Blog series: How to handle low liquid flows? Part 3

Liquid supply – a stable low liquid flow

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. 
 

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.

Tip 1: Use a pressure vessel

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.
 

Tip 2: Apply gases with low solubility

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. 
 

Tip 3: Use a degasser

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?
 

Tip 5: How to deal with corrosion issues of welded parts

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.