Diagnostics and troubleshooting of flowmeters: measurement errors

1. Description of the Problem and Scope of Application

Accurate flow measurement is critical for many industrial processes, ensuring product quality control, energy consumption optimization, accurate accounting of raw materials and finished products, and operational safety. Flow meter failures that result in inaccurate, unstable, or missing readings can have significant operational and financial consequences. This manual covers the diagnosis of common flowmeter measurement errors, including electromagnetic, ultrasonic, vortex, mass (Coriolis), and differential pressure.

Typical symptoms:

• Inaccurate Readings: A stable but systematically deviated flow value compared to the reference or expected value.
• Unstable/Erratic Readings: Sharp fluctuations in flow rates that do not correspond to a stable process.
• Zero Readings: The flow meter shows zero flow while the process is running.
• No Signal: No output signal from the device (for example, 4-20 mA, pulse signal, digital data).

Equipment involved:

This manual applies to all types of industrial flowmeters used in technological processes.

Classification of Severity:

• Critical: Affects process safety, product quality, production shutdown, or may lead to environmental disaster. Measurement error >10% or complete failure.
• Main: Affects process efficiency, accounting for raw materials/products, energy consumption. Measurement error 2-10%.
• Minor: Affects monitoring and long-term planning. Measurement error <2%.

2. Security measures

CAUTION! Before beginning any diagnostic or repair work on the flowmeter or associated piping, all standard company safety procedures must be strictly followed. Failure to follow these requirements could result in serious injury or death to personnel and significant damage to equipment. Always use appropriate personal protective equipment (PPE).

• Lockout-Tag (LOTO): Apply a lockout-tagout (LOTO) procedure in accordance with EN ISO 14118 for all power sources (electrical, pneumatic, hydraulic) powering the flowmeter and related equipment. Check for no voltage.
• Relief of Pressure and Drainage: Ensure that the section of piping where the flowmeter is installed is completely de-energized, depressurized and drained of all process fluids or gases. This includes closing shut-off valves and slowly opening drain valves.
• Hazardous Substances: If hazardous (corrosive, toxic, flammable, hot) liquids or gases are used in the process, take special measures for their safe removal and neutralization. Use appropriate PPE (acid-resistant gloves, protective suits, respirators).
• Energy Storage: Be aware of stored energy in springs, capacitors or suspended equipment. Remove this energy before work.
• Temperature: Allow hot parts of the equipment to cool to a safe temperature or use heat protective gloves.

3. Necessary Diagnostic Tools

The following set of tools is required for effective diagnostics and troubleshooting of flow meters:

4. Initial Evaluation Checklist

Before starting a detailed diagnosis, perform the following steps to collect primary information:

5. Systematic Flow of Diagnostics (Block diagram)

1. Symptom: Inaccurate or erratic flow readings
   1. Initial Check:
      1. Perform the “Initial Assessment Checklist” (Chapter 4).
      2. Which reading:
         • Missing/Nil Reading? Go to step 2.
         • The reading is there, but is it implausible or unstable? Continue from 1.b.
   2. Power and Signal Check:
      1. Measure the supply voltage of the flowmeter:
         • Use a multimeter. Check the rated voltage (eg 24 V DC).
         • If the voltage is out of ±5% or absent:
           • Root Cause: Power supply failure, cable break, short circuit.
           • Remedy: Restore power. Check the fuses.
         • If the voltage is normal: Continue.
      2. Check the output signal (eg 4-20mA):
         • Measure the current with a multimeter in "serial" mode or use a HART communicator to read the value.
         • If the current is 0 mA (or another incorrect value, for example, 3.8 mA or 21 mA):
           • Root Cause: Current loop break, flowmeter transmitter malfunction.
           • Remedy: Check the cables to the DCS/PLC. Replace the transmitter (if necessary).
         • If the current is in the range of 4-20mA but does not match the expected flow: Continue.
   3. Check Process Conditions:
      1. Compare process pressure and temperature:
         • Use calibrated pressure gauges and thermometers. Compare with design/normative values.
         • If pressure/temperature deviate significantly (>10%):
           • Root Cause: Change in fluid properties (density, viscosity), phase transitions, cavitation.
           • Remedy: Adjust the process or recalibrate the flow meter to accommodate the new conditions.
         • If normal: Continue.
      2. Check for gas bubbles/solid inclusions (for liquids) or liquid (for gases):
         • Visual inspection through sight glass (if available) or sample analysis.
         • If found:
           • Root Cause: Two-phase flow leading to incorrect readings (especially for electromagnetics and eddies).
           • Remedy: Optimize the process to avoid biphasicity or install a flow meter that is resistant to such conditions (e.g. mass Coriolis).
         • If not found: Continue.
   4. Installation and Internal Condition Check:
      1. Inspect for build-up/contamination:
         • Use inspection camera or ultrasonic thickness gauge.
         • If significant growth (>1 mm) is found:
           • Root Cause: Contamination of the sensor/walls, change in internal diameter.
           • Remedial action: Clean the flow meter and piping.
         • If clean: Continue.
      2. Check the installation requirements:
         • Compare the actual length of straight pipe sections before and after the flow meter with the manufacturer's requirements (eg EN ISO 5167-1 for differential pressure).
         • If mismatch:
           • Root Cause: Montage effects (eddy currents, velocity profile offset).
           • Remedy: Reinstall flow meter or install flow straighteners.
         • If the installation matches: Continue.
   5. Calibration and Setup:
      1. Perform field calibration:
         • Use a reference flow meter or spill stand. Compare the readings.
         • If deviation >1% of normal:
           • Root Cause: Calibration drift, sensor wear, setting error.
           • Remedy: Complete recalibration or replace faulty component.
         • If normal: Continue.
      2. Check flowmeter setup parameters:
         • Use HART communicator. Check ranges, units, ratios.
         • If parameters are incorrect:
           • Root Cause: Configuration error.
           • Remedy: Set the correct parameters.
2. Symptom: No or zero flow readings
   1. Power check:
      1. Measure the supply voltage at the flow meter terminals.
      2. If absent or <22 В DC:
         • Root Cause: Power loss, cable break, power supply failure.
         • Remedy: Restore power, check circuit.
      3. If power is normal: Continue.
   2. Signal Cable Check:
      1. Disconnect the signal cables from the flow meter and from the controller. Check the integrity of the cable (resistance) with a multimeter.
      2. If the resistance is open (infinity) or short circuit (about 0 ohm):
         • Root Cause: Cable damage.
         • Remedy: Replace or repair the cable.
      3. If the cable is intact: Continue.
   3. Sensor/Transmitter Test:
      1. Connect to the flowmeter using a HART communicator.
      2. If there is no communication or the diagnostic shows "sensor/transmitter failure":
         • Root Cause: Internal failure of the sensor or transmitter electronic unit.
         • Remedy: Replace the faulty component or the entire flow meter.
      3. If there is communication, but the readings are zero: Check for flow in the pipeline and physical patency.

6. Matrix of Malfunctions and Causes

Below is a matrix linking common symptoms to likely causes, diagnostic tests, and expected results.

7. Analysis of the Root Causes of Each Malfunction

7.1. Montage Effects

• Why it occurs: Insufficient straight sections of pipeline before and after the flow meter (for example, after elbows, valves, pumps), which leads to a distortion of the flow rate profile, the formation of eddies or turbulence. This critically affects the accuracy of most flowmeters, except mass Coriolis. For example, vortex flowmeters require significant straight sections (10D before, 5D after) to form a stable vortex plume.
• How to confirm: Visual inspection of the installation site, comparison with the manufacturer's requirements and relevant standards (EN ISO 5167-1 for differential pressure). Measuring the length of straight sections.
• Which causes, if not eliminated: Persistent, systematic measurement errors that cannot be corrected by calibration. This leads to improper control of the process, overspending of raw materials or energy, and the release of low-quality products.

7.2. Change of Process Conditions

• Why it occurs: A change in density, viscosity, temperature or pressure of a liquid/gas compared to the conditions under which the flowmeter was calibrated or configured. It also includes phase transitions (for example, the formation of bubbles of gas in a liquid or condensate in a gas), which especially affect electromagnetic, ultrasonic and vortex flowmeters.
• How to confirm: Comparison of readings of calibrated pressure and temperature sensors with the norm. Analysis of composition and phase state of liquid/gas. Visual inspection (if possible) for bubbles/droplets.
• What causes, if not eliminated: Unstable or systematically incorrect flow readings. It can lead to incorrect operation of the automatic control system, which will cause fluctuations in process parameters and product inconsistency.

7.3. Drift Calibration

• Why it occurs: Natural aging of the sensor components, mechanical wear, exposure to an aggressive environment, temperature changes, exceeding the measurement range, electrical overloads. This leads to a gradual change in the characteristics of the measuring element.
• How to confirm: Conducting a control calibration of the flow meter on a spill stand or using a reference flow meter.
• What causes, if left unchecked: A gradual increase in measurement error that can go unnoticed for a long time, leading to accumulated losses or poor quality products. May lead to unplanned stops for unscheduled calibration.

7.4. Buildup/Fouling (Coating)

• Why it occurs: Deposit of solid particles, scale, polymers, bio-film or other substances on the inner walls of the pipeline and/or on the measuring elements of the flow meter. This changes the internal diameter of the pipe, the shape of the flow velocity profile, or interferes with the operation of the sensor (for example, it covers the electrodes of the electromagnetic flowmeter).
• How to confirm: Visual inspection using an inspection camera after dismantling the flow meter. Using an ultrasonic thickness gauge to estimate the thickness of growths. Checking the resistance of the electrodes of the electromagnetic flowmeter.
• Which causes, if left unchecked: Systematic measurement errors (usually under-readings), increased hydraulic resistance leading to increased pump energy consumption, reduced product quality, and potential clogging of equipment further down the process line.

7.5. Electrical Malfunctions

• Why this happens: Open or short circuit in power cables/signal lines, power supply failure, electromagnetic interference (EMI), grounding problems, failure of transmitter electronics.
• How to confirm: Measure the supply voltage and current loop with a multimeter. Checking the integrity of cables. Diagnostics of grounding. Using an oscilloscope to detect noise in a signal.
• What causes, if not eliminated: Complete lack of readings, unstable or random readings, inability to communicate with the device. Can lead to process stoppage and significant diagnostic costs.

8. Step-by-Step Troubleshooting Procedures

8.1. Elimination of Montage Effects

1. CAUTION! Perform the LOTO and line de-energization procedure (Chapter 2).
2. Score: Disassemble the flow meter and visually inspect the pipeline section for obstructions or incorrect geometry.
3. Correction:
   • Replace the flowmeter in a new location that meets the manufacturer's requirements for straight sections (eg, minimum 10D before, 5D after for vortex and some ultrasonic flowmeters).
   • If reinstallation is not possible, install flow conditioners according to EN ISO 5167-1 to stabilize the velocity profile.
4. Verification: After restoring the process, perform test measurements. Compare readings with a reference instrument or balance data. Perform on-site calibration.

8.2. Correction of Changes in Process Conditions

1. CAUTION! Assess the risks associated with hazardous liquids/gases (Chapter 2).
2. Monitoring: Install additional temperature and pressure sensors or use existing ones to continuously monitor process conditions.
3. Correction:
   • If the changes are permanent, recalibrate the flow meter taking into account the new conditions (enter new correction factors for density/viscosity).
   • For two-phase flows: optimize the process to avoid their formation (eg pressure increase, temperature decrease). If two-phase flow is unavoidable, consider installing a flowmeter resistant to such conditions (e.g., mass Coriolis).
4. Verification: Verify stability and accuracy of readings after adjusting conditions or recalibrating.

8.3. Elimination of Calibration Drift

1. CAUTION! Perform the LOTO and line de-energization procedure (Chapter 2).
2. Calibration: Disassemble the flow meter and send it for full laboratory calibration on a certified spill bench according to DSTU EN ISO/IEC 17025. If laboratory calibration is not possible, perform field calibration using a reference flowmeter.
3. Setup: Apply the calibration correction factors to the flowmeter configuration using the HART communicator.
4. Replace: If the calibration drift is excessive and repeated, this may indicate sensor wear or damage. Consider replacing the sensitive element or the entire flowmeter.
5. Verification: After returning to service, monitor the flow meter readings for several days and compare them with other process data.

8.4. Cleansing of Growths/Contaminations

1. CAUTION! Perform the LOTO procedure and de-energize the line. Safety glasses and gloves are MANDATORY. When working with aggressive chemicals, a full protective suit and a respirator (Chapter 2).
2. Access: Remove the flow meter from the pipeline.
3. Cleaning:
   • Mechanical: Remove build-up with brushes, scrapers (make sure not to damage sensitive sensor elements).
   • Chemical: Use appropriate chemical solutions to dissolve build-up (eg acids for scale, lye for organic deposits). Follow chemical safety instructions.
   • Ultrasonic: For delicate sensors, an ultrasonic bath can be used.
4. Inspection: After cleaning, perform a thorough visual inspection for damage to the sensor or internal parts.
5. Verification: After installing and restoring the process, perform an on-site calibration to ensure accuracy is restored.

8.5. Electrical Troubleshooting

1. CAUTION! Perform the LOTO procedure and de-energize electrical circuits. Risk of electric shock. (Chapter 2).
2. Power Check: Measure the voltage at the power terminals of the flowmeter. If it is missing or unstable, check the relevant circuit breakers, fuses, terminal connections and power supply.
3. Cable Check:
   • Disconnect both ends of the signal cable and measure its resistance. It should be very low (few ohms) per core and infinite between the cores and the cores and screen.
   • Check the integrity of the shielding and grounding of the cable.
4. Grounding Check: Make sure the flow meter housing and cable shielding are securely grounded. Ground resistance should be <4 ohms.
5. Component Replacement: If all external electrical circuits are OK and the HART communicator diagnostics indicate a malfunction, the transmitter electronics module or the sensor itself has probably failed. Replace the faulty component.
6. Verification: After eliminating electrical faults, check the stability of the output signal and the accuracy of the flowmeter readings.

9. Preventive Measures

Prevention is more effective than repair. Below are strategies for minimizing flow meter measurement errors.

10. Spare Parts and Components

UNITEC-D recommends having the following spare parts in stock for quick troubleshooting. All components can be found in UNITEC E-Catalog.

11. Links

• EN ISO 5167-1:2003 - Measurement of fluid flow using pressure drop devices installed in pipelines of circular cross-section. Part 1: Basic principles and requirements.
• ISO 10816 – Mechanical vibration. Evaluation of machine vibration by measurements on stationary parts.
• EN ISO 14118:2018 - Machine safety. Unexpected startup prevention.
• DSTU EN 61010-1:2016 - Safety of measuring instruments for electrical measurement, control and laboratory use. Part 1: General requirements.
• DSTU EN ISO/IEC 17025:2017 - General requirements for the competence of testing and calibration laboratories (EN ISO/IEC 17025:2017, IDT).
• Operation and maintenance manuals from flowmeter manufacturers (Siemens, Endress+Hauser, Emerson, Krohne, Yokogawa).
• Other UNITEC-D maintenance manuals (e.g. Pumping Equipment Fault Diagnosis).