WO2003001160A1

WO2003001160A1 - Method for determination of tank bottom characterisitcs

Info

Publication number: WO2003001160A1
Application number: PCT/SE2002/001227
Authority: WO
Inventors: Tomas AKERSTRÖM; Hans Westerlind
Original assignee: Saab Marine Electronics Ab
Priority date: 2001-06-21
Filing date: 2002-06-19
Publication date: 2003-01-03
Also published as: SE517807C2; SE0102283L; EP1409970A1; NO20035776L; SE0102283D0

Abstract

A method is provided for measuring by means of radar (3) the level of the medium surface (6) in a tank (1), in which a medium (2) is stored, as well as for identifying the status of the empty tank, where the method entails that an electromagnetic signal is transmitted from the radar (3) fixed to the tank roof (4) via an antenna (5) pointing down into the tank, that an echo signal from the medium surface (6) is received by the radar (3), that the level of the medium surface is calculated from a time delay between the transmitted signal and the echo signal, that a number of characteristics of the change with time in the echo signal amplitude during emptying of the tank, based on historical and/or calculated data, are set, that a registered curve of the echo signal amplitude during emptying of the tank is compared with the set characteristics, and that when the tank is essentially empty the status for the empty tank is identified.

Description

Method for determination of tank bottom characteristics

TECHNICAL FIELD

The invention relates to a method for measuring by means of radar the level of the surface of a medium stored in a tank and in addition, when the level of the surface of the medium in the tank is low or when the tank is empty, for determining, for example, if the tank is completely empty, if the tank is clean, if there is sediment at the base of the tank, and the moisture content of any sediment present.

STATE OF THE ART

Remote methods for determining the level of a liquid or other substance in a container are widely known techniques. An example of a device for such remote measurement is provided here by the device described in patent document US 4 641 139.

In the radar level-measuring field of technology, i.e. methods for determining liquid levels in a tank using radar, it has, to date, been difficult to overcome the measuring problems which arise when the liquid surface in the tank drops to a very low level close to a metallic, that is to say strongly reflecting, tank base. Apart from the difficulties involved in resolving a weak echo from the liquid surface close to a strong echo from a metallic base, measurement is complicated by a variable tank environment, both in the short-term, for example in the event of wave formation at the surface, and in the long-term, such as when sediment from the liquid is deposited at the base time after time. The normally accurate methods for obtaining precision measurements of the liquid surface are disrupted or become unreliable.

Storage tanks and marine tanks are normally comprised of more or less horizontal metal bases, which in an empty tank provides a considerable amplitude for the echo signal in a radar level measuring device. Despite the existence of methods for neutralising the effects of interference from such strong echoes in relation to a normally weak echo from the liquid surface, there remains an unacceptable level of uncertainty for level measuring very close to the base of the tank, which provides an operator with an ambiguous or unclear picture of the actual conditions at the base of the tank. Even for tanks where signal-disturbing sediments are deposited at the base time after time, ambiguous situations for a standard level-measuring device will arise, since a permanent layer of sediments may be either dry or wet, that is to say with higher or lower moisture content, and may even display an irregular surface. In the normal procedure for discharging the cargo in a tank, any information comprising precise data about the level of the material near the base of the tank is ignored, since reliable information regarding the conditions at the base of the tank when the tank is considered to be empty is retrieved in another way, for example by probing/sounding with a lead, visual inspection, etc.

Based on experience obtained within the described technical field, it is recognized that extraordinary methods may be necessary to achieve reliable information about a) when the tank is empty and clean, b) when the tank is emptied of its material, for example a liquid, but residue remains, for example bottom sediment. Reliable information of this type is extremely important during the latter phase of emptying a tank as well as during tank cleaning processes to ensure environmentally adapted, safe and economical handling.

DESCRIPTION OF INVENTION

The invention, according to one aspect, involves the provision of a method for measuring by means of radar the level of the medium in a tank, in which a medium is stored, in addition to identifying if the tank is empty or not, where the method entails that a radar transmits an electromagnetic signal via an antenna directed down into the tank, that an echo signal from the medium surface is received by the radar, that the level of the medium surface is calculated from a time delay between the transmitted signal and the echo signal, that a number of characteristics of the change in the echo signal amplitude with time during emptying based on historical and/or calculated data are fixed, that a registered curve of the echo signal amplitude during the emptying in question is compared with the fixed characteristics, and that when the tank is essentially empty the status for the empty tank is identified.

The method is further characterised by the empty tank status being given according to a rule from a rule framework comprising at least the following fixed characteristic predetermined rules: empty tank, clean tank, tank with sediment, tank with wet sediment, tank with dry sediment. The invention is preferably employed with storage tanks and marine tanks, where the level of the surface of the contained medium is measured by radar from the tank roof.

Most liquids, here including even gases in liquid form, stored in such tanks have a known low dielectric constant. The liquid surface, when the surface level is measured by radar with a measuring signal, displays a retro-echo which has significantly lower amplitude than the amplitude of the echo signal that a flat reflecting metal base generates. Not only does the signal reflecting from the liquid surface have low amplitude; it is also the case that a significant portion of the energy of the transmitted measuring signal penetrates the upper layer of liquid. This measuring signal energy can be reflected back to the radar from other metallic objects lower down in the tank. By using, not only the calculated level of the surface, but even a time characteristic for the signal amplitude of the measured level during a relatively continuous emptying or tapping off of the contents of the tank, additional important information on the conditions at the base of the tank can be extracted, where otherwise pure radar level measuring would be incorrect because of the uncertain conditions at the tank base.

The invention indicates a method for identifying bottom conditions, by means of a set of logical decision-making rules, when all the liquid has been pumped out of a tank, or where it is presumed that all the liquid has been pumped out of the tank.

DESCRIPTION OF DRAWINGS

Fig. 1 is a schematic representation of a radar level-measuring device in a tank for determining the surface level of a medium stored in a tank.

Fig.2 is a block diagram showing the principle behind the units, which determine the level speed of the surface according to the aspect of the invention.

DESCRIPTION OF EMBODIMENTS

An example of an embodiment is described below with the aid of the attached drawing, Fig. 2. The drawing shows a block diagram of the function of a radar level-measuring device that employs the method in accordance with the aspect of the invention. The principle of a radar level-measuring device is shown in Fig. 1. A tank 1 is used for storing a medium 2. The medium may be a liquid, e.g. oil, refined products, chemicals and gas in liquid form, or be composed of particle material, that is to say a pulverized solid substance. A radar 3 is fixed to the roof 4 of the tank 1, from where a microwave signal is transmitted from the radar via an antenna 5 on the inside of the tank. The transmitted beam is reflected by the medium surface 6 and is intercepted by the antenna 5. By comparing and evaluating the time- delay between transmitted and reflected beam in a calculating and control unit 7 (see Fig. 2), the level of the medium surface 6 is determined in a known way. Fig. 1 also illustrates the radar lobe 8 and a cleaning device 9 for removing sediments 10, which may be deposited at the base of the tank. The cleaning device 9 is equipped with a spray nozzle 11.

A schematic embodiment of the invention is shown in a block and flow diagram in Fig. 2. The radar 3 used for measuring the liquid level in the tank 1 is connected to a computer 12. The computer 12 is in turn connected to a screen 13 for displaying the obtained data to an operator. The computer 12 comprises a number of operation blocks, which are illustrated in more detail in the lower part of the figure. Thus the computer 12 consists of a signal and electronic block 14, which performs signal processing of the signals the radar 3 transmits and receives. Block 15 is a unit for detecting echoes from the liquid surface in the tank 1. An additional block 16 is utilised to treat the multiple echoes. The desired echo signal is chosen and tracked in a selection block 17. The previously mentioned calculating unit 7 determines, using data from the said block, the level of the liquid surface. Finally, the measured level obtained is presented on a display in the instrument 18, which can be a screen 13 or other type of display. Thus far, the procedure illustrated by the description of the flow diagram in Fig. 2 is a known technique for determining the level of the liquid surface.

According to an example of the present invention, the data obtained from the selection block 17 is used for storing, in a characteristic block 19, historical signal amplitudes of echo signals particularly at known liquid levels close to the base of the tank. Thus, the data for the echo signal amplitude have been saved from previous occasions, when the tank has been empty, when it has been empty and clean, and when it has contained sediment with either low or high moisture content. The characteristic of the echo signal amplitude can thus be set for this known status in the tank and be saved in a memory. A rule framework for the implications of each characteristic is stored. Examples of the rules used are presented below. In a comparison block 20, the echo signal amplitude is registered during an ongoing measurement while emptying or tapping the tank. The echo signal amplitude in relation to the time elapsed for emptying and detected liquid level are registered and compared with previously recorded tapping characteristics of the tank, so that the bottom conditions can be identified based on the historical characteristics preserved in the memory.

After identification of the bottom conditions, the status is determined in the subsequent block, a status block 21, by interpretation of the relevant characteristic identified for the tank, whereupon the relevant characteristic according to the said rule framework is converted to a status for the tank, which according to the level measuring device is empty.

A set of rules are stored in the status block 21, where the rules refer to a number of previously implemented emptying events or cleaning events for the tank or tank type, applicable in the handling of storage tanks and marine tanks, where each case corresponds to a characteristic which is stored in the said memory. For each characteristic a rule is given which denotes the status of the tank or tank type as follows:

i) Typical discharge of carbohydrates and petroleum products from a tank with little or no permanent sediment at the bottom (see Fig. 3). This case is characterised by echo signal amplitudes from the liquid surface which are strongly influenced by a disturbance from an equally strong or stronger bottom echo when the medium surface level comes close to the base of the tank. Depending on the liquid's degree of transparency for microwaves, this disturbance can arise within a metre from the base of the tank. In this way, the echo signal originating from the tank base will be distinct and with increasing amplitude at a position just before the end of a liquid discharge from a tank, where a relatively low signal amplitude for the echo signal from the surface is present. The echo signal from the tank base must clearly penetrate the liquid awhile until the tank is dry, and the apparent base level must manifest itself by returning a strong and steady echo.

Rule A can be formulated: A transition (timeframe = hours) to a stable echo amplitude, stronger or weaker than the liquid surface echo, shall be interpreted as a status in which the tank is clean and dry, irrespective of whether there is any ambiguity about the level determination. ii) Typical discharge of carbohydrates, e.g. crude oil, from a tank with sediment remaining at the base (see Fig. 4). This case is characterised by a weak echo amplitude that is not affected by any disturbance from a bottom echo because of the damping layer at the bottom, when the liquid surface has reached a low level. Depending on the immediate characteristics of the sediment (thickness, composition, moisture content, etc.), the signal amplitude of the apparently detected level when the liquid has been pumped out of the tank can be anything from undetectable (fully damped) to very high (as a result of reflection from sediment due to its high water content).

Rule Bl : A transition to a stable lower echo amplitude or to a non-detectable echo shall be interpreted as a status in which the tank has sediment at the bottom.

Rule B2: A transition to an echo amplitude stronger than the echo from the liquid surface shall be carefully monitored during the time immediately following, in other words during the subsequent few minutes, since a sustained ongoing decrease in the echo signal amplitude at the same time as the apparent falling of the detected liquid level is a sign that the sediment at the base is drying out.

iii) Typical discharge of crude oil during the same time as tank cleaning (C. O. W. Crude oil washing). This case includes characteristics from both i) and ii) above, namely an initial situation with damping sediment at the tank base and consequently at the end of cleaning, a situation with a clean base.

Rule C: When cleaning is completed and the liquid has been pumped out of the tank, the echo signal signature (characteristic) shall be the same as in case i).

Claims

1. A method, for measuring by means of radar (3) the level of the medium surface (6) in a tank (1), in which a medium (2) is stored, and, on emptying the tank (1), for identifying the status of the empty tank, characterised in that the method comprises the steps:

- in a measuring duct, an electromagnetic signal is transmitted by an antenna (5) pointing down into the tank (1),

- an echo signal from the medium surface (6) is received by the radar (3),

- the level is calculated from a time delay between the transmitted signal and the echo signal, a number of characteristics of the change with time in the echo signal amplitude during emptying of the tank, based on historical and/or calculated data, are set,

- a registered curve of the echo signal amplitude during emptying of the tank is compared with the set characteristics, and - when the tank is essentially empty, the status for the empty tank is identified.

2. Method as in claim 1 , characterised in that the status for the empty tank is given according to a rule (A, Bl, B2, C) from a rule framework comprising at least the following predetermined rules: empty tank, clean tank, tank with sediment, tank with wet sediment, tank with dry sediment.

3. Method as in claim 2, characterised in that according to a first rule (A) a transition to a stable echo amplitude which is stronger or weaker than the liquid surface echo, shall be interpreted as a status where the tank is clean and dry.

4. Method as in claim 2, characterised in that according to a second rule (Bl) a transition to a stable lower echo amplitude or to a non-detectable echo shall be interpreted as a status in which the tank has sediment at the bottom.

5. Method as in claim 2, characterised in that according to a third rule (B2) a transition to a stable echo amplitude, stronger than the echo from the liquid surface, shall be interpreted as a status indicating that sediment at the base is drying out, if during the subsequent few minutes after the said transition, at the same time a continuous decrease in the echo signal amplitude and apparent falling of the detected liquid level can be identified. Method as in claim 2, characterised in that according to a fourth rule (C), and after cleaning of a tank, when the liquid has been pumped out of the tank, the characteristic of the echo signal shall be the same as in the first rule.

SUMMARY

A method is provided for measuring by means of radar (3) the level of the medium surface (6) in a tank (1), in which a medium (2) is stored, as well as for identifying the status of the empty tank, where the method entails that an electromagnetic signal is transmitted from the radar (3) fixed to the tank roof (4) via an antenna (5) pointing down into the tank, that an echo signal from the medium surface (6) is received by the radar (3), that the level of the medium surface is calculated from a time delay between the transmitted signal and the echo signal, that a number of characteristics of the change with time in the echo signal amplitude during emptying of the tank, based on historical and/or calculated data, are set, that a registered curve of the echo signal amplitude during emptying of the tank is compared with the set characteristics, and that when the tank is essentially empty the status for the empty tank is identified. (Fig. 2).