Centrifugal pump with inflow guide device

A centrifugal pump having an axial inlet and an inflow guide device mounted upstream of the impeller, which guide device is composed of several fixed guide vanes, which on the side facing the impeller each have an edge pointing in or opposite to the direction of rotation of the impeller. The edge pointing in or opposite to the direction of rotation may take the form of a swirl blade (8) which is formed on the guide vane (6) and located in the vicinity of the wall of the inlet (2), and which has a shorter radial length than the guide vane (6). This increases the efficiency of the pump and achieves an effect which increases the breadth of applicability of the pump.

BACKGROUND OF THE INVENTION
 The invention relates to a centrifugal pump with an axial inlet and an
 inflow guide device mounted upstream of the impeller, which guide device
 has several fixed guide vanes, which on the side facing the impeller each
 have an edge pointing towards or away from the direction of rotation of
 the impeller.
 Centrifugal pumps with an axial inlet often have an inflow guide device
 mounted upstream of the first impeller. Their purpose is to remove a
 possible whirl from the transported fluid, such that an axial flow is
 present downstream of the inflow guide device.
 The inflow guide device can be composed of several guide vanes, disposed
 uniformly around the perimeter of the inlet. The guide vanes can be
 attached to each other in the axis of the inlet and thus form a star or
 cross.
 Inflow guide devices are known from DE 471 149 and DE 649 668 whose blades
 can be curved towards or away from the direction of rotation of the
 impeller in order to adapt to particular inflow conditions or to generate
 them. The curvature is bow shaped and extends in each case with the same
 axial length to the entire radial span of the blade. Here, when compared
 with the blade, the curvature has only a limited axial length.
 In contrast to the previously described inflow guide device, which is
 equipped with seven blades and was relatively expensive, nowadays inflow
 crosses are preferred, that is, inflow guide devices comprising only four
 blades. These devices, inexpensive in themselves, are hardly able,
 however, to reduce an inherent disadvantage of the axial flow centrifugal
 pumps, the instability present in the pump's steady state characteristics.
 It is therefore the object of the invention to achieve such a reduction
 for a centrifugal pump of the type described above and on the whole to
 improve the flow conditions.
 SUMMARY OF THE INVENTION
 This object is achieved according to the invention in that the edge
 pointing in or opposite to the direction of rotation is formed by a swirl
 blade which is formed on the guide vane and located in the region of the
 inlet wall and which has a shorter radial length than the guide vane.
 Experiments have shown that the efficiency of the flow machine is increased
 over a wide operating range by using the inventive design with a
 turbulence bade formed on the guide vane. This is especially true when the
 angle of incidence between the guide vane and the swirl blade is negative,
 that is, when the swirl blade points against the direction of rotation of
 the impeller.
 In general, it can be said that the angle of incidence depends on the type
 of hydraulics. It can be determined based on measurements, that is, it can
 be determined with the purpose of achieving different effects on the
 pump's steady state characteristics at all operative conditions.
 The effect of the swirl blade increases nearer to the wall. Therefore, it
 is advisable to increase the axial length of the swirl blade as it
 approaches the wall. Thus it is advantageous if, in designing the swirl
 blade, one starts with a right triangle whose hypotenuse is the edge
 facing the impeller.
 The almost one-dimensional flow in the axial inlet is changed by means of
 the swirl blade into a three-dimensional whirling flow in the region of
 the wall of the inlet or of the inflow nozzle. This change leads to
 greater efficiency at all load conditions and reduces the instability of
 the pump. In particular, the kinetic energy of the gap flow and
 re-circulation flow is returned to the inflow so that energy is recovered.
 In cases of increased inlet flow, for example in the case of an overload,
 the pre-whirling increases.
 In addition, because of the swirl blades, the beginning of the
 re-circulation upstream of the impeller shifts towards relatively low
 inlet flows. Thus, the hysteresis region on the pump's characteristic
 curve, dangerous to the operational performance of the pump, is displaced
 sharply to the left, something which substantially widens the range of the
 pump's use.
 The change in the inflow which is achieved by the inflow guide device
 according to the invention leads to an adaptation of operative conditions
 which is more efficient and increases the breadth of applicability since
 the turbulence boundary is clearly displaced to the left. Due to the
 design in accordance with the invention, the flow onto the impeller is
 also improved, such that secondary currents and flow separations
 downstream from the impeller blades are minimized.
 As a further development of the invention, it is suggested that one or more
 of the guide vanes each have on the edge opposite the impeller a pre-guide
 vane which is similar in shape and size to the swirl blade. It has been
 shown that this type of pre-guide vane further improves the flow guidance.
 The invention can be realized by simple means if, as is known, the guide
 vanes are arranged in the form of a cross or a star.
 The invention will be explained in further detail with reference to an
 illustrative working embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 The centrifugal pump shown in FIG. 1 has a housing 1 with an inflow nozzle
 2 upstream from it. An impeller 4 is arranged on a shaft 3. A pressure
 pipe stub 5 forms the end in the direction of flow.
 Upstream from the impeller 4, a turbulence cross, comprised essentially of
 four guide vanes 6, is fixed rigidly in the inflow nozzle 2. A pre-guide
 vane 7 and a turbulence vane 8 are mounted on each of the guide vanes 6.
 The rotation of the shaft 3 and the impeller 4 corresponds to the
 rotational direction indicator 9. Arrows 10, illustrated inside the
 housing 1 and at the exit of the pressure pipe stub 5, show the flow path
 of the transported medium.
 It is important to note measurement S, which shows the distance between the
 center line of the guide vane 6 and the front edge of the impeller 4. The
 measurement S, which depends on the type of pump, especially on the
 impeller geometry, is to be determined, for example with the aid of
 experiments, such that the inflow guide device has an optimal influence on
 the flow and thus on the pump's characteristic curve.
 With respect to the turbulence cross illustrated in FIG. 2, the
 relationship between guide vanes 6, the pre-guide vanes 7 and the swirl
 blades 8 can be seen. Here a possible angle range between +.gamma. and
 -.gamma. is indicated for the angle of incidence for the swirl blades 8.
 The positive angle +.gamma. points in the direction of rotation of the
 impeller 4. A negative angle -.gamma. lets the guide vane 6 point against
 the rotational direction of the impeller 4. When utilized in a controlled
 manner, both directions can influence the pump characteristics in a
 desired way.
 The number of guide vanes 6 is determined by the geometry and the specific
 characteristics of the pump. It is within the knowledge and ability of an
 appropriate pump specialist to determine this.
 In order better to identify the possible angle ranges for the adjustment of
 the swirl blade 8 on the guide vane 6, these are shown in even clearer
 detail in FIGS. 3 and 4. The blade thickness D can also be seen from these
 drawings.
 Further measurements and their relationships to one another can be seen in
 FIG. 5. Accordingly, the guide vane 6 has a height H.sub.L, the pre-guide
 vane 7 has a height H.sub.V, the height of the swirl blade 8 is given as
 H.sub.D. L.sub.L is the length of the guide vane 6. The length of the
 pre-guide vane 7 is given as L.sub.V and the length of the swirl blade 8
 as L.sub.D. The lengths and the heights of the guide vanes 6, the
 pre-guide vanes 7 and the swirl blades 8 are designed to have an optimal
 influence on the characteristic curve.
 The pre-guide vane 7 and the guide vane 6, which both extend parallel to
 the axis of the inflow nozzle 2 and the impeller 4, exert a straightening
 influence on the inflow. The swirl blade 8 adjusted at an angle .gamma.
 alters the inflow direction relative to impeller 4. A desired swirl is
 brought about either with or opposite to the rotational direction of the
 impeller 4.
 The swirl blade 8 develops its greatest influence in the region of the wall
 of the inflow nozzle 2. Should a re-circulation occur before the impeller,
 the intensity is reduced by means of the inflow guide device. All in all,
 the surface of the inflow guide device should be constructed as small and
 as hydraulically smooth as possible in order to minimize the frictional
 loss. In principle, the blades 6, 7, and 8 of the pre-guide device can
 also be contoured.