Patent Description:
Impellers of early dredge pump designs consisted of two flat shrouds with simple blades extending between. Each blade was curved with a single curve, most commonly with a circular shape. The blade angles were calculated for an estimate of the best efficiency point and typical working conditions of the pump. The shape of the blade was formed by simple plate-like sides with uniform thickness along the length of the blade. Such blades are still used in dredge pumps operating around the world.

The next step in dredge pump evolution was the introduction of curved shrouds or shields and double curved blades. These curved shrouds and double curved blades improved pump performance, in particular with respect to hydraulic efficiency. The uniform thickness of the blades generally remained the same though.

The first change into a varying impeller blade thickness came with a new design for a large spherical passage pump, shown in <CIT>. For this impeller, the thickness of the blades was enlarged at the leading edge to enhance its suction capabilities. This increase in thickness, however, was only a minor change in the thickness, about a <NUM>% change in thickness between the thickest and thinnest parts of the blade cross-section, and in practice was barely noticeable. As disclosed in <CIT>, adding a strip of material to the blades along the radial inner ends to change the curvature of the blade can help in controlling flow and energy transfer from the blades to the mass being pumped. The strip is disclosed as covering up to <NUM>% of the total length of the rotor blade from the radial inner end to radial outer end. A further development of this blade was made and presented at a conference. The blade had an increased thickness on the suction side. The blade presented can be seen in <FIG>. Such an increase in thickness on only one side can result in wear that leads to sharp edged and flow separation.

The dredge pump of <CIT> comprises impellers having a thickness of the blades being enlarged at the trailing edge, so when sand circulates through the interior of the pump, the impeller can resist such an impact and wear of the trailing edge of the blade for longer period of time. Besides, as it is intended in <CIT>, the fillets comprise a number of openings that are meant to provide securing means of the impeller to the shield by means of bolts seated in those openings. The impeller disclosed in <CIT> has as a main drawback that it is subject to breakage due to presence of leading edge vortex that will have a major impact on the leading edge of the blade as well as of some other parts of the pump. Moreover, the blade itself is prone to misalignment when objects impact a number of times, which can affect the tightness of the bolts and this could create serious problems in the operation of the pump having this kind of blades. Further reference is also made to <CIT> which discloses a centrifugal pump impeller for fluids containing solids, which has a flat ascent transition between the cover plates of the impeller and the leading edge of the blades.

According to the invention, an impeller for a centrifugal pump defined by claim <NUM> is provided. Preferred embodiments thereof are further defined in dependent claims <NUM>-<NUM>.

Among other features, such an impeller for a centrifugal pump comprises a shaft shield; a suction shield connected to the rotor and axially set apart from the shaft shield; and a plurality of blades between the shaft shield and suction shield. Each blade comprises a leading edge and a trailing edge connecting between the shaft shield and the suction shield and a suction side and a pressure side, wherein each blade cross-section is thicker near the leading edge on the suction and pressure sides, and tapers to a thinner cross-section near the trailing edge. Each blade connects to the suction shield at the leading edge with an extensive fillet providing curvature toward the suction shield along the leading edge.

Providing a blade that is thicker on pressure and suction side near the leading edge and has an extensive fillet providing curvature toward the suction shield at the leading edge helps to avoid leading edge vortices and therefore increase the working performance and efficiency of the impeller and pump. The avoidance of a leading edge vortex (LEV) is a key feature in the high performance of dredge pumps. Prior art dredge pumps all suffer from LEV, causing damage to impeller or other parts of the pump. This avoidance is achieve by the extensive fillet, and improving the contact surface of the impeller with the front and/or back shrouds or shields. It should be noted that the fillet can be finished when modifying the blade or impeller such that the shape and dimensions are tailor-made for specific centrifugal dredge pumps having different dimensions or working requirements.

According to an embodiment, each blade comprises forward sweep. Providing forward sweep can help to improve flow uniformity, resulting in higher hydraulic efficiency. Additionally, wear characteristics can be improved, as the blade will wear down to an unswept state, thereby increasing the blade and/or impeller workable lifespan.

According to an embodiment, the cross-section of each blade is <NUM>% to <NUM>% thicker at the thickest point near the leading edge than near the trailing edge. By increasing the thickness near the leading edge on both the pressure and the suction side, blade is able to ensure that flow separation is avoided over a large working range, perhaps the entire working range of the specific impeller. Further, adding material for a thicker blade at the leading edge can also improve blade resistance to wear and ensure that the blade maintains a smooth, rounded shape even when the blade experiences wear. The smooth rounded shape helps to maintain smooth flow and pump performance.

The thicker cross-section is formed of a fillet which wraps around the leading edge of the blade and has a standard thickness that extends to both the pressure side and the suction side at the leading edge. This standard thickness can be, for example, about the same thickness as the original blade shape the fillet is wrapping around. The standard thickness extends about <NUM>% of the blade length between leading edge and trailing edge before tapering toward the trailing edge.

According to an embodiment, each blade has a maximum cross-sectional thickness at <NUM>% to <NUM>% along the length of the blade between the leading edge and the trailing edge, after which the blade tapers in cross-sectional thickness toward the trailing edge. Such a shape can help to ensure smooth flow and operation of the impeller.

According to an embodiment, a blade wrap angle of each blade is variable. Optionally, the blade wrap angle is between <NUM> and <NUM> degrees with a forward sweep. A variable blade wrap angle with a forward sweep can help blade wear characteristics, ensuring that incident flow does not impact perpendicular on the blade leading edge, resulting in less wear.

According to an embodiment, the fillet extends over <NUM>% to <NUM>% of the leading edge length between suction and shaft shields. Such an extensive fillet can help to protect the leading edge of the blade and ensure better flow and wear characteristics in the impeller.

According to an embodiment, the fillet height along the suction shield is <NUM>% to <NUM>% of the blade thickness. Such a fillet helps to guide flow to improve impeller performance and wear characteristics.

According to an embodiment, the blade connects to the shaft shield with a fillet at the leading edge. Such a fillet can be similar to the fillet connecting the blade to the suction shield, and can improve impeller performance and wear characteristics in a similar manner.

According to a further aspect of the invention, a centrifugal pump comprises an impeller according to the invention, i.e., as defined by claim <NUM>, and further comprises a pump housing with an axial inlet and an outlet. The impeller is connected to the pump housing through the rotor being connected to the pump housing such that the rotor can rotate around an axis A; and the shaft shield has the axial supply aligned with the axial inlet.

According to another aspect of the invention a vessel can include the centrifugal pump described above.

According to the invention, also a method of modifying a blade for a centrifugal pump defined by claim <NUM> is provided. Among other features, such a method comprises adding material to the blade at and near the leading edge to the suction and pressure sides of the blade; and tapering the added material in a direction toward the trailing edge. Such a method can adapt new or prior art blade into a blade for an impeller can help to promote smooth flow and overall impeller and pump efficiency as well as improve wear characteristics. Adding material at the leading edge can reduce or eliminate the formation of horse-shoe vortices at the leading edge and increases the range around the best efficiency point where flow remains attached to the blade.

The step of adding material to the blade at and near the leading edge comprises wrapping material around the leading edge and extending toward the trailing edge on both sides of the blade such that the material is a constant thickness for <NUM>% of the blade length between leading edge and trailing edge. By using material that is a constant thickness for about <NUM>% of the blade length, a large increase in the range at which flow remains attached to the blade can be seen. This range is also beneficial for the wear characteristics, greatly increasing the thickness near the leading edge, and then tapering to use less material and therefore have a lighter blade where the thickness is not needed.

According to an embodiment, the material added is the same material as that of the blade. This can include the exact same material, or partially the same material, for example, alloys or mixtures of the same material and another material.

<FIG> is a front view in cross section of a centrifugal pump <NUM>, and <FIG> is a side view in cross section along the line II - II in <FIG>.

Centrifugal pump <NUM> comprises a pump housing <NUM> shaped like a volute (spiral casing). The pump housing <NUM> has a circumferential wall <NUM> and a spout-shaped outlet <NUM> attached tangentially to the circumferential wall <NUM> of the pump housing <NUM>. The junction between the inner surface of the tangential outlet <NUM> and the inner surface of the circumferential wall <NUM> of the pump housing <NUM> defines what is known as a cutwater <NUM>. The pump housing <NUM> also has an axial inlet <NUM>.

A rotor <NUM> is attached in the pump housing <NUM> such that it may rotate about an axial rotation axis A. The rotor <NUM> has a central boss <NUM> which may be fastened to a drive shaft (not shown). A shaft shield <NUM> extends from the central boss <NUM>. The shaft shield <NUM> forms a first wall or shroud for delimiting the flow within the rotor <NUM>. Axially set apart from the shaft shield, rear shroud or back shroud <NUM>, the rotor has a suction shield or front shroud <NUM> which defines a second wall for delimiting the flow within the rotor <NUM>. The suction shield <NUM> has an axial supply <NUM> which is aligned with the axial inlet of the pump housing <NUM>.

A plurality (four in Fig.'s <NUM> and <NUM>) of rotor blades <NUM> are fastened between the shields <NUM>, <NUM>, whereby the blade <NUM> leading edge <NUM> and front shroud <NUM> are joined through a connection <NUM> with a fillet. In this illustrative embodiment, the rotor <NUM> comprises four rotor blades <NUM>. The rotor blades <NUM> each extend substantially radial to the rotation axis A. Each rotor blade <NUM> comprises a leading edge <NUM> and a trailing edge <NUM>. The leading and trailing edges <NUM>, <NUM> extend between the shaft shield <NUM> and the suction shield <NUM>. Between the trailing edges <NUM> of the rotor <NUM> and the inner surface of the circumferential wall <NUM> of the pump housing <NUM> there is a circumferential channel <NUM>. The circumferential channel <NUM> has a passage surface area which increases somewhat in the circumferential direction from the cutwater <NUM> toward the outlet <NUM>.

During operation, the rotor <NUM> rotates about the rotation axis A. Between the rotor blades <NUM>, the mass to be pumped is forced radially outward into the pump housing <NUM> under the influence of centrifugal forces. Said mass is then entrained in the circumferential direction of the pump housing <NUM> toward the tangential outlet spout <NUM> of the pump housing <NUM>. The pumped mass which, after leaving the rotor <NUM>, is entrained in the circumferential direction of the pump housing <NUM> flows largely out of the tangential outlet of the pump housing <NUM>. A small amount of the entrained mass recirculates, i.e. flows along the cutwater back into the pump housing <NUM>.

Said centrifugal pump <NUM> can be used in dredging operations. If the centrifugal pump <NUM> is located on board a dredging ship, such as a cutter suction dredger or hopper suction dredger, the centrifugal pump <NUM> has to fetch a loose mixture of substances, possibly including soil, stones and/or pebbles, from the sea floor. This mixture passes through pump <NUM>, and can cause a large amount of wear on pump <NUM> and pump components, particularly blades <NUM>.

<FIG> shows a cross-section of a blade <NUM>' according to the prior art, <FIG> shows a second prior art blade <NUM>". Blades <NUM>', <NUM>" includes leading edge <NUM>' and trailing edge <NUM>'. As can be seen in the cross-section, blade <NUM>' has a thickness which is substantially the same from the leading edge <NUM>' to the trailing edge <NUM>', with a small increase in thickness near the leading edge <NUM>'. The thickest section of blade <NUM>' is about <NUM>% thicker than the thinnest section in this prior art blade. Blade <NUM>" had a larger increase in thickness at leading edge, though this is only on the suction side <NUM>' and not on the pressure side <NUM>'.

In prior art pumps, blades <NUM>' typically have a rather sharp leading edge <NUM>' which is designed for the pumps best efficiency point ("BEP"). This is the design point where the blade and incident flow are usually aligned, such that the flow incidence angle is close to zero, also referred to as the shock-free entrance condition. At flow rates beyond the BEP, the incidence angle increases, and when it becomes too large, the flow is no longer able to follow the blade contour and separates from the blade surface. This has a negative effect on the suction capacity of the centrifugal pump, reducing overall pump efficiency. It also may result in cavitation and subsequent wear of the centrifugal pump.

<FIG> shows a cross-section of a blade <NUM> according to the current invention. Blade <NUM> has a leading edge <NUM>, trailing edge <NUM>, suction side <NUM> and pressure side <NUM>. At and near leading edge <NUM>, blade <NUM> has an increase in thickness around both the suction side <NUM> and pressure side <NUM>. This increase in thickness is substantial, for example in the range of <NUM>%-<NUM>% thicker at the thickest part of blade <NUM> than at the thinnest. This can be even higher in many cases, up to <NUM>%-<NUM>% thicker at the thickest part than the thickness of original blade <NUM>. There is a taper between the thicker part near the leading edge <NUM> and the trailing edge <NUM> for a smooth transition between the thickest part and the thinner part. Blade <NUM> is shown having a thickest part which is about <NUM>% thicker than the thinnest part. According to the invention, the cross-section of blade <NUM> is at least <NUM>% thicker on both suction and pressure sides <NUM>, <NUM> near leading edge <NUM> than near trailing edge <NUM>.

In <FIG>, blade <NUM> is thickened at and near the leading edge <NUM> by adding an extensive fillet <NUM> which wraps around an original blade shape <NUM> on both suction side <NUM> and pressure side <NUM> of blade <NUM>. This fillet <NUM> has a variable radius, starting with a large radius at the blade leading edge, while gradually decreasing to a small radius at the blade trailing edge. Fillet <NUM> can be the same material of original blade <NUM> or a different material. According to the invention, fillet <NUM> has a large, constant radius which wraps around about the first <NUM>% of the length of blade <NUM> between leading edge <NUM> and trailing edge <NUM> before tapering towards trailing edge <NUM>. Fillet <NUM> thus tapers toward trailing edge <NUM> such that blade cross-section is thinner at trailing edge <NUM>. The width of fillet <NUM> at leading edge <NUM> can be about the same thickness of blade <NUM>, resulting in a width of blade <NUM> near leading edge <NUM> of up to <NUM>% the thickness of blade <NUM>. Further, the width of fillet <NUM> near leading edge <NUM> can about twice as thick as fillet <NUM> thickness at an intermediate point between leading edge <NUM> and trailing edge <NUM>.

The blade <NUM> can be formed in this shape, or can be formed by adding material later to a prior formed blade <NUM>, and machined to form a smooth taper. Such a method can be used to modify prior art blades to have better flow and wear characteristics, making the formation of blades <NUM> even more economical by not having to form and replace prior art blades <NUM>', <NUM>" with entirely new blades.

According to the invention, the cross-section of blade <NUM> is at least <NUM>% thicker on both suction and pressure sides <NUM>, <NUM> near leading edge <NUM> than near trailing edge <NUM>. By making the blade <NUM> with a profile that is thicker at the leading edge <NUM> on both pressure and suction sides <NUM>, <NUM> than at a thinnest part near trailing edge <NUM> blade <NUM> is less sensitive to the flow incidence angle, allowing flow to remain attached to the blade surface even at larger incidence angles. By increasing the thickness at and near leading edges <NUM>, blade <NUM> has a larger range around its BEP where flow remains attached, keeping a smooth flow and efficiency in the pump over a large flow range. This can be especially helpful with decreased flow rates with increased incidence angles and avoiding the formation of vortices at the leading edge. Such a substantial increase in thickness can result in the blade <NUM> being able to prevent flow separation at all flow conditions within the pump working range. The ability to maintain attached flow also leads to the leading edge <NUM> maintaining its rounded shape during wear through use. Prior art blades, such as the ones shown in <FIG>, had a tendency to form a sharp edge at suction side <NUM>' as they were worn down through use. Because of the ability to maintain attached flow and fillet <NUM> wrapping around both sides of blade <NUM>, blade <NUM> maintains a smooth rounded shape with even wear, leading to better pump efficiency even as blade <NUM> experiences wear.

Additionally, this increase in thickness at leading edge <NUM> provides extra "wear material" in the region of highest wear of the blade <NUM>. This works to increase the overall blade <NUM> and pump <NUM> lifespan. Further, fillet <NUM> works to reduce horse-shoe vorticity wear on blade <NUM>. The large radius at the front portion of the blade <NUM> serves to prevent the formation of a horse-shoe vortex at the intersection of the blade leading edge <NUM> with the front and back shrouds. The horse-shoe vortex forms when the flow along the shrouds impact frontally on the blade leading edge <NUM>. The fillet <NUM> serves to avoid this frontal impact, and therefore the horse-shoe vortex formation, by gradually guiding the flow over the blade leading edge <NUM>. Note that the large fillet radius wraps around the blade leading edge <NUM>, therefore, frontal impact is avoided for a range of incidence angles corresponding to a working range around the best efficiency point. As a result, wear properties are improved not just at the best efficiency point but over a range below and above the best efficiency point.

<FIG> is a perspective view of the prior art connection <NUM>' between a blade <NUM>' leading edge <NUM>' and a front shroud <NUM>', and <FIG> is a side view of blade <NUM>', showing the prior art connection <NUM>' between the leading edge <NUM>' and the front and rear shrouds <NUM>', <NUM>'.

<FIG> shows a perspective view of the blade <NUM> leading edge <NUM> and front shroud <NUM> connection <NUM> according to an embodiment of the current invention, illustrating a further specification of the invention; and <FIG> shows a side view blade <NUM>, showing the connections <NUM> between the blade leading edge <NUM> and the front and rear shrouds <NUM>, <NUM>, showing connecting fillet <NUM> at connection <NUM>. Connecting fillet <NUM> can extend about <NUM>% to <NUM>% across the length of leading edge <NUM> between front shroud <NUM> and rear shroud <NUM>. The height of connecting fillet <NUM> along front shroud <NUM> can be about <NUM>% to <NUM>% of the thickness of blade without connecting fillet <NUM> (see <FIG>, original blade <NUM> thickness). The skilled person will appreciate that the connecting fillet <NUM> can be provided by known procedures such as casting, material deposition, welding, additive manufacturing, et cetera. By using one of these techniques, the implementation of the fillet becomes very versatile, independently of the dimensions of the pump and of the material employed for the fillet itself. Further, a connecting fillet <NUM> could also be included to connect blade <NUM> to back shroud <NUM>.

In prior art pumps with the connection shown in <FIG>, a horse-shoe vortex sometimes formed at the intersection of the blade leading edge <NUM>' with the front and/or back shrouds <NUM>', <NUM>'. The horse-shoe vortex forms when the flow along shrouds <NUM>', <NUM>' impacts the blade leading edge <NUM>'. This can cause severe local damage when pumping slurry flows, and can also increase flow non-uniformity, resulting in hydraulic efficiency reduction.

The blade <NUM> of the current invention adds connecting fillet <NUM> to curve leading edge <NUM> toward front shroud <NUM> to obtain a smooth transition, as shown in <FIG>. Such a smooth transition minimizes frontal impact of the flow along front shroud <NUM> on blade <NUM> leading edge <NUM>. Thus, the addition of fillet <NUM> helps to gradually guide flow along leading edge <NUM>, thereby minimizing or avoiding horse-shoe vortices and the associated damage. A similar fillet can also be added at the connection to back shroud <NUM>, though not shown in <FIG>.

<FIG> shows a perspective view of an impeller illustrating a further specification of the invention, and showing the blade wrap angles "E". Typical prior art impellers had a constant wrap angle from suction shield <NUM> to shaft shield <NUM>, for example about <NUM> degrees. The impeller shown in <FIG> has a variable wrap angle which increases from shaft shield <NUM> to suction shield <NUM>. This increase can be, for example, Ehub = <NUM> deg. To Eshroud = <NUM> deg. Increasing the wrap angle, for example between zero and sixty degrees, from shaft shield <NUM> to suction shield <NUM> results in forward sweep of blade <NUM>. This leads to improved flow uniformity, resulting in higher hydraulic efficiency.

Sweeping blade <NUM> also benefits wear characteristics. The incident flow will not impact perpendicular on a swept blade leading edge <NUM>, but at an angle, resulting in less wear than on a non-swept blade which has perpendicular impacts. Forward sweep also increases blade <NUM> length in the direction of increasing inlet velocity and thus in the direction of increasing wear. The inlet velocity of blade <NUM> increases from shaft shield <NUM> to suction shield <NUM> simply because the radius of the blade <NUM> leading edge <NUM> increases in this direction. In time, a forward swept blade will wear off towards a non-swept geometry whereas a non-swept blade will wear off towards a backward sweep. Thus, adding a forward sweep to blade <NUM> helps the impeller deteriorate more slowly over time. Further, forward sept blades can generate a significant increase in blade overlap, which leads to an increase in flow uniformity.

In summary, impeller with blade <NUM> which has an increased thickness at and near the leading edge <NUM> on both suction and pressure sides and an extensive connecting fillet <NUM> at connection <NUM> of leading edge <NUM> to suction shield <NUM> results in an overall more efficient pump and a blade better able to resist wear and prolong the overall working life of the blade <NUM> and overall pump <NUM>. By making blade <NUM> have a cross-sectional thickness about <NUM>%-<NUM>% thicker at or near the leading edge <NUM>; blade <NUM> is less sensitive to the incident flow beyond the best efficiency point and allows flow to remain attached to the blade surface even at larger incidence angles. This can keep smooth flow and efficiency in the pump over a large flow range, and the additional material serves to protect blade <NUM> against wear, increasing the lifespan of blade <NUM>. The ability to add material at and near leading edge <NUM> and taper toward trailing edge <NUM> allows for prior art blades <NUM>', <NUM>" to be modified and adapted to have a thicker section and thereby gain the desired flow and wear characteristics without having to totally replace all prior art blades <NUM>' in prior art pumps.

Adding extensive connecting fillet <NUM> at connection <NUM> between leading edge <NUM> and suction shield <NUM> provides a smooth transition that minimizes frontal impact of the flow along front shroud <NUM> on blade <NUM> leading edge <NUM> and helps to gradually guide flow to minimize or avoid horse-shoe vortices and the associated damage. Adding a forward sweep to blade helps to reduce the impact velocity of incident flow and creates additional blade length in the direction of increasing wear to further prolong the life of blade <NUM>.

While the embodiment has been shown with four blades, it will be understood that any suitable number of rotor blades may be provided, such as for instance three or five rotor blades <NUM>. Further, while specific blade and fillet geometries have been shown, these are for example purposes only. Within the scope of the invention which is solely defined by the appended claims,, the thickening of blade <NUM> could be different sizes as well as different thickening and tapering geometry toward trailing edge <NUM>. Further the sweep angles provided are also examples, and different pumps can have different sweep angles.

Claim 1:
Impeller for a centrifugal pump, the impeller comprising:
- a shaft shield (<NUM>);
- a suction shield (<NUM>) axially set apart from the shaft shield (<NUM>) and having an axial supply (<NUM>); and
- a plurality of blades (<NUM>) between the shaft shield (<NUM>) and suction shield (<NUM>);
- wherein each blade (<NUM>) comprises a leading edge (<NUM>) and a trailing edge (<NUM>) connecting between the shaft shield (<NUM>) and the suction shield (<NUM>), and a suction side (<NUM>) and a pressure side (<NUM>),
wherein each blade (<NUM>) connects to the suction shield (<NUM>) at the leading edge (<NUM>) with an extensive connecting fillet (<NUM>, <NUM>) providing curvature toward the suction shield (<NUM>) along the leading edge (<NUM>),
wherein the extensive connecting fillet (<NUM>, <NUM>) extends toward the trailing edge (<NUM>) and tapers toward the trailing edge (<NUM>) and defines a blade cross-section and a blade thickness,
wherein each blade cross-section is thicker near the leading edge (<NUM>) on the suction and pressure sides (<NUM>, <NUM>) and tapers such that the cross-section is thinner near the trailing edge (<NUM>),
wherein the blade (<NUM>) cross-section is at least <NUM>% thicker on both suction and pressure sides (<NUM>, <NUM>) near the leading edge than near the trailing edge, and
wherein the extensive connecting fillet (<NUM>, <NUM>) has a large constant radius which wraps around about the first <NUM>% of the blade length between the leading edge (<NUM>) and the trailing edge (<NUM>) before tapering towards the trailing edge (<NUM>).