There are several reasons why it is vital to have confidence that your flow meters are performing reliably and to their required level of accuracy. These include achieving consistent product quality, reducing waste and rework, providing a safe working environment, and ensuring that the correct amount of material is delivered, especially in fiscal measurement applications such as custody transfer.
Before flow meters leave the factory to be installed for the first time, they are calibrated to ensure they will achieve accurate and reliable measurement results. However, over time their accuracy will inevitably begin to drift, and this gradual deterioration can be caused by a range of different factors. These include the effects of corrosive or erosive fluids, dirty processes causing build-up, and the aging of the device’s electronics.
To return a flow meter to its original level of accuracy, it is necessary to recalibrate the device periodically. The time interval between calibrations typically depends on the requirements of the relevant industry standards and regulatory body, and the confidence levels demanded by customers. When used in fiscal measurement applications where absolute accuracy is critical, such as in the oil and gas or spirits industries, flow meters are usually required to be calibrated at least once a year. However, a flow meter used in an application where accuracy is non-critical can often operate for several years without needing to be calibrated.
Calibration typically involves a flow meter being taken out of the line and shipped to a flow service centre operated by an automation technology vendor and accredited to the international ISO/IEC 17025 standard covering the competence of testing and calibration laboratories. The meter is tested by comparing its accuracy against a known reference – such as a weigh scale or master meter – in controlled conditions, and it is then calibrated by being adjusted to match the reference.
The required accuracy of the master meter varies according to different standards, but it is generally accepted that the reference should be at least three times more accurate than the device under test. For example, a Coriolis flow meter claiming 0.1% mass flow accuracy would need to be tested and calibrated against a reference with at least 0.03% mass flow uncertainty.
When a meter is calibrated at a flow service centre, the uncertainty can be as low as 0.014% of the mass flow rate, providing the best possible assurance that the meter under test is performing to its original specifications. However, shipping meters for testing and calibration can result in devices being off-site for two weeks or longer, and when this causes a process shutdown it can prove extremely costly.
Flow service centres may provide premium testing and calibration services that are faster, typically taking between 48 and 96 hours, and some even offer a 24-hour emergency turnaround service – but of course, the quicker the service the more costly it inevitably becomes.
An alternative to shipping flow meters to a flow service centre is to have the testing and calibration performed on-site. Some automation technology vendors provide mobile flow calibration units, which will visit sites on arranged dates to perform the service. Emerson has several of these mobile units in operation around the world and has now expanded its service to cover the United Kingdom and the Republic of Ireland.
When an Emerson mobile unit visits a facility, typically four to six meters per day will be tested and calibrated by one of our experienced certified engineers. The service includes testing a meter’s electronics to verify that it works correctly, plus checking several calibration points against a Coriolis reference meter, and adjusting the meter under test as required, to match the reference. Flow meters up to three inches in diameter can be tested and calibrated and these can be Coriolis, turbine, magnetic or vortex devices from any vendor.
It should be noted that the reference uncertainty percentage that can be achieved at a mobile unit is not quite as low as at a flow service centre, and therefore devices should not be calibrated at a mobile unit if they are being used in fiscal measurement applications that require absolute accuracy. In these instances, the meters still need to be shipped to a flow service centre, to minimise financial risk.
In addition, mobile units typically use water as a calibration medium, but regulators demand that meters used for fiscal measurement are calibrated in an environment that replicates the real application as closely as possible. For example, the Oil & Gas Authority in the UK insists that oil must be used as the calibration medium, and that testing must take place at a temperature and pressure consistent with the real environment in which the meter operates.
However, for applications other than fiscal measurement, the reference uncertainty that can be achieved at a mobile unit is still low enough to satisfy regulatory requirements. The Coriolis reference meters used in Emerson mobile units have an industry-leading uncertainty of 0.08% of the mass flow rate, and calibration documentation is provided for your records as part of our service.
Furthermore, a range of significant benefits can be achieved when testing and calibration are performed on-site. Mobile units provide a faster, more convenient and less costly service, and being able to return a device to the line more quickly minimises process downtime, thereby increasing throughput and profitability.
Learn more about Emerson’s mobile flow calibration unit service for the UK and Ireland on Emerson.com.
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