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Foxboro Bubbler App Note Mi020328

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Text of Foxboro Bubbler App Note Mi020328

MI 020-328 InstructionSeptember 1988

Bubble Tube InstallationsFor Liquid Level, Density, and Interface Level Measurements

MI 020-328 September 1988

ContentsFigures............................................................................................................................... v Introduction ........................................................................................................................ Abbreviations ................................................................................................................. Principle of Operation .................................................................................................... Alternative to Bubble Tubes ........................................................................................... Reference Instructions .................................................................................................... Formulas for Specific Gravity Conversions ..................................................................... For Liquids LIGHTER than Water ........................................................................... For Liquids HEAVIER than Water ............................................................................ For All Liquids ........................................................................................................... Calibration ..................................................................................................................... Calculations ........................................................................................................................ Liquid Level Calculations (Figure 2) ............................................................................... Interface Calculations (Figure 3 and Figure 4) ................................................................ With Constant Tank Level (One Bubble Tube) ......................................................... With Varying Tank Level (Two Bubble Tubes) ......................................................... Density Calculations (Figure 5 and Figure 6) ................................................................. With Constant Tank Level (One Bubble Tube) ......................................................... With Varying Tank Level (Two Bubble Tubes) ......................................................... 1 1 1 2 2 3 3 3 3 3 4 4 5 5 5 6 6 7

Installation .......................................................................................................................... 8 Typical Piping Arrangements ......................................................................................... 8 Determination of Length Difference (Dimension H) With a Pair of Tubes ................. 8 Piping Parts List ............................................................................................................. 8 Installation Notes ........................................................................................................... 9 Typical Bubble Tube Installations (Figure 8 and Figure 9) ........................................... 10 Tank With One Bubble Tube .................................................................................. 10 Tank With Two Bubble Tubes ................................................................................ 11 Typical Side-Connection Installations .......................................................................... 11 Tank With One Bubble Connection At Side Of Tank ............................................. 11 Tank With Two Bubble Connections At Side Of Tank ........................................... 12 Use of a Differential Pressure Regulator ........................................................................ 12 B0107XY Differential Pressure Regulator (Figure 12) .............................................. 14 B0107XX Differential Pressure Regulator (Figure 13) .............................................. 15 Pressure Drop in Air Lines ............................................................................................ 15 Operation ......................................................................................................................... 16 Operating Notes ........................................................................................................... 16 Putting into Operation ................................................................................................. 16 Formulas to Calculate Output and Pressure Loss .............................................................. 17 Calculating Output for Any Input ................................................................................ 17 Liquid Level Formula ............................................................................................... 18iii

MI 020-328 September 1988

Contents

Density Formula ...................................................................................................... 18 Interface Level Formula (Figure 14) ......................................................................... 19 Calculating Pressure Loss in Air Line ............................................................................ 20 Calibration ........................................................................................................................ 21 Maintenance ..................................................................................................................... 21

iv

Figures1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Bubble Tube Functional Diagram ................................................................................ Liquid Level Calculation .............................................................................................. Interface Calculation - One Tube ................................................................................. Interface Calculation - Two Tubes ............................................................................... Density Calculation - One Tube .................................................................................. Density Calculation - Two Tubes ................................................................................ Bubble Tube Notch Details ......................................................................................... Bubble Tube Installation - One Tube .......................................................................... Bubble Tube Installation - Two Tubes ......................................................................... Bubble Tube Installation - One Tube at Side of Tank ................................................. Bubble Tube Installation - Two Tubes at Side of Tank ................................................ B0107XY Differential Pressure Regulator and Bubble Tube Piping ............................. B0107XX Differential Pressure Regulator and Bubble Tube Piping ............................. Interface Level Calculation Calculating Output for any Input ...................................... Bubble Tube Maintenance ........................................................................................... 2 4 6 6 7 7 9 10 11 11 12 14 15 19 21

v

MI 020-328 September 1988

Figures

vi

IntroductionThe 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.

AbbreviationsThe abbreviations below are used in this instruction. ABBREVIATIONLRV URV Sp.G.

MEANINGLower-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 OperationAir 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.

1

MI 020-328 September 1988

Introduction

Figure 1. Bubble Tube Functional Diagram

Alternative to Bubble TubesIf 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 InstructionsDOCUMENT NUMBERMI 005-529 MI 010-153 MI 011-170 MI 022-345 MI 0

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