Blog series: How to handle low liquid flows? Part 2

Tips ‘n’ tricks

Bearing in mind the above information, please read and observe the following tips with respect to low liquid flows:

Tip 1: Choose a flow meter or controller that is suited to the process and ambient conditions

The Bronkhorst website offers a product selector tool to help you choose the right low liquid flow meter or controller based on input parameters such as maximum (mass or volumetric) flow rate, operating pressure and operating temperature. 
Thermal-based μ-FLOW and LIQUI-FLOW devices can handle liquids up to 2 g/h resp. 0.25 to 5 g/h in the lowest range. Coriolis-based mini CORI-FLOW ML120 mass flow meters have 200 g/h as their maximum full scale flow rate but can be easily scaled down to a minimum full scale flow rate of 5 g/h, with the same relative accuracy. They also have a minimum flow rate of 50 mg/h.

Tip 2: Provide a stable (inlet) pressure to the fluid system

A highly stable inlet pressure of a flow controller is a prerequisite for a stable low liquid flow rate. Two methods are popular to achieve this: use a pressure vessel where gas is used to pressurise the liquid or use a pump. Please refer to parts 3 and 4 of this series of blogs for more background details.

Tip 3: In the case of a pressure vessel, minimise the containment or dissolution of gas in the liquid to be processed

 Air or other gas bubbles dissolved in the liquid, or moving along the liquid flow, have a negative effect on the flow stability. To this end:

• If gas is used to pressurise the liquid, prevent the gas from getting into direct contact with the liquid by using a membrane.
• Alternatively, use a gas with low solubility such as helium or nitrogen to pressurise the liquid when the gas needs to get into direct contact with the liquid. Apply the lowest possible pressure to the liquid and keep pressure drops throughout the fluid path as small as possible. This obviously depends on the working pressure of the application. As a last resort, use a degasser (see Part 3) to remove the gas from the liquid.

Tip 4: Choose a piezo valve to accurately control low liquid flows

The quick response time, low internal volume and low heat generation of piezo valves are especially advantageous when using gas to pressurise the liquid. For an operating pressure higher than 5 bars, solenoid valves are an alternative. Preferably install the meter part of the flow controller between the valve and the process.

Tip 5: Keep the internal volume between the flow device and the process as small as possible

This is to help minimise filling times and to limit external disturbances. For this purpose:

• Make tubing in the fluid system as short as possible and choose small diameter tubing.
• Use hard tubing such as stainless steel rather than flexible tubing.
• Avoid ‘dead volumes’ in bends and valves where air bubbles can be trapped. The μ-FLOW and mini CORI-FLOW L120 devices each contain a continuous capillary with a limited dead volume.

Tip 6: Purge the system before operation

This is especially important for skids. First connect all instruments and then purge before starting to control or measure. To this end, the Bronkhorst skids contain a purge setting.

Stay tuned for part 3, 4 & 5

In the next parts of the blog series, we will focus on low liquid flow supply using a pressure vessel or a pump and give more background details on how to manage external conditions. After all, choosing a good flow device is important, but insight into how this influences the application process is equally essential.