Centrifugal compressors are subject to transient events, such as emergency shutdowns, which can cause energetic surge events during rapid shutdown transients. Many modeling tools are used to predict the behavior of compressor systems during fast transient events. Even more, centrifugal compressor dynamic modeling is a valuable assessment tool that can help improve the design of the compressor anti-surge system to prevent harsh conditions while the unit is coasting down. Modeling of centrifugal compressor transient events requires a detailed evaluation of many system variables to obtain accurate results. Additionally, many parameters should be included and analyzed to adjust the model and obtain acceptable predictions. Those parameters can include recycle valve characteristics, the controller and actuator, suction and discharge piping volumes, aftercoolers, and operating conditions. Other parameters, such as machine coast speed and inertia and different control response times will determine if the compressor will reach its surge limit at a detrimental high head condition, avoid it or go through it at low acceptable energy conditions. Many designs use general guidelines to select and plan the compressor anti-surge system and its main components. For example, the recycle valve type, size and actuation time are key parameters during the shutdown of the compressor unit since they directly affect the amount of flow through the compressor and its reaction time. Other factors are also relevant and should be assessed in detail to assure a proper design to avoid such transient events as a sudden shutdown of a compressor from a full load or high head operating conditions resulting in damaging surge events.

In order to refine the simulation approach of the transients in centrifugal compressors, SwRI has conducted an extensive assessment of the main parameters that will affect the modeling predictions. Parameters such as recycle valve size, type, actuation time, after-cooler volume, friction factors, unit speed coast down predictions, compressor map, isolation valve control, acting time, as well as placement of the discharge check valve influence the modeling predictions considerably. Therefore, parametric studies of some of those variables have helped to refine and adjust the modeling technique while improving the accuracy of the results. Moreover, computational predictions have been compared against high fidelity - high accuracy data collected in a full scale compressor system. Initial comparisons indicated reasonable results while adjustments in the technique and main assumptions improved the modeling predictions considerably.

A generic methodology to improve modeling predictions and main considerations are part of the analysis. In addition, comparison of the modeling results and experimental data are presented and complemented with parametric studies of different variables. In general, this work should provide guidelines for advancing the modeling of centrifugal compressor transients as well as showing the application of a valuable tool for designing surge control systems for centrifugal compressors.

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