Frequently Asked Questions: Storage Vessels

View frequently asked questions about EPA's 2024 final rule for oil and natural gas operations - storage vessels . On this page:

What type of storage vessels are affected by the final rule?

The criteria for identifying storage vessels that are “affected facilities” under NSPS OOOOb is located at 40 CFR 60.5365b(e). Specifically, under NSPS OOOOb, a storage vessel for which construction, modification, or reconstruction commenced after December 6, 2022, that has potential VOC emissions equal to or greater than 6 tpy or potential for methane emissions equal to or greater than 20 tpy, as determined in accordance with 40 CFR 60.5365b(e), is an affected facility subject to NSPS OOOOb requirements. The potential emissions must be calculated as the cumulative emissions from all storage vessels within the tank battery. The emissions determination may take into account a legally and practicably enforceable limit to reduce emissions below 6 tpy VOC and 20 tpy methane. The emissions determination must be made within 30 days after actions specified in 40 CFR 60.565b(e)(2)(ii) or (iii).  

A storage vessel that demonstrates for 12 consecutive months that its uncontrolled actual VOC emissions are below 4 tpy VOC and uncontrolled actual methane emissions are less than 14 tpy methane is not required to comply with the NSPS OOOOb requirement to reduce methane and VOC emissions by 95.0 %. In such case, the storage vessel must comply with the 95% reduction requirement if its actual emissions go above 4 tpy for VOC or 14 tpy for methane. 

Existing storage vessels that are modified or reconstructed may become subject to NSPS OOOOb as well. See the provisions for modification and reconstruction at 40 CFR 60.5365b(e)(3). 

Storage vessels that commenced construction before December 6, 2022, may become subject to a future State, Tribal, or Federal Plan implementing EG OOOOc. See EG OOOOc at 40 CFR 60.5386c(e)(1) and 40 CFR 60.5396c for the storage vessel designated facility applicability requirements and presumptive standards. It is important to note that EPA has allowed states and eligible Tribes 24 months to submit plans to implement EG OOOOc. In addition to the 24 months, the EPA requires that states and eligible Tribes establish compliance deadlines that require compliance with the final state/ Tribal plan within 36 months following the state plan submittal deadline. 

What does EPA mean by “legally and practicably enforceable limits” in NSPS OOOOb and EG OOOOc?

In NSPS Subpart OOOOb and EG OOOOc, a “legally and practicably enforceable limit” may be used for purposes of determining whether a storage vessel tank battery is an affected or designated facility. The criteria that must be met for a permit limit or other requirement to qualify as “legally and practicably enforceable” are as follows:  

  1. The limit is in an operating permit or other requirement established under a Federal, state, local, or Tribal authority;
  2. The limit applies to the storage vessel at the time the applicability determination is made (i.e., within 30 days after startup of production or another action specified in NSPS OOOOb at 40 CFR 60.5365b(e)(2)(ii), or EG OOOOc at 60.5386c(e)(2)(ii); 
  3. The limit has the following elements, as set forth in NSPS OOOOb at 40 CFR 60.5365b(e)(2)(i) or EG OOOOc at 60.5386c(e)(2)(ii)
    1. A quantitative production limit and quantitative operational limit(s) for the equipment, or quantitative operational limits for the equipment; 
    2. An averaging time period for the production limit, if a production based limit is used, that is equal to or less than 30 days; and,  
    3. Established parametric limits for the production and/or operational limits; where a control device is used to achieve an operational limit, an initial compliance demonstration (i.e., a performance test) for the control device that establishes the parametric limits; 
    4. Ongoing monitoring of the parametric limits that demonstrates continuous compliance with the production and/or operational limits; 
    5. Recordkeeping by the owner or operator that demonstrates continuous compliance with the limits; and,  
    6. Periodic reporting that demonstrates continuous compliance.

Is the 6 tons per year VOC standard based on the vapor or liquid manifold of the tank battery?

The 6 tons per year (tpy) standard applies to VOC as gas that is potentially emitted from the tank battery from vents or the closed vent system. It is not clear from the question how VOC and methane gas would be emitted from a liquid line closed piping system that supplies liquid to the tanks.  

The 6 tpy VOC and 20 tpy methane thresholds in NSPS OOOOb and EG OOOOc are based on the potential to emit gas vapors.  

A storage vessel affected or designated facility subject to the standards in NSPS OOOOb or EG OOOOc is defined as a tank battery that has the potential for VOC emissions equal to or greater than 6 tpy or methane emissions equal to or greater than 20 tpy. A tank battery is a group of all storage vessels that are manifolded together for liquid transfer. For purposes of this rule, a tank battery may consist of a single storage vessel if only one storage vessel is present. See 40 CFR §60.5365b(e), §60.5395b, §60.5430b “Tank battery,” §60.5386c, §60.5396c, and §60.5430c “Tank battery” for more information.

What maintenance activities could trigger a modification of an existing storage vessel making it subject to NSPS OOOOb? Would the following activities trigger modification:

Any of the above activities (as well as other activities not specifically listed above) at a well site could trigger modification, if the action resulted in an increase in liquids received by the tank battery AND the potential for VOC or methane emissions exceed the applicable threshold (equal to or greater than 6 tpy for VOC or 20 tpy for methane).  

A ‘‘modification’’ of a tank battery at a well site or centralized production facility has occurred if the potential for VOC emissions is higher than 6 tpy or 20 tpy methane after:  

  1. 40 CFR 60.5365b(e)(3)(ii)(A) A storage vessel is added to an existing tank battery; 
  2. 40 CFR 60.5365b(e)(3)(ii)(B) One or more storage vessels are replaced such that the cumulative storage capacity of the existing tank battery increases; 
  3. 40 CFR 60.5365b(e)(3)(ii)(C) an existing tank battery receives additional crude oil, condensate, intermediate hydrocarbons, or produced water throughput from actions, including but not limited to, the addition of operations or a production well, or changes to operations or a production well (including hydraulic fracturing or refracturing of the well). 

A ‘‘modification’’ of a tank battery not located at a well site or centralized production facility, including each tank battery at compressor stations or onshore natural gas processing plants, has occurred if the potential for VOC emissions is higher than 6 tpy or 20 tpy methane after: 

  1. 40 CFR 60.5365b(e)(3)(ii)(A) A storage vessel is added to an existing tank battery; 
  2. 40 CFR 60.5365b(e)(3)(ii)(B) One or more storage vessels are replaced such that the cumulative storage capacity of the existing tank battery increases; 
  3. 40 CFR 60.5365b(e)(3)(ii)(D) An existing tank battery receives additional fluids which cumulatively exceed the throughput used in the most recent determination of the potential for VOC or methane emissions. 

Are tank batteries with low associated gas emissions exempt from the rule?

Methane and VOC emissions from tank batteries are not associated gas as defined in NSPS OOOOb and EG OOOOc.  

The definition of associated gas as stated in NSPS OOOOb and EG OOOOa is: 

Associated gas means the natural gas from wells operated primarily for oil production that is released from the liquid hydrocarbon during the initial stage of separation after the wellhead. Associated gas production begins at the startup of production after the flow back period ends. Gas from wildcat or delineation wells is not associated gas. See 40 CFR; §60.5430b; §60.5430c.   

As indicated in the regulatory definition, associated gas is produced at the first stage of separation and does not include gas from storage vessels.

What does the rule mean by tank batteries not having to be “adjacent” in order to be an affective or designated facility in the final rule?

In NSPS OOOOb and EG OOOOc, the EPA revised the definition of a tank battery such that it excludes the criteria of being adjacent. The definition of tank battery in NSPS OOOOb at 40 CFR 60.5430b (and EG OOOOc model rule at 40 CFR 60.5430c) is:

“Tank battery means a group of all storage vessels that are manifolded together for liquid transfer. A tank battery may consist of a single storage vessel if only one storage vessel is present.”

Storage vessels not connected by liquid line but physically near each other are not considered a single tank battery. The question of whether a tank connected by liquid line to another tank would be considered not part of the same facility would be a matter for a formal applicability determination if there is a question as to whether the connected tanks are part of the same facility. As such, the question of whether tank batteries are or are not adjacent to one another is irrelevant to determining whether it is a covered source in the Final Rule. 

Are all liquid line tank connections considered for NSPS OOOOb applicability determinations? Since the definition refers to lines for “liquid transfers,” should we only include lines that are meant to be normally open during normal operating conditions? Examples:

    • Fill – used to send fluid to any given tank from any given dump, not open 100% of the time. One closed, one open, fill one tank at a time then swap when selling
    • Equalizing – normally closed, used to equalize tank levels as needed
    • Overflow – normally open but used only during malfunctions to prevent spills
    • Draw off – normally closed, used for tank cleanouts

A line that transfers liquids between tanks forms a tank battery; the frequency or purpose of the liquid being transferred is not relevant to whether the storage vessel or tank battery is covered under NSPS OOOOb or EG OOOOc. In cases where liquids from the same source (such as a well or separator) can be directed by liquid line to more than one storage vessel, the receiving storage vessels constitute a single battery. As defined in the rule, a tank battery means a group of all storage vessels that are manifolded together for liquid transfer. A tank battery may consist of a single storage vessel if only one storage vessel is present. See 40 CFR Sections 60.5430b; 60.5430c. 

What are the recordkeeping requirements for affected and designated storage vessel facilities under NSPS OOOOb and EG OOOOc?

The recordkeeping, reporting, and notification requirements for NSPS OOOOb begin at 40 CFR §60.5420b and for EG OOOOc at §60.5420c.

Generally, operators of storage vessel affected facilities must report annually. In each annual report, owners and operators are required to identify each storage vessel affected facility that was constructed, modified, or reconstructed during the reporting period and must document the emission rates of both VOC and methane individually. The annual report must include: deviations that occurred during the reporting period, information for control devices tested by the manufacturer or the date, and results of the control device performance test for control devices not tested by the manufacturer. 

The report also must include the results of inspections of covers and CVS, and the identification of storage vessel affected facilities (or portion of storage vessel affected facility) removed from service or returned to service. For storage vessel affected facilities which comply with the uncontrolled 4 tpy VOC limit or 14 tpy methane limit, the report must include description of the changes which resulted in the source no longer complying with those limits and the dates that the source began to comply with the 95% reduction standard. 

The annual report must also include information on control devices used to achieve the 95% reduction standard. See section X.H of the preamble to the final rule for more information related to reporting and recordkeeping for control devices. 

Required records include: documentation of the methane and VOC emissions determination and methodology, records of deviations and the duration of such deviation, records for the number of consecutive days a skid-mounted or permanently mobile-mounted storage vessel is on the site, the latitude and longitude coordinates of each storage vessel affected facility, dates that each storage vessel affected facility (or portion of storage vessel affected facility) is removed from service or returned to service, and records associated with control devices.  

For storage vessel affected facilities that comply with the uncontrolled 4 tpy VOC or 14 tpy methane limit, owners and operators must keep records of the monthly methane and VOC determination and methodology, records of changes which resulted in the source no longer complying with those limits, and the dates that the source began to comply with the 95 percent reduction standard. All associated records that demonstrate proper design and operation of the CVS, cover and control device also must be maintained (see sections X.K and X.H of the preamble to the final rule). 

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Last updated on September 17, 2024