DE19847555A1 - Level measurement system for multi-phase offshore separator tank, comprises gamma detector to detect shadow of natural gamma radiation absorption by tank contents using long, vertical scintillation detector - Google Patents

Abstract

A psi detector (3,3a,3b) produces a level-related signal from attenuation of background psi radiation (2). An Independent claim is included for the corresponding level detection system, sensing tank vertical density profile. Preferred Features: A long psi detector, especially a scintillation detector (3a) employs a photomultiplier- (3a) or a photodiode- (3b) array. The detector (3-3b) is orientated along the filling direction of the vessel. The scintillation detector is a light-transmitting rod, an optical fiber or an optical fiber bundle. It is vertically arranged in the separator tank (4), or to a side of it which is screened from the background radiation by the tank contents (5-8). Scintillations are split into two portions, their light led by differing propagation paths to the photodetectors. Time delays of photodetector signals from correlated scintillations are measured, from which the point of scintillation is calculated. From the frequency distribution of scintillation source points, a density profile and especially levels of the vessel contents are determined. To follow level variation, the process is carried out continuously and/or drift in the electronics is eliminated, using a calibration source, especially a weak psi or alpha emitter. Photodetector signal pulse height is discriminated by the electronics. The threshold is an exponentially-decreasing function of the time delay, selected such that scintillations are evaluated in a given energy spectrum.

Description

TECHNICAL AREA

The present invention relates to the field of Level indicators. It is based on a procedure and a device for level measurement according to the Ober Concept of claims 1 and 7.

STATE OF THE ART

So-called separa are used for offshore oil production tion tanks used, in which the drilling or Promotion of different phases (sand, water, Oil and gas) due to their density differences and be carried away in separate piping systems. It it is very important to determine the height of the interface between the water and oil to know the drain valves on the tank open and close for both media in a controlled manner to be able to. For this purpose, reliable level measuring devices are used needed. Does such a level meter work not or poorly, e.g. B. Oil in the water outlet guess and cause great environmental pollution and costs stuff.

High pressure separation tanks have recently been developed that for the operation on the seabed some 100 m un below the surface of the sea. The funded and already separated oil can then be used with much less Energy expenditure can be pumped to the sea surface. Such separator tanks are very high pressures of 60 bar-180 bar and high temperatures of 50 ° C-120 ° C puts. The level measuring system has to be difficult  conditions, maintenance-free and reliable for years work because of a downtime and premature Replacement would cause high costs.

A variety of devices are in the prior art known for determining the fill level of a container, based on very different physical measuring principles pien are based. These include electrical (capacitive or resistive) and optical methods, radar reflection methods, Ultrasonic transit time methods and gamma absorption method the. Especially the determination of density by gamma absorption represents a very robust, reliable and inexpensive It is known that a gamma density pro fil sensor, which is particularly suitable for level measurement is to be realized by using a gamma source and a gamma detector synchronously along a container wall be moved up and down and out of the beam weakening the product density as a function of the sensor position is determined. This arrangement is disadvantageous the need for at least one intensive radioactive Gamma source related to appropriate precautions Shielding, handling and disposal required makes. Another big disadvantage is that an unacceptable product due to the moving components effort or an intolerable susceptibility to failure result.

There are also scintillation detectors with one or two dimensional spatial resolution known and are on the Market offered. Scintillators are available in the Form of rods, the z. B. NaJ: Tl included. The from the Szin Lightning flashing outgoing light waves are on both Detected rod ends and from the ratio of their with the optical path length exponentially decreasing intensities or pulse heights the location of the gamma photon absorption Right. Also with plastic fibers in flat parallel or crossed arrangements become spatially resolved scintiles lators realized. As also known from WO 85/04959  the location information is obtained simply by that identifies that fiber or fibers be a scintillation flash into a photo lead detector.

PRESENTATION OF THE INVENTION

The object of the invention is a gamma ray len density profile sensor for level measurement, which is characterized by a simplified, very robust construction and distinguishes a good vertical spatial resolution. This The object is achieved by the features of Claims 1 and 7 solved.

The invention is namely that in the vicinity or preferably inside a container with filling material Gamma detector is placed, with this one through the Attenuation of background gamma rays due to the filling material lung is measured and from it a level or you teprofil in the container is determined. By leaving out highly radioactive gamma sources simplify construction, Handling, safety precautions and disposal of the Level sensor crucial and customer acceptance is increased significantly.

In one embodiment, the gamma detector is on elongated scintillation detector, photomultiplier array or photodiode array, which is essentially in a Filling direction of the container is oriented. Preferably is the gamma detector in the middle of a separator tank arranged vertically or on a wall facing away from the product The other side is shielded from background gamma radiation.

Another embodiment relates to a scintilla gate in the form of a light-conducting rod, an optical one Fiber or an optical fiber bundle with at least an end-side photodetector. A density profile is made the counting rate of the scintillation flashes as a function of  Runtime difference of going in opposite directions propagating light components determined. The vertical place The resolution is essentially accurate due to the measurement given the maturity differences.

Further embodiments are variants of the Szin tillatorstabs with single or double-sided photodetector the ends and energy discrimination of the scintillati flashes for the selective acquisition of photopeak signals in the scintillator.

An advantage of the method and sensor according to the invention for level measurement consists in the reduced complexi activity and susceptibility to failure. Above all that is an advantage Simplicity, mechanical robustness and inherent reliability nonchalance by omitting the gamma sources and using of a scintillator rod or scintillator fibers compared to conventional gamma detectors.

It is particularly advantageous that with the elongated Scintillator a sufficient spatial resolution of the fill level measurement is very easy to implement.

In addition, it is advantageous that one for electri capacity measurement redundant, non-contact and sensitive level measurement method is given, which is based on is based on a completely independent measuring principle and far maintenance-free.

Other designs, advantages and applications of the Erfin dung result from the dependent claims as well as from the following description based on the figures.

BRIEF DESCRIPTION OF THE DRAWING

Show it:

Fig. 1 shows an inventive level gauge system having a scintillation detector arranged laterally in a plan view (a) and side view (b);

Figure 2 shows an arrangement of the filling level measurement system in the interior of a separator tank in cross-section.

Fig. 3 is a scintillation detector with scintillator and photodetectors at the upper and lower end;

Fig. 4 is a scintillator with retroreflector at one end according to an alternative embodiment to FIG. 3;

Fig. 5 shows an inventive level gauge system with a photomultiplier or photodiode array.

In the figures, the same parts have the same reference characters.

WAYS OF CARRYING OUT THE INVENTION

The absorption of high-energy gamma photons is based on the photo effect, the Compton scattering and the pair generation. In the photo effect, the gamma photon is completely absorbed by the electron shell of an atom and a preferably strongly bound electron is split off. The electron loses its kinetic energy due to further collisions, so that the entire gamma energy is deposited in the medium. Above the strongest binding energy of the electrons, the probability of the photo effect decreases sharply with increasing gamma energy. In the range between 100 keV and 1 MeV, the component effect becomes dominant. In addition, from 1 MeV, the gamma photon can decay into an electron-positron pair when interacting with the electrical field of an atomic nucleus or an electron. With an increasing core charge number, the probabilities for the photo effect and pair formation increase strongly and moderately for the Compton effect. Overall, the absorption coefficient is proportional to the atomic density. The resulting substance-specific density dependence of the gamma absorption is used as the measuring principle in gamma radiation density sensors. In principle, all three gamma energy consuming processes detector as proof effect in a scintillation 3a and as disruptive effect such. B. in container walls 4 a or in a scintillator housing 11 , play a role.

The invention takes advantage of the fact that mostly natural gamma background radiation 2 occurs everywhere. As causes for this come e.g. B. cosmic radiation and natural or artificial radioactivity of rock, earth, sea water or air in question. The background radiation 2 is orders of magnitude weaker than that of typical gamma sources for sensor applications. However, the invention is based on the knowledge that the background radiation 2 is generally large enough to nevertheless implement efficient fill level sensors 1 .

Fig. 1 shows schematically a first embodiment of a fill level or sealing profile sensor 1 according to the invention, which is designed for measuring gamma background radiation 2 . Background radiation 2 typically strikes a container or separator tank 4 on all sides, in the radiation shadow of which a gamma detector 3 , 3 a, 3 b is arranged. In particular, the gamma detector 3 , 3 a, 3 b is placed near or inside the container 4 . The gamma detector 3 , 3 a, 3 b is preferably elongated and oriented essentially in a filling direction of the container 4 . In particular, the gamma detector 3 , 3 a, 3 b is a scintillation detector 3 a or a photomultiplier array 3 b or photodiode array 3 b, a scintillator being arranged in front of each photomultiplier or each photodiode. Part of the background radiation 2 passes through the tank 4 , where a fraction depending on the density of the filling 5-8 , z. B. gas 5 , oil 6 , water 7 and sand 8 , is weakened, in particular absorbed and / or scattered, and is detected with the aid of the gamma detector 3 , 3 a, 3 b. It is essential to the invention that no gamma sources for the active or location-selective radiation of the container 4 are provided.

With the gamma detector 3 , 3 a, 3 b according to the invention according to FIGS . 1-5, a vertical density profile or a fill level of the tank content is determined. The positions of the interfaces 56 , 67 , 78 between the different media 5-8 can be determined from the density differences. The achievable spatial resolution when localizing a boundary layer 56 , 67 , 78 depends on the measurement or acquisition time and the strength of the background radiation 2 . For a boundary layer between sea water 7 and air 5 , a spatial resolution of 3 cm can be achieved with a measuring time of 15 s. For a boundary layer 67 between oil 6 and water 7 , a measurement time of approximately 25 minutes is to be expected with the same spatial resolution. If necessary, an additional gamma source, e.g. B. a radioactive isotope with an activity below the approval limit and in particular below the exemption limit can be arranged so that the background radiation 2 is largely homogeneously amplified in the volume of the tank 4 to be monitored. Further advantages of using gamma background radiation 2 are that the radiation 2 is available practically everywhere free of charge and remains constant over a long period of time.

Fig. 2 shows another arrangement according to the invention, in which the gamma detector 3 , 3 a, 3 b is mounted vertically in a pipe socket 9 in the separator tank 4 or - not shown - is directly immersed vertically in the filling material 6 , 7 in the separate tank 4 . The separator tank 4 is held in a frame 10 . Advantageously, the gamma detector 3 , 3 a, 3 b is arranged in the middle of the separator tank 4 , or a lateral shield 20 of the gamma detector 3 , 3 a, 3 b is provided on a side facing away from the contents 5-8 . The shield 20 is particularly useful when the gamma detector 3 , 3 a, 3 b is arranged in the vicinity of the tank 4 or on a container wall 4 a inside or outside of the tank 4 . These measures maximize a fill level-dependent signal of the gamma detector 3 , 3 a, 3 b and reduce or largely eliminate a fill level-independent background signal by gamma background radiation 2 , which has not passed through the tank 4 .

Fig. 3 shows an inventive scintillation detector 3 a. From the gamma background radiation 2 scintillation flashes 13 are triggered in the scintillator 12 with a certain response probability. A gamma photon stimulates the scintillator 12 through the above-described interaction processes to emit short scintillation light flashes 13 , the intensity of which is proportional to the deposited energy. Due to the special nature of the scintillator 12 as an elongated light guide, in particular as a light-guiding rod, optical fiber or optical fiber bundle, the scintillating flashes 13 are split into two light components which spread to both ends of the scintillator 12 . Each light wave is received by a photodetector or photo detector array 14 , 15 , converted into an electrical signal and supplied to measuring electronics 17 via lines 16 . The measuring electronics 17 measure the exact time differences of photodetector signals of related scintillation flash components in a delayed coincidence measurement. The place of origin of the scintillation flash 13 is calculated from the time delay or running time difference between the two light components, and a density profile and in particular a fill level 56 , 67 , 78 of contents 5-8 in the container 4 are determined from a frequency distribution of the places of origin. Thus, from the frequency distribution of the light pulses as a function of the time delay, the gamma absorption or density of the filling 5-8 can be determined as a function of the height in the tank 4 . In particular, the density difference between sea water 7 (1015 kg / m ³ ) and oil 6 (850 kg / m ³ ) can be detected. But gas 5 and sand 8 can also be detected in this way. By dynamically determining this frequency distribution, a momentary density profile or a variable fill level 56 , 67 , 78 can also be measured for a relatively rapidly flowing medium 5-8 and the sedimentation and separation process in tank 4 can be monitored.

As is known, the scintillator 12 can contain an inorganic material, in particular NaI: Tl in crystalline or polycrystalline form, or an organic material in crystalline, liquid or plastic-like form or a preferably doped glass. Important design parameters for a desired scintillator length and spatial resolution are the availability in lengths up to over 2 m and the mechanical stability, the optical damping and the decay time of the scintillation, ie the optical pulse width. Particularly suitable for the invention are round or angular rods made of plastic, in particular doped with NaJ: Tl, furthermore optical fibers or optical fiber bundles, in particular plastic fibers or plastic fiber bundles, or a combination of rods and fibers. Scintillator rods made of plastic can also be made of several parts in the desired length. U. pieces with optical transition, glued together. The light guides have typical optical attenuations of approx. 10 ^-2 cm ^-1 . To reduce the loss of light, the rods can be provided with a reflective coating on their outer surface. For a spatial resolution of 10 cm, transit time differences of approximately 1 ns must be detectable, which makes pulse widths in the ns range and below necessary. For this purpose, plastic scintillators and correspondingly fast photodetectors or photodetector arrays 14 , 15 with a detector surface adapted to the light guide cross section are particularly suitable.

The calibration of the density measurement is carried out by reference measurements with water 7 , oil 6 , etc. According to the invention, the synchronicity of the measuring electronics 17 , ie the two measuring channels for determining the transit time delays, can also be carried out with the aid of a calibration source 18 , which according to FIG Scintillators 12 is attached, monitored during operation and corrected if necessary. In this way it is guaranteed that the measuring electronics 17 is drift-free and long-term stable. As a calibration source 18, pulsed light or Szintillationsquellen are with light coupling in both directions of the scintillator 12 obligations suitable. In particular, weak gamma or alpha emitters 18 , e.g. B. americium with an equivalent gamma energy of 60 keV, can be used.

Fig. 4 shows a detailed view of another embodiment of the scintillator 12. At the lower end of the scintillator 12 there is a retroreflector 19 instead of the photodetector 15 , which, for. B. can be designed as a cubic or flat mirror. Both of the scintillating flashes 13 outgoing light beams are guided under different propagation paths to a single photodetector or photodetector array 14 at the upper end of the scintillator and processed as before by the measuring electronics 17 . Due to the reflection, the originally downward wave experiences a stronger damping, but also a greater delay time. In this arrangement, the spatial resolution can be doubled. In addition, the scintillator 12 is simplified by saving a photo detector 15 and the associated electrical signal line 16 and is particularly well suited for installation locations that are only accessible on one side.

A further embodiment results from Fig. 3 when the scintillator 12 is composed of two rods which are separated by a layer, not shown, reflecting on both sides. Such a composite scintillator 12 thus has an upper and lower half, in which the light propagation is completely independent of one another. The reflective layer is preferably located halfway in the scintillator 12 . In the case of a scintillation event in one scintillator half, the optical pulses are evaluated as in the exemplary embodiment according to FIG. 4. The advantage over Fig. 4 is that with the same local resolution, the light paths are halved and thus the optical attenuation is significantly reduced. In general, determining the location by measuring the transit time instead of comparing the optical attenuations is also advantageous because this method is very insensitive to changes in the optical attenuation in the scintillator 12 due to aging.

According to the invention, the behavior of the scintillation detector 3 a can also be improved by energy discrimination of the scintillation flashes 13 . The intensity or energy distribution of the scintillation events results from the stimulating gamma energy and the probabilities for the photo effect, Compton effect and possibly pair formation in the scintillator 12 . The brightest flashes are caused by the photo effect of the primary, undisturbed gamma photons and form the so-called photopeak. Weaker flashes result from secondary gamma photons, in particular from the steel walls 4 a, but also from primary gamma photons, which suffer Compton scattering in the scintillator 12 and then escape. In general, at least one discrimination threshold should be selected so that scintillation flashes 13 of a predeterminable energy spectrum caused in the scintillator 12 are evaluated. In particular, by cutting off the scintillation spectrum below a predeterminable threshold, e.g. B. the photopeak, the detection of scattered with less energy loss, z. B. Compton-ge scatterers, gamma photons are limited at the expense of the counting rate. Because of the optical attenuation in the scintillator 12 , the discrimination threshold must be chosen as an essentially exponentially decreasing function of the optical path or the time delay. The influence of parasitic gamma scattering in the container wall 4 a can be suppressed, for example, by the energy discrimination.

Fig. 5 finally shows an inventive exporting approximate shape with a linear array of photomultipliers 3 b 21 or photodiode 21, which are individually connected via signal lines 16 to the measurement electronics 17th In this example, a side shield 20 is shown on a side facing away from the filling 6 , 7 .

Overall, the invention results in a very simple and robust level sensor, which is a big one Operational safety, low maintenance and long Has lifespan and especially for use places that are difficult to access. Other applications dung, e.g. B. for snow depth determination are possible.  

Reference list

1

Gamma ray density profile sensor

2nd

Background gamma radiation

3rd

,

3rd

a,

3rd

b Gamma detector

3rd

a scintillation detector

3rd

b Photomultiplier array, photodiode array

4th

Container, separator tank

4th

a tank wall

5-8

Medium, medium

56

,

67

,

78

Boundary layers, level

5

gas

6

oil

7

water

8th

sand

9

Pipe socket

10th

frame

11

Gamma detector housing, scintillator tube

12th

Scintillator (rod, fiber, fiber bundle)

13

Scintillation light flash

14

Photodetector

1

15

Photodetector

2nd

16

electrical signal lines

17th

Measuring electronics

18th

Calibration source

19th

Retroreflector

20th

Gamma detector shield

21

Photomultiplier, photodiodes

Claims (13)

1. Method for level measurement of a container ( 4 ), particularly suitable for localizing a boundary layer ( 56 , 67 , 78 ) of two media ( 5 , 6 , 7 , 8 ) in a separator tank ( 4 ), characterized in that with the aid of at least a gamma detector ( 3 , 3 a, 3 b), a level-dependent signal is measured from an attenuation of background gamma radiation ( 2 ) in the container ( 4 ). 2. Method for level measurement according to claim 1, characterized in that

• a) an elongated gamma detector ( 3 , 3 a, 3 b), in particular a scintillation detector ( 3 a), a photomultiplier array ( 3 b) or a photodiode array ( 3 b), is used and
• b) the gamma detector ( 3 , 3 a, 3 b) is oriented essentially in a filling direction of the container ( 4 ).

3. A method for level measurement according to claim 2, characterized in that

• a) the scintillation detector ( 3 a) has a light-guiding rod, an optical fiber or an optical fiber bundle as a scintillator ( 12 ) and / or
• b scintillation detector on egg ner facing away from the filling (5-8) side in front of background gamma radiation (2) shielding (20) (3 a) or arranged vertically in the center of the separator (4)).

4. Method for level measurement according to one of claims 2-3, characterized in that

• a) in the scintillator ( 12 ), scintillation flashes ( 13 ) are split into two portions, which are guided to at least one photodetector ( 14 , 15 ) via different optical propagation paths,
• b) the time delays of photodetector signals of related scintillation flash components are measured in a measuring electronics ( 17 ) and the locations of origin of the scintillation flashes ( 13 ) are calculated therefrom and
• c) a density profile and in particular a fill level ( 56 , 67 , 78 ) of contents ( 5-8 ) in the container ( 4 ) is determined from a frequency distribution of the places of origin.

5. A method for level measurement according to claim 4, characterized in that

• a) the frequency distribution is continuously calculated to determine a variable fill level ( 56 , 67 , 78 ) and / or
• b) the measuring electronics ( 17 ) using a calibration source ( 18 ) on the scintillator ( 12 ), in particular a weak gamma or alpha emitter ( 18 ), is kept drift-free.

6. Method for level measurement according to one of claims 3-5, characterized in that

• a) in the measuring electronics ( 17 ), the photodetector signals are discriminated according to their pulse height and
• b) the discrimination threshold is chosen as an exponentially decreasing function of the time delay so that scintillation flashes ( 13 ) caused in the scintillator ( 12 ) of a predeterminable energy spectrum are evaluated.

7. Gamma-ray density profile sensor ( 1 ), in particular suitable for level measurement of a separator tank ( 4 ), comprising at least one gamma detector ( 3 , 3 a, 3 b) and measuring electronics ( 17 ), characterized in that

• a) the gamma detector ( 3 , 3 a, 3 b) is arranged near or inside the container ( 4 ),
• b) the gamma detector ( 3 , 3 a, 3 b) is designed for measuring gamma background radiation ( 2 ) and
• c) no gamma sources for active radiation of the container ( 4 ) are provided.

8. Gamma ray density profile sensor ( 1 ) according to claim 7, characterized in that

• a) the gamma detector ( 3 , 3 a, 3 b) is elongated and oriented essentially in a filling direction of the container ( 4 ) and
• b) in particular the gamma detector ( 3 , 3 a, 3 b) is mounted vertically in a pipe socket ( 9 ) in the separator tank ( 4 ) or is vertically immersed directly in the filling material ( 5-8 ) in the separator tank ( 4 ).

9. Gamma-ray density profile sensor ( 1 ) according to one of claims 7-8, characterized in that

• a) the gamma detector ( 3 , 3 a, 3 b) is arranged in the middle of the separator tank ( 4 ) or
• b) a lateral shield ( 20 ) of the gamma detector ( 3 , 3 a, 3 b) is provided on a side facing away from the filling material ( 5-8 ).

10. Gamma-ray density profile sensor ( 1 ) according to any one of claims 7-9, characterized in that

• a) the gamma detector ( 3 , 3 a, 3 b) is a scintillation detector ( 3 a) which has a light-conducting rod, an optical fiber or a fiber bundle as a scintillator ( 12 ),
• b) the scintillator ( 12 ) is at one end or at both ends with a photodetector ( 14 , 15 ) in optical connection and
• c) the measuring electronics ( 17 ) comprise means for delayed coincidence measurement.

11. Gamma ray density profile sensor ( 1 ) according to claim 10, characterized in that

• a) a retroreflector ( 19 ) with one end of the scintillator ( 12 ) is in optical connection and
• b) exactly one photodetector ( 14 , 15 ) is in optical connection with the other end of the scintillator ( 12 ).

12. Gamma-ray density profile sensor ( 1 ) according to claim 10, characterized in that

• a) the scintillator ( 12 ) has a reflective layer on both sides, preferably halfway up, and
• b) the scintillator ( 12 ) is at its two ends with exactly one photodetector ( 14 , 15 ) in optical connection.

13. Gamma-ray density profile sensor ( 1 ) according to one of claims 10-12, characterized in that

• a) the scintillator ( 12 ) contains an inorganic material, in particular NaI: Tl in crystalline or polycrystalline form, or an organic material in crystalline, liquid or plastic-like form or a preferably doped glass and
• b) in particular the scintillator ( 12 ) is a plastic rod which consists of several parts glued together and has a reflective coating on its outer surface.