An anti-vortex plate is a flat steel plate fixed over the suction outlet inside a fire water tank. It stops a whirlpool (vortex) from forming at the outlet, so the fire pump draws solid water and not air. Where a tank feeds a fire pump, NFPA 20 requires this vortex control at the tank outlet, and NFPA 22 sets how the plate is sized and installed.
On tank inspections, this is one of the parts I always ask to see before the water goes in. Once the tank is full, nobody can check it without draining the tank. I have seen engineers sign off a full pump room, test the pump, and never ask what is sitting at the bottom of the tank.
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What is an anti-vortex plate?
When water leaves a tank fast through a bottom or low side outlet, it starts to spin. You see the same thing when you pull the plug in a sink. The spinning water makes a funnel of air that reaches down into the outlet.
For a fire pump, that air is the problem. The anti-vortex plate is a horizontal steel plate placed just above the outlet. The water must flow in from the sides, under the plate, so a funnel cannot form. The plate can be square or round. Its job is simple: keep the flow into the suction pipe smooth and full of water.
Why a fire pump needs one
A centrifugal fire pump is built to move water, not air. Air pulled into the suction pipe causes three problems:
- Lower flow and pressure. Air takes space that water should fill, so the pump cannot deliver its rated performance.
- Cavitation-type damage. Air pockets and unstable flow hammer the impeller and wear it over time.
- Stress on the pump. Uneven flow adds vibration and load on the bearings and seals.
The worst moment is a real fire. The tank level drops as the pump runs, and a vortex is most likely when the water is low. That is exactly when you need the pump at full duty. The plate also lets you use more of the stored water before air reaches the pump.
NFPA 20 and NFPA 22 requirements
NFPA 20 covers the fire pump. Where a tank is the suction source, it requires an anti-vortex plate at the tank discharge outlet. NFPA 22 covers the water tank itself and gives the plate details. Authorities that review fire pump and tank drawings, such as the Los Angeles Department of Building and Safety, check these points on every tank:
| Item | Requirement |
|---|---|
| Where | At the discharge (suction) outlet of the tank feeding the fire pump |
| Plate size | At least twice the diameter of the outlet |
| Height | At least 6 in. (about 150 mm) above the bottom of the tank |
| Material and position | Horizontal steel plate, fixed on the outlet fitting |
So for a 200 mm outlet, the plate must be at least 400 mm across. Always check the edition of NFPA 20 and NFPA 22 adopted on your project, and in the UAE, the requirements of the UAE Fire and Life Safety Code and Civil Defence approval.
Installation and inspection: common site mistakes
The plate is cheap and simple. Most problems come from the timing of the work, not the design.
- Plate missing on concrete tanks. Steel tanks often come with the fitting from the supplier. Civil-built concrete tanks are where I see it forgotten.
- Plate too small. A plate sized to the pipe, not to twice the outlet diameter, does little.
- Plate too low or touching the floor. If it sits on the floor, water cannot flow in under it. Keep the minimum height.
- No inspection before filling. Inspect it after the tank is built and before the first fill. After that, it is under water.
- Not on the drawings. Show the plate, its size and height on the tank and pump room drawings, so it is checked at approval stage.
Quick checklist before you fill the tank:
- Plate installed on the suction outlet fitting, horizontal and firmly fixed.
- Plate diameter (or side) at least 2 times the outlet diameter.
- Plate at least 6 in. (150 mm) above the tank floor.
- Photos taken and added to the handover file.
Anti-vortex plate FAQ
Is an anti-vortex plate required for every fire pump?
It is required where a tank is the suction source for the fire pump. A pump fed directly from a city main does not have a tank outlet, so the plate does not apply there.
How big should the anti-vortex plate be?
At least twice the diameter of the tank outlet, and installed at least 6 in. (about 150 mm) above the bottom of the tank.
Can you check the plate after the tank is filled?
Not without draining the tank. That is why it should be inspected after construction and before the first fill, with photos for the handover file.
Keep learning
Fire pump and water supply topics also appear in the CFPS exam. Test yourself with the free CFPS Exam Simulator, or see how we prepare engineers in the CFPS Exam Preparation Course.
Want to go further? Start the free fire alarm course: 11 practical lectures for site and design engineers.
Mohammad Alhamad, CFPS, Fire & life safety expert
Sources: NFPA 20 · LADBS plan check sheet: Water Tanks for Fire Protection (NFPA 22)
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