Method for controlling a pump unit and pump unit for pumping a liquid or a suspension

Abstract

The invention relates to a pump unit (1) for pumping a liquid or suspension, the pump unit (1) comprising a centrifugal pump (2) and an inducer (3) near upstream of an inlet (210) of the centrifugal pump (2):-the centrifugal pump (2) comprising a pump housing (21) in which a flow channel (20) is formed and an impeller (22) configured to rotate in the flow channel (20) by a shaft (23) about a central axis (200), -the inducer (3) comprises an inducer rotor (32) having a sleeve-like rotor body (321), the inducer rotor (32) being configured to rotate about a central axis (200) in the flow channel (20), the rotor body (321) being provided with a plurality of blades (323) extending inwardly from the rotor body (321), -the inducer rotor (32) is separate from the impeller (22) and the rotational speed of the inducer rotor (32) is independently controllable with respect to the rotational speed of the impeller (22), the inducer (3) comprising an electric motor (35) which is an annular motor surrounding and connected to the inducer rotor (32).

Description

Technical Field

[0001] The invention relates to a method for controlling a pump unit according to the preamble of independent claim 1. In particular, the invention relates to a method for controlling a pump unit for pumping a liquid or a suspension, the pump unit comprising a centrifugal pump and an inducer upstream of an inlet of the centrifugal pump:

[0002] - the centrifugal pump comprises a pump housing and an impeller, a flow passage is formed in the pump housing, the impeller is configured to rotate in the flow passage about a central axis via a shaft,

[0003] The inducer comprises an inducer rotor having a sleeve-like rotor body, the inducer rotor being configured to rotate about a central axis in the flow channel, the rotor body being provided with a large number of blades extending inwardly from the rotor body.

[0004] The invention also relates to a pump unit according to the preamble of the second independent claim. Background Art

[0005] Centrifugal pumps with conventional inducers are used in pumping applications where there are many challenging pumping conditions, such as multiphase suspensions due to fluids consisting of gas, liquid and solid phases. These conventional inducers are mounted on the same shaft as the impeller and therefore rotate at the same speed as the impeller.

[0006] Prior art disclosure CN111828391A provides a shaftless inducer with adjustable blades for a pump. The inducer includes a pump housing and an impeller, wherein the impeller is arranged in the pump housing, a plurality of inducer blades are evenly distributed on the extended section of the impeller cover of the impeller, a positioning device is arranged at the junction between each inducer blade and the impeller cover, an external force enables the inducer blade to overcome the positioning of the positioning device and drive the inducer blade to rotate so as to adjust the angle of the inducer blade, and a thrust rotation device is arranged outside the pump housing and used to provide external force. According to the inducer, by adjusting the placement angle of the blade, the pump can operate at the optimal working condition point under different working conditions; the inducer is suitable for the condition where the inlet inlet flow is a low pressure; and the inducer adopts a hubless form, and the blades are arranged at the extension part of the impeller cover so as to reduce the weight of the inducer, provide a larger inlet area and then improve the efficiency of the pump.

[0007] The following measurement for expressing Net Positive Suction Head (NPSH, or "net positive suction head") in relation to centrifugal pumping systems will be used in this disclosure. It is equal to the total pressure expressed as the height (head in meters) of the liquid column of the pumped medium:

[0008] -NPSHa: Net Positive Suction Head Available at the pump impeller inlet. NPSHa is a value that expresses the total pressure of the liquid as it enters the pump. It is a measure of the pressure that the liquid is subjected to between its current state and the formation of vapor bubbles (boiling).

[0009] -NPSHr: The Net Positive Suction Head Required that the pump must operate without experiencing destructive cavitation and a dramatic reduction in pumping output. NPSHr is a value that expresses the minimum total head that must act on the liquid as it enters the pump impeller in order to avoid excessive cavitation and deterioration of pump performance.

[0010] The object of the present invention is to provide an inducer / centrifugal pump combination, wherein the performance is considerably improved compared to prior art solutions. One object is to enhance the versatility of the inducer / centrifugal pump combination, so that it will be suitable for pumping under various operating conditions (e.g. because of changes in the inlet pressure due to varying surface levels in the tank or due to changes in the prevailing viscosity of the fluid to be pumped). As the inducer / centrifugal pump combination is intended to be used continuously for several years 24 / 7 in the process industry, pumping efficiency is one of the highest priorities of the inducer / centrifugal pump combination. Summary of the invention

[0011] The objects of the invention may be substantially met as disclosed in the independent and other claims describing more details of different embodiments of the invention.

[0012] According to an embodiment of the present invention, there is provided a pump unit for pumping a liquid or a suspension, the pump unit comprising a centrifugal pump and an inducer located upstream of an inlet of the centrifugal pump:

[0013] - the centrifugal pump comprises a pump housing and an impeller, a flow passage is formed in the pump housing, the impeller is configured to rotate in the flow passage about a central axis via a shaft,

[0014] the inducer comprises an inducer rotor having a sleeve-like rotor body, the inducer rotor being configured to rotate about a central axis in the flow channel, the rotor body being provided with a large number of blades extending inwardly from the rotor body,

[0015] The inducer rotor is separate from the impeller and the rotational speed of the inducer rotor is independently controllable with respect to the rotational speed of the impeller, the inducer comprising an electric motor which is a ring-shaped motor surrounding and connected to the inducer rotor.

[0016] This provides an effect by means of which the performance of the pump unit can be considerably improved. In challenging pumping conditions, conventional inducers have been able to make a difference between pumping in operation / out of operation, while the present invention can achieve the best results in terms of NPSH in the required pumping conditions, net efficiency and operating flexibility. Independently controllable inducers can be used to create just the required additional NPSH in order to help the centrifugal pump and avoid cavitation, or in order to fluidize the suspension, or in order to separate the gas from the pumping medium. The rotational speed of the inducer rotor is variably and / or independently controlled with respect to the rotational speed of the impeller. There are multiple possible methods to control the rotational speed of the inducer and the impeller. This makes various different drive modes possible for the pump unit (from the start of pumping to the situation where the pumping is in a steady state mode) and for making the surface liquid level on the suction side of the pump unit change. These drive modes may include, for example, the inducer starts first, followed by the impeller; or the inducer is driven at a higher or lower number of revolutions per minute (rpm) than the impeller. Or the acceleration curves for these are different in order to achieve the desired flow to the pipeline.

[0017] The ring-shaped motor surrounding the inducer rotor makes possible in particular a compact design of the inducer. It also enables the pump unit to be constructed neatly as a single unit.

[0018] According to an embodiment, the inducer rotor and the impeller are drivable by separate electric motors electrically connected to a common control unit. Depending on the actual pumping control system, the control of the present pump unit is advantageously combined so that the inducer and the centrifugal pump can operate in a coordinated manner. Since the mass flow or volume flow rate through both the inducer and the centrifugal pump will be the same, each of the two can be driven so as to optimize the energy consumption of the pumping.

[0019] According to an embodiment of the invention, the inducer rotor can rotate in the opposite direction or in the same direction as the impeller. This direction depends on the liquid or suspension to be pumped and the design parameters of the centrifugal pump.

[0020] According to an embodiment of the invention, the inducer comprises an inlet, an inducer housing for enclosing a stator, an annular motor and a rotor, the inducer housing being connected to the pump housing via a flange attachment at a distance of 0.01 to 10 times the diameter of the flow channel. The effect of this feature is to achieve a compact pump unit in which the inducer rotor and the impeller are located at a sufficiently close distance to each other. The inducer creates a flow field in the flow channel, and if the distance between the inducer rotor and the impeller is relatively long, the flow field reaches equilibrium in front of the impeller, and then the situation will be the same as having only two pumps in series, without having the expected effect as the inducer usually has.

[0021] According to an embodiment of the invention, a sensor for detecting cavitation is provided in the flow path between the inducer rotor and the impeller. Since cavitation is one of the major limitations in pumping, it is of great interest to know when this phenomenon is about to occur or has occurred. With the present pump unit, cavitation at the impeller inlet side is the most critical since the impeller is the one that determines the required NPSH (NPSHr) of the pump unit. An alternative or complementary approach is to measure the temperature, pressure (both static and calculated dynamic pressure) and volume flow rate at the same location and calculate the required NPSH increase through the inducer to prevent cavitation.

[0022] According to an embodiment of the present invention, a method for controlling a pump unit for pumping a liquid or a suspension is provided, the pump unit comprising a centrifugal pump and an inducer located upstream of an inlet of the centrifugal pump:

[0023] - the centrifugal pump comprises a pump housing and an impeller, a flow passage is formed in the pump housing, the impeller is configured to rotate in the flow passage about a central axis via a shaft,

[0024] the inducer comprises an inducer rotor having a sleeve-like rotor body, the inducer rotor being configured to rotate about a central axis in the flow channel, the rotor body being provided with a large number of blades extending inwardly from the rotor body,

[0025] - the inducer rotor is separate from the impeller and the rotational speed of the inducer rotor is controlled independently of the rotational speed of the impeller, the inducer comprises an electric motor which is a ring-shaped motor surrounding and connected to the inducer rotor,

[0026] In the method, the control of the pump unit is determined at least on the parameters of the total volume flow rate and the pressure difference over the pump unit as a combination of an inducer and a centrifugal pump. The total volume flow rate here relates to the material flow through the flow channel. The control of the pump unit is carried out in a balanced manner by adjusting both the inducer and the centrifugal pump so that the output is NPSH as required and there is a clear margin for the occurrence of cavitation phenomena.

[0027] According to an embodiment of the invention, the pump unit is controlled such that the output power of the inducer is less than the output power of the centrifugal pump. Thus, the basic idea of ​​the feature is to use the inducer in preparing the flow to be impacted by the more powerful impeller / centrifugal pump.

[0028] According to an embodiment of the invention, the rotational speed of the inducer rotor is controlled based on the cavitation phenomenon detected at the centrifugal pump, the pressure head generated by the inducer is increased or decreased in order to maintain a margin for the occurrence of cavitation phenomena at the impeller. Because in the basic configuration of the pump unit, in particular on the inducer, there are no other adjustment parameters than the rotational speed of the inducer rotor. This means that the rotational speed is a determining parameter for controlling the inducer. Depending on the load, the same rotational speed requires different torques or power from the electric motor of the inducer. This actual control of the electric motor is done by a control unit, which is an inverter or the like.

[0029] In this specification, the term effective NPSH at the pump inlet NPSHa generally means a pump unit. NPSHa is a value expressing the total head of the liquid when it enters the pump unit. It is a measure of the pressure to which the liquid is subjected between its current state and the formation of vapor bubbles (boiling), and it is expressed as the height of the liquid column of the pumped medium. However, with the present disclosure, the flow conditions vary with position parameters along the flow channel and with respect to the inducer and the impeller, so if necessary, NPSHa is divided here into being determined at two locations, NPSHaA at the inlet of the inducer, which is the same as the inlet of the pump unit, and NPSHaB at the inlet of the impeller, therefore between the inducer and the impeller in the flow channel.

[0030] According to an embodiment of the method, the pump unit is controlled based on the surface level on the suction side of the pump, wherein the NPSHaA at the inlet before (meaning upstream) the pump unit is measured, calculated or otherwise determined and, if necessary, the output power of the inducer is increased or reduced in order to influence the NPSHaB at the flow channel before (upstream) the impeller so that during operation of the pump unit, the NPSHaB is greater than the NPSHr of the centrifugal pump. Thus, the inducer is driven so that the centrifugal pump has a margin for the occurrence of cavitation phenomena at the impeller.

[0031] Still, according to embodiments of the present method, NPSHaB is measured, calculated or otherwise determined, and the rotational speed / output power of the inducer rotor is driven along a predetermined NPSHr curve for a given operating condition.

[0032] Still, according to an embodiment of the present method, the pump unit is controlled based on the rheology of the fluid to be pumped so that the necessary fluidization parameters of the fluid are predetermined in order to enable the pump unit to operate and the rotational speed / output power of the inducer rotor can be controlled to the expected volume flow rate. This is particularly suitable for fluids of medium viscosity slurries with solids content in the range of 8% to 18% or 10% to 18%. The energy loss of the inducer is proportional to both the fluid viscosity and the shear rate, and it can be assumed that fluidization occurs when the local shear stress is equal to the yield stress of the fluid to be pumped (such as a slurry suspension). Therefore, fluidization involves the breaking of the fiber network on a local scale. For viscous fluids, the inducer can be characterized by a power number-Reynolds number relationship, which follows the equation Re=D 2 Nρ / μ, where Re is the Reynolds number, D is the diameter of the inducer rotor, N is the rotational speed, ρ is the density, and μ is the apparent viscosity of the fluid. ρ It is the nominal power N delivered to the fluid under a certain inducer geometry. ρ =P / (D 5 N 3 According to an embodiment, this calculated power number-Reynolds number curve is used as a reference curve for controlling the rotational speed / output power of the inducer rotor so that the pump unit operates at a desired operating window.

[0033] The exemplary embodiments of the invention presented in this patent application are not to be interpreted as causing limitations on the applicability of the appended claims. The verb "to comprise" is used in this patent application as an open limitation that does not exclude the existence of features that have not yet been recited. The features recited in the dependent claims are mutually freely combinable unless explicitly stated otherwise. The novel features that are considered as characteristic of the invention are particularly set forth in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In the following, the present invention will be described with reference to the attached exemplary, schematic drawings, in which

[0035] Figure 1a A pump unit according to an embodiment of the invention is shown,

[0036] Figure 1b Shown Figure 1a 3D overview of the pump unit,

[0037] Figure 2 A pump unit according to another embodiment of the invention connected to a control system is shown,

[0038] Figure 3a and 3b Different curves showing the effect of the inducer,

[0039] Figure 3c and 3d Still different curves are shown regarding the effect of the inducer.

[0040] Figure 4 An embodiment of a pump unit is shown which has guide vanes between the inducer and the centrifugal pump.

[0041] Figure 5 An example of a power number / Reynolds number curve is shown,

[0042] Figure 6 Examples of target QH curves are shown. DETAILED DESCRIPTION

[0043] Figure 1a Schematically depicted is a pump unit 1 for pumping a liquid or a suspension, the pump unit 1 comprising a centrifugal pump 2 and an inducer 3 close to upstream of an inlet 210 of the centrifugal pump 2:

[0044] the centrifugal pump 2 comprises a pump housing 21 in which a flow channel 20 is formed and an impeller 22 which is arranged to rotate in the flow channel 20 about a central axis 200 via a shaft 23,

[0045] - inducer 3 comprises an inducer rotor 32 having a sleeve-shaped rotor body 321, inducer rotor 32 being configured to rotate around central axis 200 in flow channel 20, rotor body 321 being provided with a large number of blades 323 extending inwardly from rotor body 321,

[0046] - Inducer rotor 32 is separated from impeller 22 and the rotation speed of inducer rotor 32 can be controlled independently of the rotation speed of impeller 22, and inducer 3 includes electric motor 35, which is an annular motor surrounding and connected to inducer rotor 32. With respect to the present method, inducer rotor 32 is separated from impeller 22 and the rotation speed of inducer rotor 32 is controlled independently of the rotation speed of impeller 22, and inducer 3 includes electric motor 35, which is an annular motor surrounding and connected to inducer rotor 32.

[0047] The control of the pump unit 1 is determined at least on the parameters of the total volume flow rate and the pressure difference over the pump unit 1 as a combination of the inducer 3 and the centrifugal pump 2 .

[0048] according to Figure 1aIn the embodiment illustrated in FIG. 3 , the inducer 3 comprises an inlet 310, an inducer housing 31 for enclosing a stator 351, an annular motor 352 and a rotor 32, the inducer housing 31 being connected to the pump housing 21 via a flange 211 attachment at a distance of 0.01 to 10 times the diameter of the flow channel 20. This distance may be, for example, the distance L between the inducer blades 323 and the impeller 22, with the diameter of the flow channel 20 being measured at the inducer 3. This enables a compact pump unit in which the inducer rotor and the impeller are located at a sufficiently close distance to each other and are easy to assemble. The inducer creates a flow field that starts at the inlet 310 of the inducer / pump unit and continues to the flow channel, and if the distance between the inducer rotor and the impeller is relatively long, the flow field reaches equilibrium before the impeller, and then the situation will be the same as with only two pumps in series, without the expected effect as the inducer usually has.

[0049] exist Figure 1b A total external 3D overview of the present pump unit 1 is presented in FIG. Figure 1b An embodiment of a centrifugal pump 2 and an inducer 3 is shown. The centrifugal pump 2 includes a pump housing 21 in which a flow channel 20 is formed. The inducer 3 includes an inducer rotor 32 configured to rotate in the flow channel 20 via a shaft 23 . The rotor 32 is provided with blades 323 .

[0050] according to Figure 2 In the embodiment illustrated in FIG. 1 , inducer rotor 32 and impeller 22 are drivable by separate electric motors 25, 35 electrically connected to a common control unit 5. Preferably, pump unit 1 is controlled such that the output power of inducer 3 is less than the output power of centrifugal pump 2. There are a number of operating principles that can be employed in the control of the pump unit. For example, the rotational speed of inducer rotor 32 can be controlled based on cavitation detected at centrifugal pump 2, the pressure head generated by inducer 3 being increased or decreased in order to maintain a margin for cavitation to occur at impeller 22. In FIG. Figure 2 Some possible sensors 4, 4A, 4B are also shown in order to monitor the centrifugal pump or determine the NPSHaA or NPSHaB values: pressure sensor, acoustic sensor for cavitation monitoring, vibration monitoring sensor, online viscosity sensor, online gas content meter, thermometer. The pump unit 1 may include means to monitor the centrifugal pump: calculations made by software using a variable speed drive. The control unit 5 may include executable instructions to control the rotation speed / output power of the inducer rotor 32 based on the sensor signals.

[0051] In addition, Figure 2It is shown how to control a pump unit 1 based on the surface level on the suction side of the pump unit 1, wherein the NPSHaA at the inlet 310 before the inducer / pump unit 1 is measured, calculated or otherwise determined by means of a sensor 4A and, if necessary, the output power of the inducer 3 is increased or reduced in order to influence the NPSHaB, which is measured at a sensor 4B at the flow channel 20 before the impeller 22, so that during operation of the pump unit 1, the NPSHaB is greater than the NPSHr of the centrifugal pump 2.

[0052] Hereinafter, the method of the present invention is referred to as Figure 3a , 3b , 3c and Figure 3d The attached diagram explains in more detail, Figure 3a , 3b , 3c and Figure 3d The pumping curves for different cases are shown. Figure 3a The NPSH (y-axis) is presented as a function of the volume flow rate Q (x-axis) of the centrifugal pump. There are three horizontal lines (dashed lines, NPSHa 1, NPSHa2, etc.) describing the suction head generated by different surface levels, and three curves showing the necessary NPSHr of the centrifugal pump at different rotation speeds of the impeller n1, n2 and n3 depending on the volume flow rate Q. This means that at higher Q, the necessary NPSHr of the centrifugal pump can be greater than the effective NPSHa 1, 2, etc. This would mean that the centrifugal pump would start to cavitate. At Figure 3b The influence of the inducer is illustrated by the dependence of the pressure head (or NPSH) on the volume flow rate Q at different rotation speeds n1, n2, n3 of the inducer rotor. Figure 3b It can be seen that the pressure head H decreases as the volume flow rate Q increases. However, as one can see Figure 3a As can be noted in Figure 2, at higher Q, the NPSH difference between the required NPSHr and the effective NPSHa2 of the centrifugal pump increases (as shown by arrow d) - an inducer rotating at n1, n2 or n3 can produce a NPSH deficit of d, even at higher Q, such as Figure 3b . It will therefore make it possible to achieve such a high Q with the pump unit without cavitation phenomena occurring. This means that by increasing the rotation speed of the inducer rotor it is possible to compensate for a significant amount of NPSH which would otherwise need to be arranged at the plant, for example by increasing the surface level in the suction vessel. The present invention has independently controllable inducer and centrifugal pump which makes possible an optimal and efficient performance of the pump unit over a wide range of operating conditions.

[0053] Figure 3c and 3dIt is shown how the rotational speed / output power of the inducer rotor 32 can be controlled based on the required gas separation or fluidization for a given fluid to be pumped, which can be determined based on gas detection, pre-determination or other means to determine the gas content in the fluid. Curves c1, c2 and c3 present different materials to be pumped, which can differ in viscosity or with respect to gas content. The power required by the inducer depends on the required fluidization or gas removed in order to create the required volume flow rate Q.

[0054] For a practical example in a studied pump unit 1, if the inducer rotor is driven at 1100-1780 rpm for Q=200 l / s, it will fluidize the material sufficiently and reduce the flow resistance so that the centrifugal pump can be driven at 1780 rpm to create the required head.

[0055] exist Figure 4 The inducer 3 or inducer rotor 32 and the impeller 22 ( Figure 4 In an embodiment between the two (not shown in the figure), guide vanes 24 with fixed or adjustable pitch are provided. These guide vanes can be of conventional type used only for controlling swirl in the flow channel 20, or these can be such that the guide vanes 24 include ducts 241 for removing gas from the liquid.

[0056] exist Figure 5 In, an embodiment of the method is shown, the pump unit 1 is controlled based on the rheology of the fluid to be pumped so that the necessary fluidization parameters of the fluid are predetermined to enable the pump unit to operate, and the rotational speed / output power of the inducer rotor is controlled to the desired volume flow rate. The energy loss of the inducer is proportional to both the viscosity and the shear rate of the fluid, and it can be assumed that fluidization occurs when the local shear stress is equal to the yield stress of the fluid to be pumped (such as a slurry suspension). Therefore, fluidization involves the breaking of the fiber network on a local scale. For viscous fluids, the inducer can be characterized by a power number-Reynolds number relationship, which follows the equation Re=D 2 Nρ / μ, where Re is the Reynolds number, D is the diameter of the inducer rotor, N is the rotational speed, ρ is the density, and μ is the apparent viscosity of the fluid. ρ It is the nominal power N delivered to the fluid under a certain inducer geometry. ρ =P / (D 5 N 3 ρ). Figure 5 Such a power number / Reynolds number curve for two different configurations, shown as configurations A and C, is presented in . According to an embodiment, such a calculated power number-Reynolds number curve is used as a reference curve for controlling the rotational speed / output power of the inducer rotor so that the pump unit operates at a desired operating window.

[0057] exist Figure 6 In FIG. 1 , a target QH curve is shown, with volume flow rate Q on the horizontal axis and pressure head on the vertical axis. If the pressure head Hm measured at a given Q is lower than the pressure head target Ht at a curve point, the difference between Ht and Hm is ΔH, and the inducer speed is increased to meet the target / reference curve. Because the necessary fluidization parameters of the fluid are predetermined to enable the pump unit 1 to operate, and the rotational speed / output power of the inducer rotor 32 is controlled to be such as Figure 6 The expected volume flow rate is shown in .

[0058] Although the present invention is described herein by way of example in conjunction with what is currently considered to be the most preferred embodiment, it should be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features, as well as a number of other applications within the scope of the invention as defined in the appended claims. The details mentioned in conjunction with any of the above embodiments may be used in conjunction with another embodiment, as long as such combination is technically feasible.

[0059] Parts List

[0060] 1 Pump unit

[0061] 2 Centrifugal pump

[0062] 20 Flow channels

[0063] 200 Center axis

[0064] 21 Pump housing

[0065] 210 Entrance

[0066] 211 Flange

[0067] 22 Impeller

[0068] 23 Axis

[0069] 24 Guide blades

[0070] 241 Pipe

[0071] 25 Motor

[0072] 3 Induction wheel

[0073] 31 Inducer housing

[0074] 310 Inducer / Pump Unit Inlet

[0075] 32 Inducer rotor

[0076] 321 Rotor body

[0077] 323 Blade

[0078] 35 Induction motor

[0079] 4A Sensor

[0080] 4B Sensor

[0081] Cavitation Sensors

[0082] Pressure Sensors

[0083] thermometer

[0084] 5 Control unit

[0085] ΔH Head difference

[0086] Hm Measured pressure head

[0087] Ht Head target.

Claims

1. A method for controlling a pump unit (1) for pumping a liquid or a suspension, the pump unit (1) comprising a centrifugal pump (2) and an inducer (3) located upstream of an inlet (210) of the centrifugal pump (2): - the centrifugal pump (2) comprises a pump housing (21) and an impeller (22), a flow channel (20) being formed in the pump housing (21), the impeller (22) being configured to rotate in the flow channel (20) around a central axis (200) via a shaft (23), - the inducer (3) comprises an inducer rotor (32) having a sleeve-shaped rotor body (321), the inducer rotor (32) being configured to rotate around the central axis (200) in the flow channel (20), the rotor body (321) being provided with a large number of blades (323) extending inwardly from the rotor body (321), - the inducer rotor (32) is separated from the impeller (22) and the rotation speed of the inducer rotor (32) is independently controlled with respect to the rotation speed of the impeller (22), the inducer (3) comprises an electric motor (35), the electric motor (35) is a ring-shaped motor surrounding and connected to the inducer rotor (32), - the control of the pump unit (1) is determined at least on the parameters of the total volume flow rate and the pressure difference over the pump unit (1) as a combination of the inducer (3) and the centrifugal pump (2), characterized in that The pump unit (1) is a controlled pump unit (1) based on the rheology of the fluid to be pumped, so that the necessary fluidization parameters of the fluid are predetermined to enable the pump unit (1) to operate, and the rotational speed / output power of the inducer rotor (32) is controlled to a desired volume flow rate.

2. The method according to claim 1, characterized in that The calculated power number-Reynolds number curve is used as a reference curve for controlling the rotational speed / output power of the inducer rotor so that the pump unit operates at a desired operating window.

3. The pump unit (1) is controlled so that the output power of the inducer (3) is smaller than the output power of the centrifugal pump (2).

4. The method according to any one of claims 1 or 2, characterized in that: The rotational speed of the inducer rotor (32) is controlled based on cavitation detected at the centrifugal pump (2), and the head produced by the inducer (3) is increased or decreased to maintain a margin for cavitation to occur in the impeller (22).

5. The method according to any one of claims 1 to 3, characterized in that: NPSHaB is measured, calculated or otherwise determined and the rotational speed / inducer head of the inducer rotor (32) is driven along a predetermined NPSHr curve for a given operating condition.

6. The method according to claim 1, characterized in that The rotational speed / output power of the inducer rotor (32) is controlled based on a desired gas separation for a given fluid to be pumped, which may be determined based on gas detection, pre-determination or other means to determine the gas content in the fluid.

7. A pump unit (1) for pumping a liquid or a suspension, the pump unit (1) comprising: - a centrifugal pump (2) and an inducer (3) upstream of the inlet (210) of the centrifugal pump (2): - the centrifugal pump (2) comprises a pump housing (21) and an impeller (22), a flow channel (20) being formed in the pump housing (21), the impeller (22) being configured to rotate in the flow channel (20) around a central axis (200) via a shaft (23), - the inducer (3) comprises an inducer rotor (32) having a sleeve-shaped rotor body (321), the inducer rotor (32) being configured to rotate around the central axis (200) in the flow channel (20), the rotor body (321) being provided with a large number of blades (323) extending inwardly from the rotor body (321), - the inducer rotor (32) is separated from the impeller (22) and the rotation speed of the inducer rotor (32) is independently controllable with respect to the rotation speed of the impeller (22), the inducer (3) comprises an electric motor (35), the electric motor (35) is an annular motor surrounding and connected to the inducer rotor (32), a sensor (4B) for detecting cavitation phenomena is arranged in the flow channel (20) between the inducer rotor (32) and the impeller (22), It is characterized in that - The pump unit is connected to a control unit (5) configured to perform the method according to claim 1.

8. The pump unit (1) according to claim 6, characterized in that The inducer rotor (32) and the impeller (22) can be driven by separate electric motors (25, 35) electrically connected to a common control unit (5).

9. The pump unit (1) according to claim 6 or 7, characterized in that The inducer rotor (32) is capable of rotating in an opposite direction or in the same direction as the impeller (22).

10. Pump unit (1) according to claim 6 or any one of the preceding claims, characterized in that The inducer (3) includes an inlet (310), an inducer housing (31) for enclosing a stator (351), an annular motor (352) and a rotor (32), wherein the inducer housing (31) is connected to the pump housing (21) via a flange (211) attachment at a distance of 0.01 to 10 times the diameter of the flow channel (20).

11. Pump unit (1) according to claim 6 or any one of the preceding claims, characterized in that A guide blade (24) having a fixed or adjustable pitch is arranged between the inducer (3) and the impeller (22).

12. The pump unit (1) according to claim 10, characterized in that The guide vanes (24) comprise ducts (241) for removing gas from the liquid.

13. Pump unit (1) according to claim 6 or any one of the preceding claims, characterized in that The rotational speed of the inducer rotor (32) is variably and / or independently controllable relative to the rotational speed of the impeller (22).

14. The pump unit (1) according to claim 6, comprising at least one of the following sensors (4) for monitoring the centrifugal pump or determining the NPSHaA or NPSHaB value: pressure sensor, acoustic sensor for cavitation monitoring, vibration monitoring sensor, online viscosity sensor, online gas content meter, thermometer.

15. Pump unit (1) according to claim 6, comprising means to monitor the centrifugal pump: calculations made by software using a variable speed drive.

16. A control unit (5) comprising executable instructions to control the rotational speed / output power of the inducer rotor (32) based on the sensor signal of claim 14.

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