Exercise 1. Fundamentals of the Vortex Flowmeter EXERCISE OBJECTIVE DISCUSSION OUTLINE. Introduction DISCUSSION. The vortex shedding phenomenon

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1 Vortex Flow Transmitter

2 Exercise 1 Fundamentals of the Vortex Flowmeter EXERCISE OBJECTIVE Learn the working principle of Vortex flowmeters, familiarize yourself with the Vortex flow transmitter of the instrumentation and process control training system, and learn how to commission and use it. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Introduction The vortex shedding phenomenon Reynolds number (Re). The von Kármán vortex street. Vortex detector Characteristics of Vortex flowmeters Advantages and limitations. Installation requirements. Description of the supplied vortex flowmeter Summary of technical specifications. Installing the vortex flowmeter Equipment required. Installation procedure. Calibration DISCUSSION Instrument symbol Vortex flow transmitter Introduction Flow measurement is essential in most industrial applications and the need for very accurate flow measurement devices is ever-present. One of the most versatile, yet very accurate, flowmeters used in modern plants is the Vortex flowmeter. The design of which takes advantage of the Vortex shedding effect, first described by the Hungarian-born engineer and physicist Theodore von Kármán around This type of flowmeter allows measurement of the volume flow rate by measuring the frequency of creation of vortices in the flow due to an obstacle. The sections below describe the elegant physical principles of vortex shedding and how these principles are used for flow measurement with the vortex flowmeter. The vortex shedding phenomenon This section presents the fundamental notions of classical fluid mechanics required to understand qualitatively the vortex shedding effect. However, this brief introduction is only intended to give you the basics required to understand how vortex flowmeters work and is in no way a complete description of the fluid dynamics involved. If you feel the need to push your study further, consult one of the documents listed in the bibliography. Figure 1. Theodore von Kármán ( ) - Image courtesy of NASA. Festo Didactic

3 Exercise 1 Fundamentals of the Vortex Flowmeter Discussion Reynolds number (Re) The Reynolds number is a dimensionless number that gives the ratio of the inertial forces over the viscous forces of a system. This number is omnipresent in fluid mechanics and is used to characterize flow regimes and systems. Its mathematical definition is: (1) Figure 2. Osborne Reynolds ( ). where is the Reynolds number (dimensionless) is the fluid density is the mean velocity of the fluid is the characteristic length (the width of the obstacle about which the fluid is flowing in the case of vortex formation) is the dynamic viscosity of the fluid (= or for water at ) The Reynolds number is of interest in this manual because the phenomenon of vortex shedding manifests itself in systems where. For more information regarding the Reynolds number, consult the Measurement manual. The von Kármán vortex street Theodore von Kármán discovered that an obstacle placed in a stream would cause the fluid to shed from its downstream faces in a peculiar manner. At low velocities, the flow separates symmetrically at the front of the obstacle and rejoins behind it where it forms two eddies rotating in opposite directions (see Figure 3a). As the velocity is increased above a threshold point (about for a cylindrical obstacle), the fluid alternatively sheds from one side of the obstacle and then the other, producing a pattern of propagating vortices on the downstream side (see Figure 3b). This regular pattern of vortices alternating in a constant fashion is akin to footsteps left in the snow by a pedestrian, hence the vortex street appellation. a) b) Figure 3. Streamlines around a cylindrical obstacle for different Re numbers. Von Kármán also observed that the distance ( ) between two consecutive vortices was constant with respect to the fluid velocity. Only the size of the obstacle has an effect on. This distance is also known as the wavelength of the vortex shedding. This means that the faster the fluid flows, the faster the vortices are formed but the wavelength remains the same. 2 Festo Didactic

4 Exercise 1 Fundamentals of the Vortex Flowmeter Discussion Von Kármán vortex streets are not only laboratory entities, far from it. They can be observed in many different situations such as when a rock causes vortices in a small river or when a flag flaps rapidly in the wind. Figure 4 showcases a meteorological manifestation of the von Kármán vortex street on a much larger scale. On this picture, clouds move with the wind (northward) as they meet a mountain (an obstacle) on the Alejandro Selkirk Island off the coast of Chile. This encounter generates a striking series of vortices emanating alternately from the eastern and western sides of the island. Figure 4. A von Kármán vortex street off the coast of Chile - Image courtesy of NASA. The Czech physicist Vincenc Strouhal experimented with vortex shedding in 1878 and a dimensionless number came to be associated with his name: The Strouhal number. This number characterizes mechanisms in which oscillating flows are present. Figure 5. Vincenc Strouhal ( ). Festo Didactic κ ߥ (2) 3

5 Exercise 1 Fundamentals of the Vortex Flowmeter Discussion where is the Strouhal number (dimensionless) is the frequency of vortex shedding (Hz) is the characteristic length is the mean velocity of the fluid An empirical equation was later devised to relate the Strouhal number to the Reynolds number: (3) or where the above equation is valid in the range. This last equation illustrates that, the fluid velocity, can be isolated and is found to be a function of,,, and. All these parameters except the frequency of vortex shedding are known when designing a device to produce a von Kármán vortex street. Simplifying Equation (3) by assuming the term to be negligeable compared to, we can rewrite to a good approximation the relationship between the velocity of the fluid and the frequency of shedding as: (4) where is a constant, usually determined experimentally Being able to measure the frequency of vortex shedding is the key enabling the calculation of the fluid velocity and of the flow (The flow is, where is the (known) cross-sectional area of the device). The next section explains how the vortices can be detected and their frequency of creation inferred. Vortex detector The vortices must be somehow detected by the apparatus to allow for the calculation of the frequency of vortex shedding. This can be done in a number of different ways and the exact mechanism of a specific vortex flowmeter is oftentimes a trade secret of the manufacturer. In the case of the provided vortex flowmeter, a precise mechanical sensor will allow the measurement of the frequency. A simple way to detect the vortices is to install a strain gauge (or a pressure gauge) downstream of the vortex-generating obstacle. As a vortex is shed on a 4 Festo Didactic

6 Exercise 1 Fundamentals of the Vortex Flowmeter Discussion side of the obstacle, the velocity of the fluid locally increases and the pressure decreases. On the other side, the fluid velocity is decreased and the pressure increases. The global result is that the gauge will detect the changes in pressure and its signal will oscillate at the sought-after frequency. Electronics will then be able to isolate the frequency and convert that information into a flow rate. Strain gauge Cantilevered strut Figure 6. A simple vortex detection mechanism. Other mechanisms exist such as using an ultrasonic detector to count the vortices as they cross a line transverse to the direction of motion. A piezoelectric crystal installed at the center of a diaphragm separating the lower and upper vortex regions can fulfill the same objective as the strain gauge mentioned previously. The possibilities are numerous. Characteristics of Vortex flowmeters Vortex flowmeters are a very reliable and accurate type of flowmeter which do not require much maintenance due to their simple design. The stability of the calibration is excellent as frequency measurement systems are typically free of measurement drift. Advantages and limitations Here is a brief list of some of the advantages of vortex flowmeters: Suitable for liquid, gas, or steam High accuracy (the error is typically about ±0.25% of the flow rate) and linear response Low maintenance, no moving parts Can be used with non-conductive fluids Relatively low installation cost Not subject to changes in density, pressure, or temperature. Able to withstand vibrations and thermal shocks (< 150 K/s) Festo Didactic

7 Exercise 1 Fundamentals of the Vortex Flowmeter Discussion Vortex flowmeters also have limitations and disadvantages that must be considered: Unidirectional measurement Large pressure drop Not suitable to measure the flow rate of liquids undergoing partial phase change or viscous liquids The fluids must be clean The device requires specific minimal lengths of straight pipe runs Not suitable when cavitation occurs Installation requirements This device requires a fully developed flow profile to correctly measure the flow rate of the fluid. For this reason, it is necessary for the liquids to flow upwards through the device to insure complete filling of the pipe. Measurement of gas flow rates can be performed with the flow moving horizontally. A minimum length of straight pipe runs must be respected. The lengths vary according to the nature of the upstream and downstream disturbances, the type of obstacle (sometimes called a bluff body) used to generate the vortices, and the required accuracy. Description of the supplied vortex flowmeter The vortex flow transmitter (Model 46923) is shown in Figure 7. It consists of a sensor/transmitter and a bracket for mounting the device on the process workstation. Table 1 describes the main components of the vortex flow transmitter. Figure 7. Vortex flow transmitter (Model 46923). 6 Festo Didactic

8 Exercise 1 Fundamentals of the Vortex Flowmeter Discussion Table 1. Components of the vortex flow transmitter. Component Inlet Flow orientation arrow Outlet Fault panel 24 V dc input / Analog output Digital display Description Fluid must enter the flowmeter from this port. Indicates the required direction of flow. Make sure to follow this instruction. Fluid must exit the flowmeter from this port. Contains one switch used to simulate a fault with the apparatus. Power input for the flowmeter. Used to energize the vortex flowmeter with a 24 V dc signal. The output provides a 4-20 ma signal proportional to the measured flow rate. A HART communication signal can be sent from the output. Displays the measured flow rate and other information related to the operation of the flowmeter. The protective cover can be removed to gain access to the control keys and manually control the device. The display can be used to configure several parameters and to read the flow rate directly. The function of each button is described below: Go backwards in a menu or modify a value Go forward in a menu or modify a value Select an item from a menu or store an entry ESC Press the + and buttons simultaneously to return to the previous menu or display Summary of technical specifications Some technical specifications are summarized in this section. For details, please refer to the documentation provided with the system. Device name Prowirl 72 Measured variables Volumetric flow rate Power supply 24 V dc Communication protocols HART or Fieldbus Pipe diameter 1.27 cm (0.5 in) Volumetric flow accuracy ±0.75% of full scale Operating temperature of the sensor -40 C to 70 C (-40 F to 158 F) Temperature of the process -40 C to 260 C (-40 F to 500 F) Festo Didactic

9 Exercise 1 Fundamentals of the Vortex Flowmeter Discussion Installing the vortex flowmeter The vortex flowmeter is heavy. Use proper techniques to lift it and request help whenever necessary. Always wear safety shoes when working with this equipment and make sure it is properly secured to the workstation. Equipment required The following equipment and tools are required to install the vortex flowmeter on the instrumentation workstation: Vortex flowmeter A spring nut and bolt for fixation to the support strut Ratchet with a ½ in socket or an equivalent wrench Installation procedure The vortex flowmeter is designed to be installed at the extremity of the workstation above the pump so that the input port of the flowmeter is aligned with the axis of valve HV2. The arrow cut on the plate of the mounting bracket must point in the direction of the flow. Install the flowmeter securely on the strut with a spring nut and a bolt. The recommended position is shown in Figure 8 for both the 3531 and the 3530 systems. Figure 8. Installing the vortex flowmeter on the workstation. 8 Festo Didactic

10 Exercise 1 Fundamentals of the Vortex Flowmeter Procedure Outline Calibration The vortex flowmeter is calibrated by the manufacturer at the factory. The device cannot be calibrated by the user. It must be returned to the manufacturer for calibration if it ever becomes required. PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Commissioning the vortex flow transmitter Enabling programming mode (access code). Quick setup commissioning. PROCEDURE Set up and connections 1. Connect the equipment according to the piping and instrumentation diagram (P&ID) shown in Figure 9 and use Figure 10 to position the equipment correctly on the frame of the training system. To set up your system for this exercise, start with the basic setup presented in the Familiarization with the Instrumentation and Process Control Training System manual and add the equipment listed in Table 2. Table 2. Material to add to the basic setup for this exercise. Name Model Identification Vortex flow transmitter FIT 1 Accessories Calibrator (4-20 ma) Figure 9. P&ID. Festo Didactic

11 Exercise 1 Fundamentals of the Vortex Flowmeter Procedure Figure 10. Setup. 2. Connect the control valve to the pneumatic unit. Details about the installation and operation of the control valve are available in the Familiarization with the Instrumentation and Process Control Training System manual. 3. Connect the pneumatic unit to a dry-air source with an output pressure of at least 700 kpa (100 psi). 4. Wire the emergency push-button so that you can cut power in case of emergency. The Familiarization with the Instrumentation and Process Control Training System manual covers the security issues related to the use of electricity with the system as well as the wiring of the emergency pushbutton. 5. Do not power up the instrumentation workstation yet. You should not turn the electrical panel on before your instructor has validated your setup that is not before step Be sure to install the transmitter as shown in Figure 10 and connect it to a 24 V dc power outlet on the electrical unit. Use one of the direct outputs to keep the transmitter from shutting off in case the emergency push-button or the OFF button (S2) is used. 7. Before proceeding further, complete the following checklist to make sure you have set up the system properly. The points on this checklist are crucial 10 Festo Didactic

12 Exercise 1 Fundamentals of the Vortex Flowmeter Procedure elements to the proper completion of this exercise. This checklist is not exhaustive, so be sure to follow the instructions in the Familiarization with the Instrumentation and Process Control Training System manual as well. f The hand valves are in the positions shown in the P&ID. The valve at the suction of the pump (HV1) is set so that the flow is directed toward the pump inlet. The control valve is fully open. The pneumatic connections are correct. 8. Ask your instructor to check and approve your setup. 9. Make sure it is safe to energize the system for you and for the team working on the other side of the system, if any. When ready, turn on the main power by placing the safety switch in the ON position. Do not press the S1 button yet. The transmitter initializes and the remote display turns on after a few seconds. Commissioning the vortex flow transmitter a The following procedure assumes you are using the display to program the transmitter. If you want to configure your transmitter from a computer, you can do so by accessing the same menus and inputting the same values in the appropriate fields of the Fieldcare software. Please refer to either the HART Device Configuration manual (P/N 86050) or the Foundation Fieldbus Device Configuration manual (P/N 86002) for more information on how to connect your transmitter to a computer. Enabling programming mode (access code) In many plant applications, it is crucial to the process that the settings of transmitters remain unchanged unless authorized. To avoid inadvertent changes to their settings, most devices give the possibility to protect their settings using a numerical code. The vortex flow transmitter has such a protection system and requires that the user enter a numerical code to enable the programming mode. The programming mode is disabled automatically at boot or if no key has been pressed within 60 seconds. If the programming mode is disabled and you try to change a protected setting, the transmitter prompts you to enter the ACCESS CODE. ACCESS CODE *** Festo Didactic

13 Exercise 1 Fundamentals of the Vortex Flowmeter Procedure 10. The default access code is 72. Use the + and buttons to select the digits, and the E button to confirm. This allows you to enter the code to enable the programming mode. You must input the symbol to conclude this step. ACCESS CODE 72 * 11. Once the correct access code is entered, the transmitter displays a message that indicates that the programming mode is enabled. PROGRAMMING ENABLED Quick setup commissioning 12. The easiest way to configure the Vortex flow transmitter is via the QUICK SETUP option. Press the E button to access the GROUP SELECT menu and use the + and buttons to select the QUICK SETUP function group. Press the E button once the QUICK SETUP option is highlighted. 13. Press the + button twice to change the value of the QS-COMMISSION function to YES and press the E button two times to confirm the change and proceed with the quick setup commissioning. a If the programming mode is not already enabled, the transmitter will ask for the access code. Proceed as described above and then change the value of the quick setup commissioning function to yes. 14. The quick setup commissioning consists of a series of parameters which must be set. Use the + and buttons to navigate through the available settings of each parameter as indicated in Table 3 and press the E button to confirm each selection. Each time a parameter is set, the transmitter confirms by displaying: ENTRY STORED. 12 Festo Didactic

14 Exercise 1 Fundamentals of the Vortex Flowmeter Procedure Function Default Parameter Description Table 3. Quick setup commissioning. LANGUAGE ENGLISH Sets the language used to configure the transmitter. APPLICATION MEAS. UNIT TYPE UNIT VOLUME FLOW UNIT TOTALIZER SELECT OUTPUT CURRENT SPAN VALUE 20mA TIME CONSTANT FAILSAFE MODE LIQUID VOLUME FLOW l/m (US gal/m) l (US gal) CURRENT OUTPUT 4-20mA l/min (+17 US gal/min) 5.00 s MAX. CURRENT Specifies whether the flowmeter is used with liquid or gas/steam. You can choose between: Volume flow, Calculated mass flow, or Corrected volume flow. Allows setting the units for the volume flow. Many different units are available. The transmitter calculates the total volume of fluid that passed through the flowmeter since the last reset of the totalizer. Allows configuring the output. Allows setting the current span of the transmitter. HART devices require that you choose between US or Namur versions of the 4-20 ma signal. Sets the value of the measured variable that corresponds to 100% of the span. Determines how fast the current output reacts to a change in the measured variable (i.e., damping). Determines the current output in case of a fault. Min. current: 3.75 ma (US) or 3.6 ma (NAMUR) Max. current: 22.6 ma Hold value: The current sent before the fault remains the output until the fault is cleared Actual value: The output current corresponds to the value measured despite the fault. OTHER OUTPUT? NO Allows configuring additional outputs if available. AUTOCONF. DISPL? YES Automatically configure the transmitter so that it displays the flow on the first line and the totalizer on the second line of the display. You can change the units of the totalizer or reset it by changing the parameter in the TOTALIZER function group, available from the GROUP SELECT menu. 15. Once the quick setup commissioning is completed the transmitter displays SETUP COMPLETE. Press ESC twice to return to the main display. 16. Press the S1 button to power up the drive. Set the drive to about 52.5 Hz. You should obtain a flow rate of at least 40 L/min (10 gal/min). Adjust the frequency of the drive if required. Next, use the ball valve HV2 to adjust the flow rate so that the rotameter reading is 4 L/min (1 gal/min). Festo Didactic

15 Exercise 1 Fundamentals of the Vortex Flowmeter Procedure 17. Record in Table 4 the measurements of the vortex flow transmitter. To display the other measured variables such as the vortex shedding frequency or the velocity, press the E button to access the GROUP SELECT menu. From this menu select MEASURING VALUES and the E button. In the MEASURING VALUES function, you can switch between the various measured variables using the E button. Table 4. Vortex flowmeter and rotameter flow rate readings. Flow rate Flow rate (Rotameter) (Vortex TX) L/min (gal/min) L/min (gal/min) 4 (1) 8 (2) 12 (3) 16 (4) 20 (5) 24 (6) 28 (7) 32 (8) 36 (9) 40 (10) Vortex frequency (Vortex TX) Hz 18. Use the ball valve HV2 to increase the flow rate (as read on the rotameter) by steps of 4 L/min (or 1 gal/min) until you reach 40 L/min (10 gal/min). For each flow rate, record in Table 4 the flow rate and the vortex frequency measured by the vortex flow transmitter. Stop the drive once the table is filled. 19. Compare the flow rate measurements of the vortex flow transmitter to the flow rate readings on the rotameter. How good is the agreement between the two? Which one would you say is the most precise? Is the relationship between the flow rate measured by the vortex flowmeter and the vortex shedding frequency a linear one? Using a spreadsheet software, plot a graph of the vortex shedding frequency as a function of the flow rate (as measured by the vortex flowmeter) with the data recorded in Table Turn off the electrical unit on the instrumentation station and store the equipment appropriately. Make sure the station is clean and in a proper state for its next use. 14 Festo Didactic

16 Exercise 1 Fundamentals of the Vortex Flowmeter Conclusion CONCLUSION In this exercise you have learned how to commission a vortex flow transmitter for use on the Instrumentation and Process Control system. You learned the basic principles of vortex shedding and how this is used to infer the flow rate. REVIEW QUESTIONS 1. What is the name of the aerospace scientist and engineer who is associated with the vortex street phenomenon? 2. To which physical parameter are the flow rate and velocity proportional to in a vortex flowmeter? 3. How is the distance () between two successive vortices in a vortex street going to change if the velocity of the fluid increases, all other parameters being constant? 4. An atmospheric physicist wants to study the vortex street phenomenon illustrated in Figure 4 with the help of a small-scale model. He would like the frequency of generation of the vortices ( to be the same in both the realworld phenomenon and in his small-scale model. Assuming the width of the mountain on the island is about 200 m (600 ft) and the mean air velocity to be 10 m/s (30 ft/s), what would be the required fluid velocity of the small scale model if the width of its obstacle is 10 cm (0.3 ft). Hint: the Strouhal number must be the same for both systems in order for the systems to be comparable. ( ). Festo Didactic

17 Exercise 1 Fundamentals of the Vortex Flowmeter Review Questions 5. Name two advantages and two limitations of the vortex flowmeter. 16 Festo Didactic