Patent GB-1311472-A (GB)
Title: Pipe system of heat insulated pipes including means for detecting the  presence of moisture
IPC: G01M3/18
Filed: 1969-03-18
Abstract: 1311472 District heating systems E RASMUSSEN AS 18 March 1970 [18 March 1969] 14118/69 Heading F4U [Also in Divisions F2 and G1] A method of measuring the moisture content of insulating material surrounding a pipe, for example, in a subterranean district heating system, comprises measuring the electrical conductivity between two conductors adjacent the material. If the pipe is electrically conductive it may itself constitute one of the conductors. The method provides forewarning of a situation likely to cause corrosion of the pipe. As shown in Fig. 1 an iron tube 2 is surrounded by material 6 comprising polyurethane foam and an outer sheathing 4 of polyethylene. A conductor 10 is disposed in the foam 6 and forms a circuit with a battery and the tube 2 so that a measure of the conductivity may be obtained on a meter 14. At a joint between two pipes (Fig. 2, not shown) the conductor 10 is disposed between felt layers is wrapped round the pipe 2. A second conductor 20 may be disposed outside the second layer, measurements being made between 10 and 20. If the wires 10 and 20 are of constantan (high resistivity) and copper (low resistivity) respectively, the former may be utilized for general detection of a defect area, and the latter for localization thereof. In a complex pipe system comprising a number of sections and spurs joined together (Fig. 3) localization of a fault area is detected by providing a number of different tone generators 22. Heavy lines represent pipes with water flow as indicated by the arrows, and individual sections between joints and junctions are indicated by letters a ... m. Copper wires 20 are indicated by full lines and constantan wires 10 by dotted lines, and 5 tone generators 22 (A, B, C, D and E) are provided, connections being made as shown. Each tone generator (Fig. 4, not shown) comprises an oscillator element 26, an output circuit 28 and a power supply circuit 30. The positive battery terminal is connected to the pipe 2, and the negative terminal to the upper wires 20 connected in series by the coils 28. At each tone generator connection to the -ve constantan wire (dotted) is made through a resistor 38, this circuit being completed at the far end of the section and returning down the associated wire 10. This is then connected through a wire 34 to provide the negative supply for the oscillator 26, the positive supply being derived from the pipe 2 through wire 32. Each oscillator operates at a different frequency and receivers at a central station are provided which detect the various output signals flowing in the wire 20. Moisture in any section will cause a short circuit between the constantan wire and the pipe, thereby cutting off the supply to the appropriate oscillator, so that this particular frequency will not be present in the output. Alternatives suggested in the Specification include: (a) measuring wires introduced into the insulating material from an exterior insulated wire at distinct intervals; (b) the utilization of measuring wires which readily corrode and break, thereby stopping the flow of current; (c) replacing the conductive wire by a conducting coating within the outer sheath. A one-way valve may be provided in the outer sheath so that initially trapped moisture may escape when operation first starts.