In September 2015, the EPA published a Compliance Alert addressing concerns for VOC emissions from onshore O&G storage vessels using emission control devices (VRUs, combustors, flares). The Alert discusses engineering and maintenance practices that can improve compliance and reduce air emissions. It also summarizes many of the good practices that O&G operators already use. It is not definitive guidance on good practices and following the guidance does not guarantee compliance with air quality regulations.
Technician checking for leaks
This information is timely since O&G operators must comply with the current federal NSPS OOOO regulations and any state specific regulations regarding tank emissions. The expectation is that the EPA will continue to make NSPS OOOO and storage tank emissions a priority in enforcement inspections. Implementing these practices can enhance compliance and help with negotiations with regulators should an operator have a compliance issue to resolve.
You can download a copy of the EPA Compliance Alert here: Alert LINK
Some of the engineering solutions and maintenance considerations for storage vessel discussed in the Alert include:
- Reduce Liquid Pressure Prior to Transferring the Liquid to Atmospheric Storage Vessels
- Adequate Diameter of Piping Used for Vent Lines to Control Device
- Prevent Liquid Collection in Vent Lines
- Eliminate Unintentional Natural Gas Carry-Through
- Ensure Proper Maintenance and Set Points for Pressure Relief Valves
- Minimize Venting from Thief Hatches
- Adequate Sizing of Emission Control Devices
Below is a brief summary of ways to improve emission control performance as taken from the Alert. Consult the Compliance Alert for full details.
1. Crude Oil/Condensate/Produced Water Pressure Drop
To reduce flash gas volumes and peak flowrates of vent gas during separator dumps of oil/produced water to storage vessels, use multiple stages of separation. This allows the system to operate with a smaller pressure drop between the last stage of separation (low pressure separator/heater treater) and an atmospheric tank.
Of course, to reduce air emissions, the gas liberated by the intermediate stages of separation must be collected and sent to the system (fuel, sales pipeline) and not vented to the atmosphere.
HY-BON’s IQR Emission Survey includes an optional assessment of an oil and gas process to optimize pressure drops to atmospheric tanks and reduce flash emissions.
2. Adequate Diameter of Piping Used for Vent Lines to Control Device
Use vent piping between storage tanks and emission control devices that has a diameter designed to handle the potential instantaneous peak flow of vent gas including flash gas during separator dumps. If the piping is inadequate, then a portion of the gas will not be collected by the VRU and/or combustor. This will increase the chance of creating back pressure on the storage tank and result in venting to the atmosphere at the thief hatches and/or pressure relief valves (PRV).
HY-BON includes this as a standard design criteria when sizing VRUs and combustors for a facility.
3. Prevent Liquid Collection in Vent Lines
The collection efficiency of vent gas control systems will be reduced if rich gas in the vent line between the tank and emission control device condenses and collect in vent lines – especially in low spots along the path.
In HY-BON’s VRU and combustor lines, we recommend using a sloping piping of adequate inner diameter from the tank that is routed to a drip pot (i.e., scrubber) to ensure that liquids do not collect in the line creating a blockage. Also, the scrubber can remove liquids that can harm vapor recovery compressors and cause smoking conditions in enclosed combustors/flares.
4. Eliminate Unintentional Natural Gas Carry-Through
When storage tank pressure relief devices (PRD) are opening and venting gas on a regular basis due to pressure increase in storage vessel and this caused by unintentional natural gas carry-through, take corrective action to reduce/stop venting.
This can be due to pressure increase during normal separator dump events and can also occur from separator dump valves stuck in open position (i.e., valve failed to reseat) and leaking gas into a tan.
If repeated PRD venting is not from unintentional natural gas carry-through, the following corrective actions are offered:
- Increase the PRD pressure set points if there is sufficient margin between the set point and the rated pressure of the storage vessel to do so while continuing to safeguard storage vessel integrity-
- Take steps to decrease the liquid’s pressure drop experienced at the storage vessel
- Replace the tank with a storage vessel that is rated to a higher pressure and use higher pressure set points.
HY-BON’s design services will take into account production rates, operating pressures in sizing VRUs and combustors. Our IQR services include onsite inspections for carry-through of vent gas due to stuck dump valves.
5. Ensure Proper Maintenance and Set Points for Pressure Relief Valves
By design, pressure relief valves (PRVs) are safety devices that protect vessels from over-pressurization and should remain closed during normal operations. They are not process vents that should discharge during normal operations.
The EPA alert states that PRVs should have a pressure setting that is low enough to protect vessel structural integrity and avoid over-pressurization. Also, the pressure setting should be high enough to exceed storage vessel operating pressures during normal operation.
When a PRV is found to be venting to the atmosphere, actions should be taken to verify proper valve reseating after opening.
For PRVs that are opening on a frequent basis, the operator should determine the cause. Actions to take can include
- Increase the pressure set point for the PRV. This requires an adequate safety margin between the set point and the rated pressure of the storage vessel.
- Decrease the operating pressures experienced at the storage vessel.
- Use a storage vessel rated to a higher pressure and then use a high pressure set point for the PRV. Always consider safety issues with any pressure relieving devices used and operational settings.
HY-BON’s design services will take into account production rates, operating pressures in sizing VRUs and combustors. Our IQR services include onsite inspections, measurement and sampling of leaking PRVs and thief hatches.
6. Minimize Venting from Thief Hatches
Inspect gauging/thief hatches and pressure relief devices regularly to ensure good seals. Install quality gaskets on thief hatches and regularly inspect those gaskets to ensure a tight seal. Implement procedures to ensure thief hatches are properly closed after vessel gauging, sampling and unloading.
HY-BON routinely inspects and replaces gaskets for leaking thief hatches and pressure relief devices as a part of our IQR services.
7. Proper Sizing of Emission Controls
Ensure that vent gas control devices are properly designed/sized for the specific facility’s operations. The design should be sized and operated to control for the full range of gas flowrates that are expected.
Key to ensuring proper sizing of emission controls is appropriate sampling, measurement and/or modeling to estimate potential maximum flow of vent gas from storage tanks.
HY-BON’s engineers can run process simulation calculations to estimate the potential range of flowrates (min/max) of vent gas for various operating scenarios. The assessment will take into account production rates, storage vessels used and operating pressures in sizing VRUs and combustors.
8. Sampling and Modeling to Estimate the Potential Peak Flow of Emissions
To estimate flash emissions, the Alert references the California Air Resources Board (CARB) “Test Procedure – Flash Emissions of Greenhouse Gases and Other Compounds from Crude Oil and Natural Gas Separator and Tank Systems” see: www/arb/ca/gov/cc/reporting/ ghg-rep/regulation/mrr-2013-clean/pdf (Appendix B).
HY-BON’s Complete Solution
HY-BON’s engineered vapor recovery units (VRU), vapor recovery towers (VRT) and enclosed vapor combustion units (VCU) along with our IQR and Leak Detection and Repair (LDAR) services offer a complete package for operators to comply with the storage vessel emission control requirements. We design and size our VRUs and VCUs meet and exceed the requirements of this EPA guidance.