Flow measurement plays a critical role in chemical and petrochemical, oil and gas plants. Criticality of flow measurement in the plants has become a major component in overall economic success or failure of given processes. Accurate flow measurements ensure the safety of the process and profits in plants. Better measurement can only be achieved by selecting the best/most suitable flow technology for each flow application. Sometimes, the accuracy required by the end users is the most significant factor for the specific application. The challenge is to find out the value of the product stream being measured, thus providing the most cost-effective and reliable solution to the end users.
Instrument engineers should convince the end user to not install a flowmeter that is more expensive than the yearly value of the stream and the potential loss of money caused by inaccuracies.
A diverse range of flowmeters along with the turndown factors is available for various flow applications, such as regular flow control (steam, gas, utilities, etc), process flow rates, fiscal or custody-transfer metering, and others. Most of these applications will be unidirectional, but some will be bidirectional.
The measurement of unidirectional flow rate is possible with all types of flow technologies, but the bidirectional flow measurement capability is required to measure the flow rates within the same flow loop in opposite directions. This sometimes creates difficult situations, challenges, process interruptions and/or measurement inaccuracies that can significantly affect the production and profitability of the plant.
We will further discuss the selection of the appropriate metering for bidirectional situations and applications, limitations, advantages and disadvantages, maintenance and installation cost.
Bidirectional Flow Measurement
Bidirectional flow lines are not very common in refineries and petrochemical plants, but if they are needed, they are always difficult. For bidirectional flow, the piping scheme uses the same line to accomplish delivery and/or control functions for flows moving in opposite directions (forward or reverse flow), depending upon the process conditions and objectives.
Examples of bidirectional flow are
- Raw water feed to two or more water treatment plants,
- Bidirectional steam lines supplying steam from one unit to another unit in the plant,
- Utility and circulating pumping of dielectric fluid into underground electrical cables in order to dissipate heat generated by high-voltage power lines,
- Gas injected or withdrawn from the gas storage field or reservoir, purging and blanketing of nitrogen in plants,
- Chilled water plant decoupling headers.
Bidirectional Flow Measurement Using Volumetric Flowmeter Options
The selection process of bidirectional flow metering is dependent on application requirements, process demand, end-user accuracy requirements and physical design constraints of the flowmeter itself. Various flowmeters are available with bidirectional flow capabilities, such as the DP transmitter with an orifice, the Venturi or wedge element, the Coriolis, ultrasonic, vortex, pitot, turbine and the magnetic flowmeter.
Instances where a bidirectional flow measurement is required include
- Possibility of having two different flow rates in either direction, due to the process and design conditions, and both flows need to be measured,
- Reverse-flow accuracy is required by end user or by the process.
- The need to measure reverse flow in the process.
- Bidirectional Flow Measurement Using Dual DP Transmitter Options
For bidirectional flow measurement between two process units in a process plant, two steam units linked to each other; i.e., at the time of deficiency of steam in one unit , the other unit will supply the required steam to the deficient unit and vice versa.
If reverse and forward flow rates are identical in both directions and precise accuracy is not required, then dual transmitters, one for each flow direction, are the best solutions for measuring the steam flows in/out of the plant. Two DP transmitters with an orifice plate, along with temperature compensation can be used for the bidirectional flow. In this case, a non-beveled, square-edge type orifice plate should be used, and the two edges of the orifice should comply with specifications for the upstream edge mentioned in ISO 5167 standard. It also is necessary to ensure of the full "upstream" straight lengths on both sides of flow instrument. This must be clearly communicated to the piping design team during design reviews, and before construction begins.
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.