Flapper Nozzle System
Pneumatic systems are still used in process control industries even after the entry and domination of electrical signals, electronic and digital systems. The flapper-nozzle is a basic component of pneumatic measurement, control, and transmission system.
It works as a pneumatic secondary transducer, by translating a very small displacement into a pressure signal.
Construction
The flapper or baffle is a movable flat metal and it is attached to a member whose displacement is to be detected. The flapper is positioned in front of the nozzle in such a way to cover or uncover the nozzle and vary the gap between them when moved.
It works as a pneumatic secondary transducer, by translating a very small displacement into a pressure signal.
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| The Flapper Nozzle System |
The flapper or baffle is a movable flat metal and it is attached to a member whose displacement is to be detected. The flapper is positioned in front of the nozzle in such a way to cover or uncover the nozzle and vary the gap between them when moved.
The nozzle comprises of a variable nozzle restriction in series with a fixed orifice restriction. A constant supply of pressurized air (usually 20 psi or 1.4 kg/cm2) is applied to the nozzle through the orifice restriction. The pressurized air comes out of the nozzle through the gap between the nozzle and flapper.
For the generation of sufficient back pressure and proper functioning of the system, the diameter of the nozzle has to be 1.5 to 2.5 times greater than the diameter of the orifice. In general, the orifice diameter 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 flapper gets reduced. This increases the restriction to the outflow of air through the nozzle and also increases the nozzle back pressure. 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 equals to the supply air pressure.
When the flapper is moved towards the nozzle, the gap between the nozzle and flapper gets reduced. This increases the restriction to the outflow of air through the nozzle and also increases the nozzle back pressure. 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 equals to the supply air pressure.
When the flapper is moved away from the nozzle, the gap between the nozzle and flapper is increased. The restriction to 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.
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 through the flapper nozzle system. The output pressure produced by the flapper nozzle system is proportional to the input displacement and it can be directed to operate an indicating instrument or any other system.
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| Flapper-Nozzle Gap vs Nozzle Back Pressure |
The above diagram depicts the typical graph plotted between the nozzle back pressure (Pb ) and the flapper-nozzle gap (x). The slope dPb/dx at any point of the curve is called as nozzle sensitivity or gain of the system. The curve indicates that the system exhibits an approximately linear behavior in the range between 3 and 15 psi.
Advantages
1. The flapper-nozzle system possesses the advantages of being simple to install, operate and maintain. They are explosion-proof and so they can be used in hazardous environments.
2. The flapper-nozzle system produces a linear output ranging from 3 to 15 psi, which falls under the limits of industrial control pressure making it acceptable to use for practical purposes.
Limitations
1. The flapper is positioned by the displacement producing element that is usually a low-force element. So, there are chances for the position of the flapper to be disturbed by the blast of air coming out of the nozzle. It can be nullified by implementing a feedback system to reposition the flapper.
2. The flapper moves a very small distance for its one full action from minimum to maximum nozzle back pressure. This makes the flapper nozzle system affected by vibrations.
3. The flapper-nozzle system provides amplification of the order of 105 but this output power is not sufficient to operate pneumatic actuating valves in industrial process control systems. So, the flapper nozzle system is used as a first stage amplifier and in conjunction with pneumatic relays. Pneumatic relays are known as power amplifiers and it amplifies the nozzle pressure.
4. The transmission of pneumatic signals is slow and it introduces a time lag in the system.
Application
Current to Pressure Converter
Current to Pressure converter or I/P converter, as it is shortly known, is an important application of the flapper nozzle system. As most of the control valves used in industries are pneumatic, the I/P converter is used for the conversion of 4 to 20 mA electrical signal into 3 to 15 psi pneumatic signal.
The spring holds the flapper farther to the nozzle at its zero position in such a way that the resultant pressure at the output is at its lowest (3 psi). When the current is passed through the coil, it makes the flapper move towards it. This decreases the flapper-nozzle gap and thus increases the output pressure. When the current is 20 mA, the output pressure is maximum (15 psi). There is a linear relationship between current and pressure.
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Current to Pressure Converter
very informative and accurate to clear off the doubts. thanks.
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