Thermally Activated Shutdown Seal for Rotating Shafts

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Solution Overview

Problem

In pressurized water nuclear power plants, the existing pump shaft seal assemblies are vulnerable to increased leakage due to high temperatures during loss of fuel cooling and lack a reliable backup mechanism to prevent coolant exposure to the reactor core, posing a risk of core damage.

Innovation Solution

A thermally actuated shutdown seal featuring a split ring that constricts around the shaft upon temperature rise, utilizing a meltable spacer or passively actuated device to block coolant leakage, combined with a pliable polymer seal ring to ensure a tight seal, effectively sealing the shaft even when rotating equipment slows or stops.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If standard seal assemblies are used to maintain controlled leakage, then normal sealing function is achieved, but reliability deteriorates under high temperature conditions causing excessive leakage

Engineering Contradiction:

Improveseal reliabilityVSAvoidtemperature effect on leakage

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The shutdown seal is pre-positioned in a restrained state during normal operation, ready to activate automatically when temperature reaches a predetermined threshold. The meltable spacer is installed in advance to hold the split ring in its restrained position, ensuring immediate protective action without requiring detection or control systems.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The shutdown seal utilizes the thermal energy from the high-temperature environment itself to trigger activation. The meltable spacer automatically melts when exposed to excessive temperature, causing the split ring to constrict and seal the shaft without requiring external power, control systems, or manual intervention.

Inventive Principle:

Principle #25Self-service

2Reliability

If a backup shutdown seal is added to prevent leakage under high temperature, then reliability is improved, but device complexity increases

Engineering Contradiction:

Improvebackup seal reliabilityVSAvoidseal assembly complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The shutdown seal exploits changes in physical parameters (temperature, phase state) to activate the sealing function. The meltable spacer transitions from solid to liquid at a predetermined temperature, automatically triggering the seal without requiring complex control logic or multiple components.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The shutdown seal mechanism utilizes phase transition of the meltable spacer material from solid to liquid state at a predetermined temperature threshold. This phase change automatically releases the constraint on the split ring, causing it to constrict and seal the shaft without requiring external power or control systems.

Inventive Principle:

Principle #36Phase transitions

3Reliability

If the split ring constricts to block leakage, then sealing performance is improved, but the spacer material must withstand high temperature without deforming prematurely

Engineering Contradiction:

Improveseal activation reliabilityVSAvoidspacer temperature resistance

Core Design Contradiction:

ReliabilityVSTemperature

Solution Approach 1:

Inventive Principle:

Principle #36Phase transitions

Solution Approach 2:

The shutdown seal employs composite construction combining the split ring (sealing element), meltable spacer (thermal actuator), and resilient seal ring (backup sealing element). This composite structure leverages the complementary properties of each material to achieve reliable thermal actuation and sealing under high-temperature conditions.

Inventive Principle:

Principle #40Composite materials

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The shutdown seal effectively restricts coolant leakage within 45 seconds of activation, providing a reliable backup to prevent coolant exposure and ensuring the reactor core's safety during loss of seal cooling events, meeting regulatory requirements for station blackout scenarios.

Implementation Method 1

The spacer is formed of a meltable material

Methodology Applied

Scientific EffectMelting: Melting

Implementation Method 2

a pliable polymer seal ring which is urged against the shaft

Methodology Applied

Scientific EffectElasticity: Elasticity

Data Source

PatentUS8356972B2Thermally activated shutdown seals for rotatable shafts

Publication Date: 2013.01.22 WESTINGHOUSE ELECTRIC CORP

AI summary

A thermally actuated shutdown seal for a rotating shaft having a narrow annular fluid flow path surrounding the shaft. The seal surrounds the shaft with the annulus therebetween during normal operation and constricts against the shaft when the shaft slows or stops rotating. The annulus is maintained open during normal operation by a spacer interposed between opposing ends of a split ring. When the shaft stops rotating, the temperature of the annulus rises, which actuates removal of the spacer from the split ring constricting the split ring against the shaft blocking the annulus. The blocked annulus causes a pressure differential across the seal which urges a polymer seal ring, downstream of the split ring against the shaft which seals the annulus. In one embodiment, the spacer is formed of a meltable material. In a second embodiment, the spacer is removed from the split ring by a thermally responsive actuator.