Fugitive emissions are the uncontrolled release of gases into the atmosphere. This is something that occurs regularly; in fact, it often happens when we fill our cars' gas tanks. The air being displaced inside the gas tank when it is being filled causes gasoline vapor emissions (hydrocarbons) to exit into the atmosphere in an uncontrolled manner. However, some states do require the use of 'Vapor Recovery' nozzles to capture these vapor emissions and return them to the gas station's underground gasoline tanks, where the vapors become liquid again, reducing the 'loss' of gasoline and creating less air pollution.
When controlling the emissions of Volatile Organic Compounds (VOCs) from stationary sources, this is done in two ways: the collection of VOC emissions (“capture”) and the removal of the collected VOC emissions from the airstream (“control”). Together, capture and control determine how much VOC is released to the atmosphere. The combination of capture and control is commonly referred to as “overall control” or control efficiency. Let's take a closer look at the capture strategies that are utilized in overall control:
The three types of capture strategies we will be discussing in this series are:
- Close Capture Hooding
- Permanent Total Enclosures (PTE)
- Temporary Total Enclosures (TTE)
In this blog, we will look at Permanent Total Enclosures (PTE).
A Permanent Total Enclosure (PTE) is just what it sounds like: a fixed installation that contains all of the VOC emissions so they can be collected and directed to a control device (such as a Thermal Oxidizer [TO], Regenerative Thermal Oxidizer [RTO], or Catalytic Oxidizer [CatOx]).
The design criteria for a PTE are set forth in EPA Method 204. There are five requirements:
- The total surface area of the Natural Draft Openings (NDOs) into the enclosure cannot exceed five percent of the total surface area of the PTE. The surface area of the PTE is defined as the combined area of all walls, ceilings, and floors of the PTE. A NDO is any opening that is permanently kept open to allow for air flow into or out of the PTE. Access doors that are normally closed do NOT count as a NDO.
- The distance between the fugitive VOC emission point nearest to each NDO must be at least four NDO diameters in length. For example, if a NDO has a diameter of two feet, the nearest fugitive VOC emission point must be at least eight feet away.
- The distance between the capture hood or duct opening nearest to each NDO must be at least four duct diameters away.
- The average face velocity at each NDO must be at least 200 feet per minute, and the air must flow into the enclosure at each NDO.
- Any doors or access panels that are not counted as NDOs must remain shut during normal operation of the emissions-generating unit.
In addition to the EPA design criteria described above, a PTE should be designed to minimize worker exposure to VOC fumes if they are going to regularly work within the enclosure. If workers are to gain access for any reason into a PTE, then the designer must account for worker exposure limits. Each solvent type has a Permissible Exposure Limit (PEL) that should be considered in the design strategy.
For example, if toluene is emitted from a process, the exhaust rate of the enclosure should be designed with sufficient air turnover such that the OSHA PEL of 200 PPMv of toluene will not be exceeded. When planning to build a PTE, the enclosure will first be designed to ensure that it meets EPA requirements (as described in items 1-5 above). Then, worst-case (maximum emission rate) conditions are considered, and the expected concentrations of individual compounds within the enclosure are calculated. If a comparison of these concentrations to OSHA PELs shows that one or more PELs may be exceeded, the PTE may need to be redesigned and/or exhaust rates may need to be increased.
A PTE can be designed to contain fugitive emissions from a storage and mixing operation.
If workers are expected to enter the enclosure to complete the task or for inspection, worker exposure limits must be considered.
In some instances, the PEL may effectively be reduced. In these cases, work supplied air systems may be used.
Here are a few tips when considering using a PTE:
- Enclose only what is necessary to meet design criteria.
- Follow good engineering practices - Consider access and workability.
- Seal all connections between the oven (or other sources of VOC emissions) and the enclosure.
- Do not underestimate the need for complete access for cleaning, make-ready, and equipment repairs - Time and labor to dismantle an enclosure is costly and takes away from production time.
- Use wire-reinforced glass or solvent resistant plastic for visual inspection areas - Safe and long-lasting windows resistant to damaging solvent vapors provide a longer, more useful life for the PTE.
- Whenever possible, use the oven, coater, or equipment as an exhausting mechanism. Try to eliminate fugitive blowers ducted directly to the control device - This simple technique lowers the air volume at the oxidizer for reduced overall costs.
- Get operator and maintenance input in design stages prior to implementation - Let the employee feel as if they designed the enclosure system, which encourages continued use after the initial installation.
- Consider electrical hazard ratings for instruments and electrical devices within an enclosure - Codes may dictate Class 1, Zone 1, or 11 ratings. The exact electrical requirements are based on the design details of the PTE.