Design safety steam turbines

After the process heat recovery is done, the next design step is to determine the utility supply in terms of heating and cooling, and power based on the needs and characteristics of process energy demand. In this step, the means of heat supply for the reaction and separation system will be addressed. For example, a choice for the reboiling mechanism must be made for a separation column between a fired heater and steam heater. Similarly, a choice of process driver between steam turbine and motor will be determined. Selection is made based on operation considerations, reliability and safety limits, and capital cost. Selection of process utility supply defines the basis for the design of a steam and power system. 

A steam turbine is a rotary equipment driver tiiat uses steam as its source of energy. Safety considerations relate to the intended use as well as to the selection, installation, and operation of the turbine. Steam turbines are used either as emergency backup to critical motor-operated equipment or as economical alternatives to electrical motors. When the safety of the process plant depends on the reliable operation of a steam turbine, the turbine system should be designed to start up and reach operating speed in a way that minimizes hazards. 

Safety Valve SV-2 is Set to Protect Discharge Side of Turbine,os it is Not Designed to Withstand Inlet Steam Pressure on Eihoust Side. 

A safety valve is usually needed on the steam exhaust side of the turbine to protect against high pressure on shut down. Most turbine case designs will not safely handle inlet steam pressure on the exhaust side, as the case is not designed to withstand intake pressure throughout. These valves are normally rated for 110% of the design steam rate. 

Accident evaluations specific to the GT-MHR confirmed that the passive safety characteristics of the previous steam cycle modular high temperature gas-cooled reactor designs were maintained. Events initiated by one or more turbine blade failures were assessed. It was found that the resulting differential pressure forces across the prismatic core did not exceed the allowable graphite stresses. Since the dominant risk contributor for the steam cycle design were initiated by water ingress from the steam generators, the GT-MHR is expected to have a lower risk profile to the public. References 4 and 5 provide more information on the GT-MHR safety evaluations. 

A Turbine Bypass System (TBS) is provided which passes steam directly to the main condenser under the control of the pressure regulator. The TBS has the capability to shed 40% of the turbine generator rated load without reactor trip or operation of a SRV. The TBS does not serve or support any safety-related function and has no safety design. 

The main condenser, which does not serve or support any safety fiinction and has no safety design basis, is a single-shell type deaerating unit with its shell located directly beneath the low pressure turbine. The shell has tube bundles through which circulating water flows. The condensing steam is collected in the condenser hotwells (the lower shell portion) which provide suction to the condensate pumps. 

The latest evolution made for the purpose of economics has been replacement of the Rankine steam cycle PCS with a high-efficiency Brayton (gas turbine) cycle PCS to boost the thermal conversion efficiency to -48%. The coupling of the MHR with the gas turbine cycle forms the GT-MHR. The GT-MHR retains all of the MHR passive safety characteristics but is projected to have more attractive economics than any other generation alternative (Shenoy 1996). The organization behind the MHR and GT-MHR designs is General Atomics (GA). 

SAKHA-92 is a maintenance-free nuclear power plant of increased safety. Plant design was developed on the basis of PWR technology, but implements integrated steam and gas pressurizer systems and relies on natural circulation of the primary coolant (Fig. 1). The use of such designs as leak-tight turbine-generator, canned condensate and feed pumps allows to secure the tightness of both primary and secondary circuits, which in turn make it possible to exclude some auxiliary systems. 

A steam line break (SLB) is a typical limiting accident belonging to heat removal capacity reducing accidents. A break size of 0.022 m (100 mm in diameter) inside the containment is assumed and a loss of power concurrent with a turbine trip is considered. As the MCPs coast down following a reactor trip at 115% of the nominal power, the departure from the nucleate boiling ratio (DNBR) rapidly decreases to reach the minimum value and then rises abruptly due to the increased decay heat removal with the fully-established natural circulation as shown in Figure 1-3. The primary and secondary pressures are well below the safety criteria of 110% of the design pressure, 18.7 MPa [1-14]. 

Very rare events include failure of pressure vessels (e.g. pressurizer and steam generator shell), failure of structural supports and turbine break-up. Pressure vessel and structural failures are precluded in the design by use ol)for example, the appropriate level of design and manufacturing codes and standards, quality assurance and in-service inspection. Again, safety analysis is not required. 

A layout for the Super LWR NPP is drawn in Fig. 3.2 [2]. It is a two-loop system. The supercritical steam flows into the turbine system through the outlet nozzles of the RPV and the main steam lines. The water is pumped out of the condenser by a series of pumps and back to the reactor through the inlet nozzles. A series of low pressure and high pressure feedwater heaters are placed upstream and downstream from the main feedwater pumps, respectively, to preheat the cold water before it flows into the reactor. The safety system design and characteristics are discussed in Chap. 6 in detail. 

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