Scenario: You have a hazardous material that has a really low PEL and STEL and the process where it is going to be used is an “open process”. Management ask you to perform a hazard assessment and it is determined that the room where this material will be used will need ventilation to ensure exposures are properly managed. Where do you turn to find your ventilation/exhaust system design?
OSHA’s 1910.94, Ventilation standard is VERY LIMITED in its application; in fact, the standard would NOT apply to the scenario above, as it only applies to:
- Abrasive blasting
- Grinding, polishing, and buffing operations
- Spray finishing operations
Even using OSHA’s webpage dedicated to ventilation, we see a gap in what we need and what OSHA provides. Can you guess where I am going to take us?
Yep, the International Mechanical Code. Compliance required? Maybe, but I am after SAFETY, I’m not worried about compliance with some government agency at this point!
I have used pieces of the IMC in some of my previous articles where we discuss ventilation; in this article, I want to walk us through the design of a {tip title=”Exhaust System” content=”An assembly of connected ducts, plenums, fittings, registers, grilles, and hoods through which air is conducted from the space or spaces and exhausted to the outdoor atmosphere.“}mechanical exhaust system{/tip} that would be needed for the scenario above. First, what are the difference between a ventilation system and an exhaust system?
NOTE: Links provided are to FREE access to the 2018 IMC.
The IMC defines the following:
[Mechanical] EXHAUST SYSTEM – An assembly of connected ducts, plenums, fittings, registers, grilles, and hoods through which air is conducted from the space or spaces and exhausted to the outdoor atmosphere.
VENTILATION – The natural or mechanical process of supplying conditioned or unconditioned air to, or removing such air from, any space.
A mechanical exhaust system must comply with Chapter 4, Ventilation, and Chapter 5 Exhaust Systems of the IMC.
The code is fairly broad in its application, but as we can see our scenario meets its application…
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SECTION 502 – REQUIRED SYSTEMS 502.1 General. An exhaust system shall be provided, maintained, and operated as specifically required by this section and for all occupied areas where machines, vats, tanks, furnaces, forges, salamanders and other appliances, equipment, and processes in such areas produce or throw off dust or particles sufficiently light to float in the air, or which emit heat, odors, fumes, spray, gas or smoke, in such quantities so as to be irritating or injurious to health or safety. |
NOTE: emphasis by me
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502.1.1 Exhaust location. The inlet to an exhaust system shall be located in the area of the heaviest concentration of contaminants. |
This is FUNDAMENTAL to the design of the system and in order for us to meet this basic need we MUST know how the contaminate will behave (e.g. vapor/gas density). To be effective, the system intake MUST be located where the airflow will pick up, entrain, and transport the contaminant away from the worker. The inlet has to be as close as possible to the source of contamination and should be designed to capture the contaminants before they can disperse into the space.
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502.1.4 Hazardous exhaust. The mechanical exhaust of high concentrations of dust or hazardous vapors shall conform to the requirements of Section 510. |
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510.1 General. This section shall govern the design and construction of duct systems for hazardous exhaust and shall determine where such systems are required. Hazardous exhaust systems are systems designed to capture and control hazardous emissions generated from product handling or processes and convey those emissions to the outdoors. Hazardous emissions include flammable vapors, gases, fumes, mists or dust, and volatile or airborne materials posing a health hazard, such as toxic or corrosive materials. For the purposes of this section, the health-hazard rating of materials shall be as specified in NFPA 704. For the purposes of the provisions of Section 510, a laboratory shall be defined as a facility where the use of chemicals is related to testing, analysis, teaching, research or developmental activities. Chemicals are used or synthesized on a nonproduction basis, rather than in a manufacturing process. |
emphasis by me
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510.2 Where required. A hazardous exhaust system shall be required wherever operations involving the handling or processing of hazardous materials, in the absence of such
Exception: Laboratories, as defined in Section 510.1, except where the concentrations listed in Item 1 are exceeded or a vapor, gas, fume, mist or dust with a health hazard rating of 1, 2, 3 or 4 is present in concentrations exceeding 1 percent of the median lethal concentration of the substance for acute inhalation toxicity. |
Section 510.2 specifies where the installation of a hazardous exhaust system is required based on the stated conditions. This section contains the criteria to determine whether a given operation requires a hazardous exhaust system. If, in the absence of an exhaust system, the normal operation would exceed the threshold concentrations indicated, a hazardous exhaust system is REQUIRED.
The hazardous exhaust system is intended to keep the threshold levels from being reached, either in the room containing the point source (for all types of hazardous materials) or in the exhaust duct system (for flammability/explosion hazards).
The criteria are based on NORMAL OPERATING CONDITIONS, not on accident conditions. It is important to note and emphasize that the threshold concentrations
are without the exhaust system operating. Section 510.3 requires the exhaust system to operate below threshold concentrations; therefore, the threshold limits are determined without the exhaust system operating.
Condition # 1 above requires a hazardous exhaust system where flammable vapor, gas, fume, mist, or dust reaches concentrations that exceed 25% of the LEL of the substance for the range of anticipated space temperature(s).
Condition # 2 above requires a hazardous exhaust system where the general health hazard posed by a single exposure to a vapor, gas, fume, mist, or dust is classified with a health hazard rating of 4 in accordance with NFPA 704.
Condition # 3 above requires a hazardous exhaust system where the concentration of a particular vapor, gas, fume, mist, or dust with a Health-Hazard rating of 1, 2 or 3 is present in excess of 1% of the LC50 of the substance for acute inhalation toxicity.
I am skipping the requirements for Lumber yards and woodworking facilities as that is another type of exhaust system dealing with combustible dusts. I will cover this matter in a future article.
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510.3 Design and operation. The design and operation of the exhaust system shall be such that flammable contaminants are diluted in noncontaminated air to maintain concentrations in the exhaust flow below 25 percent of the contaminant’s lower flammability limit. |
501.3 contains performance-based criteria to quantify the amount of exhaust in relation to the control of the contaminant source. 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 <25% of the LEL for a particular contaminant. Where the concentration of a flammable contaminant in air is maintained below 25% for that contaminant, there is not enough vapor/gas to sustain ignition. Managing
the contaminant(s) in a diluted state means there is less chance of catastrophic flame propagation, ignition, or explosion. The quantity or volume of exhaust required is source-specific.
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510.4 Independent system. Hazardous exhaust systems shall be independent of other types of 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. The intent is to prohibit the combining of hazardous and nonhazardous exhaust systems. 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, as defined by the IFC, and the intermixing of their exhausts does not increase the overall hazard.
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510.6 Design. Systems for the 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. |
I am not going to get into this, as this is more about moving particulates rather than hazardous gases and vapors.
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510.6.3 Hoods required. Hoods or enclosures shall be used where contaminants originate in a limited area of a 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). This section REQUIRES THE INSTALLATION OF AN EXHAUST HOOD OR ENCLOSURE AT THE LOCATION OF THE GREATEST CONCENTRATION to improve the capture ability of the exhaust system. The intent is to maximize capture efficiency by preventing contaminants from spreading beyond the immediate area of the source.
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510.6.4 Contaminant capture and dilution. The velocity and circulation of air in work areas shall 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 the thresholds specified in Section 510.2 with air that does not contain other hazardous contaminants. |
The references to “contaminated air” and “hazardous contaminants” make clear that this section is dealing with mixtures of environmental air plus hazardous contaminants. The section does not apply, for instance, to simply objectionable odor contaminants or to gaseous mixtures that are part of the processes. Rather than addressing only the flammability hazard, this section also addresses the health hazard, to be consistent with Section 510.2, because it applies to (occupied) work areas.
Dilution air may contain contaminants, as long as they are not “hazardous” contaminants. The code recognizes that the minimum transport velocity required for the capture of large particles, such as mists or dust differs from that required for contaminants, such as vapors, gases, or fumes, and must be considered in the design of exhaust systems to ensure that capture and direct transport to the exhaust duct system is achieved.
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.
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510.6.5 Makeup air. 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 located in accordance with Section 401.4. |
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. The mechanical makeup air source and 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.
The remaining parts of Section 510 cover how we run the ducts, passing through walls and floors, duct construction, etc., which we will hire a ventilation company to design these systems for us. But so you know where to find the info, here is what I am leaving out:
510.6.6 Clearances
510.6.7 Ducts
510.7 Penetrations
510.7.1 Fire dampers and smoke dampers
510.7.2 Floors
510.7.3 Wall assemblies
510.7.4 Fire walls
510.8 Suppression required
510.9 Duct construction
510.9.1 Duct joints
510.9.2 Clearance to combustibles
510.9.3 Explosion relief (Systems exhausting potentially explosive mixtures)
510.10 Supports