On October 15, 2024, EPA promulgated its awaited update to Volatile Organic Compounds (VOC) emission standards from Volatile Organic Liquid (VOL) storage vessels, 40 CFR 60, Subpart Kc. EPA also updated the rulemaking docket for the subpart in response to comments on the proposed rule. This standard covers a broader range of tank sizes and liquid volatility than its predecessor standard, Subpart Kb, which was proposed in 1984. The rulemaking alters the agency’s prior interpretation about New Source Performance Standards (NSPS) modification as a result of mere stored liquid change; it tightens design standards for rim seals and deck fittings on floating roof tanks compared with prior rules; it requires controlled degassing for certain storage vessels; and it contains an annual Lower Explosive Limit (LEL) standard for Internal Floating Roof (IFR) tanks.

Tanks Subject to NSPS Kc

The final rule applies to storage tanks with capacity of 20,000 gallons or more, as long as the maximum True Vapor Pressure (TVP) of the stored liquid is greater than 0.25 psia, and only if the tank is constructed, modified, or reconstructed after October 4, 2023. By comparison, its predecessor rule, Subpart Kb, applies to tanks of 20,000 to 40,000 gallons storing liquids with maximum TVP of 2.2 psia or more, and tanks of 40,000 gallons or more storing liquids with maximum TVP of 0.5 psia or more.

The final rule sets aside a longstanding EPA precedent that a storage vessel cannot be “modified,” and thus lose grandfathering from a new standard, merely by changing the stored liquid in the vessel. Instead, under Subpart Kc, a tank is “modified” if its liquid service is changed to “a VOL that has a greater maximum true vapor pressure than all VOL historically stored or permitted.” This reference to a piece of equipment’s air permit is novel and atypical among part 60 standards.

Subpart Kc has tighter thresholds for requiring emission control than other EPA tank standards. Subpart Kc requires a floating roof, or a Closed Vent System (CVS) to an emission control device, to be installed on tanks of 20,000 to 40,000 gallons storing liquids with maximum TVP of 1.5 psia or more, and tanks of 40,000 gallons or more storing liquids with maximum TVP of 0.5 psia or more. By comparison, Subpart Kb requires a CVS and control device or floating roof to be installed on tanks of 20,000 to 40,000 gallons storing liquids with maximum TVP of 4.0 psia or more, and tanks of 40,000 gallons or more storing liquids with maximum TVP of 0.75 psia or more. Also, by comparison, the 1.5 psia threshold is lower than the threshold of 1.9 psia for industry-specific part 63 emission standards for tanks at refineries. Subpart Kc retains the Subpart Kb and part 63 subpart WW thresholds of 11.1 psia, above which a floating roof is disallowed, and a fixed-roof tank must be used with a CVS and control device.

Rim Seal and Deck Fitting Standards in NSPS Kc

Subpart Kc applies requirements for rim seals and deck fittings, of equal or greater stringency than existing EPA tank emission standards. Notably, the final Subpart Kc rule contains language stating that its requirements may be used as a compliance demonstration method for Part 63 Subpart WW and for prior Part 60 Subparts K, Ka, and Kb.

Rim seal requirements under Subpart Kc apply to new and reconstructed IFR and External Floating Roof (EFR) tanks, and to fixed-roof tanks retrofitted with IFRs after October 4, 2023. IFR and EFR tanks will be required to have a liquid-mounted or a mechanical shoe seal primary seal and a rim-mounted secondary seal. A seal system with a vapor-mounted primary seal, such as a dual-wiper seal, will not satisfy the Subpart Kc standard even if a secondary seal is used, except that modified tanks with an “existing” IFR may instead elect to comply with Subpart Kb rim seal system requirements.

Regarding EFR tanks, most current EPA tank emission standards require EFRs to use a mechanical shoe or liquid-mounted primary seal. Subpart Kc additionally requires new and reconstructed EFRs after October 4, 2023, to choose between using a liquid-mounted primary seal or using no slotted guidepoles. Either choice comes fraught with operational complexity. Liquid-mounted seals do not have a field history of comparable service life to mechanical shoe seals, leading to unwanted roof landings and degassing events to maintain or to replace them. Whereas unslotted guidepoles cannot be readily relied on for level gauging or liquid sampling. Indeed, EPA disallows use of unslotted guidepoles for sampling in reformulated gasoline fuel regulations.

The deck fitting provisions of Subpart Kc appear to disallow use of traditional IFR sample wells with slit fabric seals.

Monitoring, Inspection, and Repair Provisions in NSPS Kc

Subpart Kc’s inspection and repair provisions only include a single 30-day extension request beyond the allotted 45-day repair timeline. In this respect, Subpart Kc follows Subpart Kb, rather than more recent tank rules, which allow two 30-day extensions and do not require prior approval from the rule administrator.

Subpart Kc follows other recent EPA tank emission standards in requiring controlled degassing. The Subpart Kc controlled degassing requirements apply to all fixed-roof tanks subject to the rule’s emission control requirements (i.e., using a CVS and control device to comply with the subpart) and to certain floating-roof tanks (i.e. with capacity of 1,000,000 gallons or more storing liquid with maximum TVP of 1.5 psia or more). The rule approves use of distillate flushing to reduce the volatility of the liquid heel remaining in the tank during roof landing or tank cleaning.

Subpart Kc also follows EPA’s 2024 Gasoline Distribution rules (Part 63 Subparts R and BBBBBB) in requiring LEL monitoring annually in an IFR tank’s vapor space. Readings greater than 25% LEL will be considered to be an inspection failure. The LEL standard will be assessed on a 5-minute rolling average basis. LEL monitoring will be required for a minimum of 20 minutes, at a location within the storage vessel no more than 3 feet above the floating roof.

In the Subpart Kc preamble, EPA writes, “We considered optical gas imaging requirements in previous reviews of storage vessels, which we reviewed again for NSPS Subpart Kc, and concluded that these requirements were not cost-effective.” Operators should be aware that EPA personnel have relied on OGI to conduct enhanced tank inspections related to current tank subparts.

The Bottom Line

Subpart Kc was published in the Federal Register on October 15, 2024, which is also the rule’s effective date. The rule retrospectively applies to tanks that were constructed, modified, or reconstructed since October 4 of last year (2023). Tank owners and operators are recommended to determine if their tanks will become subject to the rule; to develop a strategy for compliance; and to assess applicability of the subpart to tanks and tank projects going forward. Experts at Trinity Consultants are here to help. We offer unparalleled experience in the fields of tank regulation support, compliance assistance, and emission estimation.

Harold Laurence, Seattle Office, 253.867.5600 x 4811
Christie Heath, Philadelphia Office, 610.472.8349
Behdad Yazdani, Baton Rouge Office