There is a vast variety of measurement systems for volume flows. Some of these systems possess sensors that induce a temperature gradient in a silicon sensor, which is electrically maintained at a constant temperature. This gradient is recorded in the sensor and processed into the corresponding flow variables.
These flow sensors are used to measure velocities in gas pipelines. The position of the sensor in the pipeline is of utmost importance for the accuracy of the measurement. On behalf of a renowned manufacturer of measurement systems, FlowMotion has calculated and analysed the flow in various pipeline components.
Since the flow velocity in a pipeline is zero directly at the inner wall and maximal at the centre, a velocity profile is formed. It is assumed that, depending on the volume flow, a flow length of 50 to 150 times the pipe diameter is necessary for a constant velocity profile to be established in the pipe. This is also referred to as a fully developed flow. If the volume flow is calculated from a single velocity measurement, the velocity profile must be known. For this reason, it is of crucial importance to know exactly when the flow has fully developed again after elbows, branches, or fittings.
Even small percentage deviations in the calculated volume flow can lead to very high costs. To investigate the influence of various elbows and pipe expansions, Computational Fluid Dynamics (CFD) simulations were conducted. Special attention was paid to the actual flow length required for the re-establishment of a fully developed velocity profile.
The simulations have shown that, on average, a minimum of 40 pipe diameters are required for this. In this context, CFD calculations offer two possibilities: firstly, the precise positioning of the measuring probe to maximise measurement accuracy; secondly, instead of the theoretical fully developed velocity profile, the simulated profile can be used to calculate the volume flow. This is sensible when the length of the pipeline is shorter than 40 diameters due to limited installation space.
