Pipe Flow Measurement In The Process Plant

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Pipe Flow measurement

Process flow measurement is usually carried out in pipe runs between various processes in a chemical plant, where pipeline sizes and pressures can vary widely. It is also found in a transmission pipeline in a remote location to monitor consumption as in say a drinking water elutriation network or a hydroelectric scheme where pipeline sizes can be as much as 2m (72”) diameter.   Measuring flows for this diversity of applications requires a number of different principles of measurement to optimise accuracy and reliability of results.

The orifice flow meter range from Endress+Hauser is a form of measurement that uses the Bernoulli principle that accurately predicts the pressure drop across an orifice for any given flow.    This orifice plate can simply be, in essence, a thin plate with a knife-edged hole, clamped between two flanges with a sensor pipe just upstream and another just downstream of the plate orifice and connected either side of a close coupled diaphragm sensor/transmitter unit.

For large flows and greater pressures, the knife-edged orifice is not suitable in view of the excessive vortex induced pressure losses in the pipeline.   For these applications such as for high-pressure steam or gas measurement, the orifice is ‘smoothed by elongating it to reduce vortices.    Orifice flow meter models available range from 25mm (1”) diameter to about 600mm (24”) with operating temperatures between -200degC (-328degF) to as much as 1,000 degC (1,830degF) and pressures up to 420bar (6,100psig)

The vortex flow meter products from Endress+Hauser  uses the Kármán vortex street phenomenon.   In these type of meters, an angular bar is placed across the pipeline flow and as the flow increases, vortices are formed both side of the bar in a rhythmic sequence and these are detected by a pivoted sensor which oscillates at a frequency that is related to the flow.   The frequency is detected and converted to pulsed signal that is proportional to the speed of the flow and hence the flow rate in the integrated sensor/ transmitter unit.

These vortex meters are typically used in the chemical production and petrochemical refining industries, for steam, nitrogen, compressed air, carbon dioxide, water, oils, solvents and other liquids and gases.

The ultrasonic flow meter Range – Prosonic From Endress+Hauser use the principle of ultrasonic waves move slower against the flow than with the flow in a pipe.    Typical ultrasonic meters available have a number of piezo-electric sensor/transmitter units in pairs set diagonally across the pipe section so that each simultaneous pulse aimed at its paired sensor is timed electronically and those pulses going upstream through the flow is compared with those going downstream and converted to a pipe flow velocity and hence a flow rate.    A variation for very large pipelines is retrofit units which use a variation on the above wherein pulses are bounced off the pipe wall opposite the ultrasound transmitter and detected on the same side.   These meters are usually found on hydro schemes and water supply pipelines.