IMPROVING OPERATIONAL EFFICIENCY THROUGH ON-SITE CALIBRATION OF FLOW METERS

Bringing the calibration to the pipeline

The On-Site Calibration (OSC) method reverses this concept by bringing the reference measurement system directly to the installed flow meter. The technology is based on Laser Doppler Velocimetry (LDV), which measures local flow velocity from the Doppler shift of laser light scattered by tracer particles (Figure 2).

Optical access is established using a specially developed full-bore ball valve, hot tapping, and a pressure window assembly. Calibration is therefore performed while the pipeline remains fully operational, eliminating the need for service interruptions.

Figure 2: LDV measuring principle and automated profile scanning.

Measuring the real flow profile

An automated traversing system moves the laser probe across the pipe diameter, recording approximately 150 local velocity measurements per profile (Figure 2). During this velocity scan, the meter-under-test values are logged at the highest possible temporal resolution. The flow meter measurement error is calculated by comparing the calculated LDV volume flow with the time-averaged volume flow of the meter.

Instead of relying on a single measuring point, the OSC method captures the complete velocity distribution across the pipe cross-section. The resulting profile is used to calculate the reference volume flow while simultaneously revealing asymmetry and other hydraulic characteristics. Figure 3 shows typical measured profiles and the results graph from a calibration. The issued calibration certificate also includes information about the meter under test and the measurement conditions.

Probe positioning, data acquisition and measurement sequences are fully automated after parameterisation. An optical revolver enables automatic adjustment of the measuring optics for different operating conditions, while remote monitoring allows long measurement campaigns to continue overnight without permanent operator attendance.

Calibrations are typically performed at several operating points to assess meter performance across its working range. Because LDV measures velocity directly, it is independent of the measuring principle of the installed flow meter and can therefore be applied to electromagnetic, ultrasonic, orifice flow meters and other meter types.

Reliable reference measurements under fluctuating conditions

Unlike laboratory calibrations, field measurements are performed under continuously changing operating conditions. Flow rate fluctuations caused by consumer demand or pump operation are compensated by normalising the LDV measurements with data from an independently installed ultrasonic clamp-on flow meter. This approach keeps the reference measurement independent of the flow meter under test [1]. It relies only on the proven linearity of the clamp-on instrument, which is specifically qualified for this task.

Figure 3: Measured profiles (profile class “slightly disturbed”) and results diagram (profile class “fully developed”) Measured profiles (profile class “slightly disturbed”) and results diagram (profile class “fully developed”)

Accredited measurement quality

Accurate field calibration requires not only precise instrumentation but also a transparent uncertainty evaluation. The OSC method is accredited according to EN ISO/IEC 17025 [2]. To achieve the credibility and traceability required for accreditation, a wide range of uncertainties from multiple sources (see Figure 4) needed to be quantified and put together in an extensive uncertainty budget. The dominant uncertainty contribution originates from the velocity profile at the measuring location.

To quantify this influence, CFD simulations were combined with high-precision LDV measurements in a publicly funded research project [3]. Mathematical analysis of the profile allows every measurement to be assigned to one of several characteristic flow classes, each associated with a realistic measurement uncertainty [4].

Figure 5 shows the achievable total measurement uncertainties for each profile class. The minimum uncertainty for the “fully developed” profile class consists only of non-profile-related contributions.

Figure 4: Main contributors to the uncertainty budget. Main contributors to the uncertainty budget.

Figure 5: Profile classes and associated realistic expanded uncertainties (k=2). Profile classes and associated realistic expanded uncertainties (k=2).

Practical experience

More than 240 large flow meters have been calibrated in Germany, Austria, Switzerland and Australia on pipelines ranging from DN 100 to DN 1200 (Figure 6). The operating conditions of the measurements ranged from 5°C to 150°C in fluid temperature, fluid pressures ranging from 2 bar to 18 bar, and pipe sizes between DN 100 and DN 1200.

Figure 6: Typical field applications of the OSC technology.

The OSC method is also often used as a recalibration tool for flow sensors which were subsequently added as instrumentation to the grid lines. Because those flow sensors are often ultrasonic clamp-on or other comparatively easy-to-install flow meters, they often lack the needed measurement precision. An additional calibration after mounting can reduce the measurement uncertainty of these flow sensors by several percentage points.

Of the 241 flow sensors calibrated to date, 45 showed measurement deviations of more than 4%, after accounting for the inherent uncertainty of the OSC method. This means approximately 19% of all inspected flow sensors show significant deviations from their initial test-rig calibrations (Figure 7). In addition, approximately 5 % of the submitted flow sensors could not be calibrated due to faulty measurement characteristics.

At the same time, most meters showed good agreement with their original calibration, giving operators independent evidence that their installations continued to perform within specification.

Figure 7:  Results from more than 240 accredited on-site calibrations.

Benefits for utilities

On-site calibration improves confidence in customer billing, energy balances, efficiency assessments and regulatory reporting. It supports network optimisation, verifies newly installed clamp-on flow meters under real operating conditions and provides traceable evidence of the performance of the complete measuring system rather than the instrument alone. Because calibration is performed without interrupting plant operation, the method minimises operational impact while maximising practical value.