Testing HEPA filter response to high flow velocity and overpressure

In cooperation with the Los Alamos National Laboratory (LANL) HEPA filters presently on the market and modified versions were tested using the LANL blowdown facility. The filter response during the test was documented on high speed films which indicated, among other information, the load when first failure occurred and the failure mode. Differential pressure through the filter and velocity were measured with variable reluctance pressure transducers and an oscilloscope recorder. New HEPA filters show first failures already at differential pressures between some 4 kPa and 17 kPa. The better results are obtained with conventionally pleated filter paper packs sealed in a wooden frame with a polyurethane material. However, these filters cannot be used under accident conditions. Metal frame filters considered as potential filters for accident situations showed only poor resistance against overpressures. The weakest point in such filters proved to be the attachment of the filter pack in the metal frame, which limited the loadability to about 10 kPa. As soon as proper attachment is assured, the integrity of the filter pack limits the admissible differential pressure. Through some modifications of wood frame filters, physical integrity was improved to a failure pressure of 24 kPa. During operation, the filter elements are loaded with dust. Dust loading up to 1000 Pa pressure drop at rated flow, which was simulated with polystyrene latex aerosols, was found to reduce the mechanical stability by up to 40%. Pressure drop characteristics of HEPA filters influence the occurrence of high differential pressures and high mechanical loads. Pressure drops (δp) of unloaded and preloaded filters were investigated up to high flow velocities (v). Measured data can be presented in the form of δp=av+bv2, where a and b are constants. Theory, in contrast, predicts a linear relationship. The deviation is due to deformations of the pleated filter paper pack with increasing flow velocity and increasing load, respectively