Instrument Air Sizing

I want to share you how to size or select minimum capacity of air compressor and instrument air dryer. Air compressor is usually used as feed air for:

• Instrument air (for instrumentation or feed air for nitrogen generator)
• Utility air (for utility station)

Instrument Air Requirement for Instrumentation

Instrument air is required in operation of control valve and shut down valve. The rules below is used to calculated instrument air requirement for instrumentation:

• Control valve (CV) – steady state condition: 1 scfm/valve
• Control valve (CV) – transient condition: 6.25 scfm/valve
• Shut down valve (SDV) – steady state condition: 0
• Shut down valve (SDV) – transient condition: 4.8 scfm/valve

Note :  Depends on valve design, requirements, application etc… also the air consumption rate will change.

Here is calculation of instrument air requirement:

• Instrument air (normal demand, scfm) = 95% CV at steady state + 5% CV at transient + 10% SDV at transient
• Instrument air (peak demand, scfm) = 70% CV at steady state + 30% CV at transient + 20% SDV at transient

Instrument Air Requirement for Nitrogen Generator Feed

Air feed for nitrogen generator is usually from instrument air dryer. The feed requirement depends on the required purity of nitrogen. The higher the purity, the higher flow rate of instrument air to nitrogen generator. Inlet compressed air requirement to nitrogen generator is expressed as air factor. For general rule:

• Nitrogen purity 99.5%   : air factor 2.9
• Nitrogen purity 99%      : air factor 2.5
• Nitrogen purity 98%      : air factor 2.3
• Nitrogen purity 97%      : air factor 2.1
• Nitrogen purity 95%      : air factor 1.9

Utility Air Requirement

Utility air requirement for utility station is about 0.5 scfm/utility hose. Number of utility station is at least one per 30 m radius.

Sizing of Instrument Air Dryer and Compressor Capacity

After we define air requirement based on its purpose, then this is the step to determine instrument air dryer capacity:

1. Calculate required instrument air to nitrogen generator based on its purity
2. Calculate required dry air = instrument air for instrumentation + instrument air to nitrogen generator package
3. Calculate required air to instrument air dryer, considering air loss for regeneration. It is usually 1.15 times value from Step 2
4. Calculate air compressor capacity = value from Step 3 plus utility air requirement

You may also add 20% safety factor.

Example

Let’s continue with example. For example, we have:

• Control valves: 5 unit
• Shut down valve: 5 unit
• Capacity of nitrogen generator package: 25 scfm with purity 98%
• Number of utility hose: 10 unit

Instrument Air Requirement

Instrument air (peak demand, scfm) = 70% CV at steady state + 30% CV at transient + 20% SDV at transient

Instrument air (peak demand) = 70% x (5 x 1 scfm) + 30% x (5 x 6.25 scfm) + 20% x (5 x 4.8 scfm)

Instrument air (peak demand) = 17.67 scfm

Utility Air Requirement

Utility air requirement = number of hose x 0.5 scfm

Utility air requirement = 10 x 0.5 scfm = 5 scfm

Then we can continue using four steps above.

1. Instrument air to nitrogen generator package = 2.3 (air factor for 98% purity) x capacity of nitrogen generator package = 2.3 x 25 scfm = 57.5 scfm
2. Dry air requirement = dry air for instrumentation + dry air for nitrogen generator package = 17.67 scfm + 57.5 scfm = 75.175 scfm
3. Air to instrument air dryer = 1.15 x 75.175 = 86.45 scfm
4. Compressor capacity = Dry air requirement + utility air requirement = 86.45 + 5 scfm = 91.45

Credits : miss rifka

Article Shared by :
B Kumar Raj