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Instrument Air Sizing

This document discusses how to size instrument air compressors and dryers based on instrumentation and nitrogen generator requirements. It provides formulas to calculate: - Instrument air needs based on control valves and shutoff valves under steady state and transient conditions. - Nitrogen generator air feed needs based on required nitrogen purity levels. - Utility air needs based on the number of utility hoses. It then outlines a 4-step process to determine: 1) Instrument air needs for the nitrogen generator 2) Total dry air needs 3) Air requirements for the instrument air dryer 4) Final compressor capacity needs including utility air. An example application of the sizing process is

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milton1987
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2K views2 pages

Instrument Air Sizing

This document discusses how to size instrument air compressors and dryers based on instrumentation and nitrogen generator requirements. It provides formulas to calculate: - Instrument air needs based on control valves and shutoff valves under steady state and transient conditions. - Nitrogen generator air feed needs based on required nitrogen purity levels. - Utility air needs based on the number of utility hoses. It then outlines a 4-step process to determine: 1) Instrument air needs for the nitrogen generator 2) Total dry air needs 3) Air requirements for the instrument air dryer 4) Final compressor capacity needs including utility air. An example application of the sizing process is

Uploaded by

milton1987
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Sizing of Instrument Air

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

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

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