Enclosed Vapor Combustors Units and Air Quality Regulations
January 6, 2017

Vapor combustion units (VCU) also called enclosed combustion devices or enclosed flares are widely used in oil and gas operations to control vent gas VOC and methane emissions. VCUs are a good alternative control device when a vapor recovery unit (VRU) is not feasible for the application. Many operators prefer VCUs over candlestick (open tipped) flares since properly designed and sized VCUs do not have a visible flame, minimize or eliminate any smoke and have a far greater destruction efficiency.


Below is some valuable information regarding regulatory requirements, design considerations and uses of VCUs used in oil and gas production and midstream operations.

Applicable Regulations for VCUs

  • EPA federal NSPS OOOO and OOOOa
  • State environmental regulatory air quality standards and permitting rules.

State Air Permit Considerations – VCUs and Flares

  • VCUs installed for State-only air permits (e.g., non-NSPS OOOO/OOOOa) have fewer requirements for monitoring, recordkeeping and reporting. VCUs treated similar to candlestick (open-tipped) flares in many States.
  • VCUs can be a better option than a candlestick flare for meeting facility permit by rule (PBR) and general air permit VOC limitations.
    • Performance tested VCUs can have destruction efficiency of 99.9% or greater
    • Candlestick flares have a presumed destruction efficiency of 95 to 98% (depending on the State agency).
  • Lower VOC emissions (>=1.9%) from the VCU can ensure a facility’s emissions are below the PBR and general permit VOC limitations.
  • The EPA is required by a recent consent decree  to review/update candlestick flare emission factors (e.g., VOCs).

 NSPS OOOOa Requirements for VCUs

  • Destruction efficiency greater than or equal to 95%
  • Mandatory performance testing for VCUs used for storage tanks and compressor seals as follows:
    • One time manufacturer performance testing (EPA List of Performance Tested Combustion Control Devices) or
    • In-field initial performance testing by operator and after every 60 months of service – only required for models not performance tested by manufacturer
    • NOTE: no testing required for VCUs installed to comply with NSPS OOOO
  • Operate VCU within design parameters (e.g., minimum/maximum flowrate) specified by manufacturer. Follow manufacturer’s written operating instructions, procedures and maintenance schedule
  • Install and operate a continuous burning pilot flame
  • Use a monitoring device that continuously indicates the presence of the pilot flame while emissions are routed to the control device
  • No visible smoking or soot emissions
  • Conduct monthly inspections/tests for:
    • Pilot is lit when vapors are sent to the VCU
    • Visible (smoking) emissions test using EPA Method 22 for VCU exhaust
    • Monthly olfactory, visual and auditory (OVA) inspections for system integrity
  • Use an alarm system for bypasses of control device
  • Storage tanks routed to VCU mush have thief hatches equipped with a weighted mechanism or equivalent to ensure that the lid remains properly seated
  • Closed vent system (piping) used must:
    • Have a qualified professional engineer design and certify closed vent system – required by NSPS OOOOa only
    • Route all vent gas from storage tanks to the VCU
    • Conduct initial and annual inspection of tank hatches and closed vent system operating with no detectable emissions (leak free) using EPA Method 21

Destruction Efficiency

Performance tested combustors have been demonstrated to have >99% destruction efficiency based one NSPS OOOO/OOOOa performance testing.

HY-BON/EDI’s Vapor Combustion Unit (VCU) has a demonstrated destruction efficiency of ≥ 99.99%

Reasons to Use VCUs Instead of Candlestick Flares

  • No exposed flame improves safety and eliminates visible flame to public
  • Typically have greater destruction efficiency than candlestick (open tipped) flares
  • Properly designed and sized VCU’s can eliminate smoking issues

 Typical Emission Sources Controlled With VCUs

  • Storage tanks (crude oil, condensate, produced water)
  • Vapor recovery towers (VRTs)
  • Heater treater flash gas
  • Emergency and maintenance depressuring from:
    • Compressor blowdown
    • Facility blowdown
    • Low and high pressure separators

 Design Best Practices

  • Understand the process conditions before selecting the combustor. Data on the following needed:
    • Equipment routed to combustor
    • Vent gas operating pressure and temperature
    • Gas flowrates – minimum, maximum and average normal
    • Chemical makeup of gas
    • Pilot gas supply source and pressure
  • Ensure that storage tanks routed to combustors operate in a leak free manner (e.g., no leaking thief hatches) so that all gas is routed to the combustor. Consider using UWSTM Hatch Sense to detect thief hatch leaks.
  • Ensure sufficient motive force/pressure to send gas to the combustor
  • Use a flame/detonation arrestors
  • Use a knock-out vessel (scrubber) with liquids transfer pump upstream of combustor to remove free liquids – this will help minimize smoking emissions
  • Use a bird screen at burner tube exit
  • Use a burner control system connected to company communication for monitoring combustor operation
  • Ensure pilot sufficient for the application. Some State air quality regulations allow sparking devices in lieu of pilot flame – provided the VCU flame is monitored when gas sent to combustor
  • Method to ensure the presence of a pilot flame (if used) or presence of flame when gas routed to the combustor

 Other Use Options

  • For storage tank applications, consider combining a vapor combustion unit (VCU) with a vapor recovery tower (VRT) and vapor recovery unit (VRU). The VCU can be a backup control device to the VRU.
  • Also, a VCU could be used to control storage tank vent gas from a storage tank that uses a VRT to reduce flash losses.

HY-BON/EDI Solutions

Contact HY-BON/EDI regarding your vapor combustion unit (VCU) needs. Features of our VCUs include:

  • EPA 40 CFR 60, NSPS OOOO/OOOOa (Quad O) Compliant 
    EPA List of Performance Tested Combustion Control Devices
  • Completely Enclosed Combustion
  • 99.99% Destruction Efficiency
  • User Friendly Automated System
  • Operational and Quad O reporting data can be saved to a USB Key
  • RS-232 or RS-485 Communication supports satellite, cellular, or radio
  • Modbus Slave Protocol allows it to communicate with SCADA systems and other devices/software