Flapper Nozzle Working principle

Pneumatic systems are still used in process control industries even after the penetration and dominance of electrical signals, electronic and digital systems. The flapper-nozzle is a basic component of pneumatic measurement, control and transmission systems. It works as a pneumatic secondary transducer, by translating a very small displacement into a pressure signal.


The flapper or baffle is a movable flat metal and is attached to the member whose displacement is to be detected. The flapper is placed in front of the nozzle in such a way as to cover or expose the nozzle and to replace the gap between them when moved.

The nozzle includes a variable nozzle restriction in series with a fixed orifice restriction. A constant supply of pressurized air (typically 20 psi or 1.4 kg/cm2) is applied to the nozzle through an orifice restriction. The pressurized air comes out of the nozzle through the gap between the nozzle and the flapper.

In order to generate sufficient back pressure and proper functioning of the system, the diameter of the nozzle should be 1.5 to 2.5 times the diameter of the orifice. In general, the diameter of the orifice is of the order of 0.25 mm and the diameter of the nozzle is of the order of 0.625 mm.


Working principle

When the flapper is moved towards the nozzle, the gap between the nozzle and the flapper is reduced. This increases the restriction on the outflow of air through the nozzle and also increases the pressure behind the nozzle. Once the flapper completely covers the nozzle, there is no outflow of air through the nozzle. The nozzle back pressure is at its maximum and is equal to the supply air pressure.

When the flapper is moved away from the nozzle, the gap between the nozzle and the flapper increases. The restriction on the outflow of air through the nozzle is reduced and the nozzle back pressure is also reduced. The minimum value of nozzle back pressure is 2-3 psi.

flapper nozze graph

With an input supply of 20 psi (1.4 kg/cm2), an output pressure of 3–15 psi (0.2–1.0 kg/cm2) can be generated via the flapper nozzle system. The output pressure produced by a flapper nozzle system is proportional to the input displacement and can be directed to operate an indicator device or another system.

The above diagram shows the typical graph plotted between the nozzle back pressure (Pb) and the flapper-nozzle gap (x). The slope dPb/dx at any point on the curve is called the tip sensitivity or gain of the system. The curve indicates that the system exhibits a nearly linear behavior in the range between 3 and 15 psi.


  • Connect the Air supply at 20 PSI pressure
  • Keep the flapper at the center.
  • Then a constant volume of air is bleeds through the nozzle.
  • When you move flapper to left side then the bleeding decreases and the back-pressure increases as read at the gauge.
  • Similarly when you move flapper to the right the bleeding increases and the back-pressure decreases as read at the gauge

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