The present invention relates to a method for providing in a tube a sealing stopper in the form of an inflatable elastic bladder, wherein the bladder is inserted into the tube and there inflated with pressure gas into sealing engagement with the inner surface of the tube. The invention also relates to a bladder for carrying out the invention.

According to one known method, a rubber bladder is inserted into a tube and expanded with an internal overpressure, such as to be brought into sealing engage- ent with the inner surface of the tube and therewith form a sealing stopper. This stopper is held in position in the tube by the dry-friction obtained between the bladder and the inside surface of the tube. The force which strives to force-out the stopper corresponds to the pressure of the gas or liquid introduced into the tube when the tube is closed by the stopper. The co¬ efficient of friction is relatively low and must be compensated for, as far as is possible, by maintaining the inflating-gas in the bladder at a relatively high pressure. This pressure, however, is limited by the mechanical strength of the tube. Nevertheless, the frictional force can be increased by using a long stop¬ per or bladder. Stoppers of extreme lengths, however, are unwieldy and often difficult to insert into the tube.

EP, Al, 0 218 973 describes a tube-stoppering method in which an elastic bladder is inserted into a tube and filled with water, whereafter the water is frozen, such as to form an ice-plug which seals-off the tube.

According to one described embodiment of this known method, it took two hours to produce an ice-plug. One serious drawback with this method, however, is that in some cases there is considerable risk that the tube will be ruptured by the expanding ice-plug and burst, and the method i-s therefore restricted to use with tubes which are able to withstand the pressure exerted by the ice- plug.

A similar method is known from US-A-3 805 844. According to this method an elastic bladder is filled with water and the water is frozen to form a sealing and expanding ice-plug, which again involves the risk of the tube in which the plug is located bursting.

A further drawback with the known methods which rely upon the formation of an ice-plug is the relatively long time taken to melt the ice-plug, so as to enable the bladder to be removed from the tube upon completion of the repair work.

The object of the present invention is to provide a method which will enable a sealing stopper to be obtained without risk of the tube bursting as a result of an expanding ice-plug, and which will enable the stopper to be removed in a much shorter time than is possible with those methods in which the stopper is obtained with water which is first frozen to form an ice-plug and thereafter thawed out.

This object is achieved with a method according to the invention, which is characterised by the steps of provi¬ ding the bladder with a layer of freezable liquid, such as water, in a region around the periphery of the blad- der prior to inserting the bladder into the tube;

inserting the bladder into the tube and inflating the bladder into sealing abutment with the inside surface thereof; filling the bladder with a pressure liquid, such as a glycol mixture, while displacing and expelling the inflating gas; using a pressure liquid which has a lower freezing point than the freezable liquid; and cooling the freezable liquid to its freezing point such that the frozen liquid will form a sealing ring which is frozen sealingly to the tube and to the inflated bladder.

According to the invention, the bladder is first in- flated with the aid of a pressure gas, so as to fix the position of the bladder provisionally in the tube. This is necessary when the tube is vertical. Arranged peri¬ pherally around the outside of the bladder is a coating of liquid-absorbent material which is saturated with water prior to introducing the bladder into the tube. Glycol or someother liquid which has a freezing point lower than water is then introduced into the bladder. This liquid is then chilled to a temperature beneath the freezing point of water, so that the water in the absor- bent coating will freeze and form an effective seal.

Distinct from the known methods in which an ice-plug is formed within the bladder, the liquid located within the bladder when practising the present invention will always be in its liquid state. Consequently, the total time taken to establish and remove the stopper will be much shorter than the time taken when practicing the known methods, since all that is required in order to collapse the stopper and allow it to be removed from the tube, is to thaw out the relatively thin ice-annulus and empty the bladder of its pressure-liquid content.

When practicing the method according to the invention there is preferably used a bladder which has a valve- controlled inlet and outlet for a pressure gas and a pressure liquid respectively and which is characterised in that an external peripheral region of the bladder is provided with a layer of liquid-absorbent material, such as cellular rubber with open pores, such as to form a sealing ring, which layer is intended to be saturated with a freezable liquid, such as water, prior to intro- ducing the bladder into the tube concerned, whereafter the freezable liquid is frozen to form a sealing ring.

These and other characteristic features of the invention and advantages afforded thereby will be more apparent from the following description made with reference to the accompanying drawings, which illustrate an exemplifying embodiment of a bladder according to the invention.

Figure 1 illustrates schematically an inventive bladder inserted into a tube and connected to a pressure-gas container and to a pressure-liquid pump respectively, and Figure 2 is an enlarged view of an inventive bladder structure.

As will be seen from Figure 2, the bladder structure 10 has two walls and is inserted in a tube 11. The bladder comprises an inner bladder 12 and an outer bladder 14, the orifices of which are both connected to a connecting piece 16. The connecting piece has an inlet 18 and an outlet 20 with associated conduits 24, 26 for pressure liquid, which in the case of the illustrated embodiment is introduced into the inner bladder such as to extend

said bladder together with the outer bladder.

Pressure gas, such as air, is blown into the space between the inner and the outer bladders through a conduit 28 and a nipple 30, such as to inflate the outer bladder 14 in the tube 11 in a first method step, during which the inner bladder is empty and in a collapsed state, as indicated by the chain line in Figure 2.

The nipple 30 is connected to a compressed-gas bottle 32, via the conduit 28, which incorporates an over¬ pressure valve 34, which is connected to an expansion vessel 38 by a conduit 36.

In accordance with Figure 1, the pressure liquid, such as a glycol mixture, is circulated in a system which includes a pump 40 and a cooler 42 which contains, for instance, carbon dioxide snow or dry ice, so as to maintain the pressure liquid introduced into the inner bladder 12 through the conduit 24 at the desired low temperature, e.g. - 20°C. The conduit 26 is connected to the expansion vessel by a conduit 44.

As shown in Figure 2, the outer bladder 14 has provided on its outer surface a layer of liquid-absorbent material 38, such as an open-cell cellular rubber, which forms a liquid-absorbing elastic sealing ring 40.

When wishing to close-off the tube 11 by means of the inventive bladder structure, the liquid-absorbent seal¬ ing ring 40 is first saturated with a freezable liquid which has a higher freezing point than the pressure liquid.

Pressure gas is then introduced into the outer bladder 14 through the nipple 30, so as to inflate the bladder and clamp said bladder firmly against the inner surface of the tube. This is particularly important in the case of vertical tubes.

Chilled pressure liquid is now introduced into the inner bladder, so as to inflate the inner bladder and expel the gas contained by the outer bladder. Finally, the state illustrated in Figure 2 is reached, in which the liquid-filled inner bladder 12 lies sealingly against the outer bladder, which is now empty of gas. Sub¬ sequent to the pressure liquid having inflated the inner bladder and thereby pressing the outer bladder against the tube, the pressure liquid is maintained at a tem¬ perature at which the freezable liquid will freeze and form a solid sealing ring 40 which freezes firmly against the inner surface of the tube and the outer surface of the outer bladder 14.

Cold need not necessarily be transmitted via the pres¬ sure liquid and the tube can be chilled with the aid of dry ice, such as to freeze the liquid e.g. water, pre¬ sent in the radially inward sealing ring and form a solid sealing ring 40.