Vent Gas Measurement and Storage Tank Emission Controls
August 8, 2016

Existing federal and state air quality and permitting regulations require oil and gas (O&G) production facilities to control emissions from crude oil, condensate and produced water storage tanks. The primary federal EPA rules for storage tanks (and other O&G sources) are in NSPS OOOO and NSPS OOOOa. These NSPS rules affect facilities construction after August 23, 2011. State environmental regulatory agencies have their own standards and permitting regulations that require storage emission controls for existing and new facilities.

These federal and state air permitting rules require the storage tank vapor controls systems to be designed and operated such that emissions are continuously controlled. Inadequately designed storage tank emission control systems that result in venting of VOCs are a focus of the EPA.

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EPA’s Special Concerns

The EPA’s Compliance Alert of September 2015 stated that vapor control system design and operation should address worst-case or highest flowrate of vent gas from the storage tank(s).

Vapor control systems should be able to handle the peak flow of flash gas that occurs when pressurized crude oil/condensate/produced water is “dumped” from the separator (or heater treater) to the atmospheric storage vessel. One “dump event” can overwhelm an inadequately designed or sized vapor control system and create back pressure that causes gas to be released from pressure relief devices (e.g., thief hatches and pressure relief valves).

Also, inadequate operation and maintenance can also reduce the effectiveness of a vapor control system allowing vent gas to be released to the atmosphere (via thief hatches) and not routed to the VRU, Vapor Combustion Unit (enclosed combustor) or flare.

Storage Tank Vent Gas Control System Design Considerations

Design considerations for vapor control systems for storage tanks include:

  1. Piping (closed vent system) is adequate to gather and send gas to the control device such as a VRU, Vapor Combustion Unit or flare.
  2. Prevent dead legs in piping that would allow condensable liquids to gather in the piping and block the piping. Piping should slope down from the storage tank to a scrubber vessel that is routed to the vapor control device.
  3. Proper design and operating pressure settings for storage tank pressure/vacuum relief devices.
  4. Proper regulation of blanket gas system operating pressure.
  5. Adequate motive force/pressure to send the gas from the storage tank to the control device (e.g., enclosed combustor, flare).
  6. Periodic inspection for leaks from closed vent piping, tank roof, thief hatches and pressure/vacuum relief devices. Alerts from leak from
  7. Proper operating and maintenance practices for vapor control systems.
  8. Re-evaluation of system when changes occur with separator pressures, throughput of oil or produced water.
  9. Measure or model the flowrate at vent gas from storage tank for the expected range of operating parameters and throughputs. Use this information to design and operate the emission control system.
  10. Control the fluctuation in generation of vent gas at the storage tank by using throttling valves for separators.
  11. Control the flow of gas to an emission control device by using a surge tank between the storage tank and the emission control system.

NSPS OOOOa mandates an assessment of the closed vent system piping that is used for NSPS OOOOa required emission controls. A qualified professional engineer must conduct the assessment and certify that the system piping is of sufficient design and capacity to send the vent gas to the control device.

Vent Gas Measurement

Measuring the flowrate of vent gas from the storage tanks can yield essential data for the proper design of vapor control systems.

HY-BON/EDI’s IQR Team routinely measures storage tank vent gas flowrates and uses that data to design our VRUs, VRTs and VCUs. IQR stands for Identify, Quantify and Rectify.

Measurement of storage tank vent gas and use of the data gathered can include:

  1. Measure over a 24-hour period to capture data on:
    1. Flowrate of vent gas based on flowrate measurements recorded every 5 to 30 seconds – depending on project scope.
    2. Day/night diurnal changes in vent gas flowrates. 
    3. Correlate the vent gas flowrates to oil production rates.
  2. Collect a sample of the vent gas to determine the mass amounts of VOCs and methane in the vent gas.
  3.  Prepare a report with measurement data graphed for permitting or vapor control system design.
  4. Use the measurement data to assist in the design of the vapor control system.

Vent gas measurement gives data on actual flowrate conditions that can be correlated with operations (e.g., throughput, separator dump events) at the facility. Besides yielding design data, the measurement can be used adjust operations or processes at a facility. Some things that the measurement can assist with include:

  1. Consider using separator throttling valves versus snap acting dump valves for separators. These would modulate the flow of fluids to the storage and minimize instantaneous maximum flow of gas generated by the storage tank.
  2. Lower separator pressure to reduce flash gas to oil ratio (GOR) in the oil sent to the storage tank. Data from measuring the flowrates at varying separator pressures can be used to lower help minimize gas wasted (vented) and maximize gas recovered by a VRU.

Additional Systems and Backup Emission Controls

One option to handle operational changes at facilities and to better ensure compliance is to use supplemental separation equipment (Vapor Recovery Towers) and to install and operate backup emission control systems. These can include:

  1. Use a Vapor Recovery Tower (VRT) to reduce the flash gas from the storage tanks and route flash gas from the VRT to a Vapor Recovery Unit (VRU). Using a VRT will lower the pressure drop that the oil or produced water experiences when entering the storage tank and this will reduce the volume of flash gas (SCF) per barrel of fluid produced to the tank.
    When using a VRT, the flowrate of vent gas from the storage tank may be low enough to simply vent to the atmosphere or the gas could be sent to a Vapor Combustion Unit (VCU) or flare to better ensure compliance.
  2. VRU controlled storage tanks can install and operate a backup VCU or flare to combust gas not recovered by the VRU. This can assist you during times when the instantaneous flowrate of gas exceeds the capacity of the VRU or during times when the VRU is not in service (e.g., maintenance, upsets)
  3. Route vent gas emissions from upsets, unexpected surges and bypasses from VRU to a VCU or flare.

Alarms and Alerts

Consider the following to enhance early detection of leaks and timely correction actions.

  1. Install UNICO UWSTM Hatch Sense System to alert field operations when a hatch is not properly sealed.
  2. As required by NSPS OOOO and OOOOa, install an alarm and alert system for times when vent gas the vapor control device is bypassed and vented to the atmosphere.

HY-BON IQR Services

Let HY-BON/EDI assist your company with your vent gas measurement and management needs. Using our IQR services and our ongoing Vent Gas Management (VGM) system we can assist you stay in compliance with storage tank emission control requirements and make your company money. HY-BON/EDI’s vent gas management (VGM) system is a cost effective way to take this issue off of your plate. We use “best in class” vapor recovery units (VRU), Vapor Recovery Towers (VRT) and vapor combustion units (VCU) to comply with storage tank emission control requirements.

Avoid regulatory compliance issues by remotely monitoring the state of tank hatches, gates, valves, and other mechanisms. HY-BON/EDI’s. Hatch Sense is the only safe, wireless thief hatch alert system with monitoring capabilities. Since we meet the UL Class 1 Div 1 requirements for intrinsically safe equipment Hatch Sense can be deployed now at your production facilities and solve problems. Operators can use Hatch Sense to demonstrate to regulators their proactive approach to minimizing leaks and increasing compliance.