The project
Costec Systems conducted a dynamic simulation analysis of the complete steam utility system for a large oil refinery. The analysis was intended (a) to provide an overview of the coordination of the various steam producers and consumers across the site and (b) to predict transient variations of all key variables in response to a set of specified malfunctions, both for training purposes and to demonstrate the ability of the steam system to maintain supply under worst case conditions.
The plant
A process plant complex typically consists of a number of self-contained unit processes coupled together via the interchange of feedstocks and product streams. The location of intermediate storage facilities (tankage) between the individual process units allows a large degree of autonomous operation of each and prevents or reduces the spread of disturbances.
This picture of independence or autonomy is changed by the presence of common utilities. Plant subunits which might be treated from an operations viewpoint as independent are, from the disturbance viewpoint, coupled through these shared utilities. The principal utility found in most plants is steam whose piping system provides direct paths through which disturbances may be propagated. Considerable insight into the behaviour of the overall plant in response to disturbances can be gained from a detailed simulation of the steam system and the interactions between steam producers and consumers. This insight can be used to anticipate the behaviour of the plant under even extremely adverse conditions and to minimise the propagation of disturbances by developing operating strategies to return balanced conditions within the shortest possible time.
The following simplified figure depicts the system as simulated, together with steady-state steam flows through-out the three steam networks (HP, IP and LP) for the “base case”.
Conclusions
A wide variety of disturbances and operational situations, not all of them related to disturbances, were investigated. The focus was on behaviour of steam pressures and flow distributions within the various pressure networks as these show the most rapid responses. Only two sample results are quoted. Serious weaknesses were identified in the plant’s ability to handle loss of two boilers when heavily loaded, even loss of one caused concern, and problems with steam distribution within the low pressure network, due to local condensation and poor letdown valve control, were also identified.
The entire investigation, including plant data analysis, was completed within three months.
Case Study 1. A Dynamic Simulation of the Energy Plant at a Large Steelworks
Case Study 2. A Cogeneration Plant Financial Assessment
Case Study 3. A Process Plant Operations Analysis
Case Study 4. A Dynamic Analysis of Steam Utilities in an Oil Refinery