EP2837829B1 - Control of the characteristics of centrifugal pumps - Google Patents

Description

Field of the Invention

The present invention relates to a method for regulating a pump, in particular a centrifugal pump, while pumping a liquid and a corresponding device.

State of the art

Centrifugal pumps have a strong dependency of the delivery rate on the applied pressure difference and the speed. More specifically, a difference between the pump outlet side liquid pressure and the pump inlet side liquid pressure determines the flow rate (mass flow or volume flow). Each pump has a characteristic pump map that defines a relationship between the three parameters (difference between the pump-side liquid pressure and the pump-side liquid pressure, flow rate, speed). In this way, if the two parameters are known, the third can be determined from the characteristic diagram. The map can be in the form of empirical, semi-empirical or theoretical model equations. In empirical model equations, empirically recorded values can be linked to compensation functions. These empirical compensation functions can also be recorded in a table. In the case of semi-empirical model equations, both empirically determined values and physical equations are used, which e.g. Describe relationships between physical parameters. In the case of theoretical model equations, the relationships between the parameters are completely described by physical equations.

It is disadvantageous that fluctuations in the media pressure on the high and / or low pressure side cause an uneven flow (at a given speed), which in the case of processes critical to the mass flow adversely affects the Process flow can lead. The map also reduces the operating range of the pump, which can lead to process malfunctions and component damage if the limits are exceeded.

Fig. 1 shows an example of such a map. The delivery head H as a function of the speed n is plotted here via the volume flow Q. The volume flow is limited by a minimum and maximum value of the map. There is also a maximum delivery head that can only be achieved at the highest speed and minimum flow. It turns out that at a fixed speed, the flow changes greatly depending on the height. Since the delivery head is proportional to the pressure difference at the pump, fluctuations in the pressure on the high or low pressure side cause a change in the pump flow. The lower limit of the volume flow does not have to be constant as in the drawing, but can depend on the speed.

The example in Fig. 2 shows a reduction in the delivery head from H ₁ to H ₂ at constant speed n. The flow from Q ₁ to Q ₂ increases significantly due to the map behavior. Such changes can cause problems in process operation that can lead to malfunctions, downtimes and defects. In addition, in many processes changes in the flow are desired regardless of the current delivery head. This function is also affected by the influence of the map. If, for example, the flow is to be increased and the speed is increased, the increased delivery rate can result in an increase in pressure in many processes on the high-pressure side, which partly compensate for the increase in flow due to the influence of the map.

The map also shows that there are machine-specific restrictions for pump operation (such as a minimum volume flow), which must be observed to ensure that the machine functions permanently.

From the document DE 10 2011 115 244 A1 only a monitoring of the operating state of a pump is known, which includes a comparison of an actual characteristic curve with a target characteristic curve of the pump, in order to predict from this that the pump is in need of repair or replacement.

One area of application in which the safe delivery of a fluid flow is of particular importance is the pump control of a feed pump of an ORC power plant process (Organic Rankine Cycle), as schematically in Fig. 3 shown. A pump (P) is regulated in such a way that desired fresh steam parameters can be set reliably at the outlet of a heat exchanger (V) connected downstream of the pump. For this purpose, the speed of the pump is influenced by the control in such a way that the evaporation condition changes as a result of the change in flow rate in such a way that the desired pressures and temperatures of the live steam are reached and are controlled stably for stable process operation.

In this example, the delivery head of the pump depends on the live steam pressure (p _FD ) and on the pressure level _{upstream of} the pump (p _KOND ). This pressure depends on the current condensation pressure of the condenser (K) upstream of the pump. This condenser cools and liquefies the working medium in the ORC process by giving off heat to a cooling medium. This cooling medium (e.g. water from a heating network or ambient air) can fluctuate in quantity and temperature (temperature fluctuations in a heating network, wind or other environmental influences). These fluctuations affect the heat transfer in the condenser, which affects the condensation conditions and thus the condensation pressure. This means that external disturbances can affect the delivery head of the pump and therefore cause fluctuations in mass flow and live steam pressure. These possible fluctuation amplitudes must be taken into account in safety considerations and availability analyzes. Furthermore, the ORC process is a closed system, and therefore a reaction of a fluctuating live steam pressure via the expansion machine (E) to the condensation pressure cannot be ruled out. This can lead to a self-reinforcing effect that has a further negative impact on process stability.

One way to counteract these influences is to use a cascade control in accordance with Fig. 4 , In it, an internal control loop regulates the flow based on a comparison of the current actual value and the setpoint of the mass or Volume flow, while an external control loop specifies the flow setpoint for the control to the actual control variable of the pump (e.g. process pressure) to the inner loop. This allows flow deviations to be compensated for and at the same time regulated to a desired process value.

In the cascade controller, the (inner) sub-process I can be the pumping process. This contains all the components that convert the signal of the mass flow control (m control) into the delivery of a medium. This can include control / speed control of the pump, the pump motor and the pump itself. The outer subprocess II can, for example, be an evaporation process and the process value s can be the media pressure p after the evaporation. The evaporation process can thus contain all the necessary components, such as one or more heat exchangers, containers, fittings, etc.

Although this solution allows flow deviations to be detected and reacted to when they occur, the flow must have already moved away from its setpoint S _setpoint . This means that predictive compensation is not possible before fluctuations occur. An additional feedforward control is therefore necessary (not shown). In addition, this prior art solution requires a complex and often costly measurement of the mass or volume flow. Avoiding this measurement would have significant cost advantages.

The publication EP-A-1286056 discloses a method of controlling a pump in response to a signal indicating the presence and extent of cavitation. A controller receives suction and discharge pressure signals from sensors upstream and downstream of the pump.

Description of the invention

The object of the invention is to at least partially overcome the disadvantages described above.

This object is achieved by a method according to claim 1.

The method according to the invention for regulating a pump, in particular a centrifugal pump, while pumping a liquid, comprises the steps: establishing a setpoint of a flow rate of the pump; Measure an inlet pressure of the liquid upstream of the pump and an outlet pressure the liquid downstream of the pump; Determining a setpoint of a speed of the pump or a control signal determining the speed from a map of the pump, the setpoint of the flow rate and a difference between the outlet pressure and the inlet pressure being input to the map as input values; and setting the speed of the pump to the setpoint value of the speed or supplying the control signal determining the speed to the pump.

It is advantageous here that no measurement of the mass or volume flow is required for regulation or compensation by taking the characteristic diagram into account. Furthermore, the control can react when a pressure fluctuation occurs before the effects of a flow fluctuation occur (predictive control behavior), which improves the control quality.

The map of the pump can be used in the usual form, so there is a relationship between the flow rate and the differential pressure or the delivery head at different but constant speed.

The map can alternatively or additionally be used in "inverted" form (hereinafter also referred to as inverted map), in which case there is a relationship between the differential pressure or delivery head and the speed at different but constant flow rates.

In any case, the map is used in such a way that a change in the flow rate caused by a change in differential pressure is counteracted by a change in speed in order to keep the flow rate as constant as possible, which is done by finding a corresponding operating point of the pump in its map or inverted map.

The setpoint of the flow rate can in turn be determined by the control system, for example based on a specified output pressure of the pump or based on another suitable process value. On the other hand, the setpoint of the flow rate can be set by a user. In both cases, this can be done either by directly specifying the flow rate or indirectly by means of a Specification of the speed, from which the flow rate to be kept constant can then be determined.

Preferably, after determining the flow rate set point, the steps of measuring the inlet pressure of the liquid and the outlet pressure of the liquid, determining the set point of the speed of the pump, and adjusting the speed of the pump are performed continuously.

According to a further development, the determination of the setpoint value of the flow rate can comprise the following steps: determining an average over time of the difference between the outlet pressure and the inlet pressure; and determining the desired value of the flow rate from the characteristic diagram of the pump, the mean value over time of the difference between the outlet pressure and the inlet pressure and a current speed of the pump being input into the characteristic diagram. In this way, a setpoint of the flow rate that is to be maintained as possible can be determined while the pump is in operation. In this case, the setpoint of the flow rate can also be set continuously.

Another development consists in that the temporal average of the difference between the outlet pressure and the inlet pressure can be determined from a first temporal mean of the inlet pressure and a second temporal mean of the outlet pressure. It is therefore possible to use different time constants for averaging the inlet pressure and the outlet pressure if necessary.

According to another further development, the determination of the setpoint value of the speed of the pump can comprise the following further steps: checking whether a combination of the speed of the pump, the setpoint value of the flow rate and the difference between the outlet pressure and the inlet pressure lies within a map limit; Setting the speed of the pump to the setpoint value of the speed if the combination lies within the map; and setting the speed of the pump to a safety value if the combination lies outside the characteristic diagram, the safety value preferably being selected such that the deviation from the desired value of the flow rate is as small as possible.

According to another development, the setting of the speed of the pump to the setpoint value of the speed can include the output of a correction signal to an actuating signal supplied to the pump. In this way, a correction signal can be applied to the control signal. In particular, a minimum control signal can be output as a correction signal in order to prevent an operating state from being set outside the characteristic diagram.

Another development is that the map defines a relationship between the flow rate and a delivery head of the pump at different speeds, and the delivery head is determined from the pressure difference between the measured outlet pressure and the measured inlet pressure. In particular, the delivery head h can be determined from h = (p ₂ -p ₁ ) / (ρ · g), where p ₁ denotes the measured inlet pressure, p ₂ the measured outlet pressure, ρ the density of the liquid and g the standard acceleration.

According to another development, the density of the liquid can be used as a constant predetermined value, or the method can comprise the further step of measuring the temperature of the liquid and the density of the liquid can be obtained from a functional dependence of the density on the temperature or from a table can be determined, the measurement of the temperature in particular comprising an averaging of the temperature over a predetermined time interval.

Another further development is that the measurement of the inlet pressure and the outlet pressure of the liquid can take place continuously. In this way, a constant correction of the speed in the event of pressure fluctuations is possible.

The flow rate can be defined as a volume flow or as a mass flow of the liquid through the pump.

The above-mentioned object is further achieved by a device according to claim 10.

The device according to the invention for regulating a pump, in particular a centrifugal pump, during the pumping of a liquid comprises: a first one Pressure measuring device for measuring an inlet pressure of the liquid upstream of the pump; a second pressure measuring device for measuring an outlet pressure of the liquid downstream of the pump; and control means for setting a target value of a flow rate of the pump; for determining a setpoint value of a speed of rotation of the pump from a map of the pump stored in a memory, the setpoint value of the flow rate and a difference between the outlet pressure and the inlet pressure being entered as input values into the map; and to set the speed of the pump to the setpoint of the speed. The advantages correspond to those mentioned in connection with the method according to the invention. Furthermore, the device according to the invention can be designed such that it can be used to carry out the method according to the invention or one of its developments.

According to a further development, the control device can also be suitable for determining an average over time of the difference between the outlet pressure and the inlet pressure; and for determining the setpoint value of the flow rate from the characteristic diagram of the pump, the mean time value of the difference between the outlet pressure and the inlet pressure and a current speed of the pump being entered as input values in the characteristic diagram.

Another development is that the control device can be designed to output an actuating signal to the pump, and setting the speed of the pump to the setpoint value of the rotational speed can include outputting a correction signal to the actuating signal supplied to the pump.

According to a further development, the characteristic diagram can define a relationship between the flow rate and a delivery head of the pump at different speeds, wherein the control device can also be designed to add a delivery head H from H = (p ₂ -p ₁ ) / (ρ · g) determine, where p ₁ denotes the measured inlet pressure, p ₂ the measured outlet pressure, ρ the density of the liquid and g the standard acceleration.

Another development is that the device can further comprise: a temperature measuring device for measuring a temperature of the liquid and for transmitting a temperature measurement signal to the control device; wherein the control device can also be designed to determine a density of the liquid from the temperature measurement signal and to determine the density of the liquid from a functional dependence of the density on the temperature or from a table stored in the memory.

The device according to the invention or one of the developments can be part of an ORC system (Organic Rankine Cycle) with a pump for pumping a working medium of the ORC system.

The developments of the device according to the invention and its advantages correspond to those mentioned in connection with the method according to the invention.

Further features and exemplary embodiments and advantages of the present invention are explained in more detail below with reference to the drawings. It is understood that the embodiments are not exhaustive of the scope of the present invention. It is further understood that some or all of the features described below can also be combined with one another in other ways.

drawings

Figure 1

schematically shows a map of a pump.

Figure 2

shows the change in flow with pressure change and constant speed in the map of Fig. 1 ,

Figure 3

shows the essential elements of an ORC system.

Figure 4

shows a cascade controller.

Figure 5

shows the mode of operation of an embodiment of the map control according to the invention.

Figure 6

shows a compensation of the flow with a fluctuation in the pressure difference in the map of the pump.

Figure 7

shows a further embodiment of the map control according to the invention.

Figure 8

shows an example of a differential pressure and a corresponding mass flow in an ORC system.

Figure 9

shows the mass flow Fig. 8 and a corresponding steam temperature in the ORC system.

embodiments

Fig. 5 illustrates the inventive method according to one embodiment. Knowing the map of a machine allows its limitation with regard to the parameters of a process (difference between the pump outlet-side fluid pressure and the pump inlet-side fluid pressure, flow rate, speed) and their interdependency in the control system (map control). A control algorithm monitors the current delivery head (or the differential pressure) and the speed and uses this to calculate the current flow rate. For this purpose, the map is stored numerically in the algorithm.

wobei g die Normfallbeschleunigung bezeichnet.To determine the delivery head for the control, knowledge of the current pressures on the low and high pressure side (p _n , p _h ) of the pump is necessary (correspondingly: on the pump inlet side and on the pump outlet side or upstream and downstream of the pump or measured inlet pressure p ₁ and measured Outlet pressure p ₂ ). The head H can be calculated from the difference Δp = (p _h - p _n ) of these pressures and the density ρ of the medium: $$\mathrm{H}=\mathrm{\Delta }\mathrm{p}/\left(\mathrm{\rho }\cdot \mathrm{G}\right)$$

where g denotes the acceleration of the normal case.

The current density can either be determined exactly by means of an additional measurement of the temperature of the medium, or can be assumed to be constant by approximation in the operating range used. The latter simplification is at many media in the liquid phase and limited operating range (pressure and / or temperature range) in a sufficiently good approximation for the control is permissible.

A setpoint of a flow rate of the pump is determined as the currently calculated flow rate; measuring an inlet pressure of the liquid upstream of the pump and an outlet pressure of the liquid downstream of the pump; determining a setpoint value of a speed of rotation of the pump from the map of the pump, the setpoint value of the flow rate and the difference between the outlet pressure and the inlet pressure being input into the map; and finally the speed of the pump is set to the setpoint of the speed. Thus, when the differential pressure changes, the speed changes to counteract a change in flow rate that would otherwise occur. At least the change in flow rate can be reduced.

The limitation of the map (e.g. minimum flow) is also taken into account in the algorithm. This ensures both a uniform process operation and compliance with the operating limits of the pump.

Fig. 6 shows the functioning of the compensation influence of the map control, namely the correction of the speed in the event of a change in differential pressure, in order to correct the flow rate in this way. The mode of operation of the method according to this embodiment of the map control according to the invention is shown in the map of the pump. If the pressure difference or the corresponding delivery head falls from that in point 1 to that in point 2 at constant speed n ₁ , the flow rate Q increases. By reducing the speed to n ₂ , the original flow rate can now be changed at the new pressure difference or delivery head be restored in point 3.

If one looks again at the example of an ORC process already mentioned above, the measured values p _FD and p _KOND (as high pressure or low pressure) flow into the control according to the invention (see Fig. 7 ). In order to suppress the measurement of cyclical fluctuations, the measurement signal is first averaged (moving average) in a suitable averaging interval. The mean value of the live steam pressure p _{FD_M} is used with the live steam _setpoint for control deviation as the input signal of a controller (for example a PID controller). The output signal and the difference between the mean values flow into the map KF ¹ as input values. The currently expected mass flow is calculated here. This value, as well as the difference between the averaged current measured values, flow into the inverted map KF ^-1 . This provides the currently required control signal from the pump. The difference between this value and the current control signal of the controller is the deviation to be compensated for. Adding this deviation to the control signal results in the compensation of the disturbance. The influence of this activation can be adapted to the process by means of the gain K.

In this example, the map KF ¹ also delivers the currently required minimum control signal s _min to the controller. This can prevent the controller from falling below this map limit.

The predictive operating principle of this regulation offers a significant advantage of this procedure. The flow fluctuation is compensated for as soon as pressure fluctuations occur (the cause of mass flow changes and the resulting malfunctions) before a downstream measuring system or the downstream process can detect the deviation or feel its effects. By measuring the pressures and not the flow, the map control also implicitly implements the function of a feedforward control.

Fig. 8 shows an example from a measurement on an ORC system the course of differential _pressure (p _FD -p _KOND ) (upper curve in Fig. 8 ) and mass flow (lower curve in Fig. 8 ) over a period of approx. 15 minutes. You can see how pressure fluctuations show their influence on the flow. When the differential pressure drops, a higher flow is immediately measurable, and vice versa.

The effect on an evaporation process can also be measured (see Fig. 9 ). This decreases with increasing mass flow (lower curve in Fig. 9 ) the temperature of the steam (upper curve in Fig. 9 ), because the in the heat exchanger transferred power must now evaporate and overheat a higher mass flow. The steam temperature thus drops. When the flow rate drops, the temperature rises again. This shows that a reduction in flow fluctuations can lead to a stabilization of process parameters.

This stabilization can be implemented by the map control. The consequences of stabilization on design and process can mean higher process quality and availability, but also higher security against violations of process limit values. For example, with lower expected oscillations of temperatures, the safety limits are reduced depending on the now lower peak values or the process can be operated at higher temperatures (closer to the safety limits) without reductions in availability.

Furthermore, this regulation only requires two relatively inexpensive pressure measuring points, which are already available in many processes, instead of the expensive measurement of the mass or volume flow. This results in a significant cost advantage of the map control compared to conventional approaches.

The illustrated embodiments are merely exemplary and the full scope of the present invention is defined by the claims.

Claims (13)

1. A method for controlling a pump, in particular a centrifugal pump, during pumping of a liquid, comprising the following steps:

   fixing a setpoint value of a flow rate of the pump;

   measuring an inlet pressure of the liquid upstream of the pump and an outlet pressure of the liquid downstream of the pump;

   determining a setpoint value of a rotational speed of the pump from a performance map of the pump, wherein the fixed setpoint value of the flow rate and a difference between the outlet pressure and the inlet pressure are incorporated into the performance map as input values; and

   setting the rotational speed of the pump to the setpoint value of the rotational speed; wherein the fixing of the setpoint value of the flow rate comprises the following steps:

   determining a time average value of the difference between the outlet pressure and the inlet pressure; and

   fixing the setpoint value of the flow rate from the performance map of the pump, the time average value of the difference between the outlet pressure and the inlet pressure as well as a current rotational speed of the pump being incorporated into the performance map as input values.
2. The method according to claim 1, wherein the time average value of the difference between the outlet pressure and the inlet pressure is determined from a first time average value of the inlet pressure and a second time average value of the outlet pressure.
3. The method according to claim 1 or 2, wherein the determination of the setpoint value of the rotational speed of the pump comprises the following additional steps:

   checking whether a combination of the rotational speed of the pump, the fixed setpoint value of the flow rate and the difference between the outlet pressure and the inlet pressure lies within a performance map limit;

   setting the rotational speed of the pump to the setpoint value of the rotational speed, if the combination lies within the performance map; and

   setting the rotational speed of the pump to a safety value, if the combination lies outside the performance map, the safety value being preferably chosen such that the deviation from the setpoint value of the flow rate is as small as possible.
4. The method according to one of the claims 1 to 3, wherein the setting of the rotational speed of the pump to the setpoint value of the rotational speed comprises the output of a correction signal onto a control signal supplied to the pump, and wherein, in connection with claim 4, in particular a minimum control signal is outputted as a correction signal.
5. The method according to one of the claims 1 to 4, wherein the performance map defines at various rotational speeds a relation between the flow rate and a pumping head of the pump, and the pumping head is determined from the differential pressure between the measured outlet pressure and the measured inlet pressure, wherein the pumping head H is determined in particular from H=(p₂-p₁)/(ρ·g), where p₁ stands for the measured inlet pressure, p₂ for the measured outlet pressure, ρ for the density of the liquid, and g is the standard acceleration due to gravity.
6. The method according to claim 5, wherein the density of the liquid is used as a constant predetermined value, or wherein the method comprises the additional step of measuring the temperature of the liquid, and the density of the liquid is ascertained from a functional dependence of the density on the temperature or from a table, wherein the measuring of the temperature may in particular comprise averaging of the temperature over a predetermined time interval.
7. The method according to one of the claims 1 to 6, wherein the inlet pressure and the outlet pressure of the liquid are measured continuously.
8. The method according to one of the claims 1 to 7, wherein the flow rate is defined as a volume flow or as a mass flow of the liquid through the pump.
9. A device for controlling a pump, in particular a centrifugal pump, during pumping of a liquid, the device comprising:

   a first pressure meter for measuring an inlet pressure of the liquid upstream of the pump;

   a second pressure meter for measuring an outlet pressure of the liquid downstream of the pump; and

   a control unit for fixing a setpoint value of a flow rate of the pump; for determining a setpoint value of a rotational speed of the pump from a pump performance map stored in a memory, wherein the fixed setpoint value of the flow rate and a difference between the outlet pressure and the inlet pressure are incorporated into the performance map as input values; and for setting the flow rate of the pump to the setpoint value of the flow rate;

   wherein the control unit is also suitable for determining a time average value of the difference between the outlet pressure and the inlet pressure; and for fixing the setpoint value of the flow rate from the performance map of the pump, wherein the time average value of the difference between the outlet pressure and the inlet pressure as well as a current rotational speed of the pump are incorporated into the performance map as input values.
10. The device according to claim 9, wherein the control unit is configured for outputting a control signal to the pump, and the setting of the rotational speed of the pump to the setpoint value of the rotational speed comprises the output of a correction signal onto the control signal supplied to the pump.
11. The device according to claim 9 or 10, wherein the performance map defines at various rotational speeds a relation between the flow rate and a pumping head of the pump, and wherein the control unit is additionally configured for determining a pumping head h from h=(p₂-p₁)/(ρ·g), where p₁ stands for the measured inlet pressure, p₂ for the measured outlet pressure, ρ for the density of the liquid, and g is the standard acceleration due to gravity.
12. The device according to claim 11, further comprising:

    a temperature measuring device for measuring a temperature of the liquid and for transmitting a temperature measurement signal to the control unit;

    wherein the control unit may additionally be configured for determining a density of the liquid from the temperature measurement signal and for ascertaining the density of the liquid from a functional dependence of the density on the temperature or from a table stored in the memory.
13. An ORC system comprising:

    a pump for pumping a working medium, and

    a device according to one of the claims 9 to 12 for controlling the pump.

Images