If there are defects in hepa filter and its installation, such as small holes in filter itself or tiny cracks caused by loose installation, the intended purification effect will not be achieved. Therefore, after hepa filter is installed or replaced, must do leak test on filter and installation connection.
1. Purpose and scope of leak detection:
Detection purpose: By testing the leak of the hepa filter, find out the defects of the hepa filter and its installation, so as to take remedial measures.
Detection range: clean area, laminar flow work bench and hepa filter on equipment, etc.
2. Leak detection method:
The most commonly used method is the DOP method for leak detection (that is, using DOP solvent as the dust source and working with an aerosol photometer to detect leak). The dust particle counter scanning method can also be used to detect leaks (that is, using atmospheric dust as the dust source and working with a particle counter to detect leaks. leak).
However, since the particle counter reading is a cumulative reading, it is not conducive to scanning and the inspection speed is slow; in addition, on the upwind side of the hepa filter under test, the atmospheric dust concentration is often low, and supplementary smoke is required to easily detect leaks. The particle counter method is used to detect leaks. The DOP method can just make up for these deficiencies, so now the DOP method is widely used for leak detection.
3. The working principle of DOP method leak detection:
DOP aerosol is emitted as a dust source on the upwind side of the high-efficiency filter being tested (DOP is dioctyl phthalate, molecular weight is 390.57, and the particles are spherical after spraying).
An aerosol photometer is used for sampling on the downwind side. The collected air samples pass through the diffusion chamber of the photometer. The scattered light generated by the dust-containing gas passing through the photometer is converted into electricity by the photoelectric effect and linear amplification, and is quickly displayed by a microammeter, the relative concentration of the aerosol can be measured. What the DOP test actually measures is the penetration rate of the hepa filter.
The DOP generator is a device that generates smoke. After the DOP solvent is poured into the generator container, aerosol smoke is generated under a certain pressure or heating condition and is sent to the upwind side of the high-efficiency filter (the DOP liquid is heated to form DOP steam, and the steam is heated in a specific Condensate into tiny droplets under certain conditions, remove the too large and too small droplets, leaving only about 0.3um particles, and the foggy DOP enters the air duct);
Aerosol photometers (instruments for measuring and displaying aerosol concentrations should indicate the validity period of the calibration, and can only be used if they pass the calibration and are within the validity period);
4. Working procedure of leak detection test:
(1). Leak detection preparation
Prepare the equipment required for leak detection and the floor plan of the air supply duct of the purification and air-conditioning system in the area to be inspected, and notify the purification and air-conditioning equipment company to be on site on the day of leak detection to perform operations such as applying glue and replacing hepa filters.
(2). Leak detection operation
①Check whether the liquid level of DOP solvent in the aerosol generator is higher than the low level, if it is insufficient, it should be added.
②Connect the nitrogen bottle to the aerosol generator, turn on the temperature switch of the aerosol generator, and wait until the red light changes to green, which means the temperature is reached (about 390~420℃).
③Connect one end of the test hose to the upstream concentration test port of the aerosol photometer, and place the other end on the air inlet side (upstream side) of the hepa filter being tested. Turn on the photometer switch and adjust the test value to "100 ".
④Turn on the nitrogen switch, control the pressure at 0.05~0.15Mpa, slowly open the oil valve of the aerosol generator, control the test value of the photometer at 10~20, and enter the upstream measured concentration after the test value stabilizes. Carry out subsequent scanning and inspection operations.
⑤Connect one end of the test hose to the downstream concentration test port of the aerosol photometer, and use the other end, the sampling head, to scan the air outlet side of the filter and the bracket. The distance between the sampling head and the filter is about 3 to 5 cm, along the inner frame of the filter is scanned back and forth, and the inspection speed is below 5cm/s.
The scope of testing includes the filter material, the connection between the filter material and its frame, the connection between the gasket of the filter frame and the support frame of the filter group, the connection between the support frame and the wall or ceiling to check the filter medium small pinholes and other damage in filter, frame seals, gasket seals, and leaks in filter frame.
Routine leak detection of hepa filters in clean areas above class 10000 is generally once a year (semi-annual in sterile areas); when there are significant abnormalities in the number of dust particles, sedimentation bacteria, and air velocity in daily monitoring of clean areas, leak detection should also be performed.
Post time: Sep-07-2023