Mass Flow Measurement

Why is mass flow measurement important, and what are the strengths of Coriolis effect meters and regulators? Precision Fluid explains how this principle gained hold in the control and measurement of fluids

by Noemi Sala

Flow measurement, whether relative to a fluid or gas, is a common critical parameter in many processes. In most operations it is necessary to know that the right fluid is in the right place at the right time. Some critical applications require the ability to perform accurate flow measurements to ensure product quality. Health and safety are always important factors in working with liquids and gases, and require an investment to ensure that operators a safe and productive environment. Flow and pressure measurement can provide this level of safety to the process and personnel.
Most liquid and gas flow measurement instruments determine the flow rate inferentially, by measuring fluid velocity or kinetic energy change. Other factors affecting the flow rate of the liquid include the viscosity and density of the liquid itself and the friction of the liquid in contact with the pipeline. With the variations in meter technology available, it can be difficult to choose the right technology for the application. The question which needs to be asked, and which might have been ignored, is: what was the instrument designed for and what is it capable of doing?

Direct Flow Measurement
Direct flow measurement is an important development in industry as it eliminates inaccuracies caused by the physical properties of the fluid as well as the difference between mass and volume flow. Mass is not affected by changes in temperature and pressure, and alone is a valuable method of measuring fluid flow. The volumetric flow rate remains valid, in terms of accuracy, as long as the process and reference calibration conditions are met. Volumetric measurement devices, such as variable area and turbine meters, cannot distinguish temperature or pressure changes.

The Coriolis principle: what are its advantages?
A mass flow measurement method uses the Coriolis force phenomenon. This principle is all around us in the physical world: one example is the Earth’s rotation and its effect on climatic conditions.
It is a basic but extremely effective working principle. A tube is energized by a fixed vibration. When a fluid passes through this tube, the momentum of the mass flow will cause a change in the vibration of the tube, which will undergo a torsion causing a phase shift. This displacement can be measured, and a linear output proportional to the flow can be derived.
Since this principle measures the mass flow regardless of the pipe content, it may be directly applied to any fluid, liquid or gas. Besides, parallel to this phase shift in frequency between input and output, it is also possible to measure the actual change in natural frequency. This change in frequency is directly proportional to the density of the fluid, and an additional signal output may be obtained. Having measured both the mass flow rate and the density, an interesting possibility is the ability to derive the volumetric flow rate.
Here are some significant characteristics of Coriolis effect mass flow meters: it is not necessary to perform (re)calibrations on the field (control and measurement of the flow rate independently from the fluid); gases and liquids may be measured with the same sensor; it is possible to measure undefined or variable mixtures; multiple parameters.

An additional functionality
The Coriolis principle, applied as mass meters, thus gains hold among the fluid control and measurement procedures in the process industry. Bronkhorst Coriolis mass meters have the additional functionality of directly controlling a secondary instrument, which may be a pump or valve. Using the integrated PID control, fast and accurate response times can be achieved with flow rates ranging from 0.05 g/h up to 600 kg/h. All of this with an accuracy of less than 0.2% for liquids and 0.5% for gases, thus allowing considerable savings in terms of dosed products. Finally, by exploiting the control functionality provided by the Bronkhorst PID, the integration of the solution into a current application or into the manufacturing process turns out to be simple and straightforward.

How can process conditions be effectively managed?
To manage these process conditions, a meter may be added between the pump and the process. In this way the flow data from the flow meter may be read to adjust the pump speed accordingly. Traditionally, this could be managed with an analogue (4…20mA or similar) output, then taking the signal from the flow meter to a separate PID controller which compares the actual flow signal with the desired one. The electronic controller may then adjust the pump speed to obtain the desired dose and flow.
This solution will reduce errors in the original solution, but it will decrease the speed of the flow, due to filtering of the external meter/controller signal, and of the pump response, due to an additional control relay; it will also increase complexity with extra components, and the time required to achieve a stable flow can be long.

The solution to control the pump directly
Precision Fluid suggests the use of a mass flow measurement device with integrated PID control, which can directly control the pump until the desired dose or flow is reached.
With this solution, there is no need to include the pump in the control system: all that is needed is to make a set-point request to the mass flow meter, and this will guide the pump to its achievement.
Here are some of the advantages: no particular operator intervention is required; direct control of the dose/flow mass flow; accurate release which reduces normal pump emissions; low dose/flow alarm function; preventive maintenance based on pump performance over time; measurement based on passage dose/flow and not on assumed values; high accuracy and repeatability.
The device may be used in any sector where the liquid is dispensed in a container requiring quality control, generally performed on a small percentage of the vials to ensure overall compliance. If a mass flow meter is used to control the dose, it is possible to achieve 100% quality control of the product with reduced human input.
Whether dosing additives or high performance chemicals, or mixing liquids, the additive’s flow control capability and the understanding of the nature of the flow itself may represent an enormous benefit for the outcome of the application.