In a steam network system steam levels are specialized through various pressures and temperatures. To investigate the existing steam network, selection of steam levels and determination of the configuration of the operating units between the steam levels are important (Harrell, 2001). To achieve the task, a model is required. This model should allow for the steam generation potential.
Turbine hardware model, THM, is based on shaft work targeting and relies on the principle of the Willans line which provides a linear relation between the steam flow rate (Mt) and the turbine output power (E). The formula of this relation is represented via Eq. (29.1) and the Willans line can be expressed as Eq. (29.2) (Mavromatic and Kokossis, 1998).
As shown in Fig. 29.1 point A is the minimum steam flow rate needed to overcome the internal losses, Eloss, of the turbine before it starts producing total work. This amount of steam corresponds to constant point C. Point B also represents the maximum steam flow rate that the turbine can take (Mavromatic and Kokossis, 1998).
Fig. 29.1 The operation of turbine present by Willans line (Mavromatic, 1998).
Boiler hardware model, BHM, accounted for boiler efficiency to minimize the fuel cost. Moreover, BHM relies on the principle of the calculation of boiler efficiency. Most of boiler models assume boiler efficiency as a constant. For steam networks analysis, model should be capable to account real condition of boilers. BHM is based on efficiency calculation procedure that is expressed by Eq. (29.3). Steam load can be expressed as Eq. (29.4) (Shang and Kokossis, 2004):
where h1 is the boiler inner water enthalpy, h2 is the enthalpy of steam raised by boiler and Mb is the steam load produced by the boiler. Figure 29.2 shows the variation of boiler temperature versus enthalpy. As per thermodynamic principle the enthalpy difference can be written by Eq. (29.5). The boiler efficiency is expressed via Eq. (29.6) and Eq. (29.7) (Shang and Kokossis, 2004).
By substituting above equations, the fuel heat is illustrated by Eq. (29.8). Qfuel = (CpATsat + q)xMb + Qloss (29.8)
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