Testing Enclosures : Peak Pressure Versus Retention Time
This presentation by Colin Genge was prepared for the 2007 NAFED conferences held in Las Vegas, Atlantic City and Indianapolis.
Presentation Summary
During discharge, clean agent enclosures must be loose enough to prevent excessive pressures, but tight enough to satisfy NFPA retention requirements during the suggested ten minute holding period. In this session you will learn how this dilemma can be resolved. Enclosures that do not require costly relief vents to limit peak pressure can be designed. Damage caused by peak pressures may be the single largest liability that clean agent system installers face. You will also learn how to reduce the cost of airsealing to achieve agent retention.
When vents must be installed, they often do not work as designed because they are restricted, do not open at the correct pressure, do not open at all or open the wrong way! The technician will learn how to quickly verify both venting and enclosure integrity performance during the annual inspection.
This presentation is based on the results from a two year research project on peak pressure and retention time. Actual peak pressures and retention times in a test enclosure were recorded for a variety of agents over a wide range of leakages. For the first time, it is now possible to compare peak pressure and retention time predictions with actual data. The conclusions of this study can be put to good use right now to improve predictions.
Enclosure Designer Spreadsheet
Download the Enclosure Designer Spreadsheet
Enclosures must be loose enough to prevent damaging peak pressures from building up during the discharge and, on the other hand, must be tight enough to maintain agent concentration over the required retention time of ten minutes (the upcoming 2007 version of NFPA 2001 is expected to require a 10 minute hold time). All enclosures have a Minimum Leakage that the enclosure cannot go below for peak pressure and a Maximum Leakage the enclosure cannot go above for agent retention.
Currently, NFPA requires the enclosure be measured to ensure it is tight enough to meet the Minimum Leakage criteria to ensure the retention time is sufficient. No such requirement currently exists for peak pressure and as a result, actions taken to meet the retention requirements have unwittingly created peak pressure problems and enclosures have been damaged
The Retrotec Tester spreadsheet allows this calculation to be made from Door-Fan data taken during the 2 Point test. While the leak exponent can’t reasonably be specified at the start and must be set to the worst case value when designing to 0.5, the effect of the Exponent can be seen. It typically falls between 0.55 to 0.75. Input these values to see for yourself how the Gap widens.
