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When it comes to accurately measuring and controlling flow rates in the range of 1 gram/hour and lower, not only a good mass flow controller is essential, but also a proper designed system and the presence of other important components come into play. As in any system, it will be as strong as its weakest link.
For controlling low flow rates, the weakest link is usually not the mass flow sensor. A mass flow controller is capable of accurately measuring and controlling the flow rate, at its position in the system. However, there is no absolute confidence that this flow rate is accurate further up- or downstream of the flow controller. If no countermeasures are taken, the exact desired flow rate will not be delivered to the process. As the flow rate minimizes the relative internal volume of system components, such as piping, filters and valves, seem to increase. This affects the dynamics of the system as the response time will slow down resulting in a loss of direct control. So, when a set point is given and this is assumed to reach the process, expectations might not be met.
For instance, a popular setup to force a flow through the system is to make use of pressurized gas. However, gas will dissolve in the liquid to a saturation level proportional with the gas pressure. The dissolved gases appear again as bubbles downstream in the system where the pressure has decreased. If a gas bubble passes the flowmeter or valve or enters the process it disturbs the stability of the flow.
Practically for low flow rate processes, it is sometimes hard to understand why and when the system works correct. And so many questions arise. Is the purity of the media correct? Are the process temperatures as they should be? Is the set rate or dosage correct? Is the pressure stable?
For the lowest flow rates it is hard to verify if, at any time, the flow entering the process is as expected. As mentioned, there may be various underlying causes:
This blog focusses on the influence of dissolved gases in the liquid and the possible countermeasures. When dissolved gases in the liquid undergo a pressure drop through the system, gas bubbles tend to appear. The bubbles not only cause discontinuity in the flow but also tend to change the flow rate in between the gas bubbles. Several experiments have been carried out to investigate the phenomena and match it with known theories.
In figure 1 a setup is shown of two Bronkhorst low flow Coriolis mass flow instruments (mini CORI-FLOW™ ML120) in series. The first instrument is a mass flow meter. The second instrument acts as a controller, controlling the flow with an accurate onboard proportional valve positioned in front of its sensor.
In this specific case the fluid is pressurized using compressed air to force the liquid through the system. As the pressurized air comes into contact with the liquid, it will dissolve into the liquid proportional to the gas pressure. This experiment is to investigate the influence of dissolved gas in the liquid and the use of a degasser as a countermeasure for gas bubbles.
The picture 'Experiment without degasser' shows the outcome of the experiment when the setup has run for a few hours without a degasser. Clearly visible is the effect of gas bubbles passing the sensor of the flow controller. This can also be seen in the density measurement of the second instrument. The density drops each time a gas bubbles passes the instrument. The density is directly measured by the Coriolis instrument. A Coriolis instrument is capable of measuring density by a change in natural frequency of its vibrating measuring tube when liquid is flowing through it.
As expected the gas bubbles are generated by the valve in the mass flow controller as there the pressure drop occurs. As this valve is in front of its meter (in instrument 2) the mass flow meter detects the gas bubbles and thus the mass flow controller responds to it by controlling the valve. The physical effect of gas bubble generation occurs at any place in the system with large pressure drop, in most cases directly behindthe control valve. This effect is independent of measuring principle or type of control valve. Another remarkable phenomenon is that there is a difference in between the measured flows of both devices. It seems that the first instrument (mass flow meter) shows a lower flow rate of about 3% less than the flow rate measured by the second instrument (mass flow controller).
An explanation for this is that a generated bubble downstream of the control valve causes the volume flow to expand and pushes the liquid forward. As the mass flow controller will maintain its set point value of 1 gram/hour the flowrate is “slowed down” to maintain the correct mass flow. Therefore the flowrate through the first flowmeter is 3% less in between the bubbles.
There is a difference in volumetric flow rate before and after the appearance of the gas bubbles. However, the average mass flow rate in the instruments in both experiments is within specification and thus the same, as shown in table 1. This table shows the average deviation from 1 gram/hour of each instrument in both experiments over the entire dataset as shown in the charts.
The 3% error matches ‘Henrys law,’ which tells us that the solubility of air in water is 22 milligram/liter per bar. If this number is divided by the density of air, the volumetric expansion explains the 3% increase in volume flow after the gas bubbles appear. So the total volume flow is 3% higher due to the gas bubbles and the mass flow drops to nearly zero at a gas bubble in 3% of the time. This explains why the average mass flow, including the gas bubbles, remains the same compared to when the gas was disolved.
In order to take out the dissolved gas before problems appear, a HPLC (high-performance liquid chromatography) degasser is used. This device uses a permeable tube to degas the liquid. The permeable tube is positioned inside a vacuum chamber where the vacuum is maintained by a small onboard vacuum pump. The device extracts most of the dissolved gases in the used liquid.
Furthermore, as the liquid is well degassed it is capable of easily dissolving any remaining small bubbles that are left behind in the system. In this way the system will end up fully filled with liquid without any pockets left with gas. As gases are compressible, a properly degassed system makes the system stiff and very responsive. A system like this is capable of generating a continuous and stable flow towards the process with good control behaviour.
The picture 'Experiment with degasser' shows the measured outcome where the degasser is put up in front of the Coriolis mass flow instruments. It is clearly visible that the system can run for several hours without any drops or glitches in mass flow or density. Apparently, no air bubbles are present in the system or generated by the control valve. The small deviation between the instruments is within the specified accuracy of 0.2% of reading ± 20 milligram/hour zero stability.
In many low flow fluidic control systems the fluid is pressurized with a gas. When gas is entrained in the liquid flow it can appear as dissolved gas or as gas bubbles. In both cases it has no significant influence on the average mass flow. However, gas bubbles tend to disrubt the stability of the flow. The effect can be monitored by a fast and accurate flow meter. This physical effect occurs in any low liquid flow system with dissolved gases and pressure drop downstream and is independent of measuring principle or type of control valve.
It is recommended to use a degasser for generating a continuous, stable and responsive system towards the end process, especially in low flow measurements of liquids. An ideal solution for these low flow measurements would be a degasser in combination with a Bronkhorst mini CORI-FLOW ML120 mass flow meter/controller, as is used in this experiment.
As this mass flow controller has its control valve in front of the meter, the sensor is capable of monitoring the actual flow in the system. This results in an optimal and direct process control. The flow controller can be used for (ultra) low flow applications up to 200 gram/hour.