METHOD FOR SHUTTING OFF A PUMP AS WELL AS PUMP STATION ARRANGEMENT

Abstract

A pump station arrangement and a method for turning off a pump configured for pumping liquid via a conduit. The pump, before being turned off, being driven at an operational frequency (F.sub.N) by a control unit. The method is characterized by the steps of, ramping down the frequency of the pump due to a turn off instruction, the terminal frequency of the ramping down being equal to the operational frequency (F.sub.N) of the pump minus at least 10 Hz and the ramping down time being at least a reflection time (T.sub.R) for the conduit in question, and the terminal frequency of the ramping down not being less than 10 Hz, and stopping the pump after the ramping down.

Claims

1.-10. (canceled)

11. A method for turning off a pump configured for pumping liquid via a conduit, the pump before being turned off being driven at an operational frequency (FN) by a control unit, the method comprising the steps of: ramping down a frequency of the pump using the control unit due to a turn off instruction, a terminal frequency of the ramping down being equal to the operational frequency (FN) of the pump minus at least 10 Hz and the ramping down time being at least a reflection time (TR) for the conduit, and the terminal frequency of the ramping down not being less than 10 Hz, and stopping the pump after said ramping down using the control unit, the stopping step including disengagement of the pump by the control unit in order to let an impeller of the pump to freewheel until the impeller stops.

12. The method according to claim 11, wherein the step of stopping the pump after the ramping down, includes performing a second ramping down of the frequency of the pump using the control unit in such a way that a torque the motor of the pump experiences from the pumped liquid is reduced towards zero.

13. The method according to claim 11, wherein the terminal frequency of the ramping down is less than or equal to 40 Hz.

14. The method according to claim 13, wherein the terminal frequency of the ramping down is less than or equal to 35 Hz.

15. The method according to claim 11, wherein the terminal frequency of the ramping down is greater than or equal to 20 Hz.

16. The method according to claim 15, wherein the terminal frequency of the ramping down is greater than or equal to 25 Hz.

17. The method according to claim 11, wherein the ramping down time is at least 10 seconds.

18. The method according to claim 17, wherein the ramping down time is at least 15 seconds.

19. A pump station arrangement comprising a pump, a control unit and a conduit connected to an outlet of the pump, wherein, due to a turn off instruction, the control unit is configured to ramp down a frequency of the pump from an operational frequency (FN), a terminal frequency of the ramping down being equal to the operational frequency (FN) of the pump minus at least 10 Hz and the ramping down time being at least a reflection time (TR) for the conduit, and the terminal frequency of the ramping down not being less than 10 Hz, the control unit is configured to stop the pump after the ramping down by disengagement of the pump in order to let an impeller of the pump freewheel until the impeller stops.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] A more complete understanding of the above mentioned and other features and advantages of the present invention will be evident from the following detailed description of preferred embodiments having reference to the attached drawings, in which:

[0019] FIG. 1 is a schematic illustration of a pump station comprising the pump station arrangement,

[0020] FIG. 2 is a diagram that schematically disclose how the frequency, the liquid flow and the pressure of the pump are changed during turning off in accordance with the present invention, and

[0021] FIG. 3 is a diagram that schematically disclose how the frequency, the liquid flow and the pressure of the pump are changed during turning off in accordance with prior art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0022] In FIG. 1 is disclosed a pump station arrangement comprising a pump station, generally designated 1, comprising at least one speed controlled pump 2, usually two submersible pumps, configured in an active state to pump liquid from a sump 3 of the pump station 1 to a conduit 4 extending away from the pump station 1. The conduit 4 comprises a non-return valve (not disclosed) arranged in close connection with the pump 2. Thereto the pump station 1 in a conventional way comprises at least one level sensor 5 arranged to determine the liquid level in the pump station 1, the level sensor 5 may for instance be constituted by a floating level sensor that is configured to determine a predetermined liquid level or a continuous level sensor that is configured to determine different liquid levels. It shall be pointed out that the level sensor 5 may be a separate device that is operatively connected to an external control unit 6, be operatively connected to said at least one speed controlled pump 2, be built-in in said at least one speed controlled pump 2, etc. Said at lest one speed controlled pump 2 is preferably operatively connected to the external control unit 6 in order to admit adjustment of the rotational speed of the pump, alternatively said at least one speed controlled pump 2 may comprise an built-in control unit (not shown). Herein below the term control unit 6 will be used independently of its physical location.

[0023] The pump 2 and the control unit 6 together constitute at least a part of a pump arrangement, in which the pump 2 comprises an electrical motor 7 that is arranged to be driven by said control unit 6, and an impeller 8 that is connected to the motor 7 via a drive shaft 9 in a conventional way. Preferably the impeller 8 is an open impeller, and most preferably the impeller is axially displaceable in the pump 2, in relation to a suction cover/insert ring at the inlet of the pump, during operation.

[0024] The term speed controlled embrace all conceivable ways to change the rotational speed of the pump, or more precisely the rotational speed/operational speed of the motor 7, especially adjustment of the current feed frequency by means of a frequency converter (Variable Frequency Drive) is intended, that is built-in in a pump or that is external, and that constitutes an example of said control unit 6, the rotational speed being proportional to the current feed frequency during normal operation. However, internally or externally controlled adjustment of the supply power, etc. are intended. Thus, at an overall level of the invention it is not essential how the operational speed of the pump is regulated, only that the rotational speed of the pump 2 can be adjusted/controlled.

[0025] The pump 2 is configured to be operatively connected to the power mains that in different parts of the world have different power frequency, usually 50 Hz or 60 Hz. According to an alternative embodiment the pump 2 is operatively connected to a power generating unit that makes use of a diesel engine, or the like. The output frequency from the power generation unit may be constant or variable, and is usually 50 Hz or 60 Hz.

[0026] During normal operation of the pump 2 it is driven by means of the control unit 6 at an operational frequency F.sub.N, also known as operational speed. The operational frequency F.sub.N can be varied over time of be constant, and can for instance be equal to a maximum frequency, i.e. the power frequency of the power mains, or be in the range of 90-95% of the power frequency of the power mains.

[0027] When the pump 2, or the pump station arrangement, owing to certain circumstances receive an automatically or manually generated turning off instruction, for instance from the level sensor 5, the control unit 6 initiates a controlled ramping down of the frequency F of the pump 2 from the operational frequency F.sub.N downwards. This ramping down may be linear (constant decreasing rate) or un-linear (varying decrease rate) from the operational frequency F.sub.N towards a terminal frequency for the ramping down.

[0028] Reference is now made to FIG. 2 that schematically disclose a diagram having time measured in seconds at the X-axis and the frequency F of the pump 2 at the Y-axis. It shall be pointed out that the Y-axis has a comparative scale where the operational frequency F.sub.N of the pump 2 is fixed at 1 (corresponding to 100 percent, which in reality is for instance 50 Hz). The frequency F of the pump 2 is disclosed by means of the middle curve. Thereto the Y-axis also comprises the liquid flow in the conduit 4, the upper curve disclosing how the liquid flow is changed over time, and the pressure in the conduit 4 in the area downstream the pump 2, the lower curve disclosing how the pressure is changed over time. The liquid flow and the pressure are in accordance with the frequency F of the pump 2 given by means of comparative scales.

[0029] The terminal frequency of the ramping down shall be equal to the operational frequency F.sub.N of the pump 2 minus at least 10 Hz and thereto shall not fall below 10 Hz. In the embodiment disclosed in FIG. 2 the terminal frequency of the ramping down is equal to 60% of the operational frequency F.sub.N, i.e. 30 Hz if the operational frequency F.sub.N is equal to 50 Hz. Preferably the terminal frequency of the ramping down shall be less than or equal to 40 Hz, or most preferably less than or equal to 35 Hz. Thereto, it is preferable that the terminal frequency of the ramping down is greater than or equal to 20 Hz, most preferably greater than or equal to 25 Hz.

[0030] The ramping down time shall be at least one reflection time T.sub.R for the conduit 4 in question. In the example disclosed in FIG. 2 the ramping down time is approximately 15 seconds.

[0031] The reflection time T.sub.R of the conduit 4 in question is known in such pump station arrangements and refer to the time it takes for a pressure wave in the conduit 4 to move back and forth in the conduit 4. The reflection time T.sub.R is equal to 2*L/C, where L is the length of the conduit and C is a material specific constant. C is in the range 300-400 if the conduit 4 is made of plastic and C is in the range 1000-1200 if the conduit 4 is made of steel. Thus, for a conduit 4 made of plastic and having a length of 2000 meter the reflection time T.sub.R is in the range 10-13 seconds, and the corresponding value for a conduit 4 made of steel is in the range 3.5-4 seconds.

[0032] According to a preferred embodiment the ramping down time shall be at least 10 seconds, most preferably at least 15 seconds. This is applicable at least when the reflection time of the conduit 4 is note known for whatever reason.

[0033] After the ramping down the inventive turning off method comprises the step of by means of the control unit 6 stop the pump 2.

[0034] According to the most preferred embodiment, that is disclosed in FIG. 2, the step of stopping the pump 2 after the ramping down includes disengagement of the pump 2 by means of the control unit 6 in order to let the impeller 8 of the pump 2 to freewheel until it stop. According to a second embodiment (not shown) the step of stopping the pump 2 after the ramping down includes performing a second ramping down of the frequency F of the pump 2 by means of the control unit 6 in such a way that the torque the motor 7 of the pump 2 is subject to from the pumped liquid is controlled towards being equal to zero. The second ramping down is terminated when the frequency F of the pump 2 is equal to zero. In other words the second embodiment implies that a freewheeling of the impeller 8 is imitated. According to a third embodiment (nor disclosed) a second ramping down may be performed that is steeper than the first ramping down.

[0035] In FIG. 3 is disclosed a diagram of a turning off procedure according to prior art corresponding to FIG. 2, wherein the turning off of the pump 2 is performed by disengaging the impeller 8 of the pump 2 at the operational frequency and allowing the impeller 8 to freewheel until it stop. The lower curve disclose that a great under pressure arise in the conduit 4 causing extensive water hammering.

[0036] The upper curve discloses an extensive pulsation of the liquid flow in the conduit 4.

Feasible Modifications of the Invention

[0037] The invention is not limited only to the embodiments described above and shown in the drawings, which primarily have an illustrative and exemplifying purpose. This patent application is intended to cover all adjustments and variants of the preferred embodiments described herein, thus the present invention is defined by the wording of the appended claims and thus, the equipment may be modified in all kinds of ways within the scope of the appended claims.

[0038] It shall also be pointed out that all information about/concerning terms such as above, under, upper, lower, etc., shall be interpreted/read having the equipment oriented according to the figures, having the drawings oriented such that the references can be properly read. Thus, such terms only indicates mutual relations in the shown embodiments, which relations may be changed if the inventive equipment is provided with another structure/design.

[0039] It shall also be pointed out that even thus it is not explicitly stated that features from a specific embodiment may be combined with features from another embodiment, the combination shall be considered obvious, if the combination is possible.