<P> A gear pump of two parts: </P> <P> This pumps the oil in and increases the pressure in the nozzles to 15 bar maximum (217.5 psi). Usually a gerotor of the sickle type is used . Gear pumps are used frequently in oil burners because of their simplicity, stability and low price . </P> <P> To set the heat output of the burner, the rate of fuel delivery via the nozzle must be adjustable . This is often achieved by an adjustable pressure relief valve between the pump and the nozzle . When the set pressure is reached (usually 100psi), this valve opens and allows excess oil to flow through a bypass back to the fuel tank or the pump suction side . </P> <P> This allows fuel to be shut off from the sprayer by electrical control . It helps avoid drips when the valve is closed . It also eases the purging of the burner (and any boiler) of fuel mist during start up, or while restarting after a misfire . If the burner were not purged the oil / air mixture could explode dangerously . </P>

What is the typical pressure at which an oil burner operates