Level, interface, and density; open or closed, pressurized tanks and vessels
Operating Pressure
Limited only by the pressure of the available purge gas supply
Operating Temperature
As required; limited only by materials in contact with the process; has been used onsuch high-temperature processes as coal gasification
Materials
Limited only by the availability of exposed pipe materials
Costs
$200 for the simplest local indicator installation, can reach $5000 for installationsrequiring special materials and remote transmission
Inaccuracy
Function of error in the readout and other parts used,
±
0.05% to
±
2.0% of full scale
Range
Unlimited as long as purge gas supply pressure exceeds that of the process
Partial List of Suppliers
Bubbler-type level detector packages can be assembled from components describedin other sections, such as variable area flowmeters described in Section 2.27, varioustypes of pressure sensing and display devices described in Chapter 5
Prepackaged bubbler
Aalborg Instruments & Controls (www.aalborg.com)
assemblies are also
ABB Fischer & Porter (www.abb.com)
available from
Blue-White Industries (www.bluwhite.com)Brooks Instrument Div. of Emerson (www.emersonprocess.com)Dwyer Instruments Inc. (www.dwyer-inst.com)Flowmetrics Inc. (www.flowmetrics.com)Krohne America Inc. (www.krohneamerica.com)McMillan Co. (www.mcmillancompany.com)Omega Engineering Inc. (www.omega.com)Porter Instrument Co. (www.porterinstrument.com)U.S. Filter Wallace & Tiernan Inc. (www.usfwt.com)
INTRODUCTION
Many industrial accidents are caused by incomplete or inac-curate level information. Bubblers serve to solve that problemin an inexpensive and reasonably reliable manner. The oper-ation of an air bubbler is similar to blowing air into a glassof water with a straw. The more water is in the glass, theharder one needs to blow. Bubbler-type level sensors havebeen in use for as long as compressed air has. If the airpressure entering the dip pipe is greater than the hydrostatichead of the process fluid in the tank, the air will bubble outat the bottom of the pipe.
Figure 3.2a illustrates an air bubbler installation for anopen (atmospheric) tank with various purge controls. Thetransmission line should be sloped toward the tank so that,if the purge is lost and process vapors enter the transmissiontube, the condensate will drain back into the vessel. If thereadout device must be below tank level, a condensate trapcan be installed as shown by the dotted line.
The purge supply pressure should be at least 10 PSI (69kPa) higher than the highest hydrostatic pressure to begauged. The purge flow rate is kept small and relativelyconstant at about 1 SCFH (500 cm
3
/min), so there will be nosignificant pressure drop in the dip tube. Usually, the purge
media is air or inert gas, although liquids can also be used.Several methods of gas purge controls are shown in Figure3.2a to illustrate some of the installation considerations. Inmost traditional installations such as system A, nitrogen sup-ply pressure is regulated to a value corresponding to a pres-sure that is higher than the hydrostatic head when the tankis full. The purge flow rate is adjusted by a needle valve andis sent through a sight feed bubbler, which allows visualinspection of the actual flow. This system allows for detectionof levels up to 10 ft (3 m).
If higher levels are to be detected, system B, which usesa rotameter instead of a sight feed bubbler, can be considered,because a rotameter can withstand higher pressures. In sys-tems A and B, as the liquid level varies, the downstreampressure also will vary, thereby causing variations in thepurge flow rate.
Since the purge pressure at the readout device is the sumof hydrostatic head and the dynamic pressure drop in the diptube, variations in purge flow will cause errors. To correctthis condition, a differential-pressure control valve can beinstalled across the fixed restriction of the needle valve asshown in system C. This will cause the purge flow to beuniform regardless of the liquid head.
If the process material can build up or plug the dip tube,either as a result of loss of purge gas or because of the natureof the fluid, an aerator selector switch may be installed asshown on system D to allow for periodic blowing out of thetransmission line. System E can be considered in remotelocations where gas purge media is not available and a bub-bler is desired instead of a liquid purge for level detection.Here, water is jetted across a gap while air is aspirated intothe stream and compressed. The air–water mixture enters thedip tube, where the small amount of water runs down theinside of the bubbler tube while the pressure of the escaping
air is detected as a measure of level. Such a setup would bein service only when the operator wanted to make a levelreading, so the water would not flow into the vessel contin-uously. System F shows a more common approach for remotebubbler installations where a small hand pump is used tocompress the purge air.
For tanks that operate under pressure or vacuum, theinstallation of a bubbler indicator becomes slightly morecomplex, because the liquid level measurement is a functionof the difference between two bubbler pressures. Because ofthe differential measurement involved, the readout device canbe a manometer or other type of differential-pressure detec-tor. Figure 3.2b shows one of these installations. All of thepreviously discussed variations apply to both pressure andvacuum installations.
GENERAL
The bubbler detects the hydrostatic pressure in a vessel anddisplays it in a more convenient location. The pressure of aninert gas is used to transport the level information to thismore convenient location. Bubbler-type level detection hasbeen in use since compressed air became available. As illus-trated in Figure 3.2c, after the air fills the dip pipe, its pressureinside the dip tube will equal the hydrostatic head of theprocess fluid outside the dip tube, and the excess air that isintroduced will bubble out at the bottom of the tube.
If the tank is not open to the atmosphere, a second pres-sure tap is required to provide a reference pressure from thevapor space. The dip tube can enter from the top or side of
the tank as long as it extends below the minimum level thatis to be detected. As shown in Figure 3.2a, various combina-tions of valves, check valves, needle valves, and flow indi-cators may be required for various applications.
Of the bubbler design options, the
blow-back dip tube
isthe simplest and usually the least expensive. Here, piping ortubing is provided to bring the purge gas pressure to the leveldisplay or transmitter. The bubbler, because of its simplicity,is inexpensive and robust while being easy to maintain or adoptto changing process conditions. The display portion of thesystem is not wetted by the process fluid and is in a convenientlocation. Calibration and replacement of the level readout ortransmitting devices is also safe and convenient if, before ser-vice, the isolating valves (Figure 3.2c) are fully closed, and thestandard safety precautions (see Chapter 7) are observed.
If a short dip tube is inserted horizontally in the side ofthe tank, the dip tube will be easier to access and support.Figure 3.2d illustrates that a tap can be provided for cleaning(
rodding out
) the accumulated deposits. In this design anyplugging or dirt accumulation can be mechanically removedby inserting a rod into the dip tube. If it is desired to cleanout the dip tube while the process is in operation (or if thetank is full or pressurized), packing glands are provided, anda “captive rod” is permanently installed to allow clearing thedip tube at any time.
Replaceable dip tubes, with or without packing glands,have also been used on the more difficult applications. Otheroptions, such as dual or self-washing purges, will also bediscussed later in this section. In addition, jacketed dip tubesare also available and have been successfully used in appli-cations in which condensation or freezing is a concern(Figure 3.2e).
One of the advantages of the bubbler-type level measure-ment is that their readings are not affected (or affected onlyvery slightly) by foam and by variations in pressure or com-position of the vapor space above the liquid. These changes,particularly foaming, can interfere with many other types oflevel detectors, as was shown in Table 3.1b. On the otherhand, process phenomena that change the density of the liq-uid (bubble formation, boiling) will result in “understating”the level, because a drop in density reduces the hydrostatichead.
Purge Gas
Air and nitrogen are the most commonly used purge gases.Other gases can also be used if, for some reason (such astheir available maximum pressure being insufficient), thesecannot be used. The measurement itself is as reliable as theavailability of the purge gas supply. The flowing gas also
FIG. 3.2c
Bubbler-type level local indicator system.
Alternative Pressure Tap
PurgedPressure
Reference forNon-Vented
Tank
LI FI
FI
A
B
Filter
Typical for all: IsolationValve, Dirt Filter, andCheck Valve
serves to keep the inside of the dip tube dry and clean. Properfunctioning requires that the purge air or gas pressure behigher than the maximum process pressure plus the maxi-mum friction drop anticipated within the dip tube. Reliabilityis improved with increasing supply pressures.
Some bubbler-type level packages include an air pumpto generate the purge pressure and use a manometer to indi-cate the level. The danger here is that any loss of air due toair leakage or pump failure will result in a false low-levelindication. Therefore, it is more reliable to supply the bub-blers from the central air supply of the plant.
For critical applications, bottled gas, typically nitrogen,is used as a backup for the air supply. Pressure-operatedpneumatic valves can provide automatic switching of the gassupply without electrical connections (Figure 3.2f). A low-pressure detector switch can also be used and in that case;its contact not only can switch to the backup gas supply, itcan also initiate and alarm so that plant operators will beaware of the loss of air.
SIZING CALCULATIONS
The bubbler is fundamentally a mass or weight detector,because the pressure it senses is a function of both the liquidheight and density. Therefore, the pressure of the purge gasreflects level only if the liquid density (composition andphase) is constant.
Bubbler applications include level control, inventory man-agement, custody transfer, overflow protection, flow ratesmoothing, and pump suction protection. For inventory controlor for accounting purposes, the information desired is not thevolume but the mass of the liquid. Chemical reactions are alsobased on mass, and even fuels that are sold to end users byvolume (gasoline, fuel oil, natural gas) are often sold commer-cially by mass. As discussed in Section 3.18, they are sold by
“standard” volume, and this apparent volume is corrected fordensity (or temperature) difference from the standard.
A narrow-range dip tube mounted near the top of a tankcan provide accurate overflow protection because, over thatsmall range, the density correction is insignificant.
Mass and Level
Level is inferred from the pressure (
H
) measured by the bubbler.Equation 3.2(1) shows how this hydrostatic head is calculated.
H
=
(
h
) (
ρ
) (
G
/
G
c
)
3.2(1)
where
H
=
hydrostatic head
h
=
vertical depth
ρ
=
average fluid density over depth
G
=
local gravity
G
c
=
units conversion factor, not required with SI units
The total mass of liquid in the tank is obtained by Equa-tion 3.2(2).
M
=
(
H
) (
A
)
3.2(2)
where
H
=
hydrostatic head
A
=
cross-sectional area of vessel
The mass calculations must be corrected for any varia-tions in cross-sectional area over the range of interest. Theoil industry uses the term
strapping
to refer to the process ofcalibrating a tank. Internal devices, construction tolerances,and even the deformation of a large storage tank with levelvariations affect the accuracy of any level gauge. Actual testdata is required for reliable measurement accuracy.
The prudent user will not calibrate the level measurementto 100% of the tank height but will allow for errors and forchanges in density. If a tank is calibrated for 100% of fulltank level for a heavy liquid of, say, specific gravity of 1.2,it will overflow if used on water with a specific gravity of1.0. If the liquid cannot freely overflow, the hydrostatic pres-sure will build up inside the vessel and create a lifting forceon the top while pushing the walls out. As a result, the side-to-bottom joints might fail.
The Hydrostatic Tank Gauge (HTG)
Density can be measured by detecting the pressure differencefrom two dip tubes immersed in the liquid, with their bottomends vertically separated by a fixed distance “
h
” in Figure3.2g. Where needed, a third pressure, the vapor-space pres-sure above the liquid, is also measured and can be used todetermine the density of the liquid. If both level and densityare known, one can determine the mass in the tank. All threevalues (volume, mass, and density) can be reported for dif-ferent uses. Improved accuracy in pressure transmitters hasmade it possible to install hydrostatic tank gauges (HTG),which are illustrated in Figure 3.6e.
If there are two immiscible (nonmixing) liquids, then theheight of the interface above the more dense liquid may beinferred by the HTG. Note that the difference in pressures inthe two tubes may be small and require a very accuratedetector and a large suppression of zero. Note also that theinterface measurement is affected by changes in density,which can be caused by changes in composition or density.
Density
As shown in Figure 3.6e, the differential pressure can alsobe a measure of density. Figure 3.2h shows how bubblerscan also be used to correct the level for variations in density
FIG. 3.2g
The measurement of density by bubblers.
FIG. 3.2h
Density compensated interface detection with bubbler tubes.
FI
FI
h
Filter
Filter
LI
Typical for all: IsolationValve, Dirt Filter, andCheck Valve
or to measure interface or other hydrostatic-head-relatedvariables.
Calibration
The bubbler differential pressure can be calibrated in inchesor millimeters of level or in regular pressure units, but it isabsolutely vital to have good records of the units used andof all the conversion factors. For high precision at very highoperating pressures, it might also be necessary to correct forthe weight of the highly compressed gas column in the bub-bler. Another factor to consider is the thermal expansion andcontraction of the vessel and the dip tube caused by atmo-spheric or process temperature variations. In addition, pres-sure changes and gravity forces caused by level variationsshould be considered.
FLOW RATE AND PLUGGING CONSIDERATIONS
Minimum Purge Flow Rate
For accurate level signals and to keep the inside of the diptube dry, it is necessary to provide a sufficient mass flow rateof purge gas to keep the dip tube full, even during a high rateof level or vessel pressure increase. These required rates canbe calculated by first calculating the total volume of the tankcorresponding to each inch of level change and, after that,determining the corresponding mass of air in this volume.The difference in this mass divided by the time for the pres-sure or level to change is the average mass flow rate required.A typical conservative value commonly used for atmospherictank level detection is 0.5 SCFH, or 50% of full range on a0 to 1 SCFH range rotameter.
Maximum Purge Flow Rate
The tubing must be large enough to keep the pressure dropbetween the air or gas supply regulator and the end of thedip tube at a negligible value. Most users specify a minimumof 3/8-in. (10-mm) OD tubing and, preferably, 1/2-in. (12-mm) OD tubing or piping should be used. To test the maxi-mum flow limit of a bubbler installation, make a small changein purge flow rate and observe the effect. The level readoutshould not change as a result.
Most problems with excessive pressure drops are causedby damaged or deformed tubing or to partially closed valves.Sometimes, when excessive leakage from the system is noticed,maintenance technicians will respond by increasing the airflowsufficiently to keep the level detector in operation. The result-ing problem is that the level signal then will vary not onlywith level but also with the purge flow rate. On high-vacuumprocesses, the low density, and therefore high specific volumeof the purge gas, will cause high gas velocities and will alsoincrease the probability of leakage.
Dip Tube Diameter Selection
Dip tube sizing is determined both by the pressure dropthrough it, but mostly by the required mechanical strength ofthe system. Up to a length of 8 ft in un-agitated tanks (or 5ft in agitated ones), the size of the dip tube should typicallybe 1 in. diameter, Schedule 40 pipe. Longer dip tubes mustbe supported (Figure 3.2i). In case of even longer dip tubesin agitated tanks, in addition to the guide support, a 2-in. (51-mm) pipe sheath is added as shown in Figure 3.2j.
Upsets and Plugging
The most common complaint about dip tubes is plugging,whereby the purge flow is lost or is inadequate. Under theseconditions, the process liquids may rise up inside the dip tubeand coat the walls. Over time, the coating will accumulateand restrict the flow of the purge gas. The beginning ofplugging can be detected by slightly changing the purge flowrate and observing whether a change in level follows.
The probability of plugging in saturated salt solutionservices increases as the area of the tip of the dip tube isreduced. To unplug the dip pipe, we can apply “rodding out”
FIG. 3.2i
Supporting long dip tubes.
1 Inch (25 mm) Pipe (Example)
Guide Required for Over 5 ft(300 mm), for Agitated Vessel, Over 8 ft (2500 mm), to 25 ft(7500 mm) for Non-Agitated.
Level Tap
Head Pressure Tap forNon-Vented Tanks
Notes: Dimensionsand SI Equivalents areOnly Suggested andNominal
using a twist drill that has been welded to a rod so that theend can be drilled clear. Special bubbler system designs canbe used when the process liquid is a saturated salt solutionand the salts are deposited inside the dip tube as the purgegas dries the solution. In saturated salt solution service, one canuse water or a solvent to dissolve the deposits (Figure 3.2k).In viscous fluid services, solvents are used at flow rates ofabout 1 GPH to keep the dip tube clean. The solvent purgeflow rate should be enough to maintain high humidity withinthe dip tube and to wash out any salts or solids.
When measuring the level of caustic, a common practiceis to add a secondary purge of low-pressure steam, which isintroduced very close to the point at which the dip tube entersthe tank. The steam flow is restricted by an orifice union,typically bored for 0.125 in. (3 mm).
The theoretical design conditions in a plant can drasti-cally differ from real-life and upset conditions. The prudentdesign engineer understands that, at one time or another,every instrument will be exposed to the pressure at which thesystem relief valve is set to open. On the other hand, theminimum pressure for a closed system is full vacuum. Oneshould also consider the plant’s safety in terms of the qualityand availability of experienced personnel if, for example, theupset occurs at 3
A
.
M
. on a weekend.When a plant upset or system overhaul occurs, process
equipment designed to operate at a high vacuum will beexposed to positive pressures as attempts are made to unplug
piping or valves. As high-temperature processes are shut down,they might suddenly develop high vacuums, because theirvapors cool and condense. Pumps, blowers, and other equip-ment are all cycled on and off because of stuck check valvesor other system components. A power outage can also cause aloss of control and, on top of all that, human beings are notfully predictable, either. Some people, under an upset condition,will react with great confidence and do exactly the wrong thing.
INSTALLATION DETAILS
There are two fundamentally different approaches to theinstallation of bubblers, as illustrated in Figures 3.2l and3.2m. Figure 3.2l illustrates a transmitter mounted on the topof the tank and therefore has a shorter purged tubing run.This makes the system less prone to plugging but also resultsin a less convenient access for maintenance. In Figure 3.2m,where the transmitter is mounted at ground level, the purgedtubing runs are longer and more prone to plug, but access ismore convenient.
The ground-mounted installation is usually provided withdrip legs that, during upsets, can capture any liquid that mightleave the tank. The drip legs can thereby protect the trans-mitter. In either case, the most important consideration is tomaintain the adequate and reliable flow of purge gas. As wasshown in Figure 3.2a, it is also advisable to prevent the blockingof the flow of purge gas when the dip pipe rests on the bottomof the tank. A simple solution is to cut the end at about a 45°angle to prevent blocking. On the other hand, this author has
FIG. 3.2j
Sheath and bracket supports for dip tubes.
2 Inch (50 mm) Pipe Sheath
1 Inch Pipe (25 mm) Dip Tube
Drill 41/2 Inch Diameter (12 mm)Holes, 8 Inches (200 mm) from Top
Bracket for 12 ft (3600 mm) to 36ft (10,000 mm)
11/2 Inch (40 mm) Pipe
NOTES: Dimensions and SIEquivalents are Only Suggested andNominal
FIG. 3.2k
Alternate liquid purge can clean bubbler dip tubes.
not found that V-notches cut in the end of the tube will reducethe size of the bubbles or damp the small bump in pressureas each bubble escapes. This writer also believes that main-taining the purge flow rate at a constant value is not essential.
Pressure and/or Flow Regulators
Figure 3.2a shows a pressure regulator, but it is not neces-sarily true that a purge gas pressure regulator is necessary tostabilize the purge flow rate; some designs use only a flowrate regulator. For a properly designed system, the flow veloc-ity in the tubing must be low enough that any reasonablechange in flow will have practically no effect on the gaspressure at the tip of the dip pipe. If pressure changes withpurge rate, then something is wrong with the installation, andthe problem should be addressed.
The most likely cause is that the flow rate is restricted atthe wrong place. It is apparent to this writer that reducingthe purge gas pressure is not the preferred practice, becauseit increases the probability of the loss of purge flow. This canlead to plugging or to measurement errors.
The standard air supply regulator has a built-in overpres-sure vent to protect pneumatic instruments. However, in bub-bler service, this vent allows the process to back into the diptube if the process pressure exceeds the setpoint of the over-pressure vent. Therefore, the “nonbleed” regulators shouldbe used on bubbler installations, which will not vent and willtherefore make the full air supply pressure available as the flowis reduced to nearly zero. Nonbleed regulators are availablebut are not always used, because of concern that they mightbe accidentally installed on other pneumatic instruments asinstrument air supply regulators.
The purge gas supply pressure is commonly far higherthan the pressure in the dip tube, and the pressure drop acrossthe flow-restricting needle valve is large. This results in anearly constant purge flow rate, even if the pressure of theair supply or in the dip tube changes. Yet, the full supplypressure remains available if needed. This is similar to theconstant-current concept used in some electronic circuits.
The only remaining reason for having a pressure regula-tor is to protect the pressure indicator, but, if there is nopressure regulator, no gauge is needed. A modern d/p-celltype of pressure transmitter will withstand the full instrumentair supply pressure without damage. For the above reasons,it is this writer’s view that the traditional installations shownin Figures 3.2a and 3.2b can be simplified by eliminating thepressure regulators and by making sure that all instrumentsand tubing components will withstand the maximum possiblesupply pressure.
DIAPHRAGM-TYPE DIP TUBE
For some applications where the normal dip tube is not accept-able, the “dry” dip tube or plug-proof dip tube is used, asdescribed in connection with Figure 3.5d in Section 3.5. Allthat has been related here about purge supplies and sensors alsoapplies to these installations except that a barrier is providedto prevent process liquids from backing up into the dip tube.
The diaphragm is not perfectly flexible and does offersame resistance, so a small offset will exist in the resultingsignal pressure. There is a choice of diaphragm sizes, and
the larger diaphragms will have smaller offsets, whereas thesmaller diaphragms are less likely to fail.
SAMPLE CALCULATIONS
Level Detector Calibration Example
If one is to calibrate the range of the level detector (
r
) ininches of water column (
"WC
) for a 25-ft-high tank contain-ing a liquid of 0.85 specific gravity (
SG
), which correspondsto 62.4 lb/ft
3
or 999.8 kg/m
3
, and it was decided that the tankwill be considered full at 80% of tank level.
Range (
r
)
=
25
×
12
×
0.8
×
0.85
=
204
"WC
(5182 mm
WC
)
3.2(3)
If the same level detector range is to read in units of PSI,
Range (
r
)
=
(25
×
0.8
×
0.85
×
62.4)/144
=
7.36 PSI
3.2(4)
If a tank contains oil (SG
=
0.8) and water (
SG = 1.0) andwe want to detect the movement of the interface in inches ofWC over a range of 10 in.,
Range (r) = (1.0 − 0.8) 10 = 2.0 "WC (50.8 mm WC)3.2(5)
Density Detector Calibration Example
If one is to detect the average density of a 10" layer of liquidin a tank by measuring the differential pressure across that layer,which can contain any mixture of oil (SG = 0.8, density =49.92 lbm/ft3) and water (SG = 1.0, density = 62.4 lbm/ft3),the d/p cell range is:
Range (r) = 10 (1.0 − 0.8) = 2 "WC = 50.8 mm WC
= 49.92 − 62.4 lbm/ft3 3.2(6)
CONCLUSION
The bubbler remains a valuable tool in level measurementdue to its low cost, simplicity, and flexibility. It is also valu-able as an inexpensive and easily installed backup overflowprotector. For some specialized measurements, such as inter-face detection between oil and water, the capacitance gaugeis more popular because of its higher sensitivity and betterperformance.
When designing a difficult level-measurement system, aprudent design engineer might do well to specify spare noz-zles for installing a bubbler as a backup on a vessel if more
modern level sensors could fail to work properly. Successfulbubbler applications include polymers, tars, salts, and otherdifficult fluids. Failures resulting from dirt and plugging canbe simpler to live with using a bubbler than with floats orother devices that have moving parts. The advantages of theHTG system (Figure 3.6e) can also be realized with the dip-tube type of bubbler detectors.
There has been some environmental concern that, with abubbler, the purge gas that enters the tank also has to beremoved, but these purges are a very small portion of thetotal gas and vapor flow that must be removed anyway. Forexample, in the case of large storage tanks, the gas displacedduring each cycle of emptying and filling is usually morethan the volume of the gas used for purging at 1 SCFH fora week.
The main advantage of air bubbler systems is their sim-plicity and the ease with which the readout device can berelocated to just about any convenient location. For remotetank farms where compressed air is not available, one of thesimplest methods of level detection is to use a small handpump and a gauge. Bubblers are widely used in the waste-water and food industries and in some bulk storage applica-tions, but they have lost some of their earlier popularity inthe processing industries, where the trend seems to be favor-ing nonflowing, solid-state electronic devices.
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