Overcoming flow measurement challenges
Matching the flowmeter to the application can help engineers overcome flow measurement issues. Magnetic, vortex, and differential pressure (dP) flowmeters are the leading technologies used to measure volumetric flow or flow velocity. Know when to use each.
Volumetric liquid flow is one of the most important parameters for process monitoring and control in many industries including chemical, refining, oil and gas, water/wastewater, power, pharmaceutical, food and beverage, and others.
Flows must be kept within safe limits; monitoring flow rates indicates when low or high flow conditions have been breached. These excursions often are provided as alarms to operators. For real-time control, flow rate can be used as an interlock within the control system—for example, to turn on a centrifugal pump before a tank or vessel runs dry.
Perhaps the leading use of flow measurement information is to optimize continuous processes as the process variable in a proportional-integral-derivative (PID) control loop. For example, a flow measurement can be used as the process variable feedback in a control-valve positioning loop to keep the setpoint at or near its desired value.
The three leading technologies used to measure liquid volumetric flow are differential pressure (dP), vortex, and magnetic flowmeters. Selecting among these three measurement methods for a particular application is often based on which technologies have been successfully used before under similar circumstances. Consulting with a flow measurement specialist at an engineering or system integration firm or a trusted supplier can provide additional insight, including the identification of alternatives, or information regarding recent innovations that may further improve flow measurement and plant performance.
Measurement challenges with dP, vortex, and magnetic flow measurement can be overcome by employing new technologies.
Measuring dP flow
Measuring flow via differential pressure is the most common flow measurement technique and has been proven in use for decades. A dP flowmeter is a cost-effective way to measure volumetric flow, especially in applications with large line sizes—typically 8 in. dia. and more, such as those found in water feed and discharge lines. Unlike other technologies, dP flow measurement can be used with conductive and nonconductive fluids allowing it to be used with a wide range of gases and liquids. The primary element in a dP flowmeter creates a pressure drop by introducing a restriction in a pipe.
This pressure drop is then measured by the second component—a dP transmitter, which sends the readings to the control system. Depending on the exact nature of the dP flowmeter, the remaining components may include impulse piping and the connectors routing the upstream and downstream pressures to the transmitter.
By creating an engineered restriction in a pipe, typically an orifice plate, a Pitot tube (a Pitot tube measures the difference between the static pressure and the flowing pressure of the media in the pipe), or an Annubar (an Annubar is an averaging Pitot tube that takes multiple samples across a section of a pipe), Bernoulli's equation can be used to calculate flow rate because the square root of the pressure drop across the restriction is proportional to the flow rate.
A dP flow measurement has a wide range of applicability. However, challenges may arise depending on the particular application, the chief among them being wet-leg issues. "Wet leg" is the term used to describe the impulse line connection between the dP transmitter and the primary flow sensing element. A gas—air, for example—can get trapped in a wet leg and impact flow measurement accuracy. In addition, wet legs can become clogged and may freeze in cold conditions.
Existing technology allows an integrally mounted pressure transmitter to connect directly to the primary flow element, eliminating the impulse line and its wet-leg issues. These assemblies can be installed easily and quickly because no impulse lines are required. In addition, maintenance costs are decreased because leak points are reduced.
Another dP flow measurement issue can be introduced by traditional orifice plates because they require significant straight runs of pipe to reduce flow disturbances—up to 44 pipe diameters upstream of straight-run pipe and seven diameters downstream. For a 12-in. dia. pipe, 44 ft of straight pipe run would be required upstream, and 7 ft would be required downstream.
In many cases, bends in the piping within these distance limitations can cause measurement inaccuracy with traditional orifice plates. In these instances, a conditioning orifice plate can be used. A conditioning orifice plate requires only two pipe diameters upstream and downstream of the orifice plate of straight pipe run, which greatly increases the number of applications for dP flow measurement.
Vortex meters measure fluid flow according to the Von Karman effect, where a fluid striking an obstruction (shedder bar) creates alternating vortices, or low-pressure areas, behind the bar at a frequency proportional to the velocity of the fluid. The measured frequency is converted to flow velocity, which is then converted to the volumetric flow rate. Industrial vortex meters were introduced in 1968 and have been successfully used to measure the flow of steam, gases, and clean liquids. As with all flow measurement technologies, there are challenges and resolutions when applying vortex meters.
Typical vortex designs have small free spaces or crevices around sensors or shedder bars to create the movement needed to create vortices. Coating or particulate matter in the fluid can clog the crevices and inhibit sensor movement resulting in inaccurate flow measurement. All-cast vortex meters eliminate the need for free sensor movement and are thus insensitive to coating and clogging. Therefore, they can be used to reliably measure flows of dirty fluids that would plug traditional vortex sensors.
A minimum flow rate—referred to as the "low-flow cutoff"—is required to establish the vortices. Flow rates at the lower end of the desired measurement range may fall below this minimum and thus cannot be measured, frequently requiring the flowmeter and piping size to be reduced to increase velocity.
Reducer vortex meters have concentric reducers designed and built into the meter body that enable measurement of lower flow rates without expensive and disruptive process piping modifications. Factory calibration of the reducer vortex meter ensures accuracy and eliminates the need for additional upstream and downstream piping in the field.
In some cases, mass flow is the preferred measurement instead of volumetric flow. For example, in process steam efficiency calculations, the preferred unit of measure for saturated steam flow is lb/hr, as opposed to a volumetric rate. Pressure or temperature compensation can be used to enable a vortex meter to measure the mass flow of fluids. Integrating these compensating measurements into the meter reduces installed cost and complexity.
Vortex meters are often used to measure critical flows of aggressive chemicals. Traditional sensor designs provide potential leak paths, possibly allowing process fluids to escape from the line. These designs also cannot be serviced without shutting down the process and potentially exposing workers to process chemicals. For measuring these critical flows, all-cast meters with isolated sensors eliminate common leak points and can be safely serviced under process conditions.
Mag meters operate on the principle of Faraday's law of electromagnetic induction, where a voltage is developed when a conductive fluid is passed through a magnetic field. The voltage developed is proportional to the strength of the magnetic field, the length of the conductor, and the velocity of the conductor described by the equation:
- E = voltage measured at the electrode
- K = the meter constant
- B = the field density (strength of the magnetic field)
- L = the length of the conductor (length of the conductive path between electrodes)
- V = the velocity of the conductor (conductive fluid).
Flow velocity can thus be obtained, and by extension, volumetric flow.
Mag meters were introduced in 1952 and are used to measure the flow velocity of a vast number of conductive liquids. Because they introduce no obstructions in the fluid stream, they avoid many issues found with insertion meters, such as pressure drop, product shear, or wear and tear on the meter from the flowing fluid.
Mag meters can measure abrasive and high-solid slurries. In addition, wetted surfaces (liner and electrodes) can be selected for compatibility with chemically aggressive or abrasive fluids. The wetted surfaces of the meter can be selected to match the characteristics of the process fluid, extending meter life.
Advances in material technologies and installation techniques can further improve reliability and extend the life of the meter. Using meters that feature all-welded design and isolated compartments that protect against moisture and contamination can improve reliability.
Liners with increased temperature resistance, reduced permeation, and resistance to oil in applications, such as produced water measurement in oil production applications, extend service life, as do options, such as lining protectors and hardened materials to protect against abrasive wear. Installation practices can manage velocities to further reduce abrasion or to manage temperature gradients and reduce the rate of permeation.
Quality and regulatory requirements often require periodic confirmation that the meters continue to measure flow accurately. This typically requires removing the meter from the line and confirming accuracy at a flow lab or bringing additional equipment to the field so the calibration can be verified with the meter offline. Smart meter verification is an accepted method where the technology is built into the flowmeter so it can verify performance without taking the flowmeter offline and without special equipment.
Smart meter verification validates the hardware, software, coils, electrodes, and interconnecting wiring. Verification can run continuously or can be initialized through the local operator interface or via a remote software command. There is typically a significant reduction in downtime due to the elimination of calibration verification as well as errors associated with disconnecting and reconnecting functional meters. In addition, smart meter verification can simply and easily confirm initial installation, providing instant feedback that the installed meter is functioning as intended.
Fluids often contain particulates that can cause noise that interferes with the flow measurement. Examples include mining slurries, pulp and paper mill slurries, and produced water with high levels of sand carryover. Particulates striking the electrode cause high levels of noise (similar to the effect of tapping on a microphone), which can result in low signal-to-noise ratios, reduced accuracy, and poor loop performance.
Traditional means of dealing with high process noise involves the addition of damping, which can reduce loop response to unacceptable levels. Modern noise mitigation technologies can be used to provide a more accurate and responsive flow signal. These technologies include diagnostics that dynamically monitor signal-to-noise ratios, adjustable coil drive frequencies to move to a less noisy frequency, and high signal mag meters with increased signal power.
Mag, vortex, and dP meters are available as wireless devices. The transmitters in dP meters are available in versions with integral wireless, while mag and vortex meters are available with adapters to convert their outputs to a wireless signal at the transmitter. The primary value of wireless is delivered with either configuration.
Wireless installations transmit process data and diagnostic information wirelessly, eliminating the need to run signal wiring to the I/O panels. Eliminating signal wiring allows a much quicker and less expensive installation as there's no need to run wires, and it also reduces maintenance.
Wireless devices connect to a wireless gateway that can be located in close proximity and connected to the control system via hard wiring. Wireless technology also eliminates the need for additional analog input points at the control system to accept the signal wiring from the devices, which can be very expensive, particularly if a new analog input card needs to be added.
Closing the loop
Mag, vortex, and dP flowmeters are the leading technologies used to measure volumetric flow or flow velocity. Each technology has its place, and each has its challenges. The first decision is the selection of the right flow technology for the application-a task where a trusted supplier or system integrator can be enlisted to provide assistance.
The next task is to ensure the proper meter is specified for the application, including required features and options to overcome the flow measurement challenges inherent to the application.
- Wally Baker, pressure content marketing manager, Emerson Process Management; Ron Pozarski, senior industry manager, Emerson Process Management. Edited by Jack Smith, content manager, CFE Media, Control Engineering, firstname.lastname@example.org.
- Select the appropriate flow technology for the application.
- Evaluate flowmeter first, installation second, and finally lifecycle costs.
- Consider the advantages and risks of employing wireless flow measurement technology.
When evaluating flow measurement technologies, you should understand the differences among mass flow, volumetric flow, and flow velocity.
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