Building Pressurization – A False Sense of Safety for Shelter-in-Place
A common method of mitigating risk incurred by occupants of a building from external flammable and toxic hazards is to pressurize the building. A pressurized building precludes infiltration of hazardous gases by establishing outward flow through the leak paths, and it purges any hazardous gases that get in. While NFPA 497 provides guidance on best practice for how to pressurize systems for occupied buildings and other enclosures, it is based on the need to meet electrical classification requirements. Pressurization of buildings allows operators to use non-classified equipment in buildings at a huge cost savings. However, the issue with this approach is that it needs a lot of air flow from outdoors into the building to accomplish the desired differential pressure, and that air is typically drawn in from a stack that is only a little higher than the roof of the building.
This is an expensive approach because pressurizing a building means conditioning a lot more air than would be required for an unpressurized building. Depending on the size of the building and how leaky it is, pressurization may take thousands of cubic feet per minute (CFM) of outside air. This means large cooling systems running continuously during the summer and large heating systems running when the weather is cold. Installing, maintaining, and operating these systems is expensive.
Typical HVAC systems recirculate most of the air and supply only about 10% of the total flow from outside air. Depending on the capacity of the HVAC system and the size and leakiness of the building, the HVAC system on a pressurized building may recirculate only about 50% of the air, and in some cases, the HVAC system is essentially a once-through design. This may become dangerous if a flammable cloud reaches the HVAC inlet, since it can be drawn into the building and almost immediately pose a vapor cloud explosion (VCE) hazard within the building. A standard recirculation HVAC system, by comparison, only creates that hazard if the concentration at the HVAC inlet is about ten times the lower flammable limit (LFL) of the material. In either case, if flammable gases may impact the HVAC inlet, it is advisable to provide flammable gas detection at the HVAC inlet and an interlock that isolates the system if flammable gases are detected at a potentially dangerous concentration. HVAC systems are typically isolated if flammable gas concentration is detected above 10% or 25% of LFL.
Use of pressurization systems can provide a false sense of safety because in most scenarios where a dangerous concentration of hazardous gases is present at ground level, the concentration at the HVAC inlet would exceed the isolation setpoint. This is true not only of flammable gases where the difference between a dangerous concentration and the HVAC isolation concentration is a factor of 4 or 10, but is very likely the case for toxic gas scenarios where the concentration at ground level may be 100 or 1,000 times the HVAC isolation concentration. In such cases, it’s hard to imagine the pressurization system remaining in service for almost any scenario involving a deadly concentration of toxic gases. Once the HVAC system is isolated, the building loses pressurization within a few seconds, and it then responds to the hazardous gas impact the same as a building that is not pressurized.
There are cases in which a pressurization system could provide protection to occupants of a building, but it would rarely be the case that a pressurization system that draws air in from an elevation of only 20 or 30 feet remains in service during an accident scenario that poses a real hazard at ground level.
For additional Shelter-in-Place information, please see the (AIChE) American Institute of Chemical Engineers published white paper, SIP Design – How Safe is Safe Enough written by Anthony Sarrack.