Influence of Boiler Load Swing Rates on Effective Stresses of Drum Boiler Riser Tubes

In this paper, different rates of steam flow (swing rates) are used to demonstrate the transient changes in both the drum-boiler steam pressure and the wall temperature levels in addition to stresses in the walls of steam risers. A MATLAB code was written to integrate the nonlinear dynamic governing equations for solving the water volume, the drum pressure, and the steam quality at the exit of the riser tubes. The same code is also used for calculating the steam riser inner and outer wall temperatures based on the inner wall heat transfer coefficient. The temporal riser pipe pressure and the inner and outer pipe wall temperatures have been incorporated into an ANSYS model for calculating the effective combined thermal and pressure-caused stresses numerically. The combined effective stress fluctuation for the highest swing rate (40%) is found to be too low to induce any damage in the riser tube. The controlled pressure level has been compared with an actual pressure reading from an existing drum boiler under operating conditions for the purpose of validating the control parameters employed in the study. An optimization method for reducing the steam pressure overshoot level in the boiler is also proposed.