page_title: Nozzle and flapper
section_title: Application
hierarchical_section_title: Nozzle and flapper / Application
caption_attribution_description: English: Schematic showing nozzle and flapper used in a proportional controller context with field devices.
context_page_description: The nozzle and flapper mechanism is a displacement type detector which converts mechanical movement into a pressure signal, by covering the opening of a nozzle with a flat plate called the flapper. This restricts fluid flow through the nozzle and generates a pressure signal.
  It is a widely used mechanical means of creating a high gain fluid amplifier. In industrial control systems they played an important part in the development of pneumatic PID controllers and are still widely used today in pneumatic and hydraulic control and instrumentation systems.
context_section_description: The nozzle and flapper in pneumatic controls is a simple low maintenance device which operates well in a harsh industrial environment, and does not present an explosion risk in hazardous atmospheres. They were the industry controller amplifier for many decades until the advent of practical and reliable electronic high gain amplifiers. However they are still used extensively for field devices such as control valve positioners, and I to P and P to I converters.
  A proportional controller schematic is shown here.
  The set point is transmitted through the flapper plate via the fulcrum to close the orifice and increase the chamber pressure. The feedback bellows resists and the output signal goes to the control valve which opens with increasing actuator pressure. As the flow increases, the process value bellows counteracts the set point bellows until equilibrium is reached. This will be a value below the set point, as there must always be an error to generate an output. The addition of an integral or "reset" bellows would remove this error.
  The principle is also used in hydraulic systems controls.
caption: Schematic showing nozzle and flapper used in a proportional controller context with field devices.
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