A Coriolis flowmeter has been measuring a solids-containing suspension for several years. The mass flow still appears plausible, but during regular zero-point checks the value is slowly changing from one maintenance interval to the next.
Is this already an indication of worn measuring tubes?
Possibly – but not necessarily.
Abrasive particles can gradually wear away the wall of a Coriolis measuring tube over time. This changes the mechanical properties of the vibrating measuring system. At the same time, however, the following can also influence:
- deposits,
- gas bubbles,
- changing solids concentrations,
- pipeline vibrations,
- mechanical stresses,
- temperature changes
diagnostic values and, in particular, the zero point.
It is therefore problematic to regard a single process value as an unambiguous wear indicator.
A significantly more meaningful condition-monitoring concept is created when several parameters are observed together over the long term:
- zero point or zero stability,
- density measurement under defined conditions,
- measuring tube stiffness or Tube Health,
- drive power and vibration behavior,
- pickup signal quality,
- process and maintenance history.
For abrasive applications, it is therefore advisable not to wait until an obvious measurement error or even measuring tube damage occurs. The decisive factor is to record suitable reference values while the instrument is still new and then monitor how they develop over the operating life.
Why Is the Measuring Tube So Important in the Coriolis Principle?
In a Coriolis flowmeter, the measuring tube is not merely a channel through which the medium flows.
It is also an essential part of the actual measuring system.
One or more measuring tubes are excited into controlled vibration.
When mass flows through the vibrating tube, the Coriolis force causes characteristic changes in the tube movement.
Pickups detect this vibration or the phase shift between different points of the measuring tube.
This is used to determine the:
mass flow
.
The resonance frequency of the measuring system additionally provides information about the:
density of the medium
.
However, this also means:
If the measuring tube changes mechanically, an essential part of the measuring system changes as well.
This is precisely why:
- erosion,
- corrosion,
- deposits,
- mechanical damage
are particularly relevant for a Coriolis sensor.
How Does Abrasion Occur in the Measuring Tube?
Abrasion refers to mechanical material removal caused by solid particles or solids in the flowing medium.
Typical abrasive media include:
- slurries,
- mineral suspensions,
- drilling muds,
- sand-water mixtures,
- lime or chalk suspensions,
- liquids containing crystals,
- process media containing metal or mineral particles.
During flow, the particles strike the inner wall of the measuring tube.
Depending on:
- particle size,
- particle shape,
- hardness,
- solids concentration,
- flow velocity,
- impact angle
material may be removed from the measuring tube wall.
Wear does not necessarily occur uniformly over the entire sensor.
Areas with unfavorable flow deflection may be subjected to greater local stress.
Which Particle Properties Influence Wear?
The general statement:
medium contains solids
is not sufficient for selecting an instrument.
At a minimum, the following are relevant:
- solids content in mass or volume percent,
- average particle size,
- maximum particle size,
- particle size distribution,
- particle hardness,
- particle shape,
- density of the solids,
- viscosity of the carrier liquid.
Fine, soft particles in a viscous suspension can show completely different wear behavior from sharp-edged quartz sand in a low-viscosity carrier liquid.
The stability of the suspension also plays a role.
If heavy particles settle at low flow velocity, an inhomogeneous solids distribution can form inside the measuring instrument.
This can cause both:
measurement errors
and:
locally increased wear
.
Why Flow Velocity Is Critical
With abrasive media, a higher velocity is not automatically advantageous.
As the relative velocity between particles and the measuring tube wall increases, abrasive material removal can increase significantly.
A Coriolis flowmeter should therefore not be unnecessarily undersized simply to use as much of the nominal measuring range as possible.
For the same volumetric flow, a smaller nominal diameter results in a higher average flow velocity.
For abrasive media, a balance must therefore be found between:
- measurement performance,
- pressure loss,
- flow velocity,
- self-cleaning,
- wear.
The permissible velocity should be coordinated with the instrument manufacturer based on the specific particle and process data.
Consider Inlet Flow and Pipe Bends
For the measurement principle itself, Coriolis flowmeters often do not require long conventional upstream and downstream straight runs.
With abrasive media, however, the flow conditions immediately upstream of the sensor can still be important.
If, for example, directly upstream of the measuring instrument there is a:
- 90° pipe bend,
- control valve,
- reducer,
- pump outlet
strong secondary flows or turbulence may occur.
If abrasive particles then strike the measuring tube wall at an unfavorable angle or in a locally concentrated manner, wear may increase in certain areas.
For particularly abrasive applications, a more uniform inlet flow may therefore be useful even though the measurement principle itself is generally independent of straight inlet runs.
The question is then not only “What does the measuring principle require?” but also “Which flow conditions minimize mechanical attack on the measuring tube?”
Choose the Correct Installation Orientation for Suspensions
With liquids containing solids, it should be prevented that solids settle inside the sensor at low or interrupted flow.
A vertical installation with:
upward flow direction
can be particularly favorable for this purpose.
It helps transport solids evenly and reduces the risk of sedimentation in lower areas of the measuring system.
With curved measuring tubes, the specified mounting orientation is particularly important.
The manufacturer’s instructions for:
- liquids,
- gases,
- suspensions,
- partially gas-loaded media
should therefore always be observed.
An unfavorable position can otherwise promote:
- solids deposits,
- gas accumulation,
- unstable density values,
- unstable zero points.
Select the Correct Measuring Tube Material
When selecting the material, two different mechanisms must be considered separately:
corrosion
and:
abrasion
.
A material may have excellent chemical resistance to a medium and still be mechanically worn away by hard solid particles.
Conversely, a mechanically robust material may be chemically unsuitable.
At a minimum, the following must therefore be considered:
- chemical composition of the liquid,
- temperature,
- type and hardness of the solids,
- particle concentration,
- flow velocity,
- process pressure.
For the SITRANS FCS600, for example, wetted versions are available in:
- AISI 316L,
- Alloy 22
.
Which version is suitable must be determined based on the specific chemical and mechanical load.
Alloy 22 is not automatically the better choice against abrasion. The material must be selected to match the combination of chemical attack and mechanical wear.
What Changes When the Measuring Tube Wall Becomes Thinner?
If the wall of a measuring tube becomes thinner due to abrasion or corrosion, its mechanical structure changes.
In particular, the following change:
- tube stiffness,
- mass of the vibrating system,
- natural frequency or resonance behavior,
- mechanical symmetry,
- response to the drive system.
These changes can in turn influence:
- mass flow measurement,
- density measurement,
- zero-point stability.
Localized material loss can additionally create mechanical asymmetry.
Initially, the change does not necessarily have to be large enough to produce an obviously incorrect process value.
This is where long-term condition monitoring becomes particularly valuable.
What Does the Zero-Point Trend Tell You?
The zero point of a Coriolis meter describes the output signal under the condition:
sensor completely filled + actual zero flow
.
Under ideal conditions, this value should remain stable over a long period.
During recurring maintenance, for example, the following can be documented:
| Time | Zero-Point Value | Assessment |
|---|---|---|
| Commissioning | Reference value | Baseline |
| after 6 months | close to reference | normal |
| after 12 months | slightly changed | monitor trend |
| after 18 months | significantly changed | investigate cause |
A progressive change can indicate that the mechanical or process-related behavior of the sensor has changed.
It is therefore a useful part of a condition-monitoring concept.
Why Zero-Point Drift Does Not Automatically Mean Wear
The zero point does not respond exclusively to the wall thickness of the measuring tube.
It can also be influenced by:
- mechanical pipeline stresses,
- changed mounting conditions,
- vibrations,
- gas bubbles,
- solids deposits,
- inhomogeneous suspensions,
- temperature differences,
- shut-off valves that are not completely closed during the zero-point check.
A zero-point check is particularly problematic when the medium appears to be stationary but a small amount of flow is actually still present.
This residual flow is then stored as an apparent zero-point error.
For reproducible zero-point checks:
- the sensor should be completely filled,
- no gas should be trapped,
- the flow should be reliably zero,
- the process should be as stable as possible,
- the temperature should be sufficiently stable,
- the same test procedure should be used.
A changed zero point is a diagnostic indication – not proof on its own of measuring tube wear.
Can Density Measurement Indicate Measuring Tube Wear?
The density measurement of a Coriolis instrument is based on the vibration or resonance behavior of the filled measuring tube.
If the mechanical structure of the measuring tube changes, the determined density can therefore also change.
This makes a long-term density trend fundamentally interesting.
However, the following also applies here:
The process density itself can change.
In a suspension, for example, it depends on:
- solids concentration,
- composition,
- temperature,
- gas content.
A density deviation is therefore particularly meaningful as an instrument-condition indicator only when it is evaluated under:
known and reproducible reference conditions
.
Example
If a defined reference medium is always used during maintenance and the instrument shows a systematic density shift over several years at the same temperature, this change should be investigated.
A density change during an ongoing, varying production process, on the other hand, is initially a process value and not clear proof of wear.
Measuring Tube Stiffness as a Condition Parameter
For wear monitoring, a direct assessment of the mechanical properties of the measuring tube is significantly more specific than relying only on the zero-point trend.
If the tube wall becomes thinner, the stiffness of the vibrating system decreases.
Modern Coriolis flowmeters can monitor this change diagnostically.
With a Tube Health or Tube Integrity function, the current flow value itself is not simply evaluated.
Instead, the current mechanical condition is compared with a reference condition of the sensor.
A long-term change can therefore indicate:
- erosion,
- corrosion,
- material loss.
For reliable trend assessment, a reference condition should be recorded as early as possible under the actual installation conditions.
This is important because the installation situation can also influence the mechanical behavior of a Coriolis sensor.
Monitor Drive Power and Vibration Condition
The Coriolis sensor must keep its measuring tubes vibrating in a controlled manner.
It uses an electromagnetic drive for this purpose.
If the required drive power changes significantly, this can also provide diagnostic information.
An increased energy requirement can occur, for example, due to:
- gas bubbles in the medium,
- strongly damping process media,
- high viscosity,
- solids deposits,
- changed mechanical conditions.
This parameter is therefore also not specific to wear on its own.
In combination with:
- measuring tube stiffness,
- density,
- zero point,
- pickup signals
it provides a considerably more complete picture of the sensor condition.
Distinguish Between Abrasion and Deposits
Interestingly, abrasive media can cause two opposite types of change.
Abrasion
Material is removed from the measuring tube.
The wall thickness decreases.
Deposits
Material accumulates on the measuring tube wall.
The effective mass of the vibrating system increases and the free tube cross-section may also become smaller.
Both can influence:
- resonance behavior,
- density measurement,
- zero point,
- vibration damping.
A diagnostic message indicating a change in the sensor should therefore not automatically be interpreted as:
the measuring tube has become thinner
.
Depending on the diagnostic method, the following may also need to be ruled out:
- coating formation,
- crystallization,
- product buildup.
Why Reference Values in New Condition Are Important
Trend monitoring only works effectively if the starting condition is known.
Ideally, the following are documented during commissioning:
- zero point,
- density of a known medium,
- Tube Health or stiffness reference,
- drive value or drive level,
- pickup signals,
- process temperature,
- installation conditions,
- typical flow,
- solids concentration.
These data form a:
baseline
.
Later inspections can then be compared not only with a general limit, but also with the actual initial condition of the specific sensor.
This is considerably more meaningful when wear progresses slowly.
Trend Monitoring Instead of Individual Values
A single diagnostic value is rarely as meaningful as its development over time.
For example:
Tube Health = slightly changed
may initially be uncritical.
However, if the same value continues to move in the same direction during every inspection, a clear trend emerges.
The following are therefore of interest for predictive maintenance:
- absolute change since commissioning,
- change since the last inspection,
- rate of change,
- correlation with operating hours,
- correlation with processed product quantity,
- correlation with process changes.
Particularly in abrasive applications, for example, the:
cumulative processed solids mass
may be a more meaningful reference variable than calendar time alone.
A measuring instrument that has only operated sporadically for twelve months may have experienced significantly less abrasive stress than an instrument with 8,000 operating hours under high solids loading.
Define Maintenance Intervals Based on Risk
There is no universally applicable maintenance interval for abrasive applications.
It depends, among other things, on:
- particle hardness,
- particle size,
- solids content,
- flow velocity,
- operating hours,
- measuring tube material,
- permissible process risk.
For a new application, it may be useful to start with relatively short inspection intervals.
For example:
commissioning → 3 months → 6 months → 12 months
.
If the diagnostic values remain stable, the interval can then be adjusted.
If a clear trend becomes apparent, it must instead be decided whether:
- the inspection interval should be shortened,
- the flow velocity should be reduced,
- the installation conditions should be changed,
- a different sensor material or sensor type should be used.
When Recalibration Is Advisable
Internal diagnostics do not automatically replace metrological calibration.
Recalibration may be advisable, for example, if:
- a significant diagnostic trend has been detected,
- the zero point has changed permanently,
- a reference comparison is abnormal,
- a specified calibration interval has been reached,
- the sensor is to be checked after a process disturbance.
With abrasive media, particles can cause measuring tube wear and therefore, over time, a change in measurement accuracy.
Conversely, deposits can also influence the balance or vibration behavior of the measuring system.
A traceable calibration then shows whether the mass flow measurement is still within the required tolerance.
Diagnostics and calibration therefore serve different purposes:
diagnostics = monitor condition
calibration = determine measurement deviation
.
Practical Example: Abrasive Suspension in a Process Pipeline
In a production plant, a mineral suspension is measured with a Coriolis flowmeter.
The medium contains:
approx. 20% solids
.
The sensor has been operating continuously for two years.
Step 1: Reference values from commissioning
During commissioning, the following were documented:
- zero point,
- density with a reference medium,
- tube condition reference,
- typical drive values.
Step 2: Zero-point trend becomes noticeable
During the first maintenance intervals, the zero point remains largely constant.
After approximately 18 months, an increasing change becomes apparent.
However, during normal operation the system continues to display plausible flow values.
Step 3: Rule out process-related causes
The zero-point check is repeated under defined conditions.
The following are checked:
- sensor completely filled,
- shut-off valves securely closed,
- no gas bubbles,
- stable temperature,
- no unusual pipeline vibrations.
The changed zero point remains.
Step 4: Compare additional diagnostic values
The tube condition check also shows an increasing change compared with the reference condition.
The density value of a defined reference medium has also shifted slightly.
Several independent indicators are therefore pointing in the same direction.
Step 5: Investigate the process flow conditions
During plant inspection, it is found that a pipe bend is located directly upstream of the measuring instrument and that the flow velocity is relatively high.
The particles may therefore be striking the inlet area of the measuring system at an unfavorable angle.
Step 6: Maintenance action
The sensor is removed and inspected during a planned shutdown.
The inspection confirms increased wear.
Step 7: Optimize the application
For the replacement installation, the following are reassessed:
- sizing,
- flow velocity,
- installation orientation,
- inlet conditions.
In addition, automated tube condition monitoring is incorporated into the maintenance concept.
The key advantage is that the change was detected before complete sensor failure or leakage of the measuring tube occurred.
Planning and Inspection Checklist
- Fully record the medium and its chemical composition.
- Determine the type and concentration of solids.
- Consider average and maximum particle size.
- Assess particle hardness and shape.
- Check the measuring tube material for chemical and mechanical suitability.
- Do not design the flow velocity unnecessarily high.
- Take the sensor pressure loss into account.
- Avoid unfavorable pipe bends or strong turbulence directly upstream of the sensor where required for the abrasive application.
- Select a suitable installation orientation for settling solids.
- Avoid gas accumulation and two-phase conditions wherever possible.
- Document the zero point during commissioning.
- Record the Tube Health or tube condition reference.
- Document the density of a known reference medium where useful for the application.
- Store drive and diagnostic values as the initial condition.
- Perform repeat inspections under conditions that are as similar as possible.
- Assess changes as trends rather than individual values.
- Compare diagnostic trends with operating hours and processed solids quantity.
- Shorten inspection intervals if an abnormal trend is detected.
- Perform a traceable flow calibration if required.
- Document limits and alarm criteria for the specific application.
Common Mistakes
- Generally assuming that a solids-containing medium is uncritical: Particle size, hardness and velocity are major factors determining abrasive load.
- Sizing the sensor as small as possible: The resulting higher flow velocity can increase wear.
- Selecting the material only according to corrosion resistance: Chemical resistance does not automatically mean high abrasion resistance.
- Ignoring a pipe bend directly upstream of the measuring instrument: Turbulent abrasive particles can strike the measuring tube wall more intensely in localized areas.
- Allowing sedimentation inside the sensor: Deposits can influence both the measurement and vibration behavior.
- Immediately interpreting zero-point drift as abrasion: Gas bubbles, vibrations and process conditions can also change the zero point.
- Adjusting the zero point while the medium is still flowing: Residual flow is incorrectly stored as the zero point.
- Automatically interpreting a density change as tube wear: In suspensions, a different solids concentration alone changes the actual process density.
- Looking only at the current diagnostic value: The long-term change relative to the baseline is usually more meaningful.
- Not storing reference values during commissioning: Later changes then become difficult to assess.
- Equating Tube Health Check with calibration: Condition diagnostics and metrological verification are different tasks.
- Confusing deposits with material removal: Both can alter vibration behavior, but they have different causes and corrective measures.
- Defining maintenance intervals solely according to calendar time: The actual quantity of abrasive product processed may be the more useful load parameter.
Suitable Coriolis Flowmeter
For demanding industrial Coriolis applications, the Siemens SITRANS FC640 is one suitable option.
The measuring system combines:
SITRANS FCS600 sensor + SITRANS FCT040 transmitter
.
The FCS600 is designed for demanding process conditions and offers, among other things:
- double-curved measuring tubes,
- versions in AISI 316L or Alloy 22,
- nominal sizes up to DN 65,
- high process temperatures up to 350 °C,
- high-pressure versions up to 700 bar,
- compact and remote versions.
For the condition monitoring considered here, the SITRANS FCT040 is particularly relevant.
The advanced transmitter provides, among other things:
- tube condition monitoring or Tube Health Check,
- self-verification,
- monitoring of important diagnostic parameters,
- mass flow measurement,
- density measurement,
- temperature measurement,
- volumetric flow measurement,
- digital communication options.
The tube condition check examines, among other things, the mechanical stiffness of the measuring tube.
A reduction in wall thickness caused by erosion or corrosion can change tube stiffness and therefore become diagnostically visible when compared with the stored reference condition.
The function does not replace proper material and application selection. However, it provides an additional way to detect sensor changes at an early stage and to plan maintenance on a condition-based basis.
Further Coriolis systems can be found under Coriolis Flow Measurement at ICS Schneider.
Further electromagnetic, ultrasonic, vortex, Coriolis and mechanical flowmeters can be found under Flow Measurement Technology.
Conclusion
Coriolis flowmeters can also provide valuable mass flow and density information for media containing solids.
With abrasive particles, however, it must be taken into account that the measuring tube itself may be exposed to long-term mechanical wear.
The rate at which this wear progresses depends, among other things, on:
- particle type,
- particle hardness,
- solids concentration,
- flow velocity,
- flow conditions,
- measuring tube material.
A zero-point trend can provide a valuable indication, but it is not specific to measuring tube wear. Gas bubbles, deposits, vibrations and changed installation conditions can also influence the zero point.
The same applies to density measurement: a long-term shift under defined reference conditions may be diagnostically interesting, while a change during normal process operation may initially simply reflect an actual change in the medium composition.
Condition assessment becomes significantly more meaningful when mechanical diagnostic parameters such as measuring tube stiffness are also included.
Modern systems with Tube Health or tube condition monitoring can track changes in the vibrating measuring system relative to a reference condition and thereby support condition-based maintenance.
For abrasive applications, the most important strategy is therefore: do not wait until wear becomes visible through an incorrect flow value. Establish a baseline of zero point, density and condition diagnostics during commissioning and then systematically monitor how these values develop over the operating life.
FAQ: Coriolis Flowmeters with Abrasive Media
Can Coriolis flowmeters measure abrasive media?
In principle, Coriolis instruments can also measure liquids containing solids and suspensions. Whether a specific instrument is suitable for long-term use, however, depends on particle size, solids concentration, hardness, flow velocity, measuring tube geometry and material.
How does a Coriolis measuring tube wear?
Hard particles can remove material when they strike the inner wall. This abrasive or erosive wear can occur locally or over larger areas of the measuring tubes and gradually reduce their wall thickness.
Can measuring tube wear be detected from the zero point?
A long-term change in the zero point can indicate a changed sensor condition. However, it is not unambiguous because vibrations, gas bubbles, deposits, pipeline stress or an unsuitable zero-point check can also influence the value.
What is the Tube Health Check?
During a tube condition check, diagnostic properties of the vibrating measuring system are compared with a reference condition. This can include the stiffness of the measuring tube in particular. If the tube wall becomes thinner due to erosion or corrosion, its mechanical stiffness changes.
Does the Tube Health Check replace calibration?
No. The condition check is used to diagnose mechanical changes. Calibration, on the other hand, determines the actual measurement deviation compared with a traceable reference. The two methods complement each other.
Can abrasion also affect density measurement?
Yes. Density determination depends on the resonance behavior of the measuring tube. Mechanical changes can therefore also influence density measurement. However, a density change can only be clearly assessed as an instrument-related change under known reference conditions.
Why can a suspension make the zero point unstable?
Heavy particles can settle at low flow velocity and create an uneven distribution inside the sensor. Gas bubbles or a non-homogeneous mixture can also influence the vibration behavior.
Which installation orientation is suitable for solids?
For suspensions that tend to settle, vertical installation with upward flow is often advantageous. This supports particle transport and reduces the risk of deposits. The installation instructions for the specific sensor remain decisive.
Why can a pipe bend upstream of a Coriolis sensor be problematic with abrasive media?
A pipe bend can create turbulent or asymmetric flow. Abrasive particles may then strike the measuring tube locally at an unfavorable angle and increase material removal in those areas.
Is Alloy 22 automatically better for abrasive media than stainless steel?
No. Alloy 22 is mainly used because of its chemical corrosion resistance. Abrasion resistance is a different material property. The selection must therefore take both the chemical medium and the mechanical particle load into account.
How often should a Coriolis flowmeter be inspected when used with abrasive media?
There is no universally applicable interval. For a new application, shorter inspection intervals are advisable initially. Based on the observed trend, these can later be adapted to operating hours, solids loading and process risk.
Why should a reference measurement be stored immediately after commissioning?
Slow changes can only be assessed reliably if the initial condition has been documented. Baseline values for zero point, density under reference conditions and available Tube Health or sensor condition parameters are particularly useful.
Which is more meaningful: a single diagnostic value or the trend?
Usually the trend. A small one-time deviation may be caused by process conditions. A change that continues in the same direction over several inspections is significantly more meaningful and is better suited for predictive maintenance.