Working at high altitude, the low pressure, thin air, and large temperature difference add greater requirements for the airtightness of key components of power equipment. As a key flow component, the sealing performance of the impeller during application at high altitude is related to the overall efficiency, working safety, and service life of the device. In order to ensure their stable long-term operation in extreme environments, it is required to establish a scientific, rational, replicable, and mass-manufacturing-eligible airtightness test process standard.
Sealing Challenges for Impellers in High-Altitude Application Environments
On high-altitude points, especially at plateaus above 4,000 meters, air pressure is significantly lower than under normal altitudes, air is rarefied, and the sealing work environment of the seal is more complex. In such an environment, impeller airtightness comes under a number of challenging conditions. Firstly, the ambient air pressure is low, which significantly reduces the sealing effectiveness of impeller clearances. In low pressure gradients, it becomes increasingly difficult for the seals to achieve an optimum closed state, with greater possibilities of leakage. Secondly, there are gigantic temperature fluctuations in air, with large day-night temperature differences. These thermal fluctuations affect the stability of the sealing structure of the impeller, notably the effect of thermal expansion on fit tolerances, further exacerbating sealing issues.
Moreover, high operation frequency and large starting stress—due to specific work conditions in mountain areas, equipment is often started and halted, subjecting impellers to significant starting pressure and significantly lower fatigue life of seals. Working medium complexity is also an enormous challenge. In some special environments, e.g., gas compression or UAV power systems, the working medium contains impurities, i.e., low temperature, high humidity, or dust, which places additional requirements on the adaptability of the sealing material. All these reasons made higher standard requirements of impeller tightness in high-altitude areas.
Principles and Method Comparison of Airtightness Testing
The primary objective of airtightness testing of an impeller is to scientifically determine the presence and severity of leakage. In actual applications, common techniques of airtightness testing include differential pressure technique, bubble technique, helium leak detection technique, mass flowmeter technique, and vacuum decay technique, depending on different requirements and requirements of test accuracy.
Differential Pressure Method
The differential pressure technique is the most basic and most widely used leakage detection technique. Its fundamental concept is to produce pressure difference on the inside and outside surfaces of the impeller and monitor the change of pressure with time within a sealed environment in order to determine whether there is leakage. Due to its simple structure and minimal equipment investment, this method is mainly utilized in factory testing of products under normal pressure conditions. But for low-pressure conditions such as high altitude, it is difficult to create a pressure difference, and variation range is limited, so it is (highly susceptible) to system stability and temperature drift, thus (difficult to ensure) accuracy. So the single differential pressure method is inapplicable for testing at plateaus.
Bubble Method
The bubble method is a more mature visual inspection method. After the generation of a pressure difference, the test impeller is then immersed in a liquid and bubble appearance is observed to inspect leakage location and extent. This method has easy operation and little equipment and is normally employed for rapid judgment of positions of leakage in small batches or in the R&D stage. But since it relies on visual observation for leakage diagnosis, it is unable to conduct quantitative measurement, is ineffective for small leaks, and has very subjective detection results with poor repeatability. Therefore, it can only be used for experimental verification or emergency diagnosis but not for mass production lines or aerospace-level impellers with high precision requirements.
Helium Leak Detection Method
Helium leak detection is one of the most sensitive industrial-level leakage detection methods. Its basis is to fill the inside of the impeller with high-purity helium (as tracer gas) and utilize a mass spectrometer to capture escaping helium in vacuum or positive pressure space, allowing very small leakage paths to be detected highly sensitively. Helium atoms have low molecular weight, high inertness, and low adsorption, therefore are suitable for detecting leakage paths such as micro-cracks and material micro-holes, with a minimum detectable leakage rate of 10⁻⁷ Pa·m³/s. Despite having high equipment investment and environment parameter strict control during the test, its sensitivity and reliability are very high and is the preferred choice in the aerospace industry for detection technology.
Mass Flowmeter Method
Mass flowmeter method is an automatic measurement method obtained through highly accurate flow sensors. Mass flowmeter technique quantitatively indicates the leakage rate by measuring the change in mass flow due to gas leakage under pressurized or vacuum status, and usually represents the unit as Pa·m³/s or SCCM (standard cubic centimeters per minute). This technique has good data traceability and repeatability, and it is specially well-fitted for use in automated production lines, with practical benefits of quick detection of massive batches of impellers. Nevertheless, it is notable that its detection precision is greatly influenced by environmental conditions such as humidity and temperature, thus the testing conditions should be controlled stably.
Vacuum Decay Method
Vacuum decay method initially establishes a vacuum condition within the detection chamber and subsequently measures the rate of pressure recovery over a stipulated period to (deduce) the leak rate. It does not require a tracer gas and is most suitable for comparatively sealed parts or cavity-shaped structures, with moderate precision. But since impellers mostly have through-hole structures or external open conditions, the vacuum chamber would not be able to seal and enclose the entire impeller body completely, and thus this method has very limited use in testing impellers and is utilized more for overall airtightness testing of impeller parts or closed systems.
To integrate the detection needs of high-altitude impellers, a combination of the helium leak detection technique and the differential pressure method + mass flowmeter technique is a preferable choice. The extremely high sensitivity of helium can successfully detect small leaks, and a mixture of the differential pressure method with the mass flowmeter method maintains an optimal balance of efficiency and reliability.
Process Design for Airtightness Testing of High-Altitude Impellers
At high altitudes, low pressure, high temperature gradients, and light air impose higher demands on impeller sealing performance. For applications in aircraft engines, plateau power machinery, and special compressors, impeller leakage of only a few percent leads to system efficiency loss or even to functional failure. Therefore, for ensuring the reliable operation of products under plateau conditions, it is necessary to establish an exact and reproducible airtightness test process. In this section, the process is reasonably swept and optimized to ensure that the detection results are real, credible, and engineering-achievable.
Test Preparation Stage: Sealing Design and Environmental Stability Control
The foundation of airtightness testing is ensuring the sealing integrity of the test system itself. Therefore, at the test preparation stage, intentional tooling design needs to be carried out first. High-freshness fluororubber rings or multi-layer metallic sealing sheets need to be applied to cater to the complex impeller shape and to avoid any leak path. In addition, the surface of the impeller must be properly cleaned to remove oil stains, particulates, and process residues that prevent impurities from becoming trapped in the sealing surface and causing false leaks. To eliminate the influence of thermal stress in the environment, the impeller should also be warmed up following the ambient temperature of the test chamber, kept at ±1℃, so the thermal expansion would not interfere with test data and promote stability and reproducibility.
Establishment of Test Environment: Simulating Test Conditions for Plateau Low-Pressure Working Conditions
To effectively simulate high-altitude operating conditions, the pressure difference environment at 5,000 meters or higher above needs to be replicated in a laboratory. Common methods include constructing a vacuum chamber or low-pressure chamber and controlling the outer pressure of the chamber with a vacuum pump or compressor system. According to this, the interior of the workpiece is driven into a positive pressure (0.3–0.5 MPa) or negative pressure (-80 kPa) state in compliance with test requirements, forming a typical pressure difference field. The entire system must be capable of operating stably for 5–10 minutes in order to be able to supply an effective data base for subsequent leakage monitoring to avoid misjudgments or false alarms caused by pressure difference oscillations.
Implementation of Detection Link: Static and Dynamic Combined Leakage Assessment
During the detection stage, a combination of various methods is recommended to ensure a comprehensive leakage evaluation. First, through static pressure difference maintaining testing, inspect the tendency of pressure fluctuation in 5–10 minutes to initially determine if there is leakage. Second, add a mass flowmeter for dynamic detection of leakage, which detects the volume variation of leaked gas per unit of time, preferably expressed in units of Pa・m³/s or standard cubic centimeters per minute (SCCM), and is capable of quantitatively detecting the rate of leakage. For critical parts or ultra-high-precision applications, helium tracer detection technology can be further applied, i.e., injecting high-purity helium into the impeller and analyzing the external (diffused) helium concentration with a mass spectrometer. It enjoys the advantages of high sensitivity (can detect leaks of less than 10⁻⁷ Pa・m³/s) and is widely applied in airtight system detection in aerospace and aviation.
Data Judgment and Record Management: Ensuring Quality Traceability
After the detection is completed, a qualified judgment level should be set according to normal, for example, controlling the leakage rate at not more than 5×10⁻⁴ Pa·m³/s as the basis for testing. All the test data should be recorded in time, forming a complete detection file, e.g., test number, impeller model, equipment number, operator, test time, environmental parameters, and final leakage rate, etc. This not only enables follow-up technical adjustments to be traced back to product quality and responsibility defined but also provides effective data support for subsequent technical improvements. With an information-based batch workpiece quality trend analysis and process optimization closed-loop control, batch data collection is also possible, thus further improving the overall reliability of plateau equipment.
Core Control Parameters for Process Standard Formulation
In order to ensure the accuracy and consistency of detection results, strict process standards should be developed. The following are the suggested core control parameters:
| Item | Recommended Standard Parameter Range | Description |
| Detection Environment Temperature | 20℃ ± 2℃ | Ensure temperature stability and avoid measurement interference from temperature differences. |
| Test Gas | Dry air or high-purity helium | Helium purity ≥ 99.999%. |
| Pressure Difference Establishment Time | ≤ 30 seconds | Rapidly establish the pressure difference to avoid interfering with detection results. |
| Pressure Holding Time | ≥ 5 minutes | Ensure stable pressure difference for judging leakage conditions. |
| Minimum Detectable Leakage Rate | ≤ 1×10⁻⁴ Pa·m³/s | Meet high-standard detection requirements. |
| Allowable Range of Judgment Value Fluctuation | ≤ ±5% | Control equipment errors and manual intervention impacts. |
Conclusion
It is crucial to ensure long-term stable operation of equipment in severe environments to set standards for high-altitude impeller airtightness testing processes. Through the use of a combination of helium leak detection technology and mass flow control technology, detection reliability and consistency can be improved, risk of equipment leakage can be prevented, and equipment operational safety in high-altitude areas can be effectively ensured.
