Kinds of fire pumps - how they're built, driven and engaged

What counts as a fire pump

NFPA 1910 defines a fire pump as a water pump mounted on the apparatus for firefighting, rated at 250 gpm or more (but under 3000 gpm) at 150 psi net pump pressure, or 3000 gpm or more at 100 psi. Auxiliary and transfer pumps have their own definitions.

Wildland and ultra-high-pressure pumps are their own categories: NFPA 1910 tests them with 30 minutes of continuous pumping at rated capacity and rated net pump pressure, instead of the 100, 70 and 50 percent capacity points used for a fire pump.

Centrifugal pumps move the water, positive-displacement pumps prime it

Almost every main fire pump is centrifugal: a spinning impeller gives the water speed and the volute turns that speed into pressure. Pressure falls as flow rises at a given speed, and the pump can run against closed discharges for a short time without damage.

A centrifugal pump can't pull a vacuum on its own, so it can't draft without help. A positive-displacement pump does that job: usually a rotary-vane primer, which traps air and pushes it out. Once the pump is primed, the primer is shut off; running it continuously overheats it.

Single-stage and two-stage

A single-stage pump has one impeller. A two-stage pump has two, and a transfer valve that lines them up two ways.

In volume (parallel), each impeller takes water from the intake and feeds a common discharge, so their flows add at one stage's pressure. In pressure (series), the water goes through one impeller and then the other, so their pressures add but the flow is limited to about one impeller's worth. NFPA 1910 runs the 100 percent capacity test in parallel, the 50 percent test in series, and the 70 percent point in either.

The makers' trade-off: Waterous says a single-stage pump is the more efficient at rated capacity, while a two-stage pump can pass 400 psi where a single-stage is usually limited to about 300. When to switch to volume differs too: Hale says above half of rated capacity, Waterous above two-thirds.

Shift the transfer valve at low pressure, following the pump maker's procedure. Shifting to pressure (series) at high flow cuts capacity.

How the pump is driven

Split-shaft (midship) pump: a gearbox is inserted in the driveline between the chassis transmission and the rear axle. A sliding gear or collar sends engine power either to the axle (ROAD) or to the pump (PUMP), never both, so the truck can't drive while it pumps. In ROAD the gearbox carries the engine's full torque to the axle. Hale calls it its most common drive, and NFPA 1910 has its own interlock table for it.

Transmission PTO: the pump runs off a power take-off on the transmission (or a four-wheel-drive transfer case) through a short drive shaft. The PTO's own rating limits how big a pump it can turn (Hale, Waterous). On Allison 3000 and 4000 series transmissions the PTO gear is driven from the torque converter side, so it turns whenever the engine runs, whatever range is selected. That is what lets a PTO pump pump and roll. Most PTOs must be engaged with the truck stopped; only a hot-shift PTO can be engaged on the move.

Front-of-engine crankshaft and flywheel PTOs: the pump is driven straight from the engine. Like a transmission PTO, these can be built for stationary pumping only or for pump-and-roll, and NFPA 1910 tests each kind against a different interlock table.

Separate engine: portable and skid pumps have their own engine, often bolted straight to it (Waterous calls this direct engine mounting), and small portables often prime with an exhaust primer. NFPA 1910 (10.2.10) has that engine inspected and maintained like the chassis engine, and to its maker's recommendations.

Where the pump sits

Midship: under the body behind the cab, driven by a split-shaft gearbox. Waterous describes it as giving the largest stationary capacity from one engine.

Front-mount: ahead of the radiator, driven by a shaft from the front of the crankshaft. It frees space in the body and can pump stationary or pump-and-roll (Waterous).

Rear-mount: at the back of the chassis, with a split-shaft or PTO gearbox in the middle of the driveline and drive shafts carrying the power back to the pump (Waterous).

A pump built for one mounting or rotation and fitted in another delivers only 60 to 100 psi and reduced flow (Hale), which is worth knowing after any rebuild or swap.

Pump-and-roll

Pumping while the truck moves needs a pump drive that keeps turning while the wheels are driven: a transmission-mounted PTO, a front crankshaft or flywheel PTO built for it, or a separate pump engine. A split-shaft pump can't do it.

Trucks built for pump-and-roll have an extra indicator. NFPA 1910's table shows "OK to Pump-and-Roll" with the pump engaged and the parking brake released, where a stationary-only pump would show only "Pump Engaged" and lock the panel throttle out.

How to test it 🔊 Read aloud

1. Read the plate and the test labelRated capacity and pressures, the pump maker and model, and the engine speed recorded at the last test.
2. Find the driveA gearbox in the driveline behind the transmission is split-shaft. A pump driven off the transmission PTO, the front of the crankshaft or the flywheel is PTO-driven. Its own engine means a separate-engine pump.
3. Count the stagesA transfer valve control and a volume/pressure indicator on the panel mean a two-stage pump.
4. Find the primerElectric rotary-vane, oil-lubricated or oil-free, or an exhaust primer on some smaller pumps. Note whether it has a lubricant tank to check.
5. Open the right manualShift procedures, drip rates, oil types and clearances all come from the pump and transmission makers' manuals.

Numbers to work to

Common mistakes

• Assuming every pump shifts the same way - split-shaft, PTO and pump-and-roll drives each have their own sequence and lights.
• Trying to pump and roll on a split-shaft pump.
• Running a two-stage pump in pressure (series) for a high flow.
• Leaving the primer running after the pump is primed.
• Testing a wildland or ultra-high-pressure pump to the 100/70/50 percent points meant for fire pumps.
• Engaging a standard PTO with the truck rolling - only a hot-shift PTO allows it.

Practice tests that cover this