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Calibration of oil & gas flow meters

01 March 2013

Craig Marshall, project engineer at NEL, explains how operators of calibration facilities, users requesting calibrations and engineers having to establish a calibration for flow meters can best apply the general principles of calibration when measuring fluids and gas.

Calibration is not an absolute operation, but a comparison between the reading of a flow meter and that of a standard. It is, therefore, necessary to consider what properties are required from a standard. Firstly and most importantly, the standard should measure the same quantity as the flow meter. For flow measurement, the standard is a system comprising of a measure of quantity and the subsidiary measurements to determine the fluid conditions, properties and influence factors.

Another feature of the standard is that there must be confidence that the measurement taken by the standard is accurate. To achieve this all the measurements in the system have to show traceability to higher level measurements and, ultimately, to national and international standards.

However, the quantity measured by the standard may be different from the quantity passed through the test device due to changes in volume between the meter and the standard, which are usually related to the influence factors such as temperature, pressure, viscosity and expansion. As the measurement of fluid flow is dynamic and all measurement devices are affected in some way by the conditions of use, it is impossible to have a standard which fully reproduces the conditions under which the meter will be used in practice.

The combination of fluid, influence factors, the standard and the device come together to define a set of operations which are used to provide the calibration. This is expressed in a way which gives a meaningful expectation of how the device will perform in use.

Obtaining confidence
To obtain confidence in a measurement, it should be able to be repeated and give the same result. A measurement standard should, therefore, have determined repeatability and reproducibility figures which will have been included in the uncertainty determination. The repeatability of a calibration will, of course, include the repeatability of the standard, but also the repeatability of the flow meter under test.

To correctly express the ‘accuracy’ of a standard or a calibration the ‘uncertainty’ must be determined and quoted. For flow measurement the confidence in the result lying within the uncertainty is normally quoted with a ‘coverage factor’ of k=2, which is approximately 95% confidence level. All calibration results should have a stated uncertainty and ‘coverage factor’ on the calibration report or certificate.

Although it may seem obvious, the resolution of the device must also be adequate to allow a calibration to match the uncertainty required. To achieve this, the standard must be able to measure enough fluid to match the resolution of the device. For example, if a flow meter has a resolution of one litre, the standard must have a volume of significantly more than 1,000 litres to achieve an uncertainty of better than 0.1%.

The importance of calibration fluid and conditions
The nature of how flow meters interact with the flowing fluid is affected by the properties of the fluid or the velocity distribution of the fluid passing through the device. It is the changes in this interaction that alter the ability of the device to give an accurate representation of the quantity, and the magnitude of the error is different according to specific meter types and fluids.

When a fluid passes through a pipe, the distribution of velocity across the pipe alters, depending on the pipe’s internal diameter, roughness and fluid Reynolds number. The presence of any change from a straight pipe will also alter the profile drastically as bends, double bends, valves etc. all introduce asymmetry to the velocity distribution, with some introducing swirl or rotation. As the way the fluid interacts with the sensor can be highly dependent on the velocity profile, these effects must also be considered in the calibration.

Ideally, calibration should therefore be done using the same fluid and pipe work configuration within which the meter will normally operate, but in reality this is not often possible and the meter has to be installed in a test laboratory, or the calibration standard has to be installed in the process application. In either case some degree of disturbance to the meter is inevitable.

How often to calibrate?
There is no single correct answer to the question as a specific industry standard or third-party dictates the calibration frequency. In this case the meter is calibrated whether it requires it or not and is often assumed accurate between calibrations.

For most applications, however, it is the user who must define the calibration interval and the policy to determine when to calibrate, with the interval chosen to minimise the risk of an incorrect meter reading making a significant impact on the process. For example, high flow rates of oil attract huge tax liabilities. The product value is high, the risk of meter damage is high and so perhaps weekly in-situ calibrations of the meter, in the actual product, will be specified. Alternatively, metering waste water with a Venturi may only require annual inspections, irregular verification, and no flow calibration.

Other factors affecting the decision are the history of the meter, maintenance periods, or what diagnostics are regularly monitoring the meter. Whatever the frequency, it is always good practice to keep calibration graphs and control charts of the meter performance as this will assist in selecting intervals and also show changes in performance indicating degradation of meter performance.

Calibration methods for liquids and gases
Unless a liquid is volatile or hazardous it can usually be contained in an open vessel. As a result, calibration standards are usually classified as being ‘bucket and stopwatch’ systems. The ‘bucket’ is a container that is weighed or has a known volume. The ‘stopwatch’ is a method of measuring the time to fill the bucket.

‘Standing start and stop’ is the simplest method available and can be used for both high and low accuracy calibrations. The flow system is filled, all air purged and the required flowrate established. The flow is then stopped using a quick- response valve. When the container is empty, the drain valve is closed, the flow started, and when the container is full, the flow is stopped. The quantity collected is measured and compared with the meter reading, and combined with the time to fill, to give the flow rate.

The ‘flying start and finish’ method is sometimes called the diverter method, where the flow through the meter is not stopped but continues uninterrupted and is physically diverted between a return path to the liquid supply tank and the collection container. A switch on the diverter mechanism starts and stops a timer and a pulse totaliser.

Beyond ‘bucket and stopwatch’ systems, more dynamic methods are available where not only is the flow continuous through the device under test but also through the standard while it is measuring, but they are generally less accurate.

In general, all calibration methods for gas flow meters have analogies with the liquid methods, with the main difference between the calibration of a gas flow meter and a liquid device being the compressibility of the fluid. Methods used to calibrate gas flow meters include displacement, gravimetric and volumetric approaches, and the use of sonic nozzles.

Whatever methods are chosen, it is vital to remember that a calibration applies to that meter only, operating under the conditions with which it was calibrated. If in service these conditions are changed the calibration may not apply. What then are the real orders of uncertainty which might be reasonably obtained from calibrated meters?

First, the meter cannot be calibrated to an uncertainty level better than its repeatability and the uncertainty of the standard. Systematic uncertainties can only be estimated from knowledge of the calibration system and its method of traceability and transfer through to the final duty with the addition of influence factors and historical performance being added.
Liquid flow meter calibration facilities should therefore be able to measure flowrates to uncertainty levels between 0.05 and 0.5%, depending upon the complexity of the system and its design, while calibration systems for gas flow meters should be able to measure flowrate to uncertainty levels of between 0.2 and 0.5%.

* NEL is an international provider of specialist technical consultancy, research, development, testing, measurement and programme management services, to the energy, and oil & gas industries, as well as government. Part of the TÜV SÜD Group, NEL is a global centre of excellence for flow measurement and fluid flow systems and is the custodian of the UK’s National Flow Measurement Standards.

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