A ladder pump for use with a hydraulic dredge to increase the main pump efficiency comprises a casing and a vaned impeller located in the casing and mounted for rotation about a normally horizontal axis transverse to the ladder. The casing has an inlet and an outlet defining respective axes lying in a common plane perpendicular to the impeller axis and defining a deflection angle therebetween, with approximately 60.degree. being typical. The diameter of the impeller and the inner dimensions of the casing together define a substantial clearance over at least that portion of an impeller vane's travel between the inlet and the outlet, so that large solid objects entering the pump may pass under the impeller and out the pump outlet. The impeller may have substantially rigid vanes, or the vanes may be flexible. The ladder pump fits in-line with respect to the suction pipe, and has an axial extension little wider than the pipe itself. Thus it is readily positioned proximate the suction mouthpiece where it can work most effectively.

FIELD OF THE INVENTION 
This invention relates generally to pumps, and more specifically to an 
in-line centrifugal pump used in conjunction with a standard centrifugal 
pump in a hydraulic dredging operation. 
BACKGROUND OF THE INVENTION 
Hydraulic dredging provides an economical way of maintaining adequate depth 
in navigation channels necessary for commercial shipping. While hydraulic 
dredges differ in design and size, depending on the particular application 
intended, a typical hydraulic cutterhead dredge comprises a barge, a 
downwardly inclined ladder extending from the front of the barge, a 
rotating cutter at the lower end of the ladder for cutting and agitating 
material in the channel bottom, a main pump on the barge, and a suction 
pipe extending from the main pump and down the ladder, the suction pipe 
having an inlet, called the suction mouthpiece, proximate the cutter so 
that material stirred up by the cutter is drawn into the pipe by the main 
pump. The ladder is typically positioned by a ladder suspension system 
including a vertical H-frame extending upwardly from the front of the 
barge, an inclined A-frame extending forwardly and upwardly from the bow 
of the barge, and suitable rigging. A motor at the top of the ladder 
transmits rotary motion to the cutter by means of a cutter shaft extending 
down the ladder. Positioning and movement of the dredge is accomplished 
with paired vertical spuds located at the stern of the dredge along with a 
winch located on the forward deck. This winch controls wire ropes that 
pass through sheaves at the lower end of the digging ladder and terminate 
at anchors on each side of the dredge cut. During operation, the spuds are 
alternately lowered and the dredge rotated partially about the lowered 
spud. By alternately pulling on the wire ropes, the dredge is caused to 
swing back and forth at a slow speed, and so control the width of cut and 
rate of excavation. 
The main pump on the barge produces a suction at the suction mouthpiece 
proximate the cutter so that material may be drawn upwardly into the pump 
on the barge. The material exits the pump and is discharged via a pipeline 
to a location suitably removed from the channel. For example, the material 
may be discharged onto land or into the water at a distance from the 
channel. The main pump is typically a standard centrifugal pump with a 
fluid inlet coaxial with the impeller axis, and a fluid outlet generally 
tangential with respect to the impeller. By way of illustration, such a 
pump for a relatively large dredge having a 24 inch diameter discharge 
pipe might have a 28 inch diameter suction pipe. The main pump impeller 
will have a minimum clearance between the vanes of approximately 13" by 
17" for the passage of the dredged material entering the pump. This 
clearance is important as described below. The main pump impeller is 
rotated by an engine or electric motor supplying approximately 5,000 
horsepower. 
A variety of operating conditions severely compromise the efficiency and 
capacity of the dredge pump. For example, the dredge pump must accept not 
only mud, clay, sand, and gravel, but also large stones, pieces of 
water-logged wood, lengths of wire rope, rubber hose, automobile tires, 
bottles, tin cans, broken-up concrete, and the whole variety of trash 
material that finds its way into a navigation channel. This condition 
compels that there be a substantial clearance between the impeller vanes 
thus limiting the number of vanes, and also requires that there be a 
significant clearance between the vane tips and the periphery of the pump 
casing. This is in contrast to water pumps where the clearance between the 
vane tips and the pump casing is kept as small as possible at the point of 
discharge where the water exits from the pump chamber. 
When an object enters the pump that is too large to pass through, the 
dredge must cease operations until the object is removed. In most cases it 
will be found that the object has jammed against the leading edges of the 
impeller vanes, and can be removed through a manhole located in the 
suction pipe immediately in front of the pump. Occasionally the pump will 
have to be disassembled in order to remove the object. 
Another operating condition unique to suction dredging is that when 
dredging an area having a bottom characterized by gas deposits and the 
like, gas can become entrained in the fluid entering the inlet mouthpiece. 
This can cause the main pump to lose its prime, or at least cause it to 
operate in less than an optimal manner. Also, the phenomenon of cavitation 
wherein vapor bubbles form and subsequently collapse within the pump 
degrades performance and subjects the pump to a risk of damage. The 
avoidance of cavitation often entails running the pump more slowly than 
would otherwise be desirable or using an impeller with a smaller diameter. 
Problems due to loss of prime and cavitation may be reduced if the fluid 
material being dredged is pumped to the main pump inlet at a positive 
pressure. It has also been found that the main pump operates more 
efficiently if the fluid at its inlet is at a positive pressure. Such a 
positive pressure allows the pump energy to be transferred to the fluid in 
the form of kinetic energy (velocity head) rather than being transformed 
into static pressure head. Thus the fluid exits the pump at a greater 
velocity, whereby discharge transport is facilitated. 
One method of supplying this positive pressure at the pump inlet presently 
in use has been to provide a water-jet pump booster system which injects a 
high velocity stream of water into the suction pipe aft of the mouthpiece 
to add energy to the suction system. While not actually adding any energy 
to the pump itself, the provision of such a booster system does in some 
conditions allow the pump to operate more efficiently, the increase in 
efficiency more than offsetting the power required by the booster system. 
However, it will be immediately appreciated that the water-jet pump 
booster system has the disadvantage of requiring a greater main pump 
capacity due to the increased volume of fluid passing into the suction 
inlet. In cases where the main pump is already operating to capacity, the 
booster system actually degrades overall performance. 
An alternate approach has been to provide a centrifugal pump on the ladder, 
such a pump being conveniently referred to as a "ladder pump." However, as 
described above, the construction of existing centrifugal dredge pumps is 
such that fluid passing therethrough undergoes one or more right angle 
bends. This introduces frictional losses which undermine the potential 
benefits. Moreover, the most advantageous location for the ladder pump is 
as close to the suction mouthpiece as is practical. However, the 
configuration of the conventional dredge pump with the right angle bends 
precludes locating the pump proximate the mouthpiece because the inlet 
pipe or outlet pipe would extend beyond the sides of the digging ladder. 
The operation of the dredge requires that the digging ladder have as 
compact configuration as possible consistent with accomodating the cutter, 
cutter shaft, the suction pipe and the ladder suspension rigging. 
Extending the width of the digging ladder will interfere with positioning 
the cutter in close-clearance situations, as for example dredging 
alongside a dock. While the ladder pump would preferably be located as 
near the suction mouthpiece as possible, the above-mentioned clearance 
problems tend to dictate a position generally near the top of the ladder. 
Even if placement near the suction mouthpiece is feasible, the relatively 
heavy weight of a conventional centrifugal pump puts a maximum strain on 
the suspension system. This may make it difficult to retrofit dredges with 
ladder pumps without costly modification. 
Thus, while the aforementioned approaches to the problem of increasing main 
pump efficiency and capacity have provided benefits by way of increased 
production, they have been accompanied by offsetting detriments that have 
rendered their implementation less than ideal. Nevertheless, the 
disadvantages described above have been accepted as inevitable for those 
situations where the benefits outweigh the detriments. 
SUMMARY OF THE INVENTION 
The present invention provides a ladder pump that fits within the lateral 
confines of existing ladders and is accompanied by the introduction of 
only small frictional losses in the suction system. The pump of the 
present invention is easily and quickly removed from the ladder for 
maintenance purposes. 
Broadly, a pump according to the present invention is an in-line 
centrifugal pump comprising a casing and a vaned impeller located in the 
casing and mounted for rotation about a normally horizontal axis 
transverse to the ladder. The casing has an inlet and an outlet defining 
respective axes lying in a common plane perpendicular to the impeller axis 
and defining a deflection angle therebetween, with approximately 
60.degree. being typical. The impeller is driven so that the vanes are 
moving toward the barge at their lowest position and away from the barge 
at their highest position. The inlet and outlet axes are generally 
tangential with respect to the impeller, the inlet axis being aligned with 
the suction mouthpiece. Fluid thus enters the pump below the impeller axis 
in a direction generally parallel to the ladder, and exits the pump in a 
direction upwardly inclined from the ladder at the aforementioned 
relatively small angle. The diameter of the impeller and the inner 
dimensions of the casing together define a substantial clearance over at 
least that portion of an impeller vane's travel between the inlet and the 
outlet, so that large solid objects entering the pump may pass under the 
impeller and out the pump outlet. This clearance is preferably larger than 
the smallest clearances in the main pump. 
The impeller may have substantially rigid vanes, in which case the pump has 
a cutwater clearance which extends over that portion of the impeller's 
travel from the outlet to the inlet. Such a clearance is typically 
smallest near the pump outlet and gradually expands toward the pump inlet. 
The smallest clearance preferably corresponds to the smallest clearance in 
the main pump located on the barge. 
In an alternate embodiment, the vanes may be flexible, in which case the 
minimum cutwater clearance may be reduced or eliminated. For the rigid 
vane embodiment, the impeller may be closed, that is, having shrouds at 
each axial end of the vanes. Such shrouds have the effect of providing 
structural support to the vanes and possibly increasing pumping 
efficiency. An alternate embodiment of a rigid vaned impeller has a single 
shroud at one axial end. This permits the impeller to be manufactured in a 
single cavity mold in the event that it is desired to fabricate the 
impeller from a resilient material such as rubber. 
The ladder pump has a simple and compact configuration. In particular, it 
fits in-line with respect to the suction pipe, and has an axial extension 
little wider than the pipe itself. Thus it is readily positioned proximate 
the suction mouthpiece where it can work most effectively. Moreover, the 
simple and compact configuration of the ladder pump is accompanied by a 
correspondingly lower cost, thus making it economically practical to carry 
a spare ladder pump on the barge. Additionally, the compactness and light 
weight make it a relatively easy matter to retrofit existing ladders with 
such a pump without major modification. 
According to a further aspect of the present invention, the pump is held in 
position by flanges that define respective planes at a small angle with 
respect to each other, the flanges being slightly convergent at their 
lower ends, thus allowing the pump to be quickly lifted from its position 
in the suction line should it become necessary to remove the pump for 
repair, replacement, or the like. 
For a further understanding of the nature and advantages of the present 
invention, reference should be had to the remaining portions of this 
specification and to the attached drawings.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a schematic elevational view, partly cut away, showing the main 
components of a standard cutterhead hydraulic dredge 10. Dredge 10 
includes a floating barge 12 having a bow 15 and a stern 17. A downwardly 
sloped ladder 20 is pivotally connected to barge 10 at bow 15 for rotation 
about a transverse axis. A vertical H-frame 22 and a forwardly extending 
upwardly inclined A-frame 25 cooperate with suitable rigging 27 and a 
winch 30 on the barge to raise and lower ladder 20 so that its slope angle 
may be changed to suit the depth of the channel being dredged. Positioning 
and movement of dredge 10 is accomplished with paired vertical spuds, one 
spud 32 which is shown, located at stern 17. The spuds are alternately 
lowered and the dredge rotated thereabout by means of swing cables which 
are attached to swing anchors, not shown. 
With respect to components mounted on ladder 20, additional reference 
should be had to FIG. 2. A cutter shaft 35 extends along ladder 20 and 
carries a cutter 37 at a forward end thereof, cutter 37 being disposed 
immediately ahead of the forward (lower) end of ladder 20. A cutter motor 
40 is mounted to ladder 20 proximate its upper end for transmitting 
rotational motion to cutter 37 via cutter shaft 35. A suction pipe 45 has 
an open end at a suction mouthpiece 47 located immediately behind cutter 
37, and extends upwardly therefrom along ladder 20. Suction pipe 45 
includes a section on ladder 20 and a horizontal section on barge 12, with 
a suitable flexible coupling therebetween. Suction pipe 45 communicates to 
the inlet of a main pump 50 on barge 12. Main pump 50 is powered by a main 
pump engine 52, which may be a diesel engine, an electrical motor, or a 
turbine. Pump 50 is a centrifugal pump having an impeller rotatably 
mounted within a housing with the pump inlet coaxial with the impeller 
axis of rotation and the outlet perpendicular thereto and displaced 
radially therefrom. In operation, cutter 37 is rotated by cutter motor 40 
in order to agitate and cut material on the bottom of the channel, which 
material is sucked into suction mouthpiece 47, and through suction pipe 45 
by main pump 50. Main pump 50 discharges the cut material and water via a 
pipeline 55 to a location suitably removed from the channel being dredged. 
The general principles of hydraulic dredging operations as summarized 
above are set forth in considerable detail in "Hydraulic Dredging" by John 
Huston (Cornell Maritime Press, Inc., 1970) and "Coastal and Deep Ocean 
Dredging" by John B. Herbich (Gulf Publishing Company, 1975 ). 
The present invention provides a ladder pump 60 disposed in line with 
suction pipe 45 at a position as close as convenient to suction mouthpiece 
47. In order to cut down on possible clogging of ladder pump 60, suction 
mouthpiece 47 is preferably fitted with a strainer mechanism comprising 
curved strainer bars 57 rigidly mounted coaxially with cutter shaft 35. 
Cutter 37 carries a cooperating plurality of wiper blades 58 adapted to 
sweep between strainer bars 58 as cutter 37 rotates. Two such strainer 
bars and three such wiper blades are suitable. Thus oversized material is 
kept out of suction pipe 45 by strainer bars 57, and the material is 
prevented from blocking suction mouthpiece 47 by the action of wiper 
blades 58. 
The particular construction of ladder pump 60 is best seen with reference 
to FIGS. 2 and 3. Broadly, ladder pump 60 comprises a housing 65 and an 
impeller 67 mounted within housing 65 on a shaft 68 for rotation about a 
normally horizontal axis 70 transverse to the direction of extension of 
ladder 20. The sense of rotation is indicated by arrow 71, and is 
established by an electric motor or hydraulic motor 72 coupled to shaft 68 
by a flange coupling 73. Although several embodiments of impeller 67 will 
be described below, for the purpose of understanding the basic pump 
operation, a particular embodiment is shown in FIGS. 2 and 3. Impeller 67 
includes a central hub 74 and a plurality of curved back-swept vanes 75 
extending outwardly therefrom. While conventional centrifugal pumps have 
impellers with three to five vanes, impeller 67 has approximately eight 
vanes. Vanes 75 are axially bounded by paired shrouds 77 and 78, which 
shrouds are circular and coaxial with shaft 69. 
Housing 65 defines an inlet 80 and an outlet 85, each defining a respective 
direction perpendicular to impeller shaft 68 and generally displaced 
therefrom. Fluid flowing through casing 65 from inlet 80 to outlet 85 
passes underneath impeller 67 along a path generally following the 
rotation of impeller 67. The direction of flow at outlet 85 is typically 
deflected with respect to the direction of flow at inlet 80, thus defining 
a deflection angle 87. Deflection angle 87 represents a compromise. On one 
hand, a certain amount of impingement is necessary in order that the 
impeller vanes do the necessary work in pumping fluid. On the other hand, 
a large deflection angle increases frictional losses within ladder pump 60 
and necessitates other sharp bends in the ladder discharge pipe 45, 
thereby reducing the compactness of the configuration and introducing 
further frictional losses. An angle of approximately 60.degree. has been 
found suitable, but, depending on materials and conditions, other 
deflection angles less than approximately 90.degree. may be suitable. In 
some cases, the inlet and outlet may be parallel. 
For convenience, housing 65 may be considered to have a lower housing 
portion 90 extending from inlet 80 to outlet 85 below impeller 67 and an 
upper housing portion 92 extending from inlet 80 to outlet 85 generally 
above impeller 67. Lower housing portion 90 provides a smooth transition 
between inlet 80 and outlet 85, while upper housing portion 92 joins to 
the inlet and outlet at substantial angles, providing a cutwater 95 where 
it meets outlet 85. 
Lower housing portion 90 has an inner surface 96 that is preferably spaced 
radially outward from the radially outermost portion of impeller 67 to 
define a clearance dimension 97. The purpose of clearance dimension 97 is 
the accomodation without clogging of relatively large solid objects that 
are drawn into suction mouthpiece 47. Upper housing portion 92 similarly 
defines a clearance with respect to impeller 67, but this clearance is 
generally smaller than clearance dimension 97 since relatively large solid 
objects are expected to be discharged through outlet 85 and not circulate 
entirely around within the pump. This upper clearance may have a first 
dimension 100 at cutwater 95, expanding to a second larger dimension 102 
proximate inlet 80, the change in dimension being gradual, preferably 
following a volute curve. 
FIGS. 4A, 4B, and 4C are fragmentary perspective views of alternate 
embodiments of impeller 67 with like reference numerals designating 
corresponding elements. For reference purposes, FIG. 4A shows a closed 
impeller as illustrated in FIG. 2. In particular, shrouds 77 and 78 
provide structural support for vanes 75 and thus impart rigidity to 
impeller 67. FIG. 4B illustrates an impeller similar to that shown in FIG. 
4A, except that shroud 77 is the only shroud. This configuration is 
advantageous in the event that it is desired to fabricate impeller 67 from 
a resilient material by a molding process. In particular, it will be 
appreciated that a single shrouded impeller as shown may be manufactured 
in a single cavity mold. 
While the impeller embodiments of FIG. 4A and 4B have been characterized by 
one or two shrouds extending the entire radial dimension of the impeller, 
FIG. 4C shows an embodiment wherein vanes 75 extend radially beyond the 
shroud dimension. This embodiment is advantageous for an impeller made in 
whole or in part of a resilient material where it is desired to have 
flexible vanes. FIG. 4C shows an impeller having a single shroud 77' that 
has a radial extent less than that of vanes 75. Thus, vanes 75 are 
provided with a certain amount of structural reinforcement where they join 
to hub 74, but are free to flex at their outer ends. As discussed above, 
the single shroud embodiment is especially well adapted to fabricating the 
impeller from a material such as rubber. In the event that impeller 67 is 
provided with flexible vanes, either as permitted by one or more shrouds 
of lesser radial dimension or by the elimination of shrouds entirely, 
cutwater clearance dimension 100 may be substantially reduced, even being 
reduced to zero so that the vanes impinge on the inner surface of upper 
housing portion 92. 
Pump casing 65 is of standard metal construction suitable for a pump of a 
given size, and is preferably fitted with a wear liner 103 as described in 
my U.S. Pat. No. 4,120,605, issued Oct. 17, 1978, entitled "Wear Liners 
For Abrasive-Material Handling Equipment". Wear liner 105 according to the 
above referenced U.S. Patent, is of composite construction and includes a 
layer of vulcanized rubber or equivalent material with one or more sheets 
of abrasive resistant wire mesh embedded therein. Casing 65 is preferably 
provided with stuffing boxes 104 at the locations where impeller shaft 68 
penetrates the casing. 
While the size and dimensions of ladder pump 60 are dependent upon the size 
of the dredge and the main pump in conjunction with which the ladder pump 
is used, some representative dimensions will be set forth below for the 
purpose of illustration only in order to put the various clearances in a 
better perspective. In particular, the dimensions set forth below are 
reasonable for a pump having an inlet and outlet defined by a first 
transverse dimension parallel to impeller axis 70 of 32 inches and a 
second transverse dimension perpendicular to impeller axis 70 of 22 
inches. From a flow point of view, this corresponds approximately to a 
circular pipe having an inner diameter of 30 inches. In such a situation, 
impeller 67 has a diameter of 6 feet, with hub 72 having a diameter of 3 
feet. Lower clearance dimension 97 is approximately 11 inches, cutwater 
clearance dimension 100 is approximately 5 inches, and clearance dimension 
102 is approximately 9 inches. A pump having these general dimensions 
would be suitable for use in a barge having a main pump driven by a 8,000 
horsepower engine, and ladder pump motor 72 would be required to have an 
output of approximately 800 horsepower. A radial clearance of 
approximately 7 inches between adjacent strainer bars 57 would be 
appropriate for a configuration having the above dimensions. 
With specific reference to FIG. 2, it can be seen that pump 60 is located 
to one side of cutter shaft 35 and ladder pump motor 72 is located on the 
other side. This configuration is convenient and is made possible by the 
fact that pump 60 has a relatively narrow axial dimension (approximately 3 
feet) that is not significantly wider than the transverse extent of 
suction pipe 45. The relatively short portion of suction pipe 45 located 
between centrally located suction mouthpiece 47 and off-center ladder pump 
60 is angled with respect to the direction of cutter shaft 35 in order to 
achieve the needed transverse displacement. Pump 60 is fastened to 
respective inlet and outlet segments 107 and 108 of suction pipe 45 by 
means of first and second mating flange pairs 110 and 112. In order to 
facilitate removal and replacement of pump 60 within the suction line, 
flange pairs 110 and 112 are preferably not parallel, but rather converge 
slightly in the downward direction. An inclination of about 5.degree. is 
suitable. The tapered configuration provides an increasing clearance as 
pump 60 is removed from the line, and a guiding and self-positioning 
function when pump 60 is placed in position. 
Having set forth the structure of ladder pump 60 and the environment in 
which it operates, the operation may be understood. In particular, ladder 
pump motor 72 causes impeller 67 to rotate at approximately 600-900 
revolutions per minute, with the result that a suction is provided at 
suction mouthpiece 47 for drawing water and entrained solid material 
through inlet 80 into the interior of pump 60. The energy supplied to pump 
60 provides the incoming fluid with kinetic energy and discharges it 
through pump outlet 85 with sufficient head to enter the inlet of main 
pump 50 at a positive pressure. As discussed in the introductory portions 
of this patent application, a positive pressure at the inlet of the main 
pump allows the main pump to operate at a considerably higher efficiency 
and capacity, thereby allowing a greater output from the dredge. It should 
be noted that the design of ladder pump 60, dictated by a desire to avoid 
sharp bends in the fluid flow path and a desire to provide relatively 
large clearances for trash and debris, results in a pump having a 
relatively low efficiency. However, the increase in main pump efficiency 
far exceeds any inefficiencies and losses within ladder pump 60. 
In summary, it can be seen that the present invention provides a ladder 
pump having a surprisingly compact and simple configuration, thus allowing 
the pump to be situated on the ladder in a position very close to the 
suction mouthpiece without interfering with other components mounted on 
the ladder. While the above description provides a full and complete 
disclosure of the preferred embodiments of the invention, various 
modifications, alternate constructions, and equivalents may be employed 
without departing from the true spirit and scope of the invention. For 
example, the orientation and location of the pump on the ladder could be 
changed in order to accomodate different ladder configurations. Therefore, 
the above description and illustration should not be construed as limiting 
the scope of the invention which is defined by the appended claims.