Drying and gas or vapor contact with solids – Process – Treating hollow article
Reexamination Certificate
2000-05-10
2001-01-23
Wilson, Pamela (Department: 3749)
Drying and gas or vapor contact with solids
Process
Treating hollow article
C034S487000, C034S104000, C034S107000, C034S232000, C174S024000
Reexamination Certificate
active
06176022
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention lies in the field of systems of hollow guiding members for cable-like elements, such as cable ducts. In particular, it concerns a method for removing and/or detecting moisture penetrating into a hollow guiding member of a system of hollow guiding members. It further concerns a system of hollow guiding members for cable-like elements for applying the method according to the invention. Finally, it concerns coupling members for application in a system of hollow guiding members according to the invention.
2. Prior Art
It is customary not to bury cables, particularly glass-fibre cables, directly in the ground, but first to lay a tubular protection, hereinafter referred to as cable duct or duct for short, in the ground and subsequently to install the glass-fibre cable in this laid duct. Such cable duct has two major functions, namely, as guiding member for guiding a cable during installation in the desired direction, and a protection member for protecting a cable once installed in the duct against damaging outside influences. Digging trenches or ditches in the ground to subsequently lay a cable duct therein, however, is an expensive affair. Preferably, therefore, cable ducts are applied which, once buried, continue to meet future needs of new, additional or substitute cables for as long as possible. These may be cable ducts consisting of a singular tube, from which a cable once installed therein can be simply removed and replaced by a new cable. Furthermore, a cable duct with a relatively large diameter, such as, e.g., is disclosed in reference [
1
], may contain several cable ducts with smaller diameters, referred to as sub-ducts. In this connection, there is introduced into a large, primary tubular duct already buried a bundle of smaller, secondary tubular ducts, referred to as sub-ducts. Subsequently, at any desired point in time cables may be installed in the sub-ducts as the need arises. Here, such sub-ducts not only offer an additional protection to the cables, but also simplify the routing of the cables. Moreover, such sub-ducts make it possible to selectively remove cables and to replace them by new ones. This technique is not only applicable to complete cables, either electric cables or optical-fibre cables, provided with a traction element and various protective sheaths against outside influences. As is disclosed in reference [
2
], this technique is applicable down to core level, with the cable to be introduced into a sub-duct comprising only one or a few cores, such as copper wires or glass fibres. Reference [
3
] discloses a technique which, inter alia, applies a bundle of sub-ducts consisting, as it were, of a cable-sheath body provided with several tubular cavities wherein fibre elements may be installed as the need arises with the help of a blast-air technique.
A singular cable duct, but definitely a combination of a primary and a secondary duct, for each cable installed therein in fact forms a (double) wall around the cable, which may offer a (double) protection against negative outside influences, such as against penetrating moisture. Therefore, cables to be installed might be of a simpler construction if the (double) wall around the cable meets certain conditions. Thus, a cable need not be provided with a (metal) moisture screen if no moisture from outside the primary duct is able to penetrate into a secondary duct, as a result of which the cable may be implemented more lightly and cheaply, and may be smaller in size.
The current frequently used cable ducts, wherein complete optical-fibre cables are installed, often are tubes made of synthetic material, such as HDPE. Due to diffusion, however, such synthetic materials are permeable for water. This means that, even if in such a tube there is no leak, water may still gradually accumulate within such tube, which may subsequently affect a cable not protected against it. From manufacturing and cost considerations it would be a great advantage to be able to use such synthetic tubes, apart from the inner and outer diameters, without modification, both for the primary and for the secondary duct, particularly also for a cable without moisture screen.
In addition it is disclosed, such as, e.g., in references [4] and [5], particularly in the event of optical-fibre cables, to pressurize hollow spaces between the optical fibres lying within the sheath of the cable over a length of the cable with compressed gas in order to prevent moisture penetration through the cable sheath. This functions well in the event of leaks of not-too-great dimensions. Moisture penetrating due to diffusion, however, cannot be stopped in this manner, since for diffusion the driving force does not lie in the total pressure prevailing in the hollow spaces, but rather in the partial pressure of the water vapour present therein.
A variant of this technique is still disclosed in reference [6], in which an A1 tubelet, provided with a glass fibre, of a composite overhead glass-fibre cable, through an open end in a gas-tight interconnection box, is first blown through with compressed gas (dry nitrogen). Subsequently, the other end is closed off gas-tight. The nitrogen gas is retained under pressure in the interconnection box and in the tubelet, as a result of which the formation of frozen water drops in the event of a decline in ambient temperature is prevented. Furthermore there is disclosed, as in reference [7], the forcing of dry nitrogen gas under high pressure through the free space between the electric cores within the sheath of an underground cable from one cable end to the other. As a result, the cable innards are kept dry to prevent the formation of electrochemical trees in the synthetic insulation of the cable sheath.
SUMMARY OF THE INVENTION
An object of the invention is to provide the technique referred to above, with which a system of elongated hollow guiding members, wherein cable-like elements may be, or are, included without special protective sheath against moisture, is still protected against penetrating moisture. In this connection, it makes use of the fact that the moisture content in a space enclosed by walls, wherein moisture may penetrate due to diffusion, may be controlled by conveying through this space a flow of air, or more generally, of a gas with a relatively low humidity, and thereby draining off an excess of moisture. By maintaining such a flow, the humidity in the space may be kept relatively low, namely, below saturation of the moisture in the gas present in the space, so that no drops, let alone pools of water, are capable of being formed.
Basically, this method of removing, and keeping removed, moisture, may be applied to a single elongated hollow guiding member, such as a single cable duct. There is the drawback, however, that in most applications of a cable duct, the two duct ends, which are provided with an opening through which the hollow innards of such single hollow guiding member are accessible, are generally spaced far apart. Therefore, at two places lying far apart special measures are required, both to introduce the gas flow by way of a first opening and to collect the gas flowing out of the second opening. Collection of gas flowing out is primarily required if the guiding member with its second opening opens out into a building or residence, and most definitely in the event that a special gas is to be applied which is no ordinary component of the ambient air. In addition, there is a government safety guideline, which prohibits the opening out of tubes into buildings. In the event, however, that there still is a second hollow guiding member present—and such is always the case, e.g., in the event of application of the sub-ducts technique outlined above—said second hollow guiding member may function as return channel for gas flowing from the first guiding member, while at the same time said second hollow guiding member may be kept dry. The invention therefore provides for a method
Koninklijke PTT Nederland N.V.
Michaelson & Wallace
Michaelson. Peter L.
Wilson Pamela
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