (uneven thermal expansion of the flange at high temperatures leads to excessive ellipticity), which prevents the gate from vertically fitting the valve seat.

Insufficient sealing pressure or uneven distribution

Insufficient pre tightening force of bolts: thermal relaxation of flange bolts (stress attenuation of bolts at high temperatures, such as a decrease of about 10% in stress per year for carbon steel bolts above 300 ℃), or failure to tighten torque according to standards (such as the high-temperature bolt heating tightening process specified in ASME B16.5).

Excessive roughness of sealing surface: Insufficient machining accuracy (such as Ra value>0.8 μ m) or uneven grinding after repair, resulting in micro leakage channels (gas medium can leak within a gap of 0.001mm).

Defects in structural design or selection

Mismatch of hard sealing materials: Cobalt based alloys (such as Stellite 6) or hard materials such as tungsten carbide were not selected for high temperature and high pressure, and ordinary stainless steel was used for welding; Improper design of wedge angle for Wedge gate valve (such as insufficient compensation ability of 3 ° wedge angle at high temperature).

Failure of valve seat fixing method: Threaded valve seats loosen under vibration or temperature difference (such as power station valves that do not use welded valve seats), or the sealing ring (O-ring) of the pressure cover valve seat fails due to high temperature aging (nitrile rubber hardens when exceeding 200 ℃).

2、 Step by step processing plan

1. Preliminary investigation before shutdown (temporary treatment under pressure)

Confirm the leakage location: locate the leakage point by listening to the leakage rod and ultrasonic leak detector, and distinguish between internal leakage (from the valve body cavity to the downstream) and external leakage (packing box, flange interface).

Attempt to force sealing:

Manual valve: Slowly open and close the valve multiple times, using the grinding effect of the gate to remove minor impurities (suitable for soft seals or mild jamming).

Electric/pneumatic valve: recalibrate the actuator stroke, increase the clearance by 1-2% (within the allowable range of valve design), and compensate for the sealing surface clearance by overtravel.

Temporary plugging: If the medium is non flammable and non explosive, high-temperature sealing grease (such as graphite based sealant) can be injected into the grease injection hole (only for valves with grease injection structure) to fill the gap between the sealing surfaces.

2. Deep maintenance after shutdown and pressure relief

Safety prerequisite: Close the upstream and downstream valves, release pressure to 0MPa, cool down to ambient temperature, hang a "no operation" warning sign, and install blind plates for isolation if necessary.

Disassembly and cleaning

Disassemble the valve cover bolts, remove the valve cover and gate assembly, and pay attention to recording the installation position of the components (such as the inclination direction of the gate of the wedge gate valve).

Use kerosene or specialized cleaning agents to remove scale, rust, or polymer residue from the sealing surface, valve stem threads, and guide sleeve surface (avoid using metal brushes to prevent scratching the sealing surface).

Sealing surface inspection and repair

Visual inspection: Use a magnifying glass to observe whether there are radial scratches (depth>0.2mm needs to be ground), penetrating cracks (parts need to be replaced), or detachment of the welding layer (area>5% needs to be re welded) on the sealing surface.

Dye penetrant testing (PT): Conduct penetration testing on the sealing surface of hard alloy to confirm microcracks (indicated by red lines).

Grinding repair:

Mild damage (scratches ≤ 0.5mm): Use a silicon carbide grinding disc (coarse grinding 80 #~120 #, fine grinding 240 #~400 #) with a grinding platform to grind along an "8-shaped" trajectory until the sealing surface roughness Ra ≤ 0.4 μ m.

Moderate damage: An electric grinder is used, with a guide device installed to ensure the flatness of the sealing surface (deviation ≤ 0.02mm/100mm). After grinding, the contact rate is checked using the "red lead powder" coloring method (hard seal ≥ 85%, soft seal ≥ 95%).

Serious damage: Replace the valve seat or gate (if the integral valve seat can be welded with Stellite 6 and reprocessed, with a welding layer thickness of ≥ 3mm).

Component accuracy verification

Measure the straightness of the valve stem (if the total length deviation is greater than 0.1mm, it needs to be straightened), and if the thread pitch error is greater than 0.05mm, it needs to be replaced.

Check the clearance between the guide sleeve and the valve stem (normal value of 0.1~0.3mm, replacement is required if it is worn over 0.5mm), and the perpendicularity between the valve body sealing surface and the channel (correction is required if the deviation is greater than 0.1 °).

Assembly and fastening

Apply high-temperature anti seize agent (such as a mixture of molybdenum disulfide powder and high-temperature oil) to the sealing surface to avoid metal adhesion at high temperatures.

Tighten the valve cover bolts in a symmetrical order 2-3 times, using a torque wrench according to the standard value (such as Class 900 valve M24 bolt torque of about 1200N · m), and consider hot tightening of the bolts under high temperature conditions (such as adding 10% pre tightening force at 300 ℃).

3. Material and structural upgrades (for repeated leaks)

Adaptation of sealing surface material:

High temperature and high pressure (>540 ℃,> 10MPa): Cobalt based alloy (Stellite 21) or solid hard alloy (such as WC Co) is used for welding, with a hardness of ≥ HRC55.

Medium containing particles: tungsten carbide particles are added to the sealing surface overlay layer (such as Stellite 6+WC composite layer), or a metal seal+elastic compensation structure (such as spring-loaded valve seat) is used instead.

Structural optimization:

Wedge gate valve: Add a sealing surface wedge angle compensation device (such as an adjustable wedge gate) to adapt to valve body deformation at high temperatures.

Parallel double gate valve: Check the elasticity of the middle spring (replace if the compression is less than 50%) to ensure that the double gate synchronously fits the valve seat.

4. Functional testing and operational monitoring

Pressure test: After maintenance, conduct a shell pressure test (1.5 times nominal pressure) and a sealing test (1.1 times nominal pressure), and maintain the pressure for 30 minutes without any pressure drop (API 598 standard).

Operation monitoring:

Install a temperature sensor on the high-temperature valve (alarm when the valve cover temperature exceeds 10% of the design value) to monitor the thermal stress on the sealing surface.

Regularly record the operating torque (warning of sealing surface sticking when the torque curve of the electric valve suddenly changes), and when the medium is gas, it can be remotely monitored through downstream leak detectors (such as laser methane detectors).

3、 Preventive measures

Regular maintenance plan

High temperature valves undergo offline maintenance every 6-12 months, with a focus on checking the wear of the sealing surface and bolt stress (using ultrasonic bolt stress gauges).

Install a Y-shaped filter (filter accuracy ≤ 50 μ m) before the pipeline containing particulate media to reduce the erosion of solid impurities on the sealing surface.

Operating specifications

When opening and closing the valve, operate it at a constant speed to avoid rapid impact on the sealing surface (the electric valve is set with an opening and closing buffer time of ≥ 15 seconds).

It is prohibited to forcefully open or close stuck valves with force rods. Priority should be given to pressure relief maintenance rather than violent operation.

Selection optimization

For high temperature and high pressure applications, fully welded valve bodies and self sealing valve cover structures (such as API 600 gate valves) are preferred to reduce sealing points; Soft sealed valves (such as PTFE seats) should not be used at temperatures exceeding 260 ℃ to avoid melting of the sealing surface due to overheating.

summarize

The treatment of internal leakage in high-temperature and high-pressure gate valves should follow the logic of "first investigating external faults (such as stroke and bolts), then deeply inspecting the sealing surface, and finally upgrading the material structure based on the working conditions". The core lies in restoring the integrity of the sealing surface through precise damage detection, professional grinding and repair, and appropriate material selection, while avoiding repeated failures through preventive maintenance. For critical valves with frequent leaks, it is recommended to establish a full lifecycle file to record each maintenance data and failure mode, and optimize maintenance strategies accordingly.