Influence of Boiler Load Swing Rates on Response of Drum Water Level

In this study, swing rates of drum boilers are investigated in regard to maintaining proper drum level limits. The nonlinear boiler model employed in study investigations was validated by comparing simulated results against literature data. Experimental field data was recorded for variations in firing, steam, and feedwater flow rates in addition to drum pressure and water level were obtained from a Saudi Aramco power plant. The experimental results were used also for tuning the water level control loop and finding the proportional and integral gains of the boiler model that was simulated using matlab. Influence of steam demand perturbations on water drum levels was tested using four different perturbation schemes and in all cases the levels were found to be within the allowable limits. The time variation of drum water level in response to 40% ramp increase in steam demand occurring at the four different input perturbations was obtained. The considered values of swing rates of steam flow were eight steps of 5% per minute, four steps of 10% per minute, two steps of 20% per minute, or one step of 40% per minute. Although the maximum transient water level overshoot is attained due to the one step of 40% per minute perturbation, still remains within the allowable limits of either the low or high levels according to the boiler manufacturer specifications. The present results indicate that the allowable swing rates are much lower in the case of drop in steam demand in comparison to the case of rise in steam demand.