Anyone who works with pneumatic control valves will probably encounter a flapper and nozzle system, most often used to convert electric current into air pressure. But we can get into that topic in another article. For now, let’s understand how this system works.

## How does a flapper and nozzle system work?

As the name suggests, this system has a flapper and a nozzle, but also a pressure supply and a restriction. This picture will show you what it looks like:

This system senses a very small displacement of the flapper and converts that movement into a variation of the output pressure.

So a constant pressure, usually 20 pounds per square inch (psi), comes in at one end of the pipeline. This air passes through the restriction, causing the pressure to drop, and flows to the nozzle and flapper at the other end of the line. The flapper position determines the pressure inside the pipeline between the restriction and the flapper.

Therefore, the closer the flapper is to the nozzle, the closer the pressure inside the pipeline is to the supplied pressure. With the flapper closed, letting no air through, the line pressure will equal the supplied pressure minus the pressure drop in the restriction. And with the flapper fully open, the pressure will equal the atmospheric pressure.

The orifice plate and nozzle are usually very small, typical around 0.01 inch for the nozzle and  0.025 inch for the nozzle diameter. So a small change in the flapper’s displacement, like 0.0001 inch, can change the pressure value by one psi.

With this system, you’ll get a linear operation range between 3 and 15 psi from a 20-psi input.