Coriolis mass-flow controllers (MFCs) are widely used in the chemical process industries (CPI) for applications that require precise and accurate measurement and control of critical process-fluid flows. Coriolis MFCs can measure and control both gas and liquid flows, and provide a direct mass-flow measurement that is independent of fluid properties, such as density or viscosity. However, in low-flow liquid applications, Coriolis MFCs can be susceptible to clogging due to their small orifice size, which can range from 0.1 to 2 mm in diameter. Clogging can affect the performance and reliability of Coriolis MFCs, and lead to inaccurate readings, flow instability, or even damage to the instrument. Therefore, it is important to prevent clogs from forming in low-flow Coriolis MFCs by using proper filtration and maintenance techniques.
Sources and types of clogs
Clogs can form in low-flow Coriolis MFCs due to various sources and types of contaminants in the fluid. Some common sources and types of clogs are:
- Particulate matter: This can include dust, dirt, rust, metal shavings, or other solid particles that are present in the fluid or the piping system. Particulate matter can accumulate in the orifice of the Coriolis MFC and obstruct the flow, or cause erosion or abrasion of the sensor tube. Particulate matter can also affect the density and viscosity of the fluid, which can alter the Coriolis force and the mass-flow measurement.
- Gas bubbles: This can occur when the fluid contains dissolved gases, such as air, nitrogen, or hydrogen, that can come out of solution under certain conditions, such as changes in temperature, pressure, or flowrate. Gas bubbles can interfere with the vibration of the sensor tube and the Coriolis force, and cause noise or fluctuations in the mass-flow measurement. Gas bubbles can also coalesce and form larger bubbles that can block the orifice of the Coriolis MFC and reduce the flowrate.
- Precipitates: This can happen when the fluid contains dissolved salts, such as calcium carbonate, magnesium sulfate, or sodium chloride, that can precipitate out of solution under certain conditions, such as changes in pH, temperature, or concentration. Precipitates can deposit on the inner walls of the sensor tube and the orifice of the Coriolis MFC, and reduce the cross-sectional area and the flowrate. Precipitates can also affect the density and viscosity of the fluid, and the mass-flow measurement.
- Biofilms: This can occur when the fluid contains microorganisms, such as bacteria, fungi, or algae, that can grow and form a slimy layer on the inner surfaces of the sensor tube and the orifice of the Coriolis MFC. Biofilms can reduce the flowrate and the heat transfer, and cause corrosion or fouling of the sensor tube. Biofilms can also affect the density and viscosity of the fluid, and the mass-flow measurement.
Filtration and clog prevention
Filtration is the most effective and common method to prevent clogs from forming in low-flow Coriolis MFCs. Filtration can remove or reduce the amount of contaminants in the fluid before they reach the Coriolis MFC, and protect the instrument from clogging and damage. However, filtration also introduces some challenges and trade-offs, such as pressure drop, filter selection, filter placement, and filter maintenance. Therefore, it is important to consider the following factors when designing and implementing a filtration system for low-flow Coriolis MFCs:
- Pressure drop: This is the difference in pressure between the inlet and the outlet of the filter, which is caused by the resistance of the filter media to the fluid flow. Pressure drop can affect the flowrate and the performance of the Coriolis MFC, and increase the energy consumption and the operating cost of the system. Therefore, it is desirable to minimize the pressure drop by choosing a filter media that has a low resistance and a high porosity, and by sizing the filter properly to match the flowrate and the viscosity of the fluid.
- Filter selection: This is the process of choosing the appropriate type, size, and material of the filter media, based on the characteristics of the fluid and the contaminants. The type of the filter media can be either surface or depth, depending on the mechanism of filtration. Surface filters trap the contaminants on the surface of the filter media, and have a defined pore size and a high efficiency. Depth filters capture the contaminants within the depth of the filter media, and have a range of pore sizes and a high dirt-holding capacity. The size of the filter media is determined by the pore size and the surface area, which affect the filtration efficiency and the pressure drop. The material of the filter media can be either metal, ceramic, polymer, or composite, depending on the compatibility and the durability of the filter media. The material of the filter media should be resistant to the fluid, the contaminants, and the operating conditions, such as temperature, pressure, and corrosion.
- Filter placement: This is the location of the filter in relation to the Coriolis MFC and the fluid source. The filter can be either upstream or downstream of the Coriolis MFC, depending on the purpose and the preference of the filtration. Upstream filtration is more common and recommended, as it protects the Coriolis MFC from clogging and damage by removing the contaminants before they enter the instrument. Downstream filtration is less common and optional, as it protects the downstream equipment and processes from contamination by removing the contaminants after they exit the instrument. However, downstream filtration can also introduce some risks and disadvantages, such as back pressure, back flow, or recontamination of the fluid.
- Filter maintenance: This is the process of monitoring, cleaning, and replacing the filter media, based on the condition and the performance of the filter. Filter maintenance is essential to ensure the effectiveness and the reliability of the filtration system, and to prevent clogs and damage to the Coriolis MFC. Filter maintenance can be either manual or automatic, depending on the type and the design of the filter. Manual filter maintenance requires periodic inspection and intervention by the operator, and involves removing, cleaning, or replacing the filter media. Automatic filter maintenance does not require human intervention, and involves self-cleaning or self-replacing mechanisms, such as back flushing, back washing, or cartridge replacement.
