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The Florida Institute of Phosphate Research was created in 1978 bythe Florida Legislature (Chapter 378.101, Florida Statutes) andempowered to conduct research supportive to the responsibledevelopment of the state's phosphate resources. The Institute hastargeted areas of research responsibility. These are: reclamationalternatives in mining and processing, including wetlandsreclamation, phosphogypsum storage areas and phosphatic claycontainment areas; methods for more efficient, economical andenvironmentally balanced phosphate recovery and processing;disposal and utilization of phosphatic clay; and environmentaleffects involving the health and welfare of the people, includingthose effects related to radiation and water consumption.
FIPR is located in Polk County, in the heart of the central Floridaphosphate district. The Institute seeks to serve as an informationcenter on phosphate-related topics and welcomes informationrequests made in person, by mail, or by telephone.
Research Staff
Executive DirectorRichard F. McFarlin
Research Directors
G. Michael Lloyd Jr.Gordon D. NifongSteven G. RichardsonHassan El-ShallRobert S. Akins
-Chemical Processing-Environmental Services-Reclamation-Beneficiation-Mining
Florida Institute of Phosphate Research1855 West Main StreetBartow, Florida 33830
(863) 534-7160
FINAL REPORTFIPR DOE CONTRACT # 88-04-044
DEVELOPMENT OF A POSITIVE FEED SYSTEM (PFS)FOR
MATRIX TRANSPORTATION SYSTEM
GIW Industries, Inc.5000 Wrightsboro Road
Grovetown, Georgia 30813
September 15, 1990 to June 30, 1991
Performed
for
The Florida Institute of Phosphate Research1855 West Main Street
Bartow, Florida 33830
Thomas W. Hagler, Jr.Project Director
DISCLAIMER
The contents of this report are reproduced herein as receivedfrom the contractor.
The opinions, findings and conclusions expressed herein are notnecessarily those of the Florida Institute of Phosphate Research,nor does mention of company names or products constitute endorse-ment by the Florida Institute of Phosphate Research.
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FINAL REPORT PERSPECTIVE
Florida Institute of Phosphate ResearchRobert S. Akins - Research Director, Phosphate Mining
The Florida phosphate mining industry has long recognized thepotential to reduce significantly the energy required todisaggregate and transport matrix from the mine pit to the washplant. Current practice has evolved from the techniques employed inthe old land pebble mine operations of the early 1900's. Thepractice utilizes high pressure water monitors or guns todisaggregate the matrix and drift it through a stationary grizzlyinto the intake of a large diameter, skid-mounted centrifugal pumplocated well above the pit liquid level. The slurry mixture istransported in large diameter pipes several miles at high velocityfor further treatment. Over the years, improvements have been madein the equipment and controls and economies achieved through use oflarger scale operations, but the basic concepts remain unchanged.Although energy intensive, the technique has proved to be bothflexible and reliable and well suited to the wide range of matrixcharacteristics encountered in local phosphate deposits.
As recently as the mid-80's, the cost of power used in this.sequence of operations amounted to one quarter to one third of thetotal direct cost of mining. In a typical operation, 15 to 20 KWHof electrical energy is required to disaggregate and transportenough matrix to produce one ton of finished phosphate rock. Forthe Central Florida Mining District, this represents over 800million KWH per year.
Reducing the amount of water used to transport the matrixoffers one possible way to reduce the energy requirements. In themid-80's, the typical mine pumped slurry at 30% to 35% solids.This requires 2 tons of water for every ton of matrix. In 1987,the Institute funded a study with GIW Industries, Inc. (the largestsupplier of slurry transport pumps to the Florida phosphate miningindustry) to develop software for the design of the slurrytransport systems. During the course of this work, GIW tested anddemonstrated in pipelines up to 20 inches in diameter, that matrixcould be pumped at up to 50% solids. This requires only one ton ofwater for each ton of matrix and the power required was reduced bymore than one third.
Another potential area for energy savings is in the reductionor elimination of the high pressure water used to disaggreate thematrix after mining. The combined benefits of higher percentsolids and reduced gunning water could reduce the overall powerrequirements by one half.
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In present practice, however, the combination of higherpercent solids and the location of the pit pump would greatlyincrease the risk of cavitation and catastrophic pump failures. Inaddition the percent solids is frequently determined by the amountof gunning water required to disaggregate the matrix. Theseconstraints have discouraged adaption of potential energy savingtechniques.
To overcome these obstacles, GIW proposed the adaption ofcutterhead dredge technology to disaggregate the matrix and providea positive feed control into the pipeline system. To assist inimplementing the development of these concepts, GIW enlisted theaid of one of the world's leading dredge design firms, ScheepswerfStapel B.V., Spaarndam, The Netherlands. Jointly, the firmsproposed to prepare a preliminary design and feasibility study ofa 'land based, cutterhead dredge'. To assist in paying for thestudy, the Institute sought and obtained joint funding from theState of Florida's Governor's Energy Office through competitivebidding administered by The Florida Agricultural and MechanicalUniversity/Florida State University's Department of ChemicalEngineering.
This report is the product of that cooperative effort.
It is the Institute's hope and expectation that thisengineered design study will encourage the commercial applicationof the concepts and ultimately lead to significant reductions inenergy requirements for the industry -- a result that will benefitthe industry, the environment and, hence all the citizens of theState of Florida.
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LIST OF FIGURES
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General arrangement plan number DOS 291-00.00-001 . . . . . . . . . . 27
Layout of dump pit with PFS . . . . . . . . . . . . . . . . . . . . . 28
Cutter suction dredger with articulated ladder . . . . . . . . . . . 29
Start of dredging operation . . . . . . . . . . . . . . . . . . . . . 30
PFS Schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Dump pit excavation . . . . . . . . . . . . . . . . . . . . . . . . . 31
Obstruction of swing motion . . . . . . . . . . . . . . . . . . . . . 32
Proper excavation method . . . . . . . . . . . . . . . . . . . . . . 33
Cutter pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Control of cutterhead position . . . . . . . . . . . . . . . . . . . 34
Control of cutterhead position . . . . . . . . . . . . . . . . . . . 35
Pump power control . . . . . . . . . . . . . . . . . . . . . . . . . 36
Profile dredging . . . . . . . . . . . . . . . . . . . . . . . . . . 37
State-of-the-art of cutter suction automation . . . . . . . . . . . . 38
Control of PFS . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Multiple speed curve E26-91 of l8x20LHD33 pump . . . . . . . . . . . 40
Plot of energy versus tons per hour for matrixin a 17-1/4" diameter pipeline . . . . . . . . . . . . . . . . . . . 41
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I. INTRODUCTION
In September 1990, GIW Industries, Inc. (GIW) entered into a contract withthe Florida Institute of Phosphate Research and the State of Florida toprepare a design for a so called positive feed system (PFS) for operationin a phosphate matrix pipeline transportation system and for an economicevaluation of the benefits of that system in a typical Florida mine.
The following report outlines the basic design of the system as proposedand evaluates its performance in relation to the economic benefits andreturn on investment in a manner suitable for mining companies to assessits feasibility.
II. EXECUTION OF THE WORK
Under the direction of the principal investigator, Mr. T. W. Hagler,Jr.,the PFS unit evaluation and design work was subcontracted to worldrespected dredge manufacturers Stapel of Spaarndam, The Netherlands. Theeconomic evaluation and overall coordination of the work was performed byGIW and its consultants. In particular Dan Lynch, a highly respectedphosphate industry consultant, provided a review of the basic concepts andassisted with the economic evaluation.
The STAPEL report was prepared as two volumes, the first covering thedesign evaluation and the second providing the technical specification forthe PFS unit. The first volume was incorporated directly into this reportand the second is included as an appendix of this report.
III. BACKGROUND
Mining practice in Florida employs large walking draglines with 45 to 65cubic yard buckets to remove the overburden and excavate the underlying oreor "matrix".
Matrix is a geological term used to describe the unconsolidated mixture ofclay, sand and phosphate rock minerals which are present in approximatelyequal proportions. The overburden is cast to one side of the dragline intothe previously mined out area and the matrix to the other side into the"slurry pit".
After mining, the matrix must be transported to a central washing plant forfurther processing in order to separate the phosphate rock from the wastesand and clays.
Florida matrix is transported from the mine to the washing plant as aslurry by pipeline. Pipelines of 16", 18" and 20" in diameter and up toeight miles in length are used with as many as ten pumps in series. Thesebooster pumps are powered by 1000 to 1500 hp electric motors.
The Positive Feed System being proposed embodies a new PARADIGM. PARADIGMmay be defined (reference 2) as follows: "A PARADIGM is a set of rules andregulations that: (1) Defines boundaries; and (2) tells you what to do to
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be successful within those boundaries." Suggested, then, is development ofa Positive Feed System that in effect is a "PARADIGM shift"...which bydefinition is... "A change to a new game, a new set of rules" (reference 3).
The present method of mining (old PARADIGM) is based on the following "setof rules" to accomplish the objective:
1. Fixed flow high pressure hydraulic gun water in pit. "Guns" break upsolids and create a "quick sand" river of solids which will flow 40 to60 feet distance through a grizzly to the pit pump suction inlet.
2. Transportation of solids controlled by total pipeline flow velocity.Booster pump speed control in sequence increases or decreases to varythe transportation rate of solids.
This current method with pit pump located high above liquid level restrictsslurry concentration due to pit pump cavitation. Maximum averageconcentrations possible here are in the range 25-35% by weight and fallwell below those of 45 to 50% (along with the enormous energy savings)shown to be possible in reference 1.
The Positive Feed System is a different concept in which several basicchanges are proposed so as to....in effect....change the phosphate matrixpit operation from the present system to one which much more closelyduplicates the most advanced proven dredge technology.
As noted above, the "Positive Feed System" is a completely differentsystem... or new PARADIGM. With a new PARADIGM, there is a new "set ofrules". The "Positive Feed System" PARADIGM may be described as follows:
1. Water level in pit. Low pressure water....valved to maintain constantlevel. High pressure water boosted at pit when needed to undercutsolids bank buildup.
2. Matrix pipeline flow velocity. Booster pump speed control in sequenceso as to maintain a constant pipeline velocity regardless of pipelinefriction resistance. Pipeline velocity "set point" for minimum safe"carrying velocity" is controlled from a flow meter. A higher veloci-ty may be selected as desired by pit operator.
3. Density control. The positive feed system "land dredge" ladder swingsat a constant rate...back and forth...through an arc. The density isvaried by lowering or raising the ladder into or out of the matrix.If sudden high density occurs such as would be the case of a bank"cave in," a vacuum relief valve on the pump suction opens to admitdilution water.
The above described dredge type automation as applied on conventionaldredges has been utilized in Holland for approximately ten years. Thecomputer software for this computer controlled system will be adapted to
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accomplish the modified goals as applied to a land-based phosphate matrixpumping system.
IV. OBJECT OF THE PFS UNIT
The object of the PFS unit is to consistently and reliably load the highestpracticable concentration of phosphate slurry into the matrix pipeline.
It should do this in a controlled manner as automatically as practical suchthat the operation should be cavitation (and water hammer) free and asenergy efficient as possible.
V. DESCRIPTION OF THE PFS UNIT
The PFS unit is a piece of equipment that will replace or change the use ofseveral items at the phosphate matrix pit. The present high head pit pumpwill act as a counterweight to the PFS unit and become the first boosterpump. High pressure gun water used to break up solids and drift matrix tothe pump suction will be eliminated and replaced by low pressure water. Acrown type dredge cutterhead mounted on a swinging ladder will replace thegrizzly. This unit will break up or reject oversize solids and will "forcefeed" a submerged low head feeder pump which enhances its ability to handlehigher concentration of solids. High pressure "gun" water will be boostedat the pit as needed to undercut an above-water bank of matrix in the eventthis should occur.
Conceptually the unit is that of a dredge with a ladder pump, adapted foruse on land in conjunction with a dragline and a water filled dump pit.
The ladders ability to swing through an arc and articulate up and downallows the cutterhead to be brought to the matrix that is dumped by thedragline on the slopes of or inside the matrix pit. The water required tokeep a constant liquid level in the dump pit can then be supplied at lowpressure.
Physically, the PFS as shown in Figure 1 consists of a pontoon shaped skidsupporting the dredge system comprising the rotating cutterhead, suctionpipe, ladder (feeder) pump, discharge pipeline and instrumentation tomeasure flow and density.
The cutterhead, suction pipe, ladder pump and part of the discharge pipeare mounted on an articulated dredge ladder formed by three hinge connectedparts, the outer ladder, the inner ladder and the turret.
The outer ladder and the inner ladder are hinge connected allowing forrelative motion around a horizontal axis.
The turret and the outer ladder are hinge connected in a similar way againallowing for a hinge motion around a horizontal axis.
The turret is connected to the skid and allows for rotation over 225°around a vertical axis (an advantage for mobility).
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All hinge motions are powered by means of hydraulic cylinders.
Also arranged on the skid are the discharge pipeline connection to thebooster pump, the electric-hydraulic power pack serving the PFS'smachinery, a number of auxiliary pumps and two adjustable side supports(out riggers). The latter forming the means to stabilize the PFS whiledredging.
When the PFS is in operation a normal skid mounted booster pump, will beconnected to the aft side of the PFS skid. The connection is made by twohorizontal pins. The weight of the booster pump balances and helps tostabilize the PFS while dredging.
An automatic control system monitors and controls the mixture flow anddensity in the discharge pipeline.
All controls and the like are to be such that the unit can be operated byone person..... as opposed to two operators as required by the present daysystem.
A technical specification of the PFS unit is included as Appendix 1.
VI. PRODUCTION CAPABILITY OF PFS UNIT
The production capability and specific limits of the PFS unit were estab-lished with industry input at a FIPR Mining Technical Advisory Committeemeeting held 18 September 1990 at the FIPR offices. Representatives of allthe mines were invited.
The initial unit design was set to have a production capacity of 1600 to2130 US dry tons per hour. This production capacity may be achieved with aPFS unit that incorporates a feeder pump which has an 18" discharge and a20" suction located well down the ladder. Beginning with the first matrixbooster pump, the remaining matrix pumping system can be either 18 inchesor 20 inches pipeline size.
If the matrix booster pumping system is based on 17-1/4 inch insidediameter pipeline, then design flow controlled at 14 feet per second(10,200 USgpm) at a slurry density of 45 percent solids by weight willyield a transportation rate of solids of 1600 tons per hour. In the caseof a 19-1/4 inch inside diameter pipeline, if the flow is controlled tomaintain 15 feet per second (13,500 USgpm) with a slurry density of 45percent solids by weight, then in this case the resulting transportationrate of solids will be 2130 tons per hour.
Either slurry density noted above may go as high as 49 percent solids byweight at the specified (minimum safe) velocity in either pipeline withoutposing any difficulty as verified by FIPR DOE Contract No. 87-04-037R.Higher flow velocities may be selected in either matrix pipeline ifnecessary; however, this would reduce specific energy savings as well assavings expected from low wear rates in the matrix pump and pipeline, bothof which are given in the economic evaluation of the Positive Feed Systemunder Section XI.
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It was specifically requested that the PFS unit be skid mounted with amaximum ground pressure of 10 psi and that the maximum weight of the unitshould not exceed 50 tons.
A design feature of the system is that only low pressure water would besupplied to the pit but that special pumps on the PFS unit will providehigh pressure water for jetting and flushing of the dredge pump gland andthe bearing of the cutterhead.
VII. GENERAL OPERATION OF THE PFS UNIT
The PFS unit is initially installed on the side of a pre shaped dump pitand connected to the first booster. When power and water supply have beenconnected, the PFS unit with swinging ladder and cutter carries out thefinal shaping of the dump pit.
Figure 2 shows a layout of a dump pit, the position of the PFS and thefirst booster station and three dragline positions.
When the dump pit shape is established, the dragline starts dumping matrixin the dump pit, e.g. dragline position 1 corresponds with matrix dumpingon the right hand side of the dump pit.
When sufficient matrix is in the pit the PFS starts operation.
The creation of an initial stock pile of matrix in the dump pit isnecessary so that the output of the PFS will be compatible with the averagedragline production. In order to ensure continuous PFS production astockpile is necessary to cope with irregularities in the dragline matrixdigging and dumping rate.
The ladder of the PFS swings at a constant swing speed. By doing this, thedragline operator may time matrix dumping so that the bucket approaches thedump area as the PFS ladder is moving away from the dump area.
Slurry density control is obtained by raising or lowering of the swingingladder in response to density measurements. It is intended that the systembe a fully automated process; however, manual operation can be used whennecessary.
When the area to be mined from dragline position three is exhausted thedump pit has to be abandoned and relocated in the normal present daymanner.
VIII. OPERATIONAL EVALUATION
In order to identify the basic mechanisms involved and to verify that thePFS unit as designed will perform as needed an analysis and comparison ismade of material removal and injestion with existing cutter suction dredgesand the proposed PFS unit design.
This comparison of operational aspects is intended to identify a number ofsimilarities and a number of differences between the cutter suction dredgeand the PFS. The design of the PFS and its components will, as a result,
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be made to differ from the design of the cutter suction dredge in order toaccount for those operational differences.
In a normal cutter suction dredge as shown in Figure 3, the pontoon (8)supports the dredging system consisting of the cutterhead (l), suction pipe(2), dredge pump (3) and discharge pipe (4). The cutterhead, suction pipeand dredge pump are arranged on the cutter ladder (5).
The cutter ladder is hinge mounted to the pontoon and can be moved sideways (around vertical axis) and vertically (around horizontal axis) bymeans of hydraulic cylinders.
The anchor system consists of two spuds (6) mounted in slides and a singlestationary spud (7).
In normal operation, the dredge cutterhead is pressed against the face bythe ladder mechanism. When a full cutter swing is completed the workingspud slides are activated and the pontoon moves forward over the requiredlength (e.g. 60% of the cutter diameter). When the spud slides reach theend of their stroke the stationary spud is lowered. The stationary spudand one of the working spuds maintain the pontoon position while the otherworking spud is recycled over the working stroke. The spud is then droppedand the second working spud is also recycled.
When starting the dredge floats towards the dredge location. Once onlocation the dredging depth is gradually increased while the dredge ismoved forward.
The limiting parameter for the slope of the profile to be dredged is therequirement that the lower side of the ladder remains free from contactwith the bottom as shown in Figure 4.
When operating on a full face, the cutterhead describes a horizontal arcthrough the face. After each arc the cutterhead is lowered and a next arcis described in an opposite direction. The height in the face where thecutterhead starts the removing of the material depends on the characteris-tics of the material to be dredged.
In free running material it is common practice to remove the material atthe bottom level only, material supply being guaranteed by gravity feed.When too much material falls from the face on the cutterhead the dredgerretreats from the face and starts working towards the face from this newposition.
In this case, the ability to step backwards is also required to clear thematerial spilled during dredging, because that is the only way that thematerial that falls behind the cutterhead can be removed.
In the case of the PFS unit shown schematically in Figure 5, the skid 6supports the dredging system consisting of the cutterhead 1, suction pipe2, dredge pump 3, and discharge pipe 4. The cutterhead, suction pipe anddredge pump are arranged on the cutter ladder 5.
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The cutter ladder is hinge mounted to the skid and can be moved sideways(around a vertical axis) and in the vertical plane (around a horizontalaxis) by means of hydraulic cylinders. The discharge pipe 4, is connectedto the feeder pipeline to the treatment plant. Booster pump(s) 7, pump thematerial to the treatment plant.
In normal operation with the PFS unit it is planned that the cutterhead islowered into a pre-arranged dump pit. The dump pit is filled with water.Phosphate matrix is supplied to the dump pit by a dragline. Draglinebuckets are emptied on the slopes and into the dump pit. When the draglinedumps its load the ladder is moved away from the dump area. Material isdredged from the dump area when the dragline bucket is gathering a new loadof material.
The dump pit for the PFS unit will have to be arc shaped to the radius ofthe cutter and to have steep side slopes and a sloping bottom. This shouldnot restrict the draglines dumping of material.
Steep side slopes are required to obtain an optimum matrix inflow to thedeeper part of the dump pit.
A sloping bottom profile or a deep spot in the bottom will also be requiredto provide a disposal area for stones, lumps, etc., too big to be handledby the cutterhead.
An initial pit will be dug by the dragline or other type of excavator tosubmerge the ladder pump and cutter. The better it approaches the shape ofFigure 6 the less final shaping by the PFS unit will be necessary.
On the initial startup, the ladder of the PFS is lowered into the pit untilthe cutterhead touches the bottom. From that moment, the ladder starts itswing motion.
A cutter excavates with its sides. When swung at a certain depth materialmeeting the side of the rotating cutterhead will be excavated from the faceand sucked away by the dredge pump.
When a full swing at the given depth can be made without meeting obstruc-tive material the depth of the cutterhead can be increased.
Increasing the depth without moving the cutterhead towards/or away from thePFS would immediately lead to problems since the swing motion of the ladderwill be hindered by unexcavated material (see Figure 7 and 8).
It is necessary that the PFS cutterhead have the option to move back in thesame way that a cutter suction dredge can move backwards prior to the nextdeepening action.
There are two ways to obtain this degree of freedom of the cutterhead,i.e.:
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- mount the turntable on a slide which can move over the skid(comparable to the spud slides on the cutter suction dredge) or
- create a second hinge point in the ladder in between the dredge pumpand the cutter drive (comparable to backhoe or shovel arm).
The above second option was selected for the PFS unit.
The undisturbed material to be dredged will be the governing parameter forthe cutterhead design and the cutterhead drive power.
Ladder geometry
The ladder of cutter suction dredges are usually straight or have a smallbend. In the case of the PFS, it is important that the underside of theladder stays clear of the side of the pit and this requirement ismaintained over the full arc.
Furthermore, piling up of material behind and underneath the ladder has tobe prevented. Jets are therefore required at the underside of the ladder.
Material supply
Phosphate matrix is supplied by the dragline at regular intervals bydumping over the matrix pit. The material falls on the slopes or directlyinside the pit.
When an excess of material is accidently dumped on top of or flows over theladder, the PFS unit must have the capability to withdraw from thismaterial. Swinging is not possible so the PFS unit needs the capability towithdraw the ladder. While withdrawing the lower side of the ladder mustbe able to stay clear of the side of the dump pit. This requirementeffects the shape and size of the dump pit as well as the design of theladder.
In order to prevent choking of the pump when an excess of material fallsover the cutterhead, there will be a vacuum relief valve in the suctionpipe between the suction mouth (inside the cutterhead) and the submergedpump. This is common practice on dredges, however, on the PFS thesurrounding water is actually a very dense slurry which may foul the valve.For added protection, a similar arrangement could also be installed on thesuction side of the first booster pump which is attached to the rear of thePFS unit.
Stone, pebbles, etc.
Material of a size that is too big to pass through the pumps will beprevented from entering the pump by the crown cutterhead blade openings.
A cutter suction dredger when hindered by oversize lumps creates a deepspot (below the normal cutting profile) and pushes the oversize material
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into this deep spot. In the case of the PFS, the same approach can befollowed. A deep spot in the dump pit will be sufficient. In extremecases, the dragline will need to clear the pit as it sometime does now inpresent day operations.
Cutter patterns
The cutter describes arc shaped tracks in the dump pit.
Starting at a selected depth the ladder swings and the cutterhead movesalong the arc. Based on the amount of material encountered, the swing orthe height of the cutterhead is then adjusted (Figure 9 Track V2), when theconcentration increases the cutter is raised.
When a certain volume of matrix collapses on top of or next to the movingladder and makes the continuation of the swing motion impossible, thecutterhead will have to be retracted (Figure 9 e.g. Track V2 to A4).
Track A4 is completed and followed by A5 (in order to make sure that theunderside of ladder stays clear from material). From A5 dredging in TrackV1 and sequentially V2 is resumed.
IX. DESIGN REQUIREMENTS
Operational set-up
The PFS is to be located on the side of the dump pit. The dump pit depthis 7.0 meters. The water level in the dump pit varies between 4.5 metersminimum and 5.5 meters maximum above dump pit bottom.
A skid mounted typical booster pump is arranged directly behind the skid ofthe PFS. The booster pump skid is connected to the PFS skid and functionsas a counterweight during operations.
Production capacity
The production capacity of the PFS as noted earlier when operating inphosphate matrix is to be 1600 or 2130 STPH by using either 18" or 20"discharge pipeline for the high pressure booster pumping system. In eithercase, the ladder pump will only require an 18" pipe. An 18x20 reducerwould be put on the suction side of this first booster pump if needed whichis attached to the rear of the PFS unit.
Pump and drive
The pump chosen is a medium specific speed low head 18" discharge by 20"suction 33" diameter impeller (GIW l8x20LHD33) unit of 2 vane designcapable of passing 8" spheres. It operates at 350 rpm and is driven by a100 kW hydraulic drive unit. Figure 16 shows the multiple speedperformance of this unit.
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Cutter and drive
The dump pit shape governs the cutter and cutter drive design. A 100 kWcutter drive has been selected with a cutter speed adjustable in the range0-35 rpm.
Ladder geometry
The total ladder length is to be designed to fit inside an open top 40'container. The pump will be located as close as possible to the suctionmouth.
Turntable
The swing angle during operations will be 45 degrees either side of thecenterline. For transport the raised ladder can be rotated 180° forstorage on the skid of the PFS machine.
Pipeline
The diameter of discharge pipeline is set at 18" on the PFS unit. If thefirst high pressure booster pump and this pumping system uses 20" OD pipe,then an 18x20 reducer is needed.
Two hand operated valves are provided to allow the discharge flow to bedirected either to the booster pump or discharge to the side for pitshaping and overburden removal.
Transportation details
Skid mountedMaximum weight 50 ton.Easy connection to skid mounted booster pump.
Drives
Electric-hydraulic
Auxiliary pumps
A jet pump with nozzles located on the underside of the ladder is providedfor keeping the underside of the ladder clear of material. Flush pumps arealso provided for dredge pump gland and cutter bearing water flush.
Controls.
All controls are set up for a single operator located in a control cabin onthe unit.
X. DESIGN CONSIDERATIONSThe specific considerations that have led to the PFS design described inAppendix II, the technical specification Volume II of the Stapel report aredescribed below.
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Base plate
Two alternatives were considered here....that of track mounted or skidmounted. Skid mounted was selected since this is common practice in thearea, and sufficient D-8' caterpillar tractors are available to move theskid when this is necessary. It is intended that the PFS unit be no moredifficult to move than the present day pit pump.
Another factor was that the maintenance cost of a skid are very favorablecompared to the maintenance cost of a track mounted support.
Base plate stabilization
On its own, the PFS unit (over all 4x12 meters) in operation is unstablewhen set up on the edge of the pit because of the overhung weight and swingof the cutter ladder.
A weight at the non cutter end can stabilize the unit, longitudinally. Thefirst skid mounted booster pump is available to perform this task. Twovertical brackets at the stern of the PFS unit are provided to make a rigidconnection between the two units.
Stabilization in the transverse direction is required to cope with theswing motion of the ladder during operations and also when the ladder ismoved into the storage position. Two adjustable side supports are providedto give the necessary stability similar in design to those on a mobilecrane. They are arranged aft of the turret to be at a sufficiently longdistance from the pit so as to be on level ground.
Turret and turret drive
A turret is required to facilitate the swing capability of the ladder. Theturret is mounted on a roller bearing, in a way similar to a backhoesuperstructure.
Turret drive options are usually either by means of a gear and pinion or bymeans of a hydraulic cylinder.
The angle of rotation during operation is approximately 90°, but forstorage rotation over an extra 135° is required.
The advantage of the gear and pinion option is the ability to rotate overthe full angle without interruptions.
A hydraulic cylinder driven turret can rotate normally over a maximum 90°angle. Repositioning of the cylinder connection to the turntable isnecessary when an increased angle of rotation is required.
For intermittent swing motions under varying side loads (cutter force), aclearance free connection point is required. This can only be obtainedwhen the cylinder option is adopted.
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The hydraulic cylinder approach was taken. As a consequence of thischoice, storage of the ladder has to be done in three steps, changing thecylinder connection after each step.
Ladder geometry
As noted earlier, it is necessary to have some translation ability for thecutter. Here there was a choice between a ladder with the ability totranslate within itself or its base (forward, backward) or to use a doublehinge.
The double hinge option was selected for economic reasons.
Drives
Electric power is available on the locations where the PFS will be used.
A choice had to be made between electric hydraulic drives or directelectric drives.
The dredge pump, the cutterhead, and the ladder motions were selected to behydraulically driven. Hydraulic power is generated by anelectric-hydraulic power pack on the aft end of the skid. The auxiliarypumps are direct electrically driven.
The hydraulic drives for dredge pump and cutterhead were selected largelybecause of their weight. Hydraulic drives have a lower weight thancomparable electric drives so they were selected.
XI. CONTROL AND AUTOMATION
Control System
Most modern cutter suction dredges are equipped with a dredging computerfor automatic process control.
The main aim of an automatic process control system in a cutter suctiondredger is to increase its efficiency. This does this by:
- Making it possible to reach maximum production levels by optimizingprocess control of the winch systems and spud system.
- By efficient control of the pump power in connection with flow anddensity.
- Accurate profile dredging.
- Data presentation.
- Providing an automatic reporting system.
12
The computer in a dredge is normally interfaced to the:
- side winches- ladder winch- spud carriage- vacuum relief valve- bypass valve- dredge pumps
Manual control of all these systems is possible at all times.
For manual control of both side winches there exists a joy stick systemwhich combines the speed regulation of the hauling winch and the brakeregulation of the veering winch. With the joy stick the swing direction,speed and braking level are chosen together.
When switching-over to auto control it remains possible to overrule autocontrol via the joy stick. This unique combination of auto control withmanual override proves to be very versatile and user friendly with lowthresholds for acceptance by operators. The same combination of manual andauto control is used for flow control systems, control of relief valve andspud carriage.
Production control
The aim of the control system is to load the dredge near to the safe limitswithout overloading any of the drives or mechanical parts of the dredge.By reaching those limits the production is a maximum for any givensituation.
The normal dredging procedure is that the dredge swings with the cutterheadin a face. After each swing the dredge moves forward by means of its spudcarriage. In addition to or instead of taking a step forward, the ladderwinch can be operated in order to let the cutterhead dig at another depth.After taking a new step and/or changing depth, the speed of both swingwinches is controlled in such a way that the production is as high aspossible without:
2.1. overloading the cutter drive
overload of the hauling side winch
4.3. allowing too low tension in both winch lines
making possible too high a speed of veering winch5. causing cavitation or too high vacuum on dredge pump6. causing too low intermediate pressure between dredge pumps
8.7. allowing too low mixture velocity
too high mixture density
The factors (1), (2), (3) and (4) represent the cutting limits.
The factors (5) and (6) represent the suction limits.
The factors (7 ) and (8) represent the discharge limits.
13
In case of (5), (6), (7) and (8) in addition to speed control of the sidewinches the vacuum relief valve can also be opened automatically.
In case of (6), a bypass valve, i f available, can be opened as well.
Efficient use of pump power
When all pumps are running at full power on short discharge distances orwhen the average mixture density is low (for instance when cleaning up),the velocity in the pipeline can be unnecessarily high.
For each material type there is a safe carrying minimum velocity. Thevalue of this minimum velocity depends on the grain size, the nature of thematerial and also on the mixture density.
The most economic way of pumping is working with a flow which is near tothat minimum velocity. In practice, some 20% above this minimum orcritical velocity is regarded as safe.
In situations where variations in production levels occur (for instance dueto cutting limits or face dimensions), or in the case of short pumpingdistances, etc., the available pump power can be reduced considerably,without any decrease in the production.
Maintaining the flow independent of the average mixture load of the pipe-line is called flow control. Flow control applies to the total pipeline.All pump motors, including booster pumps in the line, are controlled by theflow control system.
The flow control system incorporates a sequence in the control of eachindividual pump. A number of signals per pump are used in the controlstrategy. It ensures the safe operation of all the pumps.
Flow control can be fitted on all types of drives (electric- as well asdiesel engines), assuming that a standardized control signal is availablewhich is proportional with the rotational speed of the drive.
Accurate profile dredging
In most cases, the dredge has to deliver a designed profile within accept-able tolerances. This profile can be a flat bottom and/or predefinedslopes.
Profile dredging is only possible when at all time the exact position ofthe cutterhead is known.
One of the tasks of the dredging computeractual position of the cutterhead relativeand 11.
is to calculate and present theto the dredger, see Figure 10
o a position system, the exactposition of the cutter head relative to the bottom is known.When the dredging computer is interfaced t
14
Note that the positioning system should include the horizontal and thevertical location of a reference point on the dredger since the position ofthe floating dredge will be influenced by (cutter) reaction forces, wind,etc.
By measuring and comparing the desired profile and the position of thecutterhead, the ladder winch and side winches are controlled in order tokeep the deviations within the permissible tolerances.
This means that for example in order to dredge a horizontal bottom profile,the ladder winch has to be controlled continuously.
Pre programmed dredging patterns can be used i.e. dredging toe lines inslopes.
Figure 13 is an example of a typical profile.
Data presentation
Data for all of the above as noted below are shown on a computer screenoperated by the dredging computer.
Maximum production
On the dredging data output production figures are presented that show howclose the dredge is loaded to its maximum capability.
This feedback gives the operator the opportunity to change and optimize setpoints in order to improve production. A useful tool for optimizing isthat of monitoring on the screen the most production-relevant signals on atime basis.
Accurate dredging
The path of the cutterhead in relation to the desired profile is presented.Deviations of the profile are presented and it can be observed how thedredge is performing.
This feedback gives the operator the opportunity to change the dredgingpath (work line) or dredging method in order to meet accuracy requirementsin the shortest time.
Automatic reporting system
The computer calculates all relevant data regarding production, energyusage, operational hours, dredging hours, delays, etc.
An automatic reporting system is part of the dredge control system.
Daily and weekly reports of the operation may be requested by the operatorby simply selecting a begin and end time for the reporting period.
15
The requested report is then prepared automatically.
Discharge line control
When starting, assuming the pipeline is completely empty and the pit isfilled with water. The cutterhead is lowered in the pit below water level.
The ladder pump of the PFS is started and run at low rpm. At this stage,the first part of the discharge line is filled with water.
Gradually, the rpm's of the pump are increased. After a few minutes arelative low flow and a stable discharge pressure near to nominal will beobserved on the gauges of the ladder pump.
After some time the incoming pressure of the first booster is increasingbut still unstable, indicating the flow is coming, but that the pipelinenear the booster is not completely filled.
When the incoming pressure on the first booster is stable, e.g. 30 psi,then that booster pump can be started. Initially at low speed, thengradually increasing to full speed. The criterion to switch to full speedis dependent of the amount of incoming pressure and the stability of thisincoming pressure signal. When the incoming pressure falls to zero or tovacuum, then the speed of the booster pump has to be decreased. After ashort time full speed will be possible, without loss of incoming pressure.
Switching on the following booster is only allowed when the previousbooster is running at full speed, when the incoming pressure signal isstable and when its value is well above zero. The above described proce-dure has to be followed in sequence beginning at the pit for all thebooster stations in the discharge pipeline.
The starting procedure can be conducted manually and is essentially thesame as present day practice. Automation is; however, a possibility (seepage 20 Flow Control).
Stopping procedure of the discharge pipeline
When the pumps have to be stopped then the following procedure should beadhered to. First as a normal condition, the pipeline should be cleared ofslurry by flushing with water. When the average density in the pipeline isnot extremely high and the path of the pipeline to the washing plantremains practically horizontal (no slope) and depending on the nature ofthe matrix, stopping is also possible with mixture in the pipeline as isfrequently the case today.
The last booster in the pipeline should be stopped first. The result is alower flow. On all other booster stations and the PFS ladder pump thepressures in the discharge line will remain high and well above zero. Nodangerous situations like pressure waves or vacuum in the pipeline willoccur.
16
Then the next to the last booster has to be switched off, etc. The lastpump to be switched off is the ladder pump of the PFS.
It is very important that the switching off of the boosters occurs in theright sequence: from the end of the pipeline to the beginning.
The switching off procedure can be conducted manually, automation of thisis possible as mentioned in the paragraph on Flow Control.
Control of the PFS
The PFS has the ability to move the ladder in a horizontal and a verticaldirection. The swing arc in the horizontal direction is approximately 90degrees. In the vertical direction it can move approximately from 0 to 45degrees. At the end of the ladder the cutterhead can make additionalmovement.
The PFS unit differs in some ways from the well known cutter suctiondredges but there are a lot of similarities. Most of the proven processcontrol systems can be used on the PFS.
Requirements for the control system
The PFS unit is working in a small pit which forms a buffer between thedragline and the washing plant. The PFS unit is a link in the productionchain between the dragline and plant. The production of the PFS unit inthe pit has to be adapted to the production capabilities and -requirementsof the plant. The matrix discharge to the washing plant should be at asconsistent a rate as possible.
As the supply of the matrix from the dragline is discontinuous, the PFS hasto be manipulated in such a way that the production through the dischargeline to the plant will be constant.
The first requirement is that both the flow and the mixture-density shouldbe as constant as possible.
A second requirement is that the PFS and the boosters in the discharge lineare protected against the harmful effects of cavitation.
A third requirement is that the matrix has to be pumped in the mosteconomic way from the pit to the plant.
A fourth requirement is level control of the water in the pit to ensure asafe pumping process.
Pit preparation
Prior to the actual pumping of the matrix the PFS has to shape the dumppit.
17
The pre shaped dump pit is filled with water. The cutterhead is lowered inthe dump pit. The valve in the discharge pipeline to the first boosterpump is closed and the valve in the branch line is opened if this option isused to dispose of excess dirt as the PFS unit shapes the dump pit.
The pump on the ladder of the PFS is started.
Matrix dredging
When the dragline drops the bucket content into the pit, the PFS has todredge all that material away within the production cycle time of thedragline. There should be some coordination between the cycle of thedragline and the swing cycle of the PFS.
When the dragline dumps the material on one side of the pit, ideally, thecutterhead of the PFS should be on the other side of the pit. This willprevent the bucket contents from falling directly on the cutter ladder.This means that basically the period of the swing of the PFS should beconsistant so that the dragline operator can time this dumping into thematrix pit.
The above will give limitations to the swing speed of the PFS.Undoubtedly, the dumped material will fill the pit in an irregular way.The cutter ladder has to be moved in both a horizontal and verticaldirection so as to follow the irregular face.
Mixture-density control
When the density has to be as constant as possible then the digging produc-tion of the cutterhead should be constant.
With a constant average swing speed due to dragline cycle requirements theladder should be raised or lowered according to the profile in the pit.However, profile measurements are not necessary for the automatic densitycontrol.
The basic control strategy can be described as follows. When the densityis higher than the set point density the ladder is raised; when the densityis lower, then the ladder is lowered. Lowering and raising is possiblewithin the geometric limits while raising is also limited by the pit level.Other limits in speed and position are given by the hydraulic system, likepressure- and capacity limits.
Due to the fact that the density measuring system is some 20m away from thesuction mouth, a time delay of several seconds is introduced which makesdirect control of the cutterhead movement by the density signal unsatisfac-tory.
To overcome this problem a proven mathematical suction model using thevacuum- , velocity- and density signals to forecast the density will be usedfor position control of the cutterhead.
18
Vacuum control
Vacuum or suction pressure is defined as the negative pressure referencedto surrounding air pressure which is valued as zero. In general, it can bestated that centrifugal pumps can keep their manometric head up to a vacuumvalue of -21 in Hg. Specifically every pump for a given design, speed andflow has to have a certain absolute pressure or net position suction head(NPSH) provided to its suction by the system or it will cavitate and losehead. If the NPSH available from the system is in excess of that thenthere is no problem.
When the vacuum in front of the ladder pump increases to where the NPSHavailable is insufficient, then the manometric head of the pump starts todrop due to cavitation in the pump. Most centrifugal pumps have a charac-teristic in which the cavitation effect increases by increasing vacuum. Ageneral maximum vacuum limit is -27 in Hg.
At the limiting vacuum value there will be severe cavitation causing acomplete loss of the manometric head of the pump. This can causeseparation of the flow masses fore and aft off the pump and very severepressure peaks, when flow masses meet again. Cavitation, water hammer anddamage of pumps and piping can be the result.
The vacuum control system has the task of preventing too high a vacuum fromoccurring in front of the ladder pump. This control works in two stagesgoverned by two set points.
Set point 1 represents the vacuum value which ensures a safe pumpoperation. When the vacuum value exceeds this set point the outer ladderwill be swung backwards and the ladder itself will be raised while keepingthe horizontal swing speed constant.
The effect of this action is that the dug production and consequently thevacuum will decrease. When the vacuum value is lower than the first setpoint then in principle, when there are no other limitations, the ladderwill be lowered in order to pick up more material.
It is assumed that the desired production can be dredged with an averagevacuum value which is lower than the set point 1. For example at avelocity of 15 ft/sec, a dredging depth of 18 ft, and pump depth belowwater level of 5 ft., then it can be calculated that for a S.G. of 1.4 thenecessary vacuum is in the order of -14 in Hg. A typical value of setpoint 1 could be -18 in Hg. So normally set point 1 is not exceeded.
Set point 2 which is higher than set point 1 and close to the value wherecavitation starts, represents an alarm situation. A typical value of setpoint 2 could be -21 in Hg, depending on suction capabilities of the pump.
19
When set point 2 is exceeded then the vacuum relief valve will be openedwhich causes an immediate drop in vacuum. The relief valve with amplediameter is a very effective tool in preventing cavitation in the ladderpump. Note that when set point 2 is exceeded the ladder has been raisedalso due to the fact that set point 1 has already been exceeded.
When the vacuum value moves back to a normal and safe value then the reliefvalve is closed slowly, and in a controlled way to prevent hunting andunacceptable peaks in the vacuum. Vertical ladder movements are allowed toresume after the relief valve is completely closed. This two stage vacuumcontrol utilizing the vacuum signal in combination with the density controlhas the following advantages:
- At a sudden increase of the vacuum that is still below set point 1, ahigher density will be predicted by calculation.
When the density set point is exceeded the ladder will be raised tokeep the expected production constant (see density control).
- When the vacuum value passes the set point 1 value and no densitylimit is applicable (due to change in the nature of the material),then the ladder will be raised to decrease production and vacuum in agentle manner. Normally, a small decrease in production lowers thevacuum to safe values.
- When the above mentioned control actions fail for instance when a veryfast increase of vacuum occurs due to a blockage of the suction pipe,the vacuum relief valve will be opened, This dilutes the mixtureconsiderably and lowers the vacuum to safe values. Because of itseffects on the production, the relief valve is only used as a lastresort to prevent cavitation.
Flow Control
For an optimal functioning of the PFS in combination with the boosterstation pumps on the discharge line, the implementation of a flow controlsystem is required.
A flow control system has the capability to:
- assess the ratio power-discharge distance-density and flow- select critical flow velocity- start or stop booster pumps as a result of the previous assessment.
The flow control system can be used for the starting and stopping proce-dures for the discharge line as discussed earlier, and it has the capabil-ity to react to long periodic variations in the mixture density that areexperienced.
Implementation of a flow control system requires the following instrumenta-tion and control of the booster pumps.
20
- measurement of inlet pressure of each booster station, (preferable)and power indication.
- speed control of the individual pumps.
Pit level control
Low pressure water is supplied to the pit by a separate pipeline from thewater recirculating system common to most phosphate operations.
The pipeline is connected off a manifold on the skid of the PFS andsupplies water to the jet and flushing pumps on the PFS.
The flow through an adjustable valve at the end of the pipeline into thepit is controlled by means of a level indicator in the dump pit.
The water level in the dump pit is to be kept as high as possible.
The computer which controls the valve has the capability to anticipate therequired volume of transport water based on the (calculated) mixture flowthrough the PFS.
XII. ECONOMIC EVALUATION
There is no one pipeline configuration that can be used for the economiccomparison that applies exactly to all mines. The most common pipe size inuse is the 17-1/4" diameter and a matrix of 30% pebble and 20% fines istypical. The field tests carried out as part of the FIPR DOE contract #87-04-037R showed that pipeline concentrations varied up and down signifi-cantly all the time but that 35% by weight was a more likely normal maximumaverage. A realistic average throughput for this size of pipeline usingthe current pit technology would therefore be 1600 dry tons/hour of solids.
The work done in the FIPR DOE contract #87-04-037R in the GIW HydraulicLaboratory showed that pipeline concentrations of up to 50% by weight couldbe pumped without difficulty provided that the matrix could be properlydrawn into the pipeline and considering there was no limitation due to pitpump cavitation. If we assume somewhat conservatively that the PFS makesit possible to achieve an average concentration of 45% by weight acomparison could be made and the likely benefits calculated for the same1600 tons/hour of matrix transported.
Figure 17 and the table below were calculated using the actual phosphatematrix test data and computer program developed under FIPR DOE Contract#87-04-037R.
From these it can be seen that operation at 1600 tons/hour without the PFSunit at 35% by weight concentration requires a pipeline input energy of.765 HP-HR/ton mile compared to an input energy of .407 when operating withthe PFS system at 45% by weight concentration for the same production.
21
It must be remembered that the present matrix system requires a consider-able fixed quantity of high pressure "gun water" at the pit so as to"slurry" the matrix and drift it through a grizzly to the pit pump suctioninlet. This water addition at high pressure (approximately 200 psi) is ahigh cost item. In addition, this dilution results in a higher matrixpipeline velocity well above the safe "carrying" velocity; thus, additionalpower is wasted at each booster pump while increasing both pump and pipe-line wear rates.
Since higher production is only possible in the present matrix system byincreasing total matrix pipeline velocity, the usual limitation is pit pumpcavitation ... with danger of destructive water hammer. (At higher flows, theNPSH available to the pit pump is lower...pit pump is higher).
while the NPSH required by the
Since the dragline and washing plant would most likely limit increases inproduction this economic comparison will be limited to differences in costfor the same throughput of 1600 TPH on a typical pipeline over a year'soperation.
A typical phosphate line is 3.5 miles or 18,480 feet long. The static headis almost always less than 10% of total system head so we will neglect itseffect in this comparison. The overburden stripping time varies from mineto mine and this limits the time the matrix pipeline operates. On averageit would operate 16 hours a day, five days a week for 50 weeks a year for atotal of 4000 hours a year.$O.O45/kW hour.
Electricity costs are in the range ofThe electric motors used by the pumps in the line will
have on efficiency of about 95% and the pumps (including solids effect)will be about 75% efficient.
The total pit and booster pump electrical energy cost for the original 35%by weight concentration operating condition for this line is:
If we include the cost of the 10,000 gpm of 200 psi high pressure gunwater, then this would be:
For a total of $1,001,382.
22
With the PFS unit operating at the same 1600 TPH at 45% by weight concen-tration, the total electrical input energy must include the PFS unitnoting, however, that the underwater pump is included in the pipelineenergy.
If as noted earlier with the PFS system we assume somewhat conservativelywe can reduce the gun water by half, then the total cost becomes $554,697per annum for an electrical input energy difference or savings of $446,685per year per matrix line.
In addition to the electrical energy savings, there would be savings inlabor, water usage and wear. There is also less total head and this willlikely result in less pumps being needed. The PFS unit would involve extramaintenance cost which is estimated at $50,000 per year.
The lower total system head of .0343 x 18480 = 634 feet of slurry requiredwhen operating with the PFS system compares to .0502 x 18480 = 928 feet ofslurry when operating without it. The underwater PFS pump will deliverapproximately zero pressure to the first booster as compared to anapproximate negative 20 ft slurry pressure on the suction side of thepresent pit pump. The above estimations give a reduction in total systemhead for a PFS pit operation as compared to the present arrangement of (928ft + 20 ft - 634 ft) 314 ft of slurry. This system head reduction is muchgreater than that produced by one booster pump; however, to beconservative, we will credit the PFS pipeline with one less booster station(pump and motor) at approximately $250,000 capital cost. To be veryconservative we will not deduct the operating maintenance cost of thisbooster station in this study.
23
The operating labor cost savings would, after an initial period oftraining, amount to a reduction in one person in the crew required tooperate the line. This would be largely as a result of the automatedcontrol system. The estimated savings here are $75,000 per year.
Wear varies roughly directly as the 2.5 power of the velocity and indirect-ly as the concentration. The reduction in wear in the pipes and pumpstherefore should reduce in the ratio of
Neglecting the cost of installation, the cost of 18" pipe is approximately$34.1/foot for a total typical line cost of 34.1 x 18480 = $630,000. Thecost of a set of typical pump wear parts is $20,130.
The average life of a pipeline is 2-1/2 years and the pump (parts only)could be expected to wear out in a year so if the wear rate does reduce asshown above, then the savings here would be
Operating at the higher concentration with the PFS unit, the reduction involume of water passing down the pipeline over a 4000 hour year would be
= [13,417(1-.169)-9603(1-.236)] x 60 x 4000
= (11149.5 - 7337) x 60 x 4000
= 3813 x 60 x 4000
= 915,120,000 gallons per year.
There would be some savings associated with this, but it is not beingincluded in this study.
If we assume a $250,000 contingency for the PFS unit for unforseenadditions to its capital cost, this is essentially offset by the capitalcost savings associated with the PFS pipeline requiring one less boosterstation. For this reason, a total capital investment for the PFS is takenas $1 million. The internal rate of return on this investment has beencalculated for the electrical energy savings, the pipeline and pump wearparts savings and the labor savings.... less maintenance costs associatedwith the PFS unit.
24
This calculation is included as Appendix I. With a zero intercept over aten year period it shows a total rate of return on investment of 32.7%.This pay back is estimated to be around 25 months.
An additional benefit available to a corporate investor in the projectwould be research tax credit permitted by the current Federal income taxcode. The amount of such tax credit, if any, would depend on the investingcorporation's tax situation including the extent by which their researchcosts in the year of investment exceeded their research costs in precedingyears. This credit is described in Internal Revenue Service code section41. The investing company's tax advisor will be able to more preciselydetermine the amount of any credit.
Any savings enjoyed by the down stream beneficiation processes as a resultof receiving much more consistent phosphate matrix.....constant tons perhour rate at the same slurry density..... is not being included in thisstudy. These operational benefits may be considerable.
25
REFERENCES
1. FIPR DOE Contract No. 87-04-037R
2. Discovering the Future: The Business of PARADIGMS, page 14, by JoelArthur Barker.
3. Ibid, page 15.
4. Potential Operating Cost Improvement in Florida Matrix Transportationby G. R. Addie and T. W. Hagler, Jr. at the AIME Regional PhosphateConference, September 1990, Lakeland, Florida.
26
28
CUTTER SUCTION DREDGER WITH ARTICULATED LADDER
Figure 3. C .S . D. Schematic
29
Figure 4. Start of dredging operation
Figure 5. P . F .S . Schematic
30
Figure 6 . Dump pit excavation
31
REACH
Fig. 7. Obstruction of swing motion
32
33
Fig. 10. Control of cutterhead position
34
Fig. 11 Control of cutterhead position
Fig. 12 Pump power control
Fig. 13 Profile dredging
(i) = input(m) = measurement
Fig. 14. State of the art of cutter suction automation
38
40
MATRIX ENERGY ANALYSIS17-1/4” ID PIPELINE
(30% Pebble, 20% Fines)
Figure 17
41
APPENDIX II
FLORIDA INSTITUTE OF PHOSPHATE RESEARCH
POSITIVE FEED SYSTEM
Engineering study for the designof
a Positive Feed Systemfor
Matrix transportationVolume II
Technical Specification
May 1991
GIW Industries, Inc.Grovetown, Georgia, USA
STAPELSpaarndam, the Netherlands
I I -1
TABLE OF CONTENTS
1. INTRODUCTION
2. GENERAL DESCRIPTION
PRINCIPAL CHARACTERISTICS
TECHNICAL DATA
MECHANICAL DESIGN
5.1 SKID
5.1.1 Mounted boosterpump
5.1 .2 Turret support
5 .1 .3 Side supports
5 .2 TURRET
5.2.1 Connection of the inner cutter ladder
5 .2 .2 Connection of the control cabin
5.2 .3 Turret drive
5.2 .4 Turret bearing
5.3 OPERATING CABIN
CUTTER LADDER
I I - 2
5.4
TABLE OF CONTENTS (CONT’D)
5.5
5 .6
5.7
5 .8
5.9
5.10
5.11
5.11.l
5.11.2
5.11.3
6.
7 .
7 .1
7 . 2
7 .3
CUTTER
CUTTER DRIVE
DREDGE PUMP
DREDGE PUMP DRIVE
SUCTION PIPE
DISCHARGE PIPE
AUXILIARY PUMPS
Jet pumps
Cutter bearing flushing pump
Dredge pump gland pump
INSTRUMENTATION AND AUTOMATION
FABRICATION, COMMISSIONING AND COST
FABRICATION
COMMISSIONING
COST
I I - 3
TABLE OF CONTENTS (CONT’D)
LIST OF DRAWINGS
Drawing number
DOS 291-00.00-001
DOS 291-10.00-010
DOS 291-10.00-030
DOS 291-26.20-100
DOS 291-26.20-100
DOS 291-10.00-020
DOS 291-78.00-020
DOS 291-78.00-010
Title
General arrangement
Skid
Turret
Inner ladder
Outer ladder
Side supports
Water pipe lines (schematic)
Hydraulic system (schematic)
I I - 4
1.1
1. INTRODUCTION
Under the terms of FIPR contract nr. 88-04-044 “Positive FeedSystem” GIW INDUSTRIES, INC., Grovetown, Georgia, U.S.A.is commissioned to conduct a feasibility study on a positive feedsystem for matrix transportation.
The study activities related to the technical design of themachine were subcontracted to Stapel Shipyard, Spaarndam, theNetherlands.
The report prepared by Stapel Shipyard consists of two Volumes.
This is Volume II, the technical specification of the PositiveFeed System.
I I - 5
2.1
2. GENERAL DESCRIPTION
(General Arrangement plan nr DOS 29 1-00.00-001).
The P.F.S. basically consists of a pontoon shaped skid supportingthe dredging system comprising the rotating cutter head, suctionpipe, dredge pump and discharge pipeline.
The cutter head, suction pipe, dredge pump and part of thedischarge pipe are mounted on an articulated dredge ladder formedby three hinge connected parts, the outer ladder, the inner ladderand the turret.
The outer ladder and the inner ladder are hinge connected allowingfor a relative motion around a horizontal axis.
The turret and the outer ladder are hinge connected in a similarway again allowing for a hinge motion around a horizontal axis.
The turret is connected to the skid and allows for a rotation over225° around a vertical axis.
All hinge motions are powered by means of hydraulic cylinders.
Also arranged on the skid are the discharge pipeline connecting tothe booster pump, the electric-hydraulic power pack serving thePFS’s machinery, a number of auxiliary pumps and two adjustableside supports. The latter forming the means to stabilize theP.F.S. while dredging.
When the P.F.S. is in operation a skid mounted booster pump, notbeing part of this specification, will be connected to the aftside of the PFS skid. The connection is made by two horizontalpins. The weight of the booster pump balances the P.F.S. whiledredging.
An automatic control system monitors and controls the mixture.
I I -6
3.1
3. PRINCIPAL CHARACTERISTICS
II-7
3.1
3. PRINCIPAL CHARACTERISTICS
I I - 8
4.1
4 . TECHNICAL DATA
II-9
5.1
5 . MECHANICAL DESIGN
5.1 SKID
The skid is a pontoon shaped all welded box structure (the box issealed).
Internal longitudinal and transverse frames provide ample stiff-ness and strength to receive the loads exerted by the variouspieces of equipment mounted on the skid.
The skid bottom is externally completely smooth, the leading andtrailing edges of the bottom plate are slightly tilted upwards.
At the fore and aft side of the skid brackets are arranged. Theyare the means to connect the pulling or pushing craft to the skid.Furthermore the brackets on the aft side form the means to connectthe skid to the skid of the booster pump.
On both sides of the skid brackets are arranged for the connectionof the adjustable side supports.
5.1.1 Turret support
The roller bearing of the turret is supported by a circularpedestal arranged at the front side of the skid.
The pedestal is welded to the skid.
5 .1 .2 Side supports
On each side of the skid an adjustable side support is arranged.
These supports are required to stabilize the skid when the dredgeladder is in operation.
The feet of the side supports are moved by means of a hydrauliccylinder. When the feet are in the required position (either onthe bottom or retracted) the mechanical locking system is applied.
II-10
5.2
Control of the hydraulic cylinders is from a local control boxnear the support.
5.2 TURRET
The turret is mounted on the front side of the skid. The turretcan be rotated around a vertical axis.
The turret consists of a horizontal flange to which a verticalsteel tube is welded. The turret is connected to the skid by meansof a roller bearing. Connected to the turret are the inner cutterladder, the control cabin and the turret drive.
5.2.1 Connection of the inner cutter ladder
The inner cutter ladder is connected to the turret- at three loca-tions. Clamp blocks to receive the trunnions of the ladder arelocated at two brackets protruding from the lower side of theturret.
The hydraulic cylinder controlling the position of the innercutter ladder connects the turret top with the top side of theladder.
5 .2 .2 Connection of the control cabin
The control cabin is mounted on a frame which is welded to theturret.
5 .2 .3 Turret drive
Connected to the horizontal bottom flange of the turret are threehorizontal brackets. They form the means to connect the cylinderwhich rotates the turret.
II-11
5.3
One bracket, the working bracket, is equipped with an arrangementto establish a clearance free cylinder connection. The remainingtwo brackets are used to move the turret to or from its storageposition and are provided with a quick connection arrangement.
The turret is driven by means of a trunnion mounted hydrauliccylinder of ample stroke.
During normal operations the turret drive is controlled from thecontrol cabin. When the ladder is moved to or from its storage.position the turret drive is controlled from a local control boxon the skid.
5 . 2 . 4 Turret bearing
Roller bearing.
5.3 OPERATING CABIN
The operating cabin is mounted on the left hand side of the turretand swings with the turret.
The operating cabin comprises all controls required to operate theP. F. S. See also Automation and Instrumentation.
Local controls are arranged in boxes on the skid for the adjust-ment of the skid supports and the (un) storage of the dredgeladder.
5 .4 CUTTER LADDER
The cutter ladder consists of two sections, the inner ladder andthe outer ladder. The inner ladder is hinge connected to theturret, the outer ladder is hinge connected to the innerladder.
As a result of this connection principle the two ladder sectionsare moveable in the vertical place.
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5.4
Ladder section movement is controlled by means of two hydrauliccylinders, One cylinder positioned between the turret and theinner ladder section and a second cylinder between the innerladder and the outer ladder. Both cylinders are positioned at thetopside of the ladder.
The ladder sections are made of steel tube.
Reinforcements are provided around access holes and to strengthenheavy loaded locations e.g. trunnions, cylinder brackets and sup-ports for drives and pipelines.
Inside the inner ladder are arranged the dredge pump and thedredge pump drive. The discharge pipe is connected to the upperladder.
The cutter drive is arranged in the outer ladder. At the lowerside of each ladder section a jet nozzle is provided. Covers areprovided to close all the openings at the ladder top side.
5.5 CUTTER
The cutter is a 5 bladed crown type, Stapel TSC 3. The cutterrotates clockwise seen from the control cabin. This cutter hasremovable teeth and is suitable to operate in compacted sand andclay.
5 .6 CUTTER DRIVE
The cutter is direct driven by a slow speed hydraulic motorthrough a cutter shaft.
The cutter shaft runs in a water flushed rubber bearing. The fixedpart of the bearing consists of a steel pipe provided with longi-tudinal rubber strips at the inside. The rotating part of thebearing consists of a stainless steel bushing shrinked on thecutter shaft.
The cutter motor is a slow speed hydraulic motor with a built onthrust bearing.
I I - 1 3
5.5
5.7 DREDGE PUMP
The dredge pump is a centrifugal pump with 2 bladed impeller.
The pump casing is made of cast white iron.
The suction cover and the shaft cover are provided with circularwearing and filling plates made of steel.
In order to reduce wear on the shaft side of the pump a separategland pump flushes the sealing arrangement on the pump shaft.
The impeller is mounted on the outgoing shaft of the gear box ofthe dredge pump drive.
5.8 DREDGE PUMP DRIVE
The dredge pump is driven by one hydraulic motor. This motor ismounted on a gear box.
A thrust bearing is provided on the outgoing shaft of the gearbox.The gearbox is equipped with a lubrication oil gear pump whichprovides the necessary lubrication to bearings and gear wheels.
The seals of the outgoing shaft are provided with a connection forgrease lubrication.
5.9 SUCTION PIPE
The suction pipe consists of two steel elements, the inspectionpiece in front of the dredge pump and the suction mouth, connectedby means of a flexible suction hose.
The nominal inner diameter of the suction pipe is 500 mm. Theinlet area of the mouth is approx. 1,3 times the sectional area ofthe suction pipe.
The suction mouth is connected to the underside of the outerladder by means of clamps in order to allow for easy replacement.
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5.6
The suction hose is required to allow for the relative hingemotion between the inner- and the outer ladder.
The inspection piece is required for easy inspection and main-tenance of the dredge pump.
A vacuum relief valve is arranged at the underside of the suctionpipe.
5.10 DISCHARGE PIPE
Starting at the dredge pump the discharge pipe consists of thefollowing components:
. reducer joint on the pump discharge.
. steel bend towards the inner ladder
. flexible discharge hose through the turret
. straight steel section over the top of the skid
. steel measuring loop
. steel T-piece with two gate valves
. straight section directed towards the booster pump.
The nominal diameter of the discharge pipe line is 450 mm.
The pipe line is supported by means of pipe clamps connected tosupports welded to the outer ladder on the top of the skid.
The gate valves are hand operated.
5.11 AUXILIARY PUMPS
Four auxiliary pumps are arranged on the skid of the P.F.S., twojetpumps, the glandpump and the cutter bearing flushing pump.
The water supply to all four pumps is established by means of abranch line from the water return pipeline to the dump pit.
All four pumps are driven by individual electric motors.
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6.1
6. INSTRUMENTATION AND AUTOMATION
The computer system is based on the VME-bus. It contains a power-ful processor and analogue, digital and serial interface cards toconnect the computer to the PFS and the boosters and to monitorscreens and if necessary to a printer.
To protect the computer all in and outgoing signals are galvani-cally isolated.
The computer and isolators are built in a completely closed chest.
List of necessary signals for automation.
A. Signals to the computer.
Horizontal ladder movement:
. cylinder position
. hydraulic pressure
. manual position control signal
Vertical ladder movement:
. cylinder position
. hydraulic pressure
. manual position control signal
Top of ladder movement:
. cylinder position. hydraulic pressure. manual position control signal
Cutter drive:
. cutterhead revs.. cutter head torque or hydraulic pressure
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Ladder pump drive:
. pump revs.
. pump power
Pipeline:
. flow meter
. density or S.G. meter
. vacuum meter before ladder pump
. pressure meter after ladder pump
Vacuum relieve valve:
. manual relieve valve control signal
. position signal open-closed
Pit:
. level signal
. position signal control valve
Boosters (each):
. incoming pressure
. outgoing pressure
. booster power signal (if available)
. manual speed control signal
State signals:
. ready for service signals from all drives, pumps andbooster stations.
. autocontrol on/off from various drives, pumps andboosterstations.
Signals are normally ranged: 0 - 10 Volts or 4 - 20 mA .
Signals are read in by the computer via galvanic isolators.
6.2
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6.3
Process signals from the pit and all the boosters to PFS andthe command signals from the PFS to pit and boosters are tobe sent via telemetry systems and read by the computer viaRS232.
8 . Signals from the computer.
The outgoing signals are ranged: 0 - 10 Volt.
Position control signal for horizontal ladder movement
Position control signal for vertical ladder movement
Position control signal for ladder top movement
Open - close signal for vacuum relief valve
Open - close signal for pitlevel control valve
Speed control signal for all booster stations
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7.1
7. FABRICATION, COMMISSIONING AND COST
7.1 FABRICATION
Fabrication of a complete P.F.S. system in the Netherlandsrequires a lead time period of approx. 6 months. Included in thisperiod is a ‘two week test period at the shipyard. After extensivetesting the PFS will be disassembled and prepared for shipment.
Alternatives where parts of the P.F.S. are built in the UnitedStates and assembly and testing is done in the U.S. are of coursepossible.
However for a prototype, fabrication and testing of the completeunit on one location has considerable practical advantages.
7.2 COMMISSIONING
After arrival on location the P.F.S. has to be reassembled andhooked-up to the power and water supply system available aroundthe dump pit.
Technical assistance during commissioning and crew training can begiven by the same personnel that conducted the testing at thefabrication yard.
7.3 COST
The fabrication cost in the Netherlands incl. testing, ex yard andready for shipment of the P.F.S. described in the specification isUS $ 1 ,000,000.= (one million US Dollars).
(This figure is based on 1991 Dutch florins and an exchange rateof 1 US$ = Dfl. 1,90).
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