Filtration systems are a critical part of any clean-room, with High-Efficiency Particulate Air (HEPA) filters being of particular importance as they trap harmful pathogens and other potential contaminants. HEPA filters may be part of wall or ceiling-mounted systems, or included in bio-safety cabinets. While they have a relatively long lifespan, the filters do eventually blind and need replacing. A safe change housing is used when pathogens and or harmful substances may be present in the filter media, and contact with these filters must be avoided. This filter change-out procedure is described below.
Safe change filtration methods are said to have been developed for use in the nuclear industry, to handle radioactive airflow. They evolved from rudimentary designs that first appeared in the 1950's, becoming more widespread from the 1980's on-wards. These methods have been applied to the pharmaceutical and biochemistry fields to prevent exposure to potentially harmful chemicals and micro-organisms, which are a key part of procedures in these industries. Because Safe Change filter change-outs are quite complex, they should always be left to authorized technicians.
Many HEPA filter systems use what is known as a bag in, bag out method for replacing filters. Before beginning the replacement, staff should don protective gear in the form of an isolation suit, gloves and a respirator mask. This is a precaution to ensure their safety in the event of an accident. Thereafter, the following steps should be followed.
The first step in the safe change procedure is to make sure the filtration system has been switched off, or that the airflow has been diverted elsewhere. Thereafter, it is necessary to check that the internal pressure has equalized with the external pressure. If the Safe Change Housing is fitted with isolation shutoff dampers, they must be closed before opening the housings. If the filter to be changed is in a bio-safety environment that sees the manipulation of micro-organisms, decontamination should be performed first.
After removing the lid from the filter housing, the previous filter bag should become visible. This, together with the new filter bag to be used, must be carefully inspected for any damage. If the bags are intact, the old filter can be removed – without it exiting the bag. The bag can then be welded, heat-sealed-taped or tied shut. If cable ties are used, the bag should be secured in two places. The bag can then be cut between the seals and removed and the old filter disposed of.
The new filter can then be installed, keeping it in the new safety bag. This bag should be put over the remains of the previous bag, which can be moved to the end of the new bag, and secured in place. Generally, this can be left there until the next filter change. The technician needs to ensure that the bags are properly rolled up and not stuck anywhere, or pinched in the housing cover, as this could compromise the safe change procedure.
After closing the filter housing, the airflow through the filter needs to be checked and a DOP leak test performed to ensure that it is functioning optimally.
Throughout the change procedure, it is important to remember that HEPA filters are very fragile and should be handled with great care. After the change, they need to be checked regularly to make sure that they are filtering air effectively – if the airflow drops or if they become damaged, replacement may be necessary again.
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