Lesson Learned Statement:Adequacy of downstream aerosol mixing on many older HEPA filter systems has neither been assured through system design nor validated through acceptance testing. Therefore, incorporating compensatory measures into selected test procedures may be necessary to minimize potentially overstating filter performance and to increase confidence that in-place leak tests can reliably identify adverse filter performance on less than ideal HEPA system designs.
Discussion:Acceptance testing of replacement HEPA filters serving the truck-lock of the 327 Building revealed that the facility in-place leak test procedure yielded results that were neither conservative nor sufficiently quantitative/representative of actual filter system efficiency. That finding led to developing and implementing a compensatory aerosol test method for deficient and potentially deficient HEPA system test configurations.
Analysis:Many older HEPA filter system designs predate ASME N509 and ASME AG-1 design criteria. Consequently, many lack either design features that assure fully mixed aerosol concentrations downstream of the HEPA filters or acceptance test data confirming design adequacy. Because valid in-place leak test results depend on well-mixed aerosol concentrations, compensatory measures may be necessary when testing non-ASME compliant and other potentially deficient system designs. Compensatory measures could include performing additional testing to confirm system design adequacy, relocating test ports, modifying the system to improve aerosol mixing, or changing the test process.
Achieving well-mixed aerosol concentrations should always be a primary consideration whenever addressing deficient or potentially deficient HEPA systems. However, modifying existing systems to achieve ideal test configurations, especially those with limited remaining service life, may not always be feasible or warranted. For older systems that cannot be easily modified or where geometry prevents aerosol-mixing improvements through test port relocation, a simple change to the test process may increase the reliability and conservatism of current in-place leak test practices.
Normally, test personnel assume fully mixed aerosol conditions are present when performing in-place leak tests. Therefore, aerosol measurements are usually made from a stationary sample probe inserted into the duct system test port. Held normal to the port, the probe samples the air stream at the diameter of the duct cross-section. If mixing conditions are poor, potentially higher concentrations of aerosol could easily bypass the probe and go undetected. In such cases, the performance of the filter is unknowingly overstated.
To minimize the potential for overstating filter performance, test personnel have been instructed to measure aerosol concentrations throughout the various accessible segments of the duct cross-section and to record the highest value. Modifying the aerosol test procedures to require manipulation of the sample probe within the downstream test port ensures test acceptance is based on the most conservative aerosol concentration and provides additional confidence that adverse filter performance is identified during the test process. Though not equivalent to a fully compliant test, this method has a greater potential for identifying leakage through HEPA filter systems. Depending on the filter system and actual aerosol concentration, the revised method will yield test results that are either equivalent to or more conservative than test results obtained using the conventional stationary probe method. Though effectiveness of the new method also depends on port configuration, the revised test process can be a useful tool for assessing filter performance on less than ideal system designs.
Recommended Actions:Review testable HEPA filter systems to identify potentially ASME N509/AG-1 non-compliant designs. Consider revising the filter test process to authorize sample probe manipulation for system designs where adequacy of downstream mixing has neither been validated through formal testing nor assured through system configuration, and where system modifications to improve potentially deficient mixing conditions are infeasible, cost prohibitive, or otherwise unnecessary.
Data obtained through probe manipulation in poorly mixed airstreams is not necessarily equivalent to that obtained from a properly designed and tested HEPA system. Because the practice may not always be acceptable or appropriate, application of the compensatory test practice must be carefully considered and approved on a case-by-case basis. The practice should never be considered suitable compensation for any mixing deficiencies identified in a newly installed HEPA system. Manipulating the sample probe to sample various segments of the duct and basing acceptance of in-place leak test results on the highest measured penetration value introduces an added measure of conservatism into the traditional test process. The modified test process increases the likelihood that adverse filter performance is identified, regardless of actual aerosol mixing conditions within a duct system. Contact Central Engineering for more information and history on this issue.
Originator:Fluor Hanford, Inc. Submitted by Patrick O’Brien; (509) 373-3929
Contact:Patrick O’Brien; (509) 373-3929 or Project Hanford Lessons Learned Coordinator; (509) 373-
Name Of Authorized Derivative Classifier:Not required
Name Of Reviewing Official:John Bickford
Priority Descriptor:Green / Good Work Practice
Keywords:filter, aerosol penetration testing, mixing, ventilation, HEPA filter, in-place leak test
References:Washington Administrative Code 246-247-040(4))
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DOE Function / Work Categories:Engineering Design and Construction - Non-Nuclear
Inspection & Testing
Maintenance - HVAC
ISM Category:Feedback and Improvement
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