Instruction MI 020-328 September 2013 Bubble Tube Installations For Liquid Level, Density, and Interface Level Measurements
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Contents Introduction... 5 Abbreviations... 5 Principle of Operation... 5 Alternative to Bubble Tubes... 6 Reference Instructions... 6 Formulas for Specific Gravity Conversions... 7 For Liquids LIGHTER than Water... 7 For Liquids HEAVIER than Water... 7 For All Liquids... 7 Calibration... 7 Calculations... 8 Liquid Level Calculations (Figure 2)... 8 Interface Calculations (Figure 3 and Figure 4)... 9 With Constant Tank Level (One Bubble Tube)... 9 With Varying Tank Level (Two Bubble Tubes)... 9 Density Calculations (Figure 5 and Figure 6)... 10 With Constant Tank Level (One Bubble Tube)... 10 With Varying Tank Level (Two Bubble Tubes)... 11 Installation... 12 Typical Piping Arrangements... 12 Determination of Length Difference (Dimension H ) With a Pair of Tubes... 12 Piping Parts List... 12 Installation Notes... 13 Typical Bubble Tube Installations (Figure 8 and Figure 9)... 14 Tank With One Bubble Tube... 14 Tank With Two Bubble Tubes... 15 Typical Side-Connection Installations... 15 Tank With One Bubble Connection At Side Of Tank... 15 Tank With Two Bubble Connections At Side Of Tank... 16 Use of a Differential Pressure Regulator... 16 B0107XY Differential Pressure Regulator (Figure 12)... 17 B0107XX Differential Pressure Regulator (Figure 13)... 18 Pressure Drop in Air Lines... 18 Operation... 19 Operating Notes... 19 Putting into Operation... 19 Formulas to Calculate Output and Pressure Loss... 20 Calculating Output for Any Input... 20 Liquid Level Formula... 20 Density Formula... 21 Interface Level Formula (Figure 14)... 21 Calculating Pressure Loss in Air Line... 22 Calibration... 23 Maintenance... 23 1
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Figures 1 Bubble Tube Functional Diagram...6 2 Liquid Level Calculation...8 3 Interface Calculation - One Tube...10 4 Interface Calculation - Two Tubes...10 5 Density Calculation - One Tube...11 6 Density Calculation - Two Tubes...12 7 Bubble Tube Notch Details...13 8 Bubble Tube Installation - One Tube...14 9 Bubble Tube Installation - One Tube at Side of Tank...15 10 Bubble Tube Installation - One Tube at Side of Tank...15 11 Bubble Tube Installation - Two Tubes at Side of Tank...16 12 B0107XY Differential Pressure Regulator and Bubble Tube Piping...17 13 B0107XX Differential Pressure Regulator and Bubble Tube Piping...18 14 Interface Level Calculation Calculating Output for any Input...22 15 Bubble Tube Maintenance...24 3
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Introduction The bubble tube principle of hydrostatic measurement is a convenient, low-cost method of measuring liquid level, density, or interface level in an open tank. It is particularly applicable for those installations where: Process liquid could crystallize in transmitter lines. Process temperature exceeds temperature limit of flange-mounted transmitter. Process tank does not have side connections for flange-mounted transmitter. Process liquid is corrosive and cannot have direct contact with transmitter. Abbreviations The abbreviations below are used in this instruction. Abbreviation LRV URV Sp.G. Meaning Lower-range value (measurement that produces a 4 ma output). Upper-range value (measurement that produces a 20 ma output). Specific gravity (relative density) of a liquid. Specific gravity of water in both customary and SI systems is 1.00. Principle of Operation Air is passed through a restrictor to a tube partly immersed in a liquid. The lower end of the tube is at a fixed distance above the bottom of the tank (see Figure 1). The pressure of the air supply is high enough to overcome the hydrostatic head on the tube, and the excess pressure appears as small bubbles coming out of the bottom of the tube. Thus, the back pressure in the tube is a measure of the pressure on the bottom of the tube due to the level of liquid. Since the position of the tube is fixed, any change in this back pressure is due to a change in the level of the liquid. The back pressure is connected to the high-pressure side of the transmitter, and the low- pressure side is vented. Thus, the differential pressure measured by the transmitter is a measure of the level of the liquid. If the level of the liquid is constant (continuous overflow), any change in the transmitter differential pressure must be due to a change in liquid density or interface level. Thus, density and interface level can also be measured. If the tank level varies, these measurements can still be made by using two different-length tubes connected to opposite sides of the transmitter; see Figure 4 and Figure 6. 5
Alternative to Bubble Tubes Figure 1. Bubble Tube Functional Diagram If it is impractical to immerse bubble tubes in the tank (because the tank has a mixer and/or baffles, or because the liquid is corrosive, etc.), the bubbles can be introduced through connections at the side of the tank. See Figure 10 and Figure 11 for details. Reference Instructions Description Document Number Diff. Press. Regulator MI 011-170 13A, 13H, 15A Transmitters MI 022-345 Density Measurements TI 1-50a Liquid Interface Level Measurement TI 001-051 Liquid Level Measurements TI 001-052 6
Formulas for Specific Gravity Conversions For Liquids LIGHTER than Water Baume : Sp.G. = ------------------------- 140 Be + 130 API: Sp.G. = -------------------------------- 141.5 API + 131.5 For Liquids HEAVIER than Water Baume : Sp.G. = ------------------------- 140 145 Be Twadell: Sp.G. = Tw ---------- + 1 200 For All Liquids Density [lb/ft 3 ] / 62.4 = Sp.G. Density [kg/m 3 ] / 1.00 = Sp.G. Calibration If measurement range was specified in the sales order, the transmitter was calibrated in the factory to these values, and these same values are stamped on the transmitter data plate. If measurement range was not specified, transmitter was calibrated in the factory to maximum span for the particular sensor installed in the transmitter. In this case, the measurement range area of the data plate is left blank so that the user can mark the desired calibrated range (in terms of head of water) himself. The transmitter must be calibrated to the desired range before putting it into operation. (If the transmitter was calibrated in factory, the calibration should be checked.) Use the applicable formula to determine the input pressures (in terms of head of water) corresponding to the desired measurement limits. Mark these input pressure limits in the applicable area on the transmitter data plate, and use these values as the calibrating input signals. 7
Calculations The sections that follow show typical calculations for liquid level measurement, interface measurement, and density measurement. Interface and density measurements are shown with both a constant tank level and a varying tank level. Note that the upper-range and lower-range values (URV and LRV) refer to the desired maximum and minimum measurements, respectively. The Calibrated Span = URV - LRV. The value of the suppression is the output at LRV above 0%. Liquid Level Calculations (Figure 2) Span = (A)(G L ) H w at LRV = (B)(G L ) = Suppression H w at URV = (A + B)(G L ) Calibrated Range = LRV to URV where: G L = Sp.G. of Tank Liquid H w = Equivalent Head of Water Example: A = 2.0 m, B = 0.2 m, G L = 0.8 Span = (2.0)(0.8) = 1.60 mh 2 O H w at LRV = (0.2)(0.8) = 0.16 mh 2 O = Suppression H w at URV = (2.0 + 0.2) (0.8) = 1.76 mh 2 O Calibrated Range = 0.16 to 1.76 mh 2 O Figure 2. Liquid Level Calculation 8
Interface Calculations (Figure 3 and Figure 4) With Constant Tank Level (One Bubble Tube) Span = (A)(G 2 - G 1 ) H w at LRV = (A)(G 1 ) + (B)(G 2 ) = Suppression H w at URV = (A + B) (G 2 ) Calibrated Range = LRV to URV where: G 1 = Sp.G. of Upper Liquid G 2 = Sp.G. of Lower Liquid H w = Equivalent Head of Water Example: Liquid Sp.G. varies between 0.9 and 1.8; A = 50 inches and B = 10 inches Span = (50)(1.8-0.9) = 45 inh 2 O H w at LRV = (50)(0.9) + (10)(1.8) = 63 inh 2 O H w at URV = (50 + 10)(1.8) = 108 inh 2 O Calibrated Range = 63 to 108 inh 2 O With Varying Tank Level (Two Bubble Tubes) Span = (H)(G 2 - G 1 ) H w at LRV = (H)(G 1 ) = Suppression H w at URV = (H)(G 2 ) Calibrated Range = LRV to URV where: G 1 G 2 = Sp.G. of upper liquid = Sp.G. of lower liquid H w = equivalent head of water Example: Interface liquid Sp.G. varies between 1.0 and 1.7; and H = 40 inches. Span = (40)(1.7-1.0) = 28 inh 2 O H w at LRV = (40)(1.0) = 40 inh 2 O = Suppression H w at URV = (40)(1.7) = 68 inh 2 O Calibrated Range = 40 to 68 inh 2 O 9
Figure 3. Interface Calculation - One Tube Figure 4. Interface Calculation - Two Tubes Density Calculations (Figure 5 and Figure 6) With Constant Tank Level (One Bubble Tube) Span = (A)(G 2 - G 1 ) H w at LRV = (A)(G 1 ) = Suppression H w at URV = (A)(G 2 ) Calibrated Range = LRV to URV where: G 1 G 2 H w = Minimum Sp.G. = Maximum Sp.G. = Equivalent Head of Water 10
Example: Liquid Sp.G. varies between 1.0 and 1.8; and A = 40 inches. Span = (40)(1.8-1.0) = 32 inh 2 O H w at LRV = (40)(1.0) = 40 inh 2 O = Suppression H w at URV = (40)(1.8) = 72 inh 2 O Calibrated Range = 40 to 72 inh 2 O With Varying Tank Level (Two Bubble Tubes) Span = (H)(G 2 - G 1 ) H w at LRV = (H)(G 1 ) = Suppression H w at URV = (H)(G 2 ) Calibrated Range = LRV to URV where: G 1 = Minimum Sp.G. G 2 = Maximum Sp.G. H w = Equivalent Head of Water Example: Liquid Sp.G. varies between 1.2 and 2.0; and H = 40 inches. Span = (40)(2.0-1.2) = 32 inh 2 O H w at LRV = (40)(1.2) = 48 inh 2 O = Suppression H w at URV = (40)(2.0) = 80 inh 2 O Calibrated Range = 48 to 80 inh 2 O Figure 5. Density Calculation - One Tube 11
Figure 6. Density Calculation - Two Tubes Installation Typical Piping Arrangements With bubble tube installations, refer to either Figure 8 or Figure 9, as applicable. With sideconnection installations, refer to either Figure 10 or Figure 11, as applicable. Determination of Length Difference (Dimension H ) With a Pair of Tubes With density measurement, the larger the length difference (dimension H ) between tubes, the more accurate the measurement will be. With interface measurement, dimension H is the desired level measurement range. Physical limitations are set by both the height of the tank and the amount of sludge in the bottom of the tank. The lower tube should have a minimum of 75 mm (3 in) of clear liquid below it. Piping Parts List Transmitter Piping: Pipe: DN 8 or 1/4 in; or DN 10 or 3/8 in Tubing: 10x1 mm or 0.25 in OD; or 10x1 mm or 0.375 in OD Bubble Tubes: Pipe: DN 8 or 1/4 in; or DN 10 or 3/8 in Rotameter: Not supplied by Invensys. 12
Snubbers: For installation in the process line to reduce or eliminate any unwanted pressure pulsations, 1/4 NPT at both ends. For gases and thin liquids, see list below. 0045162 Brass, 1500 psi (100 bar) 0045163 303 ss, 5000 psi (340 bar) For oils and thick liquids, see list below. 0044596 Brass 1500 psi (100 bar) 0044597 303 ss, 5000 psi (340 bar) Differential Pressure Regulator Foxboro Type 62V (Part B0107XY), or Foxboro Type 63BD (Part B0107XX). Installation Notes 1. Bubble tube and transmitter piping is supplied by user. 2. There should be a minimum of 75 mm (3 inches) of clear liquid (no sediment) below the bottom of the tube. With two-tube installations, there should also be a minimum of 75 mm (3 inches) of liquid above the upper tube. 3. Bubble tube assembly should be located in area of representative liquid, and where liquid agitation is at a minimum. 4. Bubble tube assembly must be rigidly fixed in position. 5. Bottom of bubble tubes to be notched so that air comes out in a steady stream of small bubbles (rather than an intermittent stream of large bubbles, which could introduce errors). See Figure 7 for details of this notching. (Not applicable for side-connection installations.) Figure 7. Bubble Tube Notch Details 13
6. Condition of air (temperature, moisture content, etc.) must be compatible with process liquid. 7. If air reacts with process liquid, an inert gas (such as nitrogen) can be used. 8. A differential pressure regulator may be desirable to limit use of air. 9. The rotameter needle valve should not be used for tight shut off. A hand valve should be installed upstream to permit servicing or for complete shut-off of purge medium when desired. 10. If purge supply is higher than the maximum rating of the rotameter or differential regulator, or the purge supply pressure varies greatly, install a pressure regulator downstream from the shut off valve. Typical Bubble Tube Installations (Figure 8 and Figure 9) Tank With One Bubble Tube (Liquid Level; Density or Interface with Constant Level) Figure 8. Bubble Tube Installation - One Tube 14
Tank With Two Bubble Tubes (Density or Interface with Varying Level) Figure 9. Bubble Tube Installation - One Tube at Side of Tank Typical Side-Connection Installations Tank With One Bubble Connection At Side Of Tank (Liquid Level; Density or Interface with Constant Level) Figure 10. Bubble Tube Installation - One Tube at Side of Tank 15
Tank With Two Bubble Connections At Side Of Tank (Density or Interface with Varying Level) Figure 11. Bubble Tube Installation - Two Tubes at Side of Tank Use of a Differential Pressure Regulator It may be desirable to use one or two differential pressure regulators in the piping to limit the use of air. The regulator maintains a fixed difference between the output pressure of the regulator and a varying lower pressure. In a bubble tube measuring system, if the tank level is high, or if the ends of the bubble tubes tend to clog, the pressure delivered to the bubble tube may not be high enough to generate bubbles. Or, if the tank level falls, the pressure may be so high that large bubbles are produced and air is wasted. Thus, with both situations incorrect measurements can result. However, if a differential pressure regulator is used, the bubble system supply pressure will automatically vary to adjust for changing tank conditions. Thus, desirable small bubbles are produced (with more accurate measurement readings), and purge air is minimized. If two bubble tubes (or two side tank connections) are used, either one regulator can be used as a common regulated air supply, or a regulator can be used in each line for better bubble control and highest economy of air usage. See Figure 12 and Figure 13 for piping details. The two regulators in the table below are available from Foxboro. 16
Type and Part Number Parameter Differential Pressure (fixed) Maximum Input Pressure Exhaust Flow Rate (at standard conditions) Type 62V B0107XY 10.3 kpa 1.5 psi 690 kpa 100 psi 0.03 m 3 /h 0.9 ft 3 /h Type 63BD B0107XX 20.7 kpa 3.0 psi 1720 kpa 250 psi None None Maximum Temperature 65 C 150 F 80 C 180 F Liquid Purge No Yes B0107XY Differential Pressure Regulator (Figure 12) Figure 12. B0107XY Differential Pressure Regulator and Bubble Tube Piping 17
B0107XX Differential Pressure Regulator (Figure 13) Figure 13. B0107XX Differential Pressure Regulator and Bubble Tube Piping Pressure Drop in Air Lines If the air must flow through a considerable length of pipe or tubing to the transmitter, or if the flow of air is excessive, there will be some pressure loss. Any pressure loss will cause incorrect readings. One method to determine if a pressure loss exists is to install test gauges at each end of the line going to the transmitter, and compare readings. To calculate the loss in pressure, see To Calculate Pressure Loss in Air Line section that follows. 18
(With one-tube installations, a quick check for pressure loss is to momentarily turn off the air supply and note if there is any change in transmitter output.) If a significant pressure loss exists, reduce the air flow, and/or use a larger size line, and/or move the transmitter nearer the bubble tube. Operation Operating Notes 1. Function of restrictor is to control flow of air. This is accomplished with needle valve on bottom of rotameter (except if Type 62V differential pressure regulator [B0107XY] is used). Use this needle valve to adjust air flow for optimum small bubble size. If Type 62V regulator (B0107XY) is used, needle valve on regulator is used to adjust flow of air. 2. To prevent measurement errors, open ends of the tubes should always be covered with tank liquid. 3. With interface measurement, maximum level must be below open end of upper tube; minimum level must be above open end of lower tube. 4. If range is to be changed, transmitter must be recalibrated to new range. Dimension H may require changing for new range. 5. Needle valve on rotameter should not be used as a system air shutoff; instead use upstream valve at air supply to system. 6. Do not allow level of liquid to fall below bottom of bubble tube (or tank connection). 7. Check all connections for leaks. Putting into Operation If transmitter is not calibrated to the desired range, it must be calibrated before starting this procedure. After system is installed, adjust the process liquid (level or density) so that the measurement is at some point on scale. Then complete this procedure before operating the system. 1. If transmitter is not equipped with the optional output indicator, connect an indicator in transmitter output loop to read output. 2. Turn on air supply and adjust flow of air as follows. (If system does not have a differential pressure regulator, tank liquid must be at URV.) System With Type 62V Differential Pressure Regulator (Part B0107XY) Fully open rotameter needle valve (at bottom of rotameter). Adjust regulator needle valve (at bottom of body) so that small bubbles come out of tube in a slow, steady stream. All Other Systems Adjust rotameter needle valve (at bottom of rotameter) so that small bubbles come out of tube in a slow, steady stream. 19
3. If, in Step 2, the bubbles are not visible (because of tank location, type of liquid, etc.), using the applicable air adjustments specified in Step 2, gradually increase flow of air while noting output of transmitter. Keep increasing air flow as long as output increases. At point where output stops increasing, increase air flow slightly. 4. With a one-tube installation, make the pressure drop test described in previous section. If significant error exists, make necessary corrections. If transmitter has been calibrated correctly, this completes the procedure; otherwise proceed to Steps 5, 6, and 7 (reference adjustment). Note that the accuracy of the measuring device in Step 5, will probably be less than that of the transmitter; this may degrade the accuracy of the transmitter. 5. Using a suitable measuring device (such as a dipstick with level application or a hydrometer with density application), determine actual measurement of liquid. 6. Using applicable formula listed in the section that follows, calculate transmitter output corresponding to tank measurement. 7. Adjust transmitter zero to get correct output. If necessary, also adjust zero reading of receiver. Formulas to Calculate Output and Pressure Loss Calculating Output for Any Input The formulas that follow are for various types of transmitters as listed in the table below. In using these formulas, the transmitter must already be calibrated to the desired range. If the transmitter has not been calibrated (or if the calibration is to be changed), complete the calibration first. Use the applicable formula to determine the input pressures (in terms of head of water) corresponding to the desired measurement limits. Mark these input pressure limits in the applicable areas on the transmitter data plate. Liquid Level Formula Transmitter Output Value of X Value of Y 4 to 20 ma 16 ma 4 ma 10 to 50 ma 40 ma 10 ma 3 to15 psi 12 psi 3 psi 20 to 100 kpa 80 kpa 20 kpa Output Actual Level Min. Level = X ------------------------------------------------------------- + Y Max. Level Min. Level Example: Transmitter with 4 to 20 ma Output and: Actual Level = 30 linear inches above bubble tube 20
Min. Level = 10 linear inches above bubble tube Max. Level = 90 linear inches above bubble tube Therefore, 30 10 Output = 16 ----------------- + 4 = 8.0 ma 90 10 Density Formula Output = Actual Sp.G. Min. Sp.G. X ---------------------------------------------------------------- + Y Max. Sp.G. Min. Sp.G. Example: Transmitter with 10 to 50 ma Output and: Actual Sp.G. = 1.0 Min. Sp.G. = 0.6 Max. Sp.G. = 1.4 Therefore, 1.0 0.6 Output = 40 --------------------- + 10 = 30 ma 1.4 0.6 Interface Level Formula (Figure 14) Output = X Actual Interface Level ------------------------------------------------------------- + Y Maximum Interface Level Example: Transmitter with 3 to 15 psi Output and: A = 5 linear inches above lower tube H = 20 linear inches 5 Output = 12 ----- + 3 = 6 psi 20 21
Figure 14. Interface Level Calculation Calculating Output for any Input Calculating Pressure Loss in Air Line The pressure loss in a line varies with the flow, the ID of the line, and the length of the line. The equation to determine the pressure loss is: P = (K)(F)(L) where: P = Pressure drop, inh 2 O (or mmh 2 O) F = Flow, ft 3 /h (or m 3 h) L = Length of Line/1000, feet (or m) K = Constant The following table combines the line ID and other factors into one overall constant, K. Type of Line Line Size Line ID Value of K ANSI Pipe 1/8 in 0.269 in 3.0 1/4 in 0.364 in 0.9 3/8 in 0.493 in 0.27 1/2 in 0.622 in 0.105 3/4 in 0.824 in 0.035 22
If the ID of the actual pipe used is not listed in the table above, use the formula below to calculate the approximate value of K. K = 1/(62)(ID) 4 [with line ID in inches] If the measurements are in the SI system (line ID in mm, line length in meters, and flow in m 3 /h), then use the following formula to calculate K. K SI units = (65.9/ID) 4 Example 1: Determine pressure loss in a 20 foot length of 1/8 pipe due to an air flow of 4 scfh. K = 3.0 for 1/8 in pipe (from the table) P = (K)(F)(L) = (3.0)(4)(20/1000) P = 0.24 inh 2 O Example 2: Determine pressure loss in a 10 meter length of 5 mm ID pipe due to an air flow of 0.1 m 3 /h. K SI units = (65.9/ID) 4 = (65.9/5) 4 = 30176 P = (K)(F)(L) = (30176)(0.1)(10/1000) P = 30 mmh 2 O Calibration In general, follow the calibrating procedures outlined in the appropriate transmitter instructions. If desired, instead of using air pressures as calibrating signals, these signals can be generated by varying the level (or density) of the tank liquid to values at or near each end of the range. Use the calibrating values stamped on the data plate; or if the range is to be changed, calculate the new values. In this way, the transmitter can be calibrated without removing it from the process. Maintenance Type of Line Line Size Line ID Value of K ANSI Tubing 1/8 in 0.125 in 70.0 3/16in 0.188 in 12.5 1/4 in 0.25 in 4.0 1/2 in 0.50 in 0.245 3/4 in 0.75 in 0.050 1 in 1.0 in 0.016 Use applicable reference instructions listed on page 2 when servicing the transmitter, rotameter, or differential pressure regulator. If there is any tendency for solids to crystallize in the bubble tubes, or if dirt tends to collect there, remove the cleanout plugs and push a rod down through the tubes and/or flush with a suitable liquid. Perform as often as required. 23
Figure 15. Bubble Tube Maintenance ISSUE DATES SEP 1988 SEP 2013 Vertical lines to the right of text or illustrations indicate areas changed at last issue date. Invensys 10900 Equity Drive Houston, TX 77041 United States of America http://www.invensys.com Invensys and Foxboro, are trademarks of Invensys plc, its subsidiaries, and affiliates. All other brand names may be trademarks of their respective owners. Global Customer Support Inside U.S.: 1-866-746-6477 Outside U.S.: 1-508-549-2424 or contact your local Invensys representative. Website: http://support.ips.invensys.com Copyright 1988-2013 Invensys Systems, Inc. All rights reserved MB 100 0913