Advances in Flowmeter Technology
If you haven’t looked at flowmeters lately, you may be surprised at how far many sensor and signal processing technologies have evolved in recent years.
Trevor Ball, Robert Zaun, Amy Johnson, Mark Kester, Emerson Process Management
While some flowmeters haven’t changed much in 50 years, others have made significant improvements. Some the biggest changes have been in electronics and advanced diagnostics that make flowmeters much more useful for automation and process control. Much of the instrumentation market has been in flux for the past two years, thanks to problematic economic conditions, so predictions about which flow sensor types are gaining and which are declining in use vary widely. Market analysts seem to agree, however, that where technology improvements have been made—such as differential pressure, ultrasonic, Coriolis, and vortex flowmeters—these are gaining in popularity, while some of the more traditional flow sensors, such as turbine and positive displacement, are declining. Let’s look at a group of flowmeter technologies, examine their general characteristics and see some recent improvements.
DP still a standard
Differential-pressure (DP) flowmeters all work on the same simple principal: the pressure drop across a restriction is proportional to flow rate. They can be engineered for specific characteristics to provide best results in a broad set of applications, including custody transfer, process control, and monitoring. DP flowmeters are easy to calibrate, which lowers maintenance costs, often enough to justify replacing old DP devices with newer, more sophisticated units.
Advancements in DP flow technology include both the primary element and transmitters. Even the basic orifice plate, long the industry standard, has seen many improvements:
• Conditioning orifice plates require less straight pipe up and downstream while still providing superior measurement performance.
• Improved orifice technology facilitates transmitter-direct mounting and installs in existing raised-face flanges.
• Averaging pitot tubes, with insertion technology and low pressure drop, have been made more accurate.
Transmitter advancements include more sophisticated diagnostics, wireless networking, and multivariable capabilities:
• Built-in transmitter diagnostics can go beyond internal verification, using statistical process monitoring to detect abnormal situations such as fluid composition changes or impulse line plugging.
• Careful power management can reduce consumption to the point that battery powering and wireless communication are practical. This can lower installation costs and allow measurements to be made in inaccessible locations.
• Multivariable DP flowmeters can calculate mass and energy flow with a single pipe penetration, replacing the need to install up to 10 separate devices.
With these advancements, DP flowmeters have continued to be a growing technology and remain an industry standard.
Magnetic for tough liquids
Magnetic flowmeters or magmeters function on the principle of Faraday’s Law, which states that a conductor moving through a magnetic field will generate a voltage proportional to its speed. In a magmeter, the conductor is the fluid passing through the sensor, and the relationship among voltage generated, magnetic field strength, and velocity is established through calibration.
Magmeters can provide a highly accurate volumetric flow measurement, cause no pressure loss through the sensor, and can be scaled to line sizes from 0.1 to 120 in. diameter. A magmeter can handle aggressive slurry flows such as mining slurries and pulp stock, applications that are very difficult for other types of sensors.
There are practical limitations, however. For example, the process fluid must be conductive and magmeters cannot be used with gas or steam.
Coriolis for precision
Coriolis flow measurement is based on the principle that as a fluid moves through an oscillating tube vibrating at its resonant frequency, forces are induced which cause the tube to twist. The amount of twist is directly proportional to the mass flow rate of the fluid flowing through the tube.
Density is measured at the same time, as the mass per unit volume of the fluid in the fixed volume of the tubes increases or decreases in proportion to the resonant frequency. Dividing the density by the mass provides a highly accurate calculation of volumetric flow.
By providing highly accurate multivariable measurements over a wide turndown, Coriolis meters have proven to be ideal for a wide range of applications, including process and quality control, fiscal custody transfer, batching, mass balance, and more. Because a mass measurement is unaffected by changes in pressure, temperature, density, and viscosity, they are increasingly used in place of volumetric meters. Moreover, sophisticated analysis of liquid characteristics is possible, including viscosity and specific gravity.
Because of their ability to provide increase savings and efficiency, Coriolis flowmeters are one of the few technologies that increased sales over the past two years. More recent developments, including the ability to measure accurately in the presence of entrained gas and the advent of two-wire loop-powered Coriolis meters, increase the technology’s utility.
Vortex the workhorse
Vortex flowmeters work on a principle called the von Karmen effect, which states that when flow passes by a bluff body or shedder bar, it generates vortices downstream of the bluff body. The frequency of the vortices is proportional to flow velocity. A vortex sensor typically uses an transducer to detect the frequency of the vortices and transmit a flow signal.
Vortex flowmeters are an accurate and reliable instrumentation workhorse, and can be used to measure liquid, steam, or gas and provide good accuracy, high reliability, wide turndown, and an attractive price level compared to other sensors, especially in line sizes of 6 in. or smaller. Adoption continues to grow as users recognize the advantages this metering technology.
One disadvantage of vortex meters is their inability to measure very low flows. For measurable vortices to be generated there needs to be a minimum velocity. The specific threshold depends on the device and fluid parameters.
Ultrasonics still developing
While ultrasonic flowmeters were first introduced in 1963, they have improved so much in the past 50 years that they are still considered a “high-technology” device by some market analysts.
There are two main technical approaches: Doppler and transit time:
• A Doppler meter sends an ultrasonic signal across a pipe, measures the signal reflected off particles in the moving fluid, and computes flow speed by measuring the Doppler shift.
• A transit time unit transmits two ultrasonic signals to a receiver on the other side of the pipe, one with the flow and one against the flow. It measures the difference in transit times between the two signals to calculate flow.
Recent developments in ultrasonic flowmeters include the ability to measure gas and low flows, and configurations that measure both Doppler and transit time. The fastest-growing application for ultrasonic flowmeters is custody transfer of petroleum fluids.
Thermal mass for gas
A thermal mass flowmeter measures flow by detecting the amount of heat convected from a heated surface to the fluid flowing over it. Thermal mass flow meters are almost entirely used for measuring gas.
Two methods are used: capillary and immersible:
• In a capillary sensor, a small portion of the gas is diverted to a small, heated capillary tube that has two RTD temperature sensors wrapped around the outside. The RTDs measure the rate at which the gas carries off the heat.
• An immersible sensor is completely located in the pipe, usually in the form of a probe. This makes it suitable for a wide range of pipe diameters and mass flow rates. For very large pipes, the probe may have more than one measuring point.
Sensors built around mechanical approaches date back centuries and still have a large installed base, but their market position in most areas is eroding in the face of electronically-based approaches with no moving parts. Most mechanical sensor designs available today use some form of electronic signal processing, however they still depend on a rotating element turning in bearings.
• Turbine flowmeters have been around since 1790 and rely upon a spinning rotor, which can be a paddlewheel, propeller, or a similar device. Flow is derived from the speed of the rotor as it spins in the passing fluid.
• Positive displacement (PD) flowmeters are also an old approach and measure flow by making the fluid displace a device such as a piston, gear, nutating disk, rotary vane, or diaphragm. PD flowmeters can be extremely precise and have a very wide turndown range, however they can cause a large pressure drop and accurate movements are expensive.
Not your father’s transmitter
For 50 years, flowmeters have used transmitters; that is, a device that converts the native signal from the flow sensor to a standard format analog signal, typically either a 4-20 mA current or a 3-15 psi pneumatic pressure that can be transmitted back to the control room or fed into a local controller.
Pneumatic transmitters are all but gone from the scene, but most modern field devices still communicate via a 4-20 mA signal over a twisted-pair cable. Today, they also connect via HART, wireless HART, and fieldbus networks. Modern transmitters are bristling with electronics, computers, and intelligence that allow them to perform diagnostics and advanced functions of all kinds, such as calculate mass flow in a DP or vortex flowmeter, or detect something as subtle as the changing of beer brands with a Coriolis meter.
Ever since manufacturers put the first microprocessors into transmitters, operators have had the ability to perform diagnostic and calibration functions. In the early days and into the present, this was done in the field with a handheld HART communicator, which required the operator to physically plug in to access internal transmitter data. Today, fieldbus networks and HART allow such functions to be performed from the control room. Asset management software platforms can obtain sensor data from the transmitter and perform necessary diagnostics without sending out a technician.
Diagnostic functions available today range from those that seek out specific problems to those that constantly monitor the general health of a flowmeter system. For example, the Micro Motion Smart Meter Verification system can check the health and performance of the entire Coriolis unit, sensor and electronics, while the device is in line and the process is flowing. This provides substantial savings by reducing manpower and calibration costs.
Similar technologies applied to DP flowmeters can probe beyond the health of the device by detecting process anomalies. In one case, such information from a flowmeter allowed a large chemical manufacturer to detect an upset in catalytic flow approximately 30 minutes before it reached a critical “stick-slip flow” condition. Operators were able to take preventive actions promptly and avoided a shut down.
Being able to verify calibration automatically can save considerable amounts of money. For example, breweries have to verify calibration of flowmeters used to measure the amount of alcohol produced for tax purposes. The U.S. FDA 27 CFR Part 25 requires that, “…the brewer shall periodically test the measuring device.” One brewery had 12 magmeters that fell under this requirement, so they contracted with an outside organization to verify the flowmeters twice a year. At $1,600 per meter per verification, this cost $38,400 per year. Downtime while the meters were being calibrated cost $470,000 in lost revenue.
The brewery replaced the old flowmeters with newer units that had an internal meter verification diagnostic. Now, the checks are performed automatically, with virtually no interruption to the process, saving the brewery more than $500,000 per year.
Flowmeter diagnostic functions
Diagnostic functions can be used as practical tools to diagnose process problems affecting the sensors.
Sensor failure and degradation—Most transmitters can determine when a sensor has failed, or when the sensor is reading too high or too low. Flowmeters with moving parts, such as PD and turbine meters, can suffer wear or erosion, and deposits on the inside wall of a flowmeter can affect performance. Flow transmitters can detect such problems.
Plugged line—Coriolis flowmeters can experience long-term coating buildup or plugging. By monitoring the voltage required to obtain sensor readings, diagnostics can detect the presence of coated or plugged sensor lines. DP flowmeters can also detect plugged impulse lines. A gas-to-liquids (GTL) plant was able to employ this technology to improve process quality and increase uptime by using the information to optimize maintenance schedules.
Spikes—Entrained air, gas breakout in condensate, cavitations, or other problems can cause measurement error or damage to downstream equipment. The transmitter can measure process noise and detect the presence of these process issues. At one industrial gas manufacturer’s billing site, a reciprocating gas compressor was causing pulsation in the flow measurement line leading to inaccurate billing. Advanced diagnostics incorporated in a DP flowmeter were able to detect this situation so it could be corrected.
Composition—Similar to the example where a brewery used a Coriolis meter to detect changes in beer, diagnostics in multivariable meters can determine liquid fractions in gas and pipelines, liquid condensate giveaway, and other situations where fluid composition can change.
Electrical loop diagnostics—Unwanted electrical loop changes, such as water in the housing, ground loop issues, corrosion, or an unstable power supply, are all conditions that may affect the output. Sophisticated diagnostics can monitor the integrity of the electrical loop from the flowmeter to the host system and send alerts when unwanted conditions may jeopardize the operation.
More than a process variable
Modern flowmeters do much more than just measure flow. With their advanced diagnostics, signal processing and computing functions, flowmeters can save a user a considerable amount of money in maintenance, calibration, and troubleshooting. Why is there a control problem in the process? Ask your flowmeter.
Trevor Ball, Robert Zaun, Amy Johnson, and Mark Kester work as product managers in Emerson Process Management’s various instrumentation divisions, including Rosemount, Micro Motion, and Daniel.
Every type of flowmeter has its own specific diagnostics. Using magmeters as an example, here are some special diagnostic functions they have available:
Ground and wiring fault detection—A transmitter continuously monitors signal amplitudes over a wide range of frequencies. If the amplitude of the signal at either 50 or 60 Hz (the most common ac cycle frequencies) exceeds 5 mV, that is an indication of a ground or wiring issue.
High process noise—This diagnostic checks to see if there is a process condition causing unstable or noisy readings, such as high levels of chemical reactions or entrained gas in the liquid. The transmitter monitors signal amplitudes over a wide range of frequencies, checks the signal to noise ratio, and alarms if the signal to noise ratio exceeds limits.
Empty pipe—False readings can occur when a pipe is empty, such as in batching operations. This diagnostic determines when a pipe is empty, sets the flow rate to zero, and alerts the operator.
How can updating the sensor technology benefit you?
Here are some user examples of benefitting from improved sensor and transmitter technology from a variety of industries and applications:
A pulp & paper plant in Germany was using traditional orifice plates to measure steam flow to three of their paper machines. To help increase production, it upgraded the installation with a Rosemount Annubar mass flowmeter which had a lower pressure loss and provided fully-compensated mass flow measurement.
A wastewater treatment facility had problems measuring flow of thickened sludge during custody transfer to a waste disposal company. The high solids content of the sludge, and its coating of the pipe, confused other types of flow meters. The plant was paying too much to the disposal company because of the inaccurate measurement, and the maintenance costs of flushing the lines to improve accuracy were high. The plant installed a Rosemount 8750A magnetic flowmeter (see photo), which was not affected by the high solids content or the build up.
Brewers often use the multifaceted capabilities of a Coriolis meter. One company installed a Coriolis meter on a Lauter Tun and improved efficiency of the brewing process while reducing waste and eliminating time consuming sampling. The device measures the wort concentration to ±0.1° Plato/Balling, and provides a ±0.05% volume flow rate of the wort as it passes to the brew kettle.
The large turndown range of vortex flowmeters makes them suitable for applications with a wide flow. A paper mill wanted to add a new grade of paper that required high gloss. High gloss paper demands a high steam flow rate, while low gloss paper demands a low steam flow rate. Adding the high gloss grade increased the range of steam flow seen by the flowmeter, and the low end steam flow could not be accurately measured any longer by the conventional flow meter. To solve the problem, the mill installed a Rosemount 8800 Vortex Flowmeter to measure steam flow to the supercalender, which resulted in a more reliable and accurate signal at even the lowest flow rates.
At California Paperboard, signal spikes were costing $90,000 to $180,000 per year in lost production and wasted raw materials. The flowmeter for the pulp stock sent a feedforward signal to the dryend scanner on a Fourdrinier machine. When a spike occurred, it forced the control system to disable automatic control. Product inconsistency increased, and a half-hour of production time was lost with each episode. The problem was solved by installing a magmeter with digital signal processing, which filtered out the spikes.