Predictive Maintenance of Underground Heating Lines

Authors : Andrew V. Shishkin, Oleg M. Vahitov
St. Petersburg, Russia

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Abstract:

Remote inspection of underground heat supplying systems on the basis of aerial infrared thermography (AIT) is able to find latent leaks of hot water and other imperfections (malfunctions). Moreover, AIT data allow forecasting the places of future leaks and efficiently choosing the zones of heating lines for high-prior reconstruction, according to their actual technical condition, rather than their age.

Russian underground heating networks as an object of AIT inspection

The underground heating networks of Russian cities need the aerial infrared thermography to be applied. They have a great length, and supply quite of all buildings with hot water. More than 50% of St. Petersburg heating lines are more than 20 years old. New lines occupy about 10% of the net. They have no alarm systems for the wet insulation and, as a rule, are assembled with low quality.

Technical condition of the underground heating networks is unsatisfactory. Annually, more than 2 leaks of hot water per 1 km of heating lines descend in St. Petersburg. Every day (and even several times) district repair surveys are forced to liquidate accidents caused by the leaks.

In addition to nature adverse factors, the annual testing of heating lines by surplus pressure or water temperature are compulsory in Russia. Such experiments considerably abbreviate the resource of heating lines and cause breakings not of only heating system but also destroy city area.

Restore works in the points of leaks do not improve the reliability of heating network in general. In order to decrease the number of leaks in future it is necessary to renew long pieces of heating lines with highest metal wear, dominantly caused by periphery corrosion.

The total length of bad network segments, which can be changed, is limited by financial, technical capability and the time interval between the heat supplying seasons and overpressure experiments. Therefore it is necessary to make the best choice of heating line zones for the planned high-prior changing. Such zones were the probability of leaks is peak. The exploitation term (age) is distantly not the most optimum indicator. More important factor is technical status of the heating line, which can be determined by diagnostic methods.

The maintenance of underground heating networks must to resolve the two important problems: 1) the operative detection of existing leaks and other imperfections, 2) the efficient choice of the worst zones for the consecutive planned replacement.

AIT plays the principal role in monitoring of heating networks. It brings a possibility of operative researching the large territory and registration almost instant temperature signature of underground heating networks on the land surface. Besides that, AIT overview locates aims for another, more detail and "slow", methods application.

Heating networks in St. Petersburg and other cities of Russia are wonderful polygons for applying and developing diagnostic methods and, first of all, AIT. Practically, anytime various imperfections are exists: wet and permeable isolation zones, heating lines flooded by ground water, the leaks of hot water. The statistical data about leaks can be collected rather rapidly for the creation of prognosis criteria and verifying prognosis results. Almost continuous repairing or replacing parts of heating lines allow directly verify the quality of previous remote diagnostic.

The AIT inspection of underground heating lines provides a great tool to decide (tackle) two main maintenance problems noted above.

The operative detection of leaks and imperfections on the underground heating networks

Almost all existing hot water or steam leaks and the imperfections of heating lines can be detected by AIT inspection, because they generate thermal anomalies on the land surface.

Most of anomalous zones, probably caused by underground heating line imperfection must be investigated on-site (in-situ) to determine its reason. First of all it is necessary to verify the leak existence or absence, using acoustic emission and temperature measurements. A real point of leak can be found by acoustic leak-locator. The finding of the hot water out of heating line or hot ground by the IR radiometer is an argument of leak existence.

Well skilled thermographer can limit the amount of AIT image anomalous zones for the further in-situ leak searching. From 10 to 30% of suspicious zones, segregated by such thermographer, can be really caused by leaks.

Other heat anomalies detected by AIT and partly on-site investigated should not be forgotten. It is useful to collect them in Database for served heating network and look for their dynamic. AIT images of: 1) the spots of latent underground hot water leaks, 2) heating lines flooded by ground water, 3) humid and permeable heat insulation zones are shown in the Figure.

The efficient choice of heating line parts for the high-prior planned changing

The major strategically problem of underground heating networks maintenance is the justified choice of zones for the high-prior planned reconstruction to decrease a number of future failures. It is necessary to determine such zones were the probability of leaks is peak.

The heat anomaly (where the temperature is greater then normal) on the land surface upon the underground heating line is the indicator of possibility and favorable conditions for the tube metal destroying by periphery corrosion. The increasing of thermal conductivity is caused by structural violation, humidity and permeability of insulator material. As a result the contact of tube metal with the soil will appear and sooner or later the corrosion will destroy tube wall.

Heat anomaly can be a precursor of the future hot water or steam leaks. Even such simple qualitative factor as an existence or absence of temperature anomaly is able to forecast the area of future leaks with high assurance. This factor allows taking correct decision for reconstruction of some parts of underground heating networks.

For the various regions of St. Petersburg a statistical examination had been carried out. The relationship of the conditional probabilities of the leak occurrence inside and outside of heat anomaly zones had been determined.

The heat anomalies was marked as a places of more then two times higher altitude of land surface temperature difference upon the "normal" heating line of same construction. Such anomalies had been estimated on the base of AIT data all over the served underground heating network. And then the all hot water leaks that happened during sequent three years were noted.

Statistical analysis showed that the probability of future leaks inside anomaly zones is 2.1 - 2.5 times higher than outside. Besides that the probability of more then single leak inside such zones is 3.3 - 4.6 times higher than outside. These relationships practically did not vary neither for different town districts, nor the different one-year time interval.

The leak prognoses reliability can be described by parameter Def = N/L, where N - the amount of leaks, happened into the chosen zones during some sequent time after prognostication (for example, 1 year), L - the common length of chosen zones. Indicator Def can be named "the danger of leaks".

The forecast reliability peak is attained by such choice of heating network zones of given summary length, which cover the highest amount of future leaks. In this case also would be achieved the peak effectiveness of reconstruction.

According to peak value of Def the most optimum criteria complex for leaks forecasting and designing of reconstruction can be estimated. Heat anomaly is one of the most indicial. It is also important to take into account the previous leaks and the time of exploitation.

We are annually estimate the statistical value of Def for the parts of St. Petersburg underground heating networks suggested for high-priority changing, but remained without repair.

The "danger of leaks" for heating network zones segregated for the high-priority reconstruction according to indicated criterions - Def (r) is higher 4.6 - 12.0 times than Def (o) (for the rest part of network). The value of Def(r) outnumbers Def (25) (for the heating lines more than 25 years old) in 2.1 - 6.5 times. The limits of fluctuation of these relationships are given for the various town districts and one-year time intervals.

Conclusion

The aerial infrared thermography is efficient instrument for leaks searching, condition monitoring and predictive maintenance of underground heating networks.