CENTRIFUGAL PUMP ASSEMBLY

Abstract

The invention relates to a centrifugal pump assembly having an electric drive motor (2), at least one impeller (14) driven by said electric drive motor (2) and a valve element (24; 24′) rotatable between two 5 valve positions driven by a fluid flow produced by said impeller (14), wherein the valve element (24; 24′) has a cover plate (104; 104′) extending transverse to the rotational axis (X) of the impeller (14) and facing the impeller (14), wherein the valve element (24; 24′) having protrusions (102, 102′) arranged on an outer surface side facing away 10 from the impeller (14) such that a flow can act on them for driving the valve element (24; 24′).

Claims

1. A centrifugal pump assembly, the assembly comprising: an electric drive motor; at least one impeller driven by said electric drive motor; and a valve element rotatable between two valve positions driven by a fluid flow produced by said impeller, wherein the valve element comprises a cover plate extending transverse to the rotational axis of the impeller and facing the impeller, wherein the valve element comprises protrusions arranged on an outer surface side facing away from the impeller such that a flow can act on the protrusions for driving the valve element.

2. The centrifugal pump assembly according to claim 1, wherein the protrusions are provided on an outer circumference of the valve element or a back side facing away from the impeller.

3. The centrifugal pump assembly according to claim 1, wherein the protrusions extend radially from an outer circumferential surface of the valve element and are inclined in the circumferential direction.

4. The centrifugal pump assembly according to claim 1, wherein the protrusions are extending in a radial direction related to the rotational axis of the valve element and in the radial direction the cover plate extends beyond the protrusions.

5. The centrifugal pump assembly according to claim 1, wherein the rotational axis of the valve element extends parallel and along the rotational axis of the impeller.

6. The centrifugal pump assembly according to claim 1, wherein the valve element is arranged inside a housing having a circular inner wall surrounding an outer circumference of the valve element with a ring-shaped gap between the outer circumference of the cover plate and the inner wall.

7. The centrifugal pump assembly according to claim 1, wherein the protrusions are evenly distributed over an outer circumference of the vale element.

8. The centrifugal pump assembly according to claim 1, wherein the protrusions are of toothlike shape and are extending normal to a cylindrical outer circumferential wall of the valve element.

9. The centrifugal pump assembly according to claim 1, wherein the protrusions are integrally formed with an outer circumferential wall and/or the cover plate of the valve element.

10. The centrifugal pump assembly according to claim 1, wherein the cover plate of the valve element comprises a central outlet opening being in engagement with a suction mouth of the impeller.

11. The centrifugal pump assembly according to claim 1, wherein said valve element is supported on a central bearing post and fixed in axial direction on this bearing post by an O-ring, wherein the O-ring engages into a notch on the outer circumference of the bearing post.

12. The centrifugal pump assembly according to claim 11, wherein the bearing post is attached to an internal surface of a pump housing and integrally formed with at least this internal surface of the pump housing.

13. The centrifugal pump assembly according to claim 1, wherein the valve element comprises at least one sealing portion for selectively closing a first and a second inlet port such that in a first valve position the first inlet port is closed and in a second valve position the second inlet port is closed.

14. The centrifugal pump assembly according to claim 1, wherein the valve element is movable in linear direction along its rotational axis and the valve element is movable in linear direction such that in a first axial position the at least one sealing portion is in sealing contact with an opposed valve seat and in a second axial position the sealing portion is distanced from an opposed valve seat.

15. The centrifugal pump assembly according to claim 1, wherein the valve element comprises at least one inlet opening being in flow connection with an outlet opening of the valve element and arranged such that in a first valve position the inlet opening is facing a second inlet port and in a second valve position is facing a first inlet port.

16. The centrifugal pump assembly according to claims 14, wherein the valve element comprises a sealing member surrounding the inlet opening and arranged such that in a first axial position of the valve element the sealing member is in contact with an opposing sealing surface and in a second axial position of the valve element the sealing member is distanced from this sealing surface, wherein said sealing member surrounding the inlet opening of the valve element is arranged on the outer circumference of the valve element.

17. The centrifugal pump assembly according to claim 16, wherein said sealing member surrounding the inlet opening is arranged on an axial end of the valve element opposite to the axial end formed by the cover plate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In the drawings:

[0027] FIG. 1 is an exploded view of a centrifugal pump device according to the invention;

[0028] FIG. 2 is a top view on the centrifugal pump device ac-cording to FIG. 1, on the axial end side of the electronic housing;

[0029] FIG. 3 is a sectional view of the centrifugal pump device according to FIGS. 1 and 2 along line in FIG. 2, with a valve device in its sealed position;

[0030] FIG. 4 is a sectional view similar to FIG. 3 with the valve device in its released and rotatable position;

[0031] FIG. 5 is a top view on the opened pump housing of the centrifugal pump device according to FIGS. 1 to 4;

[0032] FIG. 6 is a sectional view of the valve device in the centrifugal pump device according to FIGS. 1 to 5, with the valve element in a first valve position;

[0033] FIG. 7 is a sectional view of the valve device according to FIG. 6 with the valve element in a second valve position;

[0034] FIG. 8 is a sectional view of the centrifugal pump device along line VIII-VIII in FIG. 2, with the valve element in its second valve position;

[0035] FIG. 9 is a perspective view of the valve element in the centrifugal pump device according to FIGS. 1 to 8;

[0036] FIG. 10 is a plan view of the bottom side of the valve element containing the sealing portions;

[0037] FIG. 11 is a sectional view of the valve element with a bypass valve in its closed position;

[0038] FIG. 12 is a sectional view according to FIG. 11 with the bypass valve in its opened position;

[0039] FIG. 13 is an enlarged cross section of the bypass valve 86 as shown in FIG. 12;

[0040] FIG. 14 is an exploded view of the valve element according to FIG. 9;

[0041] FIG. 15 is an exploded view of the valve element according to seen from a different direction;

[0042] FIG. 16 is a sectional view of the pump housing along line XIII-XIII in FIG. 3 with the valve element in the second valve position; and

[0043] FIG. 17 is a sectional view according to FIG. 18 with the valve element in the first valve position;

[0044] FIG. 18 is a schematical drawing of a hydraulic circuit of a heating system including a centrifugal pump according to the invention; and

[0045] FIG. 19 is an exploded view of a valve element according to a second embodiment; and

[0046] FIG. 20 is a perspective view of an open pump housing including the valve element as shown in FIG. 19.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0047] The centrifugal pump described as an example is a centrifugal pump provided for a heating system. This centrifugal pump device includes a hydraulic valve device which can be used in the heating system to change the fluid flow between a heating circuit through a building and a heat exchanger for heating domestic water.

[0048] The centrifugal pump device has an electric drive motor 2 comprising a motor housing 4 inside which the stator and the rotor are arranged. On one axial end of the motor housing, in direction of the longitudinal axis X, there is arranged an electronics housing 6 comprising the control electronics 7 for the electric drive motor. On the opposite axial end the motor housing 4 is connected to a pump housing 8 comprising an outlet connection 10 connected to an outlet port 12 in the inside of the pump housing 8. The outlet port 12 is arranged on the outer circumference of a pump space inside which the impeller 14 is arranged. The pump housing 8, further, comprises two inlet connections 16 and 18. The first inlet connection is provided for a connection to a heating circuit in a building, whereas the second inlet connection 18 is provided for connection to a heat exchanger for warming domestic hot water. The first inlet connection 16 is in fluid connection with the first inlet port 20 inside the pump housing 8. The second inlet connection 16 is in connection with a second inlet port 22 inside the pump housing 8. The inlet ports 20 and 22 are arranged in one flat plane perpendicular to the longitudinal or rotational axis X. The rotational axis X is the rotational axis of the impeller 14 and the valve element 24 described in more detail later. The first and the second inlet ports are arranged in the bottom of the pump housing 8 seen in the longitudinal direction X.

[0049] The valve element 24 is arranged to switch over the flow path towards the impeller 14 between the two inlet connections 16 and 18. Basically, the function of this hydraulic valve device is similar as disclosed in EP 3 376 049. The valve element 24 has a central outlet opening 26 facing the suction mouth 28 of the impeller 14 or being in engagement with the suction mouth 28 such that fluid flows from the outlet opening 26 into the suction mouth 28.

[0050] The valve element 24 is rotatable about the rotational axis X which corresponds to the rotational axis X of the impeller 14. The valve element 24 is arranged on a pivot or bearing post 30 fixed in the bottom of the pump housing 8. In this embodiment the pivot is molded into the material of the pump housing 8, for example in an injection molding process. However, the bearing post may be fixed in the bottom of the pump housing 8 in different manner, for example being screwed into a threaded hole or being formed integrally with the pump housing 8. The bearing post 30 extends from the bottom of the pump housing 8 in the longitudinal direction X into the interior of the pump housing 8. The valve element 24 is rotatable about the longitudinal axis X and movable in a linear direction on the bearing post 30 along the longitudinal axis X in a certain distance. This certain distance is limited by an O-ring 32 forming an axial stop or abutment for the valve element 24. The O-ring 32 engages into a circumferential groove or notch 34 arranged close to the free distal end of the bearing post 30. The O-ring 32 forms an elastic axial stop and allows an easy assembling without special tools.

[0051] In this embodiment the valve element 24 is composed of two parts, a support member 36 and a cover member 38 which are connected by a snap fit. On the inner surface of the cover member 38 there are arranged engagement hooks 40 which embrace or engage with engagement shoulders or projections 42 in the interior of the support member 36. The cover member 38 has a cover plate 104, i.e. a cover of plate like shape, and is completely closed except the central outlet opening 36. When arranged inside the pump housing 8 the cover plate 104 of the cover member 38 forms one axial wall of the pump space 44 inside which the impeller 14 is rotating. The opposite axial wall of the pump space 44 is formed by a bearing plate 46 holding one bearing for the rotor shaft 50. Opposite to the cover member 38 there is connected a spring support 52 to the support member 36. Between the spring support 52 and the support member 36 there is arranged a helical compression spring 54. The spring 54 with one axial end abuts against an interior bottom surface of the spring support 52 and with the opposite axial end abuts against apportion of the support member 36. The spring support 52 overlaps with elastic engagement hooks 56 such that the engagement hooks 56 engage with openings or cut-outs 58 in the outer circumference of the spring support 52 from the inside of the spring support 2. Thereby the spring support 52 is guided on the outside of the legs of the engagement hooks 56 in axial direction X such that the spring support 52 is movable in this axial direction on the outside of the legs of the engagement hooks 56. Furthermore, on the support member 38 there is provided a rib 60 in the spring support 52. Rib 60 and slot 62 allow a relative movement in axial direction, but ensure a torque transfer so that the spring support 52 is connected to the support member 36 substantially torque proof except a limited play in circumferential direction between the rib 60 and the slot 62. This play ensures a damping effect provided by torsion of the compression spring 54 since the spring 54 is in the flux in rotational direction until the rib 60 abuts on one of the edges of the slot 62.

[0052] On the axial end opposite to the support member 36 the spring support 52 comprises a bearing portion 64 movably supported on the bearing post 30, i.e. sliding on the outer circumference of the bearing post 30. A further bearing portion 66 in bearing contact with the bearing post 30 is formed in the support member 36. The bearing portion 66 comprises a shoulder protruding in radial direction. Against this shoulder the axial end of the compression spring 54 abuts.

[0053] The compression spring 54 forces the bearing portions 64 and 66 away from each other and forces the valve element 24 in an axial direction towards the motor housing 4. Under compression of the spring 54 the valve element 24 may be moved towards the bottom side of the pump housing 8, i.e. away from the impeller 14 and the motor housing 4. These two possible axial positions of the valve element 24 are shown in FIGS. 3 and 4. In FIG. 4 the valve element 24 is in its first axial position in which the valve element 24 abuts against a circular shoulder 68 in the interior of the pump housing 8. The shoulder 68 extends in radial direction from the inner circumference of the pump housing 8 providing a circular sealing surface extending substantially perpendicular to the longitudinal axis X. The valve element 24 is in sealing contact with this shoulder 68 via an elastic sealing 70 on the outer circumference of the support member

[0054] This sealing 70 ensures a sealing of the pump space 44 towards the suction side of the pump device. FIG. 4 shows a second axial position of the valve element 24 in which the valve element 24 is moved towards the impeller 14 such that the sealing 70 is not in contact with the shoulder 68 anymore, but distanced from the shoulder 68. In this position the valve element 24 is freely rotatable about the longitudinal axis X. If the sealing 70, however, is in contact with the shoulder 68 a rotation of the valve element 24 is prohibited due to the frictional forces between the sealing 70 and the shoulder 68. Thus, the shoulder 68 and the sealing 70 act as a detachable coupling or clutch. The valve element 24 is moved into the released position shown in FIG. 4 by the spring forces of the compression spring 54. Into the fixed position as shown in FIG. 3, in which the sealing 70 is in contact with the shoulder 68, the valve element 24 is moved by the pressure produced by the impeller 14 and acting on the cover member 38 surrounding the outlet opening 26. Thus, the valve element 24 can selectively be moved in axial direction depending on the pressure produced by the pump on the outlet side of the impeller 14. This can be controlled by speed control and regulation carried out by the control electronics 7 arranged in the electronics housing 6.

[0055] The valve element 24 comprises two sealing portions 72 and 74, i.e. a first sealing portion 72 and a second sealing portion 74. The two sealing portions 72 and 74 are arranged on the outer axial surface of the support member 36, i.e. on the axial face side of the valve element 24 facing away from the impeller and being opposed to the first and second inlet ports 20 and 22. The two sealing portions 72 and 74 are arranged in a common plane extending perpendicular to the rotational axis X. The two sealing portions 72 and 74 are positioned diametral in relation to the axis X, i.e. in positions offset by 180° about the rotational axis X. The two sealing portions 72 and 74 each comprises an elastic sealing member 76, 78, which in this embodiment are formed integral with the sealing 70 on the outer circumference of the support member. The sealing 70 and the sealing members 76 and 78 may be formed as a separate part or sealing arrangement connected to the support member 36 or connected to the support member 36 by an injection molding process.

[0056] The first sealing portion 72 is provided to selectively close the first inlet port 20 and the second sealing portion 24 is provided to selectively close the second inlet port 22. Between the two sealing portions 72 and 74 there is provided an opening 80 in the support member 36 being in fluid connection with the outlet opening 26 and forming an entrance opening of the valve element 24.

[0057] The valve element 24 can take two different valve positions in rotational direction about the longitudinal axis X. FIG. 6 shows the first valve position in which the first sealing portion 72 closes the first inlet port. In this first valve position the second inlet port 22 is open towards the opening 80 in the valve element 24 such that a fluid flow from the inlet port 22 towards the outlet opening 26 and into the suction mouth 28 of the impeller 24 is enabled. In this first valve position, therefore, the impeller 14 and thus the entire pump sucks fluid through the first inlet connection 60 which is connected to the fist inlet port 20. In this first valve position when the valve element 24 is in its engaged or sealing position as shown in FIG. 3 the first sealing portion 72 with its sealing member 76 is pressed against a valve seat 82 formed by the surrounding circumference or edge of the inlet port 20. By this the first inlet port 20 is completely closed.

[0058] In the second valve position as shown in FIG. 7 the first sealing portion 72 is rotated aside from the first inlet port 20 such that the first inlet port 20 is opened towards the opening 80 providing a flow path from the first inlet port 20 towards the outlet opening 26 and the suction mouth 28 of the impeller 14. In this second valve position the second sealing portion 24 is moved into a position in which it covers the second inlet port 22 so that the second inlet port 22 is closed. In the engaged or sealed position of the valve element 24 the sealing member 78 of the second sealing portion 24 is pressed against a valve seat 84 formed on the outer circumference or edge of the second inlet opening 22.

[0059] Deferring from the first sealing portion 76 the second sealing portion 78 is not completely closed but contains a further valve in form of a check valve forming a bypass valve 86 as best shown in FIGS. 10-13. The bypass valve 86 has an opening 92 in the second sealing portion 74 which opening 92 is facing the second inlet port 22 in the second valve position as shown in FIG. 7. The bypass valve 86 comprises a bypass valve element 88 arranged between the support member 36 and cover member 38 of the valve element 24. The bypass valve element 88 is guided in a linear direction parallel to the rotational axis X on a guiding element 90 engaging into the bypass element 88. The bypass valve element 88 in its closed position abuts against a valve seat formed by the sealing member 78 surrounding the opening 92 or defining the opening 92 inside the second sealing portion 74. The bypass valve element 88 is hold in this closed or sealed position by a compression spring 94 forcing the bypass valve element 88 into the shown sealed or closed position. By a pressure acting on the bypass valve element 88 the bypass valve element 88 can be moved along the guiding element 90 against the force provided by the compression spring 94 to open the opening 92. The backside of the bypass valve element 88 facing away from the opening 92 is in contact with the opening 80 and the outlet opening 26, i.e. in contact with the suction side of the pump and with the flow path towards the suction mouth 28 of the impeller 14. Thus, the pressure on the suction side of the pump is acting onto the backside of the bypass valve element 88. If the pressure difference between both sides of the bypass valve 86 or bypass valve element 88, respectively, exceeds a predefined threshold, which is defined by the size of the bypass valve element 88 and the spring 94, the bypass valve 86 opens to allow a fluid flow from the second inlet port 22 towards the impeller 14 although the second inlet port 22 is closed by the second sealing portion 74. This functionality may be used in a heating system when a heating circuit in a building is connected to the first inlet connection 16. In case that all radiators in the heating circuit are closed there would be no fluid flow through this first inlet connection 16. In this condition the pressure on the suction side of the impeller 14 and, therefore, on the backside of the bypass valve element 88 will reduce to such an extend that the pressure difference across the bypass valve 86 exceeds the predefined threshold and the bypass valve 86 opens ensuring a fluid flow through the second inlet port 22 to which for example a heating exchanger for heating domestic water may be connected. In a heating system, thus, a fluid flow through the boiler can be ensured avoiding an overheating of the boiler.

[0060] The threshold for opening the bypass by the bypass valve 86 preferably it adjusted by exchanging the bypass valve element 88. There may be provided exchangeable bypass valve elements 88 of different size, in particular having different sized back surfaces onto which the pressure on the suction side of the pump acts. Since the opposite surface is always defined by the cross section of the opening 92 it is possible to adjust the forces acting in both directions onto the bypass valve element 88 by changing the size of the back surface. Alternatively, or in addition also the size of the surface closing the opening 92 can be adjusted by changing the diameter of the circular protrusion 93 on the bypass valve element 88 being in contact witch the valve seat in the sealing member 78.

[0061] The valve element 24 is moved between the two valve positions similar as known from EP 3 376 049 by the circulating flow produced by the impeller 14. If the speed of the electric drive motor is reduced or the motor is switched off by the control electronics 7 the pressure in the pump space 44 is reduced such that the compression spring 44 moves the valve element 24 in its released position as shown in FIG. 4. In this position the valve element 24 can be rotated about the rotational axis X by a circulating fluid flow inside the pump space 44. The direction of the fluid flow depends on the rotational direction of the impeller 14. The two valve positions are each defined by an end stop. For this there is provided a circular groove 96 in the bottom wall of the pump housing 8. This circular groove 96 does not define an entire circle but has an interruption in form of a web 98. The opposing surfaces of this web 98 define the two end stops for the rotational movement of the valve element 24, i.e. the end stops defining the two possible valve positions. The spring support 52 of the valve element 24 has an axial extension forming a stop element 100. The stop element 100 has a form of a finger offset to the rotational axis X and engaging into the groove 96. The stop element 100 can abut against the two opposing faces of the web 98 to define the two rotational positions corresponding to the possible valve position as described before. In this case it is advantageous that the stop is arranged in the center allowing a damping effect due to the elasticity of the parts and particularly by torsion of the compression spring 54 as described above. FIG. 13 shows the stop element 100 in the second valve position corresponding to the position shown in FIG. 7. FIG. 14 shows the stop element 100 in the first valve position corresponding to the valve position shown in FIG. 6. It can be seen that to change the valve position the valve element 24 rotates by 270°.

[0062] To enhance the rotation of the valve element 24 without increasing the flow resistance during normal operation of the pump device there are provided radial protrusions 102 distributed over the entire outer circumference of the valve element 24. The protrusions 102 are arranged on the backside of the cover plate 104 on the cover member 38 so that the cover member 36 has a cover plate 104 facing towards the impeller 14 extending in radial direction beyond these protrusions 102 so that the protrusions 102 are completely covered by this cover plate 104 on the side facing the impeller 14. Thus, the protrusions 102 are arranged on the backside of the cover plate 104. The cover plate 104 has a diameter smaller than the inner diameter of the pump housing 8 such that a circular gap 106 surrounding the outer circumference of the cover plate 104 is provided. The gap 106 provides a flow connection between the pump space 44 and the region in which the protrusions 102 are arranged. If the valve element 24 is in its sealed or engaged position as shown in FIG. 3, substantially no fluid flow through the gap 106 will occur since the flow path through the gap 106 is closed by the sealing 70 on the opposite end. However, if the valve element 24 is in its released position as shown in FIG. 4 there is a gap between the sealing 70 and the shoulder 68 opening the flow path through the gap 106 towards the opening 80 of the valve element 24, i.e. on the suction side of the valve element 24. Thus, if the impeller 14 rotates, a part of the fluid flow leaving the impeller 14 will enter the gap 106 and flow towards the opening 80 around the valve element 24 towards the outlet opening 26. Due to the rotation of the impeller 14 this side flow through the gap 106 has a spin in the rotational direction of the impeller acting on the rib or tooth shaped protrusions 102 generating a torque on the valve element 24 to rotate the valve element 24 until the stop element 100 abuts against the end stop provided by the web 98. If, now, the speed of the impeller is increased by the control electronics 7 the pressure on the outside of the impeller 14 increases so that the valve element 24 is moved into its sealed position in which the sealing 70 comes into contact with the shoulder 68 and one of the sealing portions 72, 74 comes into contact with an opposing valve seat 82, 84. In this operational condition a sealed valve position is reached. After this it is possible to quickly change the rotational direction of the impeller without moving the valve element 24 out of its present valve position. To achieve this, due to respective control by the control electronics 7 the electric drive motor is accelerated thus quickly that the pressure outside the impeller 24 generates an axial force overcoming the spring force of the compression spring 54 prior to establishing a circular flow rotating the valve element 24 into the other valve position. This allows to selectively move the valve element 24 into a desired valve position and afterwards to again change the rotational direction of the impeller 14 so that during operation of the centrifugal pump device the impeller 14 can always rotate in a desired optimized rotational direction. The arrangement of the protrusions 102 on the backside of the cover plate 104 has the advantage that the protrusions have an effect only if the valve element 24 is in its released position. During normal operation with the valve element is in its sealed position the protrusions 102 have nearly no effect, in particular they do not increase the hydraulic resistance in the pump space 44.

[0063] The electric motor inside the motor housing 4 is a wet-running electric motor having a rotor can 108 forming the rotor space inside which the rotor shaft 50 with the rotor 110 rotates. This rotor space is filled by the liquid to be pumped, i.e. preferably water. The stator 112 is arranged on the outside of the rotor can 108 in a dry stator space inside the motor housing 4.

[0064] FIG. 18 shows an example for the use of the centrifugal pump device 114 described before. The centrifugal pump device including the features described before, i.e. the valve element 24 and the bypass valve 86 are the components surrounded by the dotted line in FIG. 18. The centrifugal pump device 114 comprises the centrifugal pump 116 with the electric drive motor 2 and the impeller 14. The valve element 24 forming a switch over valve is arranged on the suction side of the centrifugal pump 116 allowing to switch the flow path between two possible inlet connections, the first inlet connection 16 and the second inlet connection 18. On the pressure side the centrifugal pump 116 is connected with the outlet connection 10. In this example the outlet connection 10 is connected to a boiler 118 heating the liquid, in particular water, in the heating circuit. On the outlet side of the boiler 118 the heating circuit branches into a first branch forming the circuit of a central heating CH which may contain several radiators 120 or one or more floor heating circuits, for example, and the second branch for heating domestic hot water DHW. The second branch comprises a heat exchanger 122 for heating domestic hot water (DHW). As can been seen, the bypass valve is in connection with the second branch, i.e. the branch containing the heat exchanger 122. In case that the valve element 24 is in the valve position in which a flow path through the central heating circuit CH is open, the bypass valve 86 can prevent an overheating of the boiler 118. In this valve position, if the radiators 120 are closed, the fluid flow through the central heating circuit CH is interrupted. In this case the bypass valve 86 can open due to a pressure difference overcoming the biasing force of the compression spring 94 such that the flow path through the heat exchanger 122 opens and the water is circulated by the centrifugal pump 116 through the second branch of the heating system, i.e. through the heat exchanger 122, thereby distributing the heat produced by the boiler 180 in the system to avoid an overheating of the boiler 118.

[0065] FIG. 19 shows a second embodiment of the valve element 24′. In this embodiment the support member 36 including the bypass valve 86 is substantially the same as described above. In this second embodiment the cover member 38′ of the valve element 24′ has a different design. In this embodiment the protrusions 102′ extend in radial direction from an outer circumferential surface 124. Furthermore, the protrusions 102′ are inclined in circumferential direction such that they form inclined vanes on the outer circumferential surface 124 of the valve element. In this embodiment the cover plate 104′ does not extend beyond the outer circumference of the circumferential wall 124, i.e. does not cover the axial end sides of the protrusions 102′. Thus, by use of this valve element 24′ the gap 106′ or ring-shaped space between the outer circumferential wall 124 of the valve element and an inner 5 circumferential wall 126 of the pump housing 8′ is open towards the space containing the impeller 14, as visible in FIG. 20. Thus, a fluid flow produced by the impeller 14 can directly impinge on the protrusions 102′. Due to their inclination the protrusions 102′ produce a higher torque acting on the valve element 24′ about the rotational axis X. The pump housing 8′ with the valve element 24′ as shown in FIG. 20 can be connected to the electric drive motor 2 as shown in FIG. 1 and described above. Furthermore, the impeller 14 can be the same as shown for the first embodiment.