Page 32 - Occupational Health & Safety, May 2017

INDUSTRIAL HYGIENE/WELDING
Monitoring the static pressure across the filter is a good indicator of the tradeoff between desired flow rate and filter efficiency. Most filter replacement indicators use this method.
best option for your welding operation and overall work environ- ment can be challenging.
Fortunately, there is the MERV industry-wide metric for rank- ing filter performance. The MERV is used to indicate the filter’s par- ticle removal efficiency (PRE) for a specific range of particle size. Rankings are made from MERV 1 to 16; the higher the number, the smaller the particle size and the higher the PRE. The details and method of measuring PRE are provided in ANSI/ASHARE Standard 52.2. Welding typically requires MERV 11, 15 or 16, but welding ap- plication and consumable type will dictate what filter MERV rating is required for your system so refer to the ANSI/ASHRAE Standard 52.5 “Understanding MERV” User Guide1 (November 2014) created by the National Air Filtration Association. (NAFA®).
For example, if you have a stainless steel welding application, a filter with a higher MERV rating will generally be required because of the low OSHA PEL for hexavalent chromium. If you use a mild steel MIG process, a filter with a lower MERV rating can be suffi- cient. Once you have selected a filter, always check to be certain it is controlling worker exposure to constituents in the fume by having a
qualified person measure exposure levels upon installation and peri- odically thereafter to ensure levels are within applicable OSHA PEL and ACGIH TLV limits.
Third, a filter should be selected based on the volume, frequency, and type of welding performed.
4. What are other indicators that can affect your fume extrac- tion filter replacement strategy?
As particles are trapped in the filter, the medium becomes more densely clogged, or opaque, and the ventilation system motor and fan are forced to work harder to maintain optimal air flow. Eventu- ally, the resistance of the filter reduces the air flow to an unacceptable level and the filter has to be replaced. Monitoring the static pressure across the filter is a good indicator of the tradeoff between desired flow rate and filter efficiency. Most filter replacement indicators use this method. If there is not a clear indicator on the system you are using for filter replacement—such as a light and/or audible tone, as mentioned previously—you can estimate a regular maintenance/ replacement strategy based on your welding process and arc time.
Filters have a specific surface life. Filter life is determined by (1) how long it takes the fine particles to penetrate into the filter weave and form a cake, and (2) the type of material (i.e., oily versus dry) penetrating the filter medium. Some filters are designed to be re- moved and properly disposed of when saturated, while others are designed to be reused if the system is equipped with a self-cleaning mechanism. In these systems, the filters are cleaned with blasts of
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Circle 3 on card.