Design of Startup Systems

The principal advantage of simultaneous, dynamic simulators is their ability to model the unsteady-state conditions that occur at startup and during fault conditions. Dynamic simulators are being increasingly used for safety studies and in the design of control systems, as discussed in Section 4.9. 

Many factors affect the mechanical design of evaporator systems, particularly of the calandrias. The two most important are the temperatures and pressures to which the equipment will be subjected. Not only are the temperatures and pressure during normal operation important, but upset, startup, shutdown, dryout, cycling, pulsating pressure, and safety relief requirements are equally important. Other considerations include external loadings from supports or piping and vibrations transmitted from external sources. Wind loadings and earthquake loads must also be considered. Anticipated life expectancy and future service should be considered. 

In spite of precautions taken during the design of evaporator systems, problems do arise during startup and operation. Parameters which cannot always be precisely determined make it nearly impossible to define all the problems during the design stage. These parameters include ... 

A primary focus in process design is to produce designs that have good overall operability characteristics, as determined by safety and environmental considerations, low product variability, low operating costs, and ease of startup/shutdown. The process design and control system... 

Startup times ranging from 1 week [95] up to 6 months, have been reported for biofilm reactors before low and stable effluent concentrations are achieved [26,37,38,42,94,100,101]. Based on lab-scale biofilm bioreactor studies problems associated with aeration [96], temperature, loading rates, biomass control [101] and pH have been reported and require special attention during the design of full-scale FBR systems. Significant pH decreases, mainly due to the nitrification of ammonia, have been observed in column systems [97,105]. In addition, pH increases can significantly impair reactor performance [37]. 

The control system of a complete plant must permit smooth, safe, and relatively fast startup and shutdown of the plant s operation. Normally, this cannot be accomplished by the material balance or product quality controllers alone, and additional control loops are needed. How to design a good startup or shutdown control system is beyond the scope of the present text. It suffices to mention that it relies heavily on the practical experience of the control designer, who must anticipate all operating contingencies during startup or shutdown. Furthermore, it must be emphasized that quite often we change the material balance or product quality control system, in order to improve the overall capability of the system to startup or shutdown the process. 

The planning and design of the steam and condensate system is based on the mass and energy balance. Startup, shutdown, and operation under partial load must also be taken into account (Table 2.5-2). 

The design of PCE safety systems must also account for maintenance and startup needs, ease of inspection and accessibility of aU components of the PCE. Manual overrides can be provided to allow inspection or repair of PCE safety instmmented systems during normal operation. 

Reactivity control for normal operabons of startup, load following, and shutdown is accomplished by bank (uniform) movement of nine control assemblies in the fuel region of the core (see Section 6.4.4.5). While the design requirement is for shutdown with up to two stuck rods, the worth of the control rods is such that any four of the nine assemblies can shutdown the reactor to cold subcritical conditions. Each assembly can be inserted into the core in three different ways a rod run-in by the PCS (a fast run-back), a fast run-in scram initiated by the RPS, and a gravity drop scram also initiated by the protection system. 

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