TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method and a system for identifying and controlling a centrifugal pump.
DESCRIPTION OF THE PRIOR ART
A pumping circuit in which a centrifugal pump is included is driven by a control system.
This control system comprises an electric motor actuating the centrifugal pump and controlled by a variable speed drive. The system also comprises a valve or a cock making it possible to control the flow rate in the pumping circuit. In normal operation, the control of the centrifugal pump is carried out on the basis of pump curves. These pump curves are defined for a single speed called the nominal speed and represent the pressure as a function of the flow rate, the mechanical power as a function of the flow rate. For a speed other than the nominal speed, there are laws of extrapolation called Rateau laws for creating the link between the magnitudes of the pumping circuit (pressure, flow rate) and the mechanical magnitudes (power, speed):
Flow rate=Flow rate_NominalSpeed×(Speed/NominalSpeed)
Pressure=Pressure_NominalSpeed×(Speed/NominalSpeed)2
Power=Power_NominalSpeed×(Speed/NominalSpeed)3
The pump curves and the extrapolation laws can already be stored in the system or reconstituted during an identification phase. In this identification phase, a pressure sensor and a flow-rate sensor are used to measure the flow rate and the pressure of the fluid flowing in the pumping circuit at each speed of the electric motor corresponding to a mechanical power supplied to the pump. Hitherto, the identification phase, carried out at a nominal speed, allows a correlation between the measurement data of the system, whether they be the data associated with the pumping circuit (pressure, flow rate) or the mechanical data (speed, power). The known drawbacks in the use of these data are of several orders:
- The collection of the various magnitudes is carried out with no guarantee of precise synchronization between these four measurements. It is known that the action of a valve or of a cock generates variations in the pumping system. Depending on the configuration of the pumping system, these variations can be oscillating and it is then difficult to collect a quadruplet of magnitudes that are coherent with one another.
- The pump curves are established at nominal speed in compliance with an ISO standard 9906 Annex A which requires 8% accuracy, that can be found insufficient for the required control activities.
- In practice, the extrapolation carried out according to theoretical laws does not always make it possible to obtain the desired accuracy.
- The use of a variable speed drive to control the electric motor and thus exclude the external mechanical measurements introduces a correlation bias between the data available in the control and estimation system, i.e. the pump curves, and the measurement or estimation data available (estimated speed, estimated mechanical power). Specifically it is known that, without a sensor of mechanical magnitudes (position, speed, torque or power) and according to the type of control algorithm used, a variable speed drive may be found to be inaccurate in the estimation of speed and power.
The object of the invention is to propose a control and identification method and system making it possible, during a dedicated phase, to be sure of the synchronized correlation between the mechanical data applied to the electric motor that are available in the variable speed drive and the data of the pumping circuit collected by the external sensors. This correlation will make it possible to estimate with very great accuracy the magnitudes of the pumping circuit (pressure, flow rate) without using sensors in the pumping circuit, independently of the manufacturer data, that is to say of the nominal pump curves.
The finality of the estimation of pressure and flow rate magnitudes can be either to diagnose the measurement elements, or to control or monitor the pumping circuit without pressure and flow-rate sensors.
SUMMARY OF THE INVENTION
This object is achieved by a method for controlling a centrifugal pump included in a pumping circuit controlled by a control system, said control system comprising:
- a variable speed drive and an electric motor controlled by the variable speed drive so as to actuate the centrifugal pump,
- a measurement or an estimate by the variable speed drive of the mechanical power supplied to the electric motor,
- a solenoid valve designed to control the pumping circuit,
- a pressure sensor designed to measure the pressure of the fluid flowing in the pumping circuit,
- a flow-rate sensor designed to measure the flow rate of the fluid flowing in the pumping circuit,
- a control and processing unit,
said method comprising a phase of identification of the centrifugal pump, carried out by the control and processing unit, prior to the normal operation of said centrifugal pump, said identification phase comprising:
- a first step of selecting a speed and of controlling the variable speed drive in order to apply said speed to the electric motor,
- a second step of controlling the solenoid valve over the whole of its operating range at said selected speed,
- a third step of measuring the pressure and the flow rate of the fluid flowing in the pumping circuit and of measuring the electric power supplied to the motor for each operating point of the solenoid valve,
- a fourth step of determining, for said selected speed, the pressure curve as a function of the flow rate and the curve of the power supplied as a function of the flow rate.
According to one particular feature, the second step, the third step and the fourth step are carried out for several speeds applied to the electric motor.
According to another particular feature of the invention, for a selected speed, the third step and the fourth step are carried out for each operating point of the solenoid valve.
The invention also relates to a system for controlling a centrifugal pump included in a pumping circuit, said control system comprising:
- a variable speed drive and an electric motor controlled by the variable speed drive so as to actuate the centrifugal pump,
- a measurement or an estimate by the variable speed drive of the mechanical power supplied to the electric motor,
- a solenoid valve designed to control the pumping circuit,
- a pressure sensor designed to measure the pressure of the fluid flowing in the pumping circuit,
- a flow-rate sensor designed to measure the flow rate of the fluid flowing in the pumping circuit,
- a control and processing unit comprising a module for identifying the centrifugal pump, activated prior to the normal operation of said centrifugal pump,
said identification module comprising:
- means for selecting a speed and means for controlling the variable speed drive in order to apply said speed to the electric motor,
- means for determining the power supplied to the electric motor for said selected speed,
- means for controlling the solenoid valve over the whole of its operating range at said selected speed,
- means for retrieving the measurements of the pressure and of the flow rate of the fluid flowing in the pumping circuit for each operating point of the solenoid valve,
- means for determining, for said selected speed, the pressure curve as a function of the flow rate and the curve of the power supplied as a function of the flow rate.
According to one particular feature, the control and processing unit is included in the variable speed drive.
BRIEF DESCRIPTION OF THE FIGURES
Other features and advantages will become apparent in the following detailed description made with respect to the appended drawings in which:
FIG. 1 represents the pumping circuit and the control system of the invention,
FIG. 2 represents the sequence applied during the phase of identifying the centrifugal pump,
FIGS. 3A and 3B represent respectively the pressure/flow rate pump curves and the motor power/flow rate pump curves characterized by the sequence of FIG. 2. Marked in bold are the pump curves at the nominal speed.
DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT
A pumping circuit comprises a centrifugal pump 1 controlled by a control system. Such a control system comprises an electric motor 2 designed to actuate the centrifugal pump and a variable speed drive 3 designed to control said electric motor.
Since the architecture of a variable speed drive is well known, it will not be described in the present application.
The characterization of the centrifugal pump 1 consists in carrying out an identification phase prior to the normal operation of the pump in the pumping circuit. This identification phase is carried out at least once, for example before the first start-up of the pumping circuit.
The identification phase of the centrifugal pump 1 consists in reconstituting the pump curves for the nominal speed but also at other operating speeds in order to cover the complete operating range. As shown in FIGS. 3A and 3B, pump curves define, for each speed of the electric motor, the pressure (Pf) of the fluid flowing in the pumping circuit as a function of the flow rate (Df) of the fluid flowing in the pumping circuit (FIG. 3A) and the motor power (Pmot) as a function of the flow rate (Df) of the fluid flowing in the pumping circuit (FIG. 3B).
In order to reconstitute the pump curves, the control system must also comprise a pressure sensor 4 designed to measure the pressure (Pf) of the fluid that flows in the pumping circuit and a flow-rate sensor 5 designed to measure the flow rate (Df) of the fluid flowing in the pumping circuit. Once the identification phase is complete, these sensors are no longer of any use and can be removed. Alternatively, the pumping actuator consisting of a variable speed drive and a centrifugal pump identified according to the proposed method can be inserted into another pumping circuit that has no sensors of the pressure and flow-rate magnitudes and thus provide a control with no sensor of the pumping circuit.
Moreover, in order to ensure the correlation between the data imposed at the input and the data collected at the output during the identification phase, the control system comprises a control and processing unit 30 making it possible to centralize the control and processing. This control and processing unit 30 may for example be a programmable controller or be that which is already present in the variable speed drive 3. In the rest of the description, it will be considered that the control and processing unit 30 is included in the variable speed drive. The control and processing unit 30 comprises a module for identifying the characteristics of the centrifugal pump 1, activated by the operator or activated automatically, for example when the system is first switched on.
The control system of the invention is shown in FIG. 1. In this FIG. 1, the variable speed drive 3 is connected to the electric motor 2 and the electric motor 2 controls the centrifugal pump 1. A solenoid valve 6, placed on the pumping circuit, is controlled directly by the control and processing unit 30. The flow-rate sensor 5 and the pressure sensor 4 are connected directly to the control and processing unit 30 in order to send their measurement data to the latter.
As shown in FIG. 2, when the identification module is activated, the phase of identifying the centrifugal pump 1 begins. In a first step (E1), the control and processing unit 30 activates means for controlling the variable speed drive 3 in order to apply a first speed to the electric motor 2. The speed is for example chosen from a range stretching from 30% to 120% of the nominal speed. In a second step (E2), for said selected speed, the control and processing unit 30 activates means for controlling the solenoid valve 6 for an operating point. Then, in a third step (E3), for each operating point of the solenoid valve 6, the control and processing unit 30 records the values supplied by the sensor 4 for sensing the pressure (Pf) of the fluid flowing in the pumping circuit and by the sensor 5 for sensing the flow rate (Df) of the fluid flowing in the pumping circuit and determines, by measurement or by estimation, the power (PMot) supplied by the variable speed drive 3. All these measurement data are sent to the control and processing unit 30 which records them in storage means (step E4). The control and processing unit 30 repeats the same operations for a new speed to be applied to the electric motor 2, until all of the speeds have been scanned. Based on the collected data, the control and processing unit 30 activates means for determining the pump curves.
Once all the data have been collected, the control and processing unit 30 establishes each pump curve in order to cover several possible cases of use:
- The pump curves, at different speeds, can be used during normal operation of the pumping circuit. An interpolation principle can be used to estimate the flow rate and the pressure for a speed comprised between two curves.
- The same principle may also be applied if only two curves are acquired for the nominal speed value and a minimum speed value.
- Another principle may be to store only the curve corresponding to the nominal speed and to calculate adjustment coefficients of the affinity laws based on other pump curves measured for lower speeds.
Irrespective of the principle of use of the pump curves, it is possible to employ said curves to estimate the pressure and the flow rate during normal operation of the pumping circuit when the pressure and flow-rate sensors are not present.
Patent Application
20130287596
