12th International Symposium on District Heating and Cooling

The <strong>12th</strong> <strong>International</strong> <strong>Symposium</strong> on District Heating and Cooling,September 5 th to September 7 th , 2010, Tallinn, Estoniacorrosion were revealed. Internal corrosion is the mostserious problem in Lääne network where an opensystem of hot water supply earlier has been used.Besides the damages caused by defects of installation,defects of construction, factory defects and impropermaintenance, the other reasons have also beenregistered.The main factors, which have an affect on the damagesin district heating networks, are the age of networks,the quality of construction works and the networkoperation conditions. The two latter can be regulatedby control authorities and proper legislation, however,the influence of these factors has been reduced incomparison with the 1970–1990 period. Then quality ofconstruction works was very low, drainage systemswere installed incorrectly or were not installed at all andisolation materials were not qualitative. As regardsdistrict heating operation conditions, theaforementioned open vented hot water supply systemused in some networks has led to intensive internalcorrosion of pipes.One important reason for damages reduction is that inrecent years the networks have significantly reducedpressure. The network works in a stable temperaturemode, the reliability of heat sources is improved andthe quantity of equipment emergency stops forced bysharp fluctuations of the heat-carrier temperature hasdecreased.Other operation condition factor which influencednumber of district heating system damages was higherwater temperatures in networks (up to 130 t °C) thannowadays (up to 110 t °C). Finally we can concludethat such factors as quality of construction works andquality of network operation are close to their optimumat present time in comparison with previous years.Damage quantity also depends on the age of networks.The number of damages can be reduced by reducingthe average age of the networks. This is possible byreplacing the old pipelines and other networks systemselements.Reconstruction and replacement works are made inTallinn, but the intensity of replacement is rather lowand not enough for a stable system operation. It isimportant to define, how intensive the networkreconstruction should be.Data for the three past years were used for defining thedamage dependence (number of damages/km/year) onthe age of networks. Data about damages werecollected for 7 age groups (0–5 years, 5–10 years,10–15 years, 15–20 years, 20–25 years, 25–30 years,30–35 years).Using least squares analysis, a regression equation forthis dependence was defined.2D 0,0096A 1.8985A 1.0496(1),whereD – Number of damages/100 km per yearA – Age of networksBefore using this regression for further calculations, weshould check if this equation is appropriate. One of themain parameters for estimation of regression equationis the correlation coefficient. It is considered, that thecorrelation is good in case when R>0.8. In the case ofdamage dependence on pipes age, R is 0.802.R2=0.643, which means that the equationcharacterizes the 64,3% of damage number changes,but the 35,7% of changes are characterized by anotherfactors. There is still an influence of other factors,which can not be changed, such as construction andinstallation problems in the past.Data about damages allocation by the group andapproximation of these data is shown in Fig. 5.The regression equation can be used for the damageforecasts in future.damages/100 kmper year7060504030201000 5 10 15 20 25 30 35Age of networksFig. 5 Damage number dependence on the age of pipes indistrict heating systems for the 2005–2007 periodAs it has been mentioned before, the age of networksdepends on the intensity of renovation works.In Fig. 6 the length of all repaired sites is shown split byyears.Since 1980 the serial repair of Tallinn district heatingsystem is being carried out.Basically the investments have been directed towardsthe increase of reliability and the reduction of quantityand duration of faults in heat supply. It has beeninvested a lot in the locking armature.For the past 10 years ~35 km of district heatingpipelines have been replaced, which is 10% of totallength of the district heating systems in Tallinn area.280