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Sizing of Compressor and Instrument Air Capacity


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

13 thoughts to “Sizing of Compressor and Instrument Air Capacity”

  1. Helo, I have a problem about sizing air capacity for some pressure vessel using air compressor. I wanna do some research about gas gun for horizontal impact testing aparatus.

    Do I need to convert all of air capacity into scfm or just using Boyle’s law for gas dynamics?

    Thank you

    1. Hello, Muhammad Baiquni! Thank you for visiting my website.
      Actually Boyle’s law and air capacity in scfm is actually the same. Boyle’s law is using this equation P1 V1 = P2 V2. We can say, condition 1 is standard condition (then we use scfm for volume and at standard pressure) and condition 2 is actual condition.

  2. Hello Miss Rifika, Yours is the only website (atleast from the ones I could locate and surf) which gives a direct formula for calculation of Air Flow Requirement for a Control Valve…!!

    So was just trying to understand the basis for the formula since it doesnt take into consideration any information on the size of the valve or actuator etc…!! Can we use the formula across all valve and actuator sizes…??

    Also are the constants like 6.25 scfm/valve etc. determined empirically / experimentally or they have been derived on the basis of some formula…!!

    Would indeed appreciate if you can help understand these queries…

    Once again, thank you very much for providing a handy tool / reference for this topic…!!

    Regards,
    Rahul

    1. Hi Rahul, thanks for leaving feedback on my blog.

      This post is preliminary guideline for sizing instrument air consumption. For accurate sizing, vendor’s data should be considered.

      The constants used in this post are based on my previous project. We usually used that constant to estimate. The value is actually used for all valve size.

      GPSA Chapter 4 also mentioned the constant used to estimate the instrument air consumption. In GPSA, 0.5 cubic foot per minute per valve is usually adequate.

      I found that almost every source has different constant for air consumption. For the best results, I suggest you to ask control valve vendors or SDV vendors, if possible. However, for preliminary calculation, I think my guideline or GPSA guideline should be enough.

  3. Thank you for your insightful post, Ms Rifka.
    I wanna ask, what did you mean by utility hose? What kind of equipment or thing is that? Sorry for my silly question.

    Thank you,

    HM

    1. Hi Hafish Mahdi,

      Thank you for leaving comment in my website. Utility hose is a kind of connection installed in several part of plant. In my experience, utility hose is part of utility station which consist of water connection, utility air connection, and instrument air connection. Whenever you need water or air, you just need to connect a hose to the connection.

  4. Well done, Good article of design practice on how for sizing Air system, if i may suggest to add IA receiver sizing, it will be good. also please add schematic diagram from Air compressor, Dryer , IA receiver, N2 package etc for easy understanding. In addition, there are some categorization of IA consumption based on CV size but as you mentioned above that vendor data is the correct one. so the calculation will take as preliminary calculation to size compressor, IA receiver etc.

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