How low can you go?

23 May 2024

Neil Hannay looks at a variety of flowmeter technologies and offers some thoughts on their suitability for low-flow measurement applications.

Many industries today rely on use of low-flow meters to ensure accurate flow rates of liquids in processes. Whether cleaning fluid additives, syrups and flavourings for beer or soda, chemical additives for oil and fuel, paint pigments or administering drugs, low flow flowmeters are required to dose concentrated fluids at the end process, dispensing the precise amount of liquid to the correct dilution.   

However, the smaller the flow the harder it is to control and measure and finding a suitable flow measuring technology can prove challenging.

There is no set definition for ‘low flow’ in terms of measurement limits for fluidics handling. However, low-flow applications encounter amplified flow stability and performance issues not seen in larger flows. The minimal liquid volume being measured in low flows renders them highly sensitive –even the slightest disruptions in process or ambient conditions can exert a substantial impact on flow stability. 

Within the markets Titan Enterprises operates in, we consider low flow rates as those below 50 ml/min, with many customers seeking flow rates of between 2 and 20 ml/min.

The amount of energy available in low liquid flow is unlikely to be sufficient to drive most mechanical flowmeters to give linear results. By comparison, electronic flow meters can be limited by sensitivity, zero drift and slow response times. 

A limiting component
Flowmeters can be the most limiting component of a low flow fluidic system, so it is essential to choose the most suitable high-precision flow sensor for a particular application. There are, typically, five types of flow meter to choose from – ultrasonic, turbine, oval gear, thermal and Coriolis – all have their pro’s and con’s.

Ultrasonic: Ultrasonic flowmeters measure the velocity of flow. Titan’s solution – which uses time of flight technology – is capable of measuring flows down to 2ml/min. Lower flow rates equate to smaller signals to determine flow rate and as such, this lower signal strength can affect the flowmeter’s capability to produce repeatable measurement results. The benefits of the technology include high accuracy. Further, it is not fluid specific, there are no pressure drop requirements and it offers a high signal to noise ratio. It is also suitable for both turbulent and laminar liquid flow. On the downside, however, the technology is susceptible to process vibrations/pulsations/noise and is sensitive to gas entrainment.

Turbine: The energy required to spin the rotor of a turbine flowmeter becomes swamped by the drag from the system at low flow rates. As flow rate reduces and transitions from turbulent to laminar flow, the linearity changes and the measurements become less accurate. Pelton wheel turbines that use low friction, precision bearings can mitigate this effect to some degree and, with the benefit of careful design, are capable of flows down to 1 or 2ml/min. The technology is capable of fast response times and operates across wide flow and operating temperature ranges.

Benefits of the technology include its low cost. It can be calibrated in-situ and it offers good accuracy and repeatability with rapid response times.

Disadvantages include the fact that the technology is susceptible to changes in fluid properties and it requires sufficient pressure to move liquid through the pipeline at a rate that causes the turbine blades to spin.

Oval gear: Positive displacement flowmeters, such as oval gear meters, are effective for measuring low flow viscous fluids, although the resolution can be quite low. To obtain good resolution, the oval gear meters need to be small in low flow applications. Installing an oval gear meter in a horizontal position will reduce rotational friction and improve low-flow measurements. The lower the flow, the smaller the gear size, which are designed with tight tolerances with small internal clearances to minimise any fluid leakage around the gears. 

The technology is well suited to use with viscous liquids. It offers precision chemical dosing and is reliable.

The technology is not, however, suited to use with low flow aqueous solutions as the slippage past the moving element is greater than the volume being measured. Trapped air can prevent small gears from rotating so it is important to ensure all gas is purged on initial startup.

Thermal: Thermal flow sensors are primarily used for monitoring gas flow. They operate on the principle of monitoring thermal transfer using a reference temperature, a heat injection and a detector. The basic approach is that heat is added to the flowing stream and a temperature imbalance is used to obtain a flow rate. The technology is fluid-specific, relying on the thermal properties of the liquid and it is generally calibrated for the specific fluid properties. 

The technology is highly sensitive and is able to measure flow rates down to nanolitres per minute. It is suitable for low pressure drop applications and is not so reliant on the dynamics of the fluid to make a measurement.

The technology is fluid specific. It is non-linear over its temperature range and so requires some correction during the process. It is not suitable for low boiling point liquids or liquid mixtures with changing composition.

Coriolis: The Coriolis is a mass flowmeter – it measures mass flow directly and independently of the properties of the liquid. It provides mass flow and density measurements that are both repeatable and highly accurate, even when the composition of the liquid is unknown or changing. Using the principle of accelerating a moving fluid and detecting the reaction on the vibrating tube with sensors, Coriolis meters are very sensitive and flows lower than 0.2ml/min are possible.

The technology offers extensive material compatibility and can be used for either liquid or gas flow measurement. It is also independent of liquid or process variables.

The primary limitation of Coriolis technology is that the flow needs to be single-phase and the material needs to be low viscosity. The devices are also costly which means they are not suitable for many low-cost low flow applications.

Neil Hannay is Senior R&D Engineer at Titan Enterprises.

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