고압 급수 가열기의 피로해석에 관한 연구

Abstract

The objective of structural analysis is to verify the fatigue life of high pressure feedwater heater by using the thermo-mechanical analysis due to the operating pressure and thermal gradients at the event of cold start, warm start, hot start, shut down and load changes.

The heater consists of tubesheet, shell and hemi-head. There are many perforated holes in tubesheet to connect U-type tubes.

The tubesheet perforated region (ligament) was modeled as an equivalent solid plate. The heat transfer coefficients HTC derived from the Dittus-Boelter equation were given on the surface contacting with feedwater, and the HTC of tube out-surface in desuper heating zone were given on shell inner surface side. The three-dimensional, isoparametric elements were used to evaluate the fatigue life in the perforated region, hemi-head and shell. The appropriate high pressure feedwater heater operation pressures and temperatures were used for the analysis.

The finite element analysis was used to evaluate stress levels and thermal gradients for the feedwater heater. Finite element analysis models were constructed for each analysis and consisted of nodes and elements representing the tubesheet solid, perforated regions, shell and hemi-head.

The first step of analysis is to determine the internal temperature distribution (thermal gradients) as a function of time for all operating mode and the internal stresses were calculated for the critical temperature distributions with operating pressure.

The second step is to calculate stress levels and evaluate code requirement and fatigue life. The heater is verified the integrity, became evaluated stresses satisfy the allowable stress and fatigue usage factors are not exceed 1.0.