What Is The Instrument Air In A Process Plant

19th  November, 2024.

In this post, we will see the concept of instrument air in a process plant.

There are many instruments and devices in an industry which require air for operation. For this purpose, the normal air pressure that we breathe daily cannot be used. This pressure is very low to operate any device or process (yes, high pressure air can move or operate a process). To achieve high pressure, the normal air must be compressed. This whole concept can be termed as instrument air in a process plant. In this post, we will see the use and theory of instrument air in a process plant.

What is instrument air?

Before understanding the concept of instrument air, let us see a simple example of a balloon blowing machine. When you blow a balloon, you need to pull the handle up to fill the pump with air. Then, you press it downwards and release the air from a small outlet. The air released from that outlet will be of such a high speed and pressure, that it will instantly start to fill the balloon. It will take time if the air volume is larger and filled from a larger area. Air will travel quickly at a high pressure through smaller areas, which means the air was compressed when filling the balloon. The same concept is used in instrument air.

The normal air is taken from outside and moved around in a closed vessel. That air is then compressed and released from a small outlet. This compressed air is of such a high speed, that it potentially becomes an energy source. This air can be used to move an object like a piston, cylinder, valve or actuator. Due to this, the instrument air acts as a utility like steam and water (used for powering a process). The devices operating with air are called pneumatic devices. It is for this reason that instrument air plays a very important role in industrial processes.

What does an instrument air system consist of?

Refer to the below image. A typical instrument air system consists of the following components - compressor, air dryer, filter, and receiver tank. The receiver tank usually has three types of valves - safety, isolation and drain. These valves are operated automatically mostly (either by a PLC or is embedded with a controller circuit in-built). The process starts with an air compressor (driven by an electric motor or gas turbine), which takes the natural air from outside as the input. Inside it, the air is compressed and pushed through the dryer. The dryer (driven by an electric motor) will remove the moisture and damp content from the air. Due to this, the dew point of the air will be mostly reduced (because moist air can further damage the devices which will use the air as it’s power supply). After this, filters are present (mostly ceramic) for removing dust, dirt, oil and other contaminants. The final air will be passed into a receiver tank, which stores the filtered compressed air. There are three valves present in the tank - the safety valve will open automatically if the pressure inside the vessel increases above a set limit and will release the extra air, the drain valve will remove the condensate wastewater from the air if present, and the isolation valve is used to cut off the air supply from going further to the final load.

Dew point is an important parameter to be considered, as it complies to ISO 8573-1 class (dew point is the temperature at which water vapor condenses and becomes dry). Also, one thing to note is that instrument air is different from plant air. Plant air may tolerate some sort of dirt, but an instrument air cannot, as the instruments and devices are very sensitive to contaminants. Also, instrument air works at a much lower pressure than plant air.

Why is it important to have clean instrument air?

1. A clean instrument air prevents the end devices from getting any corrosion or damage by accumulation of dirt. And as only air is involved in the whole process, an overall clean air is the safest mode of utility for processes (even safe for use in hazardous environments).
2. As air is free to use, clean instrument air is the cheapest mode of utility available till date for use.
3. If the receiver tank is full and you want to use it at some other location, just dismantle it from the existing setup and install it in your new setup. The overall process is very safe and easy to use as compared to combustible tanks containing fuels or steam.
4. If the air is clean, the end devices will operate very precisely and thus, their longevity will increase. This will also automatically reduce maintenance costs and manpower involved in the process.
5. Because clean air is used, the pollution levels of the plant are maintained and complied with environmental hazards and other standards. So, the system is reliable and efficient to use in this case.

I have covered a general theory related to instrument air in a process plant. I have also not attempted to cover all the topics, as it can vary from systems to systems. Once you are familiar with this engineering, you can easily tackle all types of problems in it. 

Thank you for reading the post. I hope you liked it and will find a new way in this type of technology.