When measuring gas flow in industrial applications, it’s essential to understand how temperature and pressure impact the volume of gases. This article explains the concepts of Normal Cubic Meter (NM³) and Standard Cubic Meter (SM³) — two critical terms for specifying gas flow in a consistent manner regardless of fluctuating temperature and pressure conditions.
For example, if a gas is flowing through a pipe at a temperature of 35°C and a pressure of 5 barg, the volumetric flow will be the actual cubic meter.
Table of Contents
What is AM³ (Cubic Meter of Gas)?
To begin, let’s clarify the meaning of a cubic meter (m³) when measuring gases. Unlike liquids, gases are compressible, meaning that their volume changes with variations in temperature and pressure. The actual cubic meter (m³) represents the volume of gas under the specific conditions of temperature and pressure at which the gas volume is measured. For instance, if nitrogen gas flows through a pipe at 35°C and a pressure of 5 barg (bar gauge), the volume measured at that temperature and pressure would be expressed in actual cubic meters (m³).
However, actual cubic meters can’t be used as a standard measure across different processes because each process has unique temperature and pressure conditions. To provide consistency, the industry relies on NM³ and SM³, which normalize gas volumes to predefined conditions.
What is NM³ (Normal Cubic Meter)?
The Normal Cubic Meter (NM³) represents the volume of gas at 0°C and 1 atm pressure. NM³ serves as a standardized measurement, making it easier to compare gas volumes across different systems, conditions, or industries.
Let’s take the same example, if gas flows through a pipeline at 35°C and 5 barg pressure, and we refer to the flow rate as 5 NM³/hr, this means the gas volume is equivalent to 5 meter cube per hour (NM3/hr) if it were measured at 0°C and 1 atm pressure. Using NM³ enables calculations without needing to adjust for the immediate conditions of temperature and pressure. In summary:
- NM³ measures gas flow as if it were at 0°C and 1 atm pressure.
- NM³ values are independent of actual temperature and pressure, simplifying comparisons and calculations.
What is SM³ (Standard Cubic Meter)?
The Standard Cubic Meter (SM³) is another standardized unit used in gas measurements. SM³ is defined at 20°C and 1 atm pressure.
Let’s take the same example, if gas flows through a pipeline at 35°C and 5 barg pressure, and we refer to the flow rate as 5 SM³/hr, this means the gas volume is equivalent to 5 meter cube per hour (SM3/hr) if it were measured at 20°C and 1 atm pressure.
This standard is widely used in industrial applications, particularly in regions and sectors that use 20°C as the reference temperature.
In summary:
- SM³ measures gas flow as if it were at 20°C and 1 atm pressure.
- SM³ values are independent of actual temperature and pressure and facilitate easy comparison in industrial calculations.
Example Calculation: Converting AM³ to NM³ and SM³
To illustrate the practical application of NM³ and SM³, let’s consider an example:
Suppose nitrogen gas flows through a pipeline at a rate of 10 M³/hr under actual conditions of 40°C and 10 barg. This 10 M³/hr is the actual flow rate at the actual temperature and pressure within the pipe.
If the same flow has to be converted to an NM³/hr, then the value will be
Where P1, V1 & T1 and P2, V2 & T2 stand for Pressure, Volume, and temperature of a gas at conditions 1 & 2 respectively.
Note: To eradicate the zero reading in the formula:
- The unit of temperature used is Kelvin
- The unit of pressure used is absolute pressure
Where,
Actual Conditions:
P1 = 11 Bara (as 10 Barg = 11 Bara)
V1 = 10 M3/hr
T1 = 313 K (40°C)
At 0°C temperature and 1 atm Pressure
P2 = 1 Bara (as 0 Barg = 1 Bara)
V2 = Gas Flow Rate in NM3/hr
T2 = 293 K (20°C)
Therefore:
V2 = 95.9 NM³/hr
And if the flow has to be converted to SM³/hr, then the value would be calculated as below:
Since we are using the same example, the actual conditions will remain the same, making
condition 2:
At 0°C temperature and 1 atm Pressure =
P2 = 1 Bara (as 0 Barg = 1 Bara)
V2 = Gas Flow Rate in NM3/hr
T2 = 293 K (20°C)
Therefore:
V2 = 95.9 NM³/hr
To make the calculation simpler, we have not taken into account the variation in relative humidity, and conversion of pressure from atm to bar.
Importance of Using NM³ and SM³ in Industrial Calculations
For industrial processes, maintaining constant temperature and pressure is often impractical. Equipment such as compressors, pipelines, and gas tanks operate in environments where conditions vary. Using NM³ and SM³ allows operators to express gas flow rates without needing constant recalibration for actual temperature and pressure changes. These standardized measurements also facilitate comparisons between different systems and ensure accuracy in flow calculations across diverse environments.
Conclusion
Understanding AM³, NM³, and SM³ is crucial for accurate gas flow measurement and calculation in industrial settings. NM³ standardizes gas flow to 0°C and 1 atm pressure, while SM³ uses 20°C and 1 atm pressure, both offering consistent, comparable measurements across varying environments. By using these standardized measurements, industries simplify the complexities of gas flow calculations, improving both accuracy and efficiency.
For any further questions or clarification regarding volumetric flow rates, please feel free to contact us at info@absstem.com.
