Failure Mechanisms and Selection Technology for High-Temperature and High-Pressure Valve Sealing

I.Four Core Failure Mechanisms of Sealing Under High-Temperature and High-Pressure Conditions

1. Geometric Failure Due to Thermal Expansion Mismatch

Valve components (body, gate, seat) exhibit inconsistent thermal expansion during temperature rise due to differences in material or dimensions. Taking the wedge gate valve as an example:

Radial expansion of the valve body increases the seat bore diameter;

The height of the gate increases；

If the design does not provide a compensating clearance, wedge angle mismatch will occur. In mild cases, insufficient sealing specific pressure leads to leakage; in severe cases, the gate becomes "jammed" and cannot open.

2. Material Creep and Stress Relaxation

Under sustained high temperatures, metal materials undergo irreversible creep deformation:

Sealing surfaces: Microscopic protrusions are flattened, causing a continuous decrease in effective sealing specific pressure;；

Connecting components: The preload in valve stem threads and bonnet bolts decays, resulting in insufficient closing force;

Sealing elements: Gaskets and packing undergo permanent compression, losing their resilience.

3. High-Temperature Chemical Attack

Oxidation: Loose oxide scale forms on the metal surface; spalling damages the smoothness of the sealing surface;

Sulfur corrosion: Sulfur-containing media (e.g., H₂S) react with Cr and Fe to form brittle sulfides;

Chloride stress corrosion cracking (SCC): Austenitic Stainless Steel is highly prone to intergranular cracking in high-temperature, chloride-containing environments.

4. Thermal Shock and Thermal Fatigue

Rapid temperature changes during startup and shutdown generate enormous thermal stresses:

Reticulated micro-cracks (thermal fatigue) form on the sealing surface;

The hardfacing alloy layer spalls due to differences in thermal expansion coefficients;

Stress concentration at the valve body/bonnet connection leads to bolt relaxation.

II. Precision Selection Based on Failure Mechanisms

1. Material Matching

Key principle: Avoid mating identical austenitic stainless steels. Use Stellite (cobalt-based) or nickel-based alloy hardfacing. Hardness can reach HRC 40–60, combining wear resistance, corrosion resistance, and high-temperature stability.

2. Structural Type Selection

3. Operation and Maintenance

Hot-state pre-tightening: After reaching operating temperature, retighten the bonnet bolts and packing gland to compensate for creep relaxation.

Controlled warm-up: Before opening, preheat slowly via a bypass line and drain all condensate to prevent thermal shock.

Intelligent monitoring: Perform periodic infrared thermography and acoustic emission testing to detect internal leakage at an early stage; establish procedures for lapping and repairing sealing surfaces.

III. Typical Failure Cases and Solutions

Case 1: High-Temperature Seizure of a Main Steam Gate Valve  in a Power Plant

Phenomenon: A WC9 gate valve could not be opened after closing.

Root cause: The rigid gate design did not account for thermal expansion, leading to wedge angle mismatch and mechanical seizure under high temperature.

Countermeasures:
1.Replace with a flexible gate;
2.Optimize the wedge angle tolerance fit;
3.Establish hot-state operating procedures (leave a small clearance after closing).

Case 2: Erosion Failure of a Globe Valve in a Hydrogenation Unit

Phenomenon:Severe internal leakage occurred after only three months of operation, with groove-like erosion patterns on the sealing surface.

Root cause: Long-term operation at a small opening allowed catalyst particles to erode the hardfacing layer at high velocity.

Countermeasures:
1.Replace with a multi-stage pressure-reducing control valve ;
2.Optimize the control logic to avoid valve throttling;
3.Upgrade the sealing surface to high-wear-resistant Stellite 6.

Conclusion