ML17318A691

Welded Rotors for Steam Turbines.

ML17318A691
Person / Time
Site:	Cook
Issue date:	03/17/1980
From:	BBC BROWN BOVERI, INC. (FORMERLY BROWN BOVERI CORP.
To:
Shared Package
ML17318A689	List: ML17318A690 ML17318A691 ML17318A693 ML17318A706 ML17318A707
References
CH-T-060-072-E, CH-T-60-72-E, NUDOCS 8004220023
Download: ML17318A691 (6)

Text

BBC Welded%otors BROWN BOVERI for Steam Turbines Publication No. CK-T 060 072 E

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0 Rotors for large steam turbines The rotor weld and the welding procedure The rotors for all large BBC turbines consist of a number Fig. 7 shows the various stages of development of the of disks welded together to form a single solid unit. The weld preparation during the last 40 years. The deep weld high-pressure rotor of a large turbine as shown in Fig. 4 technique adopted for today's rotors has been used since contributes 440 MW to the total turbin~enerator output about 1958 (Fig. ld). Using this procedure and BBC's of 1350 MW. advanced welding methods a rotor is produced where When building saturated steam turbines for nuclear the stress values in the welded areas are similar to power plants with an output of about 1000 MWe, the large those in the base material of the forged disks. Regular component weights of 1800 or 1500 rev/min units be- tests on rotor welds provide a solid statistical back-come very Important. In the low-pressure section for ground to the welded rotor design. Microsectlons through example, rotor weights between 150 and 200 tons and rotor welds (Fig.8) are carried out to determine the even higher are not unusual. With these large rotors the quality of the weld, the extent of the base material Brown Boverl welded disk design has a very important affected by the welding procedure and the mechanical advantage. The individual rotor components still have properties of the weld material.

moderate weights and the relatively small cross-sections allow the material to be thoroughly forged. Thus a high quality steel is guaranteed with a reduced risk of rejection.

Fig. 5 shows the welding of the low-pressure rotor of a 1000 MWe turbine for a nuclear power plant, and Fig. 6 shows the same rotor after wefdfng and annealing.

Fts. 4: Hlsh~ressuro rotor of a 135G Mw aac steam tvelne 4

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introduction Rotors of medium-sized steam turbines During the first two decades of this century, Brown Boveri Fig.1 depicts the cross-section through the rotor of a used solid torged rotors tor small turbines, and built-up 60 MW condensing turbine and clearly shows the forgings rotors, consisting of a number ot disks shrunk on to a from which the rotor has been built up. The individual central shaft, for larger units. For small machines the forgings are rough machined and ultrasonically tested at solid forged rotor is still standard, but for the large the steelworks, and in addition mechanical tests are machines BBC subsequently discarded the shrunk disk carried out on pieces from each disk before delivery is design because of Its higher stress levels. Articles from made, to ensure that the mechanical characteristics are a number ot Independent sources deal with the stress achieved. These tests determine the tensile strength, levels and quality of this type of rotor (1, 2, 3J. impact strength and yield point of the material.

After 1930 a design was adopted using a number ot disks All the disks are subJected to further standard inspection welded together to form a solid rotor. Thus all the risks procedures in the workshops, including chemical analy-inherent in large one-piece torgings were avoided and a sis, tensile and impact tests, before machining com-high standard of fault detection was achieved. since the mences. In addition the disks are ultrasonically examined individual disks delivered by the steelworks are relatively for any internal flaws, such as cracks or inclusions. Only small and can thus be very thoroughly tested. when confirmation has been received that all tests have been passed does premachining commence. This con-sists of turning the inner contour ot the disk and the weld preparation contour. The condition of the rotor during this stage of manufacture can be seen in Fig.2. After welding and machining, the rotor has the shape shown in Fig.3.

Rg. t: Section through the rotor of a 60 MW condensing turbine.

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Fig. tu Stacking the individual rotor disks in readiness for welding.

I Fig.tht Medium<ized condensing tulhine rotor alter machining.

Brown Boverl's rotor welding technology has been the After welding the root, filling is carried out on a hori-subject of a number of articles in the technical press zontal lathe using submerged arc welding. Fig. 9 gives j4, 5]. The following is a short summary of the manu- an impression of this stage of the welding process facturing procedure. and shows the equipment for preheating and maintaining Argon arc welding is used for the root weld. Fig.5 the temperature of the rotor.

shows the assembled rotor in the vertical position. The Automatic methods have been used for the two welding required preheat temperature is obtained using in- processes the argon arc root weld and the submerged duction heating. arc filler weld for many years and thus the same quality is achieved In all the welds of each rotor.

After welding, the complete rotor is subjected to heat treatment ln an oven (Fig. 6). The surfaces close to the welds are then machined to permit ultrasonic exami-nation of the welded zone.

Fltf. S: Contptetlntf the llllar weld of a rotor on the weldlntf lathe.

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'l .@RE lOvl1I lSQOEI WOwN KlvCAI >4M Fig. 5: Making the root weld on the 200 t fo~ressure rotor of an Fig. 6: Lo~ressure rotor ol an 1 t00 MW nuclear turbfne alter 1t00 MW nuclear turbine. welding and stress relieving.

Fig. 8: Mlcrosectlon ol a rotor weld.

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Fig. 7: BBC weld preparation shapes: development during the last n! IN.l 40 years.

The use of modern calculation Summary methods for the design of rotors To design a rotor making use of the latest technologies,' Brown Boveri steam turbine rotors are designed, manu-large computer is used. Finite element analysis allows factured and inspected in such a way that a maximum of the operating stresses in all parts of the rotor to be safety during operation can be guaranteed. The welded accurately calculated. Fig.10 shows the mesh used for rotor has the following positive characteristics:

the intermediate-pressure rotor of a 500 MW turbine exceptionally large flexural rigidity, favourable for a to determine Its Isothermal field and operating stresses. smooth running characteristic, For the same rotor the combined stresses due to the low stress levels since the solid disks have no central rotor speed and temperature (comparative stress) are bore [3],

shown under full load conditions In Fig.11. All points good quality of all highly stressed sections since the on any line in the figure have the same comparative small disks can be more thoroughly forged, stress level.

Extensive information Is available on the stresses in

simple inspection of the individual pieces before welding and thus a high degree of safety against steam turbine rotors during start up. A summary of this material defects, information is given in reference [7). favourable heat flow during transient operation with no appreciable axial stresses at the centre of the rotor since only a tw~imenslonal stress pattern exists.

Flp. t0: The linite element mesh for the determination ol the isothermal field and the operatln0 stresses of a Bao rotor.

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Flp. 11: Stresses ln the intermediate pressure rotor ol a aBC 000 MW turtrfne durtn0 steady-state operation.

BBC BROWN BOVERI BBC Brown, Boverl 8 Company, Ltd., CH-5401 Baden/Switzerland printed In switzerland rrrtrt-20004)

Classification rto. 01st/ot02