Process blind spot

Non-Newtonian fluids - those in which viscosity either increases or decreases with changes in shear rate - are widely used in industries such as chemicals, cosmetics, pharmaceuticals, food and oil & gas. They include high molecular weight liquids such as polymer melts and solutions, and slurries and pastes.

Despite many important industrial uses (see panel, p34), information on the flow metering of non-Newtonian fluids is very limited and their performance not fully understood - particularly as flow meters are normally calibrated using Newtonian fluids such as water, oil or gas.

Coriolis devices are the most widely used flow meters for non-Newtonian applications. Manufacturers are generally responsible for the calibrations, which they carry out in Newtonian fluids. While this procedure is accepted by many end-users, it may not pick up on uncertainties caused by differences in the behaviour from the reference fluids.

The information gap has led to costly problems for process companies over recent years, especially for processes involving aerated liquids - non-Newtonian fluids tend to be higher viscosity fluids in which aeration is a much greater problem. Proctor and Gamble, for example, has previously reported having to scrap large quantities of finished product due to inaccuracies in a Coriolis meter as a result of these issues.

Industry concern

Indeed, industry concern has now prompted work at TUV NEL in East Kilbride, Glasgow to expand the national flow measurement standard to include non-Newtonian fluids. The work, which includes developing a facility to evaluate the performance of flow meters in non-Newtonian fluids, is supported by the UK government’s National Measurement System Engineering & Flow Programme.

TUV NEL will address the many aspects of non-Newtonian fluid flow that cause inaccuracies when flow metering, explains Calum Hardie, project engineer at TUV NEL. A key goal, he said, will be to characterise when a non-Newtonian fluid moves from laminar to transitional and turbulent phase - flow meters rely on correction factors when changing to transitional and turbulent flow.

Another issue to be tackled relates to how the flow profile of non-Newtonian fluids differs from that of Newtonian fluids. In a shear-thinning, non-Newtonian fluid, for instance, the viscosity becomes lower at the wall than in the centre of the pipe due to higher shear rates at the wall. This causes the velocity profile to flatten off at the centre of the pipe where the viscosity remains high even in laminar flow, and can also be asymmetrical in transitional flows.

“Many flow meters rely on velocity profile to determine the flow rate, so the variation of velocity profile can cause large errors in these meters,” commented Hardie. This sometimes causes problems, for example when flow metering viscoelastic fluids, the TUV NEL engineer continued. This type of fluid behaviour can delay the onset of turbulence and cause cross flows and swirling, all of which can increase errors in the meter.
“These errors are increased when the fluid passes a bend or if the flow is split, for example in twin-tube Coriolis meters. It is also thought that a longer development length is needed for viscoelastic fluids,” the research engineer said.

The nature and degree of the challenge posed by non-Newtonian behaviour depends greatly on the type of flow meter being employed, TUV NEL research has found. While Coriolis meters can work within the manufacturer’s specification in shear-thinning, aqueous polymer solutions, electromagnetic meters have been shown to have errors of about 1% during transition. And ultrasonic meters can have errors of up to 15% during transition due to their reliance on velocity profile.

However, the performance of both Coriolis and electromagnetic flow meters in different types of non-Newtonian slurries can fall outside the manufacturer’s specification with inaccuracies of up to 10%, especially when working at the low end of their flow rate range.

Moreover, specifications developed from calibrations in turbulent water flow cannot be relied upon when using the flow meter in slurries. While Coriolis meters are the least prone to error in non-Newtonian fluids, this is not always the case, especially at the low end of the flow rate range, said Hardie.

Coriolis meters are also generally sized in Newtonian fluids based on pressure drop at the maximum working flow rate. The pressure drop in shear-thinning fluids is often significantly less than in Newtonian fluids, which leads to Coriolis meters being oversized for the metering of shear-thinning, non-Newtonian fluids.

“Oversized meters will not only be more expensive but will be less accurate, because the flow rates will be at the lower end of the meter’s flow range where measurement uncertainty is higher,” commented Hardie.

Better design

As well as addressing specific metering issues, the Scottish-based project team hopes that the new test facility and related project work will lead to significant improvements in the design/performance of flow meters. “Improving the understanding of the current performance of different flow meters in non-Newtonian fluids will likely lead to advances in design and performance in the long run,” said Hardie.

For example, he noted, the new test facility could be used to improve correction factors for velocity profile based on the rheological properties of the fluid so as to provide new guidance on how to install meters in non-Newtonian applications.

Other likely benefits include improvements in process efficiency by providing more representative test reference fluids that allow a better understanding of the uncertainties of measurement in a company’s process, said Hardie. The facility, he added, could also test valves, in-line viscosity measurement devices and other instruments used in non-Newtonian applications.

The project team is also hoping to make some progress in resolving problems associated with metering aerated liquids, which remains one of the biggest issues for flow metering - both of Newtonian and non-Newtonian fluids.

“I wouldn’t claim that this project alone will solve this problem,” concluded Hardie. “However, along with other projects at TUV NEL involving high viscosity oil metering, we will gain a better understanding into the effects of aerated liquids on different flow metering technologies.

“This improved understanding will be very useful for meter selection in aerated liquids and potentially for helping improve the performance of meters in these challenging fluids.”

Non-Newtonian challenges

Shear-thickening fluids include corn starch and clay slurries, though shear-thinning fluids, as used in food products, pharma products, emulsions, paints and drilling mud, are more common in industry.

One application involves adding small amounts of polymer to water to create a shear-thinning, aqueous solution that delays the onset of energy-inefficient, turbulent flow. By maintaining laminar flow, the drag-reducing polymers can deliver a significant reduction in pumping power requirements and hence lower energy consumption.

Some non-Newtonian fluids, known as rheopectic fluids, exhibit a rise in viscosity over time at a constant shear rate, while others are thixotropic, exhibiting a loss of viscosity over time at constant shear rate. Most thixotropic fluids are also shear thinning, so it is often difficult to detect between thixotropic and shear-thinning effects due to the combined shear and time effects that occur during measurement.

Many non-Newtonian fluids, such as polymer melts and polymer and soap solutions, also exhibit viscoelastic behaviour. Fluids exhibiting viscoelastic behaviour will act as both a liquid and a solid in the sense that they will flow like a liquid but will show a tendency to return to their original shape when the stress is removed.