What a fire water tank inspection covers
NFPA 25 Chapter 9 covers water storage tanks that supply a water-based fire protection system: suction tanks, gravity tanks, pressure tanks, and embankment-supported fabric tanks. The point of the inspection is to know the tank still holds water, still delivers it, and is not hiding corrosion, sediment, or a failed coating.
The work splits into what you can see from the outside on a short cycle, and what you can see only by looking inside on a 3-year or 5-year cycle. Owners miss the interior date because it falls between the annual visits and because the tank has to come out of service, or be inspected in service by a diver or a remote vehicle.
The short cycle: level, temperature, and the outside
Water level — monthly for tanks without a supervised water-level alarm connected to a constantly attended location, and quarterly for tanks with one. A tank that has dropped below its design level cannot supply the fire pump or the standpipe it was built for.
Temperature in freezing weather — water temperature is checked weekly during the heating season in tanks without a low-temperature alarm connected to a constantly attended location, and quarterly during the heating season in tanks with one. The heating system is inspected daily during the heating season without a supervised low-temperature alarm, and quarterly with one. The heating system and the low-water temperature alarms are tested before the heating season.
Exterior, supports, and the ground around the tank — quarterly. Hoops, grillage, painted or coated surfaces, and expansion joints — annually. These catch the failures you can see without opening the tank: a settling foundation, a blocked vent, a ladder that will not hold an inspector, a coating that has failed on the outside.
The interior rule: 3 years or 5 years
This is the number people get wrong. NFPA 25 puts the interior inspection of steel tanks without corrosion protection at every 3 years, and the interior of all other tanks at every 5 years. The section number depends on the edition: 9.2.6.1.1 and 9.2.6.1.2 in the 2014 edition, 9.2.5.1.1 and 9.2.5.1.2 in the 2017 edition. For the 2020 edition, which Virginia enforces, and the 2023 edition, confirm the number in your copy using Table 9.1.1.2. The rule sorts tanks by material and corrosion protection, so a pressure tank takes whichever interior row matches it. The full table by tank type and edition is on our NFPA 25 tank interior requirements page. Confirm the adopted edition with your AHJ before you quote a date.
Unprotected steel is on the shorter clock because oxygen and water attack bare plate. A lined, coated, concrete, or wooden tank is not immune. It is on the 5-year clock, and a failed lining puts it back into the problem the 3-year rule was written for.
| Item | Frequency | Reference |
|---|---|---|
| Water level — no supervised water-level alarm connected to a constantly attended location | Monthly | NFPA 25 Ch. 9 |
| Water level — supervised water-level alarm connected to a constantly attended location | Quarterly | NFPA 25 Ch. 9 |
| Water temperature — no low-temperature alarm connected to a constantly attended location | Weekly during the heating season | NFPA 25 Ch. 9 |
| Water temperature — low-temperature alarm connected to a constantly attended location | Quarterly during the heating season | NFPA 25 Ch. 9 |
| Heating system (inspection) — no supervised low-temperature alarm connected to a constantly attended location | Daily during the heating season | NFPA 25 Ch. 9 |
| Heating system (inspection) — supervised low-temperature alarm connected to a constantly attended location | Quarterly during the heating season | NFPA 25 Ch. 9 |
| Tank heating system (test) | Before the heating season | NFPA 25 Ch. 9 |
| Low-water temperature alarms (test) | Before the heating season | NFPA 25 Ch. 9 |
| Exterior, supports, and surrounding area | Quarterly | NFPA 25 Ch. 9 |
| Hoops, grillage, painted surfaces, expansion joints | Annually | NFPA 25 Ch. 9 |
| Interior — steel tank without corrosion protection | Every 3 years | 2014: 9.2.6.1.1 · 2017: 9.2.5.1.1 · 2020/2023: confirm in your copy (Table 9.1.1.2) |
| Interior — all other tanks | Every 5 years | 2014: 9.2.6.1.2 · 2017: 9.2.5.1.2 · 2020/2023: confirm in your copy (Table 9.1.1.2) |
| Level indicators | Every 5 years | NFPA 25 Ch. 9 |
| Water-level signals, high and low (test) | Annually | NFPA 25 Ch. 9 |
Virginia enforces NFPA 25-2020. Confirm the adopted edition with your AHJ. Full table by tank type and edition: NFPA 25 tank interior requirements.
What the interior inspection is looking for
The interior look is for pitting, corrosion, spalling, rot, sediment, aquatic growth, and local or general failure of the interior coating. Debris on the floor becomes the material a fire pump ingests. A failed coating is how a 5-year tank becomes a 3-year problem.
When the coating does not pass, NFPA 25 Chapter 9 calls for further evaluation. Industry practice on that follow-up includes adhesion testing, dry-film thickness, and ultrasonic thickness of the shell and floor. That follow-up is a qualified tank inspector’s work. It is not a flow test, and FLOW does not sell it as one.
Inspect it in service, or drain it with a partner
An interior inspection is either out of service or in service. Out of service means the tank is drained, opened, and looked at empty. In service means an underwater rover, a remotely operated vehicle flown by a pilot, looks at the interior while the tank stays full. FLOW does in-service rover inspections only. The AHJ and the tank’s condition decide which method is right, and when a drained inspection is needed, FLOW refers it to a licensed partner firm.
A drained inspection may be the right call when sediment has to come out, when a coating has failed, or when the inspector cannot see the floor through the water. The drain, the inspection of the empty tank, and the refill are handled by others, and FLOW refers that work to a licensed partner firm. If silt has to be removed before an underwater inspection, it is removed by others first. The rover inspection documents the silt conditions it finds.
Where FLOW fits, and where we stop
FLOW inspects the interior in service only, with an underwater rover flown by a pilot. The rover works in the tank while it stays full. The pilot flies it and records what the interior shows: silt and sediment conditions, coating loss, corrosion, and debris. FLOW does not drain, empty, or refill the tank. When the plan calls for a drained, out-of-service inspection, FLOW refers it to a licensed partner firm.
FLOW does not weld or coat the tank, run ultrasonic thickness, or stamp an engineering report. Where the AHJ wants that credential or a repair, we coordinate a licensed partner. We are not the authority having jurisdiction, and we do not sign the tank back into service.