Summary

Standard volumetric flow rates of a fluid are the equivalent of actual volumetric flow rates in the sense that they have an equal mass flow rate. This identity makes standard volumetric flow appropriate providing a common baseline for comparison of volumetric gas flow rate measurements at different conditions. This article outlines how to convert between standard and actual volumetric flow rates.

Definitions

Introduction

Standard volumetric flow rates denote volumetric flow rates of gas corrected to standardised properties of temperature, pressure and relative humidity. Its use is common across engineering and allows a direct comparison to be made between gaseous flows in a manner identical to comparing their mass flow rates. Standard volumetric flow is also commonly used by vendors when describing the capacity of vents or pressure relief devices, however for capacity checks at different conditions a comparison on a pressure loss basis is more appropriate.

The most common units to describe standard volumetric flows are standard cubic meters per hour (SCMH) in metric units and standard cubic feet per minute (SCFM) in imperial units.

However care must be taken when working with standard volumetric flows as the standard conditions may vary country to country or even region to region. The most common standards used are IUPAC (100kPa, 273.15K), ISO2533 (101.325kPa, 288.15K) and DIN1342 (101.325 kPa, 273.15K).

Conversion Calculations

Here we will adopt the convention that subscript ‘std’ denotes properties of the fluid at standard conditions and the subscript ‘actual’ denoted the process fluid at the actual process conditions. Following these conventions the equations to convert between standard volumetric flow and actual are presented below:

Known Gas Density

The conversion to and from standard volumetric flow can be completed with relative ease if the densities of the fluid at actual and standard conditions are known.

Unknown Gas Density

If the density for the two fluids is not known for the required conditions the density may be approximated using the ideal gas equation yielding the following equations:

Unknown Gas Density - Simplified

The above formula can be simplified if the same gas is considered and the pressure and temperature do not differ greatly between both sets of conditions. This simplification assumes that the molecular weight and compressibility do not change, which is generally safe for the same fluid at similar temperatures and pressures.