This invention relates to a method and apparatus for internally lining a duct, particularly an underground water supply conduit or isewer line. More particularly, the invention relates to a method and apparatus of the kind defined in the preambles of the independent claims. In this context, the term "duct" is to be construed broadly and thus is intended to encompass any duct in the shape of a pipe, conduit, tunnel or shaft. Accordingly, the invention is concerned with the technique for internally lining ducts which comprises introducing into the duct a tubular liner made of glass fibre cloth, for example, which is impregnated or coated with a curable plastics material and then pressing the liner against the internal wall of the duct by means of a pressurized bladder until the plastics material has cured.

It is known in connection with such lining technique to apply heat to the liner and to the plastics material in order to accelerate the curing. A short curing time - in actual practice meaning some ten hours or shorter - is desired, i.a. for the reason that it makes it possible during one and the same working day to commence and complete the lining of a duct or a section of a duct.

In accordance with a known method of applying heat to the liner and the plastics material (see, for example,

SE-B-435 866) , an electrical heater is introduced into the duct together with the l:.ner and the press bladder con¬ tained in the liner. After the press bladder has been pressurized and thus expanded, the heater is slowly moved from one end of the duct to the other.

It is also known to apply the heat by causing hot air or hot water to flow through the pressurized press bladder.

However, the known techniques and apparatus for applying heat to the liner and the plastics material are unsatisfactory in various respects, e.g. because they require complex and expensive equipment and/or consume much energy.

The invention aims at providing an improved method and improved apparatus for applying heat to the liner and the curable material.

According to the invention there is provided a method characterised by applying the heat for accelerating the curing from an inner conduit located in and extending along the press bladder from one end thereof to the other, the diameter of which conduit is substantially smaller than that of the duct, hot water being conveyed through the inner conduit.

The inner conduit, the diameter of which advanta¬ geously is 0.1-0.25 times the diameter of the duct, thus serves as a heater extending throughout the length of the duct or the duct section being lined. By way of the inner conduit heat is supplied to the liner and the plastics material from the flowinej hot water.

If the duct is substantially horizontal, particularly if it is an underground pipe line, the inner conduit advantageously is arranged such that it rests, throughout its length, on the inside of the lowermost portion of the press bladder, that is, as close as possible to the lowermost portion of the liner and the duct. Heat will then be transferred particularly efficiently to the lowermost portion of the liner and the plastics material so that first and foremost this portion of the liner is caused to set. The danger of the liner becoming inadver¬ tently indented by water finding its way from the sur¬ rounding ground between the liner and the inside of the duct is greatest at the lowermost portion, and for that reason it is particularly important for the plastics material to become properly cured there.

The heat transfer to the other portions of the liner and the plastics material takes place by way of the air filling the space between the press bladder and the inner conduit. This heat transfer probably is promoted by some movement of the air. Such movement takes place more or less by itself, if not for other reasons because in actual practice a certain amount of air inevitably bleeds from the press bladder and has to be replaced by new air. It is

within the scope of the invention, however, to take steps to ensure circulation of air within the press bladder using means suited to the purpose.

Surprisingly, the water temperature need not be particularly high in order that sufficient supply of heat may be achieved with an inner conduit diameter within the above-indicated range; in practice a water temperature of 45-60° C (as measured at the upstream end) has been found to be sufficient with a water flow rate of 15 1/minute, an internal diameter of the press bladder of 300-375 mm and an external diameter of the inner conduit of 50-75 mm. The just-mentioned figures, which are naturally only given by way of example, relate to an underground water supply pipe line. In certain circumstances, particularly if the water heating capacity is low, it is advantageous to cause the hot water to circulate in a closed path comprising the inner conduit and a water heater disposed outside the duct. Suitably, the inner conduit is introduced into the duct simultaneously with the tubular liner impregnated or coated with the plastics material and the press bladder disposed within the liner. In practising the method in such case, a pull cable is connected to the band-like, flexible assembly made up by the tubular liner, which may be a "sleeve" of glass fibre fabric, and the press bladder, which advantageously may be a thin-walled hose or other thin, air-tight layer of a material having some resiliency, and the inner conduit disposed within the press bladder, whereupon the assembly is pulled into the duct from one end thereof by means of the pull cable. The introduction thus is carried out basically in the way the liner and the press bladder are introduced according to the technique used previously; the only difference is that the press bladder also contains the inner conduit.

The device according to the invention is characterised in that the heater comprises an inner conduit extending along the press bladder from one end thereof to the other and serving to convey hot water, the diameter of the inner

conduit being substantially smaller than the diameter the press bladder has when it is expanded into engagement with the wall of the duct.

The invention is elucidated further hereinafter with reference to the accompanying drawing, the only figure of which shows a longitudinal sectional view of a section of a pipe line being lined in accordance with the invention.

Shown in the drawing is a section of a duct in the form of an underground pipe line L which extends between two manholes B and is to be lined internally.

The length of the pipe line section to be lined may be 30-100 m, for example.

A tubular liner ("sleeve") 11 of glass fibre cloth and impregnated or coated with a curable plastics material (epoxy resin) engages the inside of the pipe line . The liner 11 is pressed against the pipe line by a press bladder in the form of a thin-walled PVC hose 12 which is kept in an expanded state by pressurized air supplied from a compressor 13; in most cases an air pressure of 4-6 m water column is sufficient. The press hose 12 is maintained in pressurized and expanded state until the plastics material has set or cured sufficiently, whereupon the pressure is relieved and the press hose is pulled out of the lined pipe line. Disposed within the press hose 12, and extending throughout the length thereof, is an inner conduit 14 in the form of a thin-walled pliable PVC hose of the type which collapses to form a band when it is not filled with water or pressurized air, i.e., relieved of internal pressure. At the ends of the press hose 12, that is, at the manholes B, the inner conduit 14 is connected to respectively an inlet conduit 15 and an outlet conduit 16 serving to convey hot water to and from the inner conduit. The hot water is supplied by a pump 17 from a supply tank 18 in which the water is kept at a suitable tempera¬ ture by a heater 19 and to which the water is returned from the inner conduit 14. The temperature and the rate of circulation (flow rate) are chosen according to the requirements of each particular case and the desired

curing time, but generally a temperature within the range 40-70° C (as measured at the unstream manhole B or at the tank 18) is suitable. Naturally, the water pressure within the inner conduit 14 has to be sufficient to prevent the conduit from collapsing under the influence of the air pressure within the press hose 12.

The ends of the press hose 12 are connected in sub¬ stantially air-tight manner to the inlet and outlet conduits 15 and 16, and at the end where the inlet conduit 15 is connected to the imer conduit 14, a hose 20 coming from the compressor 13 opens into the press hose.

The water flow through the inner conduit 14 and the pressurization of the press hose 12 are maintained until the plastics material has cured throughout the length of the liner 11. Because the temperature of the flowing water is slightly lower at the downstream end of the inner conduit 14 the progress of the curing should preferably be checked at the downstream end of the liner.

In the illustrated example the inner conduit 14 is a single hose, but in certain cases it may be suitable to form the inner conduit of several parallel hoses, e.g. if it is desired to increase the heat dissipating surface area of the inner conduit without using a large diameter hose. As a general proposition it may be said that the inner conduit preferably should be thin-walled so that there is no substantial temperature difference between the external surface of the inner conduit and the water flowing through it.