Measurements and Basic Units

The energy balance deals with two basic forms of energy: heat and power. Fuel and steam respond basically for the heat demand and electricity and steam deliver power. Eventually, some refineries that have gas turbines may have a small amount of fuel directly linked to power, whenever it is used as a driver to impel any equipment other than a power generator.

This means that the balance has to work with two basic measurements, one linked to heat and the other to power. Electrical power can be measured directly in energy by a powermeter or wattmeter, the preferred units regularly being MWh or kWh. Heat has to be measured indirectly by mass flow of steam and fuel and converted to energy on its heat content.

For fuel, since it is burned, the heating value is the energy involved. For each fuel stream, it can be determined through laboratory analysis. If the composition of fuels produced changes frequently with different crude slates or operational mode, it is wise to analyze them regularly, using a calorimeter. For gaseous streams, this is demanding, but the method is a composition analysis, chromatog-raphy. With the basic gas components it is possible to calculate its heating value. Two heating values can be assigned to fuels depending on the amount of hydrogen, which forms water in the combustion. It depends, whether the latent heat of water formed is to be included or excluded. If it is included, this is called the fuel's high heating value or HHV. If the latent heat energy is not included, it is referred to as its low heating value or LHV. High and low heating values can both be used to calculate combustion efficiency. Depending on gas sources, especially refinery gas, generated from different process units and external natural gas supplies, it is possible that fuel gas composition may vary along the distribution system, so it is good practice to perform daily analyses at some significant points, like near to the natural gas connection, near the main producers and near high-consuming equipment, like boilers and huge furnaces.

For steam, the available energy it possesses is expressed as enthalpy that can be determined by pressure and temperature, measured over the various equipment, headers or process stages, through the whole plant. The best approach is to recognize the main steam levels that are used and assume standards of pressure and temperature for each one of them. This will simplify steam energy balance, resuming it basically to mass flow metering. To guarantee that this standard enthalpy will remain valid, just keep track of pressure and temperature at different points of the facility. If it is discovered that there are significant variations over time, operational conditions, or regions of the compound, corrections or diverse standards should be used.

After establishing these basics, it is possible to perform all the necessary calculations that generate the energy balance and some skill is in demand for reaching a solid result.

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