By Jakob Rasmussen, Fjernvarme Fyn, Jørgen Morsing, Fjernvarmecentralen Avedøre Holme and Anders Dyrelund, Rambøll
The district heating network is an important part of the energy infrastructure in many cities. It is expensive and inconvenient to replace. Therefore, a long life-time of the pipes is vital for the success and competitiveness of district heating. We have learned the lesson in Denmark, as we had huge problems 40-50 years ago - both poorly constructed concrete ducts and the first generation of preinsulated pipes caused problems. District heating know-how in 1975 meant knowing how to ensure long lifetime of the pipes. Some pipes in concrete ducts corroded after a few years, others, e.g. a 3 km long DN500 from Vestforbrænding, is still in good condition after 40 years of operation. The first generation of preinsulated pipes (invented and developed by LOGSTOR around 1970) had to be replaced after a few years, but many Danish district heating companies gave the new technology a new chance. As a result, almost all preinsulated pipes and muffs, which were constructed after 1980, are still in good shape. In fact, nobody knows the remaining life-time of these pipes. The total lifetime of new pipes with welded muffs could be 60 years or even 100 years in case of low operation temperatures.
In order to lower costs and to reduce temperatures at our efficient CHP plants we have decided to avoid heat exchangers except for production of hot tap water at each consumer. We have no heat exchangers -neither between transmission and distribution nor between distribution and radiators. Therefore, we have a relatively large water loss as our losses include losses in the buildings. One the other hand, the good water quality reduces corrosion in building installations. There is only one disadvantage: the building can be flooded in case of a severe leak. In case this is considered to be too risky, the building owner can install a leak protection system in which two valves will close automatically if there is a difference in the in- and outgoing flow. Besides, we have an old network which leaks from time to time and there can be leaks in the tanks and heat exchangers for production of hot tap water. In total, our water losses have been much larger than normal in Denmark for many years, but fortunately never larger than the capacity of our water treatment system. High level water quality and a pH 9.5 to prevent any internal corrosion are of course vital. Therefore, we have never had the problem of progressive internal corrosion. Instead, we have taken the time to implement a rehabilitation strategy with the aim to minimize the total costs. In our strategy, we have aimed at minimizing the NPV of all costs including investment in new preinsulated pipes, repair costs, costs of heat losses and costs of disruptions. A key parameter is e.g. an “acceptable” number of leaks per 100 km per year for each type of pipe and dimension. Reducing water losses and heat losses to almost zero is not cost effective, but we can talk about an optimal level. Moreover, we have considered the advantage of lowering the reduction of supply temperatures per km pipe during summer, which is particularly important for small pipes. The general outcome of the optimization is: small dimensions first - large dimensions later. Last year we replaced the last old concrete duct with preinsulated pipes. It is a 40 year old DN600 concrete duct. The concrete duct was only replaced because of renovation of the road. If not, it would probably have been in active service for 4-5 more years. The other pipes that have been replaced in these years are normally also around 45 years old. The main reason now is not that they are in a critical shape and that it is the most cost effective.
But how do we define the time for replacement ?
CASE 1: Maintenance strategy in Odense
In Fjernvarme Fyn, located in the third-largest Danish city Odense, we have been through a successful maintenance strategy with the aim to keep it simple to minimize the distribution costs for all our consumers.
Fig. 1 Supply area of Fjernvarme Fyn
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