Alarm

Monitoring of leaks in pre-insulated piping

Electronic fault monitoring (alarm) is able to detect leakage faults in the carrier (service) pipe or casing and thus timely prevent the crash and total collapse of the system. Leak detection is based on the fact that the liquid medium or water from around the pipe exhibits electrolytic conductivity, which is sensed by detection wires as it enters the insulating interlayer. There can be more than one such conductor in a single pipe, which is often practical (especially for service pipes with diameters over DN250).

In principle, monitoring systems can be divided according to the material of the detection wires used, which then determines the principle of electronic evaluation in the detector. We supply all types of alarms according to the customer's requirements, although the following are most commonly used:

• Featuring a pair of detection wires, one made of resistive material, which use the bridge method to determine the distance of the lead point (BRANDES). However, it is not often used due to several significant shortcomings.
• The most commonly used system features copper conductors. These can be further divided according to the principle of closing the measuring circuit. This is either a pair of conductors forming a separate line, or a steel heat transfer pipe is used as one of the conductors. The latter system is called NORDIC and is the most widespread system in Europe.

The detectors we supply can be categorized by their usage method as stable and portable, and by the ability to determine the distance of a possible fault into indicative and localizing. Stable detectors are fixed in a suitable building (heat exchangers, heating plants, etc.) and are usually used to continuously monitor a specific piping line. The portable instruments are designed for operational measurement at any point of access to the system (in heating ducts, in building basements - i.e. where there is a possibility of access to the conductors). Indicating detectors only indicate that a fault has occurred in the monitored section, while locating detectors are able to determine the location of the fault.

System choice

Many years of experience in systems operation for the monitoring of leaks in pre-insulated piping show that in terms of functionality and reliability, as well as minimizing acquisition, installation and operating costs, the optimal monitoring system is a combination of a stable conductivity detector for continuous monitoring of the monitored piping line and a portable reflectometric detector to provide the fault location and detailed analysis of the piping condition.

Locating the fault

The fault usually manifests itself irregularly at first, especially depending on the temperature of the medium, since the temperature coefficient of the electrical conductivity of liquids is considerable. Thanks to the high sensitivity of the stable resistive detector, it is possible to proceed to the reflectometric focusing well in advance. This may not be entirely clear at first. Increased attention should be paid to the development of the situation, and one should monitor the fluctuation of the instrument fault data. A persistent and reproducible indication of a disturbance is a sign that it has become more significant and that it is time to repair it. Before starting excavation work, it is necessary to thoroughly examine the situation. The suspected area must be defined with the help of quality construction documentation and the line of the conductor indicating the fault must be traced without error. This will also determine the location of the nearest branch or end of the pipe where this conductor is accessible from. The fault location is refined using a portable reflectometer by multiple instances of focusing from these nearest accessible points. It is essential for the evaluation that the measured data is compared with similar data gathered before the occurrence of the fault (initial system measurement). This inevitably requires the standardization of measurement methods and instrumentation.

System interconnection

The monitoring system project must be designed to allow for the most accurate targeting of the fault. It is not advisable to connect and monitor the whole system at once. This method is very confusing and does not allow for even a rough localization by identifying the section exhibiting the fault. In particular, it makes the initial targeting and necessary additional fault localization with a portable reflectometer very difficult. The system needs to be broken down into individual sections to enable targeting over shorter distances (which, in practice, is a huge problem). For similar reasons, the previously used method of connecting the detection conductor into a loop and checking its continuity in that way is not suitable. In addition, detectors using this principle cannot be used for replacement monitoring using a second intact wire if one of the wires in the pipe breaks. Quality documentation is essential. It should contain, among other things, a precise diagram of the conductor connections, precise geometric measurement, and all length measurements of separate sections and individual pipes of the pipeline (layout scheme) according to the actual realization of the construction.

Systems construction technology

Detection system wires must already be divided into shorter sections in the project documentation, so that necessary follow-up surveys can be carried out with sufficient accuracy in the event of a fault. For this purpose, it is advisable to provide access to the detection wires at shorter intervals, even in places where this is seemingly not necessary (system control points). Of the actual assembly principles, it is necessary to emphasize the careful connection of conductors by means of pressing additionally welded joints, fixing the conductors in joints into the bridges that define their position, maintaining the necessary dryness of joints until the quality fusing and sealing of casing joints is performed, easy accessibility of conductor ends and connecting loops in the taps, correct routing of conductors in the taps and at the ends to avoid short circuits and leakage into the masonry, as well as the appropriate termination of conductors at the end points using terminal boxes. When assembling the system, it is advisable to use a portable reflectometer to continuously check the connection of the detection wires, the length of the individual sections and their insulation resistance. This eliminates possible and unfortunately common wiring errors and can detect moisture-laden joints early on.

Commissioning the system

The required initial review of the system must include checking the correctness of the interconnection according to the documentation, checking and documenting the insulation resistance values of the individual sections of the system and their initial measurement by the reflectometric method. This involves measuring and archiving the impedance profiles of the individual sections, including the calibration of their electrical lengths. Comparison of the archived computer records with the eventual fault condition will then allow for the most accurate qualification and localization of the fault. The documentation should be supplemented with details of the electrical distances to all accessible points - ends and branches.