EP1516975A1

EP1516975A1 - Coverunit for covering a holder

Info

Publication number: EP1516975A1
Application number: EP04077557A
Authority: EP
Inventors: Aales Adrianus Antonie Oussoren
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-09-17
Filing date: 2004-09-15
Publication date: 2005-03-23
Also published as: NL1024311C2

Abstract

The present invention relates to a cover unit for covering a substantially circular holder for holding one or more solids, liquids and/or gases, wherein the cover unit comprises a frame which takes a substantially dome-shaped form and which is constructed from a number of rod-like structural members, a flexible cover layer for arranging over the frame, the edge portion of which cover layer can be folded back over the peripheral edge of the frame, and tensioning means engaging on the edge portion for tensioning the cover layer over the frame.

Description

The present invention relates to a cover unit for covering a substantially circular holder for holding one or more fluids and/or solids. The invention also relates to a method for manufacturing such a cover unit.
For the purpose of storing fluids (liquids and/or gases) or solids (such as for instance bulk goods) use is made of a wide variety of holders, such as liquid tanks or liquid containers, (manure) silos, reservoirs and storage systems generally. The embodiment of the holder may depend on the specific application, such as for instance manure storage, water treatment, fire prevention (sprinkler installation) and fermentation, on the quantity of fluid or solid for which the holder must provide space, on possible legal requirements laid down for the holder, etc.
The holders are often open on their top and can be covered so as to protect the internal space of the holder from outside influences. The cover may also have the function of protecting the outside environment from the release of unpleasant odours or the occurrence of other emissions adversely affecting the environment.
A variety of constructions is already known for covering such holders.
A first example of a cover construction is a floating deck wherein a floating cover layer is arranged on the liquid surface in the holder. Such a floating deck does however have a number of drawbacks. Firstly, it is relatively difficult to mix the liquid in the holder because of adhesion of the liquid, for instance manure, to the floating deck itself. A further drawback is that inspection or an operation under the floating deck is not possible, or only with difficulty. When gases are generated under the floating deck, for instance when the holder is used for the storage of liquid manure from which biogas is released, these accumulate against the underside of the cover layer. These gases cannot be readily recovered in such a construction and use of the biogas, for instance to generate energy, is hereby made more difficult.
Another construction is the so-called span cover. This is a conical tent construction consisting of an upright which is disposed in the middle of the holder and from which a cloth is suspended. The cloth is also fastened to the wall of the holder. Many tensioning straps run under the cloth from the middle to the wall. It is important in this construction that the wall can take up the forces from the tensioned cover toward the inside. For a relatively strong, for instance concrete holder wall, this need not be a problem. In the case of less strong holder walls, the transverse forces exerted by the cover on the wall are however too great, whereby damage to the wall can result. In the case of walls which are even less strong, for instance in the case of holders manufactured from sheet steel, the downward forces exerted by the cover on the holder are also too great. This may not only result in damage to the holder wall, but can also result in the cap being pulled loose under the influence of strong wind.
A further cover construction is a further development of the span cover. A wooden roof construction is herein arranged on an upright disposed in the middle of the holder. A cloth is tensioned over the wooden roof construction. A drawback of this type of cover is that only a limited span (a maximum of about 20 metres) is possible, that the span, as in the case of the span cover, is not column-free, and that the cover is relatively costly.
Yet another construction is the so-called segment cover consisting of a number of wooden trusses with roof covering on the top side of the trusses. The cover is per se form-retaining and a column need not be placed in the middle of the cover. However, the segment cap has a relatively high weight, is extremely expensive owing to the use of the wooden truss construction and moreover provides only a limited maximum span (diameter less than 16 metres).
There is also the further option of covering the holder with a flat concrete roof. The concrete roof is however rather expensive and requires a very heavy wall construction of the holder. The use of such a concrete cover is not possible with a holder of lighter form, for instance steel holders with little bending stiffness.
Finally, covers are known consisting of a frame against which a large number of form-retaining panels are attached. This cover is also very expensive and generally involves a relatively high weight. Nor therefore is this type of cover suitable, or at least less so, for application on holder constructions with relatively little bending stiffness.
It is an object of the present invention to provide a cover unit for covering such a holder in which the above stated drawbacks are obviated.
It is a further object of the invention to provide a cover unit which can be constructed at the site of the holder and placed on the holder in simple and rapid manner.
According to a first aspect of the invention, there is provided for this purpose a cover unit for covering a substantially circular holder for holding one or more fluids and/or solids, wherein the cover unit comprises:

a frame which takes a substantially dome-shaped form and which is constructed from a number of rod-like structural members;
a flexible cover layer for arranging over the frame, the edge portion of which cover layer can be folded back over the peripheral edge of the frame; and
tensioning means engaging on the edge portion for tensioning the cover layer over the frame.

Such a cover unit has a number of advantages. By erecting the support construction of the cover unit from a number of rod-like structural members the weight can remain relatively low. The separate structural members and the cover layer are moreover east to transport.
The tensioning means engage on the cover layer and thereby on the frame itself. No tensioning forces are exerted on the holder itself. In combination with the dome form of the frame, this ensures that the cover unit applies only downward forces on the holder wall and transverse forces remain absent. Because no transverse forces are exerted on the wall, the cover unit can also be applied in the case of less strong or rigid wall constructions. The cover unit can furthermore be realized with simple means, wherein all operations for constructing the cover unit and arranging thereof on the holder can basically be carried out at ground level. It has been found in practice that the cover unit can be manufactured and arranged quickly. A cover unit of average dimensions can for instance be assembled in two to three days by three to four people.
It is further a significant advantage that no intermediate support is necessary. The cover unit is therefore embodied structurally such that a free span can be realized in the holder. An intermediate support is not only perceived as a troublesome obstacle, but it also produces the danger of buckling. In the case of relatively large holders, an intermediate support moreover takes up a large amount of space during transport, which increases transport costs. Finally, an intermediate support can cause problems when the holder is also used to buffer gas. This aspect of the invention is explained later.
The outer ends of the rod-like structural members preferably meet in different nodes substantially in the plane of the dome form. The dome form can then be realized quickly and easily by supplying the structural members separately and coupling them together onsite in the intended nodes.
According to a preferred embodiment, the cover unit for covering the holder can be placed on an array of supports ordered in the form of a circle. These supports can be formed by a number of standing support elements extending in the form of a circle and placed inside the holder or outside thereof. The top side of the holder wall can however be applied equally well in many cases for this purpose (wherein the upper edge of the holder is seen as a number of supports placed side by side). The cover unit is arranged by placing the folded-back edge portion of the cover layer on said supports.
According to a preferred embodiment, the diameter of the dome form is greater than a diameter of the holder and, in the position of use, the folded-back edge portion of the cover layer rests on the top side of the holder wall. A good sealing between the cover unit and the holder wall is hereby achieved.
In a further preferred embodiment, the peripheral edge is situated lower in the position of use than the top side of the holder wall, so that the cover layer remains pressed on the holder wall as a result of the bias by the tensioning means. An even better, and preferably gastight, closure of the cover unit relative to the holder wall is hereby realized.
One of the preferred materials in which the frame is embodied is wood. In the case the material in the holder contains (aggressive) substances which can adversely affect the frame, hardwood or protected metal, such as stainless steel or aluminium, can be applied as material for the frame, or at least a part of the frame. In the case of non-aggressive or less aggressive substances it is possible to suffice with wood of lesser quality. In another preferred embodiment however, the frame is constructed from aluminium, which results in a very light construction.
The design has a low own weight through the use of a flexible cover layer. The cover layer is preferably manufactured from reinforced PVC cloth. The low weight is particularly important in the case of holder walls with very little bending stiffness, as already set forth above.
In a further preferred embodiment, the cover unit comprises half-trusses which extend radially outward from the centre of the dome form and which are formed in each case from at least two mutually coupled rod-like structural members. A greater or smaller number of structural members are successively placed depending on the intended dimensions of the cover unit. The construction of the half-trusses of a frame for a large span hereby corresponds to the construction of a frame for a relatively small span. This already results per se in a standardization of the applied structural members and of the method of manufacturing the frame.
According to a particular preferred embodiment, the cover unit comprises a number of connecting elements between the half-trusses, wherein a connecting element extends between the end of a first structural member of a first half-truss and the end of a first structural member of a second half-truss. The connecting elements hereby form "rings" in the frame. In those rings of the frame situated in the vicinity of the part of the frame which must rest on the supports there are however provided pull rod elements for the purpose of forming a connection under strain of tension between the half-trusses. In many cases the pull rod elements form the edge of the dome, though this is not essential. If the frame has dimensions such that an overhang is formed, the pull rod elements can be provided in a "ring" further to the inside.
In a particularly advantageous embodiment, the pull rod elements of the frame take a hollow form and tensioning means, preferably in the form of one or more tensioning cables 47, are arranged through pull rod elements for taking up the tensile forces in the pull rod elements. Because the tensioning means in this embodiment take up the tensile forces which occur, the pull rod elements can also be given a structurally lighter form. It is even possible to place the pull rod elements, in addition to the other elements of the frame, only under strain of pressure, which means that securing of the elements can be dispensed with.
Should the chosen structural members otherwise not provide the option of throughfeed of the tensioning means, the tensioning means can then be arranged on the outside of the ring of pull elements.
Said pull rod elements are preferably manufactured from steel, which does after all have a high maximum tensile load. Since the maximum tensile load is not an important factor in the connecting elements, other materials can also be used for these elements, such as plastic, aluminium or wood.
According to a further preferred embodiment, cables are arranged, should this be necessary, between the structural members. These cables are arranged in order to mutually connect the structural members and thus decrease the danger of the structural members buckling and/or limit the sag of the flexible cover layer.
In the case the content of the holder causes formation of gas, for instance in the case of fermenters (particularly biogas installations), manure silos and so on, a need has arisen to collect the released gas as well as the liquid. This gas can for instance be used to generate energy, for instance by using the gas as fuel for a generator and producing electric current therewith. In order to meet this need, solutions are known wherein it is assumed that the gas is collected in a gas buffer outside the holder.
Solutions are furthermore known wherein use is made of existing covering constructions, i.e. the above mentioned span covers. A flexible, gastight membrane is arranged under the conical tent cloth. The effective buffer capacity in the cone, in which a column is furthermore always present, is quite limited however, whereby external buffering usually remains necessary.
Solutions are further known wherein use is made of a very elastic membrane which, as the quantity of gas increases, is as it were inflated to a sphere. An entire support construction is however needed to prevent the sphere being blown over by the wind and/or to ensure that, despite precipitation accumulation, water continues to run off the roof construction in all conditions. Providing existing tanks with such a cover unit also remains a very expensive solution.
Finally, solutions are known wherein permanent overpressure is maintained internally in a spherical membrane by means of compressors. A second, inner membrane herein seeks equilibrium between pressure caused by the compressor and the gas pressure in the holder. Operationally and from a process engineering viewpoint, this solution has been found to be too complicated. In addition, it has been found that many of the above stated solutions are only suitable for a limited number of weather types and the associated load characteristics.
According to a further preferred embodiment of the invention, the cover unit therefore comprises a gas membrane fixed to the frame for collecting gas therewith. Owing to the fixed spherical shape of the frame and the absence of an intermediate support it is possible to store a maximal quantity of gas. The storage capacity of the gas is after all not limited by the presence of intermediate supports and the spherical space under the dome furthermore has a larger capacity than the conical form of the cover applied in many cases. This advantage is all the greater compared to the situation in which a flat cover, for instance a concrete roof, is applied.
According to a further preferred embodiment, the cover unit comprises a suspension unit for suspending the gas membrane from the frame, wherein the suspension unit is embodied for the purpose of limiting a lowest position of the gas membrane to a preadjusted level. In the lowest position the cover layer will in other words not be able to make any contact with the liquid surface. In the case for instance manure is arranged in the holder, there would otherwise be the danger of manure caking onto the membrane.
The gas membrane is preferably arranged on a suspension unit provided in the central area of the dome. At the side the membrane is preferably connected to the tensioning means, for instance the tensioning cable or the tube in which the tensioning cable is received. From this connection the gas membrane runs outward in the direction of the holder wall. Depending on the type of holder wall and the preference of the installer, the gas membrane then runs over the wall to the outside or extends downward as an apron on the inside of the wall. In the case the gas membrane hangs downward as an apron on the inside of a wall, the length of the gas membrane is made such that a natural water seal is created. As a result, the gas in the holder cannot escape. If desired, weights can be attached to the underside of the apron, preferably by incorporating these in the hem of the membrane, in order to ensure that the apron hangs in the liquid at all times. Owing to the development of the gas pressure, the apron will adjust to the greater or lesser degree of non-roundness of the holder.
According to another aspect of the invention, there is provided a method for arranging a cover unit, preferably the above described cover unit, wherein the method comprises the steps of:

coupling a number of rod-like structural members to form a dome-shaped frame;
arranging a flexible cover layer over the frame;
folding the edge portion of the cover layer back over the peripheral edge of the frame;
tensioning the cover layer on the frame by tightening tensioning means engaging on the edge portion;
placing the edge portion on the upper side of the holder wall.

In this way the cover unit can be manufactured and placed on the holder in quick and simple manner.
Further advantages, features and details of the present invention will be elucidated on the basis of the description of several preferred embodiments thereof. Reference is made in the description to the following figures.
Figure 1 shows a view in perspective of a manure silo provided with a cover according to a first embodiment of the invention;
Figure 2A shows a cross-section through the manure silo according to the first preferred embodiment wherein relatively little manure is present in the silo;
Figure 2B shows the same cross-section as that of figure 2A, but in a situation where more manure, and biogas caused by the manure, is present in the silo;
Figure 3 shows a detail section of the connection of the cover of figure 1 to the silo wall;
Figure 4 is a top view of the structure of a dome construction;
Figure 5 shows a cross-section at the position of a support at the connection of the dome to the silo wall;
Figures 6-10 show detail views of the diverse couplings between the different rod-like structural members; and
Figure 11 shows a cross-section at the position of the connection of a further preferred embodiment of the dome to the silo wall.
Figure 1 shows a manure silo 1 of circular cross-section. The walls of the manure silo have a flat top side in the shown embodiment. The material from which the silo is constructed can take many forms. In addition to concrete silos, which have a relatively great structural strength, silos are known which are manufactured from material with little bending stiffness, such as corrugated steel sheets. The walls are sometimes also constructed from plastic.
Heavy cover constructions can be used in the case of relatively strong concrete silos. However, when the silo walls are less strong, such heavy cover constructions can no longer be applied. Even if the cover construction has a relatively low weight, as for instance in the case of the above mentioned span covers, the use on silo walls with relatively little bending stiffness may be inadvisable. The span cover after all exerts not only downward force on the silo wall, but forces are also generated in transverse direction. A silo wall with little bending stiffness cannot withstand this and could deform. These drawbacks do not apply to the dome-shaped cover 2 according to a preferred embodiment of the invention shown in figure 1. This cover exerts only downward forces on the silo wall (at least when external forces, for instance as a result of wind load, are not taken into account, which external forces could cause a general shearing force in the plane of the top side of the holder) and moreover has relatively low weight. The construction of the cover will be described hereinbelow.
Figures 2A and 2B show a cross-section in more detail of the construction of cover 2. Silo 1 is constructed from a bottom 3 and a standing wall 4. Cover 2 consists of a number of rod-like structural members which meet in the middle in a central connecting member 22, which member is also referred to as the king post. The rod-like structural members form a dome-shaped frame 5 over which is tensioned a cover layer, for instance a PVC cloth of about 1 mm thickness. This is shown schematically in the figure. In practice the cloth will be arranged tightly round the frame and thereby follow the shape of the frame. It will not be a perfect spherical shape that is then discernible from the outside but rather a number of faces at different angles and tensioned by the different structural members. Tensioning takes place as follows. Cloth 6 is first arranged over frame 5. The excess of cloth 6 is folded back round edge elements 30 of frame 5 and subsequently tensioned on the inside using a tensioning cable 7. As a consequence of this tension, covering cloth 6 is pulled taut over the frame.
This construction is shown in more detail in figure 3. Figure 3 shows that covering cloth 6 is folded back round edge element 30 of frame 5. The folded-back edge portion 6' of cloth 6 is arranged on a tensioning cable 7 enclosed by a protective tube 25. Edge portion 6' is herein fastened by guiding the tensioning cable 7 through a hem 26 provided in the outer end of edge portion 6'. Once the tensioning cable 7 has been arranged in the peripheral hem 26 of edge portion 6' of cloth 6, it is tightened at one or more positions so that cloth 6 is pulled taut over frame 5.
In the pulled-taut position of covering cloth 6 the cover 2 is easily placed on the top side of silo wall 4, this such that cover 2 comes to rest, with edge portion 6' of covering cloth 6, on the top side of silo wall 4. Due to downward movement the edge element 30 of frame 5 will herein come to lie below the level of the top side of silo wall 4. A good sealing between covering cloth 6 (or in any case cover 2) and silo wall 4 is realized in this manner.
A condensation collecting layer 27 is further provided to collect possible condensed water inside the silo. The arranging of a condensation collecting layer is important if it is undesirable for this condensation, via the slope of the roof, to accumulate above the wall or drip downward along the surface on the inside or outside of the wall. This is particularly undesirable when this condensation may be aggressive to the wall. It is the intention that the moisture drops downward before reaching the wall. This is possible in at least two ways, by welding a strip against the underside of the cloth within the radius of the wall, or by folding back a strip of cloth inward from the edge tube over the tensioning unit.
Although cover 2 is laid on silo wall 4 by means of edge portion 6' in the manner shown in figure 3, there are provided a number of supports placed at equal mutual distances along the periphery of the silo wall. Figure 5 shows in cross-section a preferred embodiment of such a support 31. The support is fixed to frame 5 and has a flat underside 32. The flat underside 32 of the support is placed on the flat upper side of silo wall 4. After placing, support 31 is anchored to wall 4 by means of an anchoring element 33, which is fastened to the silo wall using for instance screws 34.
The flat underside 32 of support 31 has a relatively large width b. This has the function of being able to compensate for possible non-roundness of silo 1. Support 31 can therefore be applied within relatively large margins of non-roundness of the holder, which is advantageous in practice, and then particularly when a cover is arranged on an already existing holder. In existing silos the silo wall 4 can often no longer be described as wholly round in top elevation. Cover 2 does not however have to be made specially to size, as long as the non-roundness of the silo remains within the margin defined by the width b of the underside 32 of support 31. The dimension b is determined by the maximum tolerance associated with the holder in question.
An advantageous embodiment of the dome construction is shown in the view of figure 4 and the related cross-sections of figures 6-9. Figure 4 shows with full lines a framework with two "rings" for a dome of a determined diameter. The extension in the case of a framework with three "rings" for a dome of a larger diameter is indicated here with broken lines. The dome is constructed from the following main components:

A "peripheral" (in top view polygonal) ring or pull strap, mainly under strain of tension, of structural members in the plane of the points of support. This pull strap is formed by a number of straight structural rods (R6 and R7 in the two-ring dome and R13, R14 and R15 in the three-ring dome) which take up the "horizontal forces" (also referred to as lateral forces) from the spherical framework system.
A number of half-trusses which extend from the centre 22 and which consist of one structural rod (E1 for a one-ring dome) or a plurality of structural rods (E1,E3,E8...). The outer ends of the half-trusses are mutually connected by means of the above mentioned pull strap. As seen from above, the half-trusses each run in a straight line, from the centre 22 of the dome, radially outward to the corner points of the pull strap. If in detail implementation the angle between the relevant (half-truss) element and a connecting distributing rod element (e.g. (9)) were to become too small for practical reasons, the relevant (half-truss) element and associated distributing rod element(s) is then replaced by two spreading elements.
Where applicable, a number of intermediate rings will further be provided between the half-trusses by means of rod elements. The first intermediate ring or intermediate pull strap consists of rod elements (E2) which run (as seen from the centre 22) from the end of the first rod element (E1) of the half-truss to the same point on the adjacent truss. In the case of a dome with three or more rings, the second intermediate ring or second intermediate pull strap consists of rod elements (E6,E7) which - at the position of the second angular point on the half-truss - each connect to one of the sides of the half-truss. This results in the ring with twice as many corner points or connecting points as the first intermediate ring.

Further provided are distributing rods (E4,E5 respectively E9, E10, E11, E12) which run from the nodes on the first intermediate ring to the intermediate nodes on the second ring, and from the second ring to the nodes on the following ring.
An optional third intermediate ring (not shown in figure 4) or pull strap consists of three times as many rod elements as the number of half-trusses applied. The optional fourth intermediate ring or pull strap consists of the same number of rod elements as the third intermediate ring.
The rod elements - not being elements in the pull strap - are characterized by being mainly under strain of pressure and bending.
The building sequence for constructing a two-ring dome is as follows. Elements R6 and R7 are first fixed to each other. Elements E3,E4 and E5 are then attached, whereafter it is the turn of elements E2 to be attached. Elements E1 are the last to be attached. A suitable dome construction can hereby be built in rapid and efficient manner and with a minimum of manpower.
For the construction of the three-ring dome (shown partly in broken lines in figure 4), the rod elements of the peripheral edge of the dome (rod elements R6 and R7) are herein replaced by ordinary connecting rods. The construction sequence is now as follows. Edge elements R13, R14, and if applicable R15, are first coupled to each other. Elements E8, E9, E10, E11, and if applicable E12, are then arranged, whereafter it is the turn of elements E6 and E7. From this point the construction sequence corresponds to that of the two-ring dome.
Figure 6 shows a detail of the connection of edge elements R6 and R7 to structural member E3. The edge elements are embodied here as round tubes, the outer ends 34,35 thereof being flattened. The flattened ends of the tubes are coupled to each other with a strip 36, for instance by fastening the strip to the ends using a number of bolt connections.
In similar manner figure 7 shows a detail view of the connection of edge elements R6 and R7 to two structural members E4 and E5, which connection corresponds to that of edge elements R14 and R15 to structural members E9,E10 or E11,E12. The outer ends 34 and 35 are here coupled to each other by a strip 37.
Figure 8 shows a detail view of king post 22. The king post is arranged in the middle of the dome and functions as connecting element for all rod elements E1. A further function of the king post is to provide an optional opening in the middle of cover layer 6, which creates the possibility of gaining access to the interior of the silo.
Figure 8 shows an embodiment in which the structural members E1 all meet at one point.
Figure 9 shows an alternative embodiment in which king post 22 fulfils the same function of receiving structural members E1, wherein an opening 29 is provided in the middle of king post 22. Covering cloth 6 is herein arranged on a flange 48, wherein an opening is provided in the middle which corresponds to opening 29 in the king post. This provides the option of gaining access to the interior of the silo from above.
Figure 10 shows an embodiment in which the structural members are formed by round tubes 43, 44 and 45. These are mutually connected using a connecting piece 38 which is made from a tube 41 with a diameter smaller than that of tube 43 and 44 and a tube 40 with a diameter smaller than tube 45. Tubes 40 and 41 are mutually connected using a strip 39. A portion 42 of tube 41 is pushed into tube 43. In similar manner a portion of tube 41 is pushed into tube 44. Structural members 43-45 can be attached to each other in simple and rapid manner using connecting piece 38. When structural members 43-45 are furthermore only under strain of pressure, a separate securing can be dispensed with. Also in the case that structural members 43-44 are pull rods, it is possible to apply the shown connecting piece if there is arranged through the interior of structural members 43-44 a tensioning cable which can take up the tensile forces which occur.
A further aspect of the invention relates to the structural possibility of collecting a fluid such as manure gas G above another fluid and/or solid such as liquid manure V. For this purpose a cord 21 is fastened to king post 22, on which cord there is arranged a gastight membrane 20. This membrane hangs downward from cord 21 and is fastened along the wall to tensioning cable 7 or at least the tube 25 arranged therearound. Gas membrane 20 runs herefrom in the direction of silo wall 4. Depending on the type of silo wall and the preference of the installer, gas membrane 20 then runs first over the top side of wall 4 to the outside or gas membrane 20 runs downward as apron 20' on the inner side of silo wall 4. In the case the cloth 20 hangs downward as apron 20' on the inner side of the wall, the length of the apron will be such that a natural water seal is created through which the gas cannot escape. In the embodiment shown in figures 2A and 2B a number of weights 23 are furthermore arranged on the underside of apron 20' in order to ensure that the apron is always pulled tautly downward, and the underside thereof is at all times below the lowest liquid level and will always extend below the lowest possible liquid level in the silo.
Figure 2A shows the situation in which relatively little liquid V and little gas G is present in the silo. When the amount of liquid V and/or the amount of gas G increases, this situation being shown in figure 2B, cloth 20 will bulge out. The cloth will here ultimately follow the form of the underside of the dome, so that optimum use can be made of the storage capacity under the dome shape.
In the situation shown in figure 2A, in which there is little liquid and gas present in the holder, the length of cord 21 is chosen such that the gastight cloth 20 cannot contact the surface of the liquid V in any way whatever. This prevents possible caking of liquid (manure) on cloth 20, in which case the quality of the cloth can be adversely affected.
Figure 2A also shows that in a further preferred embodiment an insulating layer 46 is arranged against the underside of the frame. During fermenting processes it is for instance important that the mass in the holder remains at a constant temperature, which is almost always above ambient temperature. For optimum energy efficiency it must be necessary to add as little heat as possible. The arranging of an insulating layer against the relatively large roof surface is desirable in these and other cases.
In the embodiment shown in figure 11 the covering cloth 6 is folded back round a rotatable edge element 49 of frame 5. In similar manner as described above with reference to figure 3, the folded-back edge portion 6' of cloth 6 is arranged on a tensioning cable 7 enclosed by a protective tube 25. Once tensioning cable 7 has been arranged in the peripheral hem 26 of edge portion 6', it is tightened at one or more locations so that cloth 6 is pulled taut over frame 5. In this embodiment however, cloth 6 is secured mechanically to edge element 49 by providing the edge element with one or more protrusions 50 which can be guided into corresponding openings in cloth 6. This creates the possibility during tightening, or later, of rotating edge element 49 further (in direction P) or less far on its axis, so that cloth 6 (re)gains tension. In other words, the cloth can also be tensioned by fixing the membrane in the lengthwise direction of the edge element and by then rotating the edge element on its longitudinal axis. Edge element 49 is then fixed at the end of the rod at the position of the connection to other rods, so that the cloth retains the desired tension.
The invention is not limited to one or more of the above stated preferred embodiments, but is defined by the content of the following claims, within the scope of which many modifications can be envisaged.

Claims

Cover unit for covering a substantially circular holder for holding one or more fluids and/or solids,
wherein the cover unit comprises:

a frame which takes a substantially dome-shaped form and which is constructed from a number of rod-like structural members;

a flexible cover layer for arranging over the frame, the edge portion of which cover layer can be folded back over the peripheral edge of the frame; and

tensioning means engaging on the edge portion for tensioning the cover layer over the frame.
Cover unit as claimed in claim 1, which can be arranged on an array of supports ordered in a circle form, preferably formed by a number of standing support elements and/or by the holder wall, wherein the folded-back edge portion of the cover layer rests on the supports.
Cover unit as claimed in claim 1 or 2, wherein during use the peripheral edge of the dome is situated lower than the top side of the holder wall, so that the cover layer remains pressed on the holder wall as a result of the bias by the tensioning means.
Cover unit as claimed in claim 1, 2 or 3, wherein the outer ends of the rod-like structural members meet in different nodes substantially in the plane of the dome form.
Cover unit as claimed in any of the foregoing claims, wherein the cover unit is embodied to provide a free span of the holder.
Cover unit as claimed in any of the foregoing claims, wherein the rod-like structural members are arranged in each case in triangular relation.
Cover unit as claimed in any of the foregoing claims, comprising half-trusses which extend radially outward from the centre of the dome form and which are formed in each case from at least two mutually coupled rod-like structural members.
Cover unit as claimed in claim 7, comprising pull rod elements arranged between the half-trusses for the purpose of forming a connection under strain of tension between the half-trusses.
Cover unit as claimed in claim 8, wherein the pull rod elements are arranged in the immediate vicinity of that part of the frame which rests on the supports during use.
Cover unit as claimed in any of the foregoing claims, wherein the pull rod elements of the frame are hollow and wherein tensioning means, preferably a tensioning cable, are arranged through the pull rod elements for taking up the tensile forces in the pull rod elements.
Cover unit as claimed in any of the foregoing claims, wherein cables are arranged between structural members.
Cover unit as claimed in any of the foregoing claims, comprising a number of connecting elements between the half-trusses, wherein a connecting element extends between the end of a first structural member of a first half-truss and the end of a first structural member of a second half-truss.
Cover unit as claimed in claim 12, wherein the connecting element is embodied mainly for a tensile load.
Cover unit as claimed in any of the foregoing claims, wherein a row of connecting elements extending between first structural members forms a first ring and a row of connecting elements extending between second structural members forms a second ring, and wherein distributing rod elements are arranged between the first and second ring.
Cover unit preferably as claimed in any of the foregoing claims, for covering a substantially circular holder for holding one or more fluids and/or solids, wherein the cover unit comprises:

a covering of the holder;

a gas membrane fixed under the covering for collecting gas thereunder.
Cover unit as claimed in claim 15, comprising a suspension unit for suspending the gas membrane from the frame, wherein the suspension unit is embodied for the purpose of limiting the lowest position of the gas membrane to a preadjusted level.
Cover unit as claimed in claim 16, wherein the suspension unit is provided in the central area of the dome.
Cover unit as claimed in claim 16 or 17, wherein the suspension unit is a flexible cord and the length of the cord is adjusted to hold the gas membrane in the tensioned state as low as possible above the level of the fluid or solid.
Cover unit as claimed in any of the claims 16-18, wherein the cover unit is fixed to the tensioning means of the cover layer.
Cover unit as claimed in any of the claims 16-19, wherein the gas membrane is guided to a position over the holder wall and preferably hangs downward as an apron on the inside of the holder wall.
Cover unit as claimed in any of the foregoing claims, wherein the peripheral edge of the frame is provided with a number of support elements, which are provided with a flat underside for placing on the top side of the holder wall, wherein the flat underside preferably has sufficiently large dimensions to be able to compensate for non-roundness of the holder cross-section.
Cover unit as claimed in any of the foregoing claims, wherein an insulating layer is arranged under the frame.
Cover unit as claimed in any of the foregoing claims, wherein one or more of the structural members forming the peripheral edge of the frame is embodied rotatably for the purpose of increasing or decreasing the tension in the cover layer.
Method for arranging a cover unit, preferably a cover unit as claimed in any of the foregoing claims, onto a holder for holding one or more fluids and/or solids, wherein the holder has a substantially circular cross-section, the method comprising the steps of:

coupling a number of rod-like structural members to form a dome-shaped frame;

arranging a flexible cover layer over the frame;

folding the edge portion of the cover layer back over the peripheral edge of the frame;

tensioning the cover layer on the frame by tightening tensioning means engaging on the edge portion;

placing the edge portion on the upper side of the holder wall.