Storage tank for a liquid

DESCRIPTION

The invention relates to a storage tank for a liquid, which comprises a bottom and upright walls and which can be closed by means of a cover element that floats on the liquid, which cover element substantially abuts against the upright walls with its circumferential edge, whilst determining means are present for determining the liquid level in the tank.

The storage tank according to the introduction above is frequently used in particular in the process industry or in oil refineries for the storage of water or a petroleum product, such as oil or petrol. The determining means are used for determining the level of the liquid in the storage tank in an effective manner in order to thus get a perception of the amount of liquid that is still present in the storage tank. In the case of volatile and combustible petroleum products, in particular petrol, a cover element will be used in the storage tank, which cover element floats on the liquid surface and thus screens the volatile and/or combustible liquid. Since most storage tanks are cylindrical in shape, the cover element is usually circular in shape as well, its dimensions being such that the circumferential edge closely abuts against the inner side of the upright walls of the storage tank.

Although the use of a cover element on a volatile and/or combustible liquid, such as petrol, provides an effective screening, mechanical imperfections may lead to the cover element getting wedged between the upright walls of the storage tank, or the increased weight from excessive rainfall may result in the cover element moving below the liquid surface and subsequently sinking to the bottom of the storage tank as a result of the slanted position or tilting of the cover element.

The object of the invention is to overcome the above drawbacks and provide a device according to the introduction by means of which it can also be determined whether the cover element is positioned at the same level as the liquid surface in the tank and thus covers the liquid in a correct manner. In order to accomplish that object, the storage tank is according to the invention furthermore provided with detection means, which detect the position of the cover element in the tank.

In one embodiment the detection means are arranged for comparing

the detected position of the cover element with the liquid level determined by the determining means, whilst the detection means are further arranged for delivering a warning signal on the basis of the comparison between the detected position of the cover element in the tank and the liquid level determined by the determining means. This makes it possible to establish in a quick and adequate manner if and how the position of cover element in the tank is different from the liquid level therebelow.

In a functional embodiment, the detection means comprise at least one position sensor disposed above the cover element, which position sensor detects the position of the cover element on the basis of a detection signal.

In order to enable an accurate determination of the position of the cover element in the storage tank, said at least one position sensor is disposed near the geometric centre of the cover element.

A more precise determination of the position of the cover element in the storage tank, by which it can also be determined whether the cover element takes up a slanted or inclined position relative to the liquid surface can be realised in that the detection means used in such a functional embodiment comprise at least three position sensors disposed above the cover element.

By disposing the three position sensors at positions spaced 120° from each other above the cover element, an accurate three-point measurement can be carried out, on the basis of which the position and the orientation of the cover element in the storage can be unequivocally determined.

The precision of the three-point of measurement can be further enhanced in that according to the invention the detection means are arranged for comparing the positions of the cover element in the tank as detected by the various position sensors with each other and delivering a further warning signal on the basis of said comparison.

In a functional embodiment, the detection means detect the position of the cover element in the tank on the basis of the emitted and reflected radar waves, wherein said at least one position sensor is embodied as a radar antenna for emitting radar waves to the cover element and receiving radar waves reflected by the cover element.

By making use of a contactless position determination based on reflected radar signals, a quick and above all accurate measurement can be obtained for determining the position and orientation of the cover element in the

storage tank.

To realise an improved contactless position determination, the cover element is provided, near each position sensor, with a material that reflects the detection signal.

The invention will now be explained in more detail with reference to a drawing, in which:

Figure 1 shows a first embodiment of a device according to the invention;

Figure 2 shows a plan view of figure 1.

For a better understanding of the invention, identical parts will be indicated by the same numerals in the description of the figures below.

Figure 1 schematically shows an embodiment of a storage tank according to the invention. The storage tank 10 is built up of upright walls 11 , which are provided with an upper edge 13, and a bottom 12, and is filled with a liquid 1 , usually a petroleum product such as petrol or the like.

Because of the volatile and combustible nature of the liquid 1 in the storage tank 10, the liquid 1 is covered by a cover element 15, which to that end floats on the liquid surface 1a. The cover element 15 is usually embodied as a rigid element, which substantially abuts against the inner side of the upright wall 11 with its circumferential edge. The latter is made possible in particular in that such storage tanks are cylindrical in shape, which makes it possible to use circular cover elements.

The height of the liquid level relative to the bottom 12 is indicated by the letter H. To get a perception of the height H of the liquid level of the liquid 1 in the storage tank 10, every standard storage tank 10 is provided with determining means 14, which are arranged for determining said liquid level H. Such determining means are commonly known and may generally make use of a standpipe 14a, in which the height H of the liquid column collected in the standpipe is measured by means of suitable measuring methods. One measuring method is to make use of radar waves, but it is also possible to make use of a servo float construction.

As already set forth in the introduction, there is a chance that the cover element 15 will get wedged mechanically between the upright walls 11 , so that it can no longer follow the falling and/or rising of the level of the liquid 1 in the storage tank 10. This may lead to undesirable operating conditions, whilst on the

other hand the cover element may take up an inclined position, for example under the influence of the weight of the collected rainwater after heavy rainfall, and become partially submerged in the liquid 1 , in an extreme case even sinking into the liquid 1 at an angle in the direction of the bottom 12.

It is furthermore noted in this connection that the cover element 15 floats freely on the liquid 1 and that sparking may occur in the unlikely event of the cover element 15 getting wedged between the upright walls 11 and suddenly coming loose again at a later stage, which is undesirable in an environment in which a combustible, volatile liquid is present.

To get a perception of the position and the orientation of the cover element 15 in the storage tank 10, the storage tank is according to the invention provided with additional detection means 20, which are arranged for detecting the actual position of the cover element 15 in the storage tank. In this way it is possible to determine in an effective manner whether the cover element 15 is positioned at the same height as the liquid level H of the liquid 1. Furthermore, the detection means 20 may be arranged for comparing the detected position of the cover element 15 in the storage tank 10 with the liquid level H determined by the determining means 14 (by means of the standby 14a).

If a difference is found to exist between the position of the cover element 15 (as detected by the detection means 20) and the liquid level H (as determined by the determining means 14), a warning signal may be delivered in an adequate manner. On the one hand said warning signal may be generated by the detection means 20 and be delivered to the operating staff of the storage tank 10, or on the other hand the two measured values as generated by the detection means 20 and the determining means 14 may be forwarded to a control room, via means yet to be discussed hereinafter, where the values are compared with each other and further action is taken in case of a difference being detected.

To obtain a precise indication of the position of the cover element 15 in the storage tank 10, the detection means 20 comprise at least one position sensor 21 a, whilst three position sensors 21a-21 b-21c are used in the embodiment shown in figures 1 and 2. The position sensor 21 a or, in the case of a multi-sensor embodiment, the position sensors 21a-21c, are mounted to the upper edge 13 of the upright wall of 11 , being oriented above the cover element 15.

In one embodiment (not shown), the detection means 20 comprise

one position sensor 21 a, by means of which the actual position of the cover element 15 in the storage tank 10 can be detected. More specifically, said single position sensor 21a is positioned precisely above the geometric centre of the (circular) cover element 15. By comparing the detected position of the cover element 15 with the level H of the liquid surface 1a as determined by the determining means 14, it is possible to determine whether the cover element 15 is still floating on the liquid surface 1a or whether there is a difference between the position of the cover element and the liquid level H.

In the latter case a situation may occur in which the cover element 15 is wedged between the upright walls 11 and that an air chamber has formed between the liquid surface 1a and the cover element 15 as a result of the liquid surface 1 a falling. Especially in the case of combustible and volatile liquids 1 (such as petrol), an explosive gas mixture may thus form between the liquid surface 1a and the wedged cover element 15, which leads to dangerous operating conditions. The jammed cover element 15 may suddenly come loose from the upright wall 11 , for example under the influence of its own weight, in which situation sparks may form. The sudden coming loose of a jammed cover element may also be caused by heavy rainfall, when the weight of the rainwater collected on the cover element 15 causes the cover element 15 to come loose from the upright wall 11 all of a sudden.

In the latter case the cover element 15 may take up an inclined position and move partially below the liquid level 1a, possibly sinking into the liquid 1 in the direction of the bottom 2.

Submersion of the cover element 15, which may or may not be jammed, as a result of a rising liquid surface 1a of the liquid 1 may also lead to undesirable operating conditions, because the liquid 1 is now in open communication with the outside atmosphere, which is of course undesirable in the case of combustible and volatile liquids. In this case, too, the submerged cover element 15 may sink towards the bottom 12 under the influence of the weight of the liquid 1 present thereon.

A difference between the position of the cover element and the actual level H of the liquid surface 1a, with the cover element 15 being positioned either above or below the liquid surface number 1 a, can be detected with suitable equipment and may result in a warning signal. The comparison may be carried out by the detection means 20, which are to that end provided with a control unit 23. On

the other hand, said control unit 23 forming part of the detection means 20 may be disposed elsewhere, i.e. not on the storage tank, for example in a control room. Said control room may be located some distance away from the tank, where the signals delivered by the detection means 20 (position of the cover element) and by the determining means 14 (height of the liquid level) are received, processed and compared with each other, on the basis of which a suitable warning signal is generated and delivered to the operating staff.

In figure 2 a functional embodiment of the storage tank according to the invention is shown, in which the detection means 20 comprise several position sensors, in this embodiment three position sensors 21a-21c, which are mounted to the upper edge 13 of the upright wall 11 at different positions and which are disposed above the cover element 15. The various position sensors 21a-21 c are oriented in such a manner relative to each other that they are spaced about 120° from each other.

In this way a so-called three-point measurement can be carried out, in which each position sensor 21a-21 c locally determines the position of the cover element 15 with respect to the tank 10. Each position sensor 21a-21c may to that end be provided with electronics 22a-22c, which receive and process the detection signal generated by the position sensor 21a-21c for the purpose of comparing it to the liquid level determined by the determining means 14.

The three-point measurement by means of the three different position sensors 21a-21c makes it possible to determine whether the cover element 15 takes up an inclined or "slanted" position between the upright walls 11. Each position sensor 21a-21 generates a local position signal of the cover element 15, and because the detection means 20 are according to the invention further arranged for comparing said local measuring signals with each other, any inclination or slanting of the cover element between the upright walls 11 will be easy to detect.

If the various local measuring signals delivered by the various position sensors 21a-21 a do not exhibit any deviations within a predetermined bandwidth, it may be concluded that the cover element 15 takes up a horizontal position between the upright walls 11. By making a comparison according to the invention with the liquid level H determined by the determining means 14, it can be easily established whether the cover element 15 is floating on the liquid surface 1a (the detected deviation is minimal) or whether, if a deviation has been detected, the

cover element 15 is positioned far above or far below the determined liquid level H.

Both in the case of an inclined position of the cover element 15 detected by means of the various local measuring signals from the position sensors 21a-21c and in the case of a detected difference between the position of the cover element 15 and the determined liquid level H, suitable warning signals may be delivered to the operating staff of the storage tank 10, for example by means of the control unit 23.

To enable a suitable signal processing, each position sensor 21a- 21c is provided with control electronics 22a-22c, which are interconnected by means of suitable signal lines 24 and which are furthermore connected to a control unit 23, which likewise receives signals from the determining means 14. The mutual comparison of the measuring signals delivered by the various position sensors 21a- 21c (for detecting any slanting) and/or the comparison between the position of the cover element 15 detected by the detection means 20 and the liquid level H determined by the determining means 14 may also be carried out by the control unit 23 or by suitable control means located elsewhere, for example in a control room of the storage tank 10.

In a simplified construction, said electronics 22a-22c may also be incorporated in the control unit 23 for safety reasons (fire/explosion hazard), which control unit 23 may in turn also be disposed elsewhere, i.e. not on the storage tank but, for example, in a control room, and which receives and processes the measuring signals both from the various position sensors 21a-21 c and from the determining means 14. The control unit 23 can compare the various position signals delivered by the position sensors 21a-21c with each other so as to get a perception of the orientation of the cover element in the storage tank (flat or inclined/slanted). The control unit 23 can also compare the various position signals regarding the position of the cover element 15 with the liquid level H as determined by the determining means 14. The control unit 23 can deliver suitable warning signals in the case of deviations for both comparisons (between the position sensors mutually or between the position sensors and the determining means).

The position sensors 21a-21c preferably make use of radar waves for determining the position of the cover element 15 in the storage tank 10. To that end each position sensor 21a-21c possesses a radar antenna directed towards the cover element 15, which radar antenna is arranged for emitting radar waves towards

the cover element 15 and for receiving radar waves reflected by the cover element 15. The position of the cover element relative to the radar antenna of the position sensor 21a-21c can then be precisely determined on the basis of the difference in delay between the emitted radar waves and the reflected radar waves. To improve the position determination by the radar antenna, the cover element may be provided, near each position sensor 21 a-21c, with a suitable material 25 that effectively reflects the emitted radar signals to the radar antenna.

It will be understood that in this way an accurate determination of the position of the cover element 15 in the storage tank 10 can be realised and that thus any differences between the position of the cover element 15 and the actual liquid level H can be detected in an adequate manner and suitable measures can be taken in response thereto.