air lift design oct03mc5

Augmentation of Airlift Pump Performance withTapered Upriser Pipe � an Experimental Study

E Anil Kumar, Non-memberK R V Kumar, Non-memberDr A Venkata Ramayya, Non-member

The present study reports the improved performance of an airlift pump fitted with a tapered upriser pipe. The influenceof various design parameters like size, taper angle of the upriser pipe, lift and submergence as well as that of operatingparameters like air flow rate have been investigated to characterize the augmentation in performance vis-a-vis that ofan airlift pump with uniform upriser pipe dia. To understand the mechanism behind the improved performance, thehydrodynamic investigations have been carried out to bring out the influence of the adverse pressure gradient (increasingarea in flow direction) in the tapered upriser pipe on the two-phase flow regime transitions and slug flow parameters.Videographic measurements of the slug heights with perspex pipes with taper angles of 0°, 0.5° and 1° are reported andthe role of hydrodynamics on airlift pump performance is discussed. The utility of stepped upriser pipes, made of shortlength PVC pipes of increasing dia, as a means of implementation of the tapered upriser pipe in practice is also explored.To facilitate the interpretation of the experimental results, a performance simulation study has also been carried out tobring out the influence of design and operating parameters.

Keywords: Airlift pump; Tapered upriser pipe; Slug flow regime; Hydrodynamics

NOTATIONCo : drift flux profile constant in Stenning and Martin

equation5

f : friction factor

Mg : mass flow rate of air, kg/s

Ml : mass flow rate of water, kg/s

INTRODUCTIONAirlift pumps are simple devices consisting of a vertical risertube partially immersed in the fluid to be pumped and intowhich air is injected at the base to produce an upward flow.The air-fluid mixture rises to the surface because it is lighterthan the surrounding water. Even though this type ofpumping has low efficiency, simplicity in construction andabsence of mechanical moving parts are two very importantadvantages that make it useful for certain applicationsinvolving difficult pumping operations with corrosive andabrasive liquids or slurries where reliability and lowmaintenance are required.

These include pumping sandy or salty waters, viscous liquidslike hydro-carbons in oil industry, shaft and well drilling (thedrillings being lifted by underground water), under-sea mining

E Anil Kumar, K R V Kumar were with the Centre for Energy Studies,Vellore Institute of Technology (DU), Vellore 632 014 and Dr A VenkataRamayya presently is with the Department of Mechanical Engineering,Faculty of Technology, Jimma University, Jimma, PO Box 378, Ethiopia.

This paper was received on November 10, 1999. Written discussion on thispaper will be entertained till December 31, 2003.

and in bio-reactors and waste treatment installationsproviding excellent aeration of the pumped fluid1. Thesepumps are ideal also for handling highly radioactive liquidsbecause they require virtually no maintenance and can betreated as remote units2. Besides they are also being projectedfor usage in preventing icing on some high altitude and for usein combination with a wind turbine which can be placed at awindy site independent of the site chosen for the well3.

Most of the work that has been done on airlifts involves theusage of a uniform size upriser pipe wherein optimizationstudies have been reported4-5, as well as flow regime mappingand modelling work6. Numerical predictions of thehydrodynamics of airlift with uniform upriser pipe also havebeen reported7. The potential of the airlift for fluid-solidhandling has recently been reviewed by Zenz8. Even thoughtapered upriser pipes are known to be efficient, noinformation is available on the performance as well as on thehydrodynamics of such airlift configurations9-11. The presentstudy is an attempt in this regard wherein, the effect of atapered upriser pipe on airlift pump performance isexperimentally assessed in comparison with that of an airliftusing uniform diameter upriser pipe, and the correlation of thehydrodynamics of two-phase flow regime in tapered one tothe enhanced performance is evaluated.

EXPERIMENT

The experimental testing has been done on the airlift pumpwith upriser pipes made of mild steel sheet, perspex sheet andPVC pipes. The cross-sections tested include square, circularand rectangular. Two taper angles of 0.5° and 1° have been

114 IE (I) Journal�MC

employed with perspex made tapered upriser tubes of squarecross-section. The square section has been used to facilitateeasy fabrication with perspex sheets of 3 mm thickness. Thesize of upriser pipes employed ranged from 25.4 mm2 MS tube,26.7 mm × 31.7 mm rectangular perspex tube, 0.5° and 1°tapered perspex tubes with 25.4 mm starting square cross-section, 18.75 mm PVC pipe and 32 mm PVC pipe. Inaddition to implement the tapered upriser pipe, in the absenceof the availability of such tapered pipes, stepped upriser pipesare assembled using PVC pipes of different diametersconnected with appropriate coupling to result in an increaseddiameter in three discrete sizes. The different stepped upriserpipes, one with size variation from 18.75 mm to 32 mm andanother with size variation from 32 mm to60 mm are employed. A schematic of the experimental set upused is shown in Figure 1.

A mouthpiece made out of PVC pipe, 42 mm in size has beenemployed and its details are also presented in Figure 1. It hadcircumferential holes of 10 mm at a vertical pitch of 50 mm.The height of the drilled portion is 540 mm whereas the totalheight is 650 mm. The top portion of the mouthpiece has beenreduced in size from 42 mm to 32 mm. The air required hasbeen taken from a two-stage reciprocating air compressor,through a regulator of 1/2 inch size, via an orifice of 14 mmsize, made as per ASME specifications to measure the air flowrate into the airlift pump. The water flow rate has beenmeasured by using a collecting tank and a timer.

VIDEOGRAPHIC STUDIESVideography has been employed to record the two-phase flowstructure and to study the flow regime transitions involvingannular type flow and slug flow. A video camera has been usedto film the actual flow regime in airlift upriser pipe made oftransparent perspex sheet. Tapered upriser pipes of 0.5° and 1°taper as well as uniform size pipes (26 mm × 32 mm) havebeen utilized to record the flow structure at different air flowrates. These video recordings are made such that the actual airflow rate used is visible on the film by including it on a paper

and placing it in the line sight of the camera. To facilitate themeasurement of slug height, a scale is also included adjacent tothe upriser pipe of the airlifts used in the filming.

The video clips subsequently have been transferred to apersonal computer through a digital camera for the analysisand the measurement of slug flow parameters as influenced bythe size, shape and taper angle of the airlift upriser, using animage processing software.

SIMULATION OF AIRLIFT PUMP PERFORMANCEA parametric study has been conducted to bring out theinfluence of various operating parameters like lift,submergence and gas injection rate as well as designparameters like diameter of the upriser pipe etc. To simulatethe performance of airlift pump, a reliable predictive designequation is required and based on exhaustive comparativestudies9, Stenning and Martin�s equation5 is selected and usedin this study. No design equation is at present available fortapered upriser pipes. However, the simulated results aregoing to be useful in interpretation of the experimental resultsto be discussed in the next section.

Effect of Diameter of Upriser Pipe on Water Flow RateAt steady state operation of the airlift pump for a given valueof lift, submergence and diameter of the upriser pipe, thereexists a unique one to one relationship between air and waterflow rates. Under these conditions, if air flow rate is fixed, thewater flow rate is going to be automatically fixed. Figure 2shows the effect of increase in diameter on the water flow rate.It is seen that after a particular diameter, the water flow ratestarts decreasing rapidly. This brings out the importance ofcarrying out optimization of the performance of the pump.

Effect of Diameter of Upriser on LiftFor a given value of water and air flow rates, both equal to, forexample, 2.5 × 10�3 m3/s and for a submergence of 0.653 m,the lift that can be obtained is shown in Figure 3. It can beinferred from this that at a diameter of 12 mm, the liftobtained is maximum and at any other diameter the

Figure 1 Experimental set-up

Figure 2 Effect of upriser pipe diameter on liquid flow rate

Vol 84, October 2003 115

performance of the airlift pump is below par. The reason is onaccount of the interplay of various factors with the result thatthe losses are going to be minimum at the optimizedcondition.

Effect of Lift on Water Flow RateFigure 4 shows the effect of lift on the airlift pump output fora fixed value of diameter of upriser pipe, submergence and theair flow rate. This reveals that as lift to be attained increasesthe water output reduces. Here, the optimized condition islower than a lift value of 9 m since the maximum is notreflected here.

Effect of Submergence on Water Flow RateFor a given lift, diameter and air flow rate, the effect ofsubmergence on water flow rate from pump is presented inFigure 5. It can be gauged from this that when the lift is fixed,increasing the submergence level for the same diameter of theupriser pipe can increase the water flow rate. Thus, anincreased submergence level is always desirable and beneficialand as such is to be made use of as and where is possible.

Effect of Air Flow Rate on Water Flow RateFor a fixed value of lift, submergence and diameter, thedependence of water flow rate on air flow rate is given inFigure 6. This shows an important characteristic of airliftpump operation wherein for a particular value of air flow rateonly, the water flow rate is seen to be maximum. Thisoptimized operating condition has to be obtained by operatingthe airlift pump over a range of air flow rates and then fixingthe air injection rate which gives maximum water flow rate. Itis always preferable to operate the airlift pump at this air flowrate. However, even though this air flow rate gives maximumwater flow rate, the efficiency of airlift pump is found to bemaximum at a flow rate lower than this optimal air flow rate.

RESULTS AND DISCUSSIONAgainst simulated results discussed earlier, the experimentaland videographic results are presented and discussed here. Theexperimental observations on various upriser pipeconfigurations tested are listed in Tables 1-4.

PVC Stepped Upriser PipesThe summary of the performance of airlift upriser made ofPVC stepped pipe is presented in Tables 5 and 6 for lifts of 1.15 m

Figure 3 Effect of upriser pipe diameter on lift

Wate

r F

low

Rate

(10 e

xp �

5 m

3/s

)

Submergence = 6.5 mDiameter = 0.015 mf = 0.008C

O = 1.2

Air Flow Rate = 0.00003973 m3/s

Figure 4 Effect of lift on water flow rate

Diameter = 0.015 mLift = 10 mC

o = 1.2

f = 0.008Air Flow Rate = 0.00003973 m3/s

Wate

r F

low

Rate

(10 e

xp �

5 m

3/s

)

Figure 5 Effect of submergence on water flow rate

Wate

r flo

w r

ate

(10 e

xp �

5m

3/s

)

Submergence = 6.5 mLift = 4.5 mC

o = 1.2

f = 0.008Diameter = 0.015 m

Figure 6 Effect of gas injection rate on water flow rate

Air Flow Rate (10 exp � 5m3/s)

1

Submergence = 6.5 mWater Flow Rate = 2.5 (10 exp �5 m3/s)Air Flow Rate = 2.5 (10 exp �5 m3/s)


Table 1 Airlift performance with uniform tubes and stepped tubes

Lift : 1150 mm Submergence : 1470 mm

18.7 mm, 18.75 mm, 32 mm 32 mm,Uniform 25 mm, 32 mm, Uniform 47 mm, 60 mm,

Pipe Stepped Pipe SteppedPipe Pipe

M g* Ml Ml Ml Ml

103 kg/s kg/s kg/s kg/s

kg/s

3.526 0.094 0.154 0.254 0.276

4.245 0.095 0.149 0.263 0.277

4.329 0.090 0.157 0.256 0.285

4.665 0.083 0.153 0.254 0.288

4.945 0.081 0.149 0.263 0.330

Table 2 Airlift performance with rectangular duct, square duct andtapered


Rectangular 1 o 1" 0.5 o

Perspex Tapered MS Duct TaperedDuct Duct Duct

M g* Ml Ml Ml Ml


kg/s

3.520 0.181 0.048 0.119 0.181

4.245 0.183 0.083 0.121 0.211

4.329 0.183 0.110 0.127 0.238

4.665 0.184 0.143 0.122 0.265

4.945 0.184 0.160 0.121 0.278

Table 3 Airlift performance with uniform tubes and stepped tubes


18.75 mm, 18.75 mm, 32 mm 32 mm,Uniform 25 mm, 32 mm, Uniform 47 mm, 60 mm,

Pipe Stepped Pipe SteppedPipe Pipe

M g* Ml Ml Ml Ml


kg/s

3.520 0.060 0.095 0.142 0.056

4.245 0.059 0.093 0.155 0.063

4.329 0.056 0.098 0.158 0.095

4.665 0.053 0.097 0.157 0.127

4.945 0.053 0.090 0.158 0.164

Table 4 Airlift performance with rectangular duct, square duct and 0.5°tapered


Rectangular 1" 0.5 o

Perspex MS Duct TaperedDuct

M g* Ml Ml Ml

103 kg/s kg/s kg/s

kg/s

3.520 0.113 0.087 0.048

4.245 0.121 0.089 0.075

4.329 0.123 0.091 0.091

4.665 0.121 0.092 0.114

4.945 0.126 0.002 0.135


and 1.5 m. A comparison for 1.15 m lift shows that airlift withstepped upriser pipes result in substantial improvement inwater output rate compared to the airlifts with uniformupriser pipe sizes of 18.75 mm as well as 32 mm. However, theaugmentation in performance is seen to be lower for the largersize of 32 mm. This is also substantiated for the higher lift of1.5 m, even though in the case of 32 mm stepped pipe, only athigher flow rates there occurred an improvement in theperformance level as is evident from Tables 5 and 6. Theseobservations can be explained by the fact that the water outputof airlift reduces for higher upriser pipe diameter or a higherlift. The actual percentage improvement on account of steppedpipes over uniform size pipe is indicated in Tables 5 and 6, foran air flow rate of 4.948 × 10�3 kg/s.

Table 3 reveals that the performance of stepped upriser pipewith starting size of 32 mm is lower than that compared to theuni-form upriser pipe, up to air flow rates of 4.665 × 10�3 kg/s.This is on account of the higher lift compounded by a lowersubmergence which tends to reduce the water output asdiscussed in the case of simulated results. However, at a higherflow rate of 4.945 × 10�3 kg/s, there seems to be improve-ment, which again is inline with simulated trends givenearlier. The optimum air flow rate however is not determinedin the present study, on account of experimental constraints.

Tapered Upriser PipesOn perspex made rectangular cross-section duct of 26.7 mm ×31.7 mm and another one inch square duct made of MS sheetare tested and compared with the performance of 0.5° and 1°tapered upriser pipes. At the lower lift of 1.15 m tested,0.5° tapered upriser pipe affected substantial improvement inthe water flow rate at all air flow rates used, as is evident inTable 2. However in the case of higher lift of 1.5 m, theimprovement is observed only at higher air flow rates. Inaddition, it can be inferred from Table 2 and Table 4 thattapered upriser pipe water output changes substantially as theair flow rate is changed whereas the uniform size upriser pipewater output is found to be somewhat insensitive to air flowrates in the range tested. This has got an important bearing

with regard to the existence of a different kind of two phaseflow structure in the tapered upriser pipe compared to theuniform size pipe.

The actual percentage improvement in the water flow rate onaccount of these tapered upriser pipes is given in Table 7.Again, it can be noticed here that the percentage improvementis seen to be lower at higher lifts. The smaller improvement athigher lift of 1.5 m can be attributed to the lower submergencelevel of 1.12 m against 1.47 m for a lift of 1.15 m. This again isin tune with the effect of submergence on airlift performanceas outlined in the simulated results.

Two-phased Flow Structure in the UpriserEven though most often the operating regime in airlift pumpfalls under slug flow regime, it can change from one operatingregime to another depending air flow rates. In general, it isobserved that, for uniform sized pipes change over occursfrom bubbly, slug, churn turbulent to annular flow regimes inthat order6. In the absence of information regarding two-phaseflow structure in tapered upriser pipes, videographic studieshave been carried out in this study to compare thehydrodynamics in uniform size and tapered pipes.

The slug height measurements in the uniform size and 0.5°tapered ducts are given in Table 8. It can observed that the slugheight is smaller in the case of 0.5° duct compared to that of arectangular cross-section duct of the same starting (base) size.This tends to indicate that the liquid hold up is more in thecase of the tapered duct. It is felt that the slug tends to becomeunstable in the case of the tapered ducts whereas in theuniform size duct at almost all of the air flow rates tested theflow regime observed resembled like that of annular withsmaller water slugs (hence, much larger air slugs) occasionallyinterspersed with air slugs. However, this type of flow is not

Table 5 Airlift performance with stepped upriser pipe

Mg Lift, Ml

× 103 m 18.75, mm 18.85, 25, PercentagePipe 32, mm Improvement

Pipes

4.948 1.15 0.081 0.149 83.27

4.948 1.50 0.052 0.089 70.09


Mg Lift, Ml

× 103 m 32, mm 32, 47, PercentagePipe 60, mm Improvement

Pipes

4.948 1.15 0.263 0.330 25.3

4.948 1.50 0.158 0.164 4.0

Table 8 Slug height measurement in airlift upriser pipes obtained fromvideography

Mg Slug Size for Slug Size for× 103 Rectangular 0.5° Tapered

Duct, cm Duct, cm

3.521 42 18

4.245 50 19

4.330 52 25

4.665 64 30

4.948 68 31


Mg Lift, Ml

× 103 m 1" 26.7 × 0.5° Percentage Square 31.77 Taper Improvement

MS mm2 Square for 0.5° Taper Tube Pers- Tube 1" 26.7 ×

pex Starting square 31.7 mm2

Size 1"

4.948 1.15 0.12 0.184 0.278 129.60 50.70

4.948 1.50 0.09 0.126 0.135 46.53 7.14


noticed for tapered ducts. This favourable flow structure, ie,more liquid hold up and less air hold up or void fractionappears to be main reason for the augmentation inperformance of pump with tapered upriser pipes achieved inthe present study.

CONCLUSIONTapered upriser pipes with smaller taper angle are found toresult in a higher improvement in the airlift performance. Thewater output in the case of tapered upriser pipes is observed tobe more sensitive to the air flow rate than that for uniformsized pipes. A tapered upriser pipe can be implemented inpractice by employing stepped upriser pipes in the successivesize ranges, but with smaller diameters where a betterperformance is noticed. The slug height is found to be smallerin the case of tapered upriser pipes than for the uniform sizedtubes. A tapered pipe with increasing size seems to result in ahigher water hold up, which as such is beneficial for airlift.

ACKNOWLEDGEMENTFinancial support for this work from Tamil Nadu StateCouncil for Science and Technology during 1997-1999 isgratefully acknowledged.

REFERENCES1. M Giot. �Three-phase Flow.� �Handbook of Multiphase Systems�, by GHetsroni (ed), Hemisphere.� McGraw Hill, Washington, 1982.

2. N N Clark and R J Dabolt. �A General Design Equation for Airlift PumpsOperating in Slug Flow.� AIChE Journal, vol 32, 1986, p 56.

3. K A Abed. �Theoretical Study on the Performance of Airlift Pumps.� TheInstitution of Engineers (India), MC, vol 77, 1977, p 202.

4. D J Nicklin. �The Airlift Pump : Theory and Optimization.� TransactionInstn Chemical Engineers, vol 41, 1963, p 29.

5. A H Stenning and C B Martin. �An Analytical and Experimental Study ofAirlift Pump Performance.� ASME Transactions, Journal of Engineering forPower, vol 4, 1968, p 106.

6. Y Taitel, D Bornea and A E Bucker. �Modelling Flow Pattern Transitions forSteady Upward Gas Liquid Flow in Vertical Tubes.� AIChE Journal, vol 26,1980, p 345.

7. A D Nenes, A N Markatos and E Mitsailis. �Simulation of Airlift pumps forDeep Water Well.� Can Journal of Chemical Engineering, vol 74, 1996, p 448.

8. F A Zenz. �Explore the Potential of Airlift Pumps in the MultiphaseSystems.� Chemical Engineering Progress, vol 89, 1993, p 51.

9. P Madan, R Gopi and A Venkata Ramayya. �Augmentation of Airlift PumpPerformance.� Proceedings of Tamil Nadu State Council for Science andTechnology, July, 1998, p 115.

10. E Anil Kumar, K R V Kumar and A Venkata Ramayya. �HydrodynamicStudies in Airlift Pumps with Tapered Upriser Pipes.� Proceedings of TamilNadu State Council for Science and Technology, August, 1999.

11. D Venkatanarasaiah and A Vankata Ramayya. �Effect of Adverse PressureGradient on Slug Flow in an Airlift Pump.� Proceedings of Asian PacificConfederation of Chemical Engineering Congress, Seoul, South Korea,August 16-19, 1999.


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