Ensuring The Proper Ventilation Sweep for Hazardous Materials
Richard Palluzi
Pilot Plant and Laboratory Engineering, Safety, and Design Consultant at Richard P Palluzi LLC
If you look at almost any standard or code they will somewhere, someplace refer to the need for adequate ventilation of any area containing hazardous materials. I have tried to address the concept of adequate ventilation in several previous articles:
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One subject that frequently arises in actual field inspections is the need for the exhaust ventilation to adequately sweep or clear the area to ensure there are no locations where hazardous vapors can accumulate or collect. This need is often mentioned in the most common codes. For example:
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NFPA 30 The Flammable and Combustible Liquids Code in Chapter 18 Dispensing, Handling, Transfer, and Use of Ignitible (Flammable or Combustible) Liquids section 18.6.3 says “The location of both the exhaust and inlet air openings shall be arranged to provide air movement across all portions of the floor to prevent accumulation of flammable vapors.”
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NFPA 497 Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas in section 5.5.5(2) notes a location that might be considered for a lower, unclassified rating if it has adequate ventilation and “is substantially open and free of obstruction to the natural passage of air, either vertically or horizontally”.
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API 500 Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2 in section 6.3.2.4.5 says” Provisions need to be made for the introduction of air in a manner to properly distribute ventilation; that is, air should not flow directly from the air inlet to the air outlet (short-circuited) without removing air previously within the enclosed area, or from the air outlet back into the air inlet.”
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The 2021 International Fire Code (IFC) says in Chapter 50 Hazardous Materials in section 5004.3.1 (6) “The location of both the exhaust and inlet air openings shall be designed to provide air movement across all portions of the floor or room to prevent the accumulation of vapors.”
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Even a cursory reading of these and other codes show frequent references to the need to avoid accumulation of hazardous vapors with flammable and combustible liquid vapors and flammable gases being the area of most concern.
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But what does this mean in practical terms? HVAC designers try very hard to maximize personnel comfort, minimize energy use, and – of course – ensure safety. Many otherwise experienced designers have limited experience with handling hazardous gases and vapors. So, unfortunately, they often overlook problems with common designs that can allow accumulations to exist. This is aggravated by field decisions, and contractor modifications that often make the problem worse. Most of these errors are made because the personnel involved do not understand the basic concept of sweeping an area to prevent hazardous gas or vapor build up.
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To properly sweep an area and prevent build up of hazardous gas and vapor accumulations, ideally the supply should be directly across from the exhaust. It should ensure that all spaces between the two (supply and exhaust) are properly cleared (swept) of any hazardous gas and vapor. To do that properly requires careful attention to several things.
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1.??? Placement of the supply and the exhaust locations
2.??? The number of supply and exhaust locations
3.??? Anything in the way that impedes or blocks the proper exhaust movement
4.??? Any area factors that can change the desired exhaust movement.
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Placement
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The supply should always be as far from the exhaust as possible and, ideally, located in a position that the air flow moves in a relatively straight path to the exhaust. While that sounds easy many things often create issues. The farthest location may not be desirable as other equipment uses the space. The area configuration can be such that significant extra work is required to run the duct to the idea positions leading to less optimum solutions for a lower cost (AKA “value engineering” to those who do not have to live with the operational and safety impacts).
Providing supply at high locations near the roof of a taller area to meet this requirement is undesirable to an HVAC designer as the distance down to where personnel normally operate can be so far that their comfort is adversely affected. Conditioned supply air (cold or hot) rapidly equilibrates to ambient losing significant effectiveness. More conditioned air, and hence larger chillers, boilers and even fans, is required, raising capital and operating costs. As a result, designers often invariably recommend that the supply inlets be lowered to closer to the operating floor. While this reduces costs, it also greatly reduces effectiveness.
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Figure 1 shows a plan and elevation view of an ideal ventilation scheme for a room. It tacitly assumes a taller ceiling, typically 15 ft off grade or more. With this type of layout, you would expect the entire area to be well swept (with some issues near the units or equipment discussed below).
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In Figure 2, a cost conscious designer has removed the top and bottom supply vents recognizing that neither will contribute much, if anything, to the occupant’s comfort. The designer has reduced the supply duct cost by at least 30%. However, the lack of supply air at the top and the bottom of the space can easily lead to potential accumulations as shown in red.
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?Number
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More is always better in distributing exhaust but realistically more is also more expensive and space intensive. As a result, there is always a tendency to reduce the number of supply and exhaust points to the bare minimum to save costs. This reduction, however, often creates potential hazards often not recognized by the designer or owner. HVAC designers extremely rarely use CFD modeling to identify the proper duct locations. Even the few times this effort is performed it is often not very realistic in its results. As shown in Figure 3 below, there is now the potential for another area where hazardous gas and vapor could accumulate.
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Every supply and exhaust has somewhat of a dead space to the top, bottom and sides where the sir movement is more restricted. Fewer locations increase the quantity and to some degree size of these dead spaces. Too few locations almost always result in significant areas of concern.
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Congested equipment
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Units or equipment in the space between the supply and the exhaust can also create problems. If the density of equipment is high enough and/or the spacing around individual components small enough, air flow will be reduced as air seeks the path of least resistance. This reduction in exhaust flow in areas around equipment, where it is most often needed, makes its effectiveness significantly less. This is well recognized. Many design guides and codes suggest a reduction in effectiveness from 2 to 5 depending on the degree of congestion. (So, a designer should consider the airflow 20 to 80% less effective than their calculations would indicate requiring 200 to 500% more exhaust to achieve the same effectiveness.) ?Failing to recognize and address this issue can lead to a significantly increased risk of hazardous gas and vapor accumulating around components most likely to be ignition sources.
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Practically speaking the equipment location and density is usually fixed by operations and cannot be adjusted by the HVAC designer. However, it is critical that the designer recognize this reduced effectiveness and compensate for it by providing either higher flow rates, more supply and exhaust locations, or better supply and exhaust locations. This issue has even been noticed in such high velocity areas as laboratory hoods as noted in ?“Why Don’t We Just Put It in the Hood?”: Issues with Degrading Hood Effectiveness Due to Equipment Placement, (ACS J. Chemical Health & Safety, Jan, 2020) and “Why Can’t We Put It In the Hood?”, https://www.dhirubhai.net/pulse/why-cant-we-put-hood-richard-palluzi/.
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Area Effects
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Many things about the area can also adversely affect the effectiveness of the exhaust. A large garage door or even a standard personnel door that is frequently left open for equipment movement or operator comfort can short circuit the entire exhaust pattern by drawing in supply air from this huge opening and leaving significant spaces within the area effectively without exhaust. Fans and similar ventilated equipment can easily create very different exhaust patterns which push supply and exhaust away from certain areas to others. This can create pockets in the areas not adequately swept.
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Trying to prove that a proposed design has issues is always difficult. The only foolproof way to show a design does or does not work is to build it and conduct a larger scale smoke test in the actual space. This is not only expensive but if the test fails the corrective measures usually range from very expensive to prohibitively expense forcing the organization to live with a recognized problem. I have seen many organizations be forced to significantly change their operations or accept a much higher than desired level of risk. Hence prevention is a much more effective mitigation.
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How does one avoid these problems? There are a few simple guidelines.
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Many other configurations and local issues can reduce the effectiveness of an exhaust to such a point that hazardous accumulations of gases and vapors can easily be created. Circumstances that allow this to occur are often much more prevalent and frequent than designers, owners, and safety people recognize. While the limited examples above can only show some blatantly obvious examples I have seen hundreds of others in my 49 years of experience. So, when someone suggests that the proposed HVAC design does not sweep the area you should pay careful attention and take proactive steps to avoid it.