Bidirectional Flow Measurement

The Right Flowmeter Is a Balance Between Technical Needs and Cost-Efficiency

By Ruchika Kalyani

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The square root function is complicated by the one-transmitter option because reconfiguration of the transmitter signal (4-12 mA and 12-20 mA) requires added function blocks and, subsequently, corresponding function blocks or logic at the distributed control system (DCS) side.

In cases where it's only a matter of knowing the reverse flow direction, and accuracy is not important, then the existing DP set without configuration can be used. At zero flow, 4mA is shown, and an output less than 4 mA can be used to alarm for reverse flow even when the square root function is on.

With newer, smarter flowmeter techniques, transmitters are equipped with a feature that allows reconfiguration of the DP transmitter range, such as split-range output signal (4-20 mA) to the system side (DCS, PLC). The bidirectional function, such as square root functions, can be directly applied to the transmitter by either installing special bidirectionality software at the control system side, or by using the built-in capability of the flowmeter to be used in both forward and reverse flow directions.

With equal or unequal flow rates, flow direction will be indicated as the output value (4-12mA = Reverse and 12-20 mA = Forward). With equal flows, zero flow point is established based on the DP range of forward and reverse flow, and for unequal flow rates, zero flow point will be a calculated value.

Bidirectional Flow Measurement with Vortex Flowmeters

The other option of two vortex flowmeters can also be used for steam bidirectional flow if higher accuracy is required than can be achieved using the orifice solution. However, this application is limited to smaller line sizes because vortex meters are more economical up to 4-in. (100-mm) pipe size. Beyond this size, orifice plates are more economical. In addition, the selection of a vortex-shedding flowmeter may increase the maintenance and installation cost.

Wherever higher accuracy is required, vortex flowmeters are not a good option, as vortices shed by both bluff bodies propagate really far beyond the pipe and may affect the other meters' readings. Another drawback is that the straight pipe run distance required between two vortex meters is unpredictable. For example, in the case of no obstructions, the meter required the run of 10 D (diameters) to 15 D, and if there is a control valve in either direction, the meter may require a higher run of 25 D to 30 D or even more. In comparison to the options of dual transmitters for bidirectional flow measurement between the two process units, DP flow measurement may be the most cost-effective solution.

Bidirectional Flow Measurement with Turbine and Magnetic Flowmeters

Bidirectional flow measurement is always a challenge when there are changes in process parameters, such as viscosity, conductivity, etc. It is always worth keeping these specific situations in mind while selecting any flowmeter technology, but with bidirectional flowmeter applications, it is especially important. DP type meters are usually not really well-suited to handle these process parameter variations.

Again, an example is utility pumping and circulating plants pumping dielectric fluid into underground electrical cables in order to dissipate heat generated by high-voltage power lines. This application requires flow rate monitoring upstream and downstream because it involves dielectric fluid; therefore, it requires viscosity compensation as the temperature of the dielectric fluid changes. In this application, turbine flowmeters can provide the solution for bidirectional flow measurement with moderate accuracy. However, drawbacks associated with this technology include a poor response of the flowmeter at low flows due to bearing friction; lack of suitability for high-viscosity fluids because the high friction of the fluid causes excessive losses; as well as the requirement for regular maintenance and calibration to maintain its accuracy.

The magnetic flowmeter can also be used for bidirectional flow measurement. It has the advantages of no pressure drop, linear output, short inlet/outlet pipe runs (five diameters upstream of the electrode plane and two diameters downstream), and good turndown. Magnetic flowmeters are relatively expensive and are mainly limited to conductive fluid applications, such as acids, bases and slurries, as well as water. A pre-requisite for this type of flowmeter is that the fluid is electrically conductive with an absolute minimum conductivity of 2-5 µSiemens.

Bidirectional Gas Flow Measurement with Ultrasonic Flowmeters

At gas storage fields or natural gas reservoirs, accurate gas flow measurements are required for tasks such as injection and withdrawal of gas from these reservoirs. Reservoirs are used as buffers between suppliers and consumers. In order to maintain the balance for the entire reservoir, it's necessary to monitor bidirectional flow at the wellhead.

For this purpose, conventional DP flowmeters with an orifice are far from a suitable solution, as they lack accuracy and reliability. Orifice plates are subject to wear and tear. Secondly, regular inspections and maintenance are required. While measuring the dirty gas, the pressure taps of the orifice plates are particularly exposed to clogging due to the solid particles which may be present in the dirty gas. These will definitely distort the accuracy of measurement.

In these cases, an ultrasonic flowmeter may be a far better solution because this type of flowmeter has no pressure drop, no flow blockage, no moving parts, and is suitable for high-volume bidirectional flow and also for low-flow measurements where other types of flowmeters do not provide the required results.

The advantage of using the clamp-on gas flowmeter transducer on the outside of the pipe is that it doesn't require any pipe work or any kind of process interruption. With this type of flowmeter even a little moisture content present in the gas can't significantly affect the measurement.

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