Failures of boiler pressure parts, which working in high temperature and pressure conditions are often caused by overheating or corrosion. These two parameters are decisive, but not the only ones. Local stress concentration also depends on the type of headers support and external loads from pipelines. Boiler pressure parts subjected to all loads mentioned before are steam superheaters. Thermal expansion, high pressure and temperature lead to shortening superheaters lifetime. In the places with significant stress caused by all load combinations it is difficult to predict creep strains and material structure changes. This paper shows superheater in which considering external loads from pipeline and their influence on the stress concentration in the superheater tubes. This article also shows steel S304H creep analysis for 100k [h] results and creep equation with experimental developed constants.
The paper presents the main types of power boiler steam superheater loads together with relevant standards applicable to superheater calculations. In the case of horizontal superheaters, due to their thermal expansion and assembly errors which are the effect of non-uniform support of the hanger tubes, extra loads are generated on the coil pipes. An FEM analysis performed for a selected superheater structure indicates that maximum stress values and the location where the stresses occur depend on the method of support of the superheater tubes. It is shown that maximum stresses in this structural element can be reduced by introducing appropriate tension of hanger bars supporting the superheater hanger tubes.Keywords: power boiler, creep, stress, FEM analysis Streszczenie W artykule przedstawiono główne rodzaje obciążeń przegrzewaczy pary kotłów energetycznych oraz normy obowiązujące przy ich obliczeniach wytrzymałościowych. W przypadku przegrzewaczy poziomych, ze względu na ich rozszerzalność cieplną oraz błędy montażowe spowodowane nierównomiernym zawieszeniem rur przegrzewacza następuje generowanie dodatkowych obciążeń na rurach wężownicy. Wykonana analiza MES dla wybranej konstrukcji przegrzewacza wykazała zależność maksymalnych naprężeń i ich miejsca występowania od sposobu zawieszenia rur przegrzewacza. Pokazano, że wprowadzenie odpowiednich naciągów w prętach wieszakowych podtrzymujących rury wieszakowe przegrzewacza może obniżyć maksymalne naprężenia w tym elemencie konstrukcyjnym.
Higher temperatures of the power boiler superheater operation may lead to high strains caused by the creep phenomenon. This paper presents a determination of the maximum allowable operating temperature limited by the creep phenomenon for steam superheater SH3. The calculations are carried out first on the basis of applicable European standards. Then, calculations are performed based on conducted creep tests, a proposed creep model, and a finite element method (FEM) model. A detailed creep phenomenon analysis based on the conducted creep tests shows that stresses that determine the creep process are mainly caused by pressure. Normal stresses resulting from bending are mainly the effect of thermal expansion. These stresses undergo significant relaxation because of creep. The creep phenomenon analysis explains the equations of the European standards. The presented calculations enable estimation of a safe value of the operating temperature which is constant over time. The estimated time of safe operation does not take account of temperature spikes. For this reason, pressure elements working at high temperatures must be inspected regularly to assess their wear state.
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