Increasing competition in the process industry means that operations and service work have to be optimised continuously. Environmental regulations for flaring have also become stricter. As a result, while ultrasonic flow meters have been used for over 30 years to measure the flow to flare stacks, today flare flow measurement is one of the most challenging flow applications, as velocities and gas compositions may change rapidly.
Flare stacks can be found in many industries as an integral part of the safety and waste gas system, as they allow controlled venting for large volumes of combustible components during upset conditions. By their nature, pressure, volume flow and composition of flare gases can change considerably in short periods of time.
Compact and more optimised production facilities have higher capacities and therefore result in higher maximum flow conditions over the flare. Flow rates of more than 120 m/s have been recorded during shutdown conditions in refineries and petrochemical plants. Waste or flare gas can vary greatly in its composition and abrupt flow condition changes should be expected. Furthermore, low flow conditions have became more important for process control and optimisation, and for identification of valve leakage in normal operational conditions
Minimum flow rates demand high-resolution transducer technology and advanced signal processing algorithms to ensure exact time measurement. High flow velocities will introduce soaring noise within the flowing gas when valves (for example) are installed before the acoustic ultrasonic measurement. Even drift of the ‘ultrasonic beam’ has to be considered at high gas velocities.
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