How does a gas station pump work

How does a centrifugal fire pump work?

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On this page we would like to provide more detailed information on the function, mode of operation and operation of the front pump of our fire engine for those particularly interested.


Fire-fighting centrifugal pumps, as the correct designation is, have the task of drawing water from open bodies of water, hydrants or the vehicle tank and under

forward controllable pressure increase.

The picture on the right shows a section of an FPV 8/8, which was made in-house and is used in the Schönborn fire department for machinist training.


Have the letters and numbers of the type designation

the following meaning:

  • FP centrifugal fire pump
  • V front-mounted pump
  • 8 Nominal flow rate x 100 = 800 l / min
  • 8 nominal outlet pressure = 8 bar



On the pump one can distinguish outer and inner parts. The pump housing (here painted silver) is made of a light metal casting and is also anodized. The outer parts also include the A suction inlet (110 mm) and the two B pressure connections (75 mm) with self-closing screw-down valves.


Self-closing means that springs are fitted in the valves that hold the valve head

Keep it closed even when the valve is turned on so that the pump can be properly vented. There is an inlet pressure gauge, an outlet pressure gauge and a tachometer on the pump to monitor the pump activity. Other external parts are the drainage cock, the grease nipple for the pump shaft bearing on the suction side and the rear pump seal.



1 Suction inlet seal

2 Fixed coupling suction inlet

3 Seal to the pump housing

4 nozzles for filling the pump

5 Channel to the ventilation device

6 pressure room

7 distributor

8 2nd stage impeller

9 pump shaft

10 Rear pump seal

11 spacer sleeve

12 print room

13 pump cover

14 1st stage impeller

15 Slide bearing on the suction side

16 protective screen

17 retaining ring


The inner parts include the pump shaft, the impeller with its tight fit on the shaft and the fixed diffuser. It is a two-stage pump. A stage consists of an impeller and a diffuser.


fixed distributorWheel


The water enters the inside of the pump housing (suction chamber) through the suction nozzle, flows through the impeller mouth and is caught by the blades of the rotating impeller and thrown outwards into the diffuser by the centrifugal forces that occur. The diffuser converts the speed energy into pressure energy and directs the water to the impeller mouth of the second stage.


Here the pressure of the first stage is almost doubled before the water is fed from the distributor to the pressure chamber with the pressure outlets. The pressure level depends on the set engine speed.



Pressure increase in the diffuser:


The water is dammed up in the distributor, which reduces the water speed. Since speed and pressure are inversely related, the pressure increases as the water speed decreases. When the screw-down valves are closed, all of the water is retained.


The water speed is reduced to zero, which means that the entire speed energy is converted into pressure. Under these circumstances the pump will reach the highest pressure. This pressure is also called closing pressure. If you now open a valve, the stagnation of the water decreases, the flow rate increases and the pressure of the pump decreases.


Two measuring instruments are attached to check the operating status of the centrifugal fire extinguishing pump. It is an overpressure and underpressure meter as an inlet pressure meter on the suction side of the pump and an overpressure meter as an outlet pressure meter at the pressure outlet.


When sucking from open bodies of water or wells, the measuring device constantly shows negative pressure (more correctly said: negative overpressure). The pointer moves in the red area and indicates the height of the water level.


Inlet pressure meterOutlet pressure meter


The centrifugal fire extinguishing pumps are not able to draw in the water by themselves. For this reason, a device must also be installed that lowers the pressure in the pump and the suction line below atmospheric air pressure. As a result, the water is pressed into the pump by the air pressure on the surface of the water until it is caught by the impellers and the water pumping starts. Therefore the term "suction" is actually not quite correct, because it is only vented and the pressure does not drop below zero.


Now the venting device can be switched off, which works fully automatically with modern pumps, but has to be done by hand here. The venting devices are designed so that venting does not take longer than 60 seconds at a suction height of 7.5 m, a suction line length of 10 m and an air pressure of 760 mbar.


In the event that the ventilation device fails, water can be filled into the pump via the filler neck on the pump cover. When the suction line and the complete pump are filled, normal pump operation can also be started. For this purpose there is a check valve in the suction strainer.


The housing of the exhaust gas jet is provided with cooling fins and built into the exhaust line from the engine to the muffler. Inside the housing there is a flap (red) that accumulates the exhaust gases. It is operated via a linkage and in both of its end positions either lets the exhaust gases escape through the exhaust pipe or directs them to the nozzle of the gas ejector.


Here the exhaust gases are pressed through the driving nozzle (blue) into the collecting nozzle (green). At the end of the propellant nozzle, the high flow velocity creates a negative pressure which entrains the air in the collector nozzle. Since the collecting nozzle is connected to the pump via a valve, the pump chamber and suction hoses are vented. The external air pressure pushes the water down.


Operation of the front pump


Water withdrawal from open waters:


  • Set the gearbox shift lever to "neutral" (the green control lamp on the instrument panel must light up).
  • Open the bonnet and both doors for better ventilation, cover the driver's seat and steering wheel.
  • Connect the suction line and pressure hoses to the pump, opening all pressure outlets.
  • Close the screw-down valves and drain cock.
  • Open the vent valve (behind the instrument panel).
  • Couple the pump with the motor stopped and secure the lever against unintentional disengagement.
  • Start the motor.
  • Put the gas jet into operation by operating the control lever.
  • Increase the speed with the throttle (must be increased almost to full throttle so that the gas jet reaches its full output)
  • Monitor the venting process via the inlet pressure gauge.
  • When pressure is displayed on the outlet pressure gauge, open the screw-down valves a little, then close the vent valve.
  • Put the gas heater out of operation.
  • Reduce speed.
  • Slowly open the screw-down valve.
  • Adjust the commanded pump outlet pressure.


Water supply from hydrant or fire pumps:


  • Set the transfer box to "neutral" (green control lamp lights up).
  • Open the bonnet and both doors for better ventilation, cover the driver's seat and steering wheel.
  • Connect the claw to the suction nozzle.
  • Connect the hose line supplying water to the claw.
  • Connect the pressure hoses to the pump, opening all pressure outlets.
  • Couple the pump with the motor stopped and secure the lever against unintentional disengagement.
  • Start the motor.
  • After the water supply has been completed, screw-down valves open and regulate the output pressure commanded.


It is forbidden to increase the outlet pressure above 12 bar. If a higher pressure is required, no one is allowed to stand in front of the pump (hose bursts!). The pressure at the inlet pressure gauge should not fall below a value of 1.5 bar.