EP1936205A1 - Method for operating a speed controllable centrifugal pump power unit - Google Patents

Abstract

The operating process is for a centrifugal pump, the revs of which can be regulated, with an unstable pump characteristic (9). The effective pump characteristic (9') at a low throughput rate is raised sufficiently to ensure that there is only one intersection point between it and the actual pipeline network characteristic (Rb).

Description

The invention relates to a method for operating a variable speed centrifugal pump assembly with the features specified in the preamble of claim 1.

The characteristic curve of a pump indicates the behavior of the pump as a function of the operating conditions. In modern speed-controllable centrifugal pump units, the effective pump characteristic can be varied within wide ranges by changing the speed. However, a centrifugal pump due to the given mechanical / hydraulic conditions for each speed on a certain pump characteristic (throttle curve), which indicates the head depending on the flow rate. There are pump characteristics, in which the delivery height decreases with increasing flow and which are referred to as stable. If, on the other hand, there are areas in which the delivery head increases with an increasing flow rate, this is called an unstable pump curve or throttle curve, which, in particular in conjunction with a flat pipe network characteristic, can result in no clear intersection between the pipe network characteristic and the pump characteristic but typically two intersections. Then, however, the operating point can change abruptly, which can lead to oscillations in the system and is highly undesirable because the operating point is not predictable. On the other hand, however, centrifugal pumps with an unstable pump characteristic often have a particularly good efficiency, which is why this has been inevitably accepted or has taken care of, that the pump did not work in this unstable area if possible.

Against this background, the invention has for its object to provide a method for operating a variable speed centrifugal pump assembly with unstable pump characteristic that avoids the aforementioned disadvantages and in particular ensures that there is always only one intersection between the pump characteristic and system characteristic.

This object is achieved according to the invention by the features specified in claim 1. Advantageous embodiments of this method are specified in the subclaims of the following description and in the drawing.

According to the invention, the speed-controllable centrifugal pump assembly is operated with unstable pump characteristic so that the effective pump characteristic is raised to lower delivery heights by increasing the speed so far that always results in only one intersection with the current pipe network characteristic. The basic idea of the present invention is thus, only if, on the one hand, the system characteristic curve is relatively flat and the pump is to be operated in the range of small flow rates, the pump characteristic curve is raised by increasing the speed so that it is ensured even in this area that only a clear point of intersection with the system characteristic so the pipe network characteristic of the system in which promotes the centrifugal pump is given.

This method according to the invention is particularly advantageous, since it makes it possible to drive centrifugal pump units with unstable pump curve, possibly also in the per se unstable conveying region of the characteristic curve stable.

The raising of the speed to low flow rates, according to an embodiment of the invention advantageously by a controller (open loop), which raises the power consumption of the pump set to low heads.

Alternatively, according to the invention, a controller may also be provided which raises the rotational speed of the pump assembly to low delivery heights. Such a targeted increase in speed is technically more complex than the control of power consumption, but allows a relatively accurate vote on the current system requirements.

It is particularly advantageous to provide a preferably subordinate control (closed loop), which regulates the power consumption of the pump unit constantly to low delivery heights. Such a control is easy to implement and is usually sufficient to avoid the problem of the unstable pump characteristic as described in the introduction.

It is expedient for the aforementioned control / regulation that, if it takes place as a subordinate regulation or control, it takes place so quickly that when the delivery rate drops, the normally resulting unstable states of the pump are not reached at all. This can be ensured by the subordinate fast control being designed to be faster than the inertia of the conveyed liquid column. Then it is ensured that the increase in the speed always takes place before the delivery has dropped so far that an unstable operating state could result.

Since the required speed of the control is dependent on the inertia of the pumped liquid column, the speed of the control will have to be adjusted to this case in individual cases. In practice, however, it has been found that it is sufficient for almost all applications if the subordinate fast control / regulation has a time constant or control time constant of less than one second preferably less than 300 ms. The last-mentioned time constant is sufficient even with comparatively small tube lengths to safely master the aforementioned problem and, moreover, can be implemented with relatively little technical effort in terms of control / regulation.

Figur 1

zwei Rohrnetzkennlinien mit einer instabilen Pumpenkennlinie und die sich dabei ergebenden Betriebspunkte,

Figur 2

einen Hydraulikschaltplan einer Anlage, bei der sich die in Figur 1 dargestellten Betriebszustände einstellen,

Figur 3

eine instabile Pumpenkennlinie sowie die Anhebung dieser Pumpenkennlinie im instabilen Bereich,

Figur 4

eine Anhebung gemäß Figur 3 mit anderen Steuerungsverfahren.

The invention is explained in more detail with reference to embodiments shown in the drawing. Show it

FIG. 1

two pipe network characteristics with an unstable pump characteristic and the resulting operating points,

FIG. 2

a hydraulic circuit diagram of a system in which the in FIG. 1 set operating states,

FIG. 3

an unstable pump characteristic and the increase of this pump characteristic in the unstable region,

FIG. 4

an increase according to FIG. 3 with other control methods.

Based on FIG. 2 is shown a promotion situation, which should first explain the problem of an unstable pump characteristic using a practical example. A centrifugal pump 1 delivers from a storage tank 2, a liquid via a line 3 with the interposition of a check valve 4 and a check valve 5 in a container 6. At the beginning of the conveying process, a height difference Hg1 is overcome, which substantially the height difference between the liquid surface 0 of the comparatively corresponding to the level fluctuation substantially constant storage tank 2 to the liquid surface 7 of the comparatively small container 6, which it is necessary to fill. When the valve 5 is open, the centrifugal pump 1 - which is a variable-speed centrifugal pump with an unstable pump characteristic - conveys a large amount Flow rate Q up to a head Hg 1. The valve 5 is closed with increasing reaching the level 7, whereby the flow rate decreases slowly. These operating points are in the diagram according to FIG. 1 marked with a or b.

By slow closing of the valve 5, the pipe network characteristic of Ra changes to Rb, etc., then adjust according to operating points c, d, e, f, g, h and i, the further the valve 5 is closed. Due to sinking liquid level from level 7 to the level 8 is then to be overcome head Hg2 significantly lower than before during subsequent opening of the valve 5, which is why the flow rate then either increased abruptly, or almost dried there, as the FIG. 1 shows, the pump characteristic 9 has at least with the pipe network characteristic Rb two points of intersection and it is not predictable which of the two operating points adjusts itself.

Such behavior is highly undesirable if steady and stable operation is to be achieved in a flow range that is skipped, as it were. Such an operating state can occur, for example, when pumping water into a steam boiler (boiler feed application) while using waste heat to heat the feed water.

In order to avoid this undesirable operating state, the effective pump curve 9 is raised to falling flow rates so far that instead of the in FIG. 3 With 9 marked course a course 9 'results in which it is ensured that only one intersection with the pipe network characteristic is always given. In the presentation after FIG. 3 For comparison, the pipe network characteristic Rb is shown, which is almost horizontal and therefore particularly critical for this application. As such, the pipe network characteristic Rb can be detected in a manner known per se, so that the self-adjusting pipe network characteristics are stored in the control or regulation of the pump and corresponding Speed increases can be made only in this critical area, as determined by FIG. 3 is shown.

To increase the speed to falling flow rates out, it is necessary on the pump side to detect the flow rate - also called flow. This can be done either by suitable sensors by direct measurement, or preferably by indirect measurement based on the electrical variables of the pump driving the electric motor. Thus, by measuring the electrical power consumption of the motor, the delivery rate of the pump can be detected, at least in the region of instability of the pump characteristic, if, as is typically the case, the power consumption with the delivery rate increases or decreases severely monotonically.

Based on FIG. 4 It is shown how the power consumption of the pump P driving motor P varies depending on the flow rate (the flow rate) Q. It is therefore particularly favorable, with decreasing delivery rate, to regulate the power consumption of the pump constantly, as described in US Pat FIG. 4 is shown by the curve part 10 '. To regulate the power consumption is technically feasible in a simple manner and causes the basis of FIG. 3 described effect that the pump curve 9 is raised in the range of decreasing flow rates Q.

The increase in the speed with decreasing flow rate Q or the corresponding reduction in the speed with increasing flow Q is always so fast that the inertia of the pipe system and funded by the pump fluid is so large that the increase in speed has occurred before an unstable operating point K can be set at all. With a few meters of line length, time constants or control time constants of 300 ms are sufficient, but usually time constants or control time constants that are less than one second are sufficient.

LIST OF REFERENCE NUMBERS

0

Liquid level in the tank 2

1

rotary pump

2

storage tank

3

management

4

check valve

5

shut-off valve

6

container

7

Level 1

8th

Level 2

9

Pump curve

10

Power curve

9 '

raised pump characteristic

10 '

about constant power

hg1

head

hg2

head

Claims (6)

Method for operating a speed-controllable centrifugal pump assembly with unstable pump characteristic, characterized in that the effective pump characteristic is increased to low flow rates through speed increase so far that only results in an intersection with the current pipe network characteristic. A method according to claim 1, characterized in that a control (open loop) is provided, which raises the power consumption of the pump unit to low flow rates. Method according to one of the preceding claims, characterized in that a control is provided which raises the speed of the pump unit to low flow rates. Method according to one of the preceding claims, characterized in that a preferably subordinate control is provided, which regulates the power consumption of the pump unit to low flow rates constant. Method according to one of the preceding claims, characterized in that the preferably subordinate control / regulation is designed so that it is faster than the inertia of the conveyed liquid column. A method according to claim 5, characterized in that the control / regulation has a time constant / control time constant of less than one second, preferably less than 300 ms.

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