Oil, gas & renewables T
CALIBRATION TRANSFERABILITY
he transferability of a calibration from one fluid to another fluid has been a topic of interest for several years now. Many flow calibration facilities are limited in the fluids and calibration conditions that
they can offer. If certain flow meters can be calibrated in one fluid and then successfully operated in another then this could reduce calibration costs, improve flexibility, and ease maintenance requirements.
There are many factors when specifying the calibration philosophy for a flow meter. The measurement uncertainty, calibration frequency, flow range, number of calibration points, temperature, pressure, viscosity and even fluid type must be considered. It must be remembered that measurement is not an absolute operation, but instead an estimate of a true value with an associated uncertainty. The ’performance accuracy’ (measurement uncertainty) of a measurement is a function of both the quality of the device but also the calibration. To provide confidence that the measurement taken by the device is accurate, all the measurements in the system should demonstrate traceability to a higher-level standard. Whilst a device might be high quality, it can still drift over time. Specific industry standards or agreements normally dictate the calibration frequency. For most applications however, it is the user who must define the calibration methodology and conditions. The calibration temperature, pressure, fluid viscosity and flow range often are not fully detailed, and the specifications are dependent on the end users’ decisions. However, the temperature, pressure and fluid properties of a process can differ considerably from conditions at the calibration laboratory. The standard practice for calibrating flow meters has been to match the fluid viscosity and, if possible, the fluid temperature and pressure. Unfortunately, matching all parameters is seldom possible due to the replication limitations of the calibration facilities. For challenging fluids such as Liquified
Natural Gas (LNG), heavy oil and CO2, there are limited traceable calibration
facilities available. In the extremely important area of Carbon Capture, Utilisation and Storage (CCUS), there are a
limited number of gaseous CO2 calibrations facilities but no liquid or supercritical CO2 facilities. As such, calibrating in a more
readily available fluid such as water or nitrogen would be extremely advantageous. As these fluids are readily available at flow calibration facilities around the world, the cost of a calibration is much lower and also easier to arrange. Whilst calibrating in one fluid and operating in a different fluid sounds like a really promising solution, there are several matters to consider such as meter technology and measurement uncertainty. All flow meters are affected to some degree by fluid properties and velocity profiles. The nature of how flow meters interact with the flowing fluid is a function of the fluid properties of and/or the velocity distribution of the fluid passing through the device. 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 bends and valves can introduce asymmetry to the velocity distribution and potentially swirl. All of these effects must also be considered in the calibration. The most fundamental consideration for fluid calibration transferability is simply whether a flow meter can be operated in a
By Dr Chris Mills, senior consultant engineer at TÜV SÜD National Engineering Laboratory
different fluid medium, e.g., can a liquid flow meter that was designed for oil be used with water (potential rust issues), or can it be used in gas (potential damage to bearings)? Most flow meters with moving parts, e.g., turbine and positive displacement meters are designed for operation in one fluid type and cannot be simply relocated from liquid to gas operation or vice versa. Even newer technologies such as ultrasonic flow meters cannot be relocated from liquid to gas operation or vice versa due to the different meter designs and underlying principles such as transducer frequencies and meter design. The two most appropriate meter technologies that can be operated in multiple different single phase fluids, are Coriolis flow meters and Differential Pressure meters. Both technologies are single design and are not
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March 2023 Instrumentation Monthly
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