Skadliga nollflödespunkter i Fortum Värmes fjärrvärmenät i Stockholm
En analys av dess förekomst och effekter
When several production units provide a district heating network, water is pumped from different directions. When the flows from the plants finally meet in the network, the water can be completely still. These places are called zero flow zones. In district heating networks with several plants, which also have different supply temperatures, large and frequent temperature fluctuations can occur when the zero flow zones move. This may cause wear on the pipes, a phenomenon called low cycle fatigue.For this reason it is important to examine where and when zero flow zones that cause temperature changes occur. The purpose of this thesis was therefore to investigate how temperature changes caused by zero flow zones influence Fortum?s district heating network. Simulations in the software TERMIS and literature reviews regarding temperature variations in district heating pipes were used.The results showed that low cycle fatigue in district heating pipes is rare. The design criteria that the European standard, EN13941, contain are well proportioned. However, pipe manufacturers agree to that temperature changes should not occur too quickly to minimize potentially harmful bending moments. Most of the surveyed manufacturers recommend that temperature changes should not be faster than 10°C/h.The simulations showed that temperature variations occurred exclusively in an area where the flows from two different production units with a significant difference in supply temperature met. The conclusion is thus that no specific actions need to be implemented to prevent the temperature variations. This is because they are not considered as a major issue.