Cleanroom HEPA filter leak detection PAO leak detection process

Maintaining the cleanliness of the clean room and properly maintaining the high-efficiency filter are important issues. 

Let us first understand what the high-efficiency filter is: the high-efficiency filter is mainly used to collect dust and various suspended solids below 0.3um. Use ultra-fine glass fiber paper as the filter material, use offset paper, aluminum film and other materials as the diverter plate, and use the high-efficiency filter frame. 

Each unit has been tested and has the characteristics of high filtration efficiency, low resistance and large dust holding capacity. High-efficiency filter with cleanliness equal to or higher than Class 100 clean room. Before installation, it should be leaked in accordance with the method specified in the "Clean room construction and Acceptance Specifications" [JGJ71-90] and meet the specified requirements.

(Figure 1: cleanroom HEPA filter leak detection)

When does the high-efficiency filter need to be replaced

For high-efficiency filters, when the resistance value of the filter is greater than 450Pa;

Or, when the airflow velocity on the windward side is reduced to the minimum, even if the medium-efficiency filter is replaced, the airflow velocity cannot be increased.

If there is an irreparable leak on the filter surface, a new high-efficiency filter must be replaced, and Clean Room detection and high-efficiency filter leak detection are required after replacement.

If the above conditions are not available, it can be replaced every 1-2 years according to environmental conditions.

leak detection method of high efficiency filter

The particle counter sampling head must be inserted into the exhaust static pressure tank (or pipeline) connected to the exhaust high efficiency filter for leak detection of high efficiency filter (this is different from the scanning leak detection of high supply Air volume high efficiency filter) because the leak detection side of the supply air high efficiency filter is exposed in the room, and the leak detection side of the exhaust high efficiency filter is located deep in the static pressure box or pipeline, so the above-mentioned leak detection side of the exhaust high efficiency filter can be pressed as mentioned above. 

The PAO leak detection of the high efficiency filter installed in the clean room can confirm the integrity of the high efficiency filter and the sealing of the equipment.

Test requirements

Test method: PAO leak detection method

Test scope: For all clean rooms with high efficiency filters installed in the company, the high efficiency filters must be inspected and leaked on site in clean areas such as production workshops and laboratories.

Working principle: PAO aerosol generates dust source on the upwind side of the tested high-efficiency filter, and is sampled and detected by the downwind photometer. The scattered light generated by the dust gas during the photometric time is converted into electricity through the photoelectric effect and linear amplification, and is quickly displayed by the microammeter. The collected air sample passes through the diffusion chamber of the photometer. 

(Figure 2: Aerosol)

High-efficiency filter leak detection range

• Filter material

• The filter material of the filter is connected to the inside of the frame

• The filter frame between the gasket and the filter group of the support bracket

• The filter between the support frame and the wall or ceiling.

High-efficiency filter leak detection status and prerequisites

1. Static

2. The air volume/Wind speed test has been completed and the results meet the requirements;

3. Qualification standard: The maximum permeability measured by the inspected high-efficiency filter shall not exceed 2 times the "factory qualified rate". (The "factory pass rate" can be calculated based on the filtration efficiency marked on the HEPA filter nameplate)

1k = (1-α) × 100%

In the formula: k ---- factory qualified permeability.

α----filtration efficiency indicated on the nameplate.

Note: Before testing, you need to fill in the relevant records of the "high efficiency filter leakage record".

4. High efficiency filter leak detection process

High efficiency filter leak detection preparation: remove the diffusion plate of the high efficiency filter and clean the cabinet and frame to avoid interference from dust. Prepare nitrogen cylinders and related connecting pipes.

(Figure 3: High efficiency filter leak detection)

Smoke generation: The inert gas inlet of the smoke generator is connected to the outlet of the nitrogen cylinder equipped with a pressure regulating valve through a plastic tube with an outer diameter of 1/4. The nozzle of the aerosol generator is connected to the intermediate storage nozzle of the gas cabinet of the air purification system through a plastic tube. Press the "TDA-5C aerosol generator" to heat it to 393.3°C-407.4°C. Open the nozzle regulating valve, open the nitrogen source and its regulating valve to adjust the output of the smoke.

Parameter setting: Aerosol photometer initialization, 0% reference standard value setting. Before each high efficiency filter scan, check the 100% reference value and connect the DOP detection hole retained on the sampling tube channel, one end is connected to the upstream (high efficiency filter inlet side) sampling port, and the other end is connected to the upstream aerosol photometer. Open and measure the upstream aerosol concentration. Adjust the opening of the aerosol generator output valve until it is confirmed that the upstream concentration reaches 10-20 ug/L, and set this benchmark to 100%.

Adjust the aerosol generator and aerosol photometer according to the "TDA-2H Aerosol Photometer Standard Operating Procedure" and "TDA-5C Aerosol Generator Operating Procedure".

Photometer scanning: Scan the entire working surface of the high efficiency filter (including between the filter and the frame, between the frame and the frame, and between the frame and the static pressure box) with the sampling head of the photometer. When scanning, the sampling head is about 2-4 cm away from the filter, and the speed is about 3-5 cm/second. When scanning, the line will reciprocate back and forth, and the lines should overlap. 

Note: During the scanning process, avoid ash interference with the cabinet, especially when the sampling head cannot contact the high-efficiency air outlet or filter frame.

Test result record: Record the measured permeability data of each clean room high-efficiency filter in the "High-efficiency Filter Leakage Detection Record".