On-site calibration of bulk flow meters at operational conditions

Bulk water meter errors are a major contributor to ‘non-revenue water’ for utilities, costing approximately $15 billion/year globally. On-site calibrations under real operation conditions are recommended. However, until now, it has been difficult to obtain accurate recalibration of these meters within their actual on-site operation conditions.

Large flow meters are often used continuously for decades after an initial calibration is done using external test benches. In the best-case scenario, the flow meters are unmounted and recalibrated on an external test bench. But this method has the drawback of product supply interruption in addition to its already high costs. Also, flow meter behaviour under actual operation conditions can differ greatly compared to test bench conditions, so even if calibrated, the measurement uncertainty in operation remains fairly unknown. This often leads to more difficult fluid distribution optimisations and inaccuracies in billing. More precisely measured values of flow sensors may uncover hidden saving potentials within the fluid distribution system and allow more exact performance indicators, which can also lead to improvements in energy efficiency.

German company OPTOLUTION has developed an innovative calibration technology for large thermal (district heating and cooling) and water meters. The company can calibrate the flow meters on-site without demounting the sensors and therefore without interrupting supply. The procedure is said to have many advantages. For example, the calibration can be done under the real operational conditions instead of calibrating at idealised conditions on classical test-benches, and effort and costs are reduced as the dismantling process is not required.

Queensland case study example

Having assessed currently available techniques, Edgar Johnson of engineering and consulting company GHD facilitated the use of an accurate laser Doppler velocimetry (LDV) system, developed by OPTOLUTION for the in situ calibration of large flow meters for bulk water supplier Queensland Urban Utilities (QUU).

LDV operations involve splitting a primary laser beam into two parallel beams that intersect at the focal point, which are directed into the flow of water along a transept of the pipeline diameter. Minute particulate matter or bubbles passing through this point of intersection reflect the laser light at a fixed frequency (the Doppler effect), which is used to calculate the point velocity with an accuracy of ±0.1%. The LDV apparatus is robotically advanced in small increments across the pipeline diameter to compile a complete velocity profile.

The goal of this project was the assessment of measurement uncertainties on a selection of important flow meters in distribution lines for potable water.

In doing so, the following points were addressed:

• Finding of financial losses resulting from measurement errors.
• Avoidance of unfair billing and cost allocations.
• Reduction of inefficiencies due to miscalculations in water loss components.
• Further optimisation of network development and planning solutions.
• Security in compliance with local and national regulations.

The on-site calibrations were performed on nine magnetic-inductive flow meters with sizes ranging from DN 600 up to DN 1050. All of those pipes are located underground in either densely populated areas or open country. Therefore, all pipes had to be exposed first. The subsequent creation of the optical accesses was performed by local specialists without the need of interruption of the bulk water supply.

Due to the concrete inner layer of the bulk water pipes, measurements of the inner pipe geometry were performed with added care and specialised measurement equipment.

Limited by the bulk water network topology, volume flows for calibration purposes were only available for short periods. The on-site calibrations were conducted at two characteristic network volume flows with each calibration achieving uncertainties around ±1% of the measured value. The results of the on-site calibrations showed that the flow meters under test have measurement errors ranging from -12.6% up to +3.9% compared to the reference flow rate. In two calibration sites, the calibration measurements identified irregular flow meter data acquisition due to electrical issues or asymmetric flow conditions, deviating from the manufacturer guidelines, increasing measurement uncertainties of the meter under test and the calibration itself.

Edgar Johnson, GHD.

“By reducing non-revenue water, the LDV calibration trial had indicative payback periods of between 18 and 24 months, where the magnitude of financial losses due to meter errors would provide the necessary justification for investment.” (Johnson, et al, 2016)

“The LDV (laser-Doppler-velocimetry) technology was selected because of its unique capability to carry out very accurate in situ calibrations of large flow meters with a metrological accredited traceability to a national flow reference standard. The thoroughness and meticulous emphasis on the details required for application of the method under field conditions was demonstrated by OPTOLUTION,” said Johnson.

OPTOLUTION’s calibration laboratory is accredited by the standard of DIN EN ISO/IEC 17025, which means it is entitled to use the ILAC MRA mark for its calibration certificates. ILAC MRA is a multilateral agreement for calibration standards and for accepting measuring results internationally. The actual measurement uncertainty of the calibration depends on the on-site flow conditions. The minimum measurement uncertainty of 0.7% of the measuring value of the flow rate can be reached for fully developed flow conditions.