How to perform DOP leak detection on HEPA filters?

If there are defects in the high efficiency filter and its installation, such as the filter itself has small holes or the installation is not tight to form small cracks, it will not achieve the predetermined purification effect. Therefore, after the installation or replacement of the high efficiency filter, the filter and the installation connection must be detected.

I. Purpose and scope of leak detection:

Detection purpose: By testing the leakage of high efficiency filter, find the defects of high efficiency filter and its installation, so as to take remedial measures.

Detection range: clean area, laminar flow table and high efficiency filter on equipment.

2. Leak detection method:

Commonly used is the DOP method of leak detection (that is, the DOP solvent is used as a dust source, and the aerosol photometer is used to detect the leak), and the dust particle counter scanning method can also be used to detect the leak (that is, the atmospheric dust is used as a dust source, and the particle counter is used to detect the leak).

However, because the particle counter reading is cumulative reading, it is not conducive to scanning, and the inspection speed is slow; In addition, on the upwind side of the high efficiency filter being measured, the atmospheric dust concentration is often low, and it is easy to find the leak by adding smoke, and these deficiencies of the particle counter leak detection can be made up by the DOP method, so the DOP method is now widely used for leak detection.

Three, DOP method leak detection principle:

The DOP aerosol is sent as the dust source on the upwind side of the HEPA filter (DOP is dioctyl phthalate, molecular weight 390.57, the particles are ball shaped after spray).

The aerosol photometer is used for sampling on the downwind side. The collected air sample passes through the diffusion chamber of the photometer, and the scattered light generated by the dusty gas passes through the photometer is converted into electricity by photoelectric effect and linear amplification, and is quickly displayed by the microammeter to measure the relative concentration of the aerosol. DOP test actually measured the penetration rate of high efficiency filter.

DOP generator is a device for producing smoke. After DOP solvent is poured into the generator container, aerosol smoke is generated under certain pressure or heating conditions and sent to the upwind side of the high efficiency filter (heating DOP liquid to form DOP steam, steam condenses into tiny droplets under specific conditions, removing too large and too small droplets, leaving only 0.3um particles. Fog DOP into the air duct);

Aerosol photometer (measuring and displaying the aerosol concentration of the instrument, should indicate the validation period, must be verified and within the validity period can be used);

4. Working procedure of leak detection test:

1. Prepare for leak detection

Prepare the equipment required for leak detection and the air supply pipe layout of the purification air conditioning system in the area to be inspected, and notify the purification air conditioning equipment company to perform operations such as gluing and replacing high efficiency filters on the day of leak detection.

⒉ Leak detection operation

(1) Check whether the liquid level of the DOP solvent in the aerosol generator is higher than the low liquid level, and it should be added if insufficient.

(2) Connect the nitrogen bottle with the aerosol generator, turn on the temperature switch of the aerosol generator, and convert the red light to green light, that is, to the temperature (about 390~420 ° C).

(3) Connect 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 high efficiency filter being tested, open the photometer switch, and adjust the test value to "100".

(4) Turn on the nitrogen switch, control the pressure between 0.05 and 0.15Mpa, slowly open the oil valve of the aerosol generator, and control the test value of the photometer within 10 to 20. After the test value is stable, input the measured concentration of the upstream, and then perform subsequent scanning inspection operations.

(5) Connect 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 support, the sampling head is about 3 ~5cm from the filter, and scan back and forth along the inner border of the filter, the inspection speed is less than 5cm/s.

The test covers the filter material of the filter, the connection between the filter material and its internal frame, the connection between the gasket of the filter frame and the support frame of the filter group, and the connection between the support frame and the wall or ceiling to check for small pinholes and other damage in the filter media, the frame seal, the gasket seal, and the leak in the filter frame.

The daily leak detection of high efficiency filters in clean areas above 10,000 is generally once a year (sterile areas should be six months); When there are significant abnormalities in the number of dust particles, settling bacteria, and wind speed in the daily monitoring of the clean area, leaks should also be detected.