(148d) Asset Performance Modeling for Refinery Steam Systems

As part of a utilities modernization study, an asset performance study was carried out for a refinery steam system. Asset Performance Modeling consists of both stochastic reliability modeling and deterministic dynamic simulation, used in conjunction to optimize operating parameters while ensuring reliable production (in this case, steam production). The study assisted the refinery in assessing current reliability performance and in ensuring that the improvement/enhancement options being considered will result in a reliable and stable steam system which would help achieve the target reduction in refinery-wide production losses.

Operating reality for utility systems involves stochastic (random, unplanned and sequential) events. A reliability study, therefore, attempts to emulate real-life behavior as it takes into account the stochastic operational behavior of both producers and users in a utility (or any production) system. Reliability modeling was used to:

? Ensure that the modifications of the steam system could meet current and expected future steam demand.

? Make objective data-driven decisions around appropriate equipment sparing philosophy and system/hardware selection options to meet project premises, i.e., get the emotion out, and achieve project objectives at the lowest possible investment.

For the present study, a reliability model was built and validated with historical plant data. The model confirmed that, in general, there was sufficient overcapacity in the steam system to meet steam demand. The model was also used to examine the effect of steam producer outages at the different pressure levels. The model showed that the level of overcapacity was sufficient to sustain the loss of one boiler at any one of the three pressure levels, without significant impact on the system. However, during periods when all steam users were running at maximum rates, the system was shown to be vulnerable to boiler outages.

A dynamic simulation model was also used to examine the deterministic, operational issues in the steam system. The model's purposes included:

? Representing the existing steam system to understand the causes of instabilities in the system

? Proposing improvements to address any identified system weaknesses

? Using the model to test proposed modifications

The dynamic model was built and validated with plant data. A series of upset conditions were simulated by the model, to determine the dynamic response of the steam system. The dynamic modeling showed that the proposed control scheme was not consistent with the plant's control philosophy, and was used to verify that a modified control scheme could adequately handle upset conditions.

This presentation will give an overview of the technologies and methodologies used in this study, as well as highlight the specific application of asset performance modeling for refinery steam systems.