Most of us know that a properly designed and installed ventilation system is our FIRST option for controlling hazardous concentrations of flammable vapors, gases, fumes, mists or dust, and volatile or airborne materials posing a health hazard, such as toxic or corrosive materials. And thankfully, most workstations and areas that need proper ventilation have it these days; however, we tend to find these systems were often times designed and/or installed by unqualified personnel and thus oftentimes do not FULLY COMPLY with ACGIH or AIHA standards, and thus they may not be as effective as needed. Here is an easy checklist to determine if your ventilation system meets “code”:
1) The design and operation of the exhaust system MUST be such that flammable contaminants are diluted in non-contaminated air to maintain concentrations in the exhaust flow BELOW 25% of the contaminant’s LFL/LEL.
In other words, a direct solution is to restrict the concentration of all known contaminants of concern to a specified and acceptable level. That “level” is below 25% of the LFL/LEL for a particular flammable. Where the concentration of a flammable contaminant in the air is maintained < 25% of the LFL/LELfor that contaminant, there is insufficient fuel vapor in the gas phase to sustain homogeneous ignition. Managing the contaminant(s) in a diluted state means there is less chance of catastrophic flame propagation, ignition, or explosion. Because maintaining acceptable contaminant levels is not easily or exactly determined, the services of concentration that triggers the requirement for the hazardous exhaust system prescribes the operating parameters of the system.
2) Hazardous exhaust systems MUST be INDEPENDENT of other types of exhaust systems.
The intent is to PROHIBIT the combining of hazardous and nonhazardous exhaust systems. To minimize the potential for spreading hazardous exhaust to other parts of a building, hazardous exhaust systems MUST NOT connect with any other exhaust system of the building. Without COMPLETE ISOLATION, the fire, health, and explosion hazards inherent in hazardous exhaust systems cannot be confined only to the hazardous system and could jeopardize other systems and other parts of the building.
For example, an exhaust system conveying flammable vapors MUST NOT share ducts or exhaust equipment with a toilet room exhaust system.
This does not, however, prohibit multiple hazardous exhaust ducts of the same type from connecting to a common trunk, riser, or system.
It also does not prohibit dissimilar hazardous systems from sharing common ducts if the dissimilar exhausts are compatible and the intermixing of their exhausts does not increase the overall hazard. When combined, dissimilar exhausts could react to form an exhaust mixture that is more combustible, flammable, explosive, or otherwise more hazardous than the individual components.
3) INCOMPATIBLE materials MUST NOT be exhausted through the same hazardous exhaust system.
Hazardous exhaust systems MUST NOT share common shafts with other duct systems, except where such systems are hazardous exhaust systems originating in the SAME FIRE AREA.
Exception: The provisions of this section may not apply to laboratory exhaust systems. See code Section 510.5 Incompatible materials and common shafts
This section has two (2) separate requirements:
- one related to connecting together (manifolding) of hazardous exhaust ducts and
- the other related ducts sharing common shafts up through the building
The second sentence says that HVAC ductwork and nonhazardous exhaust ducts CANNOT BE ENCLOSED IN THE SAME SHAFT ENCLOSURE with the hazardous exhaust system and ducts.
4) Systems for removal of vapors, gases, and smoke shall be designed by the CONSTANT VELOCITY or EQUAL FRICTION methods. Systems conveying PARTICULATE MATTER shall be designed employing the CONSTANT VELOCITY method.
Exhaust systems conveying vapors, gases, and smoke must be designed using either the constant velocity or the equal friction method.
In the equal friction method, ducts are sized for a constant pressure loss per unit length.
In the constant velocity method, ducts are sized for a constant velocity per unit length to maintain minimum contaminant transport velocities.
MINIMUM TRANSPORT VELOCITY is the velocity required to transport particulates without settling. The American Conference of Governmental Industrial Hygienists (ACGIH) Industrial Ventilation—Manual of Recommended Practice lists some generally accepted transport velocities as a function of the nature of the contaminant. Duct velocities can be higher than the minimum transport velocities but can never be lower. Round ducts are preferred because they offer a more uniform air velocity to resist settling of material and can withstand the higher static pressures normally found in exhaust systems.
For additional guidance on duct sizing using one of the methods above, consult a standard design handbook such as ASHRAE’s Handbook of Fundamentals or the ACGIH Industrial Ventilation—Manual of Recommended Practice.
5) The design of the system shall be such that the emissions are confined to the area in which they are generated by air currents, hoods, or enclosures and shall be exhausted by a duct system to a SAFE LOCATION or treated by removing contaminants.
Exhaust air systems are either general systems that remove air from large spaces or local systems that capture heat, vapors, gases, fumes, mists, or dust at source-specific locations within a room or space. Hazardous exhaust systems MUST be designed to prevent the spread of contaminants beyond the area of origin. Confinement is accomplished using controlled air currents or barriers such as hoods, booths, and similar enclosures.
Additional information concerning procedures for evaluating and controlling contaminant levels can be found in ACGIH Industrial Ventilation—Manual of Recommended Practice or ACGIH Threshold Limit Values for Chemical Substances in the Work Environment.
6) Hoods or enclosures MUST BE used where contaminants originate in a limited area of space. The design of the hood or enclosure shall be such that air currents
created by the exhaust systems will capture the contaminants and transport them directly to the exhaust duct.
When sources within the building generate hazardous contaminants, direct exhaust through hoods is MORE EFFECTIVE than control by general ventilation (dilution). We are required to install an exhaust hood or enclosure at the location of greatest concentration to improve the capture ability of the exhaust system.
Most often, this is at the source of contamination such as a particular piece of equipment or appliance or a particular process or operation such as evaporation; plating; container filling; welding; chute loading of conveyors; crushing; cool or hot shakeout processes; grinding; blasting or tumbling.
The intent is to maximize capture efficiency by preventing contaminants from spreading beyond the immediate area of the source.
7) The velocity and circulation of air in work areas MUST BE such that contaminants are captured by an airstream at the area where the emissions are generated and conveyed into a product-conveying duct system. Contaminated air from work areas where hazardous contaminants are generated shall be diluted BELOW THEIR SAFETY THRESHOLDS with air that does not contain other hazardous contaminants.
Dilution air may contain contaminants, as long as they are NOT “hazardous” contaminants. Capture velocities are air velocities at the point of contaminant generation upstream of the hood or inlet. The contaminant enters the airstream at the point of generation and is conducted along with the air into the hood and from there directly to the exhaust duct system. This section also prescribes the maximum contaminant concentration allowed in occupant work areas where the contaminants are generated. At lower levels, the contaminant is considered diluted or innocuous.
8) MAKEUP AIR shall be provided at a rate approximately equal to the rate that air is exhausted by the hazardous exhaust system. Makeup air intakes shall be properly located and sized. See my more detailed articles:
- The Ventilation Design Basis begins with our intakes
- FRESH AIR intakes for a ventilation system need to be from a source of FRESH AIR!
- FRESH AIR intakes for a ventilation system need to be from a source of FRESH AIR! (Part II)
Exhaust flow can occur only if the air is constantly supplied to replace the air being exhausted. The air exhausted from a hazardous exhaust system must be replaced with air at the REQUIRED EXHAUST FLOW RATE. Hazardous exhaust systems are usually designed with the quantity of makeup air being SLIGHTLY LESS than that exhausted, thereby creating a slight negative pressure that helps confine contaminants to the area of origin.
The introduction of makeup air is critical to the proper operation of all hazardous exhaust systems. When the make-up air is provided via a mechanical makeup air source, the hazardous exhaust system should be electrically interlocked and controlled by a single start switch to make certain makeup air is supplied when the exhaust hood is in operation.
Makeup air intakes must be located at sufficient distances from exhaust outlets to prevent exhaust discharge from contaminating the makeup air.
9) Hazardous exhaust duct systems shall extend directly to the exterior of the building and shall not extend into or through ducts and plenums (i.e., space between the structural ceiling and the dropped ceiling or under a raised floor)
The intent of this section is to minimize the potential for spreading hazardous exhaust to other parts of the building as a result of duct leakage or failure. In the event of a duct fire or explosion, other areas of the building could be jeopardized. The intent is to REQUIRE ROUTING OF HAZARDOUS EXHAUST DUCTS TO THE OUTDOORS AS DIRECTLY AS PRACTICABLE, thereby avoiding unnecessary duct lengths and travel through other spaces. In all cases, ducts conveying hazardous exhaust MUST NOT EXTEND INTO OR THROUGH OTHER DUCTS OR PLENUM SPACES.
10) Fire dampers and smoke dampers are PROHIBITED in hazardous exhaust ducts.
Fire and smoke dampers MUST NOT be installed within hazardous exhaust systems because a closed damper will prevent all or part of the exhaust system from functioning and could possibly create a dangerous condition in the room or space where the exhaust system originates. The purpose of the hazardous exhaust system is to dilute and remove the hazardous materials so that they are not a threat to the building occupants. The installation of fire or smoke dampers may adversely affect the operation of the exhaust system, thereby preventing the exhaust system from achieving its intended purpose. Additionally, the materials conveyed in hazardous exhaust ducts could damage, impair or obstruct the dampers, seriously affecting their performance.
Hazardous exhaust systems are permitted to penetrate fire-resistance-rated assemblies, other than firewalls, and fire and smoke dampers are not required at those penetrations because protection is provided by the fire-resistance-rated enclosure required by the IMC and IBC.
11) Hazardous exhaust systems that penetrate a floor/ceiling assembly MUST BE enclosed in a fire-resistance-rated shaft.
To reduce the risk of spreading fire from the hazardous exhaust system to other parts of the building, the hazardous exhaust system must be enclosed in a fire-resistance-rated shaft enclosure from the point where the system penetrates a floor level to the termination outdoors. A MINIMUM 1-hour fire-resistance-rated shaft enclosure is required even though the floor/ceiling assembly itself may not be fire-resistance-rated.
The requirements for constructing the shaft enclosure and for the minimum fire-resistance ratings are given in the IBC. In addition to reducing the risk associated with fire, the rated enclosures will also help protect the duct from physical damage and may provide some protection against the spread of hazardous exhaust in the event of an explosion or duct failure.
12) Ducts shall be protected with an approved automatic fire suppression system installed in accordance with the International Building Code.
Exceptions:
- An approved automatic fire suppression system shall not be required in ducts conveying materials, fumes, mists, and vapors that are nonflammable and noncombustible under all conditions and at any concentrations.
- Automatic fire suppression systems shall not be required in metallic and noncombustible, nonmetallic exhaust ducts in semiconductor fabrication facilities.
- An approved automatic fire suppression system shall not be required in ducts where the largest cross-sectional diameter of the duct is less than 10 inches (254 mm).
- For laboratories, as defined in Section 510.1, automatic fire protection systems shall not be required in laboratory hoods or exhaust systems.
13) Ducts used to convey hazardous exhaust MUST BE constructed of materials approved for installation in such an exhaust system and shall comply with one of the following:
- Ducts shall be constructed of approved G90 galvanized sheet steel, with a minimum nominal thickness as specified in Table 510.9.
- Ducts used in systems exhausting nonflammable corrosive fumes or vapors shall be constructed of nonmetallic materials that exhibit a flame spread index of 25 or less and a smoke-developed index of 50 or less when tested in accordance with ASTM E84 or UL 723 and that are listed and labeled for the application.
Where the products being exhausted are detrimental to the duct material, the ducts shall be constructed of alternative materials that are compatible with the exhaust.
14) Ducts shall be supported at intervals NOT EXCEEDING 10 feet. Supports shall be constructed of NONCOMBUSTIBLE MATERIAL.
So there we have it, 14 items we can visually verify to help ensure our hazmat ventilation systems are designed, constructed, and operated properly so as to ensure we are achieving maximum protections with our ENGINEERING CONTROLS. Don’t be fooled by what you see at first glance; use these basic requirements to ensure what is in place is actually proper for its intended purpose.