With the influx of new and repositioned life science buildings coming to market in recent years, a key attribute leading to the success of these buildings has been the ability to maximize available quantities for tenant chemical storage. To achieve this, centralized chemical storage rooms are often provided (designed as either Control Area or Group H, High Hazard storage rooms), allowing chemicals to be remotely stored without detracting from the maximum allowable quantities (MAQ’s) of chemicals available for use within tenant spaces.
Within such rooms, a component that requires significant thought and coordination is the ventilation systems used to mitigate dangerous vapor accumulation. Chemical storage rooms that accommodate small, normally closed vessels may not need as robust of a ventilation system as rooms containing large vessels and/or dispensing operations. Of the latter, two distinct ventilation system types are often found in the larger chemical storage applications:
- Group H Exhaust (per International Mechanical Code [IMC] Section 502.8), for rooms with large chemical quantities, and
- Hazardous Exhaust (per IMC Section 510), for rooms with open use operations that generate dangerous vapors.
Group H Exhaust (IMC 502.8.1) is required for any indoor storage area where the quantity of chemicals exceeds the MAQs per Control Area. The requirements for a Group H Exhaust system are outlined in IMC Section 508.1.1, and include, but are not limited to:
- Exhaust rate of 1 CFM per square foot of room area
- High/low exhaust (within 12” of the ceiling/floor depending on the weight of expected fumes),
- Manual ventilation shutoff switch exterior to the room,
- Continuous operation of equipment, etc.
Also note that specific hazards such as compressed/cryogenic gases, fumes posing health hazards (and associated point-collection systems), and other unique hazards may require additional protection.
Hazardous Exhaust (IMC 510), on the contrary, is not dependent on the MAQs. Rather, IMC 510 is applicable where any one of the following thresholds may be exceeded during normal operations, in the absence of the exhaust system running:
- A flammable vapor, gas, fume, mist, or dust is present in concentrations exceeding 25 percent of the lower flammability limit of the substance at the expected room temperature;
- A vapor, gas, fume, mist, or dust with a health-hazard rating of 4 is present in any concentration.
- A vapor, gas, fume, mist, or dust with a health hazard rating of 1, 2, or 3 is present in concentrations exceeding 1 percent of the medial lethal concentration of the substance for acute inhalation toxicity.
One of the major features of a Hazardous Exhaust system is that it must be independent from other ventilation systems, typically requiring separation by rated shaft construction and/or fire-rated ducts. Other design requirements may include internal duct suppression, redundant fans, standby power, etc., which should be reviewed with respect to the installation and design goals.
The misconception we often see with the two system types is that they are either thought of as one in the same, or mutually exclusive. To help clarify some ambiguity surrounding these system types, we offer the following:
- A Hazardous Exhaust system (IMC 510) may sometimes be required in a non-Group H (i.e. Control Area) occupancy, depending on the operations taking place.
- A Group H room with Group H Exhaust (IMC 502.8) may sometimes also require a local Hazardous Exhaust system (IMC 510) given the operations taking place within.
- Where both types of systems are warranted, it’s possible to achieve both goals with a single HVAC system designed to meet both criteria.