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**Keywords:** helium leak detector, leak detector, vacuum system, leak rate, hood method, blowing method
With the rapid advancement of science and technology, cryogenic liquids have become increasingly common in industrial applications. This has led to the widespread use of high-vacuum multi-layer insulation transport vehicles, where the quality of the vacuum in the sandwich tank is critical for performance. Leak detection plays a vital role in ensuring the integrity of these systems, as the leak rate directly affects the lifespan and efficiency of the tank. This paper explores the physical process of helium leak detection and introduces practical methods for detecting leaks in cryogenic liquid transport vehicles.
### 1. Physical Process of Helium Leak Detection
The helium mass spectrometer leak detection method is considered the most sensitive and non-destructive technique for identifying leaks. When using this method, the test specimen is either pressurized or evacuated, while the leak detector remains on the low-pressure side. Before any helium reaches the detector, part of it is removed by an auxiliary vacuum system, and the rest enters the detector. The system operates once it reaches normal conditions, with the background value of the leak detector assumed to be zero (or compensated).
Once helium is introduced into the system, the partial pressure of helium increases until it reaches a steady state—where the amount entering equals the amount being pumped out. If the helium supply is stopped, the pressure gradually decreases back to the initial level. This process helps identify the presence and location of leaks.
### 2. Helium Leak Detection Process for Cryogenic Liquid Transport Vehicles
A typical cryogenic liquid transport vehicle consists of an inner cylinder, outer cylinder, support structure, multi-layer insulation, adsorbent, and suction nozzle. The inner and outer cylinders are made of 0Cr18Ni9 stainless steel, and the insulation layer includes aluminum foil and glass fiber paper. The interlayer is evacuated using a suction nozzle to achieve vacuum insulation. Helium leak detection is used to check for leaks in both the inner cylinder and the interlayer.
#### 2.1 Preparation for Helium Leak Detection
Before conducting the leak detection, the vacuum system must be thoroughly cleaned to remove welding slag, grease, and other contaminants that could block leaks or damage the equipment. The system should also be pre-evacuated to improve detection accuracy and conserve helium gas.
##### 2.1.1 Cleaning the Vacuum System
The tank is cleaned and dried to prevent contamination. After cleaning, the vacuum system is pre-evacuated to reduce the risk of false readings. This step is crucial for ensuring accurate results during the actual leak detection process.
##### 2.1.2 Pre-Evacuation of the Vacuum System
The tank is placed in a heating furnace and subjected to multiple cycles of nitrogen flushing, heating, and evacuation. This process is repeated at least eight to ten times to ensure the system is free from residual gases. A cold trap is installed to maintain stable temperature conditions, and after heating, the cold trap is defrosted and dried before proceeding with the leak test.
#### 2.2 Leak Detection Methods
Two commonly used methods are the **hood method** and the **blowing method**. The hood method is used first to measure the total leak rate, and if it exceeds acceptable limits, the blowing method is employed to locate the exact leak point.
##### 2.2.1 Hood Method
In the hood method, a plastic cover is placed over the suspected area, and helium is introduced. If the leak detector detects a change in signal, it indicates a leak. The process involves covering welds with plastic and sealing them with tape, ensuring a tight seal. For full coverage, the entire tank is wrapped in a plastic film, and the lowest point is used for helium injection.
##### 2.2.2 Blowing Method
The blowing method involves directing a helium jet toward suspected leak points. It is particularly useful for locating small or hidden leaks. Key considerations include starting from the top, using different nozzle sizes for precision, and ensuring proper ventilation and environmental control during testing.
After identifying a leak, the affected area is repaired, and the process is repeated to confirm the fix. Standard leak devices are used to calibrate the detector and ensure accurate sensitivity measurements.
This detailed approach ensures that cryogenic liquid transport vehicles meet strict vacuum requirements, enhancing safety and reliability in industrial applications.Food and Beverage Service Equipment
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Technical application of leak detection in tank sandwich of cryogenic liquid transport vehicle