Oil & gas


fluid with the calibration standard being brought in as needed. However, this is not often possible owing to costs, availability and facility infrastructure. In addition, the same fluid and installation considerations would have to be considered for the calibration standard. Typically, operational meters are removed


from service and installed in an accredited calibration laboratory where the standard has full traceability and confidence it its performance. Under these conditions, the physical meter installation can be replicated quite easily, and the test fluids are chosen to be as representative as possible to the operational fluids. Of course, a perfect match is not always possible, but any errors would be minimal and accounted for in the measurement uncertainty of the device.


How often sHould a flow meter be calibrated?


There is no single correct answer to this question and it depends heavily on what measurements are being made, and more importantly why they are being made. For very high accuracy service, for example in custody transfer or fiscal measurements, there may be mandates provided in sales agreements or through regulatory guidelines that dictate measurement uncertainty and calibration periods that must be adhered to. For general process measurements, it may be up to the end user to specify their own maintenance and calibration philosophy. The basis of the calibration interval can be


based on a number of factors such as meter size, location, weight, criticality and stability. In addition, there may be extenuating factors that limit the choices available, such as meters that are integral to safety of process equipment and therefore can only be removed during shutdowns. In general, the choice can be broken down


into three categories which are: calendar- based, risk-based and condition-based


Instrumentation Monthly March 2019


calibration intervals. Calendar-based intervals, as the name suggests, rely on an arbitrary time period for recalibration. Risk-based intervals use previous calibration data to estimate drift in measurement. This provides a realistic and evidence-based calibration interval representative of the technology and meter. Lastly, condition-based calibration intervals are calculated using meter diagnostics where an estimate of the meter’s health and therefore measurement quality is provided. In this instance the meter tells the user when a calibration is required. 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


At the heart of it, a calibration is a comparison of one device with another and there are many different calibration methods available for flowmeters which vary in their techniques, methods and implementation. It is useful to consider them under three categories which are: laboratory calibration methods, master meters and verification methods. Unless a liquid is volatile or hazardous, it can


usually be contained in an open vessel. As a result, laboratory calibration standards can be classified as being ‘bucket and stopwatch’ systems. The ‘bucket’ is a container which is weighed or has a known volume. The ‘stopwatch’ is a method of measuring the time to fill the bucket. Both the flying and standing start and stop methods are two techniques of collecting a quantity of fluid that differ only in their starting and ending cycle where the former leaves the device under test at a constant flowrate and the latter brings the flow to a complete stop. 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. These are typically called master meter systems and are generally less accurate than laboratory calibration methods. However, they do have the advantage that they are much more compact and can easily be transferred between locations. A master meter is any device that can be installed in series with a device under test where the same quantity of fluid can pass through each device uninterrupted. Normally these are other flow meters but for very high accuracy applications a prover loop is used. A prover loop consists of a dedicated flow stream with a known volume between detectors. As the fluid flows through the prover, a timer is started at detector one and stopped at detector two thus an accurate volume flowrate is determined. Provers can use different fluids and operate at different temperatures and pressures and as long as the appropriate corrections are applied, the results will be accurate. Provers offer similar measurement uncertainty


to laboratory calibrations and allow for live fluids to be used. Their use typically provides a very low overall measurement uncertainty, but they are extremely costly and require a large space and weight footprint. The cost is easily justified in high accuracy applications. Technically, a verification method is not a


calibration as the check meter will be of comparable or even poorer accuracy than the device under test. It is mentioned here as there are some cases where meters cannot be removed from service, but an indication of its performance must be acquired. It is important to be aware of some of the options available for this application. Typically, these verifications devices fall into the categories of non-invasive (or clamp-on) technology or insertion methods. These methods tend to be portable (although insertion methods require some infrastructure to be in place), can be installed when the line is operating and provide a rough indication of the flowrate (one to five per cent). 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.


Calibration is an essential part of successful


process operations and without it there would be little confidence in measured values. For flow meters, there are many more considerations the end user must be aware of to ensure a correct and transferable calibration is performed on their equipment. Care must be taken deciding when, where and how to complete the calibration with guidance in this article providing some key information for end users to use.


TUV SUD NEL www.tuvnel.com 43


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