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

February 18, 2020 Allard Overmeen

Part 2: Tips for flow meter selection

As you might remember from reading the first part of this blog series, for (ultra) low liquid flows, tiny disturbances in the process or ambient conditions can have a large impact on flow stability. This second blog aims to help users of low-flow fluid systems optimise the stability and performance of their system, by giving tips and recommendations for dimensioning, choice of material and best practice procedures, based on years of experience, gathered by Bronkhorst High-Tech.

The core of the low-flow fluid system is obviously a liquid flow meter/controller. Deciding which instrument type is best for the low-flow application largely depends on accuracy and stability requirements. However, ambient or media conditions which are difficult to control can also play an important role in choosing the best solution.

Blog series: Tips for Flow Meter Selection

When to choose a thermal or Coriolis flow meter or controller?

Within the Bronkhorst range of products, some thermal and Coriolis flow devices are particularly suitable for (ultra) low liquid flow applications. The different working principles for thermal and Coriolis devices make them specifically suitable for different applications, requirements and conditions. In thermal-based mass flow devices, a constant temperature difference is created between two positions along a (capillary) tube. When liquid flows through this tube, the energy needed to maintain this temperature difference is proportional to the mass flow rate. 

Read more about the thermal-based mass flow measurement working principle.

In the Coriolis setup, the liquid flow through a tube with a small diameter causes this tube to twist, and the change in deflection is a direct measure of the mass flow through the tube. Moreover, the resulting change in vibration frequency of the (filled) tube is proportional to the medium density.

Read more about the Coriolis mass flow measurement principle

Generally speaking, a Coriolis flow meter/controller ... 

  • performs very well in situations where absolute accuracy and flow stability are essential; 
  • has long-term stability and negligible thermal sensitivity;
  • is a good choice when the medium density needs to be measured or monitored, in addition to the flow;
  • can be used for mixtures of liquids with unknown properties (i.e. is media independent);
  • is somewhat receptive to vibrations around the resonance frequency, which may necessitate the use of shock absorbing measures.

Blog series mini cori flow ML120

​On the other hand, a thermal flow meter/controller ...

  • is a more economical choice if the processed liquids and (ambient) temperatures are stable and distributed evenly;
  • performs well when reproducibility is more essential than accuracy;
  • requires specification of the liquid density, viscosity, thermal conductivity and heat capacity;
  • generally causes a relatively small pressure drop, which can help to keep the flow stable if the liquid contains a considerable amount of dissolved gas.

Blog series EL Flow Prestige Flow meter

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) mass 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.

Blogseries ES-FLOW liquid flow

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: De-aerate the system before operation

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

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. 

Blog series skid solution an dosing for solar panels flow control

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