Pressure Vessel Leak Tightness Testing

Leak tightness testing is an important method for evaluating the sealing performance of pressure vessels. By taking advantage of the higher permeability of gases compared to liquids, this test can effectively detect unacceptable leaks in the vessel's weld joints and sealing structures, ensuring the safety and reliability of the pressure vessel during use. Leak tightness testing is typically used to verify whether a pressure vessel can meet specific gas leakage requirements, especially for pressure vessels that pose potential threats to the environment or personnel safety.

The Necessity of Leak Tightness Testing

Not all pressure vessels require leak tightness testing. Most pressure vessels do not have strict leakage requirements, and pressure tests generally include some leakage detection capabilities. However, leak tightness testing is essential in the following situations.

1. Pressure Vessels Containing Highly Toxic Media

If the pressure vessel contains highly toxic or hazardous substances, even a small leak could endanger lives or cause severe environmental pollution. For such vessels, any minor leak must be strictly prevented.

2. Pressure Vessels Designed to Have No Leaks

Some pressure vessels are designed with the requirement that leakage is absolutely unacceptable, often because leaks could compromise the safety, proper operation, or result in economic loss due to the loss of valuable media. For example, pressure vessels containing high-pressure, high-temperature, or high-purity media often require extremely high sealing standards.

3. Pressure Vessels Containing Flammable or Liquefied Gases

For vessels storing flammable compressed gases or liquefied gases, leaks pose significant safety hazards. Leak tightness testing is crucial to prevent gas leaks.

4. Pressure Vessels with Strict Vacuum Requirements

Some equipment requires very high vacuum conditions, such as those used in scientific research or specialized industrial processes. Leak tightness testing is necessary to ensure that the vessel maintains the required vacuum state during use.

5. Spherical Tanks and Heat Exchangers

Equipment like spherical tanks and shell-and-tube heat exchangers often need leak tightness testing due to their special structure and the nature of the media they handle, particularly the connection points between tubes and tube sheets in heat exchangers, to prevent leakage that could cause harm.

Media and Operational Requirements for Leak Tightness Testing

Common media used for leak tightness testing include air, ammonia gas, and ammonia-air mixtures. The choice of testing medium affects the testing process and leak detection methods. For example, ammonia gas, being more permeable, typically requires higher testing standards and more rigorous procedures. Regardless of the medium chosen, leak tightness testing must be performed after the pressure test, as this ensures that the pressure vessel's overall strength has been verified and the safety of the leak tightness test is assured.

Design and Implementation of Leak Tightness Testing

The implementation of leak tightness testing should follow the specific requirements outlined in the design documents. Designers must consider the pressure vessel's operating environment, the characteristics of the media it will contain, and the vessel's structural features to determine whether leak tightness testing is necessary and to define the test's specific requirements. Key design considerations include:

Defining the Test Medium: The design document should specify the type of medium used for the leak tightness test, such as air or ammonia gas.
Test Parameters: The design document should specify parameters for the test, including test pressure, test temperature, and test duration.
Leakage Standards and Detection Methods: Detailed leak detection methods and acceptance criteria should be defined. For example, for large gas storage tanks, a simple pressure gauge reading that shows a drop in pressure over time may not adequately assess the pressure vessel's sealing performance. More precise standards should be established to ensure the vessel’s actual sealing ability.
Leakage Tolerance: For some special pressure vessels, designers need to assess the acceptable leakage limit. This assessment should consider not only the properties of the media within the pressure vessel but also environmental factors such as population density and potential for environmental pollution.

Relationship Between Pressure Testing and Leak Tightness Testing

While pressure testing and leak tightness testing are related, their purposes and requirements differ. Pressure testing primarily verifies the pressure vessel's ability to withstand internal pressure, ensuring that the pressure vessel will not rupture or deform under external pressure or temperature changes. In contrast, leak tightness testing checks the vessel's sealing performance to ensure there are no leaks.
In principle, pressure vessels that pass a pressure test do not need to undergo a separate leak tightness test, especially if the pressure test indicates no leaks. However, for components that did not undergo pressure testing (such as safety attachments), a leak tightness test may still be required. Additionally, given the differences in the permeability of various testing media, leak tightness testing should not be omitted. For instance, the absence of leaks in an air pressure test does not guarantee the same result when using ammonia or other more permeable media. Therefore, extra caution is necessary when using highly permeable testing media.

Conclusion

Leak tightness testing is a critical part of pressure vessel inspection, particularly for pressure vessels containing hazardous, toxic, or flammable media. Designers must carefully consider the vessel's operating conditions, the properties of the media, and environmental impacts when deciding whether leak tightness testing is necessary and ensuring that the testing process is conducted in compliance with the required standards. By performing rigorous leak tightness tests, the safety and reliability of pressure vessels can be ensured, effectively preventing potential safety hazards and environmental risks during use.