Performance map control of centrifugal pumps

Abstract

The present invention relates to 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. Furthermore, the invention relates to a corresponding device for controlling a pump.

Claims

1. A method for controlling a pump during pumping of working medium in an Organic Rankine Cycle (ORC) system, comprising the following steps: fixing a setpoint value of a flow rate of the pump, the pump pumping the working medium to a heat exchanger of the ORC system, and the heat exchanger evaporating the working medium; measuring a condensation pressure of the working medium upstream of the pump at a location between a condenser of the ORC system and the pump and a live steam pressure of the working medium downstream of the pump at a location between the heat exchanger and an expander of the ORC system; determining a setpoint value of a rotational speed of the pump from an inverted performance map of the pump, the inverted performance map of the pump being a relation between a differential pressure across the pump and a rotational speed of the pump for a particular flow rate of the pump, wherein the setpoint value of the flow rate and a difference between the live steam pressure and the condensation pressure are incorporated into the inverted performance map as input values; and setting the rotational speed of the pump to the setpoint value of the rotational speed, wherein an influence of fluctuations of the difference between the live steam pressure and the condensation pressure on the flow rate is compensated by determining the setpoint value of the rotational speed of the pump and setting the rotational speed of the pump to the setpoint value, thereby stabilizing process parameters including at least one selected from the group consisting of (i) the live steam pressure, and (ii) a live steam temperature.

2. The method according to claim 1, 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 live steam pressure and the condensation pressure; and fixing the setpoint value of the flow rate from a performance map of the pump, the time average value of the difference between the live steam pressure and the condensation pressure as well as a current rotational speed of the pump being incorporated into the performance map as input values, the performance map of the pump being a relation between the flow rate of the pump and the differential pressure across the pump for a particular rotational speed of the pump.

3. The method according to claim 2, wherein the time average value of the difference between the live steam pressure and the condensation pressure is determined from a first time average value of the condensation pressure and a second time average value of the live steam pressure.

4. The method according to claim 2, 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 live steam pressure and the measured condensation pressure.

5. The method according to claim 4, wherein a density of the working medium is used as a constant predetermined value.

6. The method according to claim 4, wherein the pumping head H is determined from H=(p.sub.2p.sub.1)/(.Math.g), where p.sub.1 stands for the measured condensation pressure, p.sub.2 for the measured live steam pressure, for a density of the working medium, and g is a standard acceleration due to gravity.

7. The method according to claim 4, wherein the method comprises the additional step of measuring a temperature of the working medium, and a density of the working medium is ascertained from a functional dependence of the density of the working medium on the temperature or from a table, wherein the measuring of the temperature comprises averaging of the temperature of the working medium over a predetermined time interval.

8. The method according to claim 2, wherein the fixed setpoint value of the flow rate and an unaveraged difference between the live steam pressure and the condensation pressure are incorporated into the inverted performance map as input values to determine the setpoint value of the rotational speed of the pump.

9. The method according to claim 8, 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, the control signal being based on the time average value of the difference between the live steam pressure and the condensation pressure and the correction signal being based on the unaveraged difference between the live steam pressure and the condensation pressure.

10. The method according to claim 1, wherein the step of determining 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 live steam pressure and the condensation 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.

11. The method according to claim 10, wherein the safety value is chosen such that the deviation from the setpoint value of the flow rate is as small as possible.

12. The method according to claim 10, wherein the setting of the rotational speed of the pump to the setpoint value of the rotational speed comprises an output of a correction signal onto a control signal supplied to the pump, and wherein a minimum control signal is outputted as a correction signal.

13. The method according to claim 1, wherein the condensation pressure and the live steam pressure of the working medium are measured continuously.

14. The method according to claim 1, wherein the flow rate is defined as a volume flow or as a mass flow of the working medium through the pump.

15. An Organic Rankine Cycle (ORC) system comprising: a pump for pumping a working medium to a heat exchanger of the ORC system, the heat exchanger evaporating the working medium, and a device for controlling the pump during pumping of the working medium the device comprising: a first pressure meter for measuring a condensation pressure of the working medium upstream of the pump at a location between a condenser of the ORC system and the pump; a second pressure meter for measuring a live steam pressure of the working medium downstream of the pump at a location between the heat exchanger and an expander of the ORC system; 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 an inverted pump performance map stored in a memory, the inverted performance map of the pump being a relation between a differential pressure across the pump and a rotational speed of the pump for a particular flow rate of the pump, wherein the fixed setpoint value of the flow rate and a difference between the a live steam pressure and the condensation pressure are incorporated into the inverted performance map as input values; and for setting the flow rate of the pump to the setpoint value of the flow rate, wherein an influence of fluctuations of the difference between the live steam pressure and the condensation pressure on the flow rate is compensated by determining the setpoint value of the rotational speed of the pump and setting the rotational speed of the pump to the setpoint value, thereby stabilizing process parameters including at least one selected from the group consisting of (i) the live steam pressure, and (ii) a live steam temperature.

16. The ORC system according to claim 15, wherein the control unit is also suitable for determining a time average value of the difference between the live steam pressure and the condensation pressure; and for fixing the setpoint value of the flow rate from a performance map of the pump, the performance map of the pump being a relation between the flow rate of the pump and the differential pressure across the pump for a particular rotational speed of the pump wherein the time average value of the difference between the live steam pressure and the condensation pressure as well as a current rotational speed of the pump are incorporated into the performance map as input values.

17. The ORC system according to claim 16, wherein the fixed setpoint value of the flow rate and an unaveraged difference between the live steam pressure and the condensation pressure are incorporated into the inverted performance map as input values to determine the setpoint value of the rotational speed of the pump.

18. The ORC system according to claim 17, 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; the control signal being based on the time average value of the difference between the live steam pressure and the condensation pressure and the correction signal being based on the unaveraged difference between the live steam pressure and the condensation pressure.

19. The ORC system according to claim 15, 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=(p.sub.2p.sub.1)/(.Math.g), where p.sub.1 stands for the measured condensation pressure, p.sub.2 for the measured live steam pressure, for the density of the working medium, and g is the standard acceleration due to gravity.

20. The ORC system according to claim 19, further comprising: a temperature measuring device for measuring a temperature of the working medium and for transmitting a temperature measurement signal to the control unit; wherein the control unit is additionally configured for determining a density of the working medium from the temperature measurement signal and for ascertaining the density of the working medium from a functional dependence of a density on the temperature or from a table stored in the memory.

Description

DRAWINGS

(1) FIG. 1 shows schematically a performance map of a pump.

(2) FIG. 2 shows the change of the flow rate in the case of a change of pressure and a constant rotational speed in the performance map of FIG. 1.

(3) FIG. 3 shows the essential elements of an ORC system.

(4) FIG. 4 shows a cascade controller.

(5) FIG. 5 shows the mode of operation of an embodiment of the performance map control according to the present invention.

(6) FIG. 6 shows a compensation of the flow rate in the case of fluctuations of the differential pressure in the performance map of the pump.

(7) FIG. 7 shows a further embodiment of the performance map control according to the present invention.

(8) FIG. 8 shows, exemplarily, a differential pressure and a corresponding mass flow in an ORC system.

(9) FIG. 9 shows the mass flow according to FIG. 8 and a corresponding steam temperature in the ORC system.

EMBODIMENTS

(10) FIG. 5 illustrates the method according to an embodiment disclosed in the present invention. The knowledge of the performance map of a machine allows to implement in the control (performance map control) the machine limitation with respect to the parameters of a process (difference between the liquid pressure on the pump outlet side and the liquid pressure on the pump inlet side, flow rate, rotational speed) and the parameter interdependence. A control algorithm monitors here the current pumping head (and the differential pressure, respectively) as well as the rotational speed and calculates therefrom the current flow rate. To this end, the performance map is stored numerically in the algorithm.

(11) For ascertaining the pumping head for the control, it is necessary to know the current pressures on the low and high pressure sides (p.sub.n, p.sub.h) of the pump (i.e.: inlet pressure p.sub.1 and outlet pressure p.sub.2 measured on the inlet side and on the outlet side of the pump and upstream and downstream of the pump, respectively). The pumping head H can be calculated from the difference p=(p.sub.hp.sub.n) between these pressures and the density of the medium:
H=p/(g)
where g stands for the standard acceleration due to gravity.

(12) The current density may either be determined precisely by an additional measurement of the temperature of the medium, or it may, through an approximation, be assumed to be constant in the operating range used. The latter simplification is admissible for many media in a liquid phase and in the case of a limited operating range (pressure and/or temperature range) in an approximation that is sufficiently good for the control.

(13) A setpoint value of a flow rate of the pump is set as the currently calculated flow rate; an inlet pressure of the liquid is measured upstream of the pump and an outlet pressure of the liquid is measured downstream of the pump; a setpoint value of a rotational speed of the pump is determined from the performance map of the pump, the fixed setpoint value of the flow rate and the difference between the outlet pressure and the inlet pressure being incorporated into the performance map as input values; and, finally, the rotational speed of the pump is set to the setpoint value of the flow rate. It follows that a change in the differential pressure will cause a change in the rotational speed so as to counter a change in the flow rate, which would otherwise occur. The change in the flow rate can at least be reduced.

(14) In addition, the limitation of the performance map (e.g. minimum flow) is taken into account in the algorithm. A uniform process operation as well as compliance with the operating limits of the pump can be guaranteed in this way.

(15) FIG. 6 shows the functionality of the compensation influence of the performance map control, viz. the correction of the rotational speed in response to a differential pressure change for correcting the flow rate in this way. The mode of operation of the method according to this embodiment of the performance map control according to the present invention is shown in the performance map of the pump. If, at a constant rotational speed n.sub.1, the differential pressure or the corresponding pumping head decreases from that at point 1 to that at point 2, there will be an increase in the flow rate Q. By reducing the rotational speed to n.sub.2, the original flow rate can be reestablished at point 3 in the case of the new differential pressure or pumping head.

(16) Referring again to the above-mentioned example of an ORC process, the measurement values p.sub.FD and p.sub.COND (as high pressure and low pressure) are incorporated into the control according to the present invention (cf. FIG. 7). For suppressing the measurement of cyclical fluctuations, the measurement signal is first subjected to averaging (moving average) in a suitable averaging interval. The average value of the live steam pressure p.sub.FD.sub._.sub.M is used with the live steam setpoint value concerning the control deviation as an input signal of a controller (e.g. a PID controller). The output signal and the difference between the average values are incorporated as input values into the performance map KF.sup.1, where the currently expected mass flow is calculated. This value as well as the difference of the unaveraged current measurement values are incorporated into the inverted performance map KF.sup.1. The latter provides the currently necessary pump control signal. The difference between this value and the current control signal of the controller is the searched-for deviation to be compensated. By adding this deviation onto the control signal, a superimposition of the compensation of the disturbance is obtained. Through the gain K, the influence of this superimposition can be adapted to the process.

(17) In this example, the performance map KF.sup.1 also supplies to the controller the currently necessary minimum control signal s.sub.min. The controller can thus be prevented from falling below this performance map limit.

(18) A significant advantage of this approach is offered by the anticipatory operating principle of this control. The flow rate fluctuation is already compensated upon occurrence of pressure fluctuations (which cause mass flow changes and the resultant disturbances), before a downstream measurement system or the subsequent process could be able to detect the deviation and register the effects thereof. By measuring the pressures instead of the flow, the performance map control implicitly realizes also the function of a disturbance feedforward control.

(19) FIG. 8 shows exemplarily, on the basis of a measurement at an ORC system, the profile of the differential pressure (p.sub.FD-P.sub.COND) (upper curve in FIG. 8) and of the mass flow (lower curve in FIG. 8) over a period of approx. 15 minutes. It can be seen how pressure fluctuations show their influence on the flow rate. When the differential pressure decreases, a higher flow rate is immediately measurable, and vice versa.

(20) In addition, also the effect on an evaporation process is measurable (cf. FIG. 9). Here, the temperature of the steam (upper curve in FIG. 9) decreases in response to an increase in the mass flow (lower curve in FIG. 9), since the power transmitted in the heat exchanger must now vaporize and superheat a higher mass flow. Therefore, the steam temperature decreases. When the flow rate decreases, the temperature will increase again. Hence, it can be seen that a reduction of the flow fluctuations can lead to a stabilization of process parameters.

(21) The performance map control allows this stabilization to be realized. The effects of the stabilization on the structural design and the process can be a higher process quality and availability as well as a higher reliability of observing process limit values. For example, if the temperature oscillations to be expected are not so high, the safety limits may be reduced in accordance with the now lower peak values and the process can be performed at higher temperatures (closer to the safety limits) without the availability being reduced in any way.

(22) In addition, this control only requires two comparatively economy-priced pressure measuring points, which are already available in many processes, instead of the expensive mass flow or volume flow measurement. A significant cost advantage of performance map control in comparison with conventional approaches is obtained in this way.

(23) The embodiments shown are only exemplary and the full scope of the present invention is defined by the claims.