Inst ToolsInst ToolsInst Tools
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Search
  • Books
  • Software
  • Projects
  • Process
  • Tools
  • Basics
  • Formula
  • Power Plant
  • Root Cause Analysis
  • Electrical Basics
  • Animation
  • Standards
  • 4-20 mA Course
  • Siemens PLC Course
Reading: Instrument Air Sizing
Share
Font ResizerAa
Inst ToolsInst Tools
Font ResizerAa
  • Courses
  • Design
  • PLC
  • Interview
  • Control System
Search
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Inst Tools > Blog > Guest Articles > Instrument Air Sizing

Instrument Air Sizing

Last updated: August 3, 2019 11:25 am
Editorial Staff
Guest Articles Instrumentation Design
9 Comments
Share
5 Min Read
SHARE

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
Don't Miss Our Updates
Be the first to get exclusive content straight to your email.
We promise not to spam you. You can unsubscribe at any time.
Invalid email address
You've successfully subscribed !

Continue Reading

Why Orifice Flow Meter Flange is Not Recommended 150# Rating?
How to Minimize Water Vapor in Instrument Air Lines?
Failsafe Wiring Practices
Thermowell Outer Diameter Larger than Nozzle Inside Diameter?
Control Valve Recommended Practices for Harsh Process Conditions
Circular Chart Recorder Calibration Procedure
Share This Article
Facebook Whatsapp Whatsapp LinkedIn Copy Link
Share
9 Comments
  • azetou says:
    October 29, 2017 at 11:33 am

    Hi, thank you very much for your effort. Could you please provide a reference for these calculations, it means what standard is used ?

    Reply
  • Vignesh says:
    November 30, 2017 at 6:00 am

    Please explain the Instrument air peak demand calculation,ho it is came as 17.67 sqfm

    Reply
    • azetou says:
      November 30, 2017 at 7:05 am

      The explanation is already there :

      Instrument air peak demand = 70% of Control valves air consumption @ steady state + 30% Control valves air consumption @ transient + 20% of Shutdown down valves @ transient.

      Valves air consumption of valves is from manufacturer.

      The article, however, lack some reference to where the percentages in the equation came from. example : ISA standard or other.

      Reply
  • azetou says:
    November 30, 2017 at 7:49 am

    for the admin : Could you please add functionality for editing our comments ? thanks.

    Reply
    • S Bharadwaj Reddy says:
      November 30, 2017 at 2:11 pm

      Thanks for the suggestion. Right now there is no Authentication (login) for User Comments, so editing a comment after certain time is not possible for the users. I will see the possibilities in future updates but not sure for any time frame as it involves some code changes. Meanwhile please mail me for any edits of posted comments ( [email protected] )

      Reply
      • azetou says:
        November 30, 2017 at 2:15 pm

        Thank you for you efforts.

        Reply
  • daz says:
    May 9, 2019 at 3:36 pm

    hi can you provide basis on this calculation to support it thanks..ISA,API etc

    Reply
    • Jannat says:
      January 30, 2023 at 10:38 pm

      Good design information.

      Reply
  • SOLY says:
    June 11, 2024 at 5:52 pm

    WHAT IS THE REFERENCE OF: “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 ” ?

    Reply

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Stay Connected

128.3kFollowersLike
69.1kFollowersFollow
210kSubscribersSubscribe
38kFollowersFollow

Categories

Explore More

Pneumatic Signal Piping and Fittings
Pressure Gauge Problems
Difference Between a Flow Control & a Needle Valve
Electrical Heat Tracing
Difference between PT100, PT1000, Nickel RTD and Balco RTD
Monsoon Protection for Field Instruments and Junction Boxes
Level Instruments Design Rules
Where should we Terminate the Cable Shields ?

Keep Learning

Process Plant Risk and Safety Management

Safety Roles & Responsibilities of Process Engineering (Detail Design)

Marshalling Panel

What is Marshalling Cabinet?

Thermowell Immersion Length

Thermowell Insertion and Immersion Length

Thermowell Problems

Thermowell Problems

Level Gauges

Level Gauge Design

PLC Program for an Alternate output circuit

PLC Program for Alternate Output Circuit with Latched Function

Gas Detectors Location Installation

Gas Detectors Installation Techniques

Instrumentation and Control (I&C) Design

Instrumentation and Control (I&C) Design

Learn More

PLC to Start or Stop 3 Machines using 1 Switch Program

PLC to Start or Stop 3 Machines using 1 Switch Program

Vibration Measurement Sensors

Types of Sensors used in Vibration Measurement

instrumentation engineering standards

ISA codes for Process Instrumentation

WirelessHART Principle

Wireless HART Communication Protocol Overview

Furnace Draft Control System

Furnace Draft Control System

Centrifugal Machine Field

Batch Type Centrifugal Automation in Sugar Industries

Fixed Type Motor control center (MCC)

Motor Control Center (MCC) Signal Interface Termination (SIT)

Electromagnetic

Types of Signals Based on Electromagnetic Compatibility (EMC Performance)

Menu

  • About
  • Privacy Policy
  • Copyright

Quick Links

  • Learn PLC
  • Helping Hand
  • Part Time Job

YouTube Subscribe

Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Welcome Back!

Sign in to your account

Username or Email Address
Password

Lost your password?