Bidirectional Flow Measurement

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

By Ruchika Kalyani

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With this combination, do not expect high accuracy and turndown. This combination will provide the lowest installed cost with acceptable accuracy, as it is easy to maintain and replace. Also, this dual transmitter combination option will be ideal in cases where the transmitter will experience reverse flow once every four or five years for a four- or five- day period.

Bidirectional Flow Measurement with Single DP Transmitter Option

A single DP flow transmitter coupled to a primary element option, such as the special orifice plate mentioned above, can also be adopted for cheap reverse-flow measurement. This arrangement will cut down the installing expense of another (second) DP transmitter, orifice plate, additional hardware, meter installation requirements and the complexity of signal switching. The square root function is complicated with the single- transmitter option, as reconfiguration of the transmitter signal (4-12 mA and 12-20 mA) requires additional function blocks and, subsequently, corresponding function blocks or logic at the distributed control system (DCS) side.

In cases where it is only a matter of knowing the reverse flow direction and accuracy is not important, then existing the 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; i.e., split-range output signal (4-20 mA) to system side (DCS, PLC). The bidirectional function (i.e. square root functions) can be directly applied to the transmitter by either just by installing special bidirectionality software from the flowmeter vendor at the control system side, or by using the inbuilt 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 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, but this application is limited to smaller line sizes, as 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 to 15 D (diameters), and if there is a control valve in either direction, the meter may require the 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 very important. DP type meters are usually not really well-suited to handle these process parameter variations.

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, as 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. 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; and 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 (5 diameters upstream of the electrode plane and 2 diameters downstream), and good turndown. Magnetic flowmeters are relatively expensive, and are mainly limited to conductive fluid applications such as acids, bases, 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.

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