UST overfill prevention: shutoff valves, alarms, and ball floats
Overfill prevention stops or warns before a delivery overfills an underground tank. The federal rule allows three devices: an automatic shutoff valve in the drop tube, an overfill alarm, or a ball float valve, and each has to be inspected at least every 3 years (EPA, Musts for USTs).
A tank that never receives more than 25 gallons at a time doesn't need it (EPA).
The three types
Each device has a level it has to act by.
| Device | What it does | Must act by |
|---|---|---|
| Automatic shutoff valve (flapper in the drop tube) | Closes and stops the delivery | 95% full, and before the fittings on top of the tank are covered |
| Overfill alarm | Warns the driver with a horn or light they can see or hear at the fill | 90% full, or 1 minute before overfilling |
| Ball float valve (vent line) | Blocks vapor from leaving, which slows the delivery | 90% full, or 30 minutes before overfilling |
Sources: EPA, Musts for USTs; EPA, Spill and Overfill Prevention (2024).
Ball floats are on their way out. They can't be used when overfill prevention is installed or replaced after October 13, 2015 (EPA). They also only work with tight-fill gravity deliveries, not pressurized (pumped) deliveries or tanks with suction pumps or coaxial drop tubes (NEIWPCC inspector training, 2021).
Field note: On the sites I work on, overfill prevention is alarms and drop-tube shutoff valves. Ball floats aren't an option for new or replacement equipment.
The 3-year inspection
The inspection has to confirm the device is set to act at the right level and will actually act when fuel gets there (Mich. Admin. Code R 29.2120, adopting 40 CFR 280.35). It follows the manufacturer's instructions or a code of practice (EPA).
For a drop-tube valve, that usually means pulling the drop tube. Testing in place misses many problems (NEIWPCC). The inspector measures the tube and the tank's manway height to confirm where the valve will shut off, and accounts for tank tilt.
What fails. At one first round of inspections, half the devices were installed wrong or didn't work. Failures included corroded parts, missing gaskets, loose adapters, valves set at the wrong height (one at 97% instead of 93% because the tank was tilted), and tampering (NEIWPCC). EPA also says to check that a shutoff hasn't been tampered with, disabled, or removed (EPA).
How to order fuel so it never gets close
The device is a backup. The real overfill prevention is ordering the right amount. The rule says to make sure the delivery will fit in the empty space in the tank and to watch the delivery the whole time (EPA).
EPA's formula: order no more than (tank capacity × 0.90) − fuel already in the tank (EPA).
Example: A 10,000-gallon tank holding 3,500 gallons can take 10,000 × 0.90 − 3,500 = 5,500 gallons.
FAQ
Can I still use my ball float?
Under the federal rule, an existing ball float installed on or before October 13, 2015 can stay if it works properly. Once you replace your overfill prevention, the replacement can't be a ball float (EPA). Check your state's rule too.
Where does the overfill alarm have to be?
Where the delivery driver can easily see or hear it (EPA).
Does a spill bucket count as overfill prevention?
No. A spill bucket catches drips at the fill when the hose comes off; overfill prevention stops the tank itself from overfilling. You need both. See spill bucket testing.
Sources
- EPA: Musts for USTs (EPA 510-K-15-001, Nov. 2015)
- EPA: UST Spill and Overfill Prevention (EPA 510-F-24-006, July 2024)
- NEIWPCC: UST inspector training, overfill prevention (Dec. 2021)
- Michigan Admin. Code R 29.2120
This guide is general information. Your state UST program decides what's required at your site.