Why is Mass Flow Measurement important within process industries and what are the strengths of Coriolis Flow Meters and Controllers?
Measurement of flow, whether it is a liquid or gas, is commonly a critical parameter in many processes. In most operations it is important to know that the right fluid is at the right place at the right time. Some critical applications require the ability to conduct accurate flow measurements to ensure product quality. Health & Safety is always an important factor when working with liquids and gases, investment in ensuring your team can operate in a safe and productive environment is very important. Measuring flow and pressure can provide this security to the process and personnel.
With most liquid & gas flow measurement instruments, the flow rate is determined inferentially by measuring the fluids velocity or the change in kinetic energy.
Other factors that affect liquid flow rate include the liquid's viscosity and density, and the friction of the liquid in contact with the pipe.
With the many variations of flowmeter technology available it can be challenging to make a decision on which technology is right for the application.
An important and perhaps overlooked question, is what the instrument is supposed to do versus what is it able to do?
Direct flow measurement
Direct mass flow measurement is an important development across industry as it eliminates inaccuracies caused by the physical properties of the fluid, not least being the difference between mass and volumetric flow. Mass is not affected by changing temperature and pressure. This alone makes it an important method of fluid flow measurement. Volumetric flow remains valid, in terms of accuracy, provided that the process conditions and calibration reference conditions are adhered to. Volumetric measuring devices, such as variable area meters and turbine flow meters, are unable to distinguish temperature or pressure changes.
One method of Mass Flow measurement employs the phenomenon of Coriolis force.
The Coriolis principle
This long understood principle is all around us in the physical world; for example the earth’s rotation and its effect on the weather. The operating principle is basic but very effective.
A tube is energised by a fixed vibration. When a fluid passes through this tube the mass flow momentum will cause a change in the tube vibration, the tube will twist resulting in a phase shift. This phase shift can be measured and a linear output derived proportional to flow.
As this principle measures mass flow independent of what is within the tube it can be directly applied to any fluid flowing through it, liquid or gas. Furthermore, in parallel with the phase shift in frequency between inlet and outlet it is also possible to measure the actual change in natural frequency. This change in frequency is in direct proportion to the density of the fluid – and a further signal output can be derived. Having measured both the mass flow rate and the density it is, interestingly, possible to derive the volume flow rate.
The Coriolis mass flow meters offer significant features compared to other principles:
- No need for (re)calibration in the field – fluid independent flow measurement and control
- Gas and liquid can be measured with the same sensor
- Ability to measure undefined or variable mixture
- Multi parameters
The Coriolis principle, applied as a mass flow meter, therefore has its place within fluid measurement and control within the Process Industry.