Steam system desuperheater

Fig. 31. Steam system of a pulp and paper mill where PRV = pressure reducing valve, DSH = desuperheater, and DA = deaerating. To convert MPa to...

The purpose is to develop a steam balance for operational supervision as well as for identification of improvement opportunities in the steam system. Models for boilers, turbines, deaerators (DAs), letdown valves, desuperheaters, and steam flash tanks are discussed in the previous chapter. Historian and distributed control system (DCS) data will be coimected to steam balance so that the steam balance is capable of dynamically balancing the steam and power demands due to process variations, units on or off, and weather change. 

Calculate the steam prices based on enthalpy-based method for the steam system in Figure 17.1, which consists of major components of a complex steam system boilers with deaeration and makeup, back-pressure steam turbine, condensing turbine, process steam demand, steam letdown vale, desuperheater, and so on. The economic data are provided in Table 17.1. 

The maximum or minimum temperature attainable in a vessel can be limited by properly designed jacket heating systems. If steam heating is used, maximum temperatures can be limited by controlling steam pressure. A steam desuperheater may be needed to avoid excessive temperature of superheated steam from a pressure letdown station. 

On this system, the turbine is speed controlled and passes steam, depending on the electrical demand. The bypass-reducing valve with integral desuperheater makes up any deficiency in the steam requirements and creates an exhaust steam pressure control. Alternatively, any surplus steam can be bypassed to a dump condenser, either water or air cooled, and returned to the boiler as clear condensate. 

System = Contents of desuperheater (open, steady state) Superheated steam... 

Steam condensate is a valuable resource. It has considerable thermal value and some potential uses in the process. However, intensively treated boiler feed water must replace any condensate lost from the system. Accordingly, most plants have systems to capture most of the condensate and return it to the boilers. With steam distributed at several different pressures, it is also possible to allow condensate collected at one pressure to flash down to a lower pressure and in the process form more steam at the lower pressure. Steam is often available at the evaporators well above its condensing pressure, in which case it is worthwhile to install a desuperheater. Condensate is a good source of the coolant. 

In the base case, the liquid vaporizes at 130°C, and the steam used is passed through a desuperheater to create saturated steam at 155°C from saturated, low-pressure steam at 160°C. It is now necessary to increase the flowrate of the vaporized stream Pump and system curves are provided in Figures P22.15a and P22.15b. Assume that the pressure entering the vaporizer remains constant. 

Indirect systems often require relatively larger equipment and much more costly instrumentation plus the maintenance and supervision that goes with that instrumentation. Direct heated vaporizers require simpler controls than indirect heaters plus avoid any danger of condensate fireezeup except if the steam trap gets blocked and traps condensate inside the shell. Desuperheaters can be added to give maximtun flexibility to exit vapor control temperature. 

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