Controlling Fugitive Emissions, Part 3: Temporary Total Enclosure

    Fugitive emissions are what we call the uncontrolled release of gases into the atmosphere. This is something that happens regularly; in fact, fugitive emissions can be released when we fill our cars' gas tanks. The air that is displaced inside a gas tank that is being filled causes hydrocarbons in the form of gasoline vapor emissions to exit from the fill spout into the atmosphere in an uncontrolled manner. Some states seek to limit these fugitive emissions by requiring gas stations to use "Vapor Recovery" nozzles which capture this vapor and return it to the gas station's underground gasoline tanks, reducing the "loss" of gasoline and creating less air pollution.  

    Controlling emissions of Volatile Organic Compounds (VOCs) from stationary sources is done in two ways: the collection of VOC emissions (“capture”) and the removal of these VOC emissions from the captured airstream (“control”). Together, capture and control determine how much VOC is released to the atmosphere, and the combination of these two methods is commonly referred to as “overall control” or control efficiency. Here on the CPI blog, we've been taking a closer look at the capture strategies that are utilized in overall control:

    Capture Strategies

    The three types of capture strategies we will be discussing in this series are:

    In this blog, we will look at Temporary Total Enclosures (TTE).   

    A Temporary Total Enclosure (TTE) is, as the name implies, a temporary installation that is used for testing purposes only. The purpose of a TTE is to collect fugitive VOC emissions that would ordinarily be emitted into ambient air. This could happen by emissions escaping through doors and windows in the plant, or by passing through the facility’s HVAC system. A TTE allows an emissions testing firm to quantify these fugitive emissions. 

    TTEs usually come in one of two types: 

    1. A truly temporary structure is built around the emission unit to be tested. This type of TTE very often consists of frames made with lumber and covered in a plastic material, such as a sheet of the polyethylene material Visqueen.
    2. A building or room can be designed to act as a TTE, under the “Building Enclosure” option. In these cases, normally open doors may be closed, passageways temporarily sealed with tarps, and/or HVAC systems turned off for the duration of the test period. As is the case with PTEs, the EPA has strict design criteria for TTEs. 

    The tests performed with a TTE to determine capture efficiency are long and, relative to other types of emissions tests, expensive. A typical TTE test involves at least four 10-hour days of work. During this time, a testing company is required to conduct three eight-hour capture test runs, in addition to other, less time-consuming tests to support the main effort. A TTE test can also be very disruptive to production, although usually much less so when the “Building Enclosure” option is chosen. 

    Given the time, expense, and disruption involved in testing for fugitive VOC emissions using a TTE, operators are well advised to ensure that their local capture systems are performing at optimum efficiency and meet all applicable regulatory and permit requirements before formal testing begins. For most processes, it is possible to accurately determine VOC capture efficiencies using information measurements before the formal emission test begins, at a fraction of the cost of formal testing. Typically, information measurements are conducted by comparing VOC usage rates to the VOC emission rates within the ductwork leading to the control device. Face velocities at hoods and other collection points can be measured using anemometers, pitot tubes, smoke tubes, and other techniques. 

    If you are considering using a TTE, here are some tips to follow: 

    • Use this approach to avoid improperly sized rooms: Too big = too much air = excessive compliance costs.
    • Rooms with specific traffic patterns may not be capable of meeting requirements when considering routine operation. Buildings with low negative pressure may be too costly when air make up systems are factored in, and the overall cost may be higher than that of a well-designed PTE.
    • Make sure to seal all windows, doors, roof vents, etc. Even room to room ventilation systems will need modifications.
    • Consider installing close capture hooding to improve the room environment. OSHA guidelines for room ventilation and air changes will apply. Get operator and maintenance input in design stages prior to implementation.
    • Make sure all processes are balanced. Designing a capture system for an unbalanced process can be costly and inefficient.  

    For more information about fugitive emissions, please read our previous blogs in this series:

    Controlling Fugitive Emissions, Part 1: Close Capture Hooding

    Controlling Fugitive Emissions, Part 2: Permanent Total Enclosure

    We are happy to answer any questions you may have in the meantime; please contact us

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    Topics: Air Pollution Control, VOC Emissions, Blog, Emissions Capture

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