You prevent seal leaks in a fire pump by keeping the seal type the pump is listed for and removing what pushes that seal past normal leakage: misalignment and pipe strain, air or a weak suction, sand in the water, packing set too tight or too loose, and long idle spells between test runs.

Why Fire Pump Seals Leak

Fire pump seals leak for the same reasons other pump seals do, and three conditions of fire pump duty make them worse: long standby, raw water and engine vibration.

  • Long standby: debris in the water settles into the seal area while the pump stands idle between runs.
  • Raw water: pumps fed from wells, ponds, rivers and open tanks take in sand and silt that wear mechanical seal faces and the shaft sleeve under packing.
  • Engine vibration: on diesel-driven pumps, engine vibration degrades mechanical seal faces over time.

On a packed pump, most leaks start with the packing itself. A gland run too tight burns the rings and grooves the sleeve. Rings that are worn, of the wrong type or badly installed leak from the start, and a blocked water-seal line leaves the rings without the water that cools them.

Choose the Right Seal and Materials

For a listed horizontal split-case or vertical turbine fire pump, the right seal is compression packing. In the cycle that produced the 2025 edition of NFPA 20, NFPA’s fire pump committee voted to make packing the required sealing type for centrifugal fire pumps and for vertical shaft turbine fire pumps. Do not convert a packed fire pump to a mechanical seal to cure a drip.

Mechanical seals stay in NFPA 20 for the pumps that still use them, mainly positive displacement fire pumps and a few FM-listed units. The standard limits them to clean water: a public main, a closed-top tank, or a supply that meets the pump maker’s water quality specification. The pump should also see positive pressure at its inlet at all times, and any seal flush must be at positive pressure.

NFPA 20 sets no face material, elastomer or flush-rate requirements for those seals, so replace one with the configuration the pump maker specifies for that pump. For packing, use what the pump’s manual calls for.

Align the Shaft and Install the Seal Correctly

Correct pipe strain and soft foot first, then align the coupling with both shafts turned, then fit the packing or seal clean on an undamaged sleeve.

Pipe strain should not move the shaft alignment more than 2 mils (50 µm) vertically or horizontally at the coupling. A movement of 2 mils at any foot is a soft foot, corrected with shims.

The coupling tolerance tightens as speed rises. At 1,800 rpm, an acceptable alignment allows about 1.3 mils of offset per inch of flexplane separation; a faster pump needs less.

NFPA 25 has coupling alignment and shaft end play checked every year.

On a vertical turbine, check that the lineshaft sits centred in the stuffing box and runs true before the packing or seal goes in.

Keep the Seal From Running Dry

Keep the casing full of water and the packing supplied with leakage. Keep the automatic air release valve on the casing working, and keep the suction pressure positive.

Treat any run without water at a mechanical seal as damage and check the seal for the signs of dry running.

On packing, running dry means running without leakage. Before starting a packed pump, set the gland loose enough to leak.

On a vertical turbine the column, head and stuffing box sit above the water. Follow the pump maker’s instructions for what they need before start.

Keep Suction Conditions Within Limits

The suction must deliver water to the flange at positive pressure (−3 psi is allowed from a grade-level tank) while the pump flows 150% of its rated capacity, with the net positive suction head available above what the pump requires at that flow.

Weekly and monthly test runs at churn never load the suction. Only a flow test shows whether it holds at peak flow.

Check the piping around the suction flange as well as the supply:

  • Velocity: the suction pipe should be sized so the velocity stays at or below 15 ft/s (4.5 m/s) at 150% of rated capacity.
  • Elbows and tees: NFPA 20 restricts elbows and tees close to the suction flange of a split-case pump. A relief valve tee back into the suction should sit at least 10 pipe diameters from the flange, its plane at right angles to the shaft.
  • Reducers: use an eccentric reducer at the suction flange.
  • Valves: no control valve other than an OS&Y gate valve within 50 ft of the suction flange, and no pressure-regulating valve on the suction side.
  • Tank outlets: an anti-vortex plate at the tank’s outlet keeps the pump from drawing air.

Keep Debris Out of the Seal

Stop debris at the intake, and keep fine solids away from the sealing surfaces with clean water. For open water sources NFPA 20 calls for a double intake screen at the suction intake, and a vertical turbine’s suction strainer has to stop a 0.5 in sphere.

Neither stops sand or silt, so on a packed pump with a lantern ring, connect the sealing chamber to a source of clean fresh water where one is available.

Check NFPA 20’s rules on suction devices before adding a strainer to a horizontal fire pump’s suction pipe.

Adjust Packing Correctly

Adjust the gland with the pump running until the packing drips steadily, about one drop per second, and never tighten it to stop all leakage. Where your pump’s manual sets its own rate, use it.

Take up the gland nuts evenly, a little at a time, and let the leakage settle before the next turn. Steam at the gland means it is far too tight.

New packing needs several runs of adjustment. Start with the gland finger-tight, let water run freely at first start, then take it up over the following test runs.

When the gland has no take-up left, or tightening no longer cuts the leakage, replace the packing and check the sleeve for wear or a rough finish.

Repack with rings of the right size and stagger the ring joints to the angle your pump’s manual gives. Line the lantern ring up with the water-seal connection.

Inspect the Seal During Every Test Run

At every test run, check the seal’s leakage and temperature and the pump’s noise, and record the gland position:

  • Packing glands: a steady drip at the rate they were set to, with no spray and no steam. Adjust the gland nuts during the run if the rate has drifted.
  • Mechanical seals: no visible drip. A drip that continues once the pump is running is a leak.
  • Heat: no packing box, bearing or casing running hot. At churn, the casing relief valve must discharge enough to keep the case from overheating.
  • Noise and vibration: anything new is a reason to check alignment and suction before it damages the seal.

A gland near the end of its travel, or leakage that climbs from run to run, means it is time to plan the repack and sleeve check before the packing gives out.

Protect the Seal During Standby

Run the pump on its test schedule: the runs themselves are the main protection for a seal on standby.

For a mechanical seal, check the water quality while the pump stands, including on closed-loop supplies where nobody monitors it.

Where the pump room needs heating, NFPA 20 requires it kept above 40°F (4°C).