Design of Instrument Air Systems

What is Instrument Air?

Most power plants and process industries require the instrument air to run their process.\u00a0\u00a0 The instrument air is compressed air that is free from moisture and dust particles.

The pneumatic equipment such as control valves, solenoid-operated valves, cylinders, actuators, and positioners require the instrument air to control the process.

It is referred to as the circulating system of the instrumentation systems.

How do we calculate the requirement of instrument air in a process plant?

The requirement of instrument air varies from plant to plant. And the design requires surveying the amount of air required for the process plant.

The instrument air required must be calculated by considering all the peak and working parameters of all pneumatic systems in the plant and their consumption.

To meet the system requirements the instrument air systems must be designed in such a way that the airflow must be at least 800 CFM or more with a pressure of 8 bars to the pneumatic systems in the plant.

Depending upon the capacity of the receiver tank, the charging and discharging of the receiver tank is about a 2 min to 5 min cycle and is a continuous type.

The operating pressure band between the lower and upper pressure of the receiver is 10 psi.

Instrument air specification

• Airflow must be 600-800 CFM.
• Air pressure 6.5 to 7.0 bar.
• The temperature must be around an ambient of 26 Degrees Celsius.
• The relative humidity must be 55% to 65%.
• CFM is the cubic feet per minute.

What are the main components of an instrument air system?

The instrument air system consists of

1.Instrument air compressor:

The instrument air compressor is a device or equipment that produces compressed air. The air is drawn from the atmosphere and compressed to a final pressure. The compressor can be a single or double-stage compressor.

2.Inlet air filter:

The inlet air filter in the compressor system is a dry cartridge that filters or removes the solid dirt particles from the atmosphere entering the system.

3.Air after cooler:

The air after cooler reduces the temperature of the instrument air before entering the receiver tank.

4.Air receiver:

In this unit, compressed air is received and stored. Air receivers should be sized to provide an adequate volume of air surge to provide a sufficient quantity of air to the pneumatic systems.

5.Instrument air or oil separator:

In the case of a reciprocating oiled piston-type compressor, the oily content is made to eliminate from the air.

6.Pressure reducer:

This unit reduces the air pressure to the desired value. The pressure in the receiver tank must be maintained as per design.

The ambient temperature of the receiver must be lower than the dew point temperature of the air entering the receiver. Due to this, the moisture present in the air is condensed at the bottom of the receiver tank.

7.The instrument air header:

Air Header is a series of multiple valve assemblies that operate from one originating point. The instrument air header is meant to distribute the required air to all the pneumatic systems within the plant.

Quality Standards of Instrument Air:

In all process industries the most commonly used instrument air standard must be ANSI/ISA \u2013 S7.0.01-1996.\u00a0 The few air standards are as shown below:

A. Pressure Dew Point:

It is defined as the temperature at which free moisture condenses out from the instrument air into liquid water for a specific pressure.

The pressure dew point at the air dryer outlet must be a minimum of 13 \u00b0C or below.

B. Particle Size:

The instrument air supplied to the pneumatic system through headers is expected to contain particulate matter at an acceptable size of 40\u00b5m to all pneumatically operated devices.

The additional filtration modules need to be installed if the particulate size required is less than 40 \u00b5m.

C. Lubricant Content:

In cases where the installed air compressor in the instrument air system is lubricated with lube oil, there is always a risk of oil carryover along with the compressed air.

This creates a blockage to the pneumatic devices that receive the instrument air and affects their operation. Hence the lubricant content should be close to 0 ppm but should not exceed 1 ppm.

D. Location of the system:

The location of the instrument air system in the plant is most important to prevent contaminants, hazardous and flammable gases from being drawn into the inlet air filters.

What are the various types of compressors available?

The various types of compressors available are

1.\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Reciprocating oiled piston.

2.\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Reciprocating oil-less piston.

3.\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Rotary vane.

4.\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Rotary liquid ring.

5.\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Diaphragm.

6.\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Rotary screw.

7.\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Centrifugal.

Design considerations of instrument air receiver tank:

The following are the design considerations for the receiver tank.

1.Pressure fluctuations must be minimized

During an emergency shutdown the pressure fluctuations must be reduced.\u00a0 The instrument air is measured in units of time,

2.Short-term air demand

In process facilities often the demand for instrument air (IA) can fluctuate sometimes reaching a peak.

3.Energy Savings

Instrument Air Systems run frequently consuming power & it becomes imperative to achieve power savings by operating (loading/unloading) the air compressor only as and when required.

Water Condensation in Instrument Air Receiver:

Instrument Air Receiver Size:

\u00a0The size of the air receiver is based on the principle of excess pressure

The net volume of the receiver is calculated by the relation 

V = [(Q*f*Pa) / (P)]

Where

Q= Capacity of air in CFM

f = Loading and unloading time in sec

Pa = Barometric Pressure of tank

P = Pressure difference of Receiver Tank

Sizing and distribution of instrumentation air systems

The instrument air required by the pneumatic systems in the process plant is distributed through carbon steel headers. The ball valves are used in all sub-headers for controlling the air.

The connection to the instrument air main header will always be at the upper section of the pipe.

The sub-headers are provided with drain valves at dead ends. The sizing of the sub-header is shown depending on the number of air users. The below table shows the size of sub-headers.

What are accessories that are required to be mounted to the instrument air system?

The following are the accessories that need to be mounted are

1.Pressure gauge:

A dial-type pressure gauge is mounted on the receiver body to indicate the pressure of the air inside the system.

2.Pressure Regulators:

Pressure regulators are provided to control the pressure on downstream devices. Any failure in the pressure regulator will result in full system pressure on downstream system devices.

3.Globe Valves:

Globe valves regulate system flow rates and provide tight shut-off.

4.Gate Valves and Ball Valves:

Gate valves provide a full, line-size port for airflow with minimum pressure drop and are conducive to internal cleaning. And are typically used for on/off isolation. To provide better shutoff the ball valves are most preferred.

Why is an interlock provided to the instrument air system?\u00a0\u00a0

• The primary function of an interlock is to control the pressure of instrument air inside the receiver tank to the desired set point.
• And to protect the system from damage due to a rise in pressure.
• The process interlock is assigned to the system.
• It uses the pressure loop control system.
• The setpoint of the instrument air receiver is
  1. For loading, it is set to 8 bars
  2. For unloading, it is set to 4 bars.

The process of loading and unloading is a continuous type process.

During the time of loading if the instrument air pressure in the receiver tank reaches desired Set Point (8 bar). Then the compressor system is subjected to a hold state. And is again started if the instrument air pressure drops to the minimum value (4 bars).