40 CFR 60 Subpart Kc Explained Like You’re on a Tank

You’re up on the roof. Wind’s picking up. Somewhere below your boots, volatile organic liquid is doing what volatile organic liquids do, trying to become vapor and drift off into the air you breathe. That’s the entire problem EPA regulations like 40 CFR 60 are trying to solve, and the newest chapter in that story is Subpart Kc.

If you own, operate,engineer, inspect, or maintain aboveground storage tanks, you’ve probably heard the name tossed around in permitting meetings, tank inspections, or panicked emails from your compliance team. Let’s break it down in plain English, so that you won’t need a law degree to understand what it means for the tank sitting under your feet.

First, What Even Is 40 CFR 60 Subpart Kc?

40 CFR 60 Subpart Kc is the EPA’s newest New Source Performance Standard (NSPS) for volatile organic liquid (VOL) storage vessels, including petroleum storage tanks. It was finalized on October 15, 2024, and it applies to storage vessels that were constructed, reconstructed, or modified after October 4, 2023.

Think of it as the next generation of a regulatory family that includes Subparts K, Ka, and Kb, each written for tanks built in a different era. Subpart Kc is the newest sibling, and true to form, it’s stricter than all the ones that came before it.

In short: if your tank is new, rebuilt, or has recently changed what it stores, Kc might already apply to you whether you’ve realized it yet or not.

So, Who’s Standing in the 40 CFR 60 Kc Blast Zone?

Subpart Kc applies to storage vessels that meet both of these conditions:

Capacity of 20,000 gallons or more
Used to store a volatile organic liquid with a maximum true vapor pressure (MTVP) of at least 0.25 psia

That vapor pressure threshold is a big deal. It’s substantially lower than the thresholds under the older Subpart Kb rule, which means tanks storing liquids that would have flown under the radar a few years ago may now be squarely in scope.

And here’s the part that catches a lot of tank owners off guard: changing what’s in the tank can trigger Kc all by itself. Under the final rule, if you switch a tank’s service to a liquid with a higher MTVP than anything historically stored or permitted in that vessel, that’s considered a “modification.”

For example, your 28,000 gallon diesel tank is emptied and is being turned around to hold conventional gasoline. Since the gasoline has a higher MVTP than diesel, that tank is now considered to have undergone a “modification” making it susceptible to 40 CFR 60 Kc’s new requirements.

Suddenly the tank that’s been sitting quietly for years can be a brand-new regulatory migraine.

The Number Everyone’s Talking About: 98%

Here’s the headline that should matter most to you if you’re standing on that roof: Subpart Kc requires 98% control efficiency, compared to 95% under 40 CFR 60 Subpart Kb.

That jump from 95% to 98% sounds small on paper. In practice, it’s a completely different performance bar, especially for the seal systems doing the day-to-day work of keeping vapor where it belongs: inside the tank.

Depending on your tank’s vapor pressure, Kc lays out the control hierarchy like this:

MTVP below 11.1 psia: You can use an internal floating roof, an external floating roof, or a closed vent system routed to a control device, fuel gas system, or process.
MTVP of 11.1 psia or greater: Floating roofs are off the table. You need a closed vent system routed to a control device (or fuel gas system/process) achieving that 98% efficiency.

Your Seal Selection Just Got A Lot Slimmer

If your tank has a floating roof, this is where Kc really tightens the screws.

Under Subpart Kc, floating roof tanks must use:

A liquid-mounted primary seal or mechanical shoe primary seal, plus
A rim-mounted secondary seal

Here’s the part that trips people up: vapor-mounted primary seals no longer make the cut, even when paired with a secondary seal. If your current setup relies on a vapor-mounted primary seal, it satisfies Kb. It does not satisfy Kc.

There’s also a new decision to make on external floating roofs regarding guidepoles: choose a liquid-mounted primary seal, or go with unslotted guidepoles. There’s no perfect answer here, just the one that fits your tank, your product, and your operations team.

“But What If My Tank Was Built Under Kb…”

Good question! And good news, sort of. 40 CFR 60 Kc doesn’t automatically reach back and grab tanks that are properly grandfathered under Subparts K, Ka, or Kb. Those rules still apply to the vessels that were built under them.

But two things are worth sitting with:

Kc can apply voluntarily. Owners and operators may choose to comply with Kc to demonstrate compliance with K, Ka, or Kb. Meeting Kc’s tougher standard automatically checks the box for the older ones.
Modifications can pull you in. As mentioned above, changing to a higher-vapor-pressure liquid can turn an “existing” Kb tank into a “modified” Kc tank.

So even if you’re confident your tank predates October 2023, it’s worth double-checking your liquid service history before you assume you’re in the clear.

Why This Matters More Than a Line Item in a Permit

Emissions compliance isn’t paperwork for its own sake. Every percentage of control efficiency is volatile organic compound emissions that either escape into the air your community breathes, or don’t. Subpart Kc exists because the EPA’s periodic technology reviews found that better control was achievable with today’s seal materials, monitoring practices, and closed vent system design. Subpart Kc is the codification of “we can do better,” and now the regulation expects you to.

For tank owners, that translates into real decisions: which seal system to install, how to document vapor pressure history, and how to build inspection and monitoring records that hold up if the EPA (or your own EHS and compliance team) comes asking.

Standing on the Roof, Looking Forward

Subpart Kc isn’t a hypothetical future regulation. It’s already in effect, already applying retroactively to vessels built or modified since October 4, 2023, and already reshaping how new tanks get designed from the ground up.

If you’re staring down a new build, a tank reconstruction, or a change in liquid service, the smartest move is figuring out where you stand before the inspector does.

Mesa ETP Was Built for 40 CFR 60 Kc

For over 50 years, Mesa ETP has engineered the seal systems, floating roof drains, and vapor control products that keep tanks like yours compliant and performing, long before “98% control efficiency” was even a phrase anyone used.

That’s exactly why we developed FlexCore^®, our patent-pending liquid-mounted seal built with literal tougher-than-steel Armor Fabric™. FlexCore^® was engineered specifically to meet the demands of stricter storage tank emission regulations like 40 CFR 60 Subpart Kc, delivering industry-best emission control, durability, and application flexibility, without making your installation crew’s life miserable.

Standing on a tank roof shouldn’t feel like standing on a question mark. Talk to a Tank Expert at Mesa ETP, and let’s figure out exactly what Subpart Kc means for your tank, and just how to get ahead of it.

References

U.S. EPA. 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. eCFR. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-C/part-60/subpart-Kc
U.S. EPA. Volatile Organic Liquid Storage Vessels – Including Petroleum Storage Vessels: New Source Performance Standards (NSPS). https://www.epa.gov/stationary-sources-air-pollution/volatile-organic-liquid-storage-vessels-including-petroleum
U.S. EPA. Volatile Organic Liquid Storage Vessels – Including Petroleum Storage Vessels: NSPS – Frequent Questions. https://www.epa.gov/stationary-sources-air-pollution/volatile-organic-liquid-storage-vessels-including-petroleum-0
Federal Register. New Source Performance Standards Review for Volatile Organic Liquid Storage Vessels (Including Petroleum Liquid Storage Vessels), 89 FR 83319 (Oct. 15, 2024). https://www.federalregister.gov/documents/2024/10/15/2024-22823/new-source-performance-standards-review-for-volatile-organic-liquid-storage-vessels-including
Trinity Consultants. EPA Finalizes New Source Performance Standards Applicable to Volatile Organic Liquid Storage Vessels. https://trinityconsultants.com/resources/epa-finalizes-new-source-performance-standards-applicable-to-volatile-organic-liquid-storage-vessels/
Civil & Environmental Consultants, Inc. EPA Finalizes New Rule for Volatile Organic Liquid Storage Tanks (NSPS Subpart Kc). https://www.cecinc.com/blog/2024/11/04/epa-finalizes-new-rule-for-volatile-organic-liquid-storage-tanks-nsps-subpart-kc/