Published on 10-Apr-2024

How is Leak Testing Done in the NDT industry?

How is Leak Testing Done in the NDT industry?

Table of Content

  • What is Leak Testing?
  • Leak Testing Methods
  • Leak Testing Considerations
  • Leak Testing Equipment
  • Leak Testing Standards and Codes
  • Conclusion
  • 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.

It qualifies as a Non-destructive Testing (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 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

Some of the most popular leak test techniques are listed below.

1. Spark Coil Technique

The electromagnetic radiation that generates glow discharge in nearby evacuated ampoules is produced using the spark coil approach by 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 quite distinct, and a knowledgeable individual can infer the interior pressure from the plasma's colour.

This straightforward method, however, has a number of downsides since, in addition to its limited applicability, it must also be avoided due to radio disturbances.

2. Pressure Change Method

Pressure gauges, which are 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 temporarily stuff a 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 overpressured 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 - A Non-destructive Testing Technique

Leak Testing Considerations

Leakage inspections have some special considerations as an NDT approach because they necessitate the application of pressure to an object in order to detect leaks.

Below is a summary of important considerations.

1. Acceptable Leak Rate

When doing leak testing, it's critical for inspectors and maintenance staff to be aware of the permissible leak rate for a given item or system.

Not every leak needs to be fixed; others may only need more observation or possibly no action at all.

There are usually regulations outlining 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 taken into account. 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

Here are some examples of the equipment types that are frequently used for various leak testing techniques.

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 being used for the test.

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.

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.

For code-based inspections, leak testing is frequently employed, and most nations that employ this type of testing for inspections will have a standard for leak testing.

Here are a few of the 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.

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Leak testing considerations, including acceptable leak rates, manufacturing considerations, and medium considerations, help tailor the testing approach to the specific object and its intended use.

The choice of pressure levels and understanding the properties of the substance being tested are crucial factors.

Various types of leak testing equipment, such as air leakage inspection devices, compact pressure decay leak testers, and large display leak testers, enable inspectors to conduct tests efficiently and accurately, depending on the application.

In essence, leak testing is a vital non-destructive testing method that ensures the safety and reliability of systems and objects that handle liquids and gases, contributing to overall quality control and maintenance procedures in various industries.

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.

It is commonly used for items that handle or transport liquids or gases and is a Non-destructive Testing (NDT) method.

Leaks occur as substances flow from areas of high pressure to low pressure.

Pressure is used to create flow towards areas of lower pressure, revealing the location of leaks.

Leak testing is typically used for closed systems to check for defects.

Involves monitoring pressure gauges while introducing a solvent to spot pressure spikes caused by leaks.

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.

Pressure is a critical factor, as various compounds react differently to different pressure ranges.

Reduces connection volume and enhances sensitivity for faster testing.

Provides larger screens, more storage, and faster testing capabilities.

Leak standards are used to compare systems and establish test parameters.

Common leak testing codes include ASME Section V Article 10, API 570, ISO 9712 , and MIL-STD-750E.


Q: What is leak testing in NDT?

A: Leak testing in Non-destructive Testing (NDT) is a method used to detect and evaluate the presence of leaks or unintended fluid or gas escapes in a sealed or pressurised system.

It is a non-destructive way of assessing the integrity of a component or system to ensure it doesn't allow substances to leak, which could be hazardous or result in operational issues.

Q: What is an example of a leak test?

A: An example of a leak test is the use of a pressure decay test. In this method, a component or system is pressurised to a specified level, and the pressure is monitored over a set period.

If the pressure drops, it indicates the presence of a leak. Another example is the bubble test, where a liquid or gas is applied to the surface of a component, and the formation of bubbles indicates a leak.

Q: Why do we need leak testing? 

A: Leak testing is essential for several reasons:

Safety: It ensures that pressurised systems do not leak potentially dangerous substances, preventing accidents or environmental harm.

Quality Control: It verifies the quality and reliability of components or systems, reducing the risk of product failures.

Compliance: Many industries have strict regulations and standards that require leak testing to meet safety and environmental requirements.

Cost Reduction: Early detection of leaks can prevent costly repairs or product recalls.

Q: What is a leak and why is it used? 

A: 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.

Q: 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.

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.

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.

Hydrostatic testing focuses on structural integrity, while leak testing focuses on detecting and quantifying leaks in a system or component.

Both tests are crucial for ensuring the reliability and safety of pressurised systems in various industries.


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