Coriolis for Viscous Media: Consider Mass Flow, Pressure Loss and Installation

→ Product category: Coriolis flow meters

 

Highly viscous liquids present particular challenges for flowmeters. Oils, concentrates, resins, paints, pastes and chemical intermediates flow much less easily than water. Their viscosity can also change significantly due to temperature, shear or product composition. This increases the pressure loss and the required pump capacity, while many conventional measuring principles reach their limits at low flow velocities.

A Coriolis flowmeter can nevertheless be highly suitable for such applications. It measures mass flow directly and requires neither a known density nor a minimum electrical conductivity of the medium. Depending on the measuring system, density, temperature, volumetric flow and other derived process variables may also be available. This is particularly valuable when raw materials are dosed by mass, formulations are monitored or valuable products are balanced.

Direct mass-flow measurement does not, however, mean that viscosity is irrelevant. It influences the pressure loss in the measuring tube, the damping of the tube vibration, the required pump capacity, cleanability and the selection of the nominal size. A Coriolis instrument may be metrologically suitable but still fail to fit the hydraulic conditions of the plant.

The decisive question is therefore not whether Coriolis can generally measure highly viscous media. What matters is whether the benefit of direct mass, density and temperature measurement justifies the additional pressure loss, instrument size and investment. Suitable instruments can be found in the Coriolis and vortex flowmeters category. The flow measurement technology category provides an overview of additional measuring principles.

How does a Coriolis flowmeter work?

A Coriolis flowmeter contains one or more measuring tubes that are set into a defined vibration by an electromagnetic drive. With no flow, the inlet and outlet sides move largely synchronously. When mass flows through the vibrating tube, Coriolis forces are generated. These produce a measurable time shift or twisting between the tube sections.

This phase shift is directly related to the mass flow. The instrument therefore does not first have to convert volumetric flow into mass using an assumed density. In addition, the mass contained in the measuring tube changes the natural frequency of the vibrating system. The transmitter can use this to determine the density of the medium. An integrated temperature sensor measures the measuring-tube temperature and supports, among other things, compensation for material properties.

The measuring principle requires no moving gears, turbines or vanes in the medium. It is also independent of electrical conductivity. This makes Coriolis fundamentally attractive for oils, solvents, concentrates and many chemical products.

What role does viscosity play?

It is often said that Coriolis measurement is independent of viscosity. This statement applies to only part of the measuring task. Mass flow is not calculated using a viscosity value. A change in viscosity therefore does not have to be entered directly as a correction factor in the flow calculation, as it does with some other measuring principles.

Viscosity is nevertheless crucial for hydraulic and mechanical design. A viscous medium requires more pressure to be conveyed through the pipework, fittings and measuring tubes. At the same time, it damps the vibration of the measuring tube more strongly. The tube drive must therefore provide more energy to maintain the intended vibration amplitude.

Viscosity must also not always be regarded as a single fixed numerical value. Many oils behave approximately as Newtonian fluids: their viscosity depends mainly on temperature. Pastes, adhesives, coatings or concentrates, by contrast, may be shear-thinning, thixotropic or have a yield point. With such non-Newtonian media, the apparent viscosity also depends on the shear rate and flow condition.

For sizing purposes, the actual process values during start-up, normal operation, shutdown and cleaning should therefore be available, rather than only a data-sheet value measured at room temperature.

When is Coriolis worthwhile despite the pressure loss?

The additional pressure loss may be acceptable if direct mass-flow measurement provides a clear process benefit. This applies, for example, to formulations, dosing, raw-material balances and filling processes. If an ingredient is required in kilograms, direct mass measurement is usually more meaningful than volumetric measurement followed by density correction.

Coriolis is also attractive when the density of the medium changes. With oils, concentrates or mixtures, temperature and composition may mean that the same volume does not always contain the same mass. A purely volumetric meter does not detect this change without additional compensation.

Another advantage is the combination of several process variables in one measuring system. Mass flow, density and temperature can be used together to monitor product quality, concentration or formulation. With valuable media, this additional information may be economically more important than the extra pumping energy.

Coriolis is particularly suitable when the following conditions are met:

• Mass is the decisive production or billing variable.
• The density or temperature of the medium changes during operation.
• High accuracy and repeatability are required.
• Small batches or low flow rates must also be measured reliably.
• The medium is homogeneous and largely single-phase.
• The available pump pressure is sufficient for the calculated pressure loss.
• Density or temperature values provide an additional process benefit.

Coriolis may be less suitable when only a basic volumetric-flow indication is required, the available differential pressure is very low, or the product contains large amounts of gas, is inhomogeneous or cannot be pumped sufficiently well.

Why does pressure loss increase with viscous media?

Pressure loss is caused by friction between the medium and the measuring tube, as well as by changes in direction and cross-section within the sensor. In principle, the higher the viscosity and flow rate, the greater the required pressure differential. The exact relationship is also determined by the tube geometry, internal diameter, tube length, density and flow behaviour.

With low-viscosity media, a relatively small sensor may be useful in order to obtain a good measuring signal at low flow rates. With a highly viscous medium, however, the same reduction in nominal size may result in an unacceptable pressure loss. The instrument must therefore not be selected solely on the basis of the required measuring range.

The calculation must cover the entire operating range. The most critical condition is often not normal production, but cold start-up. An oil that flows well at 70 °C may be many times more viscous at 15 °C. If the pump is started under these conditions, the pressure loss and load on the measuring system may be considerably higher than during warm continuous operation.

Filters, valves, pipe bends, heat exchangers and other installations must also be considered. The Coriolis sensor is only one component of the overall hydraulic resistance.

Considering temperature, viscosity and flow behaviour together

With many viscous liquids, viscosity falls sharply as the temperature rises. Moderate heating can therefore reduce the pressure loss much more significantly than a small change in pump speed. For oils, greases, resins or polymer-containing media, temperature control consequently becomes an essential part of the flow measurement.

At least the minimum, normal and maximum process temperatures should be specified for sizing. The temperature during plant start-up, production interruptions and cleaning is also relevant. A medium may remain fluid during operation, but thicken or harden in the measuring tube during a shutdown.

For non-Newtonian media, the supplier should, where possible, receive rheological data or a viscosity curve for the relevant shear and temperature range. A single value such as “10,000 mPa·s” is often insufficient to describe a paste. The conditions under which this value was determined are decisive.

Trace heating or thermal insulation can help maintain stable viscosity. However, the permissible sensor temperature, temperature distribution and the requirements for the transmitter and connection cable must be taken into account.

Understanding measuring-tube frequency and damping

The measuring-tube drive keeps the tubes vibrating at their resonant frequency. The resonant frequency changes with the mass of the medium contained in the tube and therefore forms the basis for density measurement.

A highly viscous medium damps the tube movement more strongly than a low-viscosity liquid. The transmitter therefore increases the drive power so that the required vibration amplitude is maintained. Many modern systems monitor diagnostic values such as drive energy, vibration amplitude or frequency stability for this purpose.

Increased damping does not automatically mean that the measurement is incorrect. It may be a normal effect of the medium. If the drive reaches its power limit, however, the vibration can no longer be maintained with sufficient stability. Gas bubbles, deposits or a partially filled measuring tube may also increase damping and generate similar diagnostic messages.

For very viscous or molten products, it should therefore be checked whether the selected sensor has a sufficiently powerful tube drive and suitable diagnostic functions. A maximum viscosity value alone is not always sufficient for this assessment.

Correctly sizing the nominal diameter and measuring range

The optimum nominal diameter is a compromise. A smaller measuring tube often increases the usable signal at low flow rates and improves the relationship between the process value and zero-point stability. At the same time, the flow velocity and pressure loss increase.

A larger sensor reduces the hydraulic resistance, but at very low flow rates it may operate outside its optimum range. Directly adopting the existing pipe diameter is therefore just as problematic as reducing it as a general rule.

Sizing	Advantage	Possible disadvantage
Smaller sensor nominal size	Better signal at low mass-flow rates, compact sensor	Higher velocity and significantly greater pressure loss
Larger sensor nominal size	Lower pressure loss and lower flow velocity	Low flow rates may be close to the lower measuring limit
Sensor nominal size matches the pipework	Simple mechanical integration	Not automatically the best metrological or hydraulic solution

Reliable sizing requires the minimum, normal and maximum mass-flow rates. Density, viscosity at several temperatures, available inlet and outlet pressure, and the maximum permissible pressure loss must also be known.

Measuring low flow rates at high viscosity

Low flow rates are particularly demanding with highly viscous media. On the one hand, the sensor must be small enough to generate an adequate measuring signal. On the other hand, the cross-section must not become so small that the pressure loss rises disproportionately or the product can no longer be conveyed safely.

In dosing systems, the dynamics of the process must also be considered. Valve opening time, pump overrun, flexible hoses and compressible product components can influence the quantity actually dispensed. High measuring accuracy of the Coriolis sensor alone therefore does not guarantee an accurate batch.

For very small quantities, the sensor should be installed as close as possible to the dosing point. Dead volume between the measuring instrument and filling valve produces an overrun quantity that may account for a significant proportion of a small batch.

The zero point also becomes relatively more important. A small absolute zero-point error may be insignificant at a high flow rate, but account for a relevant proportion of the reading at a very low mass-flow rate. Zero adjustment may only be carried out with a completely filled, temperature-stable measuring tube and confirmed zero flow.

Pump capacity and available pressure

The Coriolis flowmeter must be included in the pump characteristic and the total system resistance. In addition to the sensor, the pump must overcome pipework, elevation differences, filters, valves and process vessels.

A larger pump is not automatically the best solution. Excessive pressure peaks can place a load on seals, hoses and process connections. Positive-displacement pumps must also be provided with suitable overpressure protection. Pulsations can influence the measurement and rapid dosing processes in particular.

Special care is required when installing the sensor on the suction side. High viscosity and low inlet pressure can encourage cavitation, outgassing or incomplete filling of the measuring tube. In many applications, installation on the discharge side of the pump is more favourable. Sufficient back pressure helps to keep dissolved gases in the medium and ensure a single-phase product.

Installation, gas bubbles and pipe forces

Coriolis flowmeters generally do not require the long inlet and outlet runs needed by measuring principles that depend on the flow profile. Mechanical installation nevertheless remains decisive. The sensor must be installed without mechanical stress and the pipework must be adequately supported. Misalignment must not be pulled into the measuring instrument by tightening the flange bolts.

The mounting position should be selected so that the measuring tube remains completely filled and no gas or solids pockets can form. With liquids, an arrangement that reliably carries gas bubbles onwards is advantageous. The exact orientation depends on the sensor design, medium and manufacturer’s instructions.

Gas bubbles are particularly critical with viscous media because they often escape from the product only slowly. They alter the damping, density indication and measuring stability. Possible causes include a leaking suction line, an insufficient vessel level, foaming, cavitation or an agitator introducing air into the product.

Strong external vibrations should also be avoided. Pumps, agitators and flexible pipework can introduce mechanical vibrations into the sensor. Although modern instruments provide good isolation, an unfavourable mechanical installation can still impair the measurement quality.

Cleaning, deposits and trace heating

With highly viscous media, cleanability is often more important than the nominal measuring accuracy. If product remains in the measuring tube, it may thicken, crystallise or harden during a shutdown. Deposits change the vibrating mass of the measuring tube and may therefore affect the zero point, density indication and diagnostic values.

The cleaning strategy should be defined during instrument selection. Depending on the application, possible methods include flushing with product, solvent, water or cleaning medium, CIP processes or thermal cleaning. Materials, seals and surfaces must be compatible with both the process medium and cleaning medium.

In heated pipework, the Coriolis sensor should be installed without cold spots wherever possible. An unheated sensor between two warm pipe sections can become the point with the highest viscosity. Insulation or trace heating must be designed so that the sensor and connections are heated evenly and the permissible temperature limits are maintained.

For products that solidify completely during a shutdown, it must also be clarified whether changes in volume or improper restarting could place a mechanical load on the measuring tube.

Coriolis, oval gear or gear meter?

In addition to Coriolis, oval gear or gear flowmeters are frequently considered for viscous media. These instruments operate according to the positive-displacement principle. Defined partial volumes are measured by rotating elements and output as pulses or a flow value.

Positive-displacement meters can operate very accurately with homogeneous, lubricating and sufficiently clean liquids. Their mechanical clearances, bearings and gears make them more sensitive to abrasive particles, hardened residues or unsuitable foreign matter. The pressure loss of these measuring principles can also become considerable as viscosity increases.

Criterion	Coriolis	Oval gear / gear meter
Primary measured variable	Direct mass flow	Direct volumetric flow
Changes in density	Mass remains directly measurable; density may also be available depending on the model	Density information or compensation is required for mass
Moving parts in the medium	No rotating measuring elements	Mechanical oval gears or gear wheels
Viscous liquids	Well suited if pressure loss and tube damping can be controlled	Very well suited to appropriate homogeneous and clean liquids
Particles and abrasion	Must be assessed according to the measuring tube and application	May cause wear or blockage
Additional measured variables	Density and temperature may be available depending on the model	Additional sensors or electronics are generally required
Maintenance	No mechanically rotating internal parts	Wear parts and clearances must be considered

Suitable positive-displacement meters can be found in the gear sensors and oval gear meters category. One example is the Flowal OF oval gear meter for Newtonian, non-abrasive liquids such as oils, greases, fuels, paints and selected chemicals.

Practical example: Concentrate in a mixing plant

A liquid concentrate is to be dosed by mass in a mixing plant. The product is stored in a heated feed vessel. At its normal processing temperature, it can be pumped easily. After a longer production interruption, however, it cools down and becomes significantly more viscous.

Purely volumetric measurement is unsuitable because the density changes with the temperature and product batch. The actual mass added is decisive for the formulation. A Coriolis flowmeter therefore offers a clear benefit.

The initial design uses a small sensor so that even low dosing quantities can be measured with good resolution. The pressure-loss calculation shows, however, that this sensor would create excessive resistance during cold start-up. A larger nominal size is therefore selected and the minimum flow is compared with the achievable zero-point stability.

The sensor is installed on the discharge side of a positive-displacement pump and insulated together with the product pipe. Controlled trace heating is provided for shutdown periods. The pipework is routed so that gas bubbles cannot remain trapped in the measuring tube.

During operation, mass flow and density are monitored together. An unexpected change in density provides an indication of a changed product composition or air ingress. The higher investment is justified in this case because the formulation is controlled directly by mass while additional quality information is obtained at the same time.

Selection guide for viscous media

For reliable instrument sizing, the following data should be provided as completely as possible:

• exact product designation and chemical composition,
• Newtonian or non-Newtonian flow behaviour,
• viscosity at the minimum, normal and maximum temperature,
• density and possible density fluctuations,
• minimum, normal and maximum mass flow,
• process pressure and available pump differential pressure,
• maximum permissible pressure loss at the measuring point,
• pipe nominal size and existing process connections,
• gas, solids or abrasive content,
• cleaning procedure and cleaning media,
• shutdown behaviour, solidification or crystallisation,
• required measured variables such as mass, density, temperature or concentration,
• hygienic, hazardous-area, pressure-equipment or material requirements.

The sizing process should then assess not only the measuring accuracy, but also the pressure loss, pump reserve, start-up condition, cleaning and possible product changes. Only this overall assessment shows whether Coriolis is technically and economically suitable.

Which products are suitable?

The Coriolis and vortex flowmeters category includes various Coriolis sensors and transmitters for low, medium and high flow rates. The specific selection must be made on the basis of the medium and permissible pressure loss.

The SITRANS FC520/540 is a universal multiparameter measuring system for standard and hygienic applications. It combines an FCS500 sensor with an FCT020 or FCT040 transmitter. Depending on the model, a viscosity function, density measurement and concentration calculations are available.

The SITRANS FCS600 is designed for demanding process conditions. Insulation and trace-heating options make it particularly suitable for viscous or molten liquids. The material, temperature range, pressure rating and connection must be selected for the specific application.

The SITRANS FCS500 covers nominal sizes from DN 10 to DN 80 and is suitable for numerous routine and hygienic processes. Its pressure-loss-optimised design is useful when precise mass-flow measurement is to be combined with the lowest possible hydraulic load.

For large pipes and high flow rates, the SITRANS FCS700 may be suitable. Large tube diameters, short flow paths and a powerful measuring-tube drive are advantageous for high flow rates and strongly damping applications.

Additional measuring principles for viscous liquids, including oval gear and gear sensors, can be found in the flow measurement technology section. The suitable solution depends on whether mass or volume is required and on the requirements for pressure loss, cleaning, wear and process diagnostics.

Conclusion: Direct mass-flow measurement must be hydraulically compatible with the plant

Coriolis flowmeters can also be an excellent solution for highly viscous media. Their strength lies in the direct measurement of mass flow and the additional measurement of density and temperature. This is particularly beneficial for formulations, dosing, valuable raw materials and fluctuating product density.

Viscosity nevertheless remains a key sizing parameter. It influences pressure loss, pump capacity, measuring-tube damping, cleanability and safe plant start-up. A sensor that is metrologically suitable may be hydraulically unsuitable if the nominal size and temperature conditions have not been considered.

The optimum solution results from a compromise between a small measuring tube for low flow rates and a sufficiently large cross-section for an acceptable pressure loss. With very viscous products, trace heating, shutdown behaviour, gas bubbles and cleaning must also be considered.

If mass is the decisive process variable and the additional density information provides a genuine benefit, Coriolis can justify the higher pressure loss. If only the volumetric flow of a clean, homogeneous and sufficiently lubricating liquid is required, an oval gear or gear flowmeter may be the more economical alternative.

Frequently asked questions about Coriolis measurement with viscous media

Can a Coriolis flowmeter measure highly viscous oil?

In principle, yes. The actual viscosity range, temperature, flow rate, selected nominal size and available pump pressure are decisive. The application must be designed for the entire operating and start-up range.

Is Coriolis measurement completely independent of viscosity?

No. The mass flow is not calculated from viscosity. However, viscosity influences pressure loss, tube damping, pump capacity and, in some cases, the achievable measurement dynamics.

Why does a small Coriolis sensor cause a greater pressure loss?

At the same flow rate, the flow velocity increases in a smaller measuring tube. Combined with the high internal friction of a viscous medium, this results in a greater pressure loss.

Can trace heating reduce the pressure loss?

With media whose viscosity decreases as the temperature rises, stable heating can significantly reduce the pressure loss. Temperature limits, material compatibility and product safety must be observed.

What must be considered with pastes and non-Newtonian media?

With these media, the apparent viscosity depends on shear, temperature and flow condition. The most complete possible rheological data and information on the yield point are therefore required for sizing.

Is an oval gear meter better than Coriolis for viscous media?

Not in general. Oval gear meters are very well suited to many clean, homogeneous and viscous liquids and measure volumetric flow directly. Coriolis is advantageous when mass, density, changing product conditions or the absence of moving measuring elements are important.

Does a Coriolis flowmeter require inlet runs?

Long straightening runs are generally not required. Nevertheless, the measuring tube must remain completely filled, the sensor must be installed without mechanical stress, and gas bubbles, strong vibrations and unfavourable pipe forces must be avoided.