February 12, 2026

Best Practices for Reducing and Controlling Methane and VOC Emissions from Crude Oil Bulk Storage Tanks

A.   Introduction

Bulk crude oil storage terminals are facilities designed to store large quantities of crude oil or condensate oil temporarily before it is transported to refineries, export terminals, or other destinations. These terminals are a part of the midstream segment of the oil and gas industry, acting as logistical hubs for crude oil distribution.

U.S. Energy Information Agency (EIA) data from February 2024, reported that U.S. net stocks (including tank farms and refineries) of crude oil storage was 302 million barrels.

Air pollution emission standards from the United States Environmental Protection Agency (EPA) and state air permitting agencies require bulk crude oil storage tanks to control volatile organic compounds (VOC) and hazardous air pollutant (HAP) emissions. Based on these EPA emission standards, companies may be required to use inherently low emitting storage tank designs (e.g., floating roof tanks) or use some type of end-of-pipe emissions control for vent gas from storage tanks (e.g., fixed roof tanks and external floating roof tanks).

This white paper aims to help achieve consistent and adequate emission reductions from storage tanks by improving understanding of the following:

Section	Section Description
A	Introduction
B	Bulk Crude Oil Storage Tank Process and Emissions
C	Air pollutants of concern
D	Sources of Air Emissions from Bulk Crude Oil Storage Tanks
E	Quantification of Storage Tank Emissions
F	Options for storage tank emission control systems
G	Best Practices for Combustion Control Devices
H	Air Quality Regulations
I	Sources of Leaks from Emission Controlled Storage Tanks
J	Summary
K	Conclusions

B. Bulk Crude Oil Storage Tanks Process 

Storage tanks operating at crude oil bulk storage facilities emit vent gas containing volatile organic compounds (VOC). VOCs are photochemically reactive organic compounds that can form ground-level ozone – a component of smog. VOCs from crude oil storage tanks at bulk storage facilities consist primarily of nonmethane, nonethane (C3+) hydrocarbons. A small portion of the VOCs include hazardous air pollutants (VOC HAPs) such as benzene, toluene, ethylbenzene, xylenes, and n-hexane. Vent gas emitted from the bulk crude oil storage predominantly consists of VOCs.

Crude oil sent to bulk storage tanks will typically be stabilized or “dead oil.”

Stabilized oil is crude oil or condensate oil that is treated with heat to drive off lighter hydrocarbons (C1-C4) from the crude oil (other than simple flashing). Stabilization processes used include simple treatment with a heater treater or partial distillation using heat with a pack column tray or packed tower column. This treatment lowers the Reid Vapor Pressure (RVP) of the oil.

Dead oil is a general term for crude oil or condensate oil that has “flashed” lighter hydrocarbon gases such as methane (CH4) and ethane. The flashing of CH4 and other lighter hydrocarbons occurs at O&G production sites when the oil takes a pressure drop between the last separator/heater treater and storage tank. Most of the lighter hydrocarbons from crude oil are generated/recovered/controlled at the O&G production well site. Also, much of the crude oil produced in the U.S. is transported by tank trucks or railcar. When the oil is transferred from the well site storage tank to the tank truck or rail car, more of the lighter components (e.g., CH4) are released from the oil. This wellsite oil arrives at the bulk crude oil storage tanks with negligible amounts of dissolved CH4 compared to crude oil stored at O&G production facilities.

Bulk crude oil storage is used to temporarily store oil prior to shipment via pipeline to a refinery, other larger regional storage hubs (e.g., U.S. Strategic Oil Reserve), blending and distribution hubs or loaded to export terminal marine vessels for transport to international markets.

The types of storage tanks used for bulk crude oil storage include:

• Fixed roof storage tanks
• Internal floating roof tanks
• External floating roof tanks

B.1. Fixed Roof Storage Tanks Process and Emissions

Fixed roof storage tanks are used at bulk crude oil storage facilities. Cylindrical, welded steel shell tanks with a permanent fixed roof are the common type of fixed roof tank used. The roof is typically cone-shaped or dome-shaped. These tanks operate at or near atmospheric pressure and temperature. These liquids can be transported offsite using pipelines, tank trucks, overwater barges, oil tankers or pipelines. The transport methods typically operate at or near atmospheric conditions of pressure and temperature.

Routine vent gas emissions from fixed roof storage tanks are categorized into the following:

• Standing losses
• Working losses

Nonroutine emissions from fixed roof tanks include:

• Tank cleaning losses

For fixed roof storage tanks containing liquid hydrocarbons, standing losses, or breathing losses, occur when hydrocarbons evaporate into the vapor space. During the day, as the vapor space or liquid warms, the vapors expand, increasing pressure within the tank. This pressure rise causes vapors to be expelled through the roof vents. These standing losses happen without any change in the liquid level inside the tank.

Working losses are due to increase in the tank’s liquid level based on the piston effect. As the oil fills the storage tank, the liquid level rises which increases the pressure in the vapor space. The increased pressure forces vapor to be expelled from the tank through the vents on the roof.

Tank cleaning losses arise during periodic tank emptying and cleaning operations. These emissions occur when vapors are released through forced ventilation of storage tanks containing crude oil or other VOC liquids. Such emissions can occur even if no physical cleaning of the tank takes place.

Facilities with multiple fixed-roof storage tanks can manage venting of vapor spaces in various ways. Each tank may have its own vent to the atmosphere. Alternatively, a closed vent system (header) can connect the vapor spaces of multiple tanks. For tanks without emission control requirements, the header may vent to the atmosphere through an elevated pipe. For tanks requiring emission controls, the header collects vapors from one or more tanks and directs them to an emission control system.

Pressure and vacuum relief valves are used to maintain the integrity of storage tanks and prevent over-pressure or implosion. However, these valves are potential sources of atmospheric leaks.

Bulk crude oil storage tanks primarily rely on automatic tank gauging systems (ATG) to measure existing volumes and changes in the amount of stored oil. However, some tanks use manual gauging, which involves measuring the oil level with a weighted tape to calculate the volume produced based on changes since the last gauging. Manual gauging is conducted through openings in the tank roof called thief hatches. These hatches can also be used for product sampling, a common practice to analyze basic sediment and water (BS&W), density, Reid Vapor Pressure (RVP), and chemical composition. When a thief hatch is open, the tank vents vapors into the atmosphere.

Many storage tanks can be retrofitted or equipped with automatic tank gauging systems to prevent emissions associated with manual gauging. Thief hatches are typically secured with spring-loaded, deadweight, or locking mechanisms to ensure they remain closed. However, they can become a source of unwanted leaks if they fail to close or seat properly after gauging or sampling.

Blanket gas systems are frequently employed as a safety measure for storage tanks to maintain a positive pressure in the tank’s vapor space. This positive pressure prevents air from entering the tank during unloading of product or cooling of the vapor space. When liquid is removed from the tank (or the vapor space cools), the blanket gas system maintains a positive pressure inside the tank by releasing gas into the tank vapor space. This gas fills the space left by the liquid removed and maintains a constant pressure inside the tank, which helps to prevent implosion. The inert gas nitrogen (N2) is a commonly used blanket gas. Pressure regulators are typically used in blanket gas systems to maintain a constant pressure in the storage tank.

Generally, the amount of routine air emissions from a fixed roof storage tank is a function of the following:

• Throughput of product
• Chemical makeup of product
• Crude oil or condensate product vapor pressure
• Storage tank dimensions
• Storage tank roof and sides color
• Insolation factor based on tank color
• Product temperature
• Geographic location air temperatures 
• Geographic location insolation factor
• Use of heated tanks
• Use of tank insulation

B.2. Floating Roof Storage Tanks Process and Emissions

Floating roof storage tanks are commonly used for bulk crude oil storage. Floating roof tanks are well suited for stabilized or dead oil applications. These tanks are commonly used in bulk crude oil storage, bulk gasoline storage and distribution facilities, natural gas processing plants, oil storage and petroleum refineries. 

When the vapor pressure of the stored crude oil equals or exceeds 11.1 psia, EPA regulations (40 CFR 60 Subpart Kb and Kc, and 40 CFR 63 subpart WW) require the use of fixed-roof tanks equipped with a closed vent system and a control device. Floating roofs are not allowed in these cases.

The two types of floating roof storage tanks include:

• External floating roof (EFR) tanks
• Internal floating roof (IFR) tanks

All floating roof storage tanks have a roof deck that rests on the surface of the stored liquid, moving up and down as product is added or withdrawn from the tank. The floating roof reduces the amount of vapor space in the tank, which in turn reduces the amount of hydrocarbon emissions that are generated and released into the atmosphere. The floating roof consists of a deck, deck fittings, and a rim seal system.

The rim seal system is attached to the deck perimeter and contacts the inside tank wall. Primary rim seals include mechanical shoe, liquid-mounted and vapor-mounted. Secondary seals installed above the primary seal may be used. These secondary seals can be flexible wiper seals or resilient filled seals (core of open-cell foam encapsulated in a coated fabric).

Routine emissions from EFR tanks and IFR tanks result from:

• Working losses
• Standing losses

Working losses in EFR and IFR tanks, also called withdrawal losses, occur as the liquid level and the floating roof descend. This process leaves a film of liquid that adheres to the inner tank walls, which then evaporates.

Standing losses in EFR and IFR tanks result from the routine evaporation of the stored liquid through the rim seal system and deck fittings.

Nonroutine emissions from EFR and IFR tanks include: 

• Floating roof landing losses

 o   Standing idle losses

 o   Filling losses

• Tank cleaning losses

Floating roof tanks minimize evaporative losses during routine operations by floating directly on the liquid’s surface. However, when the tank is emptied to the point where the roof rests on deck legs or hangers (roof landing), floating roof landing emissions occur because the roof is no longer floating. These emissions persist until the liquid level rises enough for the roof to float again.

After the roof is landed, the following floating roof landing emissions occur: 

• Standing Idle Losses occur when the floating roof lands, the dropping liquid level creates a vacuum that risks roof collapse. To equalize pressure, a breather vent (vacuum breaker) is used, forming a vapor space between the roof and the liquid. This vent may remain open until the roof refloats, allowing vapor to escape through the vent, deck fittings, and rim seal.
• Filling Losses occur while the tank is being refilled until the liquid reaches the level needed to refloat the roof.

Tank cleaning emissions result from vapors released from forced ventilation of a storage tank that contains crude oil or other VOC liquids. These occur regardless of whether any tank cleaning occurs.

B.2.1. External Floating Roof Tanks Process

The typical external floating roof (EFR) tank is an open-top cylindrical steel shell with a roof that floats on the surface of the stored liquid. The floating roof consists of a deck, deck fittings and a rim seal system. The typical floating decks use welded steel plate and can be double-deck or pontoon types.

For double-deck equipped tanks (contact deck), the deck rests (floats) on the surface of the stored liquid. For pontoon floating decks (noncontact deck), the deck rests on pontoons that float on the surface of the stored liquid. For both types of decks, the decks move up and down as product is added or withdrawn from the tank. The floating roof reduces the amount of vapor space in the tank, which in turn reduces the amount of hydrocarbon emissions that are generated and released into the atmosphere.

EFR tanks experience wind-induced emissions because of rim seal evaporative losses. In contrast, no wind loss mechanism has been identified for rim seal losses in IFR tanks or domed EFR tanks.

Deck drains are used to remove rainwater from the floating deck.

Due to the design of EFR tanks, they are not equipped with any type of secondary control system other than good operating work practices.

B.2.2. Internal Floating Roof Tanks Process

IFR tanks use a floating roof within a permanently fixed roof enclosure. The fixed roof can be either column-supported, resting on internal columns within the tank, or self-supported, requiring no internal columns for structural integrity. Self-supporting fixed roof tanks include domed floating roof tanks. Domed IFR tanks include EFR tanks that were retrofitted and IFR tanks equipped with a domed roof. Many air quality regulations consider a domed EFR tank a type of IFR tank.

The function of the fixed roof or domed roof is to block the wind and stop wind induced evaporative losses.

The deck in internal floating roof tanks rises and falls with the liquid level and either floats directly on the liquid surface (contact deck) or rests on pontoons several inches above the liquid surface (noncontact deck). Deck materials include aluminum, steel and resin-coated, fiberglass reinforced polyester (FRP). Many aluminum internal floating roofs currently in service have noncontact decks, according to the EPA AP-42, Chapter 7: Liquid Storage Tanks.

Working and standing evaporative losses accumulate in the vapor space between the floating roof and the fixed roof. These vapors can either be vented to the atmosphere or directed to a closed vent system connected to an emission control device, such as a vapor recovery unit, enclosed combustion device, or flare.

IFR tanks generally are not equipped with secondary control systems beyond adherence to proper operating practices for the floating roof system. However, some IFR fixed roof or dome roof tanks can be configured to route vent gas to a combustion device, such as an enclosed combustor or flare.

B.3. Loading Losses Process and Emissions

In addition to storage tank emissions there are associated loading losses. Loading losses refer to the evaporative emissions that occur when crude oil or refined petroleum product is transferred (loaded) into tank trucks, rail tank cars, or marine barges. These emissions are a source of VOCs released during petroleum handling operations.

The primary cause of emissions during loading operations is the displacement of vapor-laden air from the receiving vessel (tank truck, railcar, barge) as liquid crude oil fills the tank. The displaced vapors can be categorized as follows:

• Residual Vapors – Leftover vapors from previous cargo that remain in the tank.
• Newly Generated Vapors – Vapors formed due to the agitation and evaporation of fresh crude oil during the loading process.

These vapors escape into the atmosphere if not captured and controlled.

Loading losses and their emission control strategies will be covered in a separate blog post by Cimarron.

C. Air Pollutants of Concern

The air pollutants of concern for crude oil and condensate storage tanks include the following:

D. Sources of Air Emissions from Bulk Crude Oil Storage Tanks 

As stated earlier the categories of emissions for bulk crude oil storage tanks are listed below.

Emissions from fixed roof storage tanks include:

• Standing (breathing) losses
• Working losses
• Tank Cleaning Losses

Emissions from floating roof tanks include:

• Standing (breathing) losses
• Working losses
• Roof Landing Losses

 o   Standing idle losses during roof landing

 o   Filling losses during roof landing

• Tank Cleaning Losses

E. Quantification of Storage Tank Emissions

Quantifying storage tank emissions is necessary to ensure compliance with air quality permitting and regulatory requirements. Precise emission quantification also facilitates the selection and design of appropriately sized emission control systems for the facility.

E.1. Direct Measurement Tank Emissions – Fixed Roof Tanks and IFR Tanks

Some operations use direct measurement to determine the vent gas flowrate from fixed roof tanks and IFR tanks. Direct measurement collects data to determine the minimum, maximum, and average flowrate of vent gas. This data can be used to size an emission control system.

Neither the EPA nor the industry has published reference methods specifically for the direct measurement of emissions from storage tanks.

Typical direct measurement of storage tank vents involve:

1.    Measurement of vent gas flowrate from a hatch of a single tank or multiple tanks that share vapor spaces through a common closed vent piping system.

2.    Make sure that all vent gas flows through the sample piping meter run. Use audio, visual, or olfactory (AVO) methods or optical gas imaging (OGI) to check for leaks.

3.    Use a thermal mass flow meter (commonly used) or ultrasonic meter to measure the low pressure vent gas.

4.    Measure vent gas flowrate over a 24-hour period to obtain data on minimum, maximum and average flowrate.  Measurements should be recorded at least every 5 minutes.

5.    Collect a sample of the vent gas and send it to a laboratory for an extended chemical analysis. The laboratory should analyze the sample for CO2, C1 through C10+.

Check with your State regulatory agencies to determine if they accept direct measurement for vent gas quantification for air permitting.

E.2. Fixed Roof and Floating Roof Vent Loss Quantification – AP-42

Standing and working losses of VOCs are calculated using methods found in the EPA AP-42, Compilation of Air Pollutant Emissions Factors, Fifth Edition, Volume I Chapter 7: Liquid Storage Tanks.  The emission equations and associated factors in AP-42, Chapter 7 were developed by the American Petroleum Institute (API) and updated in October 2024. These methods are used to calculate emissions from fixed and floating roof storage tanks storing products such as crude oil, condensate, and other VOCs and chemical mixtures (e.g., gasoline).

Also in October 2024, the Environmental Protection Agency (EPA) released the TANKS 5.1 web application. Tanks 5.1 uses the storage tank calculation methods in AP-42, Chapter 7. TANKS 5.1 calculates standing, working and tank cleaning losses for fixed roof tanks and standing, working, landing and tank cleaning losses for IFR and EFR tanks.

The data needed for the calculations include product throughput, tank color, dimensions, roof type, exterior surface color, operating pressure, product vapor pressure and temperature and atmospheric conditions of pressure and temperature and customized chemical makeup data for organic liquids.

All state regulatory agencies accept the methods in AP-42, Chapter 7 for calculating tank emissions.

Process simulation software such as ProMax and Aspen HYSYS include the methods in AP-42, Chapter 7 for calculating standing and working losses from storage tanks.

E.2.1. Speciation of Tank Emissions

Some air permitting agencies and emission inventories require speciation of storage tank vent gas released. Speciation refers to identifying and quantifying the individual chemical components present in the emitted vent gas. Speciation of tank venting emissions is used to calculate the amounts of individual HAPs in the vent gas. For fixed-roof and internal floating roof (IFR) tanks, this can be achieved through direct sampling and chemical analysis of the vent gas, if feasible. In cases where vent gas analysis data is unavailable, AP-42, Chapter 7 provides a methodology to speciate vent gas emissions using Raoult’s Law and Antoine’s Equation to estimate individual component concentrations. Detailed equations and data for this approach can be found in AP-42, Chapter 7, section 7.1.4. Speciation Methodology.

Process simulation software such as ProMax and Aspen HYSYS include methods to speciate storage tank vent gas.

F. Options for Storage Tank Emission Control Systems

The commonly used emission controls for fixed roof storage tanks and crude oil loading facilities include:

• Thermal Oxidation Systems

 o   Thermal oxidizers

 o   Vapor Combustion Units (marine and onshore facilities)

 o   Incinerators

 o   Flares (open-tipped)

• Vapor Recovery Systems

 o   Vapor recovery system, also called vapor recovery unit or “VRU”

The floating roof of an IFR tank is considered an emissions control method by environmental regulations. Secondary emission controls for IFR tanks are not commonly used, unless needed to meet air permitting requirements. If secondary controls are needed, then a thermal oxidation system can be used for IFR tanks for certain applications.

Environmental regulatory agencies often use the term “flaring” for the combustion of waste gas using a flare or an ECD.

F.1. Thermal Oxidizers

If it is not possible to recover hydrocarbon vapors from storage tanks or loading operations, thermal oxidizers can be used. Various types of thermal oxidation systems fall under this term. Direct fired thermal oxidizers are simple combustion systems that use natural or forced draft burners to achieve the necessary temperature and residence time for high destruction efficiencies (typically 98–99.99%). These systems are engineered to precisely regulate air and assist gas flow, maintaining optimal combustion conditions within the chamber.

Thermal oxidizers use some type of enclosure around the flame.  Then can be used in sensitive locations requiring minimal flame visibility, low emissions, reduced heat and noise output. The systems allow for easy emissions sampling and can be configured in vertical or horizontal orientations based on process requirements and available space. Systems are optimized to operate smokeless.

Fixed and portable thermal oxidizers can be used to control tanks cleaning operations for fixed roof, IFR and EFR tanks.

Applications by Cimmaron include low NOx burners and continuous emission monitoring systems (CEMS).

F.2. Vapor Combustor Units

Vapor combustor unit (VCU) are a type of thermal oxidizer commonly used to control emissions from storage tanks and  loading operations.

VCUs are primarily designed to manage waste gases generated during gasoline and refined product terminal loading. They are also applicable in ethanol facilities and landfills, handling waste gas vapors with varying hydrocarbon content. Compared to open flares, VCUs feature a larger diameter combustion chamber, ensuring sufficient residence time to achieve the desired destruction efficiency, usually between 98-99.9%. For lean waste gas streams, assist gas is required to facilitate combustion. For very lean (endothermic) waste gas streams, it is often best advised to implement an Incinerator.

In many applications, a vapor combustor provides a safe and cost-effective solution for managing vapors generated during the handling, storage, or loading of volatile liquid hydrocarbons or similar substances. VCUs are specifically designed to safely burn hydrocarbon vapor mixtures, whether or not the mixture falls within the flammability range of the vapors being combusted. These units are commonly utilized for waste streams requiring high destruction rate efficiency (DRE), such as:

• Tank breathing and cleaning facilities
• Truck loading terminals
• Rail loading terminals
• Marine loading facilities
• Barge cleaning and depressurizing facilities

Fixed and portable VCUs can be used to control tanks cleaning operations for fixed roof, IFR and EFR tanks.

F.3. Incinerators

An incinerator is a type of thermal oxidizer designed to destroy volatile organic compounds (VOCs)  by combusting vapors at high temperatures within a controlled chamber. Incinerators are typically selected for lean waste gas streams or where very high destruction efficiency may be needed. It typically features precise control of temperature, residence time, and oxygen levels. Incinerators can achieve destruction efficiencies up to 99.99%. Incinerators are commonly used in plant and industrial settings where high-level emission control is required, such as at crude storage, refined product storage terminals or chemical processing facilities.

F.4. Flares

Flares, also called open-tipped or candlestick flares, are used to control waste gas from fixed roof storage tanks. There are various flare designs depending on the application and chemical makeup of the gas sent to the flare. Flare designs can be unassisted, air-assisted, pressure-assisted and steam-assisted and serve low-pressure and high-pressure gas sources.

Fixed and portable flares can be used to control tanks cleaning operations for fixed roof, IFR and EFR tanks.

Flares used to comply with certain EPA emission standards and State air permits are required to meet EPA regulations in 40 CFR 60.18.

Historically, the default destruction efficiency value used for flares was 98%.

One important consideration for combustion-based emission controls is the preference for enclosed systems—such as VCUs, thermal oxidizers, incinerators, and enclosed flares—over open flares. This is largely because many terminals are located near populated areas and communities typically oppose visible flames or light emissions. Additionally, from a risk management perspective, exposed flames from flares present greater safety concerns, especially at crude oil storage or fuel-handling facilities.

F.5.1. Vapor Recovery Systems – Fixed Roof Storage Tanks

Vapor recovery systems, often called vapor recovery units (VRU), are used to recover vent gas from O&G storage tanks. The typical VRU receives low pressure vent gas and, depending on the application, separates inerts if present through an adsorption step, then will either condense or absorb the vent gas to recover it into liquid form. VRUs can recover 98% or more of storage tank vent gas depending on the tank product and system sizing.

Some facilities use a flare or ECD as a backup emission control device when the VRU is out of service.

G. Best Practices for Combustion Control Devices

Some design and monitoring practices for flares and ECDs that can maximize the control efficiency and are also required by EPA and environmental regulatory agencies include the following:

Design Practices

• Use a continuously burning pilot flame and a system to continuously detect pilot flame. Send alerts to operations if pilot is not operating.
• Storage tank vent gas contains a high percentage of higher molecular weight hydrocarbons (pentanes plus, C5+). These hydrocarbons can condense in closed vent piping between the storage tank and emission control device, creating a blockage. To prevent this, follow the following practices:

 o   Use a separator to condense and collect condensable liquids upstream of the control device.

 o   Design the vent piping between the storage tank and emission control device with a diameter large enough to accommodate the potential instantaneous maximum flow of vent gas.

 o   To prevent liquids from condensing and blocking the closed vent piping, use a downward-sloping pipe from the storage tank to a separator upstream of the control device.

• Ensure that the vent gas has enough motive force to reach the flare tip or ECD burner. Blowers can be used to provide motive force to the vent gas.
• 40 CFR 60.18 requires the following net heating value (NHV) of vent gas being combusted:

 o   200 BTU/scf or greater for nonassisted flares

 o   300 BTU/scf or greater for air-assisted or steam-assisted flares

NOTE: Typical fixed-roof storage tank vent gas will have a BTU/scf that is greater than 1500.

• Design flares to meet the exit velocity requirements in EPA regulations in 40 CFR 60.18.

Monitoring Practices

• Continuously measure and record flare gas flowrate using a meter. If vent gas volumes exceed expected flowrates, then investigate to determine if corrective actions are needed.
• Continuously detect and record the presence of a pilot flame or combustion flame using monitoring devices, such as thermocouples, ultraviolet beam sensors, or infrared sensors.
• Use an automated alert system to notify company of pilot or combustion flame malfunctions so timely corrective actions can be taken.
• Periodically monitor the device discharge for visible smoke emissions using EPA Method 22 (40 CFR 60). Take needed corrective actions to eliminate visible emissions.
• Implement a leak detection and repair (LDAR) program for closed vent system piping, thief hatches and pressure/vacuum relief valves using methods such as:

 o   Periodic auditory, visual or olfactory (AVO) methods

 o   Periodic EPA Method 21 leak monitoring

 o   Continuous monitoring using infrared spectroscopy, thermal imaging or tunable diode laser spectrometer (TDLS) sensors or other EPA approved methods

G.1. Best Practices for Vapor Recovery Systems – Fixed Roof Storage Tanks

Some design and monitoring practices to use for vapor recovery systems include the following:

Design Practices

• Design vent piping between storage tanks and emission control devices with a diameter large enough to handle the potential instantaneous peak flow of vent gas.
• Use a separator to condense and collect condensable liquids upstream of the control device.
• To prevent liquids from condensing and blocking the closed vent piping, use a downward-sloping pipe from the storage tank to a separator upstream of the control device.
• Maintain the temperature of vent gas above its dew point to prevent liquids from condensing and degrading the operation of the vapor recovery compressor.

Monitoring Practices

• Continuously measure and record vent gas flowrate to the VRU using a vent analyzer.
• Implement a leak detection and repair (LDAR) program for closed vent system piping, thief hatches and pressure/vacuum relief valves using methods such as:

 o   Periodic auditory, visual, or olfactory (AVO) methods

 o   Periodic EPA Method 21 leak monitoring

 o   Continuous monitoring using infrared spectroscopy, thermal imaging or tunable diode laser spectrometer (TDLS) sensors or other EPA approved methods.

H. Air Quality Regulations

Air quality regulations for bulk crude oil storage and loading facilities primarily target emissions of VOCs and HAPs. The following EPA air quality regulations apply to storage tanks operating at bulk storage facilities.

• 40 CFR 60 Subpart K – Standards of Performance for Storage Vessels for Petroleum Liquids for Which Construction, Reconstruction, or Modification Commenced After June 11, 1973, and Prior to May 19, 1978
• 40 CFR 60 Subpart Ka—Standards of Performance for Storage Vessels for Petroleum Liquids for Which Construction, Reconstruction, or Modification Commenced After May 18, 1978, and Prior to July 23, 1984
• 40 CFR 60 Subpart Kb—Standards of Performance for Volatile Organic Liquid Storage Vessels (Including Petroleum Liquid Storage Vessels) for Which Construction, Reconstruction, or Modification Commenced After July 23, 1984, and On or Before October 4, 2023
• 40 CFR 60 Subpart Kc – Standards of Performance for Volatile Organic Liquid Storage Vessels (Including Petroleum Liquid Storage Vessels) for Which Construction, Reconstruction, or Modification Commenced After October 4, 2023
• 40 CFR Part 63, Subpart CC: National Emission Standards for Hazardous Air Pollutants (NESHAP) for Petroleum Refineries. Only applies to storage tanks operating at refineries.

Rules in 40 CFR 60 Subparts OOOO, OOOOa, OOOOb and OOOOc do not apply to crude oil bulk storage facilities. These rules apply to crude oil production facilities, which includes the well and extends to the point of custody transfer to the crude oil transmission pipeline or any other forms of transportation.

Currently, there are no specific Subparts of 40 CFR Part 98 – Mandatory Greenhouse Gas Reporting that apply to bulk crude oil storage facilities. These would have to report GHGs if associated with a petroleum refinery (40 CFR 98 Subpart Y) or suppliers of petroleum products (40 CFR 98 Subpart MM). Bulk crude oil storage facilities are not a part of 40 CFR 98 Subpart W.

I. Sources of Leaks from Emission Controlled Fixed Roof Storage Tanks

Emission controls for storage tanks include vapor recovery systems, flares and enclosed combustion devices along with the closed vent piping systems that connect the vapor spaces of the tanks to control devices.

 The following are potential leaks to the atmosphere from storage tanks equipped with emission controls.

• Condensation of heavier hydrocarbons can block the closed vent piping of a storage tank causing the tank to vent to the atmosphere via the pressure/vacuum relief valve or thief hatch. This can occur in areas where the pipe is cooled (e.g., underground), areas with pipe constrictions or where the slope of the pipe between the tank and the emission control device.
• Thief hatches that are left open when not in use, inadequately seated hatches and over-pressuring of tanks causing the hatch to open and vent gas.
• Pressure/vacuum relief valves venting due to over-pressuring of the storage tank and mechanical or material failure.
• Blanket gas regulator pressure setting not optimized for the storage tank operating conditions.

Periodic or continuous leak detection and repair (LDAR) monitoring systems, along with an alert system, can be used to detect and stop leaks in a timely manner. To prevent recurrence of leaks, companies can perform a root cause analysis to identify the cause of the leak and take appropriate steps.

J. Summary

This white paper provides an in-depth analysis of air emissions and control methods associated with bulk crude oil storage tanks.  These bulk crude oil storage facilities serve as temporary storage hubs for crude oil before transportation to refineries, export terminals, or distribution facilities. The U.S. Energy Information Administration (EIA) reported crude oil storage levels exceeding 302 million barrels as of February 2024, emphasizing the significant role of these facilities in the energy supply chain.

To comply with U.S. Environmental Protection Agency (EPA) and state air pollution standards, bulk crude oil storage facilities must control emissions of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). These controls include inherently low-emission designs such as floating roof tanks and end-of-pipe solutions like vapor recovery systems, flares, and enclosed combustion devices for fixed roof tanks.

The white paper addresses the following key aspects:

1. Emissions sources and pollutants of concern, including VOCs and HAPs.
2. The operational processes and emissions associated with fixed roof and floating roof storage tanks.
3. Quantification methods for storage tank emissions, including EPA-approved approaches and direct measurement techniques.
4. Best practices for emission control systems, focusing on combustion devices and vapor recovery units.
5. Key regulations impacting bulk crude oil storage, such as 40 CFR 60 (Subparts K, Ka, Kb, and Kc) and relevant portions of 40 CFR 63 and 40 CFR 98.
6. Potential leak sources in emission-controlled storage tanks and the role of leak detection and repair (LDAR) programs in minimizing emissions.

This guidance aims to help operators ensure compliance, optimize emission controls, and implement best practices to reduce air pollution from bulk crude oil storage operations.

K. Conclusions

Effective emissions control for storage tanks requires a combination of advanced technologies, adequate monitoring, and adherence to best design and operational practices. These measures enable companies to address regulatory requirements, minimize environmental impacts, and contribute to broader sustainability goals. By implementing technologies such as VRUs, flares and other combustion devices, alongside continuous monitoring and preventative measures, companies can achieve significant reductions in VOC and HAP emissions. Proactive management of storage tanks not only ensures regulatory compliance but also positions the industry as a responsible contributor in reducing air pollution.

Cimarron – Who We Are

With decades of operating history and innovation across our trusted brands, Cimarron provides technology-driven emissions management solutions for the global energy system. Our leading-edge products, services, and real-time monitoring systems reduce emissions, optimize operations, ensure regulatory compliance, and drive sustainability progress for our customers operating in oil & natural gas production, energy storage & distribution, renewables & biogas, coal mine methane, and certain industrial end markets.

Cimarron boasts a collection of well-established technologies which have been assembled and innovated from trusted industry brands. Our vast global experience, spanning tens of thousands of equipment installations, serves as a testament to our ability to achieve success in every project upon which we embark.

Cimarron is headquartered in Houston, Texas with approximately 550 employees serving our global customer base. In addition to being present in all major regions in the U.S., Cimarron operates across more than 45 countries around the world. We support our customers from sales, engineering, manufacturing, and field service locations across the United States, Italy, India, England, and the United Arab Emirates, further supported by our network of international partners.

Please contact us to learn more about our products and services and about all our solutions at sales@cimarron.comor visit our websitewww.cimarron.com.