Statistical evaluation comparing strength of new and aged nuclear grade HEPA filters

Abstract

Nuclear grade high-efficiency particulate air (HEPA) filter service life is generally limited to 10 years from the date of manufacture at Department of Energy facilities. Filter life extension offers a substantial cost savings and decreased facility downtime. This study compared tensile strength results between unused new and unused aged filters (7, 10, and 13 years old) to discern age related differences. Unused HEPA filters were disassembled. The filter sheets were preconditioned in a controlled environment for 24 h to resolve any atmospheric history of the filter media and to remove any effects of hysteresis on the results. Media used in HEPA filters are fragile, and damage occurs outside of the sample area when the material is cut. Basis weight measurements were recorded. Machine direction (MD) and cross direction (CD) tensile strength specimens were taken from the top or bottom of each filter pleat. Tensile strength specimens were also obtained of the pleat fold in the machine direction. Burst strength specimens were taken from across each filter pleat from top to bottom. Analysis of the raw data suggests that differences in tensile strength between new and aged HEPA filters are not significant. These results complement previous studies that have evaluated the relationships between the postulated 10-year HEPA filter lifespan, other performance indicators, and assessed cohort of aged filters as a function of normal operations. Those results along with the tensile strength results reported here suggest that HEPA filters, when properly maintained, can reasonably operate safely and efficiently for extended periods beyond 10 years when managed with well-defined performance and operational requirements.