Calculi and Approximation

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After converting all measurements in energy, a basic calculation on an energy balance is matching production and consumption of each energy form. Ideally, with all energy inputs and outputs measured, the difference between the sum of productions minus the sum of consumptions should be positive and would represent the losses. And the next step would be tracking these losses to try to avoid or reduce them. But this fortunate option scarcely happens most of the time in the real world. There are many reasons for this to be rare, like inaccurate instrument readings, arithmetic and recording errors, wrong or inappropriate use of conversion factors or of standard conditions. There is also much lack of metering, which eventually leads to poor estimates of quantities.

The challenge lies in selecting the essential measurements to reach a level of accuracy that suits the needs of the energy efficiency potential. Good practice is to guarantee that precise meters are installed on the main production and consumption energy streams, including imports and exports points, like:

• refinery gas producing units;

• boilers and furnaces;

• electric motors, gas and steam turbines (big drivers);

• catalytic cracking unit flue gas (CO gas) (when it is the case).

In general, it is imperative to measure all sources and if possible to address something like 70% of the consumers. When it gets down to process unit level, around 60% of energy use should be related to very few pieces of equipment and the other 40% spread among many smaller ones, not worth metering individually. In these cases it is indispensable to have a unit battery limit meter for each individual energy stream. With the consumption details of the main equipment and the overall measurement it is possible to make good tracking of energy; what is consumed by whom, where and when. In this trail, losses, wastes and poor efficiencies will be discovered.

One should record not only the results, but the concept that helped built this methodology; make it simple to understand. If it is a continuing and evolving job, it has to be traceable and reconcilable, to give every technical person that runs into it the ability to comprehend, check its accuracy and improve it. Explain assumptions underlying the calculations, like what is the time period considered, which unmetered flows are estimated, and how this estimative is done. Detail all functions that evaluate any dependent consumption, like steam for stripping in a distillation column, explaining how it was built and how it is expected to be used. The refinery scenario changes over time, for seasonal operational reasons like turnaround maintenance, or new projects and adjustments will be needed. This registry and improvement process will give the balance accuracy, consistency, reconcilability and legitimacy. All these values are very important to reliability in estimating the energy efficiency opportunities.

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