The Discussion of this exercise covers the following points: Range with an elevated or suppressed zero Suppressed-zero range Elevated-zero range

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1 Exercise 4-3 Zero Suppression and Zero Elevation EXERCISE OBJECTIVE In this exercise, you will learn the effect that mounting a pressure transmitter above or below the reference level has on the hydrostatic pressure sensed by this element. You will also learn how zero suppression and zero elevation can be achieved. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Range with an elevated or suppressed zero Suppressed-zero range Elevated-zero range DISCUSSION Range with an elevated or suppressed zero As mentioned in Ex. 4-2, hydrostatic pressure measurement allows the level of liquid in a vessel to be inferred. In many applications, the reference level is at the bottom of the vessel. However, it may be impossible to position the primary sensing element of the pressure transmitter at exactly the same height as the reference level. For example, if the vessel is an elevated water storage tank, it may be better for maintenance reasons to position the transmitter below the tank. If, instead of an elevated tank, it is an underground tank, the transmitter must be positioned above the bottom of the tank. In either case, the transmitter is not at the same level as the bottom of the vessel. This difference between the reference level and the position of the transmitter must be compensated for by setting a suppressed-zero range or an elevated-zero range (also known as zero elevation and zero suppression respectively). The suppressed-zero range and the elevated-zero range are frequently confused or misunderstood. Many alternative terms are used to describe these two ranges. These terms only tend to further confuse the distinction between the two types of ranges and you should avoid using them. Suppressed-zero range By definition, a suppressed-zero range is a range where the value of the measured variable zero is less than the lower-range value. That is, the zero does not appear on the scale. For example, if the differential-pressure transmitter reads 30% when the measured variable (i.e., the level) is zero, the range is a suppressed-zero range. A suppressed-zero range is required if the pressure transmitter used to measure a gauge pressure is installed under the highpressure tap of the vessel as Figure 4-12 shows. Festo Didactic

Ex. 4-3 Zero Suppression and Zero Elevation Discussion High-pressure tap Minimum measurable level Open to atmosphere Figure 4-12. Set up where a transmitter is installed below the high-pressure tap.

2 Ex. 4-3 Zero Suppression and Zero Elevation Discussion High-pressure tap Minimum measurable level Open to atmosphere Figure Set up where a transmitter is installed below the high-pressure tap. Be careful. In many installations, the fluid in the impulse line is not the process fluid, thus its density may differ from the density of the process fluid. If a pressure transmitter is installed under the high-pressure tap of the vessel, the fluid in the impulse line exerts a pressure on the transmitter sensor when the vessel is empty. Thus, even if the vessel is empty, the transmitter still reads a positive pressure. This pressure is constant and always present since the impulse line always stays filled. To compensate for this extra pressure, the transmitter must be set so that the extra pressure is suppressed. The pressure due to the liquid in the impulse line is proportional to the distance h between the transmitter and the vessel high-pressure tap as Figure 4-12 shows. This pressure also depends on the density of the fluid in the impulse line. Mathematically, the pressure on the primary element of the differential-pressure transmitter when the vessel is empty is: (4-2) where is the pressure that exerts the fluid in the impulse line on the transmitter sensor is the density of the fluid in the impulse line is the acceleration due to gravity is the distance between the transmitter and the high-pressure tap of the vessel 182 Festo Didactic

3 Ex. 4-3 Zero Suppression and Zero Elevation Discussion When the vessel is full, the pressure that the column of fluid exerts at the highpressure tap of the vessel depends on the distance H100% between the tap and the surface of the fluid. This pressure determines the span of the level measurement. The level is at 0% of the span when it is at the high-pressure tap of the vessel and it is at 100% span when it is at a distance H100% of the highpressure tap. The static pressure that the fluid exerts at the high-pressure tap of the vessel when the level is at 100% of the span is: (4-3) where is the static that the fluid exerts at the high-pressure tap when the level is at 100% of the span is the density of the fluid in vessel is the acceleration due to gravity is the distance between the high-pressure tap of the vessel and the surface of the fluid when the level is at 100% of the span To infer the level in a vessel from a pressure measurement, the differentialpressure transmitter must be connected as Figure 4-12 shows. With such a connection, the pressure at the low-pressure port of the transmitter is the atmospheric pressure : (4-4) The pressure at the high-pressure port of the transmitter is the sum of the static pressure due to the height of fluid above the high-pressure tap of the vessel, the pressure due to the fluid in the impulse line, and the atmospheric pressure. Equation (4-5) gives the pressure at the high-pressure port ( is the measured level). Other names for suppressed-zero range: zero suppression, elevation, elevated range, elevated span. (4-5) Therefore, the differential pressure read by the differential-pressure transmitter is: (4-6) It is clear from Equation (4-6) that the pressure due to the fluid in the impulse line must be suppressed to obtain the level of liquid from the differential pressure read by the transmitter. On modern equipment, this extra pressure is usually suppressed by setting the zero of the high-pressure port of the transmitter when the transmitter is in position and the impulse line is full of fluid. Elevated-zero range By definition, an elevated-zero range is a range where the value of the measured variable zero is greater than the lower-range value. An elevated-zero range is required if the pressure transmitter used to measure a gauge pressure is installed above the high-pressure tap of the vessel. In this case, the transmitter may read, for example, -30% when the level is zero. An elevated-zero range is also required if a differential-pressure transmitter is used to measure the level in Festo Didactic

4 Ex. 4-3 Zero Suppression and Zero Elevation Discussion a pressurized tank. Figure 4-13 shows a case where an elevated-zero range is required. Atmospheric pressure Open to atmosphere Minimum measurable level High-pressure tap Figure Set up where an elevated-zero range is required. Other names for elevatedzero range: zero elevation, suppression, suppressed range, suppressed span. When an elevated-zero range is required, the pressure transmitter reads a negative pressure when the vessel is empty. Depending on the set up, this negative pressure is caused by the liquid in the impulse line or by a pressure on the low-pressure port of the transmitter that is higher than the pressure on the high-pressure port of the transmitter ( ). To compensate for this negative pressure, the transmitter must be set so that the differential-pressure transmitter reads a pressure differential of zero if the vessel is empty. In other words, the zero must be elevated. For an installation where an elevated-zero range is required, such as with the set up shown in Figure 4-13, the pressure at the low-pressure port of the transmitter is proportional to the height. The negative pressure due to the zero elevation is expressed as: (4-7) The static pressure exerted by the fluid at the high-pressure tap of the vessel when the level is at 100% of the span is (4-8) Thus, the differential pressure read by the differential-pressure transmitter is: (4-9) To compensate for the negative pressure that the fluid in the impulse line exerts, the zero of the differential-pressure transmitter must be set so that the pressure differential is zero when the vessel is empty. 184 Festo Didactic

5 Ex. 4-3 Zero Suppression and Zero Elevation Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Purging the air from the bottom hose (column pressurization) Transmitter calibration Effect of lowering the transmitter on the measurable level range Zero suppression End of the exercise PROCEDURE Set up and connections This exercise can also be accomplished using the optional industrial differentialpressure transmitter (Model 46929). Should you choose this piece of equipment, refer to Appendix I for instructions on how to install and use the transmitter for level measurements. 1. Set up the system shown in Figure a Mount the DP transmitter so that the pressure ports are one row of perforations above the bottom of the column, and at least four rows of perforations above the bottom of the expanding work surface. Connect the rotameter outlet to the port of the column on which a pipe extends down into the column. Make sure the top cap of the column is tightened firmly. The column is first operated in the pressurized mode in order to purge air from the hose connecting the bottom of the column to the pumping unit. Failure to purge air from this hose can prevent the water in the column from decreasing below a certain level when the pump speed is decreased or the pump is stopped. Plug Plug Figure Zero-suppression application. Festo Didactic

6 Ex. 4-3 Zero Suppression and Zero Elevation Procedure 2. Make sure the reservoir of the pumping unit is filled with about 12 liters (3.2 gallons) of water. Make sure the baffle plate is properly installed at the bottom of the reservoir. 3. On the pumping unit, adjust pump valves HV1 to HV3 as follows: Open HV1 completely. Close HV2 completely. Set HV3 for directing the full reservoir flow to the pump inlet. 4. Turn on the pumping unit. Purging the air from the bottom hose (column pressurization) 5. Make the pump rotate at maximum speed. This causes the water level to rise in the column. 6. Close valve HV1 completely. This causes the water level to rise further in the column. 7. Bleed the high-pressure port of the DP transmitter. 8. Stop the pump. This causes part of the water in the column to siphon back out. 9. Remove the plug connected to the hose port at the top of the column. Connect this port to either of the auxiliary return ports of the pumping unit using an extra-long hose. This hose serves as an overflow if the column gets full. It also allows the column to be open to atmosphere through the reservoir of the pumping unit. 10. Set the pump speed to 50% to make the water level rise about halfway up the column. Then, open valve HV1 completely. The level should remain relatively stable. Transmitter calibration In steps 11 through 17, you will adjust the ZERO and SPAN knobs of the DP transmitter so that its output current varies between 4 ma and 20 ma when the level of the water in the column is varied between 5 cm and 55 cm (2 in and 22 in). 11. Connect a multimeter to the 4-20 ma output of the DP transmitter. 186 Festo Didactic

7 Ex. 4-3 Zero Suppression and Zero Elevation Procedure 12. Make the following settings on the DP transmitter: ZERO adjustment knob: MAX. SPAN adjustment knob: MAX. LOW PASS FILTER switch: I (ON) 13. Adjust the pump speed until the water level is stable at 5 cm (2 in) in the column. This is the reference level. a To prevent air from entering the hose that connects the bottom of the column to the pumping unit, do not allow the water level to fall below 4 cm (1.5 in) in the column throughout the exercise. 14. While observing the multimeter reading, turn the ZERO adjustment knob of the DP transmitter counterclockwise to decrease the current and stop turning it as soon as the multimeter reads 4.00 ma. 15. Readjust the pump speed to raise and stabilize the water level to 55 cm (22 in) in the column. 16. Adjust the SPAN knob of the DP transmitter until the multimeter reads 20.0 ma. 17. Due to interaction between the ZERO and SPAN adjustments, repeat steps 13 through 16 until the DP transmitter output actually varies between 4.00 ma and 20.0 ma when the level of the water is varied between 5 cm and 55 cm (2 in and 22 in). Effect of lowering the transmitter on the measurable level range 18. Now that the DP transmitter is calibrated, lower its mounting so that the pressure ports are approximately 3 rows of perforations below the bottom of the column. While doing this, be careful not to modify the setting of the ZERO and SPAN adjustment knobs. 19. By varying the pump speed, raise the water level in the column from 5 cm to 55 cm by steps of 5 cm (or from 2 in to 22 in by steps of 2 in). After each new level setting, measure the analog output generated by the DP transmitter and record it in Table 4-3. Festo Didactic

8 Ex. 4-3 Zero Suppression and Zero Elevation Procedure Table 4-3. DP transmitter output as a function of the level without zero suppression. Level cm (in) 5 (2) 10 (4) 15 (6) 20 (8) 25 (10) 30 (12) 35 (14) 40 (16) 45 (18) 50 (20) 55 (22) Analog output without zero suppression ma Analog output with zero suppression ma 20. Using Table 4-3, plot the relationship between the analog output and the level without zero suppression. 21. From the curve obtained, can the measurable level range for which the DP transmitter was initially calibrated [5 to 55 cm (2 to 22 in)] still be obtained? Explain. Zero suppression 22. Recalibrate the DP transmitter so as to take account of the depression of its sensing element with respect to the reference level of 5 cm (2 in). To do so, redo procedure steps 11 through Vary the pump speed to raise the water level from 5 cm to 55 cm by steps of 5 cm (or from 2 in to 22 in by steps of 2 in). After each new level setting, measure the analog output generated by the DP transmitter and record it in Table Using Table 4-3, plot the relationship between the analog output and the level with zero suppression on the graph of step Stop the pump and turn off the pumping unit. 188 Festo Didactic

9 Ex. 4-3 Zero Suppression and Zero Elevation Conclusion 26. Has zero suppression reestablished the measurable level range for which the DP transmitter was initially calibrated [5 to 55 cm (2 to 22 in)]? Explain. End of the exercise 27. Disconnect the circuit. Return the components and hoses to their storage location. 28. Wipe off any water from the floor and the training system. CONCLUSION In this exercise, you calibrated the DP transmitter in order to measure a specific level range in the open column. Then, you lowered the mounting of the DP transmitter with respect to the reference level. This caused the lower portion of the transmitter output range to become useless and the upper portion of the initial measurable range to become undetectable. You then performed zero suppression by recalibrating the DP transmitter so as to take account of the lower transmitter position. This allowed the proper measurement of the entire initial level range. REVIEW QUESTIONS 1. In applications where the level of liquid in a vessel is inferred using hydrostatic pressure measurement, what effect does mounting the primary sensing element of the pressure transmitter below the reference level have on the pressure sensed by this element? 2. What does "zero suppression" mean? 3. If the primary sensing element of the pressure transmitter is lowered with respect to the reference level and the transmitter is not recalibrated accordingly, what happens to the measurable level range? Festo Didactic

10 Ex. 4-3 Zero Suppression and Zero Elevation Review Questions 4. In applications where the level of liquid in the vessel cannot be seen and the primary sensing element of the pressure transmitter is located below the reference level, how can zero suppression of the transmitter output be achieved? 5. In applications where the level of liquid in the vessel can be seen and measured directly on a calibrated scale, as in the case of the column of the training system, how can zero suppression be achieved? 190 Festo Didactic

The Discussion of this exercise covers the following points:

The Discussion of this exercise covers the following points: Exercise 3-2 Orifice Plates EXERCISE OBJECTIVE In this exercise, you will study how differential pressure flowmeters operate. You will describe the relationship between the flow rate and the pressure drop