There are two prominent types of mass flowmeters currently on the market – thermal mass and Coriolis mass. We’ll consider the capabilities of each of these flowmeter types here.
A Coriolis mass flowmeter is a device that measures mass flow rate of a fluid traveling through a tube. The mass flow rate is the mass of the fluid traveling past a fixed point per unit time.[1] Thermal flowmeters measure the mass of gas molecules causing heat transfer to derive a flow measurement.
If density is constant, then the relationship is simple. If the fluid has varying density, then the relationship is not simple. The density of the fluid may change with temperature, pressure, or composition, for example. The fluid may also be a combination of phases such as a fluid with entrained bubbles.[1]
A key advantage of mass flow devices is that they alleviate some of the potential complications associated with correcting for temperature or pressure variation. In gas flow measurement, for example, common units of measure are volumetric flow units that are corrected to base conditions or standard temperature and pressure (STP). This can be accomplished with many technologies using a multivariable transmitter or done through the control system or a flow computer. Mass flow devices output in volumetric or mass units at STP or base conditions as a standard offering without any additional pressure and temperature correction necessary on the end-user’s part. An example of a common unit of measure that is corrected back to base conditions in the natural gas industry is standard cubic feet per hour (SCFH). In the U.S. oil and gas industry, it is common to have base temperature and pressure for natural gas of 60 F and 1 atm. Having a set of base conditions like this allows consistent reporting of flows or volumes that aren’t going to vary by operating conditions.
How Thermal Flowmeters Work
Thermal flowmeters introduce heat into the flowstream and measure the rate of heat dissipation. The heat dissipation is measured by one or more temperature sensors. This method works best with gas flow because the much greater heat absorption capacity of liquids rapidly saturates the signal, leading to a loss of measurement resolution.
While all thermal flowmeters inject heat into the flowstream, there are two different methods used to measure the rate of heat dissipation. One method is called constant temperature differential. Thermal flowmeters using this method utilize a heated RTD as a velocity sensor and another sensor that measures the temperature of the gas. The flowmeter attempts to maintain a constant difference in temperature between the two sensors. Mass flowrate is computed based on the amount of electrical power added to heat the velocity sensor to maintain this constant difference in temperature.
The second method is called constant current. It also uses a heated RTD as a velocity sensor and another temperature sensor to measure the temperature of the flowstream. With this method, the power to the heated sensor is kept constant. Mass flow is computed based on the difference in temperature between the heated velocity sensor and the temperature of the flowstream. Both methods make use of the principle that higher velocity flows produce a greater cooling effect. And both methods measure mass flow based on measuring the amount of cooling that occurs in the flowstream.[2]
How Coriolis Flowmeters Work
Coriolis mass flowmeters use a curved or straight tube as a sensor and apply Newton’s second law of motion to determine the flow rate. An electromagnetic drive coil is located in the center of the tube bend (if the meter is a bent-tube design), and it causes the tubes to vibrate like a tuning fork. There is a pickoff coil located on the inlet and outlet side of the sensor. At no-flow conditions, these pick off coils create voltages that are in phase with each other. When flow is introduced into the sensor, the voltage created by the pickoff coils will be out of phase, and that amount of phase shift is measured as a time difference. This time difference is directly proportional to the mass flow in the sensor.[3]
Applications of Thermal Mass Measurement
Thermal flowmeters are used for a wide variety of gas measurement applications. Notably, there are two periods in the history of thermal meters when environmental applications have boosted sales. The first period was in the early 1990s, when the need for continuous emissions monitoring (CEMS) required measurement of sulfur dioxide (SO2) and nitrous oxides (NOx). Thermal flowmeters were ideal for this purpose. By combining a measurement of the concentration of SO2 and NOx with a measurement of flowrate, it can be determined how much of these gases are released into the atmosphere. This is important since they have been identified as the primary causes of acid rain. At that time, thermal flowmeters competed with averaging Pitot tubes and ultrasonic flowmeters for this measurement.
The broad acceptance of global warming as scientific fact rather than as mere theory has presented a second opportunity for thermal flowmeters. Beginning with the election of the Obama administration in 2008, the U.S. government has made the identification and reduction of greenhouse gases a major priority. For example, the Obama administration has the stated goal of reducing greenhouse gas emissions by 80 percent by 2050.
Other countries have also joined in the effort to reduce greenhouse gases. The Kyoto Accord has resulted in the creation of several mechanisms for measurement of greenhouses gases internationally. These include Certified Emission Reductions (CER), a credit system designed to help European countries achieve reduced emission targets. A second program is the Clean Development Mechanism, which allows countries to help reduce emissions in developing countries by investing in sustainable development programs that have that result.
Some of the applications that have opened up as a result of efforts to reduce greenhouse gases include the following:
Biogas
Ethanol distillation and refining
Recovery of methane from coal mines
Measuring emissions from boilers, process heaters, and steam generators
Measurement of recovery of landfill gases
Measurement and monitoring of flue gases and flare gases
Applications of Coriolis Mass Measurement
Coriolis flowmeters can measure both liquids and gases. However, Coriolis flowmeters have an easier time with liquids. This is because liquids are denser than gases, and Coriolis meters rely on momentum of the fluid to deflect the flowtube. Gases are lighter than liquids, and have a more difficult time deflecting the flowtube. While suppliers have had good success with straight-tube meters for liquids, applying them to gas flow measurement has been more challenging.
One major application area for Coriolis flowmeters is downstream applications for petroleum liquids. Here they are displacing positive-displacement meters in some applications. Unlike positive-displacement meters, Coriolis meters do not have moving parts, apart from the tube vibration. While bent-tube meters can introduce some pressure drop, straight-tube meters have virtually no pressure drop. Straight-tube meters also do well in sanitary applications for the food & beverage and pharmaceutical industries. Fluid does not build up in straight-tube meters as it sometimes does on the curved surfaces of bent-tube meters.[4]
Final Thoughts
Coriolis flowmeters are the most accurate meter made. As a result, they are used for custody transfer of both petroleum liquids and gases. Thermal flowmeters, by contrast, are less accurate than Coriolis, which is why they are not employed for custody-transfer applications. That said, thermal flowmeters cost far less than Coriolis meters, which makes them an excellent fit for noncustody-transfer gas measurements where top-rated accuracies aren’t required.
The thermal flowmeter category has been responsible for some compelling technology innovations, a trend beginning with their introduction to the marketplace in the 1970s and one that continues to this day. The thermal flowmeter market is currently being boosted by the drive to measure greenhouse gas emissions, as well as other energy management applications. As a result, suppliers are responding with more feature-rich products that have enhanced performance levels. Notably, modern thermal flowmeters have improved low-flow sensitivity and enhanced diagnostics for field calibration verification without the need for external hardware.
This content is sponsored by Magnetrol International. Sponsored content is authorized by the client and does not necessarily reflect the views of the Process Flow Network.
References:
“Mass flow meter,” Wikipedia, wikipedia.org/wiki/Mass_flow_meter.
“The history and evolution of thermal flowmeters,” Flow Control magazine, http://www.flowcontrolnetwork.com/the-history-evolution-of-thermal-flowmeters/.
“Q&A: Advances in Coriolis flow meter technology,” Flow Control magazine, http://www.flowcontrolnetwork.com/qa-advances-coriolis-flow-meter-technology/.
“Coriolis flow measurement – Past, present & future,” Flow Control magazine, http://www.flowcontrolnetwork.com/coriolis-flow-measurement-past-present-future/.
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