Factors Influencing HEPA Filter Performance – 9060

Properly functioning HEPA air filtration systems depend on a variety of factors that start with the use of fully characterized challenge conditions for system design and then process control during operation. This paper addresses factors that should be considered during the design phase as well as operating parameters that can be monitored to ensure filter function and lifetime. HEPA filters used in nuclear applications are expected to meet design, fabrication, and performance requirements set forth in the ASME AG-1 standard. The DOE publication Nuclear Air Cleaning Handbook (NACH) is an additional guidance document for design and operation HEPA filter systems in DOE facilities. These two guidelines establish basic maximum operating parameters for temperature, maximum aerosol particle size, maximum particulate matter mass concentration, acceptable differential pressure range, and filter media velocity. Each of these parameters is discussed along with data linking variability of each parameter with filter function and lifetime. Temporal uncertainty associated with gas composition, temperature, and absolute pressure of the air flow can have a direct impact on the volumetric flow rate of the system with a corresponding impact on filter media velocity. Correlations between standard units of flow rate (standard meters per minute or cubic feet per minute) versus actual units of volumetric flow rate are shown for variations in relative humidity for a 70 C to 200 C temperature range as an example of gas composition that, uncorrected, will influence media velocity. The AG-1 standard establishes a 2.5 cm/s (5 feet per minute) ceiling for media velocities of nuclear grade HEPA filters. Data are presented that show the impact of media velocities from 2.0 to 4.0 cm/s media velocities (4 to 8 fpm) on differential pressure, filter efficiency, and filter lifetime. Data will also be presented correlating media velocity effects with two different particle size distributions. INTRODUCTION HEPA filters are commonly employed to control particulate matter (PM) emissions from processes that involve management or treatment of radioactive materials. Facilities within the US Department of Energy (DOE) complex are particularly likely to make use of HEPA filters in the processing of exhaust gases prior to release to the environment. In May of 1999 the Defense Nuclear Facilities Safety Board (DNFSB) released Technical Report 23 entitled HEPA Filters Used in the Department of Energy’s Hazardous Facilities [1]. This report expressed concerns for the potential vulnerability of HEPA filters used in vital safety systems. Later that same year DOE initiated a response to the DNFSB’s Recommendation 2000-2 by implementing measures with regard to 100 percent quality assurance testing of HEPA filters and a review of vital safety systems in general [2]. DOE’s actions in this matter were also timely with regard to concerns being voiced by citizen groups over the performance of HEPA filters and how their functional status is monitored. Of particular concern are the threats to filter performance posed by water and smoke. While these two threats are both associated with fire scenarios, leaking reheaters can also pose a wetting threat to filters. WM2009 Conference, March 1-5, 2009 Phoenix, AZ For the past several years, the Institute for Clean Energy Technology at Mississippi State University has conducted extensive research under its DOE sponsored HEPA Filter Monitoring Project. Studies have included moisture failure, source term loading, seal and pinhole leak tests, and media velocity. Details related to design, construction, and operation of the test stands utilized in these research efforts have been published [3]. Discussion of the experimental design related to these research efforts as well as results has been presented at numerous conferences [4, 5, 6] and published [7]. These discussions include aerosol generation, filters tested, and aerosol measurement instrumentation utilized. RESULTS AND DISCUSSION Moisture Failure/Wetting The studies undertaken as part of this project were designed by a national Technical Working Group (TWG) as a part of joint effort by the DOE and the US Environmental Protection Agency (EPA) to coordinate research efforts to the maximum extent possible for issues associated with treatment and disposal of mixed wastes. During the early planning stages of this project it became clear to the TWG from the input gathered from facility personnel, permit writers, and other stakeholders that the most important data to collect would be for effects caused by the wetting of HEPA filters. A series of three filters were tested by subjecting them to repeated cycles of challenge with increasing relative humidity. A test cycle began by challenging a filter under baseline conditions with a relative humidity (RH) of approximately 15%. After collection of a full suite of data at this RH, the humidity was raised to approximately 50%. Challenge of the filter was held constant at this RH while another set of data were collected and then the RH was raised to between 90 and 100%. Data from this set of test conditions were collected and then the particle generator was turned off and RH in the test stand was returned to 15%. The filter was dried overnight at this low RH and the test cycle was then repeated. This process was followed until the filter failed to demonstrate a filter efficiency of 99.97% when it was dry. Table I contains a summary of the test conditions used for one of the filters. Figure 1A contains a representative example of the data collected during these series of tests. This figure demonstrates the correlation between RH (red) and differential pressure (dP, blue) across the filter and differential temperature (dT, green) across the filter housing. Elevated RH challenge conditions were achieved by injecting water aerosol into the test stand approximately 15 diameters (7.5 feet) upstream of the filter. The RH of the flue gas was measured up and downstream of the filter. No liquid water was detected at the 15 or 50% RH test levels. However, the filter became wet and liquid water started to accumulate in the housing in front of the filter in a short period of time after the RH was raised to 90%. Table I. Average Test Conditions of the ICET HEPA Filter Test Stand During Testing Activities for the Moisture Failure Mode Study. Volumetric Flowrate (cfm) 250 Media velocity (ft/sec) 4 6 Temperature (F) 77 RH Low: 13.7% Mid: 51.1% High: 91.6% Static Pressure on Test Stand (in WC) 3.2 in. wc subatmospheric Particle loading rate: mg/m #/cm 25 5x10 WM2009 Conference, March 1-5, 2009 Phoenix, AZ PM KCl PM: CMD (nm) GSD 130 2.00 Figure 1A displays the correlation of dP (blue), dT (green) and RH (red) for the testing of a partially loaded HEPA filter with an ambient (room temperature) air flow. It is clear from the data in this plot that monitoring dP is not as sensitive or as rapidly responding as monitoring differential temperature for sensing the presence of liquid water in the air flow upstream of the filter. The dT curve (green) responds in concert with an increase in addition of moisture, either as a negative inflection (downstream T > upstream T) at low RH or as a much larger positive value (upstream T > downstream T) at an RH above 60%. It can be deduced that at high RH and low temperature air flows moisture rapidly converts the HEPA filter into an evaporative cooler. In this type of application monitoring dT across the filter housing and can serve as a very inexpensive and effective method for detecting liquid water reaching the filter. Figure 1B shows the relative humidity curves and downstream PM concentrations for a filter as it is cycled through a moisture failure test. Notice that initial test conditions include approximately 15% RH followed by periods of exposure to 50 and 90+% RH. This particular filter underwent three days of testing. Table II contains numerical data for downstream PM concentrations under the different RH test conditions. Trends seen in data for testing of the specific filter presented in Figure 1 and Table II are representative for all filters tested. The following observations can be made: (1) repeated wetting of a filter results in a deterioration of filter performance, (2) it is possible for a filter to “fail” (or demonstrate a filter efficiency less than 99.97%) when wet and yet recover filter efficiency when dry, and (3) no filter was found to fail irreversibly the first time it became wet. WM2009 Conference, March 1-5, 2009 Phoenix, AZ