Low Flow Rates With Large Pulsations: Solving Flow Measurement Issues
Commonly low flow liquid movements are generated by a reciprocating motive action peristaltic, piston or diaphragm pump. Using such pumps can cause very large pulsations in liquid pipelines resulting in measurement problems with most inferential flow meters. Likewise turbine flow meters, differential pressure devices and velocity measuring flow meters are also likely to experience problems. Though positive displacement and Coriolis meters will normally handle pulse variations, even these flow devices can have problems.
Removing Flow Pulsations: Tuned Pulsation Dampers
The best solution for any flow meter, particularly with low flow rate measurement, is to remove the pulsations. This is best done in two stages. First regulate the pressure and then install a tuned pulsation damper. The balance between the two will depend on the frequency and amplitude of the pulsations and the solution is only likely to be effective at a given flow, temperature and pressure condition. In some cases the plumbing in the system can work in the users favour. If the meter is located after a long length of flexible tubing a lot of the pulsations will have been reduced to an acceptable level.
Low Flow Rate Measurement: Flexible Lines Can Be Problematic With Low-Flow Systems
If a dead ended column is added where the amount of air stored can be varied with a release valve – an in-built pulsation damper can be formed. From experience it will require some trial and error to establish the correct volume of air for the line pressure but this can be done. However using flexible lines and air after the flow meter can also cause problems as these can induce reverse flow. Depending on the meter type this will manifest as positive flow adding to the metered volume.
Low Flow Rate Measurement: Solid Pipework Often Better For Low Flow Systems
Overall with low flow systems/low flow rate measurement it is often better to have solid pipework after the flow meter and flexible before. Also take care when designing your installation to ensure that there are no spaces for any unwanted vapour to collect before or after the flow meter as this can exacerbate pulsation problems.