Accuracy is the ability of the meter to measure close to the true value of the flow. Manufacturers usually state the accuracy of their flowmeters.
This is often the accuracy of the flowmeter in the calibration lab, but it can be affected by many installation parameters including temperature and density changes, piping configuration, and obstructions in the flow upstream of the flowmeter.
Also, vibration (noise) or flow pulsations from nearby rotating equipment can interfere with an ultrasonic sensor’s measurement or affect a Coriolis meter.
Total calculated measurement uncertainty of the installed flow meter takes into account: the accuracy of the flowmeter itself, the contribution to inaccuracy of piping and obstructions, and the accuracy of the flow computer and other electronics, including the flow computer’s A/D converter.
Because of the significant financial risks in custody transfer and fiscal metering applications, close attention must be paid to small details that would be ignored in a process application.
Alignment of the metering tube and the upstream piping is critical. If the misalignment is less than 1/8” (3.2 mm) and the misalignment is concentric, an ultrasonic flowmeter can handle the discrepancy. If the misalignment is eccentric, errors up to 0.2% can be caused.
Such an error can result in under- or over-billing of very large amounts over a year’s time.
As with process flow applications, upstream obstructions and disturbances in the flow stream must be reduced. However, due to the accuracy requirements of a fiscal metering or custody transfer application, it is even more critical to reduce noise and flow disturbance from control valves, thermowells, elbows, wyes, and tees upstream of the meter.
Where practical, there should be a sufficient straight run of piping upstream and downstream of the flowmeter. Also, control valves and temperature instruments that protrude into the pipe should be located downstream of the flowmeter.
Noise and vibration damping devices may be required, especially when using Coriolis and ultrasonic flowmeters.
In many custody transfer installations, multiple meters are installed off a single header. This permits each flowmeter to be operated independently of any other meter, allows one meter to be used as a master meter, and gives the operator and maintenance technician the ability to isolate one flowmeter for repair, calibration and maintenance without shutting off the flow.
It is also important to size headers correctly as header sizing can be critical to the performance of the system. Header sizing is especially important when retrofitting an existing metering skid or metering installation. Care must be taken to ensure that headers are actually built as depicted in the drawings.
Thermowells are challenging, though necessary. AGA 9, for gas metering, recommends installing thermowells at least 2 to 5 pipe diameters downstream of the flowmeter. For bi-directional systems, the standard recommends 3 diameters from the meter. Many flow experts consider these distances to be too close, and add a margin of safety to increase measurement certainty. Vortices shed from thermowells disrupt the flow profile and can reduce installed metering accuracy.
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