Volatile Organic Liquid Storage Vessels - Including Petroleum Storage Vessels: NSPS - Frequent Questions

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These frequently asked questions (FAQs) do not impose legally binding requirements on the U.S. Environmental Protection Agency, states, Tribes, or the regulated community, nor do they confer legal rights or impose legal obligations upon any member of the public. The Clean Air Act provisions and the EPA rules and regulations described in this document contain legally binding requirements. These FAQs do not constitute a rule or regulation, nor do they change or substitute for any Clean Air Act provision or EPA rules or regulations.

The general answers provided here may not apply to a particular situation based upon the circumstances. Interested parties are free to raise questions about the substance of these FAQs and the appropriateness of their application to a particular situation. EPA will evaluate particular situations as they arise in the future on a case-by-case basis. These FAQs are a living document and may be revised periodically without public notice.

This page contains answers to frequently asked questions about 40 CFR part 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. The final rule was published in the Federal Register and became effective on October 15, 2024.  

What constitutes a “modification” under NSPS subpart Kc?

The General Provisions for 40 CFR Part 60 - Standards of Performance for New Stationary Sources define the term “modification” as “any physical change in, or change in the method of operation of, an existing facility which increases the amount of any air pollutant (to which a standard applies) emitted into the atmosphere by that facility or which results in the emission of any air pollutant (to which a standard applies) into the atmosphere not previously emitted.” 40 CFR 60.2. 

The General Provisions address the criteria for modifications at 40 CFR 60.14(a), which states “any physical or operational change to an existing facility which results in an increase in the emission rate to the atmosphere of any pollutant to which a standard applies shall be considered a modification within the meaning of section 111 of the Act. Upon modification, an existing facility shall become an affected facility for each pollutant to which a standard applies and for which there is an increase in the emission rate to the atmosphere.” 40 CFR 60.14(a) also provides for exceptions to this general definition of changes that constitute a modification, as identified at 40 CFR 60.14(e)-(f): 

(e) The following shall not, by themselves, be considered modifications under this part: 

(1) Maintenance, repair, and replacement which the Administrator determines to be routine for a source category, subject to the provisions of paragraph (c) of this section and § 60.15. 

(2) An increase in production rate of an existing facility, if that increase can be accomplished without a capital expenditure on that facility. 

(3) An increase in the hours of operation. 

(4) Use of an alternative fuel or raw material if, prior to the date any standard under this part becomes applicable to that source type, as provided by § 60.1, the existing facility was designed to accommodate that alternative use. A facility shall be considered to be designed to accommodate an alternative fuel or raw material if that use could be accomplished under the facility's construction specifications as amended prior to the change. Conversion to coal required for energy considerations, as specified in section 111(a)(8) of the Act, shall not be considered a modification. 

(5) The addition or use of any system or device whose primary function is the reduction of air pollutants, except when an emission control system is removed or is replaced by a system which the Administrator determines to be less environmentally beneficial. 

(6) The relocation or change in ownership of an existing facility. 

(f) Special provisions set forth under an applicable subpart of this part shall supersede any conflicting provisions of this section. 

For the purposes of New Source Performance Standards (NSPS) subpart Kc, EPA developed special provisions consistent with 40 CFR 60.14(f) that specifically apply to and supersede the exemption listed under 40 CFR 60.14(e)(4). EPA previously had cited this provision to assert that a change in the type of material stored in a storage vessel is not, by itself, a modification if the storage vessel is capable of accommodating the storage of the new material. As part of the NSPS subpart Kc review, EPA determined that if changing the volatile organic liquid (VOL) stored results in a greater maximum true vapor pressure (MTVP) than the previously stored VOL, then a modification occurs because of the resulting higher emissions rate. In addition, EPA determined that VOLs stored in the storage vessel are neither raw materials nor fuel inputs to a process at the affected facility itself and clarified that the new provisions, finalized under 40 CFR 60.110c(e), would preempt the exemption provisions at 40 CFR 60.14(e)-(f). The new provision at 40 CFR 60.110c(e) states: 

  (e) For the purposes of this subpart,  

 (1) The phrase “change in the method of operation of, an existing facility which increases the amount of any air pollutant” in the definition of modification in § 60.2 or “operational change to an existing facility which results in an increase in the emission rate to the atmosphere of any pollutant to which a standard applies” in § 60.14(a) means a change in operation occurs if the storage vessel is used to store a VOL that has a greater maximum true vapor pressure than all VOL historically stored or permitted; and  

 (2) The exemption in § 60.14(e)(4) regarding alternative fuel or raw material does not apply to storage vessels.  

As stated above, pursuant to 60.14(f), the finalized provision 40 CFR 60.110c(e), which establishes modification standards consistent with the statutory definition and based on operations of the affected facility, supersedes the exemption under 40 CFR 60.14(e)(4). EPA always has maintained that the basic exemptions from the standard definition of a modification may not be appropriate for all sources, and 40 CFR 60.14(f) was included in the general provisions to allow EPA to supersede the general modification regulations, including the exemption provisions in 40 CFR 60.14(e), for sources where these provisions may not reasonably be applied. 

The applicability of the remaining exemptions established under 40 CFR 60.14(e)(1)-(3) and 40 CFR 60.14(e)(5)-(6) remain unchanged.

What records or information can be used to evaluate whether a modification occurs under 40 CFR 60.110c(e)?

Owners and operators may use a variety of records and other information to determine whether a change in the stored VOL triggers the modification provisions under 40 CFR 60.110c(e). In developing the 40 CFR 60.110c(e) modification provision, EPA recognized that the criteria would need to be applied broadly for both permitted and unpermitted storage vessels. Further, EPA recognized that permits vary between delegated authorities and may not specify the stored VOLs authorized for a particular storage vessel. As such, a multitude of records and information can be used to determine whether a change in the stored VOL triggers modification. 

Facilities may reference (but are not limited to referencing) the following types of information: 

  • Air permit (e.g., NSR, Title V, etc.) specifications, which state either the VOL vapor pressure properties associated with a specific vessel (or group of vessels) or information sufficient for determining MTVP for VOLs; 

  • Air permit applications, which indicate MTVP for permit emission limitations; 

  • Historical emissions calculations; 

  • Determinations of MTVP or Maximum Reid Vapor Pressure in accordance with 40 CFR 60.113c(d);   

  • Historical VOL and MTVP records maintained over the lifetime of the storage vessel; and 

  • Documentation submitted to a delegated authority that include MTVP records or information sufficient for determining MTVP for VOLs. 

Can EPA provide examples of actions that would not constitute a “modification” under NSPS subpart Kc?

The provisions in 40 CFR 60.14(e) outline potential actions that are not considered modifications. The applicability for the provisions 40 CFR 60.14(e)(1)-(3) and 40 CFR 60.14(e)(5)-(6) remains unchanged as part of this rulemaking action. A number of potential actions do not qualify as modifications under NSPS subpart Kc; these include (but are not limited to) the following: 

  • Maintenance operations, component repairs, and replacements that are consistent with the criteria of 40 CFR 60.14(e)(1);  

  • Changes in the throughput of the stored VOL that are consistent with the criteria of 40 CFR 60.14(e)(2); 

  • The addition or use of systems or devices to reduce air pollutants that are consistent with the criteria of 40 CFR 60.14(e)(5); 

  • Relocation and changes in ownership that are exempt under the provisions of 40 CFR 60.14(e)(6); and 

  • The introduction of new VOLs with vapor pressures that do not exceed the MTVP for all VOLs historically stored or permitted in accordance with 40 CFR 60.110c(e)(1). 

EPA recognizes that some facilities and permitting authorities may desire to take an administrative action to update a storage vessel’s existing permit for documenting historical service. An administrative action or permit modification to document historical service does not itself meet the definition of modification under 40 CFR 60.110c(e), unless accompanied by a physical or operational change at the affected facility that results in increased emissions.   

Do process upsets result in modifications?

Process upsets can occur within a refining or chemical manufacturing process. These are generally non-routine, infrequent, unscheduled and short-duration events and can cause diversions of a VOL to a storage vessel affected facility not typically associated with the storage of such materials. Under most scenarios, EPA expects that the vapor properties of such upsets would generally not exceed a greater MTVP than all VOL historically stored or permitted. However, in the rare instance that a process upset results in a VOL being stored in a storage vessel with a MTVP that exceeds that which has been “historically stored or permitted,” the owner or operator should expeditiously remove the material from the storage vessel. Generally, EPA would not consider this type of non-routine, infrequent, unscheduled and short-duration event to be a modification. 

Facilities with process upsets (regardless of MTVP) should evaluate whether the event qualifies as a compliance deviation or violation under NSPS Kc, NSPS Kb, or other programs. These issues should be properly reported and addressed with the appropriate delegated authority.

When are floating roof landings allowable?​

Floating roofs must be floated on the liquid surface at all times except during the initial fill and during those intervals when the storage vessel is completely emptied or subsequently emptied and refilled, including when the grade of the stored VOL is changed. 40 CFR 60.112c(b)(1) and (c)(1) specify that the when the floating roof is resting on the roof supports (i.e., landed), the process of filling, emptying, or refilling must be continuous and must be accomplished as rapidly as possible. Instances where the storage vessel is storing liquid without sufficient liquid depth to float the floating roof or without being completely emptied, or subsequently emptied and refilled, may constitute a deviation of the standard. 

When do the storage vessel degassing provisions under 40 CFR 60.112c(e) apply? ​

As established under 40 CFR 60.112c(a)(3), the degassing requirements of 40 CFR 60.112c(e) apply to all storage vessels that are vented to add-on controls regardless of size. The degassing requirements also apply to all storage vessels that comply with the floating roof control requirements that have a design capacity greater than or equal to 1,000,000 gal containing a VOL with a MTVP of 1.5 psia or more. The degassing provisions detailed in 40 CFR 60.112c(e) apply at all times in which owners and operators of affected facilities subject to the standard elect to degas their storage vessels. The “degassing” definition in 40 CFR 60.111c clarifies when degassing is required:  

Degassing means the process of removing VOL vapors from a storage vessel during, or in preparation of, cleaning or when taking the storage vessel out of service for inspection, repair, or other reason. 

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Last updated on April 17, 2025