FLOW, LEVEL & CONTROL FEATURE T


he metering of liquids at low flow rates can present problems,


with the low energy available from the fluid and the miniaturisation of flowmeter components being the most obvious. Fluidic considerations can, however, also be daunting. For flow in a pipe or tube, the Reynolds number (Re) – see Figure 1, right – is a dimensionless, mathematical term which takes into consideration density, viscosity, velocity, pressure and various other operating parameters. Apart from laminar flow meters, which rely on measuring fluids with low Reynolds numbers, most traditional turbine type flow meters


Figure 1:


Reynolds number, which takes into account various operating parameters


EXAMINING THE TECHNIQUES for measuring flow in small bore tubes


As with many measurement technologies, no one flow measurement technique is ideal for all applications. Trevor Forster, managing director of Titan Enterprises, looks into the variety of technologies available for measuring flow in small bore tubes


require turbulent flow, which equates to higher Reynolds numbers. Low flows therefore usually equate


to laminar flow, and the relative characteristics of the fluid are more like treacle that water. Probably the earliest successful low


flow metering technology was a vertically mounted ball in a clear tapered tube (see Figure 2, below), which worked well at both high and low Reynolds numbers. In this type of meter a ball, or shaped float, is fitted to a tube with a bore in which the cross sectional area changes along


Figure 2:


An early low flow metering technology. Target applications included low cost low indications of non-opaque fluids


its length. The float rises up against a calibrated scale on the tube as the flow increases. Due to the low energy in the system, gravity is used as the reactive force against the float along with the relative buoyancy of the float and the viscosity and density in the fluid being metered. However, if the viscosity or density of the liquid changes, so does the drag or lift on the float, setting a limitation on the accuracy of this type of meter. With these, tiny changes in the


operating conditions give a disproportionate change in the flow reading, so for best performance these meters have to be calibrated and used in their actual operating conditions. Typically, these meters do not give an output for remote instrumentation or totalising.


DEVELOPMENTS Another early flow meter development was the differential pressure (∆P) meter which used a laminar flow element. Unfortunately, these flowmeters require fluids with a high Reynolds number to work efficiently. At Reynolds numbers below around 2000 the flow is termed laminar. The easiest way to visualise this is to


imagine a tap on your kitchen sink being opened. At low laminar flows the water is clear like an icicle; but when the tap is


/ INSTRUMENTATION


opened further the water becomes turbulent and rough in appearance. When liquid is flowing in a conduit with


turbulent flow the pressure drop along a length of pipe can be shown to be related to a square law, i.e. the pressure drop quadruples for every doubling of flow rate. With low Reynolds numbers the pressure drop is linear with flow rate. Laminar flowmeters use this phenomenon by deliberately creating these linear elements with a pressure drop that can be easily measured. Positive displacement flowmeter


technology exploits the high viscosity of some fluids at low flow rates. Varieties of these flowmeters (see Figure 3, over page) include gear, oval gear, nutating disc, screw, piston, oscillating piston or a new generation of wobbling diaphragm meters. A positive displacement meter


requires fluid to mechanically displace components in the meter in order to carry out flow measurement. While these are best suited to measurement of more viscous lubricating fluids, some will operate with non-lubricating fluids providing there are no particles which can jam the meter. As the viscosity has to be high the pressure drop is often high as well. Positive displacement meters produce signals which can be used for remote outputs but operational resolution is


INSTRUMENTATION | JUNE 2014


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