In a refinery, steam utility is one of the most important utilities, which can be used at different temperature ranges. Proper handling of the available steam is an important factor for economic aspects of the plant. Instability in steam header pressures restricts optimum use of resources and it can also create safety issues. Constantly changing demands at various steam pressures challenge proper management and control of existing steam supply capabilities. Some emergency situations, such as boiler trip, add complexity to the above problem. Steam load shedding is required due to reduction in steam supply while demand being the same. The choice of steam load shedding location can affect economy of the process. Robust and reliable controller strategy becomes critical in such situations.
Aspen Dynamics was used as dynamic process simulation tool. A typical refinery steam system was considered as base case. A controller strategy was developed to handle various plant modes viz. normal, optimum, process upsets, and emergency (boiler trips). It also keeps the steam header pressures within specified limits. Changing steam demands for high pressure, medium pressure and low pressure steam were considered to test the controller strategy. Various situations of outage in steam supply were also considered. The methodology was to maintain header pressure, minimize steam venting and minimize steam letdowns. The target was to minimize need for external power supply while maintaining safety, and stability of the plant. This also includes reduction in wastage of steam and maximizing on-site power generation. The study has shown significant savings in the utility costs due to proper steam load shedding.