Patent Application
20250130092
The present invention relates to a guided wave radar level gauge and to a method for operating the guided wave radar level gauge. In particular, the present invention is aimed at guided wave radar level gauge for saturated steam applications.
Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined. More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
The guided wave radar is particularly advantageous in high pressure saturated steam applications, such as boiler drums, high pressure feedwater heaters and steam separators where the high pressures and temperatures together with vibrations and potential magnetite coating of metallic parts may cause problems for other measurement techniques. Guided wave radar is used for direct level measurement and is completely independent of density, and with no moving parts, it offers the advantages of lower maintenance and greater reliability.
An example implementation of a guided wave radar for pressurized steam applications uses a reference reflector at a known location to be able compensate for variations in operating conditions due to vapor in the tank. However, the system requires that the position of the reference reflector is calibrated when commissioning the system or when replacing components of the system, and such a calibration needs to be performed in an empty tank or at least in near ambient conditions. Thereby, a process in the tank may need to be interrupted when changing parts of the radar level gauge.
Accordingly, it is desirable to provide improved solutions for providing accurate level measurements in high pressure saturated steam applications.
In view of above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide a radar level gauge and a method for determining a fill level of a product with improved calibration.
According to a first aspect of the invention, there is provided a guided wave radar level gauge for determining a fill level of a product contained in a tank for high-pressure applications comprising: a transceiver configured to provide a transmit signal, and to receive a reflected signal resulting from a reflection of the transmit signal at a surface of the product; a probe connected to the transceiver and configured to propagate the transmit signal towards the surface and to return the reflected signal to the transceiver; a reference reflector arranged at a reference position on the probe; control circuitry configured to determine the fill level based on the received reflected signal, wherein the control circuitry is further configured to: perform a distance measurement to determine a measured distance to the reference reflector; determine a first dielectric constant of a vapor in the tank based on the measured distance to the reference reflector and a reference distance to the reference reflector; acquire a temperature or pressure of the vapor in the tank; determine a second dielectric constant of the vapor in the tank based on the acquired temperature or pressure; and compare a difference between the first and second dielectric constant of the vapor in the tank with a predetermined difference threshold value.
Through the described radar level gauge, it can be determined if the measured position of the reference reflector corresponds to the calibrated position of the reference reflector, i.e. if the calibration is correct, without having to empty the tank. In other words, the control can be performed during operation of the tank. Accordingly, the described radar level gauge can perform a proof test during operation which both reduces the risk of erroneous level measurements, and which simplifies replacement of parts in the radar level gauge since parts can be installed and calibration can be performed without opening, depressurizing or emptying the tank.
According to an example embodiment, the control circuitry is further configured to: determine that the difference between the first and second dielectric constant is above the predetermined threshold value; determine an updated position of the refence reflector based on the second dielectric constant; and set the reference position of the reference reflector to the updated position. The calibrated position of the reference reflector can thereby be automatically updated as required in response to changing conditions in the tank, ensuring that level measurement can be performed with the required accuracy.
According to an example embodiment, the control circuitry is further configured to determine that the difference between the first and second dielectric constant is above the predetermined threshold value and to provide a notification to a user that a calibrated position of the reference reflector is incorrect. In some situation, it may be desirable to also notify an operator of a change in conditions in the tank resulting in an erroneous calibrated position of the reference reflector since this may be an indication of a malfunction or the like which needs to be attended to. A notification can for example be provided in the form of a visual or audible alert.
According to an example embodiment, the control circuitry is further configured to determine that the difference between the first and second dielectric constant is below the predetermined threshold value and to provide a notification to a user that a calibrated position of the reference reflector is correct. The confirmation that the calibrated position of the reference reflector corresponds to the measured position acts as a proof check and verification that the calibration is correct, ensuring that the level measurement is reliable.
According to an example embodiment, the radar level gauge may further comprise a temperature sensor and/or a pressure sensor arranged in the tank to determine a temperature and/or a pressure in the tank. Accordingly, both temperature sensing and pressure sensing can then be integrated in the radar level gauge system and be controlled by the same measurement circuitry as the one controlling the level measurement, thereby facilitating integration of the proof check in a radar level gauge system. It would however also be possible to acquire the temperature and/or pressure from other sensors, or to manually input temperature and/or pressure values into the radar level gauge system for verification of the reference reflector position.
According to an example embodiment, the control circuitry is further configured to acquire a pressure in the tank, and to determine that the tank is pressurized when the pressure is above a first predetermined pressure threshold value.
Moreover, the control circuitry may be further configured to acquire a pressure in the tank and a temperature in the tank, and to determine that the tank comprises saturated steam when the pressure is above a second predetermined pressure threshold value and a temperature is above a predetermined temperature threshold value. Since the known relations between temperature, gas pressure and dielectric constant generally holds true for saturated steam, and not necessarily for non-saturated steam, it may be desirable that the radar level gauge system ensures that the tank is pressurized and that it contains saturated steam before determining a dielectric constant of the vapor in the tank based on temperature or pressure and before recalibrating a position of the reference reflector.
According to a second aspect of the invention, there is provided a method of determining a dielectric constant in a guided wave radar level gauge for determining a fill level of a product contained in a tank for high-pressure applications, the radar level gauge comprising a transceiver configured to provide a transmit signal, and to receive a reflected signal resulting from a reflection of the transmit signal at a surface of the product; a probe connected to the transceiver and configured to propagate the transmit signal towards the surface and to return the reflected signal to the transceiver; a reference reflector arranged at a reference position on the probe; control circuitry configured to determine the fill level based on the received reflected signal, the method comprising, by the control circuitry: performing a distance measurement determining a measured distance to the reference reflector; determining a first dielectric constant of vapor in the tank based on the measured distance to the reference reflector and a reference distance to the reference reflector; acquiring a temperature or pressure of the vapor in the tank; determining a second dielectric constant of the vapor in the tank based on the acquired temperature or pressure; and comparing a difference between the first and second dielectric constant of the vapor in the tank with a predetermined difference threshold value.
Effects and features of this second aspect of the present invention are largely analogous to those described above in connection with the first aspect of the invention.
Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.
These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing example embodiments of the invention, wherein:
In the present detailed description, various embodiments of the system and method according to the present invention are mainly described with reference to a guided wave radar level gauge installed in a tank located on land. However, the described system and method is suitable for use in other areas such as in marine applications. Moreover, various embodiments of the present invention are mainly discussed with reference to a radar level gauge system with a signal propagation device in the form of a probe, and wireless communication capabilities.
It should be noted that this by no means limits the scope of the present invention, which also covers a radar level gauge system with another type of signal guiding device, as well as a radar level gauge system configured for wired communication, for example using a 4-20 mA current loop and/or other wired means for communication.
The guided wave radar level gauge system 100 is installed to measure the filling level of a product 102 in the tank 104. Moreover, the described radar level gauge system 100 is configured to determine the fill level of product in a tank 104 in a high pressure steam application where the tank comprises a liquid and pressurized vapor 112. It is here assumed that the volume above a determined fill level comprises saturated vapor 112. In the illustrated example, the probe 108 is arranged in a still pipe 114 arranged separately from the tank 104. The probe 108 is extending from the measurement unit 106 towards and into the product 102 in the still pipe 114. The still pipe 114 is connected to the tank so that the fill level of the tank 104 is the same as the fill level in the still pipe 114, thereby enabling measurement of the fill level in the tank by measuring the level of the product in the still pipe 114.
By analyzing a transmitted signal being guided by the probe 108 towards the surface 116 of the product 102, and a reflection signal traveling back from the surface 116, a filling level of the product 102 in the tank 104 can be determined. It should be noted that, although a tank 104 containing a single product 102 is discussed herein, the distance to any material interface along the probe can be measured in a similar manner.
The radar level gauge system 100 in
As is schematically illustrated in
The MCU 200 determines the filling level of the product 102 in the tank 104 and provides a value indicative of the filling level to an external device, such as a control center, from the MCU 200 via the WCU 202 through the communication antenna 204. The radar level gauge system 100 may advantageously be configured according to the so-called WirelessHART communication protocol (IEC 620691).
Although the measurement unit 106 is shown to comprise an energy store (battery 206) and to comprise devices (such as the WCU 202 and the communication antenna 204) for allowing wireless communication, it should be understood that both power supply and communication may be provided in different ways, such as through communication lines (for example 4-20 mA lines).
The local energy store 206 need not (only) comprise a battery, but may alternatively, or in combination, comprise a capacitor or super-capacitor.
Moreover, the measurement control unit (MCU) 200 may more generally be referred to as control circuitry 200, and the control circuitry 200 may include a microprocessor, microcontroller, programmable digital signal processor or another programmable device. The control circuitry may also, or instead, include an application specific integrated circuit, a programmable gate array or programmable array logic, a programmable logic device, or a digital signal processor. Where the control circuitry includes a programmable device such as the microprocessor, microcontroller or programmable digital signal processor mentioned above, the processor may further include computer executable code that controls operation of the programmable device.
The first and largest echo 300 as seen from the direction of the measurement unit 106 is the same for both the calibrated and uncalibrated measurement, identifying a transition from the measurement unit 106 to the pipe 114.
The next echoes 302, 304 represent the echoes from the reference reflector 110. Here it should be noted that the reference reflector 100 may be configured in many different ways, for example depending on the type of probe used. The reference reflector 110 may for example vary in length, and for a single probe the reference reflector 110 may be configured as a change in diameter of the probe 108 where a transition from a larger diameter to a smaller diameter of the probe 108 gives rise to a reference echo. The reference reflector 108 may also comprise a separate reflector element attached to the probe 108.
The lower 302 of the two reference reflector echoes represents the echo 302 from the measured and uncalibrated signal and the upper reference reflector echo 304 represents the echo for the calibrated signal.
Similarly, in the next two echoes at the location of the surface 116 of the product, the lower 306 of the two surface echoes illustrates the echo 306 from the measured and uncalibrated signal and the upper echo 308 represents the actual fill level for the correctly calibrated signal.
The reason for the erroneous echo positions in the measurement is due to that the dielectric constant of the vapor 112 in the pipe is different from the dielectric constant of air and that the propagation velocity of the signal is dependent on the dielectric constant of the material surrounding the probe. In particular, the dielectric constant in the vapor 112 is higher than for air, which gives an erroneous result if an original assumption is made that the dielectric constant of the ambient surrounding the probe is equal to 1. The dielectric constant for pressurized steam increases with increasing pressure and temperature, which also leads to an increasing error in the uncalibrated signal.
By knowing the actual physical position of the reference reflector 110 and thereby the distance from the measurement unit 106 to the reference reflector, the measured signal can be calibrated, and the correct fill level can be determined. This also makes it possible to determine the dielectric constant of the vapor in the tank based on the distance between the uncalibrated echo 302 and the actual position of the reference reflector 110.
However, in some circumstances the physical position of the reference reflector 110 may be unknown, or it may be desirable to verify that the assumed position of the reference reflector 110 is correct. The position of the reference reflector 110 may for example be unknown, or at least unverified, if components of the radar level gauge have been replaced or reinstalled, or if information has been lost for other reasons. In such situations it is advantageous to be able to determine the position of the reference reflector 110 without having to empty and depressurize the tank to perform measurements in ambient air where the dielectric constant is equal to one.
A determination of the conditions in the tank and thereby of the position of the reference reflector can be performed by the system and method of the present disclosure.
A requirement for performing the described measurements is that the reference reflector is located above the fill level of the tank. It can be verified that the location of the reference reflector is located above the fill level by comparing the sign of the echo signal where the reference reflector is configured to provide a negative echo and the surface provides a positive echo. By observing the respective position of the first positive echo and the first negative echo as seen from the start of the probe, it can be established if the reflector is above or below the fill level.
An example method for of determining a dielectric constant in a guided wave radar level gauge is performed by the control circuitry 200 of the radar level gauge system 100 and will be described with further reference to
The method comprises, by the control circuitry 200, performing 400 a distance measurement determining a measured distance to the reference reflector 110. The distance to the reference reflector 110 can be determined during a level measurement where different echoes in the reflected signal can be distinguished and distances from the measurement unit 106 to the reference reflector 110 and to the surface 116 can be determined. Referring to
Next, a first dielectric constant, ϵ1, of vapor in the tank can be determined 402 a based on the measured distance to the reference reflector 110 and a known reference distance to the reference reflector 110. A difference between the measured distance and the reference distance is indicative of a dielectric constant of the vapor surrounding the probe different from 1, and since the relation between dielectric constant and propagation velocity along is known, the first dielectric constant, ϵ1, can be determined.
The next step comprises acquiring 404 a temperature, T, or a pressure, P, of the vapor in the tank. In the example embodiment illustrated in
A current temperature or pressure in the tank may also be input manually by an operator to the radar level gauge system 100 based on a reading from a separate temperature or pressure sensor.
Once a temperature and/or pressure has been determined, a second dielectric constant, ϵ2, of the vapor in the tank can be determined 406 based on the acquired temperature or pressure, for example by using values given in Table 1 below describing the relationship between temperature, pressure and dielectric constant for saturated water vapor. Even though the present description describes relationships between temperature, pressure and dielectric constant for saturated water vapor, the same general principle can be used for saturated vapors of other materials. Furthermore, in addition to the use of tabulated values, it is also possible to use analytical expressions describing the relation between temperature, pressure and dielectric constant. Such analytical expressions can for example be derived by curve-fitting to measured values, through simulations and modelling or by theoretical calculations.
When the first and second dielectric constants ϵ1 and ϵ2 have been determined, a difference between the first and second dielectric constant of the vapor in the tank is compared 408 with a predetermined difference threshold value. The difference threshold value may for example be a percentage of the measurement value or it may be an absolute value which can be set for a given implementation of the radar level gauge system. The predetermined threshold value may for example be 4% of the dielectric constant calculated from the distance measurement.
Once the difference between ϵ1 and ϵ2 have been determined, further action may be taken depending on the outcome of the comparison and depending on the requirements for a specific application.
In some applications, it is assumed that the tank comprises saturated steam when the measurement is performed. A verification that the tank comprises saturated steam can also be given by an operator.
However, it is also possible to verify that the tank comprises saturated steam by comparing the temperature and/or pressure in the tank with corresponding temperature and/or pressure threshold values.
The method may further comprise acquiring a pressure 700 in the tank and a temperature 706 in the tank and determining 710 that the tank comprises saturated steam when it is determined 704 that the pressure is above a second predetermined pressure threshold value, and it is subsequently determined 708 that a temperature is above a predetermined temperature threshold value. The threshold values for the temperature and pressure can be determined based on known properties of saturated steam in a pressurized tank.
The described method thereby provides a proof testing procedure where a correct calibration of a reference reflector position can be verified. In particular, the proof test checks that a dielectric constant acquired from a lookup table and based on a measured temperature and/or pressure correspond to the dielectric constant from the measured distance to the reference reflector. The proof test can be configured to be performed automatically or manually, and it provides a verification that the level measurement is correct and that level measurements are correctly calibrated also under changing conditions in the tank.
Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Also, it should be noted that parts of the system and method may be omitted, interchanged or arranged in various ways, the system and method yet being able to perform the functionality of the present invention.
Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
| Number | Date | Country | Kind |
|---|---|---|---|
| 23205331.4 | Oct 2023 | EP | regional |
