DAMAGE ANALYSIS OF CONDENSER COOLING TUBES IN A THERMAL POWER PLANT
Abstract
A steam turbine condenser (STC) is a surface, shell, and tube type vacuum condenser, cooled by a water system supplied from a river. The STC consists of two water passes and two water flows. Its condenser shell is constructed from carbon steel, and contains 4910 cooling tubes made of CuZn28Sn1As brass, each with dimensions of 23.0 x 1.0 mm and a length of 6,400 mm.
Three new steam dump devices (SDDs) have been installed on the condenser. Two SDDs are designated for high-pressure (HP) steam, while one is allocated for intermedia-pressure (IP) steam. The primary function of the SDDs is to divert steam from the boiler through the bypass system into the STC. The bypass system is utilised primarily during start-up, shutdown, and for managing excess steam transfer.
The Commissioning and Trials Board conducted tests on the new SDDs, which lasted for five hours. However, during the commissioning phase, high condensate levels in the STC caused trips. Additionally, online monitoring indicated high condensate conductance. Upon opening the condenser water chamber doors and filling the steam side of the condenser with water, it was discovered that twenty-eight of the cooling tubes were leaking. These leaking tubes were subsequently plugged. For further analysis, four cooling tubes were extracted, and two condensate samples were obtained. While extracting the cooling tubes, the other tubes were inspected visually using a borescope to assess their internal condition.
The analysis results indicated that all the cracks on the extracted tubes were located approximately 5 mm from the tube sheet on the front side of the condenser, where the cooling water inlet and outlet are situated. Furthermore, grooves were observed on all the other tubes at the same location, characteristic of stress-corrosion cracking influenced by ammonia.
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