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DOP Leak Detection Method of HEPA Filter in Cleanroom

POST TIME:2019-12-23 16:06:59 Click:

DOP Leak Detection Method of HEPA Filter in Cleanroom

HEPA (high efficiency filter) generally refers to the filter with a collection efficiency of more than 99.97% for particles with a particle size ≥ 0.3 μ m, which is usually used as the end filter device of the enterprise clean room to provide clean air. Whether the clean room can achieve and maintain the designed cleanliness level is related to the performance and installation of high efficiency filter to a certain extent. Therefore, it is one of the important means to test the high efficiency filter in the clean workshop to ensure that it meets the requirements.

FDA and GMP in the United States also clearly pointed out that in-situ leak test should be carried out after the installation of the high-efficiency filter to check the tightness of the filter gasket, frame and filter material. For the sterile preparation production workshop, the leak test of the high-efficiency filter should be carried out regularly.

1. Leakage detection purpose of high efficiency filter: the filter efficiency of high efficiency filter itself is generally tested by the manufacturer, and the filter efficiency report and qualification certificate are attached when leaving the factory. For pharmaceutical enterprises, high efficiency filter leak detection refers to the on-site in-situ leak detection after the installation of high efficiency filter and its system, mainly to check the small pinholes and other damages in the filter material, such as frame seal, gasket seal and the leak on the filter frame. The purpose of leak detection is to find out the defects of the high-efficiency filter and its installation in time by checking the tightness of the connection between the high-efficiency filter and the installation frame, and to take corresponding remedial measures to ensure the cleanliness of the area.

2. Principle of DOP (PAO) leak detection method: DOP (PAO) generator is usually used to generate dust in the upstream of the filter for leak detection of high efficiency filter, and photometer is used to detect the aerosol concentration in the upstream and downstream of the filter to determine whether the filter has leakage. The purpose of dust generation is because the dust concentration upstream of the high-efficiency filter is relatively low, and only particle counter is used to detect without dust generation, so it is difficult to find leakage, so it is necessary to supplement the dust generation to find leakage obviously and easily. The artificial aerosol DOP has a history of nearly 40 years. For a period of time, because it is suspected to have carcinogenic effect on human beings, it is often replaced by the DOS (dioctylsebaeate dioctyl sebacate) or dehs (DI (2-ethylxyl) sebacate) and Pao (polyaphaolefin polyolefin), but the experimental method is still called "DOP (PAO) method". Because the concentration of atmospheric dust varies with the place and time, sometimes it is higher, sometimes it is lower, so it is generally not used for leak detection. FDA points out that the selected aerosol should meet certain physical and chemical requirements during leak detection, and should not use the aerosol that will cause microbial pollution and microbial breeding.

DOP (PAO) generator can be divided into heat generation and cold generation. The heat generator uses the principle of evaporation and condensation to evaporate the atomized aerosol particles with a heater and condense them into tiny droplets under specific conditions. After removing the oversized and undersized droplets, it leaves about 0.3 μ m of fog DOP (PAO) in the air duct, with the particle size distribution of 0.1 μ m ~ 0.3 μ M. The cold generator is a kind of multiphase DOP (PAO) aerosol with a maximum particle size of about 0.65 μ m, which is produced by bubbling compressed air in liquid and splashing through Laskin nozzle. Cold DOP (PAO) is often used when scanning filters for leakage detection. There are two kinds of testing instruments, one is aerosol photometer, the other is particle counter. The commonly used testing instrument in high-efficiency filter leak detection is aerosol photometer (hereinafter referred to as photometer), which is a kind of pre scattering linear photometer. It consists of vacuum pump, light scattering chamber, photomultiplier tube, signal processing converter and microprocessor. Its working principle is: when the air flow is pumped to the light scattering chamber by the vacuum pump, the particles in it scatter the light to the photomultiplier tube. In photomultiplier tubes, light is converted into electrical signals, which are amplified and digitized and then analyzed by microprocessor to determine the intensity of scattered light. The mass concentration of particulate matter in gas can be directly measured by comparing with the signal generated by reference material.

Therefore, it is widely used. However, the particle counter, whose test value reflects the concentration and particle size of the number of particles in the air flow, and specifies the particle size range, has high sensitivity, is applicable to all dust source aerosols, and has a large choice, but is seldom used in the leak detection of high-efficiency filter, and the test results of the two instruments are difficult to be compared quantitatively.

3. Test method: 3.1 to determine whether there is obvious leakage in the high efficiency filter itself and its installation, the following parts must be tested on site: filter material of the filter; the connection between the filter material of the filter and its frame; between the gasket of the filter frame and the support frame of the filter group; between the support frame and the wall or ceiling. DOP (PAO) leak detection materials and instruments are: dust source (PAO solvent), aerosol generator, aerosol photometer. The aerosol generator used by our company is TDA? 2H handheld Laskin nozzle type aerosol generator, which directly uses air without compressed gas as power. Under the working pressure of 20 PA, when the air flow rate is 50-2025 F3 / min, the multi dispersive submicron oil dust aerosol with the concentration of 10 μ g / ml-100 μ g / ml can be produced. The aerosol photometer used is ATI 2H type photometer, the dynamic measurement range is 0.00005 μ g / L ~ 120 μ g / L, and the sampling flow is LF3 / min (28.3 L / min). Pao aerosol is introduced into HEPA of HVAC system at the upstream side of HEPA to be tested. To make the concentration of aerosol reach HEPA uniform, aerosol can be directly introduced from the negative pressure side of the system fan. If it is to be introduced from the air duct, it should be introduced at least 10 times the diameter of the air duct from HEPA, and the turning should be minimized (American Society of environmental science and Technology). Generally, keep the upstream aerosol to reach the required concentration, and the concentration fluctuation in a certain range. For HEPA on laminar flow hood and super clean platform, aerosol is directly introduced from the negative pressure side of the system fan.

3.2 initialize aerosol photometer, set 100% and 0% reference standard value. Initialize and set the alarm value according to the aerosol photometer operation requirements. Connect the upstream sampling tube with the upstream sampling port to measure the concentration of the upstream aerosol. Adjust the aerosol concentration according to the operation requirements of the aerosol generator to make the upstream aerosol concentration reach 10 mg / L ~ 30 mg / L. 3.3 scanning Leak Detection remove the diffuser of HEPA, and scan the sealing of the whole filter face, between the filter and the frame, between the frame and the frame, and between the frame and the static pressure box. During scanning, the distance between the sampling head and the filter surface is about 1 inch (about 2.54cm), and the scanning speed is not more than 5cm / s. The scanning shall be conducted back and forth in a straight line, and the lines shall overlap. During the detection, if there is an alarm sound (i.e. the% leakage rate exceeds 0.01%), it indicates that there is leakage. Special glue shall be used to stop the leakage or fasten the leakage before scanning and patrol inspection. Check a filter for about 5 minutes. During the test, always confirm the concentration of the upstream aerosol. Pay attention to wearing a protective mask and eye mask during the test.

4. Results judgment and treatment: the leakage rate of high efficiency filter should be ≤ 0.01%. If the% leakage (leakage rate%) of all points is less than 0.01% in the process of HEPA detection, then the HEPA is qualified; if one point is more than 0.01%, it is unqualified, and the point is marked out, which needs to be repaired or replaced. Special glue is allowed to repair the leakage of filter material of high efficiency filter, but the area of single leakage shall not be greater than 1% of the total area, and the area of all leakage shall not be greater than 5% of the total area, otherwise it must be replaced. Through the leakage test of the high-efficiency filter on site, we suggest that the sealing method between the filter element of the high-efficiency filter and the frame of the static pressure tank should be mechanical seal (gasket as the sealing material). When installing the high-efficiency filter, pay attention to the tightening force of the liquid tank and the position of the blade in the middle of the liquid tank.

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