Classification and Characteristics of Leak Detection Technologies

Release Date:

2022/03/24

During the manufacturing or operation of process equipment, it is not only necessary to determine whether leaks are present, but also to quantify the leak rate. In leak detection technology, the concept of “leak” is inherently linked to the notion of the maximum allowable leak rate. A leak is an absolute condition, whereas “no leak” is a relative one. For vacuum systems, as long as the pressure within the system can be maintained below the specified vacuum level over a given time interval, the system can be considered leak-free even if leak holes do exist. For pressurized systems, as long as the pressure drop remains within the permissible limit and does not adversely affect the system…

During manufacturing or operation, process equipment not only needs to be monitored for leaks but also for the magnitude of the leak rate.

In leak detection technology, the concept of “leak” is closely linked to the concept of the maximum allowable leakage rate.

Leakage is absolute, whereas the absence of leakage is relative. For vacuum systems, as long as the pressure within the system can be maintained below the specified allowable vacuum level over a given time interval, the system can be considered leak-free even if leak holes are present.

For pressure systems, as long as the pressure drop remains below the allowable limit and does not impair normal operation, the system can likewise be considered leak-free.

For sealed systems handling toxic, flammable and explosive, environmentally harmful, or valuable media, the system’s leakage rate must be kept below the maximum allowable leakage rate specified by environmental protection, safety, and economic considerations.

Leak detection involves introducing a tracer substance into one side of the equipment or sealing assembly being tested, and then using instruments or a specific method to detect the tracer substance that has leaked through any suspected leaks on the opposite side, thereby achieving the purpose of leak detection.

The task of leak detection is to determine, during manufacturing, installation, and commissioning, whether leaks are present and to quantify the leak rate, as well as to locate the exact positions of any leak holes; and, during operation, to monitor for potential leaks and their evolution.

I. Selection of Leak Detection Methods

There are numerous leak detection methods, each with its own characteristics; therefore, prior to conducting a leak test, it is essential to select the most appropriate method based on the specific leak detection requirements and the testing environment.

The selection of a leak detection method should take the following factors into consideration:

(1) Leak Detection Principle

Regardless of the leak detection method employed, it is essential to understand its fundamental principles. Leak detection encompasses a broad range of techniques and embodies various metrological and testing methodologies. Achieving a thorough understanding of the underlying principles of many such methods, however, is no easy task.

(2) Sensitivity

The sensitivity of a leak detection method can be expressed as the minimum leak rate that the method is capable of detecting.

When selecting a leak detection method, the sensitivity of each method should be considered, i.e., which level of leakage can be detected by each method.

For example, when detecting a leak rate of 10⁻⁵ cm³/s, employing a method with a sensitivity of 10⁻² cm³/s is entirely pointless.

Conversely, when detecting leaks at a rate of 10⁻² cm³/s, although a method with a sensitivity of 10⁻⁵ cm³/s may be theoretically feasible, it is likely to be economically impractical in practice.

(3) Response Time

Regardless of the method used, detecting a leak rate always requires a certain amount of time. The length of the response time can affect the accuracy and sensitivity of the leak detection. Generally, extending the detection time improves sensitivity; however, if the detection time is too long, changes in environmental conditions may reduce detection accuracy. The response time comprises the instrument’s own response time, the lag in gas flow, and the time required for various preparatory steps. This factor must be taken into account when selecting a leak detection method.

(4) Identification of Leak Points

Some leak-detection methods can only determine whether a system has a leak, but cannot pinpoint the location of the leak; others can not only locate the leak but also quantify its rate. If the sole objective is to verify whether the equipment meets specifications, the former approach is sufficient. However, when performing maintenance or identifying the root cause of a leak, the latter approach must be used—and even with this method, there is still a risk of missed leaks.

(5) Consistency

For some leak-detection methods, the results are essentially the same regardless of the operator’s level of expertise; for others, however, the outcomes differ markedly depending on whether the operator is a specialist or a novice. Whenever feasible, methods that can be performed correctly without requiring specialized technical skills should be used. Each method has its own unique technical critical points, and different leak detectors may not arrive at consistent detection results.

(6) Stability

Leak detection is an integrated technique that combines measurement and testing. If the data obtained from the test are unstable, the results are meaningless. Proper leak detection requires not only stable instrumentation but also a testing method that exhibits robust stability.

(7) Reliability

The absence of detected leaks does not necessarily mean that no leaks are present; this must be carefully assessed. When employing a particular leak-detection method, it is essential to determine its reliability. The reliability of the leak-detection results depends on various factors, including the consistency and stability of the method as described above.

(8) Economic Efficiency

Economic efficiency is one of the key factors in selecting a leak detection method. While it is relatively straightforward to evaluate the cost-effectiveness of a leak detection method on its own, comprehensively assessing its economic viability—taking into account the required leak detection equipment, the technical skills required of personnel, and the reliability of the detection results—is considerably more challenging.

For example, soap-solution leak detection is a very cost-effective method; however, it cannot detect small leaks and therefore is not suitable for applications with stringent leakage requirements.

When using an expensive helium mass spectrometer leak detector, multiple small leaks can be quickly identified.

It is difficult to make a blanket statement as to which of the two methods is more economical and which is less so. Therefore, when selecting a leak detection method, in addition to considering its cost-effectiveness, it is essential to conduct a comprehensive evaluation of factors such as sensitivity, response time, and detection requirements, so that the chosen method both meets the leak detection requirements and is economically sound and reasonable.

II. Classification of Leak Detection Methods

There are numerous leak detection methods and instruments; based on the equipment used, they can be categorized into helium mass spectrometry leak detection, halogen leak detection, vacuum gauge leak detection, and others.

Based on the detectable leak size, leak detection methods can be further classified into quantitative leak detection methods and qualitative leak detection methods.

Based on the condition of the equipment being tested, leak detection methods can be further classified into pressure leak testing and vacuum leak testing.

Below is a brief explanation based on the latter classification method.

1. Pressure Leak Detection Method

The equipment or sealing assembly under test is pressurized with a tracer substance. If leaks are present in the equipment or sealing assembly, the tracer substance will escape through these leak paths. By employing appropriate methods or instruments to detect the tracer substance outside the equipment, one can confirm the presence of leaks, pinpoint their locations, and quantify the leakage rate.

Pressure leak detection methods include water-pressure testing, pressure-drop testing, acoustic leak detection, ultrasonic leak detection, bubble testing, pressurization of collection cavities, ammonia-based leak detection, halogen-based leak detection, radioactive-isotope tracing, and helium-mass-spectrometer sniffing.

2. Vacuum Leak Detection Method

Both the equipment under test and the sensitive elements of the leak detection instrument are maintained in a vacuum, while the tracer gas is introduced externally to the equipment. If the equipment has a leak, the tracer gas will pass through the leak and enter both the interior of the equipment and the space surrounding the sensitive elements of the leak detection instrument, where it is detected by these elements, thereby enabling determination of the presence of the leak, its precise location, and the magnitude of the leak rate.

Vacuum leak detection methods include the static pressure-rise method, liquid coating method, discharge-tube method, high-frequency spark leak detection method, vacuum-gauge leak detection method, halogen leak detection method, and helium mass-spectrometry leak detection method, among others.

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