The group "tests of filters" is part of the Internal Service for Prevention and Protection at Work (SIPPT/IDPBW). The main activity is to control the efficiency of installations with absolute filters (HEPA = High Efficiency Particulate Air filters) at SCK•CEN, in Belgonucleaire, at FBFC and in the nuclear power plants of Doel and Tihange.
As subcontractor of AV international in Doel and Tihange, the group works according to a strict quality system (QA). Besides, several security prescriptions are fulfilled (VCA (LSC), the 10CFR5° system).
A DOP generator generates some di-octylphtalates (DOP) aerosols in the conduct before the filters installation to be tested. The aerosol concentration is then measured by a particle counter before and after the installation. The efficiency is calculated based on these measurements.
• DOP generator
The purpose of the DOP (di-octylphtalates ) generator is to generate a quantity of DOP aerosols in the conduct. This aerosol can then be measured by a particle counting system in order to evaluate the efficiency of an installation with HEPA filter.
DOP (di-octylphtalate) is stored in a liquid form in a container. The aerosol is produced by using:
- an inert gas, for instance nitrogen for the TDA-5B generator
- pressurised air for the type F1000 generator
on the liquid DOP. The mix gas-liquid DOP is heated in a heating block (a spiral conduct inside an aluminium block) up to 360 °C. The DOP condenses at the outlet of the generator and forms condensation aerosols. 99% of the particles are smaller than 2 µm.
The photometer is a particle counter that is used to calculate the efficiency of a ventilation installation with HEPA filters.
The air or gas is drawn through the scattering chamber, particulate matter in the sample passes through the focal point of the scattering chamber. Particulate matter light into the dark cone and onto the photomultiplier tube, which converts the light into an electrical signal.
The signal is amplified and digitised, then analysed by a microprocessor to determine the intensity of the light scattered by the signal. This signal is then compared to a reference signal (0% base line) that is set using some clean air that had been cleaned through an ULPA filter.
The photometer can not classify the particulate matter in various ranges. This can only be done by using a laser spectrometer, but measuring with a photometer is much faster than with a laser spectrometer.
• Laser spectrometer
The laser spectrometer is a particle counter that is used to calculate the efficiency of a ventilation installation with HEPA filters.
Particles passing through the laser beam (generated by a He-NE laser tube) in the sampling aperture scatter light. This scattered light is measured by a photodetector module. The amount of scattering depends on the particle size. This allows counting the number of particles in defined diameter ranges.
A too high particle concentration results in the saturation of the laser spectrometer. Therefore a dilution system needs to be used in order to dilute the particle concentration. This is used in the measurement bore, the filter where the concentration is high. This system allows the measurement of the efficiency of 2 filters in series which is not possible when using a photometer.
The efficiency of filter reaches a minimum for the MPP particle (Most penetrating particules). For HEPA filters this diameter is about 0.15 µm. With a laser spectrometer it is possible to measure the efficiency around the MPP and for a higher particle diameter.
• Absolute filter
The absolute filters or the real HEPA (High Efficiency Particulate Air) filters were designed during the Second World War. They were originally made to collect radioactive dust to clean the air. Because of their high efficiency, they have quickly been considered as the best way to filter the air. Later on, they have often been used in laboratories, clean rooms and hospitals.
The tested installations are equipped with ‘High Efficiency Air filters’ (HEPA). These HEPA filters are usually used in nuclear power plants and have a global efficiency of 99.95% (EU type H13) or 99.995% (EU type H14). This classification is in accordance with the European norm: EN 1822-1: 'High efficiency air filters (HEPA and ULPA)'.
Contact: Majkowski Isabelle