| The operational objects in high-level biosafety laboratories are highly pathogenic microorganisms. Harmful aerosols may be released during most of regular experimental operations. Once laboratory carries bacteria and the contaminated indoor air is discharged to atmosphere, population and animals will be infected, which will cause epidemic outbreak, threaten severely people's health, and even induce magnitude public health incident. Thus, treatment of contaminated air in biosafety laboratory is the key point to ensure laboratory biosafety.At present, HEPA filter is still the main method to protect from air pollution of bio-safety laboratory. HEPA filter has the risk of leakage and pathogenic microorganism surviving/propagating on the surface. It is required by WHO in"Laboratory Biosafety Manual"(3rd Edition) that all the HEPA filters in laboratory must be installed in a manner that permits gaseous decontamination and testing. This requirement is also prescribed in GB19489-2008 that the decontamination and testing should be executed in-situ.The building of high-level biosafety laboratory started relatively late in our country. The applied technology of laboratory exhaust air treatment is rather backward compared with foreign countries, and cannot meet the requirement of in-situ leak detection and in-situ decontamination. In order to solve the technical difficulty that how to improve the high-level biosafety laboratories built or being built to meet the requirements of GB19489-2008, it is called to study on the in-situ leak detection technology for HEPA filter unit basing on the existing standards and other technical requirements, and develop exhaust treating equipments for high-level biosafety laboratory suitable for our country situation.Scanning leak test and total efficiency leak test methods commonly used in the world were studied. Combined with the performance requirements of related standards for HEPA tester, as well as the actual situation of the built and being built high-level biosafety laboratories in our country, a research program of in-situ leak testing technology for HEPA filter of biosafety laboratory is proposed.This paper firstly studied the technical method that how to apply total efficiency leak test to exhaust filter equipment, and designed the downstream mixing device on the equipment through theoretic analysis, experimental comparison and CFD simulation, which ensured a representative downstream sampling. It is recommended to evaluate the property of mixture of aerosol at the downstream sampling point according to ASHERE52.2-1999"Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size". Inject DOP aerosol manually in turn at the 9 dust generating points of the filter's downstream and sample at the probes. For the particulars of sized between 0.3~0.5μm, the relative deviations of the test results inferior to 3% where the aerosol are collected at all downstream sampling probes corresponding to the dust generating points, which be able to meet the standard requirements for not more than 10%.According to the standards, it is required to ensure a spatial uniformity of concentration of the aerosol closing to filter's upstream for the HEPA filter units with upstream dust generation and sampling functions. To meet the requirement, mixing board is usually installed at upstream. While it still cannot mix enough in some cases, thus ring dust generating probe is taken instead of conventional probes. Experimental results indicate that ring dust generating probe can accelerate aerosol mixing, which achieves a good result.Test HEPA filter paper with leakage of vary aperture by stage filter media tester TSI3160 US, of which the filter MPPS efficiency is more than 99.97% and MPPS particle size is 0.1μm. The test result shows that it has little effect on the overall efficiency of filter paper when the leakage diameter is inferior to 0.4mm, while the efficiency of filter paper decreases obviously when the diameter is superior to 0.4mm. Consequently, it is recommended to choose the leakage diameter as 0.4mm when testing the leak identification of scanning probe and performance of leak location. The leakage diameter indicates here the round pin diameter used to make a leak.The proposal of developing line scan sampling probe was put forward regarding the problems when applying conventional sampling probes to auto-scanning system of exhaust filter units. Firstly test the 2 designs of line scan sampling probe according to the air flow velocity consistency at sampling probe, where the sampled air flow rate is 28.3L/min. Based on the test result, a primary choice is made between the 2 designs. The line scanning sampling probe is selected because the average values of air flow velocities are all within the range of±20% and it can meet the requirement of related standards.In order to test the leakage identification performance of line scanning probe, a comparison test of filter leakage identification has been taken between conventional scanning probes and line scanning probes. 2 leaks of 0.4mm were made artificially in the filter paper and separately at inlet and outlet surfaces, The 2 kinds of probe are fixed at the distance of 25mm to filter to sample around the leakage. The test result shows that the transmittance get for fixed leakage by line scanning probe is 0.7 times of which by conventional probes. The leakage identifying range of line scanning probe is about 7mm with transmittance presenting a parabola shape. But the leakage identifying range of conventional probe is about 11mm with transmittance presenting a trapezoid shape. Furthermore, line scanning probe has less difference of test results at various leakages. It is proved to be available for filter scanning test.According to the characteristics of the line-scanning sampling probe, the movement of the probe is set to intermittent operation mode, which pauses 0.5s after moving every 5mm. The reason to select this mode is to provide time for date collection and processing, because the particle counter cannot collect data and transmit data to computer simultaneously, this operation mode can effectively prevent from missing leakage identification. Taking into account the delay of data collection caused by the length of sampling pipe, the sampling time was corrected in the process of scanning. The leak area can be located accurately when filter with leaks is scanned.According to the performance requirements of exhaust filter units, the automatic line-scanning leak detection system was developed. Besides the sampling probe, the system also includes motion control system, sampling pipeline switch systems and data acquisition and processing systems. The advantage of such system is short scan cycle, simple mechanical structure, less matching equipments, high automation and more humanized. All electrical components of the automatic scanning systems have been installed outside the units, so it can be fully applicable to filter leak detection of filter of the exhaust filter units in situ.High-level biosafety laboratory should select the appropriate exhaust filter units based on its construction, and also select the appropriate leak detection method in accordance with its installation manner of HEPA filter. In this thesis series of biosafety laboratory exhaust filter units were developed, which could provide the essential technical and equipment support for high-level biosafety laboratory in our country. The performances of these apparatus have reached the same level of those abroad, which could be used widely in high-level biosafety laboratory in our country in the near future. Keywords: high-level biosafety laboratory; HEPA filter; HEPA filter unit; leak detection in-suit; total efficiency leak test; scanning leak test. |
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