Summary of high efficiency filter leak detection methods for operating room purification, workshop purification and laboratory purification projects
Summary of high efficiency filter leak detection methods for operating room purification, workshop purification and laboratory purification projects:
1. sodium flame method
Principle: the principle of sodium flame is to spray and dry sodium chloride solution to form a sodium chloride aerosol with a mass median diameter of about 0.4 m. Before and after the tested high-efficiency filter material, the dust-bearing air is sampled and led to the sodium flame photometer to measure the photoelectric flow value related to the dust concentration, so as to calculate the transmittance of the filter material.
Test principle: the test dust source is sodium chloride salt spray of single dispersed phase, and the "quantity" is the brightness of hydrogen flame when salt fog is contained. The main instrument is flame photometer. Salt water splashes under the agitation of compressed air, forms a small salt mist after drying and enters the air duct. Before and after the filter, samples were taken respectively, and the salt fog made the color of hydrogen flame blue and brightness increased. According to the brightness of the flame, the salt mist concentration of the air is determined, and the filter efficiency of the salt mist is determined. According to the national standard, the average diameter of salt spray particles is 0.4 μ m, but the actual measurement result is 0.5 μ M. In Europe, the measurement result of the middle diameter of the actual salt spray particles is 0.65 μ M. With the popularity of other detection methods, sodium flame method is no longer used in Europe. The relevant domestic departments are revising the original national standards, and whether to abolish or continue to use the sodium flame method has not yet been implemented.
2. Counting scanning method
According to the code for construction and acceptance of clean room (jgj71-90), the tested high-efficiency filter must have detected the air volume, and the designed wind speed must be between 80% and 120%. For the concentration of particles on the upper wind side of the tested high-efficiency filter to the controlled particle size, it must be ≥ 3.5 × 104pc / L. for the concentration of particles with the controlled particle size ≥ 0.1 μ m, it must be ≥ 3.5 × 106-3.5 × 107pc / L 。 The particle counter scanning method with the minimum sampling amount > 1L / min is used to scan and detect the installation joint and main section of the high efficiency filter. The detection point shall be 20-30mm away from the tested surface, and the probe shall move at the speed of 5-20mm / s to scan the whole section, sealing head and installation frame of the tested filter. According to the code for construction and acceptance of clean room, the penetration rate converted from the leakage concentration at the lower air side of high efficiency filter is used to measure whether it is qualified.
Actual problems: the high efficiency filter is generally carried out after the system air volume and the air volume of each air outlet are adjusted and balanced. According to the specification, the deviation between the air volume of each air outlet and the designed air volume is less than 15%, which meets the condition that the inspected air outlet is carried out close to the designed air speed. Therefore, when the air volume is well balanced, the leakage detection of high efficiency filter should be carried out in time.
In engineering, the minimum particle size channel of particle counter is 0.3 μ m, which is generally used in the purification system of more than 100 levels. Therefore, the controlled particle size of the tested high-efficiency filter at the windward side is ≥ 0.5 μ m, and its concentration must be ≥ 3.5 × 104pc / L, while the general atmospheric dust concentration is 5.3 × 104-2.5 × 105pc / L, which reaches the upper limit of the particle counter.
In many purification systems, the concentration of fresh air entering the circulating air-conditioning box is far less than that of the atmospheric dust after being treated by the filter, and the concentration will be lower after mixing with the return air. Therefore, the introduction of air into the installed purification air conditioning system is a problem worthy of careful consideration. In order to ensure the particle concentration requirements upstream of the tested air filter without destroying the air volume balance of the system, it is an ideal way to introduce an artificial aerosol with uniform concentration upstream.
How much aerosol concentration is needed depends on the efficiency of the filter and the resolution of the particle counter. The minimum bit of particle counter is a bit, and the minimum number is zero. Generally, there will be dead zone in the testing instrument. If the required downstream concentration is less than 10, it is qualified. It is difficult to guarantee the validity of these data according to the principle of statistics.
At present, most of the domestic high-efficiency filters are classified according to the filtration efficiency of 0.3 μ m particles in DOP test of the United States. Therefore, when the counting scanning method is used for leak detection, the particle size to be measured should be ≥ 0.3 μ m, which requires better upstream concentration. According to the efficiency of high efficiency filter 99.97% and the downstream concentration within three effective figures, it is required that the concentration of particles with particle size ≥ 0.3 μ m in the upstream air is at least 6 × 104pc / L. If the high-efficiency filter with 99.99% efficiency is used, the particle concentration with the upstream ≥ 0.3 μ m is about 2 × 105pc / L, and the particle concentration with the upstream ≥ 0.5 μ m is far more than 3.5 × 104pc / L. Therefore, using atmosphere as dust source can not meet the test requirements.
The particle counter with large flow rate is more suitable to detect the installation leakage of high efficiency filter. The larger the air sampling amount is, the more representative the test results are and the higher the accuracy is. The sampling quantity required by the specification is 1L / min, while the sampling quantity we often use is 0.1cfm (2.83l / min), so as to meet the requirements of the test instrument, but this will affect the work efficiency of leak detection. Taking 610 × 610 high-efficiency filter as an example, when using the mobile sampling head to sample under the high-efficiency filter, the sampling speed is 20 mm / s, and the rectangular 100 × 11.33 sampler needs at least 244 seconds; the sampling speed of the circular sampler is slower, and the time of leak detection scanning is longer. However, the calculation method of scanning rate and acceptable observation counting method in iso14644-3 is difficult for general engineers and technicians to master. Therefore, when the country formulates this specification in the future, it is suggested to adopt appropriate sampling rate and acceptable observation counting for different levels of high-efficiency filters to facilitate the actual operation of Engineering testers.
3. DOP particle scanning positive pressure leak detection method
The inspector must pay attention to the operability of the parameters such as sampling time and upstream particle concentration required in the specification in practical application. When participating in the leak detection of the high efficiency filter in the purification workshop of a pharmaceutical factory, combined with jis-b-9927 of Japan and fedstd-209d standard of the United States, Laskin DOP particle generator was used to generate a large number of stable particles that meet the requirements, so that the concentration of particles ≥ 0.3 μ m upstream was greater than or equal to 1 × 105pc / L. In the upstream of the filter, DOP particles are uniformly injected into the system through compressed air (cold generation). Because the concentration is generally greater than 1 × 105pc / L, which is beyond the measurement range of the detection instrument, the particle concentration diluter is connected in series before the detection instrument to make the measured concentration within the measurement range of the detection instrument.
For the high efficiency filter with 99.97% ex factory efficiency, the measured transmissivity shall be less than 0.06%, that is, the measured particle concentration shall be less than 0.06% C1 (C1 is the upstream concentration). When the particle concentration emitted from the particle generator upstream of the filter is ≥ 0.3 μ m, it is 3.3 × 106pc / 2.83l. For different air volume systems, it is converted to the downstream limit concentration. For example, a certain design air volume is 150000m 3 / h, the downstream limit concentration of the purification air conditioning system is 1960pc / 2.83l. After the aerosol concentration from the particle generator is stable, the particle counter is used to test the leakage of the installed high efficiency filter. The particle counter must scan the outer frame and inner surface of the high-efficiency filter twice at the same time. The upstream of the filter must also sample and test the aerosol concentration in the air duct twice. The period of sampling and sweeping must be equal, and the starting and ending time should be consistent as far as possible.
Taking the 610 × 610 high efficiency filter as an example, it is suggested to adopt a sampling period of one minute and a speed of 50 mm / s. Before leak detection, a 500mm high enclosure is placed around the high-efficiency filter to prevent the air flow outside the high-efficiency filter frame from forming eddy current, which will affect the results of scanning test. The main reason for the leakage of the filter is that the sealing effect of the outer frame sealing ring is not good, or the filter material is not smooth and tight. In practical application, the installation leakage mainly comes from the outer frame, and the quality of the outer frame sealing strip has a great relationship with the leakage.
Therefore, during leak detection in the project, enclosure will be added around the high-efficiency filter to reduce the area of the outlet at the lower part of the air outlet enclosure, and a certain positive pressure will be formed in the enclosure. If the high-efficiency filter is installed with leakage, particles will leak out through the enclosure. At this time, the particle counter can scan for 1 minute at a speed of about 50 mm / s at 25 mm under the high-efficiency filter to determine whether the filter leaks, which is the "positive pressure leak detection method". For the purification workshop with a large number of high-efficiency filter tuyeres, the test effect of high-efficiency positive pressure leak detection method is equivalent to the traditional internal and external frame scanning method, but the efficiency is doubled. Taking a purification air-conditioning system with 60 610 × 610mm high efficiency filter outlets as an example, it takes at least two hours to scan by traditional methods, while it takes only one hour to scan by positive pressure leak detection method, which saves more than half of the time.
4. oil mist method
Test principle: the dust source is oil mist, the "quantity" is the turbidity of oil mist air, and the instrument is turbidity meter. According to the turbidity difference of the gas sample, the filtration efficiency of the filter for oil mist particles is determined. Paraffin oil is specified in Germany, and the particle size of oil mist is 0.3-0.5 μ M. The average diameter of oil mist specified in the Chinese standard is 0.28-0.34 μ m. There is no specific provision for the type of oil. Although the Chinese standard stipulates that the oil mist method can be used, domestic manufacturers prefer to use another sodium flame method with the same standard specification. Only some manufacturers still use the oil mist method when measuring the filter materials.
5. Particle counter method
Test principle: pass the dusty air flow through the strong light illumination area at a very small flow rate. When the dust particles in the tested air pass in turn, each dust particle will generate a light scattering, forming a light pulse signal. According to the relationship between the amplitude of the light pulse signal and the size of the particle surface, the number and brightness of particles are measured by the photomultiplier tube, and the filtration efficiency is determined
6. PAO method
Purpose and principle: leakage detection of high efficiency filter after field installation is to check whether there is leakage of high efficiency filter and supporting hydrostatic tank or whether the leakage is within the scope allowed by the specification. If the high-efficiency filter device is qualified after leak detection, it can ensure the safe and reliable operation of the clean room. At this time, the indoor cleanliness is not up to the standard, the reasons should be found out from other aspects of the clean room.
Pao aerosol is sent to the upper wind side of the tested high-efficiency filter as the dust source, and a photometer is used to sample and detect the dust containing gas at the lower wind side. The scattered light generated by the photometer is converted into electricity through photoelectric effect and linear amplification, and it is quickly displayed by the microammeter. The collected air sample passes through the diffusion chamber of the photometer, because of the difference of light intensity caused by the diffusion of particles, the The relative concentration of aerosol can be measured by light intensity and photometer
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