Design of Ship Loading Chutesto Reduce Dust EmissionsCraig Wheeler, Tobias Krull, Alan Roberts, and Stephen WicheCentre for Bulk Solids and Particulate Technologies, The University of Newcastle, NSW, 2308, Australia;craig.wheeler@newcastle.edu.au (for correspondence)

Published online 13 December 2006 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/prs.10180

This article presents an industrial case study toreduce dust emissions from a grain handling shiploader. The primary objective of the study was toreduce dust emissions to within acceptable environ-mental levels during ship loading. Several constraintswere imposed on the solution as the result of timeand budgetary restrictions, and the inability to add adust suppression agent to the grain for quality rea-sons. Although this article specifically deals withgrain, application of this technology is also equallysuitable for reducing dust emissions while handlingother particulate commodities.

A number of alternative loading chute configura-tions and delivery spoon (a discharge device) profileswere examined in a pilot-scale test facility. This arti-cle discusses a number of alternative solutions thatwere investigated during the course of the study andthe critical parameters of the final design. Testsshowed that it was not beneficial to decelerate theproduct stream to keep the relative velocity of the airstream over the grain below the minimum pickup ve-locity. Instead, it was found that concentrating theproduct stream and keeping the product velocity highproved to be more beneficial in reducing dust emis-sions. A reduction of 50% in dust emission wasachieved through the use of specifically designed con-stant-radius and parabolic-profile loading spoons.The product stream exiting the curved spoons wasfound to be concentrated and streamlined, resultingin the dust being contained within the productstream. � 2006 American Institute of Chemical Engi-neers Process Saf Prog 26: 229–234, 2007

1. INTRODUCTIONOne of the primary considerations in loading grain

into ships is to minimize dust generation because ofenvironmental and occupational health and safetyreasons. This article presents an industrial case study

(see Krull [1]) that involved a review of several load-ing options aimed at minimizing fugitive dust at theloading point. Details of this study can be found in arecent article by Wheeler et al. [2]. The projectevolved, primarily as the result of unacceptable dustlevels arising during the loading of particularly dustygrain. As a consequence of environmental restrictionsand planned decommissioning of the existing shiploader in the near future, several constraints wereimposed on the solution, including:

• Modifications to the ship loader should be ableto be undertaken in a minimum period of timeand, if possible, not involve any significant struc-tural work to the ship loader.

• Ability to be retrofitted to the existing booms ofthe ship loaders.

• Existing filling levels of ships are to be main-tained and the ship loader should be able toload grain beneath the top of the ship’s cargohold.

• Because of strict export quality requirements, noadditives including water were to be added tothe grain as a dust-suppression agent.

2. PROJECT OVERVIEWThe project involved a systematic review of the

existing grain handling terminal. The review identi-fied several key dust-generation points throughoutthe terminal. Conveyor transfer points were identifiedas areas requiring significant redesign to eliminateimpact points. Curved hoods and loading spoonswere recommended to minimize impact velocitiesand maintain the momentum of the grain throughoutthe transfer points, as noted by Roberts [3]. Of keyconcern was the dust generated at the dischargepoint of the grain loader. Tests were undertaken thatmonitored the dust levels on the ships’ decks duringloading. The dust concentration levels were measuredusing a DUSTTRAKTM aerosol monitor (TSI Inc.,Shoreview, MN), which is a portable laser-photometer� 2006 American Institute of Chemical Engineers

Process Safety Progress (Vol.26, No.3) September 2007 229

that measures airborne dust concentration. The initialtests served as a benchmark from which the results ofany future modifications can be compared.

Initially an ‘‘Options Study’’ was undertaken fromwhich potential solutions were brainstormed andranked according to a number of weighted criteria.To evaluate each of the proposed solutions a pilot-scale test facility was built. Although the absolutechange in dust concentration cannot be determinedaccurately from the pilot-scale testing, the test facilityprovides a means of experimentally quantifying thelikely relative reduction in dust levels arising fromeach solution.

The study involved building a scale model of theexisting ship loader chute and delivery spoon. Thescale model was tested, using a variety of representa-tive grain samples. Dust concentration levels weremeasured for the duration of the loading using theDUSTTRAKTM aerosol monitor. These initial testsserved as a benchmark from which all other solutionswere compared and evaluated.

3. CHARACTERIZATION TESTSThe first stage in the test program involved charac-

terizing a number of grain samples that were to betested in the pilot-scale test facility. The purpose ofcharacterizing the grain samples was twofold. First,the grain was characterized to determine the ‘‘as-sup-plied’’ properties of the grain samples so as to estab-lish the suitability of particular handling methodsand, second, to determine whether there was anydegradation of the sample during testing. Characteri-zation tests that were undertaken included a particlesize distribution, minimum pickup velocity, moisturecontent determination, and dustiness tests.

A particle size analysis was undertaken to deter-mine the size fractions for testing the minimumpickup velocity and to monitor the degradation ofthe grain during the testing. The ‘‘worst case’’ graintested showed 15% by volume was under 45 �m.This indicated that to significantly minimize dust gen-eration during loading, the relative velocity of the airstream over the grain must be kept below the mini-mum pickup velocity for this size fraction.

The characterization tests showed that for all sizefractions tested, the minimum pickup velocity wasfound to be very low (0.2 to 0.5 m/s). Because it willbe practically impossible to reduce the product exitvelocity from the spoon to values below this limitingvalue and maintain throughput, research focused onconcentrating the product stream and thus entrainingthe dust.

Dustiness tests were carried out on the grain at thesupplied moisture content. The dustiness tests servedto quantify the level of dust in the grain ‘‘as supplied’’to ensure the testing did not degrade the grain andeffectively increase the level of dust in the sample.The dustiness test rig, shown in Figure 1, consists ofa rotating drum in which the grain sample to betested is placed. The drum is rotated around its hori-zontal axis at a speed of 30 rpm for a period of5 min while an air flow rate of 170 L/m is drawn

through a hole in the drum lid, then through a hol-low drive shaft and a paper filter bag that collects thedust generated in the drum. The weight of the filterbag is measured before and after the test to determinethe quantity of dust collected. A dust number, calcu-lated using the formula given in the following equation,provides a relative means to compare samples:

Dust number ¼ Mb �Ma

Ms� 100;000 ð1Þ

where Mb is mass of the filter bag and dust, Ma ismass of the filter bag, and Ms is mass of the samplein the drum (all variables in grams).

4. CHUTE FLOW ANALYSISGiven the low minimum pickup velocity values

measured, research focused on concentrating theproduct stream and thus entraining the dust throughthe application of curved loading spoons. Roberts [3]analyzed the flow of a bulk material through hoodand spoon chutes within conveyor transfer stationsbased on a lumped parameter model. The basis ofthis methodology is detailed in Figure 2.

The governing differential equation for the velocityof the bulk material as a function of the angular posi-tion � for the spoon is

dv

d�þ �ev � gR

vðcos �� �e sin �Þ ¼ 0 (2)

where �e is the equivalent drag friction factor, given by

�e ¼ � 1þ C1

v

� �(3)

C1 ¼ KvBoHovo

ðBo � 2s tan�Þ2 1þ tan�

�

8>>:9>>; (4)

Figure 1. TUNRA bulk solids dustiness tester. [Colorfigure can be viewed in the online issue, which isavailable at www.interscience.wiley.com.]

230 September 2007 Published on behalf of the AIChE DOI 10.1002/prs Process Safety Progress (Vol.26, No.3)

where B0 is the initial chute width, v0 is the initial ve-locity, � is the convergence angle (see Figure 2), and� is the boundary friction coefficient.

For the case of a parallel chute, � ¼ 0 and C1 ¼Kv(v0H0/b). The pressure ratio Kv is based on the stressfield of the bulk solid in the chute and the measuredvalue of the effective angle of internal friction.

The above equations allow the velocity, as a functionof position as defined by � or s, to be determined forthe spoon. From the continuity of flow, the streamthickness variations around the chute profiles denotedby H can then be determined. Furthermore, the combi-nation of the velocity and normal pressure distributions,combined with the measured boundary friction charac-teristics, allow the chute wear profiles to be determined.

The analytical procedures readily permit variouschute geometric profiles to be examined with a viewto achieving optimum flow conditions with minimumwear. Charlton et al. [4] showed that a parabolicchute profile is the best practical solution for opti-mum flow with minimum abrasive wear.

4.1. Design Objectives and Dust ControlFor good dust control, severe impact zones and

unconstrained or dispersed flow of the bulk solidin its passage through the chute are to be avoidedat all costs. Fundamentally, the objective is toachieve streamlined flow of the bulk solid through-out the chute. The analytical procedures describedallow this performance objective to be met. Main-taining streamlined flow with minimal impactlosses ensures that the dust particles are substan-tially entrained with the stream of larger particles.In effect, the dust is encapsulated by the mainstream.

5. PILOT-SCALE TEST FACILITYTo experimentally quantify the relative change in

dust generation, likely through the use of differentloading methods, a pilot-scale test facility was con-structed (see Figure 3).

The test facility consisted of an enclosure to simu-late the comparatively large size of the ship’s cargohold relative to the size and diameter of the chute.The test enclosure consists of a rectangular sectionwith a lower wedge-shaped hopper for ease of emp-tying. The test enclosure is supported on four legs,resting on four load cells, to monitor the flow rate ofgrain into the enclosure. The front face of the enclo-sure is made from Perspex to enable visual observa-tion of the spoon/chute performance and dust con-centration. The test enclosure is well sealed to pre-vent dust from escaping during conveying trials. Avacuum system is attached to the bin to evacuate thedusty air from the enclosure after each test.

The testing procedure involved fitting a number ofdifferent chutes and discharge spoons designs to theinclined screw conveyor. The system facilitates therecirculation of the grain as it discharges from theinclined screw conveyor. The product flow rate is con-trolled by a slide gate on the storage bin (not shownfor clarity), which feeds onto the belt conveyor. A rela-tive measure of the effectiveness of each solution toreduce dust was undertaken by measuring the dustemission within the enclosure during each test. Dustconcentration levels inside the enclosure were meas-ured using a DUSTTRAKTM aerosol monitor at 2-s inter-vals for the duration of the tests and for about 5 minafter. During the emptying stage, the vacuum system isswitched on to evacuate the dusty air from the enclo-sure. After each test the dust collected by the vacuumsystem is returned to the grain sample.

6. RESULTS AND DISCUSSION

6.1. Alternative Spoon DesignsTo concentrate the material stream and thus

entrain the dust during discharge into the ship’s cargohold the use of long-radius spoons was investigated.Both constant-radius and parabolic profiles weredesigned using the methodology outlined in Section 4.

Figure 3. Pilot-scale test facility.Figure 2. Chute flow models [2]. [Color figure can beviewed in the online issue, which is available atwww.interscience.wiley.com.]

Process Safety Progress (Vol.26, No.3) Published on behalf of the AIChE DOI 10.1002/prs September 2007 231

6.1.1. Constant-Radius SpoonFigure 4 shows the pilot-scale constant-radius

spoon. The spoon was manufactured with a stainlesssteel base and Perspex sides to visualize the flow.The spoon was installed so that the outlet of thespoon is at the same height from the bottom of thebin compared to the original spoon.

Initial observations showed a significant reductionin airborne dust within the enclosure. As opposed tothe original spoon, the product stream exiting thespoon was very well defined, with a sharp boundarylayer between the air and grain. Having determinedthe product flow rate and the height of the grainstream, we estimated the exit velocity of the grain tobe about 4.5 to 5 m/s. These results compare favor-ably with the predicted velocity profile and streamthickness, calculated using the methodology outlinedin Section 4. The increased component of the exit ve-locity in the horizontal direction compared to theoriginal spoon improved the loading reach of thespoon and therefore reduced the required inclinationangle of the vertical chute.

The average dust concentration level for the seriesof tests showed a reduction in dust of >50% com-pared to that of the original loading spoon. Thereduction in dust levels were attributed to the con-centrated product stream entraining the dust.

6.1.2. Parabolic SpoonFigure 5 shows the pilot-scale parabolic-profile

spoon. Once again, the spoon was manufacturedwith a stainless steel base and Perspex sides to visual-ize the flow. The spoon was installed so that the out-let of the spoon is at the same height from the bot-tom of the bin compared to the original spoon.

The product exit velocity and stream thicknessfrom the parabolic spoon also compared favorably tothe predicted values. From visual observations, bothspoons perform similarly, with the parabolic spoonreaching further because of its extended horizontaldimension. Measured dust concentration levelsshowed an average level representing a reduction of

>50% of the original spoon, making it equally suc-cessful compared to the constant-radius spoon.

Both spoon profiles resulted in the grain exiting ina highly concentrated and fast moving stream, furtherresulting in increased reach with minimal dust emis-sions. Testing showed that ideally the stream shouldbe directed into the side of the stockpile during fill-ing so the dust remains encapsulated within the pile,rather than falling from above, as was the case withthe original loading system.

6.1.3. Further Spoon OptimizationAs an additional modification to the constant-ra-

dius spoon, an air-restrictive flap was designed andincorporated into the design. For a given grain massflow rate, the flap is designed to rest just above thegrain stream exiting the spoon. The flap consists of athin steel backing plate with attached reinforced rub-ber lip to allow for automatic adjustment for suddenchanges in the product mass flow rate. A schematic

Figure 4. Constant-radius spoon. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]

Figure 5. Parabolic spoon. [Color figure can beviewed in the online issue, which is available atwww.interscience.wiley.com.]

232 September 2007 Published on behalf of the AIChE DOI 10.1002/prs Process Safety Progress (Vol.26, No.3)

of the constant-radius spoon with flap is shown inFigure 6. From visual observation, the flap success-fully diverts the air stream into the product streamexiting the spoon outlet. This was confirmed byreduced dust concentration measurements comparedto the constant-radius arrangement without the flap.An average dust concentration equivalent to a further30% reduction in dust was measured compared withthe design without the flap.

To examine the risk of the flap resulting in a pres-sure increase within the vertical chute, the top sectionof the chute was vented to atmosphere, simulatingthe conditions onsite. The tests were repeated and novisible dust was noticed escaping from the top vent.

6.1.4. Tapered ChuteA tapered chute was designed and manufactured,

based on the principle that a gently tapered reductionof the cross-sectional area of the chute leads to amore concentrated stream. By selecting a small incli-nation angle to the vertical axis, impact zonesbetween the stream of grains and the wall are signifi-cantly reduced. An area reduction also forces thestream of dust-laden air to mix with the grain stream,thus reconcentrating the stream and reducing theamount of mobilized dust. The size of the area reduc-tion has to be carefully selected based on theexpected product velocity and tonnage to avoidchoking the chute. A safety factor was included toallow for sudden increases in the tonnage and a 60%cross-sectional area reduction resulted.

Figure 7 illustrates the assembled tapered chute incombination with the constant-radius spoon. Again,the assembled chute section is attached to a straightvertical piece of PVC pipe, maintaining the same dis-tance between the outlet of the spoon and the bot-tom of the test enclosure.

During testing the product stream appears to rec-oncentrate even before it collides with the wall. Thesloped wall is believed to guide the product streamto the opposite side, creating an ‘‘air buffer.’’ Thetapered section successfully assists in reconcentrating

the stream and minimizing the amount of dust-ladenair traveling with the product stream. The productstream exiting the spoon is highly concentrated andfast moving, resulting in an increased reach with min-imal dust emissions. Ideally, the stream should bedirected into the side of the stockpile so the dustremains encapsulated within the pile, rather than fall-ing from above.

Testing on the tapered chute was undertaken incombination with the constant-radius spoon, withand without the air-restrictive flap. The average dustconcentration level recorded showed only a slightimprovement to the constant-radius spoon with theair-restrictive flap.

6.2. Vertical Chute DesignsA variety of cascade-type chutes were also

designed and tested with the aim of replacing thevertical drop chute. The principal aim within this ele-ment of the project was to slow the velocity of grainduring the vertical drop, rather than allowing thegrain to freefall. Given the constraints of the existingship loader and the implied design considerationsseveral cascade-type configurations were explored.However, from the experimental investigation it wasconcluded that the dust emissions were not reducedbut shifted. The results from the tests undertaken onthe cascade-type chutes can be summarized as fol-lows:

• Overall dust emissions are higher than having astraight vertical fall as a result of the increasednumber of impact zones.

Figure 7. Tapered chute with constant-radius spoon.[Color figure can be viewed in the online issue,which is available at www.interscience.wiley.com.]

Figure 6. Constant-radius spoon with air restrictionflap [5].

Process Safety Progress (Vol.26, No.3) Published on behalf of the AIChE DOI 10.1002/prs September 2007 233

• Problems of dust emissions are moved from thespoon outlet to the head chute.

• Outlet velocities are decreased, thus reducingthe reach of the spoon.

7. CONCLUSIONSThis article has presented an industrial case study

that experimentally investigated a number of loadingspoon and chute configurations with the aim ofreducing dust emissions from a grain handling shiploader. A pilot-scale test facility was developed forthe purpose of benchmarking the existing ship load-ing arrangement and to evaluate proposed solutions.A series of tests were undertaken to determine theinfluence of curved loading spoons. A reduction of50% in dust emissions was achieved through the useof a specifically designed constant-radius chute,whereas similar results were achieved with a para-bolic-profile spoon. The product stream exiting thecurved spoons was found to be concentrated andstreamlined, resulting in the dust being containedwithin the product stream. Furthermore, the spoonsresulted in the product having a greater horizontalvelocity, resulting in the grain being able to be dis-charged further, thus improving the filling capacity ofthe ship loader.

A further reduction was accomplished by incorpo-rating an air-restrictive flap, which forces the dust-laden air traveling with the grain stream back into thestream. This modification resulted in a further 30%dust reduction in addition to the reduction alreadyachieved by the constant-radius spoon. A taperedchute was also developed as a lower-maintenance al-ternative to the air-restrictive flap in the constant-ra-dius spoon. This type of chute was found to be anequally efficient alternative. Because of its steepslope, impact zones are minimized and the dust-laden air is forced to merge with the product stream,minimizing dust emissions.

Various cascade-type chutes were also tested,although they proved to be unsuitable in reducingthe overall dust emissions. High-impact zones at thetop of the cascade chutes generated more dust com-pared with an unrestricted vertical fall. Dust emis-sions were moved from the spoon outlet to the headchute. Furthermore, outlet velocities were decreased,thus reducing the effective reach of the spoon. At thetime of writing this article a prototype loading spoonhad been successfully tested on the ship loader.Results from initial testing showed reductions in dustemissions of the order of 50–60% from that of theoriginal loading system, confirming the pilot-scale testresults. Minor modifications to the loading spoonhave been planned before fitting the spoons to allship loaders at the terminal.

LITERATURE CITED1. T. Krull, Grain terminal dust suppression, Report

No. 6371, The University of Newcastle ResearchAssociates (TUNRA) Bulk Solids, Callaghan, NSW,Australia, April 2004.

2. C.A. Wheeler, T. Krull, A.W. Roberts, and S.J.Wiche, Reducing dust emissions from grain han-dling ship loaders, Proc 40th Annual Loss Preven-tion Symposium, April 2006, pp. 549–561.

3. A.W. Roberts, Chute performance and design forrapid flow conditions, Chem Eng Technol 26(2003), 163–170.

4. W.H. Charlton, C. Chiarella, and A.W. Roberts,Gravity flow of granular materials in chutes: Opti-mising flow properties, J Agric Eng Res 20 (1975),39–45.

5. A.W. Roberts, C.A. Wheeler, T. Krull, and S.J.Wiche, Dust reduction in delivery of particulatecommodities, Provisional Patent filed with Austra-lian Patent Office, US Patent Office, and Cana-dian Patent Office, April 2006.

234 September 2007 Published on behalf of the AIChE DOI 10.1002/prs Process Safety Progress (Vol.26, No.3)