Steam de-superheaters

Steam can get overheated by the superheater if the flow is less due to any reason. This can be harmful to the turbine blades if the temperature of steam is too high. The de-superheaters are used to keep the steam temperature in control. 

The temperature of the superheated steam can also be controlled to the desired temperature by bypassing the hot gases around the superheater. 

The de-superheater has specially designed nozzles for spraying a controlled amount of treated water in the superheated steam. An enquiry for steam de-superheater shall include the following  

Material of construction of de-superheater body and water injection nozzles. Provision of automatic control valve for feed water injection. 

Provision of external reinforcements for the shell (it may collapse due to spray of excess water when steam supply is too less.) 

In certain situations (on very low load) there is a very small flow of steam through the super heaters and as a result, the steam can become overheated beyond permissible limits The de-superheaters are employed for taking care of such situations. 

The de-superheater can generate additional steam by spraying an optimum amount of water through specially designed nozzles in the superheated steam. Following points should be considered while procuring a steam de-supeiheater  

Steam conditions at Met pressure, temperature, and specific volume. Steam flow rate in kg/hr. 

Steam conditions desired at the outlet pressure and temperature. The temperature should correspond to about 15-20 °C superheat to prevent condensation of steam till it reaches the steam turbine inlet. 

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From steam tables, the outlet temperature is 251°C, which corresponds to a superheat of 67°C. Although steam for process heating is preferred at saturated conditions, it is not desirable in this case to de-superheat by boiler feedwater injection to bring to saturation conditions. If saturated steam is fed to the main, then the heat losses from the main will cause a large amount of condensation in the main, which is undesirable. Hence, it is standard practice to feed steam to the main with a superheat of at least 10°C to avoid condensation in the main. 

Also shown in Figure 23.17 are let-down stations between the steam mains to control the mains pressures via a pressure control system. The let-down stations in Figure 23.17 also have de-superheaters. When steam is let down from a high to a low pressure under adiabatic conditions, the amount of superheat increases. Desuperheating is achieved by the injection of boiler feed-water under temperature control, which evaporates and reduces the superheat. There are two important factors determining the desirable amount of superheat in the steam mains. 

Cogeneration units—monitor inlet and outlet steam pressure, temperature and flow of steam and water through superheaters and de-superheaters regularly is to ensure proper steam quality at the inlet of steam turbine. 

The hot gas fiom the incinerator is partially cooled to about 500°F by a heat recovery steam generator. The gas then flows through a de-superheater and a quench vessel. The quench vessel temperature is monitored to ensure that the gas entering the first absorber stage is cool enough to avoid damage to the fiber reinforced plastic absorption vessels. The gas then flows countercurrently through three absorbers where the SO2 is reacted with aqueous ammonia and converted to an aqueous solution of ammonium bisulfite and ammonium sulfite ... 

Superheating steam for reboilers and steam heaters is mostly a waste, as it s a standard industry practice to inject condensate into the steam to bring the steam back to its saturation temperature. Superheated steam does not transfer heat as well as saturated steam. Hence, the de-superheating stations. As there is typically another convective section (usually BFW preheat) above the steam superheat tubes, no net energy savings result from steam superheat. 

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