APPARATUS"

CROSS-REFERENCE TO RELATED APPLICATIONS This patent application claims priority from Italian patent application no. 102020000017122 filed on July 15, 2020, the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD The present invention relates to a device and a method for measuring a liquid level in an apparatus, in particular a pressure apparatus and even more particularly in an apparatus of a urea plant.

The invention finds a preferred application, in particular, in apparatuses forming part of a high-pressure section of a urea production plant (urea plant), such as a urea synthesis reactor, a high-pressure stripper, a high- pressure carbamate separator, etc.

BACKGROUND OF THE INVENTION Measurement of the level of a fluid in high-pressure apparatuses for urea production is typically done with nucleonic (radioactive) or radar type instruments.

The operating principle of nucleonic type instruments is based on measuring the radiation emitted by a nuclear source which is not absorbed by the fluid whose level is to be measured in the apparatuses.

Typically, a nucleonic instrument has an emitting unit (source), installed inside a special (so-called "drywell") container housed inside the apparatus whose level must be measured and consisting of a wire or a series of point sources distributed longitudinally so as to emit radiation along the entire length of the measurement field, and a receiving unit (detector) installed outside the apparatus and generally consisting of one or more receivers. Radioactive instruments generally provide satisfactory performance in terms of reliability and resolution (i.e. the smallest appreciable variation of the quantity under examination throughout the whole measurement range).

However, given the position of the source and receivers, measurements made with such instruments are strongly influenced by the thicknesses of the walls of the apparatus (in the specific application of the high-pressure apparatuses of a urea plant, as is general the case for apparatuses operating at high pressure, relatively large wall thicknesses are required), and by the geometry of the apparatus.

Therefore, when the apparatus has thick walls and/or complex geometry, measurements with poor resolution are obtained. This is the case, for example, of a stripper of a urea plant in the high-pressure section (i.e. the apparatus of a high-pressure section of a urea plant located downstream of the urea synthesis reactor and in which the carbamate is decomposed), which typically has a complex shape (in particular, a cylindrical bottom portion protruding inferiorly from a hemispherical dome) right at the bottom of the apparatus, where the liquid to be measured collects.

Due to the geometric complexity and in particular the diameter variations at the bottom of the apparatus, as well as the high thickness of the steel walls of the apparatus, the measurement resolution is very poor precisely in the low-level zone, i.e. where the liquid level is normally maintained while the system is running and which needs to be measured.

Other technical issues and disadvantages of radioactive meters result from:

- the need for highly radioactive sources due to the high thickness of apparatuses and their complex geometry;

- the need to confine the radioactive sources in order to ensure an adequate level of safety for operators; - the need for the user to comply with applicable legal requirements for the introduction, handling and disposal of radioactive materials.

As an alternative to measuring with nucleonic

(radioactive) instruments, it is known to use level meters based on radar technology. This type of measurement is carried out by sending microwave pulses, generated by a transmitter, into the liquid whose level is to be determined. When a microwave pulse reaches the liquid phase which has a different dielectric constant than the gas phase, part of the pulse is reflected back to a receiver. The time difference between the transmitted and reflected pulse is converted into a distance, from which the liquid level is subsequently calculated.

The intensity of the reflection depends on the dielectric constant of the liquid whose level is to be measured: the higher the value of the dielectric constant, the more intense the reflection.

Typically, the solution developed for high-pressure urea apparatuses involves the use of a tube (a so-called "standpipe") connected at one end to a microwave transmitter and having the other end immersed in the liquid whose level is to be measured and which penetrates inside the tube. The function of this tube is to guide the microwaves emitted by the transmitter ("Guided Wave Radar") inside the process fluid.

Such a solution is shown for example in

WO2013/036108A1 .

However, configurations of the type now described (with a waveguide defined by a standpipe, i.e. a hollow tube) present additional issues, in particular: - difficulty in detecting the reflected pulse that determines the measurement of the level of the process fluid inside the confined volume of the standpipe, especially when the process conditions are supercritical, i.e. the fluids have properties intermediate between those of gas and liquid (as for example in the case of a urea reactor) and therefore a low differentiation between the dielectric constants of the phases;

- difficulty in detecting the free liquid surface within the confined volume of the standpipe, accentuated by turbulent conditions for example during the chemical reaction of urea synthesis;

- the occurrence, inside the standpipe, of liquid phase crystallization and corrosion phenomena typical of a high- pressure urea environment;

- delay and discrepancy in the measurement when the volume change is abrupt due to the reduced communication area between the inside and outside of the standpipe;

- complex design/construction of the sealing system because the microwave transmission antenna is in direct contact with a process environment characterized by high temperatures and pressures and high chemical aggressiveness.

To overcome these problems at least in part, systems are available on the market in which the radar signal is transmitted through a waveguide in the form of an elongated solid rod (and not an internally hollow tube), with the lower end immersed in liquid.

An example is described in WO2019/096623A1.

However, using a solid rod as a waveguide has some drawbacks.

Firstly, since level measurement is carried out under dynamic conditions, i.e. a certain amount of liquid is continuously flowing in and out of the apparatus, the measurement is affected by the fact that liquid, flowing from the top to the bottom of the apparatus (as for example in the case of the high-pressure stripper of the urea plant) falls on the waveguide, altering the signal in transit along the waveguide and therefore the accuracy of the measurement.

In addition, the use of solid waveguides immersed in the process liquid results in some scattering of the measurement signal and the reflected signal in the surrounding environment.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a device and a method for measuring a liquid level in a pressurized apparatus, in particular an apparatus of a urea plant, which are substantially free of the drawbacks evidenced herein of the known art; in particular, an object of the invention is to provide a device and a method of measurement which permit the use of a radar instrument (thereby avoiding the use of radioactive materials) while providing high resolution and accuracy of level measurement.

The present invention thus relates to a device and a method for measuring a liquid level in a pressurized apparatus, in particular a pressurized apparatus of a urea plant, as respectively defined in essential terms in the attached claims 1 and 16.

The invention also relates to a pressurized apparatus, in particular an apparatus of a urea plant, as defined in claim 14.

Further preferred characteristics of the invention are indicated in the dependent claims.

The measuring device of the invention makes it possible to resolve the technical problems of traditional solutions, allowing reliable and accurate measurements of the level of process fluids in high-pressure urea apparatuses.

The device of the invention employs a radar-type instrument and a probe operating as a waveguide to guide microwaves in the process liquid: the probe is in the form of a solid rod (and not a hollow rod as in "standpipe" systems) and is associated with a special confinement cover that covers and protects the probe from the process liquid.

In this way, the measurement is always accurate and reliable, even in dynamic conditions (i.e. even if a certain quantity of liquid continuously enters and exits the apparatus, directly hitting the measuring probe): in fact, the cover, made of material suitable for the process conditions, protects the probe (the waveguide that transmits the microwaves) from the liquid flowing from the top to the bottom, preventing contact with the falling liquid from altering the measurement.

The cover, also made of conductive material (like the probe) and electrically connected to the probe, further serves to minimize dispersion of the measurement signal and the reflected signal in the surrounding environment, ensuring a significant improvement in the measurement.

At the same time, the device of the invention is not affected by any rapid changes in the level of the liquid to be measured or turbulence due to vapour generation, since the measurement is carried out in an open environment and not in a confined space as in the standpipe application.

The device and the measurement method in accordance with the invention thus make it possible to perform a level measurement which: - is direct and does not require compensation for varying process conditions (i.e. density, conductivity, temperature and pressure);

- is independent of the geometry of the apparatus; occurs in the absence of radioactive materials and therefore without all the problems associated with the use of radioactive materials;

- has a simple and reliable sealing system; is reliable even under turbulent conditions, in supercritical phases and with an aggressive process fluid that tends to crystallize.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and benefits of the present invention will be apparent from the following description of a non-limiting example of an embodiment of it, with reference to the Figures of the attached drawings, wherein:

- Figure 1 is a schematic longitudinal sectional view of a lower end part of a pressurized apparatus, in particular a high-pressure stripper of a urea plant, provided with a measuring device for measuring a liquid level according to the invention;

- Figure 2 shows a variant of the measuring device shown in Figure 1;

- Figure 3 shows a further variant of the measuring device shown in Figure 1; - Figure 4 shows a cross-sectional detail, according to the trace plane IV-IV in Figures 1, 2 and 3, of the device of the invention;

- Figure 5 shows a further embodiment of the measuring device of the invention;

- Figure 6 shows a cross-sectional detail according to the trace plane VI-VI in Figure 5;

- Figure 7 shows in enlarged scale a detail of the measurement device of Figure 5.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 shows a pressurized apparatus 1, in particular a high-pressure stripper of a urea plant, of which only a lower part 2 is illustrated.

The apparatus 1 is provided with a measuring device 3 for measuring the level of a process liquid that collects in the lower part 2 of the apparatus 1, where the liquid attains a free surface H (which is assumed, here and hereafter, to be substantially horizontal in the absence of turbulence).

Although in the example described and illustrated herein the apparatus 1 is a stripper, the measuring device 3 of the invention also finds application in other apparatuses, for example (but not necessarily) in other apparatuses of the high-pressure section of a urea plant, such as in particular a urea synthesis reactor, a carbamate separator, or a carbamate condenser. It is also understood that the measuring device 3 of the invention may be used not only in apparatuses of the high-pressure section, but also in apparatuses in other sections of a urea plant.

In general, the measuring device 3 of the invention may be used in various apparatuses, even outside the urea production industry. The apparatus 1 extends along a longitudinal axis A (generally vertical in use) and comprises a casing 4 having a side wall 5, arranged around the axis A, and delimiting an internal process chamber 6. The casing 4 comprises, beginning at a lower end 7 of the apparatus 1: a substantially cylindrical end portion 8, enclosed by a bottom wall 9, located at the end 7; an intermediate domed (for example, substantially hemispherical) portion 10, located above the end portion 8; and a substantially cylindrical main portion 11, positioned above the intermediate portion 10 and having a diameter ID1 greater than the diameter ID2 of the end portion 8.

During normal operation of the apparatus 1, the casing 4 contains, in particular in the lower end part 2, a certain quantity of a process liquid, which in the present case (high-pressure stripper of a urea plant) flows from the top to the bottom of the apparatus 1 and collects in the process chamber 6 reaching a level defined by the free surface H; the level of the process liquid in the apparatus 1 is variable during the operation of the apparatus 1, hence the need to measure this level and monitor its changes over time.

The end portion 8 is provided with an outlet conduit 12, arranged for example through the side wall 5 of the casing 4 and elbow-bent towards the bottom wall 9.

The measuring device 3 comprises: a radar instrument 18 having a microwave emitter and a microwave receiver, in particular capable of emitting/receiving microwaves with a frequency between 100 MHz and 1.5 GHz; a waveguide probe 19 for transmitting microwaves; and a confinement cover 20 associated with the probe 19 and serving to protect the probe

19 from falling process liquid in the apparatus 1 and to minimize dispersion of the signal from the probe 19.

The radar instrument 18 is arranged outside the casing 4 and faces and is connected to a nozzle 21 that penetrates the casing 4 through the side wall 5 and is attached to the casing 4, for example, by means of a flange coupling (known and not illustrated). The radar instrument 18 is connected and secured to the nozzle 21 by means of a connector 22.

The radar instrument 18 and its related nozzle 21 may be located at different levels on the apparatus 1.

In general, the radar instrument 18 is positioned on the apparatus 1 at a level, measured parallel to the axis A, above the maximum level of the process liquid to be measured in the apparatus 1. In the example shown in Figure 1, the radar instrument

18 and the nozzle 21 are arranged on the main (cylindrical) portion 11. Alternatively, also depending on the effective size of the apparatus 1, the radar instrument 18 and the nozzle 21 are arranged on the intermediate (domed) portion 10. The nozzle 21 is arranged substantially transversely to the axis A through the side wall 5. In the embodiment of Figure 1, but not necessarily, the nozzle 21 is substantially perpendicular to the axis A and substantially horizontal. Beneficially, the radar instrument 18 integrates both an emitter and a receiver, located in the radar instrument 18 itself.

The probe 19 enters laterally into the apparatus 1 and in particular inside the process chamber 6 through the nozzle 21 and extends between two opposite ends 24, 25 and in detail: a proximal end 24, connected to the radar instrument 18, and a distal end 25, positioned inside the process chamber 6 near the bottom wall 9 so as to be immersed in use in the process liquid. The proximal end 24 of the probe 19 is coupled to the radar instrument 18 by means of the connector 22, which also acts as a sealing element around the probe 19.

The probe 19 has a solid rod body 26, made, entirely or at least for all its part intended, in use, to be submerged in the process liquid, of a conductive material, in particular a metallic material, capable of transmitting along the rod body 26 the microwaves generated by the radar instrument 18 and reflected towards the radar instrument 18.

The rod body 26 of the probe 19 is made of a material suitable for use in the process conditions in which the measuring device 3 is used, i.e. depending on the apparatus 1 in which the measuring device 3 is mounted. For example, in the exemplative case described herein, the rod body 26 is made of a material suitable to withstand the conditions typical of a urea environment, for example (316L urea-grade type) stainless steel, titanium, zirconium, or duplex or super duplex steel, preferably having a 25Cr/22Ni/2Mo composition .

The rod body 26 extends along a longitudinal profile that may have various shapes and different lengths, also depending on the size and type of the apparatus 1.

In general, the probe 19 (i.e. its rod body 26) comprises an upper connection portion 27, which extends from the proximal end 24 (connected to the radar instrument 18) and penetrates the apparatus 1 through the nozzle 21; and a lower measurement portion 28, which in use is at least partially immersed in the process liquid and terminates at the distal end 25.

The connection portion 27 and the measurement portion 28 may be variously oriented or arranged relative to one another.

For example, in the embodiment of Figure 1, the rod body 26 of the probe 19 follows an L-shaped longitudinal profile: the connection portion 27 and the measurement portion 28 are straight and substantially orthogonal to each other and joined by a curved joint portion 29; the connection portion 27 extends radially into the apparatus 1 substantially orthogonally to the axis A, and the measurement portion 28 extends parallel to the axis A. In this configuration, the measurement portion 28

(defining the part of the probe 19 immersed in the process liquid) is immersed into the process liquid in an axial direction, that is, it extends parallel to the (vertical) axis A of the apparatus 1 and perpendicular to the free surface H of the process liquid (understood, as already indicated above, to be in the absence of turbulence).

The rod body 26 may have a different cross-sectional shape, for example circular (as shown in Figure 4) or in the shape of a regular polygon (square, hexagon, etc.). Preferably, the rod body 26 has a constant cross- section along the entire length of the rod body 26.

For example, the rod body 26 has a circular cross section and a diameter of between 5 mm and 30 mm (preferably approximately 20 mm). The probe 19 is mechanically supported by the connector

22 and/or support elements (known and not shown), for example positioned in the nozzle 21 and/or on the wall 5.

With reference to Figures 1 and 4, the cover 20 is arranged around the probe 19 and in particular around the rod body 26 of the probe 19 and extends along the probe 19 following its longitudinal profile, whose shape it replicates.

The cover 20 surrounds the probe 19 externally and is radially spaced from a lateral surface 31 of the rod body 26.

The cover 20 has a longitudinally open cross-section so that it only partially surrounds the probe 19 angularly. In other words, the cover 20 is open around the probe 19 and is not closed in a ring around the probe 19, leaving a longitudinal opening 32 facing the lateral surface 31 of the rod body 26.

In this way, the formation of liquid stagnation zones is avoided, and it is possible for the gas phase present in the apparatus 1 to pass through. The opening 32 is delimited laterally by a pair of opposing longitudinal edges 33 of the cover 20, facing and parallel to each other (Figure 4).

The cover 20 has a concave inner lateral surface 34 facing the lateral surface 31 of the rod body 26 of the probe 19, and a convex outer lateral surface 35 opposite the concave inner lateral surface 34.

Preferably, the cover 20 extends around the probe 19 for an angular extent of at least 120° and less than 360°, between approximately 150° and 250°.

For example, the cover 20 has a cross-section in the shape of an arc of a circle or an arc of an ellipse, parabola, or other conic.

In the embodiment shown in Figure 1, the cover 20 extends from the proximal end 24 of the probe 19, where the cover 20 is connected to the nozzle 21, to the distal end 25, where the probe 19 optionally protrudes out of the cover 20 with a short end section.

It is understood that the cover 20 may extend over a different length of the probe 19. The cover 20 is positioned so that the inner lateral surface 34 faces the lateral surface 31 of the probe 19.

The cover 20 is positioned above the probe 19 so as to fully cover the vertical projection of the probe 19, meaning the projection of the probe 19 on a horizontal plane parallel to the free surface H of the process liquid and perpendicular to the axis A.

In particular, the cover 20 is located above the connection portion 27 and the joint portion 29, so as to protect the probe 19 from the process liquid flowing in the apparatus 1 from top to bottom; and is arranged alongside the measurement portion 28.

The cover 20 is also made of a material suitable for use in the process conditions of the apparatus 1.

In particular, the cover 20 is made of a conductive material, in particular a metal material, preferably a material of the same type, or the same material, as the probe 19.

The cover 20 is mechanically and electrically connected to the probe 19 by means of tie rods 37 extending, for example, radially between the inner lateral surface 34 of the cover 20 and the lateral surface 31 of the probe.

The tie rods 37 are arranged, for example, at the ends 24, 25 and/or spaced along the cover 20.

The tie rods 37, in addition to mechanically supporting the cover 20 by holding it in position with respect to the probe 19, also make the electrical connection between the cover 20 and the probe 19.

Appropriately, the tie rods 37 are made of the same material (or a material of the same type), in particular a metal material, as the cover 20 (and/or the probe 19).

In use, the measuring device 3 operates by implementing the measuring method of the invention as follows.

The emitter of the radar instrument 18 emits microwave pulses that travel along the probe 19 through the rod body 26 to the process liquid contained in the apparatus 1. The microwave pulse is reflected when it reaches the interface between the liquid phase and the gas phase (i.e. the free surface H), which have different dielectric constants. The reflected pulse is in turn transmitted from the probe 19 to the receiver of the radar instrument 18. The level of liquid contained in the apparatus 1 is calculated by converting the time difference of the transmitted and reflected waves into distance, in particular by TDR (Time Domain Reflectometry). The calculation is carried out, for example, by a processing unit which is either integrated into the radar instrument or connected to it.

In the embodiment of Figure 2, in which details similar to or the same as those already described are indicated by the same numbers, the probe 19 again enters the apparatus 1 laterally, in particular through the nozzle 21. The probe 19 again comprises an upper connection portion 27 and a lower measurement portion 28, which are straight and joined by a curved joint portion 29. The connection portion 27 extends radially in the apparatus 1 and substantially orthogonally to the axis A; while the measurement portion 28 is inclined with respect to the axis A and the free surface H of the process liquid (and not parallel to the axis A and perpendicular to the free surface H, as described above with reference to Figure 1). The measurement portion 28 and the connection portion 27 are inclined relative to each other by an angle greater than 90°, preferably between 120° and 180°.

In this configuration, therefore, the measurement portion 28 is immersed in the process liquid at an inclination: the portion of the probe 19 immersed in the process liquid extends obliquely with respect to the axis A of the apparatus 1 and the free surface H of the process liquid. In this embodiment, as already described with reference to Figure 1, the probe 19 is also associated with a cover 20 covering the probe 19.

Again in this case, the cover 20 is arranged above the probe 19 so as to cover the vertical projection of the probe 19.

In particular, the cover 20 is positioned above the connection portion 27, the joint portion 29, and also the inclined measurement portion 28, so as to protect the entire probe 19 from the process liquid flowing in the apparatus 1 from top to bottom. The entire outer lateral surface 35 of the cover 20 faces upward (albeit partially inclined) and the entire opening 32 faces downward (partially inclined).

In the further embodiment of Figure 3, in which details similar to or the same as those already described are indicated by the same numbers, the nozzle 21 is again arranged through the side wall 5 of the casing 4, but instead of being perpendicular to the axis A it is inclined with respect to the axis A and the side wall 5.

In this case, the probe 19 is completely straight, i.e. it has an entirely straight rod body 26, and it is aligned with the nozzle 21. The probe 19 thus extends along a straight axis inclined with respect to the axis A.

The connection portion 27 and the measurement portion 28 are aligned with each other along a common straight axis inclined with respect to the axis A and the free surface H of the process liquid.

Accordingly, the cover 20 also extends parallel to the probe 19 and straight, with the outer lateral surface 35 inclined parallel to the probe 19 and facing upwards. For example, the probe 19 is inclined at an angle of between 30° and 60° with respect to the axis A.

In the embodiment of Figure 5, in which details similar to or the same as those already described are indicated by the same numbers, the probe 19 is inserted into the apparatus 1 through the bottom wall 9.

The nozzle 21 is thus arranged through the bottom wall 9 and the radar instrument 18 is arranged below the nozzle 21 and the apparatus 1.

The probe 19 again extends from the radar instrument 18, to which it is connected by means of the proximal end 24.

In this case, the probe 19 has a profile which is bent by 180° and the rod body 26 of the probe 19 has a U-shaped longitudinal profile.

In particular, the probe 19 comprises a connection portion 27, which extends from the proximal end 24 connected to the radar instrument 18 and penetrates the apparatus 1 through the nozzle 21, and a measurement portion 28, which in use is at least partially immersed in the process liquid and terminates at the distal end 25.

The connection portion 27 and the measurement portion 28 are straight and substantially parallel to each other and to the axis A, and are joined by a U-shaped joint portion 29. In this configuration, the measurement portion 28

(which is immersed in the process liquid) also extends parallel to the (vertical) axis A of the apparatus 1 and perpendicular to the free surface H of the process liquid.

With reference also to Figures 6-7, the probe 19 is partially covered by a covering sheath 40, arranged around the rod body 26 of the probe 19 on the lateral surface 31 of the probe 19.

The sheath 40, optionally made from several pieces joined together, has a closed cross-section around the rod body 26 and covers the lateral surface 31 of the probe 19, being in contact therewith.

The sheath 40 extends to cover in particular the connection portion 27 and the U-shaped joint portion 29. In contrast, the sheath 40 leaves at least part of the measurement portion 28 uncovered so that it is exposed to the process liquid.

The purpose of the sheath 40 is to isolate the rod body 26 of the probe 19 from the process environment at the connection portion 27, and it is therefore made of electrically insulating material capable of withstanding process conditions (particularly the conditions of a urea environment), such as ceramic materials or polymeric materials such as PTFE or PEEK.

Similarly to the description above, the probe 19 is again associated, at least in the part exposed to the process liquid, with the cover 20.

In particular, the cover 20 extends along the measurement portion 28, to which it is flanked laterally and faces as described above. Optionally, the cover 20 extends above the joint portion 29.

Again in this case, the cover 20 is connected to the probe 19 by tie rods 37 which support the cover 20 and provide electrical continuity with the probe 19.

Finally, it is understood that further modifications and variations may be made to the device and the measurement method described and illustrated herein without thereby departing from the scope of the attached claims.

For example, according to further embodiments not shown, the probe 19 is inserted into the apparatus 1 from above, through an upper wall of the casing 4. The nozzle 21 is therefore arranged at the top of the apparatus 1 and is, appropriately, parallel to the axis A.

In this case, similarly to the description above with reference to Figure 3, the probe 19 may be completely straight, i.e. have an entirely straight rod body 26. The probe 19 thus extends along a straight axis and the cover 20 accordingly also extends straight and parallel to the probe 19.

The probe 19 may be substantially vertical and parallel to the axis A, with the cover 20 arranged laterally beside the probe 19 and the inner lateral surface 34 facing the lateral surface 31 of the probe 19; or inclined with respect to the axis A, with the cover 20 arranged above the probe 19, with the inner lateral surface 34 facing and above the lateral surface 31 of the probe 19.