WO2007049966A1

WO2007049966A1 - Level gauge

Info

Publication number: WO2007049966A1
Application number: PCT/NO2006/000135
Authority: WO
Inventors: Øivind JENSEN; Øivind TØMMERÅS; Oddbjørn MALMO
Original assignee: Kongsberg Maritime As
Priority date: 2005-10-28
Filing date: 2006-04-11
Publication date: 2007-05-03
Also published as: JP2009513971A; CN101297182B; JP4757920B2; NO323548B1; KR100891572B1; NO20055030D0; CN101297182A; KR20070045888A

Abstract

The present invention relates to an apparatus for determining fluid level in a container, comprising a radar measuring device with an antenna for sending signals towards the fluid surface and means for detecting echo signals, and characterized in that it comprises a device for providing a container reference signal. The invention comprises also a method and a system for determining fluid level in a container.

Description

Level gauge

The present invention is related to an apparatus for determining fluid level as described in the Applicant's Norwegian Patent Application NO 20054466, the contents of which are hereby incorporated by reference.

Said apparatus comprises a wave shield for fluid level measurement by means of radar instruments and in some embodiments a bottom part.

The invention relates thus to an apparatus for measuring fluid level in a container, and will be explained in detail by means of an example where the fluid is LNG and the container is a tank situated in a vessel, but it can be used in any other radar measurement system e.g. land based LNG storage tanks, petroleum product tanks, chemical tanks and liquid nutrition tanks.

Conventional level gauging radars are supported by the roof of the tank. A certain distance is assumed to exist between the radar source and the bottom of the tank (tank depth). The gauging radars measure the ullage (that is the distance between the source and the fluid surface) and the fluid level is calculated as the difference between the tank depth, and the measured ullage. This method is for various reasons not accurate enough to measure low levels in deep tanks.

According to ISO 13689 the level measurements can be related to a datum level which is "The reference level, equal to the zero level in the tank calibration table, from which liquid depths are measured" (ISO/CD 13689.3 § 3.6) . The liquid level is "The distance between the liquid surface in the tank and the datum level" (§3.7). This invention deals with the practical problems of establishing a fixed reference position for the radar measurement (at the top of the container) which can be related directly to the tank datum level (at the bottom of the container). Thermal expansion of the tank roof has alone a potential to influence the tank depth (by altering the distance between the radar and the bottom) more than the required accuracy for CTS applications. Thermal expansion of the tank roof, where the radar is fastened is mainly due to solar radiation on deck, since the top of the tank is thermally insulated from the cold liquid gas inside the container. This phenomenon is not taken into consideration in the prior art systems. In LNG applications there are also significant sources of error in the methods used to correct the level measurement for thermal contraction of the radar still pipe.

WO 2004/083791 shows a system where the level of a surface of a product is measured. This system comprises two level gauges, a first one arranged above the surface of the product and a second one arranged close to the bottom of the container to determine the product level when the measured value is below a predetermined value. This device attempts to provide accurate measurements for low levels and is primarily included to be able to measure the level of the higher density liquid in a stratified storage tank (i.e. water at the bottom of an oil tank). The inclusion of a second level gauge has however the following disadvantages, namely additional cost and complexity in the installation phase, and increased risk of malfunction during operation as it will be submerged at the extremely low temperatures in LNG storage tanks.

The prior art does not provide any satisfactory method for measuring fluid level in deep tanks because the reference level used for the tank's bottom does not take into consideration height variations in sufficiently precise manner.

An object of this invention is to provide a container reference device which provides an accurate reference signal for level measuring relative to the bottom of the container.

Another object is to take into consideration thermal expansion/contraction of the container and/or the measurement system when measuring fluid level.

Another object of the invention is to provide a device which is easily secured to the container.

Another object of the invention is to provide a device which takes into consideration thermal contraction/expansion of the still pipe.

Another object of the invention is to provide a method to determine the wave propagation speed or the permittivity (dielectric constant) in the fluid preferably directly (i.e. without need for separate measurements nor operator inputted values), so that the travel distance of the radar signal through the liquid phase can be calculated from the measured time delay.

These and other objects are attained by means of an apparatus for determining fluid level in a container, comprising a radar measuring device with an antenna for sending signals towards the fluid surface and means for detecting echo signals, and characterized in that it comprises a device for providing a container reference signal.

In one embodiment the radar measuring device comprises a still pipe and a sheltering device. In a variant of this embodiment the container reference device comprises a reference pin adapted for sliding in a mainly vertical slit in the still pipe and adapted to be received in openings in the sheltering device. In another variant the container reference device comprises a disc and a support pin, where the disc is a reference marker and rests on the support pin. In another variant the container reference device comprises a reflector and the sheltering device comprises a bottom part so that the reflector is used to delay the bottom reference signal by reflecting parts of the energy deflected from the bottom part of the shielding device back into the still pipe. In a variant of this embodiment the reflector is a collar ring situated on the upper edge or on the walls of the shielding device and in another variant reflector is a separate reflector adapted to provide flexibility regarding offset distance.

In another embodiment of the invention the apparatus comprises further means for measuring wave propagation speed. In a variant of this embodiment said means comprise two pins adapted for sliding in mainly vertical splits in the still pipe and in another variant it comprises fixed markers.

The invention comprises also a method for measuring fluid level in a container by means of a radar system, comprising sending signals towards the fluid surface, characterised in that it further comprises detecting a first echo signal from the fluid surface, detecting a second signal from a container reference device, and based on said first and second echo signals, preferably on the time delay between these and the wave propagation speed providing a signal representative of the fluid level. In one embodiment the characteristic properties of the fluid (and hence the wave propagation speed) are known, in another embodiment the characteristic properties of the fluid are unknown, and the permittivity or the wave propagation speed is directly measured by the system.

The invention comprises also a system for determining fluid level in a container, comprising a radar measuring device with an antenna for sending signals towards the fluid surface, and means for detecting a first echo signal from the fluid surface, characterized in that it further comprises: a container reference device for providing a second echo signal, means for detecting said second signal and means for based on said first and second echo signals providing a signal representative of the fluid level.

The time delay between the first and the second echo signals is measured to provide the signal representative of fluid level by multiplication of the time delay by the wave propagation speed and then applying the appropriate correction for the distance between the true container bottom and the container reference position.

In one embodiment of the invention the reference device is connected to the container's bottom. In another it is situated on the container's bottom. In another embodiment the reference device is connected to a sheltering device and in a variant of this the reference device is connected to a sheltering device's wave screen. Different features of the invention are presented as different embodiments for the sake of clarity and the skilled man will understand that it is possible to combine these to adapt the apparatus to a concrete use.

The invention comprises further use of two or more container reference devices to provide two or more echo signals. Use of still pipe reference devices that is devices which are connected to the still pipe and track its movement together with container reference devices is also envisaged.

According to the invention the bottom echo or an echo from a marker which follows the bottom at a fixed or calculable distance from the bottom is used to measure or calculate the fluid level based on the time delay between the fluid surface echo and the true or delayed/advanced bottom echo.

This feature can be combined with a sheltering device provided with a bottom echo attenuator as described in Norwegian Patent Application NO 20054466. Using a marker echo which is either advanced or delayed relative to the bottom echo (this being controlled by the reference device's geometry) permits tracking of the fluid surface echo with higher accuracy and closer to the bottom than in the prior art.

The invention will now be described by means of an example illustrated in the attached drawings, where: figure 1 shows a first embodiment of the invention, figure 2 shows a second embodiment of the invention, figure 3 shows a third embodiment of the invention, figure 4 shows a variant of the embodiment in figure 3, figures 5 and 6 shows embodiments of the invention which permit measurement of fluid properties.

Figure 1 shows a sheltering device 13 with walls 100 and a bottom part 101 (preferably comprising an attenuator). The bottom part 101 is connected to the tank's bottom or situated on the tank's bottom. Walls 100 are equipped with openings 102 adapted to receive a reference pin 10 and which provide a fixed distance between pin 10 and bottom part 101. The container reference device according to the invention comprises thus in this embodiment reference pin 10 which is thread through and is allowed to slide in two diametrally opposite slits 11 in the radar still pipe 12. Reference pin 10 rests on the top edge of shielding device 13 and thereby follows the movements of said shielding device 13 and thus the tank bottom. The slit 11 height allows for the maximum thermal contraction of the still pipe 12 combined with a curving of the container's top part (thermal expansion of the roof). The radar is fixed to the still pipe and this is connected to the roof or top part of the container. The slits 11 permit a displacement of the pin in height due to these phenomena. Slits 11 will be typically 90-120 mm and preferably 100mm for a 30 m deep tank. The width of the slits 11 is preferably less than 5 mm to avoid unwanted disturbance of the radar signal. The pin 10 can e.g. be made in metal or plastic material, and the signature is chosen so that it is easy to detect but lower than the level echo (preferably 10-20 dB below the level echo) to minimize the interference with the level echo. The distance from the bottom of the shielding device 13 to the reference pin 11 is preferably more than 20 cm and preferably more than 25 cm to avoid any unwanted interference with the level echo in the critical bottom zone.

The radar signal will be reflected by the fluid surface and by the reference pin and comparison of these signals will provide a measure of the fluid level as the distance between the tank bottom and the reference pin is fixed and known.

In figure 2 the same objectives are realised by a second embodiment of the invention where the container reference device comprises a reference marker (disc 14) resting on a support pin 15 made of plastic or other non-metallic material. Support pin 15 is fastened to the bottom part 101 and the radar signal will in this case be reflected by disc 14 and by the fluid surface.

In the third embodiment of the invention, shown in figure 3 the container reference device comprises a collar ring 16 situated on the upper edge of the shielding device 13. Collar ring 16 can also be situated on the shielding device's 13 walls at a lower level than the shielding device's upper edge, in this case it will be designed as a ring protrusion on the walls. Collar ring 16 is designed to reflect an appropriate part of the radar energy via a second reflection at the container bottom 101 back into the still pipe. The strength of this reflection should preferably be of the same magnitude as the level echo, but because it is delayed relative to the level echo, a stronger signal can be accepted without deterioration of the level measuring accuracy, even at low liquid levels. Collar ring 16 is thus used to delay the bottom reference signal by reflecting parts of the energy deflected from the bottom part 101 of the shielding device 13 back into the still pipe 12. The signal sent by the radar will then through three reflection steps (arrows referenced by 200) before reaching the receiver be delayed relative to the direct fluid level echo (fluid surface echo), and thus it will not interfere with the fluid level echo even the fluid level is low (close to the bottom of the tank). The delay of this echo will however depend on the distance between the radar and the bottom of the tank, and thus be a measure of the tank depth.

In figure 4 the same effect as in figure 3 is obtained by means of a fourth embodiment of the invention where the container reference device comprises a separate reflector 17. This embodiment is more flexible as to offset distance than the one in figure 3.

As mentioned in NO 20054466 the bottom part 101 of the shielding device 13 can e.g. be an attenuator, a deflector or a quarter wave step device.

By situating the reference device before the bottom part (as in the embodiments shown in figures 1 and 2) or after the bottom part (as in the embodiments shown in figures 3 and 4 where the reference device's echo has a reflected signal with longer path than the bottom reflection), the marker echo will be advanced or delayed relative to the bottom echo and high accuracy close to the bottom is achieved.

The embodiments in figures 3 and 4 are based on reflection from the bottom part so that a sufficient part of the radar energy is deflected towards the reflector by a quarter wave step or another type of deflector 18. In these embodiments the wave propagation speed in the fluid (liquid) must either be known or measured differentially by the radar during loading/unloading of the tank. It can also be measured directly (figures 5 and 6) for instance by applying (figure 5) two still pipe reference pins 19, 20, 21, 22 either sliding or in a known fixed distance as illustrated in figures 5 & 6 respectively. The still pipe reference pins 19, 20, 21, 22 can be separated by a distance holder 201 made in Invar or the actual distance in LNG can be calculated using known thermal correction factors if it is made in a material which is not invariant to temperature changes.

Figure 6 shows two fixed markers 21 and 22 and a movable reference pin 23 (marker which follows the bottom, corresponding to the embodiment in figure 1). Reference pin 23 will be in contact with the shielding device 13 (not shown).

When submerged, any pair of reflectors (disks as used in Kongbergs Autrocal® system ) which are used to calculate the wave propagation speed in vapour (and hence the ullage), can be used to calculate the permittivity in the LNG, or any combination of discs, additional reference pins and markers as described in the embodiments illustrated in figures 1-4 can be used, as long as the distance between them is known.

If the wave propagation speed (i.e. the electrical permittivity) is measured directly, a priori knowledge about the fractions of the various hydrocarbons in the LNG is not needed, and the distance from the radar to the tank bottom can be calculated from the radar measurements described above for all levels, not only when the level is in the bottom zone.

Claims

1. Apparatus for determining fluid level in a container, comprising a radar measuring device with an antenna for sending signals towards the fluid surface and means for detecting echo signals, characterised in that it comprises a device for providing a container reference signal.

2. Apparatus according to claim 1, cha racterised in that the radar measuring device comprises a still pipe (12) and a sheltering device (13).

3. Apparatus according to claim 2, cha racterised in that the container reference device comprises a reference pin (10) adapted for sliding in mainly vertical slits (11) in the still pipe (12) and adapted to be received in openings (102) in the sheltering device (13).

4. Apparatus according to claim 2, cha racterised in that the container reference device comprises a disc (14) and a support pin (15), where the disc is a reference marker and rests on the support pin.

5. Apparatus according to claim 2, cha racterised in that the container reference device comprises a reflector and the sheltering device comprises a bottom part (101) so that the reflector is used to delay the bottom reference signal by reflecting parts of the energy deflected from the bottom part (101) of the shielding device (13) back into the still pipe (12).

6. Apparatus according to claim 5, cha racterised in that the reflector is a collar ring (16) situated on the upper edge or on the walls (100) of the shielding device (13).

7. Apparatus according to claim 5, cha racterised in that the reflector is a separate reflector (17) adapted to provide flexibility regarding offset distance.

8. Apparatus according to any of the preceding claims, cha racterised in that it comprises further means for measuring wave propa ggaattiioonn ssppeeeedd..

9. Apparatus according to claim 8, cha racterised in that said means comprise two pins (19,20) adapted for sliding in mainly vertical splits in the still pipe (12).

10. Apparatus according to claim 8, cha racterised in that it comprises fixed markers (21, 22).

11. Method for measuring fluid level in a container by means of a radar system, comprising sending signals towards the fluid surface, characterised in that the method further comprises:

- detecting a first echo signal from the fluid surface,

- detecting a second signal from a container reference device,

- based on said first and second echo signals providing a signal representative of the fluid level.

12. Method according to claim 11, characterised in that it comprises comparing characteristic parameters for the first and the second echo signals.

13. Method according to claim 12, characterised in that the signal representative of the fluid level is provided based on the time delay between said first and second signals and the wave propagation speed.

14. Method according to claim 13, characterised in providing the characteristic properties of the fluid and hence the wave propagation speed.

15. Method according to claim 14, characterised in that the characteristic properties of the fluid are unknown, and the method comprises measuring permittivity or the wave propagation speed

16. Method according to claim 11, characterised in that the reference device is connected to the container's bottom or is situated on the container's bottom or is connected to a sheltering device.

17. Method according to claim 16, characterised in that the reference device is connected to a sheltering device's wave screen.

18. System for determining fluid level in a container, comprising a radar measuring device with an antenna for sending signals towards the fluid surface, and means for detecting a first echo signal from the fluid surface, characterised in that it comprises

- a container reference device for providing a second echo signal,

- means for detecting said second signal,

- and means for based on said first and second echo signals providing a signal representative of the fluid level.

19. System according to claim 18, characterised in that it comprises means for comparing the first and the second echo signals.

20. System according to one of the preceding claims 17-18, integrated into a radar level gauge for determining the wave propagation speed or the electrical permittivity of the fluid in the bottom zone of the tank.

21. System integrated into a radar level gauge for determining the wave propagation speed or the electrical permittivity of the fluid in the bottom zone of the tank, characterised in that it comprises an apparatus according to claims 1-10.

22. System integrated into a radar level gauge for determining the wave propagation speed or the electrical permittivity of the fluid in the bottom zone of the tank.