EP2837829A1 - Control of the characteristics of centrifugal pumps - Google Patents

Abstract

The present invention relates to a method for controlling a pump, in particular a centrifugal pump, during the pumping of a liquid, comprising the steps of: setting a target 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 of a speed of the pump from a map of the pump, wherein the setpoint set the flow rate and a difference between the output pressure and the input pressure as input values in the map received; and adjusting the speed of the pump to the speed reference. Furthermore, the invention relates to a corresponding device for controlling a pump.

Description

Field of the invention

The present invention relates to a method for controlling a pump, in particular a centrifugal pump, during the pumping of a liquid and a corresponding device.

State of the art

Centrifugal pumps have a strong dependence of the flow rate on the applied pressure difference and the speed. More specifically, a difference between the pump-side fluid pressure and the pump-side fluid pressure determines the flow rate (mass flow or volumetric flow). Each pump has a characterizing pump map that defines a relationship between the three parameters (difference between the pump output side fluid pressure and the pump input side fluid pressure, flow rate, speed). In this way, the third can be determined from the characteristic map with knowledge of two of the parameters. The map may be in the form of empirical, semi-empirical or theoretical model equations. In empirical model equations empirically recorded values can be associated with compensation functions. These empirical compensation functions can also be recorded as an illustration in a table. In the case of semi-empirical model equations, empirical values as well as physical equations are used, which are e.g. Describe correlations of physical parameters. In the case of theoretical model equations, the relationships of the parameters are fully described by physical equations.

The disadvantage is that fluctuations in the medium pressure on high and / or low pressure side cause a non-uniform flow (at a given speed), which in mass flow critical processes to impairments of Process flow can lead. Furthermore, the map reduces the operating range of the pump, which can lead to process disturbances and component damage when the limits are exceeded.

Fig. 1 shows an example of such a map. Herein, the delivery head H is plotted as a function of the rotational speed n 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 height, which can only be achieved at maximum speed and minimum flow. It turns out that at fixed speed, the flow changes greatly depending on the altitude. Since the delivery head is proportional to the applied pressure difference across the pump, fluctuations in the pressure on the high or low pressure side cause a change in the pump flow. The limitation of the volume flow downwards does not have to be constant as in the drawing, but may depend on the speed.

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

Furthermore, the map also shows that there are machine-specific restrictions for the pump operation (such as a minimum volume flow), the compliance of which is necessary to permanently ensure the machine function.

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

One area of application in which the safe delivery of a fluid stream is of particular importance is the pump control of a feed pump of an ORC (Organic Rankine Cycle) power plant process, as shown schematically in FIG Fig. 3 shown. Therein, a pump (P) is controlled in the form that desired live steam parameters at the output of a heat exchanger downstream of the pump (V) can be set safely. For this purpose, the speed of the pump is influenced by the control so that change the so changed flow, the evaporation condition such that the desired pressures and temperatures of the live steam and stabilized for a stable process operation are controlled.

In this example, the head of the pump depends on the live steam pressure (p _FD ) and on the pressure level in front of the pump (p _KOND ). This pressure depends on the actual condensation pressure of the condenser (K) upstream of the pump. This condenser cools and liquefies the working fluid in the ORC process by releasing heat to a cooling medium. This cooling medium (eg water of a heating network or ambient air) may 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. Thus, external disturbances can affect the delivery head of the pump and therefore cause fluctuations in mass flow and live steam pressure. These potential fluctuation amplitudes must be taken into account in safety considerations and availability analyzes. Furthermore, the ORC process is a closed system, and thus a retroactive effect of a fluctuating live steam pressure on the expansion machine (E) on the condensation pressure can not be excluded. Therefore, a self-reinforcing effect may occur which further adversely affects process stability.

One way to counteract these influences is the use of a cascade control according to Fig. 4 , An internal control circuit regulates the flow rate on the basis of a comparison of the actual and setpoint values of the mass or mass flow rate. Volume flow, while an external control loop sets the flow rate setpoint for control to the actual control variable of the pump (eg process pressure) the inner circle. This allows flow deviations to be compensated 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. Here are all the components that convert the signal of the mass flow control (m-control) in the delivery of a medium. This may include a control / speed control of the pump, the pump motor and the pump itself. The outer sub-process II may be, for example, an evaporation process and the process value s may be the media pressure p after evaporation. The evaporation process can thus contain all the necessary components, such as one or more heat exchangers, containers, fittings, etc.

Although this solution makes it possible to detect and respond to flow deviations on occurrence, the flow must have already departed from its setpoint S _setpoint . Thus, no predictive compensation before the occurrence of fluctuations is possible. Thus, an additional feedforward control is necessary (not shown). In addition, this solution requires a complex and often costly measurement of the mass or volume flow according to the prior art. Avoiding this measurement would have significant cost advantages.

Description of the invention

The object of the invention is 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 controlling a pump, in particular a centrifugal pump, during the pumping of a liquid, comprises the steps of: determining a nominal value of a flow rate of the pump; Measuring an inlet pressure of the liquid upstream of the pump and an outlet pressure the liquid downstream of the pump; Determining a desired value of a rotational speed of the pump or of a speed-determining actuating signal from a characteristic map of the pump, the specified nominal value of the flow rate and a difference between the output pressure and the input pressure being input into the characteristic map as input values; and adjusting the speed of the pump to the setpoint of the speed or supplying the speed-determining control signal to the pump.

It is advantageous that, due to the consideration of the characteristic field, no measurement of the mass or volume flow is required for regulation or compensation. Furthermore, the control can already react when a pressure fluctuation occurs before the effects of a flow fluctuation occur (predictive control behavior), which improves the control performance.

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

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

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

The desired value of the flow rate can in turn be determined by the control, for example based on a specified outlet 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 a Specification of the speed, from which then the constant flow rate can be determined, done.

Preferably, after setting the target value of the flow rate, continuously performing the steps of measuring the input pressure of the liquid and the output pressure of the liquid, determining the target value of the rotational speed of the pump and adjusting the rotational speed of the pump.

According to a development, the setting of the target value of the flow rate may comprise the following steps: determining a time average of the difference of the outlet pressure and the inlet pressure; and determining the setpoint of the flow rate from the map of the pump, wherein the time average of the difference of the output pressure and the input pressure and a current speed of the pump as input values are included in the map. In this way, during the operation of the pump, a setpoint value of the flow rate which is to be maintained as far as possible can be determined. In this case, the setting of the target value of the flow rate can also be carried out continuously.

Another development is that the time average of the difference of the output pressure and the input pressure from a first time average of the input pressure and a second time average of the output pressure can be determined. Thus, it is possible to use, if necessary, different time constants for the averaging of the input pressure and the output pressure.

In another embodiment, determining the pump speed setpoint may include the further steps of: checking whether a combination of pump speed, set flow rate set point, and the difference between the output pressure and the input pressure is within a map boundary; Adjusting the speed of the pump to the speed reference if the combination is within the map; and adjusting the speed of the pump to a safety value when the combination is outside the map, wherein the safety value is preferably selected so that the deviation from the desired value of the flow rate is as small as possible.

According to another development, adjusting the rotational speed of the pump to the target value of the rotational speed may include outputting a correction signal to a control signal supplied to the pump. In this way, a correction signal can be switched to the control signal. In particular, a minimum control signal can be output as a correction signal in order to avoid that an operating state is set outside the map.

Another development is that the map at different speeds defines a relationship between the flow rate and a delivery of the pump, and the head is determined from the pressure difference between the measured output pressure and the measured input 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 fluid and g the normal fall acceleration.

According to another embodiment, the density of the liquid may be used as a constant predetermined value or the method may comprise the further step of measuring the temperature of the liquid and the density of the liquid may be from a functional dependence of the density on the temperature or from a table In particular, measuring the temperature may include averaging the temperature over a predetermined time interval.

Another development is that the measurement of the inlet pressure and the outlet pressure of the liquid can be carried out continuously. In this way, a constant correction of the speed at pressure fluctuations is possible.

The flow rate may be defined as a volumetric flow or as a mass flow of the fluid through the pump.

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

The device according to the invention for controlling a pump, in particular a centrifugal pump, during the pumping of a liquid comprises: a first A pressure gauge for measuring an inlet pressure of the fluid upstream of the pump; a second pressure gauge for measuring an output pressure of the liquid downstream of the pump; and a controller for setting a target value of a flow rate of the pump; for determining a target value of a rotational speed of the pump from a map of the pump stored in a memory, wherein the setpoint value of the flow rate and a difference between the outlet pressure and the inlet pressure are input into the map as input values; and for adjusting the speed of the pump to the setpoint of the speed. The advantages correspond to those which have been mentioned in connection with the method according to the invention. Furthermore, the device according to the invention can be designed so that the method according to the invention or one of its developments can be carried out with it.

According to a development, the control device may furthermore be suitable for determining a time average of the difference of the outlet pressure and the inlet pressure; and for setting the target value of the flow rate from the map of the pump, wherein the time average of the difference of the output pressure and the input pressure and a current speed of the pump as input values are included in the map.

Another development consists in that the control device can be designed for outputting an actuating signal to the pump and setting the rotational speed of the pump to the desired 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 map can define a relationship between the flow rate and a delivery height of the pump at different rotational speeds, wherein the control device can also be designed to deliver a delivery height H from H = (p ₂ -p ₁ ) / (ρ · g) where p _{1 is} the measured input pressure, p _{2 is} the measured output pressure, ρ is the density of the fluid, and g is the standard acceleration.

Another development is that the device may further comprise: a temperature measuring device for measuring a temperature of the liquid and for transmitting a temperature measuring signal to the control device; wherein the control device may further be configured 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 further developments can be part of an Organic Rankine Cycle (ORC) system 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 which have been mentioned in connection with the method according to the invention.

Further features and exemplary embodiments and advantages of the present invention will be explained in more detail with reference to the drawings. It is understood that the embodiments do not exhaust the scope of the present invention. It is further understood that some or all of the features described below may be combined with each other in other ways.

drawings

FIG. 1

schematically shows a map of a pump.

FIG. 2

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

FIG. 3

shows the essential elements of an ORC system.

FIG. 4

shows a cascade controller.

FIG. 5

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

FIG. 6

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

FIG. 7

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

FIG. 8

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

FIG. 9

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

embodiments

Fig. 5 illustrates the method according to the invention according to one embodiment. The knowledge of the map of a machine allows its limitation with respect to the parameters of a process (difference between the pump output side fluid pressure and the pump input side fluid pressure, flow rate, speed) and their mutual dependence in the control to implement (map control). A control algorithm monitors the actual head (or differential pressure) as well as 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 head for the control of the current pressures on the low and high pressure side (p _n , p _h ) of the pump is necessary (corresponding: pump inlet side and pump outlet side or upstream and downstream of the pump or measured input pressure p ₁ and measured Outlet pressure p ₂ ). From the difference Δp = (p _h -p _n ) of these pressures and the density ρ of the medium, the head H can be calculated: $$\mathrm{H}=\mathrm{Ap}/\left(\mathrm{\rho }\cdot \mathrm{G}\right)$$

where g denotes the normal acceleration.

The actual density can either be determined exactly via an additional measurement of the temperature of the medium, or can be assumed to be constant by an approximation in the operating range used. The latter simplification is included Many media in liquid phase and limited operating range (pressure and / or temperature range) in a sufficiently good approximation for the regulation allowed.

A setpoint of a flow rate of the pump is set 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 of a speed of the pump from the map of the pump, wherein the setpoint of the flow rate and the difference between the output pressure and the input pressure are included as input values in the map; and finally, adjusting the speed of the pump to the setpoint speed. Thus, as the differential pressure changes, there is a change in rotational speed to counteract a change in flow rate that would otherwise occur. At least the change in the flow rate can be reduced.

Further, the limitation of the map (e.g., minimum flow) is considered in the algorithm. As a result, both a uniform process operation and compliance with the operating limits of the pump can be ensured.

Fig. 6 shows the operation of the compensation effect of the map control, namely the correction of the speed at a differential pressure change, so as to correct the flow rate. The 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 drops from that in point 1 to that at point 2, the flow Q increases at a constant speed n _1. By reducing the speed to n ₂ , the original flow at the new pressure difference or delivery head can now be reduced be restored in point 3.

Considering again the above-mentioned example of an ORC process, the measured values p _FD and p _KOND (as high-pressure or low-pressure) flow into the regulation according to the invention (see Fig. 7 ). To suppress the measurement of cyclical fluctuations, the measurement signal first passes through an averaging (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 the control deviation as the input signal of a controller (eg a PID controller). The output signal and the difference of the average values flow as input values into the map KF ¹ . Here the currently expected mass flow is calculated. This value, as well as the difference between the unmediated current measured values, flow into the inverted characteristic map KF ^-1 . This supplies the currently required actuating signal of the pump. The difference between this value and the current control signal of the controller is the sought-after deviation to be compensated. By adding this deviation to the control signal results in an activation of the compensation of the fault. The gain K can be used to adjust the influence of this connection on the process.

In this example, the map KF ¹ also provides to the controller the currently required minimum control signal S _min . This can be prevented by the controller below this map limit.

A significant advantage of this approach is provided by the predictive effect principle of this regulation. Even when pressure fluctuations occur (the cause of mass flow changes and resulting disturbances), the flow fluctuation is compensated before a downstream measuring system or the downstream process could detect the deviation or feel its effects. By measuring the pressures and not the flow, the map control also implicitly implements the function of feedforward control.

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

It is also the impact on an evaporation process measurable (see Fig. 9 ). This decreases with increasing mass flow (lower curve in Fig. 9 ) the temperature of the steam (upper curve in Fig. 9 ), as in the heat exchanger transmitted power must now evaporate a higher mass flow and overheat. The steam temperature thus drops. When the flow drops, the temperature increases again. This shows that a reduction of flow fluctuations can lead to a stabilization of process parameters.

The map control allows this stabilization to be implemented. The consequences of stabilization on design and process can mean higher process quality and availability, but also higher safety against breaches of process limits. Thus, e.g. With lower expected oscillations of temperatures, the safety limits are reduced as a function of the now lower peak values or the process with higher temperatures (closer to the safety limits) are operated without availability reductions.

Furthermore, this regulation requires only two relatively favorable 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 map control over conventional approaches.

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

Claims (15)

Method for controlling a pump, in particular a centrifugal pump, during the pumping of a liquid, comprising the steps: Establishing a setpoint 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 of a speed of the pump from a map of the pump, wherein the setpoint set the flow rate and a difference between the output pressure and the input pressure as input values in the map received; and Set the speed of the pump to the speed reference. The method of claim 1, wherein setting the target value of the flow rate comprises the steps of: Determining a time average of the difference of the output pressure and the input pressure; and Determining the setpoint of the flow rate from the map of the pump, wherein the time average of the difference of the output pressure and the input pressure and a current speed of the pump as input values are included in the map. The method of claim 2, wherein the time average of the difference of the output pressure and the input pressure from a first time average of the input pressure and a second time average of the output pressure is determined. Method according to one of claims 1 to 3, wherein determining the target value of the rotational speed of the pump comprises the following further steps: Check that a combination of pump speed, set flow rate set point, and the difference between output pressure and input pressure is within a map limit; Adjusting the speed of the pump to the speed reference if the combination is within the map; and Setting the speed of the pump to a safety value when the combination is outside the map, the safety value is preferably selected so that the deviation from the target value of the flow rate is as small as possible. Method according to one of claims 1 to 4, wherein the setting of the rotational speed of the pump to the desired value of the rotational speed comprises outputting a correction signal to a control signal supplied to the pump, and wherein in connection with claim 4, in particular a minimum control signal is output as a correction signal. Method according to one of claims 1 to 5, wherein the characteristic field at different speeds, a relationship between the flow rate and a delivery height of the pump defined, and the delivery height of the pressure difference between the measured output pressure and the measured input pressure is determined, the delivery height H in particular H = (p ₂ -p ₁ ) / (ρ * g) where p _{1 is} the measured input pressure, p _{2 is} the measured output pressure, ρ is the density of the fluid, and g is the standard acceleration. The method of claim 6, wherein the density of the liquid is used as a constant predetermined value, or wherein the method comprises the further step of measuring the temperature of the liquid and the density of the liquid from a functional dependence of the density on the temperature or from a table In particular, measuring the temperature may include averaging the temperature over a predetermined time interval. Method according to one of claims 1 to 7, wherein the measurement of the inlet pressure and the outlet pressure of the liquid is carried out continuously. Method according to one of claims 1 to 8, wherein the flow rate is defined as a volume flow or as a mass flow of the liquid through the pump. Device for controlling a pump, in particular a centrifugal pump, during the pumping of a liquid, the device comprising: a first pressure gauge for measuring an inlet pressure of the fluid upstream of the pump; a second pressure gauge for measuring an output pressure of the liquid downstream of the pump; and a controller for setting a target value of a flow rate of the pump; for determining a target value of a rotational speed of the pump from a map of the pump stored in a memory, wherein the setpoint value of the flow rate and a difference between the outlet pressure and the inlet pressure are input into the map as input values; and for adjusting the speed of the pump to the setpoint of the speed. An apparatus according to claim 10, wherein the control means is further adapted to determine a time average of the difference of the output pressure and the input pressure; and for setting the target value of the flow rate from the map of the pump, wherein the time average of the difference of the output pressure and the input pressure and a current speed of the pump as input values are included in the map. Apparatus according to claim 10 or 11, wherein the control means is adapted to output a control signal to the pump and adjusting the speed of the pump to the target value of the speed comprises outputting a correction signal to the control signal supplied to the pump. Device according to one of claims 10 to 12, wherein the map at different speeds, a relationship between the flow rate and a head of the pump defines, and wherein the control device is further adapted to a head h 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 fluid and g the normal acceleration of the fall. The device of claim 13, further comprising: a temperature measuring device for measuring a temperature of the liquid and for transmitting a temperature measuring signal to the control device; wherein the control device is further configured 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. ORC system comprising: a pump for pumping a working medium, and An apparatus according to any one of claims 10 to 14 for controlling the pump.

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