Understanding Leak Testing in NDT: A Comprehensive Guide

Table of Content

• What is Leak Testing?
• Leak Testing Methods
• How Does a Leak Detection System Work? 
• Leak Testing Considerations
• Leak Testing Equipment
• Leak Testing Standards and Codes
• Leak Testing Benefits
• Advances in Leak Testing
• Key Takeaways
• FAQs

Inspectors conduct leak testing to ascertain whether a system or object is operating within a predetermined leak limit. When an object has a flaw, such as a hole, crack, or other type of flaw, the liquid or gas it is holding can leak out. This is known as a leak. Pressure is used in leak testing to identify these flaws so that they can be fixed during routine maintenance procedures.

Leak tests are typically carried out on items that are used to hold or transport liquids or gases. One of the most commonly employed inspection techniques is leak testing which is an NDT method since inspectors can use it without causing long-term alterations to or harm to the object they're checking.

What is Leak Testing?

Leak testing is the process where inspectors apply pressure to an object to locate defects that are causing leaks. When anything leaks out of a container, it goes from a high-pressure area to a low-pressure area. Leak testing applies this phenomenon by closely observing the flow and utilising pressure to create flow towards lower pressure, or the location of leaks. 

Which fluid is used in leak tests?

In Leak Testing, various fluids are used for the leak test procedure. Common fluids include air, water, inert gases like nitrogen or helium, hydrogen, and tracer gases like helium or hydrogen. The choice depends on system type, required sensitivity, and safety considerations.

What circumstances call for leak testing?

Most often when testing closed systems for bugs. The success or failure of a leak test depends on the object being tested. Different types of materials and objects may respond differently to the high pressures that are typically employed in leak testing to force a liquid or gas out of a defect, revealing its presence and location.

Leak testing is the process where inspectors apply pressure to an object to locate defects that are causing leaks. When anything leaks out of a container, it goes from a high-pressure area to a low-pressure area. Leak testing makes use of this phenomenon by closely observing the flow and utilising pressure to create flow towards lower pressure, or the location of leaks. 

What circumstances call for leak testing? most often when testing closed systems for bugs. The success or failure of a leak test depends on the object being tested. Different types of materials and objects may respond differently to the high pressures that are typically employed in leak testing to force a liquid or gas out of a defect, revealing its presence and location.

Leak Testing Methods

Inspectors utilise various techniques to detect leaks in sealed systems, ensuring their integrity and safety. Some of the most popular leak test techniques are as follows:

1. Spark Coil Technique

The electromagnetic radiation that generates glow discharge in nearby evacuated ampoules is produced using the spark coil approach using a high voltage or Tesla coil and sparkling point. Typically, it is only feasible in non-metal containers, such as glass and plastic components or tubes.

We can observe plasma inside the tested element as we draw the leak antenna along it. As we approach the leak, a sharp arc passage between the plasma and antenna appears. The fault patch is distinct, and a knowledgeable individual can infer the interior pressure from the plasma's color. This straightforward method, however, has a few downsides since, in addition to its limited applicability, it must also be avoided due to radio disturbances.

2. Pressure Change Method

Pressure gauges, frequently used to track system performance, are employed in the pressure change approach. A solvent, such as acetone or something similar, can be squirted over suspected leak spots while you keep an eye on the gauge for a pressure spike that happens when the solvent enters the leak. This method has some drawbacks, including low sensitivity, the potential for solvent freezing to stuff the leak, and the potential for solvents to destroy vacuum grease and elastomer gaskets.

3. Overpressure Methods

The tested element must be filled with fluid or gas to perform overpressure procedures. Typically, water from domestic installations is used as a fluid. The wet portions of the exterior surface reveal significant leakage as well as smaller ones up to around I mbarl/s.

In a gas testing procedure, the vessel is immersed in water and over-pressured by a certain amount (depending on the material and thickness of the wall). Gas bubbles start to escape from leaks Leaks up to 10-3 mbarl/s can be found in this way. The suspected points should be sprayed with a soap solution if the vessel is too large for immersion. This time, we can observe the bubbles exiting if there is a leak. This technique can be used for very big systems and can detect leaks up to 10-5 mbarl/s.

4. Halogen Leak Detectors 

The system must be pressured with a gas containing an organic halide, such as one of the Freons when using halogen leak detectors in the detector-probe mode (to 10-3 mbarl/s). Next, a sniffer probe sensitive to traces of the halogen-containing gas scans the system's exterior.

The theory is based on the enhanced emission of positive ions (K or Na) brought on by the abrupt appearance of halide composition. The metric for leak size is the ion current. Halogen detectors can also be used in reverse mode, which involves connecting an evacuated vessel to the detector and spraying freon into it. In this way, its performance is used in rough, medium, and high vacuums and is up to 10-7 mbarl/s.

5. Dye Penetrant Method

The Dye Penetrant Method is a modification of a method used to detect weld flaws and metal fissures. It makes use of a fluid with low viscosity and rapid surface movement. When a leak is suspected, this fluid is painted on one side of the wall, and after some time, it is discovered on the opposite side. The test is straightforward, inexpensive, leaves records, and has a sensitivity of up to 10-6 mbarl/s.

There's More to Explore, Leak Testing Methods

How Does a Leak Detection System Work?

A leak detection system identifies leaks or abnormalities in a sealed system without causing damage to the object under inspection. Its working consists of many factors, which include:

1. Leak Test Pressure Limit:

The system subjects the sealed component or system to a predetermined pressure limit, often higher than its normal operating pressure.

2. Leakage Test Procedure:

• Initial Preparation: Ensure the object or system under test is properly cleaned and free from contaminants that could interfere with the leak testing process.
• Sealing: Seal off all openings or connections to create a closed system, preventing external factors from affecting the test results.
• Pressurisation: Apply pressure to the sealed system, gradually increasing it to the predetermined leak test pressure limit specified for the application.
• Monitoring: Continuously monitor the pressure within the system using pressure gauges or sensors to detect any deviations that could indicate a leak.
• Detection: Employ Leak Detection Methods such as pressure decay, mass spectrometry, ultrasonic testing, or bubble testing to identify any leaks within the system.
• Analysis: Analyse the results to pinpoint the location and severity of any detected leaks, determining whether they exceed acceptable leak rates.
• Documentation: Document all test procedures, results, and corrective actions taken for future reference and compliance with regulatory standards.

3. Integration Solutions:

• Automated Systems: Implement automated leak testing systems equipped with sensors, actuators, and data acquisition systems to streamline the testing process and improve accuracy.
• Data Integration: Integrate leak testing systems with existing quality control or production monitoring systems to enable real-time data analysis and proactive maintenance.
• Remote Monitoring: Utilise remote monitoring capabilities to monitor leak testing processes from a central control room or via mobile devices, enabling timely intervention in case of abnormalities.
• Interdisciplinary Collaboration: Foster collaboration between engineering, quality assurance, and maintenance teams to ensure effective integration of leak testing solutions into overall production processes.
• Training and Education: Provide comprehensive training and education for personnel involved in leak testing procedures to ensure proper execution and interpretation of results.

By following a systematic leakage test procedure and implementing effective integration solutions, organisations can enhance the reliability, efficiency, and safety of their products and processes while adhering to regulatory standards and quality requirements.

Leak Testing Considerations

Leakage inspections have some special considerations as an NDT approach because they require applying pressure to an object to detect leaks.

Below is a summary of important considerations.

1. Acceptable Leak Rate:

Inspectors and maintenance staff must be aware of the permissible leak rate for a given item or system when doing leak testing. Not every leak needs fixing; others may only need more observation or no action. Regulations outline permissible leak rates for certain goods and toxins in various businesses.

2. Manufacturing Considerations:

Before performing a leak test, it’s important to consider the function for which a system, part, or object was originally made. The target use case for a given object may require the manufacturer to have created it in such a way that it will either retain or allow liquids to pass through it. For example, a car part may be designed specifically so that gases can’t escape from it, or an IV may be designed to keep liquids inside it.

3. Medium Considerations:

When organising a leak test, the substance that the object is intended to hold must be considered. The sizes of molecules vary among various substances. Knowing the difference between a flaw that would be considered acceptable and one that would be large enough to let a particular liquid or gas escape is crucial when conducting a leak test.

Pressure is a relevant factor since various compounds will react differently to various pressure ranges. While a pressure range that is too low could produce ambiguous findings, a pressure range that is too high could potentially harm the object being tested.

Leak Testing Equipment

Equipment types frequently used for various leak testing techniques include the following:

1. Air Leakage Inspection Devices

Inspectors can view data from ongoing leak tests on displays attached to air leak testing equipment. These tools can be applied to numerous leak testing techniques, including vacuum decay, pressure decay, burst, chamber, and others.

2. Compact Pressure Decay Leak Tester

Inspectors can limit the amount of connection volume required for the test by placing this small leak tester next to the fixtures used. Due to the volume reduction, the leak test's duration can be shortened, and its sensitivity raised.

3. Large Display Leak Tester

Larger display leak testers, like this one from Zaxis (called the 7i), have larger screens, more internal storage, larger test volumes, and faster testing.

Read More About Leak Testing Considerations and Equipments

Leak Testing Standards and Codes

Inspectors use leak standards to contrast various leak systems or to establish the parameters of their leak test by simulating a leak in the component under test. Leak testing is frequently employed for code-based inspections, and most nations that utilise this method will have a relevant standard or operating code set.

Here are a few leak testing codes that are more frequently used:

1. ASME Section V Article 10 Leak Testing (Pneumatic and Hydrostatic)

In the ASME Boiler and Pressure Vessel Code, Section V, Article 10 provides detailed requirements and guidelines for conducting both pneumatic and hydrostatic leak tests on pressure vessels, piping, and other components.

2. API 570 Inspection, Repair, Alteration, and Rerating of In-Service Piping Systems

The American Petroleum Institute (API) standard API 570 includes requirements for leak testing of piping systems in the petrochemical and refining industries. It outlines procedures for conducting tests and evaluating the results.

3. ISO 9712 Non-destructive Testing – Qualification and Certification of NDT Personnel

ISO 9712 is an international standard that includes provisions for the certification of personnel involved in Non-destructive Testing, including leak testing. It outlines the qualifications and requirements for individuals performing leak tests.

4. MIL-STD-750E Test Method Standard for Semiconductor Devices – Test Method 1071.1 Leak Detection

MIL-STD-750E is a military standard that specifies test methods for semiconductor devices. Test Method 1071.1 covers leak detection, outlining procedures and criteria for leak testing in semiconductor manufacturing.

Leak Testing Benefits

Leak testing offers numerous benefits from a non-destructive testing perspective. They include:

Enhanced Product Reliability:

Leak testing ensures the integrity of components and systems, reducing the risk of failure or malfunction.

Improved Safety:

By detecting leaks early, leak testing helps prevent hazardous situations and potential harm to personnel or the environment.

Compliance with Regulations:

Meeting leak test pressure limits and adhering to industry standards ensures compliance with regulatory requirements.

Cost Savings:

Early detection of leaks minimises costly repairs, product recalls, and potential liabilities associated with faulty products.

Quality Assurance:

Leak testing identifies defects and discrepancies, maintaining product quality and customer satisfaction.

Process Optimisation:

Integration of leak testing equipment enhances production efficiency and streamlines quality control processes.

Environmental Protection:

Preventing leaks minimises the release of harmful substances into the environment, supporting sustainability initiatives.

Data-driven Decision Making:

Leak testing equipment provides valuable data for analysis and continuous improvement efforts.

Risk Mitigation:

Identifying and addressing leaks proactively mitigates operational risks and ensures operational continuity.

Leak testing considerations and equipment play a crucial role in achieving these benefits by enabling comprehensive testing procedures and ensuring the reliability and safety of products and systems. Advanced pipeline leak detection methods further enhance the effectiveness of leak testing, offering innovative solutions for detecting and mitigating leaks in complex infrastructure networks.

Advances in Leak Testing

Advances in Leak Testing have revolutionised detecting and mitigating leaks, enhancing safety and reliability across industries. These, coupled with Leak Testing considerations, including acceptable leak rates, manufacturing considerations, and medium considerations, have refined testing methodologies to suit specific objects and intended uses. Understanding the properties of the substance being tested and selecting appropriate pressure levels are crucial factors in Leak Testing procedures. 

This ensures the accurate detection of leaks while minimising false positives or negatives. Various types of Leak Testing equipment, such as air leakage inspection devices, Compact Pressure Decay Leak Testers, and large display leak testers, provide inspectors with efficient and accurate testing capabilities for different applications.

Advancements in leak testing include:

- Advanced Sensor Technology and Improved Leak Detection Algorithms: Advanced sensor technology and algorithms increase sensitivity and accuracy in leak detection, enabling the identification of smaller leaks.

- Integration with Digital Platforms for Real-time Monitoring and Data Management: Integration with digital platforms allows for real-time monitoring and data management, facilitating instant feedback and analysis of leak testing processes.

- Remote Control Capabilities and Portability for On-site Testing and Monitoring: Leak testing equipment with remote control capabilities and portability enables on-site testing and monitoring, enhancing flexibility and efficiency.

- Enhanced User Interfaces and Automated Reporting Features for Improved Usability: User-friendly interfaces and automated reporting features streamline operation and documentation, reducing training time and potential errors.

- Multi-channel Testing Systems for Simultaneous Testing and Increased Throughput: Multi-channel testing systems enable simultaneous testing of multiple components or systems, increasing testing throughput and productivity.

- Calibration Enhancements for Accurate Leak Detection: Calibration enhancements ensure accurate leak detection by properly tuning leak detectors to helium and correctly reporting leaks.

- Gas Tracing Probes for Pinpointing Leaks in Pipes: Gas tracing probes are highly accurate for pinpointing leaks by pumping gas into the pipework and measuring where the gas escapes.

- Pressure Testing Methods for Accurate Leak Detection in Plumbing Systems: Pressure testing methods precisely measure pressure levels to detect leaks in plumbing systems, with fluctuating pressure indicating the presence of a leak.

- Acoustic Listening Devices for Effective Leak Detection: Acoustic listening devices differentiate between regular water flow and breaks, effectively detecting leaks.

- Enhanced Durability and Reliability of Leak Testing Equipment: Improved materials and construction techniques result in leak testing equipment that is more durable and reliable, reducing maintenance requirements and downtime.

Leak Testing contributes to quality control and maintenance procedures, ensuring compliance with regulatory standards and industry requirements. With technological evolution, Leak Testing will continue to play a pivotal role in ensuring the integrity of critical infrastructure and products.

Key Takeaways

• Inspectors use leak testing to determine if a system or object operates within a specified leak limit.
• Leaks occur when objects have flaws like holes, cracks, or other defects that allow liquids or gases to escape.
• Involves monitoring pressure gauges while introducing a solvent to spot pressure spikes caused by leaks. It fills the tested element with fluid or gas, allowing gas bubbles to indicate leaks.
• It's essential to know the permissible leak rate for an item or system, as not all leaks require immediate action.
• Consider the object's original design and intended use, as it may need to retain or allow specific substances to pass. Different substances have varying molecule sizes, affecting what constitutes an acceptable flaw.
• Common leak testing codes include ASME Section V Article 10, API 570, ISO 9712, and MIL-STD-750E.

FAQs

1. What is a leak and why is it used? 

A: Leak is the unintended passage of a substance (such as gas or liquid) from a contained space to the external environment. It is used as a term to describe a flaw or failure in a sealed system or component. Leak testing is used to identify and quantify these leaks to ensure the integrity and safety of the system or product.

2. What is the difference between a Hydrotest and a leak test? 

A: Hydrostatic testing (Hydrotest) and leak testing are both methods used to assess the integrity of pressurised systems, but they differ in their approach and purpose.

i. Hydrostatic Testing (Hydrotest):

• Method: A hydro test involves pressurising a system or component with a liquid, typically water, to a predetermined pressure level.
• Purpose: It is primarily used to check for structural integrity and evaluate the strength of the component or system. It ensures that the item can withstand the expected operating pressure without permanent deformation.

ii. Leak Testing:

• Method: Leak testing involves pressurising a system or component with gas or liquid and monitoring for the presence of leaks.
• Purpose: The primary purpose of leak testing is to identify and quantify leaks or unintended fluid or gas escapes in a system or component. It ensures that the system or product is free from leaks that could compromise safety, quality, or performance.

References:

1. FasTestInc

2. Flyability

3. TQC

4. BOC Ireland