High Viscosity Fluid Flow Meters. High viscosity liquids pose a problem for many flow meter types, such as the vortex meter discussed in the newsletter. Turbine meters, differential pressure devices and quite frequently velocity measuring devices rely on a minimum liquid speed and consequently are challenged by high viscosity fluids. To use these techniques with high viscosity fluids it is common to use larger diameter pipes to maintain line pressure drop and keep liquid transport costs as low as possible. Many high viscosity fluid flow meter technologies operate best in the turbulent flow region i.e. with Reynolds numbers above ~2000. This is a dimensionless number that characterises fluid motion. In Figure 1 below the tap on the left shows laminar flow where the stream is clean. The tap illustrated on the right shows turbulent flow where the surface, and indeed the internal flow is random. Most high viscosity flows will be laminar.
High Viscosity Fluid Flow Meters: Velocity Profiles
This laminar flow affects the velocity profile of the fluid within the tube. With high viscosity fluid flow
meters which are operating at high viscosity (low Reynolds numbers), the velocity profile of a fluid is rather like a parabola with the centre liquid traveling at a higher speed than the fluid close to the pipe wall (see Figure 2 below). This is because of the viscous drag created against the static boundary. In lower viscosity fluids by comparison the velocity profile is “fully developed” i.e. the flow is mostly traveling at the same speed except for a small area close to the pipe wall.
These are representative illustrations for high viscosity fluid flow meters and reality will be somewhere in between in each case. Ideally the flow will be steady and driven by perhaps pressure but usually a pump is used. In addition to varying velocity profile according to a fluids viscosity is the fact that flow will also vary within the pipe and sometimes may even be momentarily stationary. Diaphragm, piston, gear and even peristaltic pumps are used to pump viscous fluids, all introduce pulsations into the flow. The frequency and amplitude of these pulsations can cause problems to some flow meter types and should therefore be removed. With electronic meters the danger comes with aliasing where the sample time for the meter and the ripple in the flow are close to or at the same frequencies. For all these reasons positive displacement flow meters (also see oval gear flow meters) are often the preferred choice for viscous fluids.
Positive displacement flow meters vary in design and working principle but the fundamental operation is the same. They all take a discrete package of fluid and pass it from the inlet to outlet with little or no liquid loss or slippage. Gear flow meters are very popular but almost by definition can have problems operating as ‘high viscosity fluid flow meters’ as the gear teeth themselves can cause “hydraulic locks”. With a classical gear tooth profile the volume of fluid at the tooth root cavity has effectively no escape. Gear meter manufacturers counteract this by either making the tooth engagement deliberately “loose” or by removing certain parts of the gear profiles to permit the excess fluid to extrude and escape the gear root rather than lock the gears. Screw gear devices are very efficient with very high viscosity liquids and return startling performance but are also expensive.
Titan Enterprises welcome enquiries for high viscosity fluid flow meters, please contact us here