Polymer Drag Reduction in Large Pipes and Sewers: Results of Recent Field Trials

Polymer Drag Reduction in Large Pipes and Sewers: Results of RecentField TrialsR. H. J. Sellin and M. Ollis Citation: J. Rheol. 24, 667 (1980); doi: 10.1122/1.549598 View online: http://dx.doi.org/10.1122/1.549598 View Table of Contents: http://www.journalofrheology.org/resource/1/JORHD2/v24/i5 Published by the The Society of Rheology Additional information on J. Rheol.Journal Homepage: http://www.journalofrheology.org/ Journal Information: http://www.journalofrheology.org/about Top downloads: http://www.journalofrheology.org/most_downloaded Information for Authors: http://www.journalofrheology.org/author_information

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Polymer Drag Reduction in Large Pipes and

Sewers: Results of Recent Field Trials

R. H. J. SELLIN and M. OLLIS, Department o{ CivilEngineering, University of Bristol, Bristol BS8 ITR, England

Synopsis

Economically attractive applications for drag-reducing polymers include their usein sewers subjeet to occasional overload, If polymer dosing can be shown to providethe required extra discharge capacity in a particular case, then costly sewer rebuildingcan be delayed or avoided. A permanent and fully instrumented dosing station hasbeen constructed on a 305 mrn sewer in Bristol and preliminary tests showed that sewervelocities could be increased by up to 60% using 40 wppm Polyox WSR-301. Furthertrials have confirmed these results for WSR-30l but yielded lower velocity increasesfor a high-molecular-weight grade of polyacrylamide. Records of automatic dosingduring recent rain storms show a 25% increase in capacity in this sewer. Tests in a 760mm pumped main sewer at Bath show 200/0 drag reduction over the pipe's 8 km length.An emulsion-type liquid polymer containing 50% by weight active polymer was usedin this test,

1 INTRODUCTION

1.1 Earlier Studies

Numerous reports have been published of laboratory-scale experiments using drag-reducing fluids in pipes, but few experiments havebeen reported using either large pipe or open-channel flow systems.Reports containing the results of field trials are even less frequent.

The most extensive study so far of the behavior of drag-reducingfluids in large pipes is that ofTullis and Ramu.! who made their experiments in a 305 mm diam pipe 60 m long using solutions of PolyoxWSR-301 in water. Forester et a1.2 also obtained data in a 254 mmpipe using Separan AP30. Both these studies have shown that a dragreduction of 40---50% can be achieved in these pipe sizes althoughhigher polymer concentrations (40-60 wpprn) are required to do this

© 1980 by The Society of Rheology, Inc, Published by John Wiley & Sons, Inc.Journal of Rheology, 24(5), 667-684 (1980) 0148-6055/80/0024-0667$01.80


668 SELLIN AND OLLIS

than in smaller pipes. Tullis prepared a predissolved master solutionof the polymer before eaeh test, a procedure which has severe limitations in engineering applications where either large discharges (bulkof stored liquid) or long storage periods (deterioration by aging) arepossible. Forester used an emulsion suspended in a nonsolvent gelwhich he injected into the main water flow where it quiekly dispersedand dissolved. This method of polymer injection is expensive butavoids the two problems noted above.

The first large pipe use of polymers that were stored in powder formuntil immediately prior to the injection was made by Sellinf usingPolyox WSR-301 and water pumped through a 200 mm diam pipe,4.2 km long. The polymer powder was metered into a vortex mixingchamber (see Fig. 1) where it was wetted out grain-by-grain whilefalling through the air eore of a forced vortex produced by a small flowof water which was then pumped into the test pipeline using a positivedisplacement pump to minimize the risk of shear degradation. Theexperiments gave drag reductions in the range of 40-45% averagedover the whole pipe length.

Because unsureharged sewers behave hydraulieally as open channels, the effect of drag-reducing polymers on open-channel flow is ofsome interest. Results published by Sellin and Barnard.! Peterson

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DRAG REDUCTION IN PIPES AND SEWERS 669

et al.,5 and Derick and Logie'' show that there is no essential differencebetween drag reduction in open channels and that in pipes runningfull. It is interesting to read" that the intervention of a hydraulicjump, below a control structure, improves drag reduction downstream,presumably due to its vigorous mixing action.

The earliest detailed report of drag-reducing polymers being testedin sewers is that published by the Western Company in 1969. 7 Partof this report was concerned with the injection of a polymer slurry(WSR-301) into a manhole chamber on a 610 mm sanitary sewer usinga portable truck-mounted injection system. Water levels at a numberof downstream manholes were measured and the results show clearbut unquantified signs of drag reduction.

During 1970 a feasibility study was carried out by Hereules Inc, toinvestigate the effectiveness of polymers in preventing overflows fromcertain sanitary sewers in the County of Milwaukee, Wisconsin.Again portable injection equipment was used this time in a 1070 mmsewer and polymers from five manufacturers were used to make acomparative assessment. On the first six study days overflows werereported" to have been effectively eliminated but on the last five daystheir incidence was as high as on the control days. This change didnot correlate with the use of particular polymers and remains unexplained.

More recently a fully automatie polymer dosing station was constructed on the Bachman Creek sanitary sewer in Dallas, Texas.fThis system is installed on a 610 mm gravity sewer and uses powder-form polymer which it mixes into a slurry with water before injecting it into the sewer. The control system provided is very comprehensive and allows for variation in the polymer feed rate dependingupon the degree of overloading in the sewer. There are also a numberof safety features including a shut-down circuit which can sense apolymer blockage in the feed unit.

In 1978 a report-? appeared of a polymer-dosing system installedin the Tenafly pumping station in Bergen County, New Jersey, toaugment the peak discharge capacity of a 406 mm pumped mainsewer. Using Hercofloc 831 a 16% increase in capacitywas achievedfor a dosing rate of 40 wppm and a 29% increase for 100 wppm.Problems encountered with the very simple dosing system used couldbe overcome because the equipment was installed in a mannedpumping station.


670 SELLIN AND OLUS

1.2 Polymer-Sewer Studies at Bristol

Following the laboratory tests on circular pipes flowing part-fulljand the successful polymer injection tests in the long pipeline at Avonmouth'' a eost-benefit study was made to calculate the financialeffects of using polymers to achieve a 25% increase in capacity for awide range of sewer sizes and gradients. Full information is pubIished-! on an these points as weH as the effect of sewer roughness,polymer concentration, total dose duration, and different ways ofproviding the extra sewer capacity required. The results show thatpolymers can be very cost effective in medium sewer sizes (300-500mm), but tbat benefits are inversely linked to sewer gradient for thelarger sizes. Accordingly, when the opportunity arose in 1976 to builda trial automatie polymer dosing station on a 305 mm sanitary sewer(the Knowle sewer) for which a 25% increase in capacity was required,this was adopted as being the natural development in this line of investigation. Some early test results, together with a detailed description of the polymer injection system used, have already appeared,12 but more recent modifications to the system are describedin Sec. 3.1.

2 TEST PROGRAM

2.1 Outline of Test Facilities

The test results presented in this paper are the outcome of a program carried out during the summer of 1979, the objective being togain further experience in the use of drag-reducing polymers to increase the flow capacity of sewers. The following test sites wereused.

2.1.1 Knowle Seuier Site

This is a 305 mm diam gravity flow sewer serving a largely residential area on the Eastern edge of Bristol. Although designed as afoul-water sewer, it responds strongly to rainfall due to the combinedeffects of wrong surface connections and groundwater infiltration.A permanent polymer dosing station (shown schematically in Fig. 2)was constructed on the lower section of this sewer in 1977, and the trialsystem also incorporates two sewer water-level recorders and a recording electromagnetic flowmeter. The arrangement of the experimental components of this sewer is shown in Fig. 3.



f\.oieters arKf fU5eS

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Fig. 2. Polymer dosing building-arrangement of instruments and equipment.

2.1.2 Bath Pumped Main Sewer

As the Bath sewage treatment works is same distance from the city,all the gravity flow sewers concentrate on a pumping station not farfrom the city center from where the sewage is pumped out of a wet weilby up to three centifrugal pumps into a cast-iron pipe 760 mm in diamand 8 km in length. A temporary polymer injection system was installed in this pumping station (Fig. 4) and tests were carried out overone week during August 1979. Storm tanks at the pumping stationwere filled before each test and used to maintain steady pumping ratesduring the 5 hr test period,

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Fig. 4. Bath pumped main sewer-polymer emulsion injection system.

2.2 Description of Tests

The results obtained from these two sewer test facilities during thisperiod fall into the following categories.

2.2.1 Knowle Low Flow Tests

Manually controlled polymer injection tests were carried out in theKnowle sewer using the low (dry weather) flow available on the testdate (about 20% of sewer capacity). Two polymere were tested:Union Carbide's Polyox WSR-301, a high-molecular-weight poly(ethylene oxide) ofproven drag-reducing efficiency; and Allied Colloids' Magnafloc 156, a high-molecular-weight anionic polyacrylamidewhich had shown itself in laboratory tests to be only slightly less effective than the Polyox. Four tests, each at different polymer concentrations, were carried out for each polymer. These tests data,together with some obtained under similar conditions on an earlieroccasion, are shown in Figs. 5-7.

2.2.2 Knowle Storm Flow Operation

Storm event records of water level, polymer injection duration, andsewer discharge were obtained on a total of five occasions during Julyand August in the Knowle sewer using Polyox WSR-301. The polymer dosing periods were initiated by the water level in the sewer exceeding the preset activating level on the doser control system. Thedosing period was either the minimum value set-usually 15 min-orthe duration of the high sewer level, whichever was longer. Thedischarge and dose duration period for three of these storm events



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Fig.5. Knowle sewer low flow tests: Polyox WSR-301.

are shown in Figs. 8-10, and for the first of these the correspondingsection of the sewer water level (chart 1) and dosing event recorder(chart 2) are also shown superimposed on the discharge record.

2.2.3 Bath Sewer Tests

These test runs were carried out using an emulsion-dispersedpolymer which was premixed with water immediately before beingadded to the large pipe. This polymer emulsion, designated AlcomerllOL, contains 50% active material by weight of a high-molecularweight polyacrylamide copolymer of approximately 30% anioniccharacter and is manufactured by Allied Colloids Ltd. The tests weredesigned partly to determine how much drag reduction could be obtained by adding polymer to sewage in a pipe of this size and length,and partly to gain experience of emulsion-dispersed polymers which,so far as is known, had not been used before in a full-scale pipe flow in-


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stallation. The considerable simplification of the dosing plant andconsequent savings that would result if an emulsion could be usedeffectively in large pipes makes this a point of great interest. In theevent only two polymer concentrations were tested, each at seven flowrates.

3 POLYMER INJECTION SYSTEMS

Laboratory experiments with drag-reducing polymers are normallyconducted using pre-prepared master solutions ofthe polymer whichis either diluted to the test concentration in a holding tank immediately prior to use or added to the main water flow in suitable proportions as it approaches the test range. Applications in large pipeswould entail the preparation and storage of such large volumes ofmaster solution (normally 0.1%) that this approach is not practical.



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Fig.8. Sewer dosing system records-night of 30/31 July 1979.

To avoid the deterioration of the polymer during storage it is alsopreferable to delay the preparation of a solution as late as possible,Consequently an automatie makeup unit is required for preparinga polymer solution from a powder or an injeetion pump if the polymeris stored in emulsion form.

3.1 Polymer Injection at Sewer Manhole

The injection system adopted for the Knowle sewer used polymersupplied and stored as a powder and involved the preparation of anaqueous slurry immediately before use. The storage and injectionsystem is shown in outline in Fig. 2 and is deseribed in greater detail



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Fig.9. Sewer discharge record-evening of 16 August 1979.

elsewhere.P Early experience with this equipment had establishedthat the control system worked weIl but highlighted problems withthe polymer feed unit. Some caking of the powder was taking placeduring storage periods particularly in the screw feed tube. Theselumps were passed out of the horizontal feed tube without trouble butwere blocking the funnelleading to the eductor mixing unit which inturn led to apower blockage spreading back into the feed tube unit.This eductor has now been changed to a vortex mixing chamber, asshown in Fig. 1, because operation of this type ofmixing unit allowsa larger access hole for the polymer powder which can accommodatelumps. The vortex mixing chamber also uses a smaller amount ofprimary feed water to make up the polymer slurry. In this modifiedinjection system the powder feed funnel is made of metal and bothit and the horizontal feed tube above it are provided with trace heatingelements to avoid the risk of condensation.

3.2 Polymer Injection into a Rising Main

In order to dose the flow in a rising main it is necessary either to addthe polymer to the liquid in the wet weIl, so that it passee through the


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pumps into the main, or else to pump it directly into the main on thehigh pressure side of these pumps using a small positive displacementpump, as shown in Fig. 4. The second alternative is to be preferredas the polymer may suffer shear degradation passing through the maincentrifugal pumps. In the present tests the use of polymer emulsionmade its direct injection into the pumped main feasible and the monopump used was chosen to minimize shear degradation. Beforeentering the main sewer the emulsion was added to a small feed watersupplyas shown in Fig. 4 as this amount of premixing in a small-borepipe helped the inversion of the polymer emulsion.

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4 RESULTS

4.1 Knowle Low-Flow Tests

Figures 5 and 6 show values ofthe fiow cross-sectional area in.thesewer under the dry weather conditions occurring on the day of thesetests. The values of fiow area are based on water level measurementsobtained using an electrically driven dipper-type instrument mountedat an intermediate manhole situated 258 m downstream from thepolymer dosing station (see Fig. 3).

The discharge trend during the test period can be inferred from theupper plateau levels in these records (Figs. 5 and 6), which correspondto the intertest periods. The arrival of polymer-dosed fiow at themeasuring manhole is marked by a strong but short-lived surge(positive disturbance). The end of each test period is similarlymarked by the passage of a negative disturbance.

The positive wave that marked the arrival of the polymer-dosedfiow at the measuring manhole is due to the piston-like action of thislow friction fluid attempting to travel faster along the sewer (fixedenergy gradient) than the higher friction undosed fluid in front of it.The opposite effect occurs when the polymer dosing is stopped. Theeffect of these solitary surge waves during fiood fiow dosing periodsis not known.

In between these transient events aperiod of steady flow occurs,resulting in a low-level plateau. The equation of continuity can beused during these steady flow periods to enable mean velocities 10 becalculated from which the percent velocity increase values are calculated, which appear in Fig. 7. The vertical error bars in this figuretake account of the variation in the flow area data within each testperiod which makes the calculation ofmore precise values meaningless. Horizontal error bars representing variations in polymer feedrate and in sewer discharge measurement could equally well be calculated but these have not been shown because of the fiat responseof drag reduction to polymer concentration in the high-drag-reductionregion.

Similar tests were carried out two years earlier, using WSR-301only, and the data points so obtained are plotted in Fig. 7. It can beseen that the results obtained with the same polymer grade on thesetwo different occasions are quite consistent within experimentallimits. From Fig. 5 it is shown that test 1 was interrupted soon afterit commenced and the shutdown period was such that two distinct



positive waves were detected at the measuring point. Similarly, inFig. 6 the consequences of a short break in dosing toward the end ofthe test 3 period result in the negative wave before the break beingovertaken by the positive surge following the resumption of dosing.The results of test 4 in Fig. 6 suggest that the polymer feed rate wasnot being maintained, perhaps due to the natural anxiety of the operator to avoid being left with a hopper full of polymer when a changewas scheduled before the next test.

Magnafloc-156 was in a fine bead form rather than the more normalpowder and this had two unexpected effects. First the feed rate fromthe hopper was much high er than expected and this necessitated feedrecalibration after the tests and resulted in evalues for this polymerhigher than had been intended. Secondly, it is suspected that thebeadlets dissolved in water more slowly than the powder so that thepolymer solution process was less advanced at the measurementsection than was the case with the powder. Whatever the reason, itis clear from Fig. 7 that the polyacrylamide (M-156) is producing lessdrag reduction in this sewer than the poly(ethylene oxide) (WSR301).

4.2 Knowle Flood Dosings

The three traces shown in Fig. 8 were explained in Sec. 2.2.2. Thisparticular flood event was selected for plotting in this way becausethe relationship between the three traces shows very clearly the effectof polymer dosing on a storm hydrograph peak. The dose durationon this night was the minimum value set (15 min) as the water levelat the dosing manhole had started to fall very shortly after triggeringthe system. Both Figs. 9 and 10 show events where the dose durationexceeded the minimum value because surcharge conditions persistedlonger. The broken lines in Figs. 9 and 10 represent estimates of thesewer discharge capacity during their respective storms if polymerdosing had not been used.

For all these storm dosings the polymer feed system had been setto deliver 2 g/sec, which corresponds to polymer concentrations in thesewer in the range 15-18 wppm. The increase in discharge recordedby the flowmeter in each case for Polyox WSR-301 is in the range of20-25%, which gives an operating zone in Fig. 7 only slightly belowthe trend established by the low-flow tests. In light of these resultsthe feed system will be set in the future at 4 g/sec to see if a corre-



spondingly larger increase occurs in the recorded flow rate. Duringthe low-flow tests described in Sec. 4.1 the sewer discharge was approximately 1 cfs compared with the value of 5 cfs achieved withpolymer dosing of 2 g/sec under flood conditions.

4.3 Bath Sewer Tests

The tests conducted in this pipeline were more speculative as regards their expected outcome than the Knowle experiments. Figure11 shows values of friction coefficient A plotted against Reynoldsnumber for the flow in this 760 mm pipe.

Although the drag reduction achieved (10-20%) with 10-20 wppmwas less than hoped for, it is not out of line with previously publishedresults of a comparable nature. The only closely related project isthat in the Tenafly pumping station.!? where a freshly producedpolymer/water mixture was introduced directly into the inlet of oneof two centrifugal pumps supplying the pressure main sewer-anarrangement similar in some respects to the Bath one. The Tenaflypolymer-dosing system yielded a 16% flow increase at 40 wppm anda 20% increase at 100 wppm. The emulsion injection system (see Sec.3.2) worked well in the Bath tests and shows a number of advantagesover a dry-powder feed system. One of these is the absence of polymer dust which invariably causes problems with powder howevermuch care is taken to screen the apparatus from draughts and in fillingthe storage hopper. Any connection to a gravity feed sewer will always experience air currents because the flow in the sewer generatesair movement between manholes. The results obtained with the Bathsystem point toward a clear "onset" point for drag reduction whichfor this pipe is at a Reynolds number of 3 X 105 . Achieving onsetconditions appears to be more of a problem in large diameter pipes(over 500 mm) than in smaller ones.

5 CONCLUSIONS

The conclusions that can be drawn from these results may besummarized as follows:

(1) A range of drag-reducing polymers may be selected that willgive good results when used in sanitary sewers.

(2) Relatively low cost and reliable dosing equipment can be designed to operate automatically on sewer water level signals.



(3) Increases in flow velocity in excess of 60% have been obtainedby adding 40 wppm or more of Polyox WSR-301 to sanitary sewerdischarges under free surface flow conditions.

(4) Increases in sewer storm discharge capacity in the range of20-25% have been recorded using 15-18 wppm of the samepolymer.

(5) Test results available suggest that 30-40% increases shouldbe possible in the smaller sewers if sufficient polymer is used.

(6) Experiments carried out in a pumped main sewer 760 mmdiam by 8 km long demonstrated drag reduction of up to 20% usingan emulsion-type polymer suspension in a nonaqueous carrierliquid.

(7) Magnafloc 156, although a good drag reducer in clean waterin laboratory pipe systems, shows up less weHin sewers. Bead formpolymers seem to dissolve more slowly than powders, which is a disadvantage when used where critical sewer bottlenecks are short (lessthan 1 km).

Further work is required to develop more reliable automatie dosingequipment. The present equipment in the Knowle dosing stationis visited monthly to change chart rolls, to check for polymer blockages, and to refill the polymer power hopper if required. A morethorough maintenance period is needed annually including thecomplete emptying and cleaning of the polymer hopper and feedpassages. The adoption of liquid-carrier polymer systems wouldreduce the complexityand cost of present-day dosing equipment andmake mechanical breakdowns less likely. However, more development work is needed in this direction to ensure a trouble-free interfacebetween the emulsion and the water and also to see that rapid dissolving of the polymer follows its injection.

The financial and technical assistance ofthe Bristol City Engineer's Drainage Departrnent, the Wessex Water Authority, and Allied Colloids Ltd. is gratefully acknowledged both in the the work at Knowle and also at Bath.

References

1. J. P. Tullis and K. L. V. Ramu, "Drag reduction in developing pipe flow withpolymer injection," in l st International Conference on Drag Reduction. (B.H.R.A.,Cambridge, U.K., 1975), pp. G3-31.

2. R. H. Forester et al., J. Hytlronaut., 3 (1), 59 (1969).3. R. H. J. Sellin, "Experiments with polymer additives in a lang pipeline," in Ref.

1, pp. G2-19.4. R. H. J. Sellin and B. J. S. Bamard, J. Hydraul. Res., 8 (2),219 (1970).


684 SELLIN AND OLUS

5. J. P. Peterson et al., AlChE Symp. Sero 130,82 (1973).6. C. Derick and K. Logie, "Flow augmenting effects of additives an open channel

flows," report No. EPA-R2-73-238, V.S. Environmental Protection Agency, June 1973,74 pp.

7. Western Company, TX, "Polymers for sewer flow control," report No. WP-20-22,U.8. Federal Water Quality Administration, 1969, 178 pp.

8. Hereules Ine., "Polymer injeetion studies in a Milwaukee sewer," report presented to the Metropolitan Sewage District of Milwaukee, Wisconsin, 1971.

9. Private communication, Bachman Creek polymer dosing system, Dallas WaterUtility, TX, 1977.

10. D. H. Hull, Water Wastes Eng. (May 1978), p. 55.11. R. H. J. Sellin, Proc. Inst. Civil Eng., Part 2, 63, (1977).12. R. H. J. Sellin, J. Hydraul. Res., 16 (4), 337 (1978).

Received November 1, 1979Accepted as revised March 6, 1980


Documents

Polymer Drag Reduction in Large Pipes and Sewers: Results of Recent Field Trials