Operating safety steam turbines

In steam turbine power plants, steam pipes carry super heated vapor under high temperature (1050°F/565.5°C) and high pressure often at 3500 psiMPa or greater. In a jet engine temperatures may reach to 1000°C, which may initiate creep deformation in a weak zone. For these reasons, it is crucial for public and operational safety to understand creep deformation behavior of engineering materials. 

New concepts based on passive safety presently under study are the Pebble Bed Modular Reactor (PBMR - gas cooled, high temperature, helium operated, direct cycle turbine generators) supported by an international group based in South Africa, the IRIS reactor (a PWR with steam generators integrated in the reactor pressure vessel) and the already mentioned APIOOO. Other concepts still under study but already proposed exist... 

High-pressure steam may also serve as a driver for turbines. Chlor-alkali plants, which are usually placed in areas of low electrical power cost, are less likely than most other types to justify the use of steam-turbine drives but still may use them as a backup source of power. Furthermore, one of the ways to cope with a major electrical failure is to use steam to operate critical drives imtil all systems are shut down or electrical power is restored. An example of a critical service is the caustic circulation pump on the emergency vent scrubber. A spare pump is always necessary, and it should have an independent source of power. One way to accomplish this is with a steam-turbine drive. Other services may also be considered critical for personnel safety or process security. The latter is especially true in a membrane-cell plant, where some systems are vital for the protection of the membranes from damage. 

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 pressure relief function is used to control large pressure transients. This system will operate safety/relief valves following closure of the main steam isolation valves or the sudden closure of the turbine admission or stop valves and failure of the turbine bypass system to relieve the excess pressure. For this fimction, the safety/relief valves discharge steam from the steam lines inside the drywell to the suppression chamber. Each safety/ relief valve is operated from its own overpressure signal for the relief fimction, and by direct spring action for the safety function. 

Both EdF reference sets of EOPs have four separate procedures for the reactor operator, for the water and steam (turbine) operator, for the shift supervisor (containing a combination of reactor, water and steam operator procedures) and for the safety engineer. All procedures are in the colour flow chart format (paper based). The recent four loop N4 plants have fully computerized procedures (with computerized operating actions actuated from the operator video display units) as well as a complete backup paper based set of procedures for operating from the auxiliary panel if the computer system fails. 

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. 

Some more details of the plant system of the Super LWR and Super FR are shown in Fig. 1.7. The RPV and control rods are similar to those of PWRs, the containment and safety systems are similar to those of BWRs and the balance of plant (BOP) is like that of supercritical FPPs. All RPV walls are cooled by inlet coolant as in PWRs. The operating temperatures of major components such as the RPV, control rods, steam turbines, pipings and pumps are within the experiences of those of LWRs and supercritical FPPs. 

The upset in Event 4 is similar to the upset in Event 1. Operator intervention can stop this runaway by starting the steam turbine driven water pumps, or adding Shortstop. While this operator action was judged to be very effective, no risk reduction credit was taken because of operator availability. The analysis shown in Table 7 led to safety integrity level 3 for SIF S-1. 

It is important to note safety differences between the SRS reactors and LWRs. Since the SRS reactors are not for power production they operate at a maximum temperature of 90° C and about 200 psi pressure. Thus, there are no concerns with steam blowdown, turbine trip, or other scenarios related to the high temperature and pressure aspects of an LWR. On the of nd, uranium-aluminum alloy fuel clad with aluminum for the SRS reactors melts at a m ver... 

Examples of common safe practices are pressure relief valves, vent systems, flare stacks, snuffing steam and fire water, escape hatches in explosive areas, dikes around tanks storing hazardous materials, turbine drives as spares for electrical motors in case of power failure, and others. Safety considerations are paramount in the layout of the plant, particularly isolation of especially hazardous operations and accessibility for corrective action when necessary. 

FBTR is a 40 MWt/13.2 MWe, mixed carbide fuelled, sodium cooled, loop type reactor. It has been provided with two once through serpentine type steam generators (SG) in each of the two secondary loops. The reactor has 100% steam dump capacity, in order to continue reactor operation, when turbine generator (TG) is not available. The reactor achieved its first criticality in October 1985. Since then it has been operated at various power levels in stages upto 10.5 MWt. The small carbide core with 26 fuel subassemblies has been licensed to operate upto 10.5 MWt with 320 W/cm peak linear heat rating of fuel. All the works in TG system have been completed and the turbine has been rolled upto its synchronous speed of 3000 rpm. In March 1997, clearance from safety authorities has been obtained for further loading of the fuel subassemblies and to operate the reactor upto a power level 12.5 MWt. Synchronisation of TG with grid will be done shortly. 

The RPV is provided with a pressure relief system which consists of 12 safety (relief) valves connected evenly onto the four steam lines, with blowdown pipes leading down into the condensation pool. The safety (relief) valves are own medium operated valves, each being controlled by two pilot valves, one pressure activated and one electrically controlled this means that actuation can be initiated in a controlled way by pressure monitoring equipment, to avoid over pressurization or to achieve depressurization. In addition, control valves are provided downstream two of the safety valves, in order to enable proper pressure control of the reactor also in the event of isolation (loss of the turbine condenser function). 

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. 

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