|Raising Steam Superheat
Many Ammonia plants have observed declining high pressure and medium pressure steam superheat, resulting
from plant expansion and efficiency modifications. Greater load on the steam system results with expansion to higher
capacity, while furnace and boiler flue gases decline from efficiency refinements, with limiting surface area of steam
superheating coils getting caught in the middle.
For all of the examples in this article, refer to Figure 1 steam balance as the "Base Case", with subsequent changes shown
on their respective figures, described below.
In Figure 3 we see the impact of increasing high pressure steam superheat on the overall steam balance. For this example,
high pressure steam superheat has been increased by 30 degrees Fahrenheit (Deg F, hereafter.) In the Base Case (Figure
1) the 103-JAT extraction turbine operates fully loaded and additional steam is let-down through valve PV-13 to supply the
550 psi header demand and control that pressure. In the upgrade case, capital investment is made to replace the high
pressure steam superheat coil to obtain higher performance, improving superheat to 860 Deg F, as shown in Figure 3 on
Page 4. Several typical "standard" assumptions are made to perform these calculations. The efficiencies of all steam
turbines are held constant. The steam flow through the topping turbine is maintained the same as the Base Case and the
power produced increases from the effect of higher inlet steam superheat, with resulting higher exhaust superheat as well,
to the medium pressure header. Subsequently, the turbines supplied with medium pressure steam improve in their
respective performance, due to higher superheat, with reduced steam requirements (at constant efficiency.) In reality the
high pressure extraction turbine may not quite pass the original Base Case steam flow at the higher superheat.
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