Pitot Tubes and Industrial DP Transmitters (Optional Exercise)

Transcription

1 Exercise 3-4 Pitot Tubes and Industrial DP Transmitters (Optional Exercise) EXERCISE OBJECTIVE In this exercise, you will study how pitot tubes operate. You will measure flow using a pitot tube and a differential-pressure transmitter. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Introduction Industrial applications Measuring principle Advantages and limitations Description of the supplied pitot tube assembly DISCUSSION Introduction A pitot tube (Figure 3-23) is a differential pressure type flow rate measuring device. This primary element consists of a tube pointing toward the fluid flow to obtain the total pressure (PT). A second tube is usually positioned to measure the static pressure (PS). The dynamic pressure (q) is obtained using Bernoulli s transfer equation (neglecting the hydrostatic component): (3-21) where is the dynamic pressure is the total pressure is the static pressure flow Figure 3-23 Side view of a common pitot tube design (pitot static). Festo Didactic

2 Ex. 3-4 Pitot Tubes and Industrial DP Transmitters (Optional Exercise) Discussion The pitot tube is used with a secondary device such as a differential-pressure transmitter. The DP transmitter converts the pressure differential measured at the two pressure taps into a signal that can be interpreted by a PID controller or a PLC. a Installation and commissioning of the industrial DP transmitter is detailed in Appendix I. Industrial applications Pitot tubes are primarily used with gas flows (e.g., in airplanes); but they are also suitable for clean liquids. Solid or highly viscous material can easily block the small openings of the pitot tubes. Different pitot tube designs exist for diverse applications. Some examples are shown in Figure When only the total pressure is needed, a basic pitot with one opening may be appropriate. An alternate pitot-static design can be a combined tube with an impact hole facing upstream and static holes on the side. The reversed pitot (or pitometer) is a variant of the pitot-static design where the high-pressure (or impact) opening faces upstream and the low-pressure opening faces downstream. Figure Different pitot designs. Most pitot tubes develop relatively small differential pressure. The reversed pitot design enables greater differential pressure (of about 40%). Therefore, it is more reliable with lower flow velocities. One way of improving the reliability of the pitot device is to have a series of openings (multiport) across the pipe. This method, used with the supplied pitot assembly (see Figure 3-25), provides an average pressure that compensates for non-flat velocity profiles. 148 Festo Didactic

3 Ex. 3-4 Pitot Tubes and Industrial DP Transmitters (Optional Exercise) Discussion Measuring principle The method used for measuring flow rate with a pitot tube is similar to the one used for orifice plates and other primary elements as it uses differential pressure and is based on Bernoulli s principle. Refer to Figure 3-25 while reading the following description to understand how the supplied pitot assembly is used for measuring the flow rate: flow High pressure tap (H) Low pressure tap (L) Figure Pitot measuring principle (side view). 1. The total pressure ( ) is measured through the openings facing upstream. 2. The static pressure ( ) is measured via the openings facing downstream. The volumetric flow rate is related to the pressure differential measured between the two pressure taps. If we assume that the measured liquid is incompressible and that the flow is turbulent, the volumetric flow can be determined with the following formula: (3-22) where is the volumetric flow rate is the area of the pipe is the pressure differential between the high and low pressure taps is the fluid density This relationship shows that the flow rate is proportional to the square root of the measured pressure differential or (3-23) If the mass flow rate must be determined, the following formula is used: (3-24) Festo Didactic

4 Ex. 3-4 Pitot Tubes and Industrial DP Transmitters (Optional Exercise) Discussion Advantages and limitations Pitot tubes are commonly used in industry. The main advantages are listed below: Low cost Easy installation However, some limitations are to be considered: Low accuracy because of the low differential pressure developed Slurries and dirty liquids can block the pitot tube openings Description of the supplied pitot tube assembly The pitot tube assembly (Model 6554) designed for the training system is shown in Figure Figure Pitot tube assembly, Model Inlet port 2. High (H) pressure tap 3. Low (L) pressure tap 4. Outlet port The main technical specifications for the pitot device are summarized below: Mounting position Direction Pipe fill Homogeneity Min. Reynolds number Pipe inside diameter (D) Horizontal, vertical, or angular flow must be in the direction indicated by the arrow pipe must be full at all times no change of state may occur 5000, equivalent to a flow of 3.8 L/min (1.0 gal/min) with the following pipe inside diameter 15.8 mm (0.622 in) 150 Festo Didactic

5 Ex. 3-4 Pitot Tubes and Industrial DP Transmitters (Optional Exercise) Discussion The model-specific reference curve that gives the measured pressure differential that corresponds to a flow rate is given below for SI units (Figure 3-27) and US customary units (Figure 3-28). You will need this curve when configuring the differential-pressure transmitter P(kPa) Q (L/min) Figure Pitot tube pressure curve (SI units) P(psi) Q (gal/min) Figure Pitot tube pressure curve (US customary units). Festo Didactic

6 Ex. 3-4 Pitot Tubes and Industrial DP Transmitters (Optional Exercise) Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Measuring the pressure drop-versus-flow curve Measuring flow rates using a pitot tube End of the exercise PROCEDURE Set up and connections 1. Set up the system shown in Figure The industrial DP transmitter must be connected to the high- and low- pressure taps of the pitot tube. Figure Measuring flow rate with a pitot tube. 2. Make sure the reservoir of the pumping unit is filled with about 12 L (3.2 gal) of water. Make sure the baffle plate is properly installed at the bottom of the reservoir. 3. On the pumping unit, adjust valves HV1 to HV3 as follows: Open HV1 completely. Close HV2 completely. Set HV3 for directing the full reservoir flow to the pump inlet. DP transmitter commissioning Refer to Appendix I for instructions specific to the industrial DP transmitter. 4. Using Figure 3-27 or Figure 3-28, what should be the differential pressure for the (maximum) flow rate of 12 L/min (3 gal/min)? 152 Festo Didactic

7 Ex. 3-4 Pitot Tubes and Industrial DP Transmitters (Optional Exercise) Procedure 5. Configure the DP transmitter for differential pressure measurements in the desired unit. Use the pressure value of step 4 for the SET URV parameter corresponding to a 20 ma output signal. 6. Turn on the pumping unit. Start the pump at a moderate speed. 7. Bleed air from the impulse lines. 8. Stop the pump. 9. Adjust the zero of the differential pressure transmitters. Measuring the pressure drop-versus-flow curve 10. Adjust the pump speed until you read a flow rate of 4 L/min (1 gal/min) on the rotameter. This minimum flow rate ensures a turbulent flow inside the pitot tube. 11. Record your results in Table 3-6. Increase water flow by 1 L/min (0.25 gal/min) until you obtain 12 L/min (3 gal/min) on the rotameter. Table 3-6. Comparison between readings. Rotameter flow L/min (gal/min) 4 (1.00) 5 (1.25) 6 (1.50) 7 (1.75) 8 (2.00) 9 (2.25) 10 (2.50) 11 (2.75) 12 (3.00) DP transmitter pressure kpa (psi) P 1/2 kpa 1/2 (psi 1/2 ) 12. Stop the pump. Festo Didactic

8 Ex. 3-4 Pitot Tubes and Industrial DP Transmitters (Optional Exercise) Procedure 13. From the data recorded in Table 3-6, plot the relationship between flow rate and DP transmitter readings. 14. Do these results follow the reference curve of Figure 3-27 or Figure 3-28? Explain. 15. Calculate the square root of the pressure drop for each flow rate and fill the last column of Table Plot a graph of the square root of the pressure loss as a function of the flow rate using the data in Table 3-6. Can you verify that? Yes No Measuring flow rates using a pitot tube 17. Configure the DP transmitter for flow rate measurement. Refer to Appendix I for details. Use data from Figure 3-27 and Figure 3-28 and consider the maximum flow rate to be 12 L/min (3 gal/min). 18. Adjust the pump speed until you read a flow rate of 4 L/min (1 gal/min) on the rotameter. 19. Record your results in Table 3-7. Increase water flow by 1 L/min (0.25 gal/min) steps until you obtain 12 L/min (3 gal/min) on the rotameter. Table 3-7. Flow readings. Rotameter flow L/min (gal/min) 4 (1.00) 5 (1.25) 6 (1.50) 7 (1.75) 8 (2.00) 9 (2.25) 10 (2.50) 11 (2.75) 12 (3.00) DP transmitter flow L/min (gal/min) Q L/min (gal/min) 20. Stop the pump. 154 Festo Didactic

9 Ex. 3-4 Pitot Tubes and Industrial DP Transmitters (Optional Exercise) Conclusion 21. Calculate the difference between the rotameter and DP transmitter for each flow rate and fill the last column of Table Is the flow rate difference relatively constant? End of the exercise 23. Turn off the pumping unit. 24. Disconnect the circuit. Return the components and hoses to their storage location. 25. Wipe off any water from the floor and the training system. CONCLUSION In this exercise, you became acquainted with the use and commissioning of a pitot tube with an industrial DP transmitter. You also verified that flow across a pitot tube is proportional to the square root of the pressure differential between the two pressure taps. REVIEW QUESTIONS 1. Why can t pitot tubes be used to measure the flow of highly viscous material? 2. What measurement can be accomplished with the tube pointing toward the flow? 3. Which pitot design is more reliable with lower flow velocities? Festo Didactic

10 Ex. 3-4 Pitot Tubes and Industrial DP Transmitters (Optional Exercise) Review Questions 4. Why do some pitot tubes have a series of openings across the pipe? 5. What do you need for configuring the DP transmitter for flow measurement using a pitot tube? 156 Festo Didactic