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This is the 3rd part of our blog series ‘How to hand low liquid flows’. In this part we share our insights about liquid supply using a pressure vessel. A pressure vessel is one of the two methods you can use to provide a stable inlet pressure. The other one is using a pump. You can read more about liquid supply with a pump in our next blog, part 4.
Did you miss out on the previous parts?
In part 1 we explained the definition of (ultra) low liquid flows and in part 2 we shared our tips & tricks about how to select a flow meter to optimise the stability and performance of your process.
Download the e-book 'How to handle low liquid flows' to find out more about 'low flows'. Including in-depth information, technical advice and insider tips from our experts.
As indicated in the previous part when we provided tips on how to select a suitable flow meter or controller, a highly stable inlet pressure is necessary to obtain a stable low liquid flow rate. While a gas as a compressible medium can have a cushioning effect in levelling the pressure of a system, a liquid is not very resilient. Although Bronkhorst flow controllers are capable of neutralising pressure fluctuations to some extent, rapid changes in the inlet pressure can destabilise the flow.
In general, two methods are suitable for providing a stable inlet pressure to the liquid system: using a pressure vessel where gas is applied to pressurise the liquid or a pump. Whichever method you choose is partly a practical decision based on what infrastructure is available at the customer’s site.
A pressure vessel is a relatively safe option without needing electricity or moving parts, and it is a plus for some volatile liquids. However, gas bubbles dissolved in the liquid have a negative effect on the flow stability.
Frequently, it is a balance between the volume of liquid to be dosed, the pressure and the application. In a research lab, for example, helium gas for pressurising purposes may be available, whilst in a production environment another solution needs to be found, usually pump controlled. Read Part 4 of this blog series for more details about using a pump to provide a stable inlet pressure.
When using a pressure vessel, the main challenge is to minimise the dissolution of gas in the liquid to be processed. The less gas that comes into contact with the liquid, the better it is. Henry’s law states that the amount of dissolved gas in a liquid is proportional to the pressure of the gas that is in direct contact with the liquid. This has some practical implications.
Gas used to pressurise the liquid can be dissolved in that liquid at a high pressures. However, when the pressure lowers further downstream in the process, this dissolved gas will be released as gas bubbles again. This is usually an unwanted phenomenon. Moreover, the solubility of a gas in a liquid decreases as the temperature gets higher. A temperature rise of the processed liquid will therefore also result in released gas bubbles.
In this respect, it is recommended that the pressure and temperature drop over the liquid path is kept as small as possible.
If gas is used to pressurise the liquid, an important recommendation is therefore to prevent the gas from coming into direct contact with the liquid. For example, use a pressure vessel with a membrane that physically separates the liquid from the pressurising gas - in a similar way as an expansion tank is used in your central heating system at home.
If it is necessary or inevitable for the pressurising gas to come into direct contact with the liquid, there are some solutions. For example: apply gases with a low solubility. Helium is usually the best choice for water-based liquids, followed by nitrogen. If possible, apply the lowest possible pressure to the liquid. This obviously depends on the working pressure of the application. In this respect, it may help to position the liquid vessel substantially higher than the flow controller - to let gravity do its work and thus require a lower gas pressure.
As a last resort, use a degasser to remove the gas from the liquid. This is a device with a porous hose to which a vacuum is applied on the outside, causing gas bubbles to be drawn out of the liquid inside the hose. Standard degassers go well with water-like media. For low liquid flow rates below 50 g/h and depending on the type of application, we recommend using a degasser if stability and maximum accuracy are key and if gas bubble disturbances need to be deleted. A degasser is considered an expensive option, but if you have a high-end application, you look for the best available solution. Metaphorically speaking, you don't buy 25 euro tyres for a very expensive racing car, or do you?
If the inlet pressure fluctuations in the pressure vessel setup are too fast for the flow controller to be able to compensate, there are some solutions. A pressure relief or a buffer tank between the pressure regulator and the liquid vessel can smooth out these fluctuations to provide a stable inlet pressure.
For use with water-based media, Bronkhorst recommends having a vessel made out of one piece of passivated steel to deal with corrosion issues of especially welded parts. With organic solvents like methanol, toluene or acetone, corrosion of welded parts in a vessel is not usually a problem. Because liquid flow control with a pressure vessel is usually a batch process, make sure that the vessel is large enough to provide a stable flow for a sufficient amount of time between refills. To prevent splashing and liquid spills when filling, use a filling funnel at the inlet.
Blog series - Part 1/5: What are low liquid flows? A blog series about how to handle low liquid flows including the definitions and tips.
Blog series - Part 2/5: Tips for flow meter selection. A blog series about how to optimise the stability and performance of your process.
Blog series – Part 4/5: How to use a pump with your flow meter for a highly stable inlet pressure.
Blog series – Part 5/5: How can external conditions influence your flow meter and what to do about it?