Actual Flow-Standard Flow-Normal Flow: Instroduction
Gas flow measurement is one of the most misunderstood topics in instrumentation. Many engineers see values like Am³/h, Nm³/h, Sm³/h, or SCFM and assume they all mean the same thing. In reality, they do not.
A gas volume is not fixed. It changes with pressure and temperature. That is why gas flow must always be defined using actual, standard, or normal conditions. Without this clarification, flow data can easily be misinterpreted.
This article explains actual flow, standard flow, and normal flow in simple language using practical industrial examples.
What Is Actual Flow?
Actual-flow is the real volume of gas flowing through a system at its operating conditions. These conditions include the actual pressure, actual temperature, and the resulting gas density.
Actual-flow is commonly expressed in units such as Am³/h (actual cubic meters per hour) or ACFM (actual cubic feet per minute).
Actual flow represents the space the gas occupies inside the pipe at that moment
It changes whenever pressure or temperature changes
It does not directly represent the quantity of gas
Example
If natural gas is flowing at a pressure of 50 bar and a temperature of 35°C, the measured flow is an actual flow. If the pressure drops or the temperature increases, the actual flow value will change even though the amount of gas passing through the system remains the same.
Why Actual Flow Alone Is Not Reliable
Gases are compressible. This means the same gas occupies different volumes at different pressures and temperatures.
High pressure compresses gas and reduces volume
High temperature expands gas and increases volume
Volume alone cannot represent gas quantity
Because of this behavior, actual-flow values cannot be directly compared between different systems, locations, or time periods unless pressure and temperature are also considered.
What Is Standard Flow?
Standard-flow is a volumetric flow rate that has been mathematically corrected to a defined set of reference conditions known as Standard Temperature and Pressure (STP).
The main purpose of standard-flow is to remove the influence of pressure and temperature so that gas flows can be compared accurately.
Standard flow allows fair comparison
It ensures consistency across locations
It is essential for billing and reporting
Standard flow is typically expressed as Sm³/h or SCFM.
Example
A gas flow of 1000 m³/h at STP means the gas volume has been converted to standard conditions, regardless of the actual operating pressure and temperature in the pipeline.
Where Standard Flow Is Used
Standard-flow measurement is critical in several industries.
Custody transfer of natural gas
Billing and invoicing
Regulatory compliance
Energy content calculations
In gas pipelines, ownership transfer is always based on standard or normal flow, not actual volumetric flow.
What Is Normal Flow?
Normal-flow is similar to standard flow but uses a different reference condition known as Normal Temperature and Pressure (NTP).
Normal conditions are typically defined as:
Temperature: 20°C
Pressure: 101.325 kPa
Normal flow is widely used in industrial and European applications and is usually expressed as Nm³/h.
Example
A flow rate of 100 Nm³/h means the gas flow has been corrected to normal conditions, even if the actual gas is flowing at a different pressure and temperature.
What is Nm³/h ?
Nm³/h does not represent the actual volume inside the pipe. It represents the quantity of gas expressed as a volume at normal reference conditions.
Nm³/h allows easy comparison of gas flows
It eliminates pressure and temperature influence
It represents gas quantity, not pipe volume
This is why most industrial gas flow meters display Nm³/h.
Actual Flow vs Standard Flow vs Normal Flow
The key difference between these flow types lies in how pressure and temperature are handled.
Actual flow changes with operating conditions
Standard flow is corrected to STP
Normal flow is corrected to NTP
For reporting, analysis, and billing, standard or normal flow is always preferred.
STP vs NTP – Key Differences
STP and NTP are often confused, but they are not the same.
STP (Standard Temperature and Pressure)
Commonly defined as 0°C and 100 kPa (IUPAC)
Sometimes defined as 0°C and 101.325 kPa (NIST)
Mainly used in laboratories and scientific calculations
NTP (Normal Temperature and Pressure)
Defined as 20°C and 101.325 kPa
Closer to ambient industrial conditions
Widely used in flow measurement and utilities
Why NTP Is Preferred in Industrial Applications
Normal conditions better represent real operating environments.
Industrial systems usually operate near ambient temperature
NTP reduces correction errors
It simplifies pipeline and compressor design
This is why many datasheets and flow meters specify Nm³/h
Gas Behavior with Pressure and Temperature
Gas behavior is governed by physical laws.
Gas volume decreases when pressure increases
Gas volume increases when temperature rises
Gases are highly compressible compared to liquids
These properties make flow correction essential in gas measurement.
Ideal Gas Law Explained in Simple Terms
The relationship between pressure, volume, and temperature is described by the Ideal Gas Law:
PV = nRT
In practical terms:
Pressure and temperature changes affect gas volume
If mass remains constant, volume must adjust
Flow measurement must account for these changes
Volumetric Flow vs Mass Flow
Volumetric flow refers to the gas volume at operating conditions, while mass flow refers to the actual amount of gas.
Volumetric flow changes with pressure and temperature
Mass flow remains constant
Mass flow is easier to compare and calculate
This is why many systems prefer mass or standard flow measurement.
Why Mass Flow Is More Stable
Gas mass does not change when pressure or temperature changes.
Mass flow remains constant through compressors
It simplifies energy calculations
It avoids confusion caused by volume expansion or compression
Mass flow is often calculated using gas density and volumetric flow.
What Does “Normal Volume” Mean?
Normal volume does not represent physical volume inside a pipe. It represents gas quantity converted to reference conditions.
One Nm³ represents the mass of gas at NTP
It is a calculated value, not a measured volume
It allows accurate comparison between systems
Molar Volume at Reference Conditions
At defined reference conditions, gases occupy known molar volumes.
At 0°C and 101.325 kPa: 22.414 m³/kmol
At 0°C and 100 kPa: 22.711 m³/kmol
These values are used to convert between actual and reference conditions.
Why Flow Must Be Converted to Reference Conditions
Consider the following comparison.
One cubic meter of air at 100 bar and 40°C contains about 112 kg
One cubic meter of air at 1 atm and 0°C contains about 1.3 kg
The volume is the same, but the gas quantity is completely different. This proves why reference conditions are essential.
Two Correct Ways to Specify Gas Flow
There are only two technically correct methods.
Stating flow at actual operating conditions
Stating flow at defined reference conditions such as Nm³/h
The second method is always preferred in industrial practice.
Industrial Applications of STP and NTP
Standardized gas flow is used across industries.
Custody transfer and billing
Energy and efficiency calculations
Pipeline and compressor sizing
HVAC and utility systems
Scientific testing and calibration
What we learn today?
Gas flow measurement is not just about reading a number from a flow meter. It is about understanding what that number truly represents.
Once actual flow, standard flow, and normal flow are clearly understood, engineers can design better systems, avoid billing disputes, and interpret flow data correctly.
This understanding separates routine measurement from professional engineering judgment.
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