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What Is Pressurized Thermal Shock (PTS)? | Process Engineering Glossary
What Is Pressurized Thermal Shock (PTS)?
In piping engineering and process engineering, pressurized thermal shock (PTS), also spelled pressurised thermal shock, is the combination of rapid cooling and sustained internal pressure in a thick-walled vessel or component. When cold fluid suddenly contacts the hot inner wall while the equipment remains pressurized, thermal stresses and pressure stresses add together at the inner surface, increasing the risk of crack growth.
Why Pressure Makes Thermal Shock Worse
Ordinary thermal shock occurs when a fast temperature change creates a steep gradient through a wall. During rapid cooling, the inner surface contracts while the hotter outer wall resists, putting the inner surface in tension. If the component is also under pressure, the pressure adds further tension at the same location. A small existing flaw that would be harmless under either load alone can then grow.
The risk increases sharply when the material has lost toughness, either because it is cold or because it has been embrittled by long service. Under these conditions, PTS can lead to brittle fracture, a sudden failure with little or no warning. PTS is best known as a key safety concern for nuclear reactor pressure vessels, where neutron radiation gradually reduces the toughness of the vessel steel.
Applications
Nuclear Reactor Vessels
Regulators require nuclear plants to evaluate PTS during emergency cooling scenarios and to show that the embrittled vessel wall can withstand them. Operating limits and inspection programs are built around these evaluations.
Process Pressure Vessels
Thick-walled reactors, separators, and exchangers can see PTS-like conditions during emergency depressurization, cold quench, or cold fluid injection, so pressure vessel design and operating limits consider these events.
Life Extension Studies
PTS is evaluated in remaining life assessment for older vessels whose material toughness may have declined over decades of service.
Benefits
Prevents Catastrophic Failure
Understanding PTS helps engineers avoid conditions that could cause sudden fracture of high-energy equipment. This protects people and assets from very severe consequences.
Informs Safe Operating Limits
PTS analysis sets limits on cooldown rates and pressure during cooling, which are written into operating procedures. Operators can then respond to upsets without creating new hazards.
Guides Material Choices
Toughness requirements for thick sections are a key part of material selection for vessels exposed to rapid cooling. Better toughness provides a larger margin against crack growth.
Limitations
Complex Analysis
PTS assessment uses fracture mechanics, transient thermal analysis, and material toughness data, which require specialist expertise. Results are sensitive to assumptions about flaw size and material properties.
Material Data Uncertainty
Toughness after long service may not be known precisely without testing or surveillance samples. Conservative assumptions are often needed as a result.
Emergency Scenarios Are Hard to Avoid
Some events that cause rapid cooling, such as emergency cooling or depressurization, are themselves safety actions. Designers must balance the benefits of those actions against PTS risk.
FAQ
How is PTS different from ordinary thermal shock?
Thermal shock refers to stress from a rapid temperature change alone. PTS adds sustained internal pressure, so the combined tensile stress at the inner wall is higher and more likely to drive cracks.
Why is the inner surface most at risk?
The inner surface cools first and fastest, while the outer wall stays warm. The resulting contraction mismatch creates tension at the inner surface, and the steepness of the gradient depends on thermal conductivity and wall thickness.
How can PTS risk be reduced?
Common measures include limiting cooldown rates, lowering pressure before or during cooling, warming injected fluids where possible, and maintaining adequate material toughness.
Does thermal expansion analysis cover PTS?
No. Normal thermal expansion analysis looks at overall movement and stress in a system, while PTS requires a transient analysis of temperature and stress through the wall combined with fracture mechanics.
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