ML17318A690

1160 MW Turbine for Facility.

ML17318A690
Person / Time
Site:	Cook
Issue date:	03/17/1980
From:	Hossli W BBC BROWN BOVERI, INC. (FORMERLY BROWN BOVERI CORP.
To:
Shared Package
ML17318A689	List: ML17318A690 ML17318A691 ML17318A693 ML17318A706 ML17318A707
References
CH-T-060030-E, CH-T-60030-E, NUDOCS 8004220017
Download: ML17318A690 (20)

Text

The 116%MW Turbine BBC BROWN SOVERI for "Donald C. Cook" Nuclear Power Plant of AEP Publication No CH-T060030 E sooasaeOI l

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The 1160 MW Turbine for "Donald C. Cook" Nuclear Power Plant of AEP IV. Hossli comprise a four-flow high-pressure section and a twelve to Follotving some introductory comments on the reasons for sixteen-flow low-pressure section. Considering the overall building halfspeed machines, the turbine for "Donald planning, the construction of such a machine in one line C. Cook" nuclear potver plant is described. In dealing uith (tandem compound) would become somewhat question-some elements, the author goes into details in e.explaining the able, even if the design problems (e.g. differential expan-basic ideas ivhich governed the designs adopted. Some notes sion) could be solved without great difficulty. Breaking the on the tests performed and on special safety considerations unit down into two machines is usually uneconomical, so round off the article. that the next obvious solution was to revert to "half-speed" machines (see Fig. 1).

The preliminary design work was therefore put in hand, in order that the answers to fundamental questions could be found before enquiries were received. In this process the design principles that have proved so successful in the past were adopted, as they not only offer numerous advantages, they can also be extrapolated with remarkable accuracy.

About ten years ago nuclear power plants emerged from Nevertheless the new dimensions did pose some major the experimental stage into the everyday industrial field. problems which had to be solved first and made additional The reactors used today may be divided into three main tests and experiments necessary.

groups: Having closely examined the detailed proposals regarding design, production and operation of half-speed machines,

- Gaswooled reactors (e.g. AGR or THTR) which, up to the American Electric Power System (AEP) placed an now, have only made headway in Britain; more recent order with Brown Boveri early in 1968 for a machine rated developments will confirm whether they are'economically 1160 MW (53 bar, 268 'C saturated steam. back pressure competitive. 0 051 bar), comprising turbine and generator, destined for

- Pressurized-water reactors (PWR), representing the C. Cook" nuclear power plant. 'Donald "light-water" line of reactors, which today are being built by a number of manufacturers and are in use throughout the world.

- Boiling-water reactors (BWR), to which the same applies.

When the success of the light-water reactors began to become apparent, some years ago, it was soon seen to affect two aspects of steam turbine design:

Fig. I Comparison of th>> size and capacity of I'ull.speed and half. speed

- Light-water reactors generally provide'saturated steam machines at a pressure of 40 to 80 bar. i turbine. 3000 rcvimin. l00% output Consequently the heat drop that is available is only about B ~ Z turbines. 3000 rcvimin, each 50% output C ~ I turbine, I500 revlmin, l00% output 65% of that used by modern reheat turbines. The specific steam consumption can therefore be 60 to 90% higher At than in conventional plants, depending on the specific s conditions.

- The corresponding nuclear power plants can only be justified economically if unit ratings are made very high.

These factors led to the problem of handling very large steam volumes, especially at the low-pressure end of the turbine and this induced Brown Boveri to reconsider the construction of "half-speed" turbines (running at 1500 or 1800rev/min instead of 3000 or 3600rev/min) after a break of some 40 years. It was assumed that, having regard C to the specific conditions, the configuration of a machine running at the normal 3000 rev/min would, for example, BROWN aovaiu 1eooec ~ I

"Donald C. Cook" Nuclear Power Plant The Turboset The plant is located on the east bank of Lake Michigan in Layout St.Joseph's County and is therefore directly accessible by ship from Europe through the St. Lawrence Seaway. This The turbine consists of a double-flow h.p. section is important in view of the considerable transport prob- and three double.Row I.p. sections driving a four-pole lems. Fig.2 shows an artist's impression of the phnt alternator. On either side of the h.p. cylinder there is a pair which willcontain two units. Each turbine will be supplied of combined stop and control (throttle) valves. The with steam from a Westinghouse pressurized-water reac- general arrangement of the set is illustrated in Fig. 4, 5 tor. The first turboset is being provided by an American and 6. The enormous size of the combined water separa-m'anufacturer, while set No. 2 is being supplied by Brown tors/reheaters between the h.p. and I.p. sections of the Boveri. The selfwxplanatory schematic layout can be seen turbine can be seen at a glance.

in Fig. 3.

The two feed pumps, each of 50% capacity, are to be driven by steam turbines [3).

Fig. 2 - View of a model of-Donald C. Cook" nuclear power plant of

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Fig. 3 - Schematic layout of the "Donald C. Cook" plant R ~ Reactor HP High-pressure turbine HP LP LP LP LP LP LP LP Low.prcssure turbine 0 Generator C ~ Condenser H ~ Fccd.heaters tYS Water separator RH Reheater Steam W'S RH ~ 'tVatcr 0

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I Hg Hg aaowN aovEAI tcoccs ~ I Inlet Valves consequently, of not being able to fulfil deadlines, was relatively high. Therefore, for the next pair of similar The saturated steam, at 53 bar and 268'C, from the reactor turbines that are on hand for TVA, as well as for a enters the turbine through four live-steam lines to the four 1200 MW set for Jersey Central, a modified design was combined valves (Fig. 7) which are equipped with steam developed in which the outer casing is a welded fabrication strainers to prevent solid matter entering the turbine and (Fig. I2).

also to stabilize the flow. In their dimensions and design The cast h.p. casing of thc "Cook" turbine is a single-shell pressure these valves resemble the interceptor valves design and features blade carriers. The latter are not supplied for the 1300 MW reheat turbine for Cumberland attached to the casing itself, but form an integral casting power plant of TVA [Ij. However, the much lower together with the casing. In this way the risk of erosion due temperature at "Cook" allows the use of ordinary, low- to leakage in the region of the attachment points was alloy cast steel. Fig. 8 gives some idea of thc proportions completely avoided. The critical parts of the flanges werc of these huge inlet valves. From them four pipescarry the faced with high-alloy material, the blade carriers being steam into ihe h.p. cylinder. additionally bolted together, so as to prevent leakage. Due allowance had to be made for accessibility to these bolts from outside. To minimize distortion of the large end faces High-Pressure Section under the internal pressure, tensioners were incorporated.

This is particularly important as it prevents any displace-The h.p. section is of double-flow design; as the turbine is ment and distortion of the gland sections, the whole casing intended for base-load duty, a regulating stage can be being mounted direct on the foundations at the sides, on dispensed with, the steam flowing directly through the account of the very large torque, and not supported in the reaction blading. usual manner by feet on the end faces and pedestal The casing is of low-alloy cast steel. Thc size and weight of bearings.

its two halves made it necessary to adopt a composite The advanced welded design used I'or TVA and Jersey assembly, each half of the casing being made of six Central is based on similar considerations. Two solid rings sections which were afterwards welded together (Fig. 9, in the outcr casing absorb the torque exerted on the l0, l I). stationary part and transfer the forces direct to the When these large castings were ordered each half weighs foundations through feet. The end faces are similarly fixed about l00 t-it was found that the risk of rejection and, by means of tensioners. Of course, thc points at which

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iu Ici there might be some risk of leakage (blade carrier Crossover Pipes, lVater Separators/Reheaters. Interceptor attachments, flange joint, etc.) are faced with high-alloy Va/ves material. If necessary, the blade carriers can be made of high-alloy material for instance, should there be any risk From the h.p. section the steam passes through four lines of erosion. into the combined water separators/reheaters (Fig. 13).

For some time now it has been possible to calculate the There the steam. is first dried in mats of wire mesh,,

highly complex shapes of these casings with a high degree following which it is superheated by flowing through nests of accuracy. Numerical methods, using finite elements, for of tubes heated with live steam.

modern large computers have been developed to such an The reheating part can be designed in two stages, the lower extent that they enable these problems to be solved. stage being heated by bled steam from the h.p. cylinder Fig. 1 Section through a combi.

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and the upper by live steam. The decision as to whether a steam regulating system; it may even be necessary to single or two-stage design should bc adopted is largely provide two such valves, one of which is controlled, as dependent on the result of an economic calculation. From before, by the steam regulating system, thc other being the tanks housing the separators/reheaters the steam flows actuated by the trip system. These valves are well-tried to the I.p. sections. Each of the two reheaters has three components (Fig. l4, l5), having already been used in a outlets, so that each I.p. section is fed through a pair of number of district-heating turbines for control duties.

symmetrical lines (see Fig. l4).

Reheat stop and interceptor valves are incorporated in Lrptt~Pressure Secffon these crossover lines, their task in the event of a shutdown being to block the large amount of energy in the steam The enormous dimensions compared to a full-speed stored in the separator/reheater tanks and thus to prevent machine of similar output in a fossil plant are still more the turbine from overspeeding. An overspeed calculation obvious in the I.p. sections (compare with the height of the confirmed by measurement is needed to determine the man in Fig. 5). Here. too, the established design has been number of valves required. Dependin'g on the result, either adapted to meet the requirements of the enlarged dimen-no valve at all is provided. or one valve controlled by tlic sions.

Ffg. 9 - Longitudinal section through the high. pressure section {cast casing) of th>> -Cook" turbine

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A consistent distinction between the various functions has Since these casings arc exposed to an external pressure, been observed. The outer casing serves only to confine the duc allowance has to be made for unstable implosion in steam and to absorb the forces of external prcssure. Thc addition to the normal stresses. The procedure adopted for blade carriers are supported by cantilever beams fastened the hood is that it is assumed that a state of stress as in a direct to the foundations. The glands are attached to the membrane is present (i.e. tension and compression, but no bearing pedestals and also joined to the outer casing bending stress). As with Euler's bending equation, plau-through flexible elements rather like expansion joints. sible deformation is assumed (longitudinal or peripheral ln this way it is possible to prevent the inevitable elastic waves), as known from observations. The resultant addi-distortion of such a large outer casing from having a tional stresses in the case of instability are then larger than negative effect on the rotating parts (e.g. rubbing due to the opposing forces due to resilience. Since the hoods of reduction of clearances, fouling, etc.). such l.p. casings are, in principle. cylindrical shells, thc The problems connected with production and transport problem has to be tackled by means of partial differential were solved by designing the I.p. section in several parts. equations. the solutions of which yield two types of The hood and the sides ol the lower part can. ifnecessary, bending characteristics in the longitudinal and peripheral be welded together on site after they have been assembled. directions. The combination which is able to withstand the Special precautions have been taken in the design to minimum loading usually it leads to roughly square prevent distortion due to welding at critical points, c.g. at indentations is decisive. Fig. I6 shows such indentations the horizontal flange joint. in a model casing. It is worth noting that measurements Fig. lz - Longitudinal section through the h.p. section or a hall'-

spced.saturatcdsteam turbine with welded casing I IJersey Central, TVA) r lcooro I

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agree very well with the calculated values. Also, it may be pointed out that the model casing had no internal struts whatsoever.

To establish the degree of safety with respect to implosion allowance must naturally be made for additional bound-ary conditions. such as pre-shaping for production rea-sons, and the like, which reduce the implosion limit.

Exhaustive tests on models (Fig. I7) provided ample proof that the calculated values were correct. For the I.p. casings of'alf-speed machines, however, the need to be strictly true to scale (e.g. welds) makes it necessary to use such large models that a most unconventional method had to be adopted for the pressure test to destruction.

Determination of the safety with respect to implosion is naturally somewhat uncertain. In order to be sure of the results the model casing of the I.p. section of the "Cook" turbine was lowered to a depth of 65 m in Lake Constance (Fig. I8), at which depth it was subjected to an external prcssure which caused it to buckle inwards.

With this casing the safety factor with respect to implosion was found to be more than 6. The method of calculation was calibrated against the results of this test, so that it is Fig. l7 - Tests on a model of the l.p. casing ol'he "Cook" turbine now possible to interpolate and extrapolate the calcula-Using dial gauges and strain gauges the deformaiion and stresses on the tions.

evacuated casing were measured. The inner casings are welded fabrications and, like the cast blade carriers, are only noteworthy for their large dimen-sions (Fig. I9, 20). The rotor blades of the final I.p. stage (Fig. 2l), which in the "Cook" machine are 52 inches long Fig. lg- Test to destruction on the model casing of the -Cook" turbine (l320 mm), were derived by means of model laws from The necessary external pressure was obtained by lowering the model. those normally made for, and currently in service in, internally at atmospheric pressure, into Lake Conuancc until a depth of machines for 3000/3600rcv/min. In this way it was over 65 m was reached.

possible to utilize the wealth of experience gained hitherto and so ensure enhanced reliability (Fig. 22).

Rotors The welded sectional design of thc rotors [2], though demanding comprehensive experience gained over many years, oHers so many advantages that its use is likely to increase. The ever-growing unit sizes, especially those of turbines for l500/l800 rev/min, makes this design almost inevitable. One I.p. rotor of the l800rev/min "Cook" turbine weighs almost 200 t without its blading. In view of Ci ~g the quality requirements that have to be satisfied, it is impossible to obtain a suitable one-piece forging anywhere in the world at the present time. This problem is neatly solved by the welded sectional design.

The various aspects which favour the adoption of this I system will therefore be brielly recapitulated.

t, The sections can be designed so that they come very close to the ideal, uniformly stressed disc. It is possible to OIIOAII ~ construct rotors of almost unlimited size, as the size of the l3

individual disc sections is small compared with that of the The ratio of the weight of such rotors to their moment of rotor as a whole. On account of their relatively small inertia is extremely good. Owing to the specific stress in cross-sectional area, the sections can be annealed right l.p. rotors being low, they can be made with much larger through with comparative ease; consequently the quality diameters; the Brown Boveri steam turbines have the of the material at the centre ol'the disc is first~lass. highest ratio of shaft diameter to blade length. Each I.p.

Material samples for destructive testing are taken from the rotor of the "Cook" turbine is composed of ten sections most highly stressed region, close to the centre of the discs. (the h.p. rotors of four). The weight of the heaviest section Owing to the <<ase with which such sections can also be is not more than 30t. which is well within the power of nondestructively tested there is no need for a central hole. most steelworks, and enables them to produce flawless which would give rise to double, local stresses. In designs forgings of excellent quality.

with shrunk-on sections it is even possible for some of the Of course, the necessary production and testing facilities disc material to become plastic (Fig. 23). This could affect had to be provided. The requirements of future turbines the shrink fit to such an extent that the smooth running were duly taken into account. Finally, the cranes and properties would be adversely affected. This cannot occur lifting gear had to be dimensioned accordingly (Fig. 24).

with welded rotor assemblies. To enable these rotors to be transported, the last two rows Constructing the rotor of several sections eliminates of blades have to be removed. With the fir-tree root thermal instability. Owing to the ease with which sections method of fixing, this does not present any particular can be replaced at short notice, it is possible to be much problems when these blades are inserted on site.

stricter as regards rejection, without the risk of exceeding Special containers were made to protect the rotors with the delivery deadlines; in other words, still better quality. rest of their blading during shipment.

Fig. lp- One half of the inner I.p. ~

casing during production

Bearings Together with the constantly growing unit ratings, thc size of these important components have also increased. For a long time it was possible to extrapolate existing bearing designs which have rendered good service, without en-e countering any unexpected problems. In the meantime,

~ 1 ,I however, the necessary dimensions have become so )argc I that, owing to the increased Reynolds number, flow in the lubricating gap has become turbulent. As a result of this, I the losses and the temperature rise of the oil increase much more rapidly than would be expected from simple extra-polation. A possible way of avoiding these difficulties is to I use pivoted segmental bearings.

During the development of these bearings exhaustive trials had to be carried out on the test-bed in order to determine the most favourable design, as well as thc maximum load capacity and the loss power. During the past five years considerable expeiience has been gained with this kind of bearing, so that there was nothing to prevent their use in the "Cook",turbine.

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/ Length of blade (mm; inch) 3 P.

n 3000 x I.2 3600 x0.5 l800 D 2376 93,S x0,$ 3 l980 78 x2 3960 IS6 498 l634 xI 498 l634 xI 498 l634

/ 792 3l 2 xo83 660 26 x2 l320 S2 D//

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between a welded rotor design and a shan with shrunken disa o< ~ Radial stress rt .,

sv as ~ Tangential stress atter plastic yield m~ ~ Tangential stress (elastic) o, ~ Yield point Fig. 24 - Low.prcssure rotor of the "Cook" turbine twctght approx.

as ~ Blading tension 00 t) being removed from the furnace aner being wcldcd and stress a<< ~ Radial stress due to blading relieved Bearing Arrangement This system has been employed on large turbines for over twenty years. The excellent results obtained prove that the For the assembly of the shaft line the neatest and also the principle adopted was correct.

technically most consistent solution was adopted, with only one bearing between two turbine casings and between the turbine and the generator. Draining, Protection against Eiasion This arrangement offers the following advantages:

In the h.p. section the expansion of the steam continues Alignment of the turboset is simple well into the wet-steam region. To protect it against

- The critical speed is clearly defined (no uncertain erosion caused by the film of water that condenses inside bearing loads during assembly or in operation due to the the turbine casing, the inner surface is lined with stainless bearing pedestal heating up unevenly) steel (Fig. 25); in the Anal stage of the blading, slots ensure Insensitivity to deformation of the foundations that this film is drained oK The bearing load is relatively high, therefore the The blades of the last I.p. stage are protected against tendency for oil whip is minimized erosion by having their leading edge hardened by an induction process.

In the I.p. section the same criteria apply as to convention-On the other hand, this single-bearing design must pay al machines because. with the arrangement adopted, the particuhr attention to the following points: expansion line yields the same steam conditions as in reheat turbines. As additional safeguards, special draining

- Different methods of assembly (though not more diff- facilities are incorporated in the crossover lines to the I.p.

icul) sections which, in the event of failure of the reheaters,

- Designing the coupling as an auxiliary bearing remove the film of condensate that forms on the walls of

- Auxiliary stub shaft, needed for overspeed test on the the pipes and thus prevent the formation of dangerous individual shaft, and for assembly water streaks at the inlet.to the I.p. blading.

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0verspeed tions, the assumptions originally made, for instance regarding the thickness of the film of water, the rate of Saturated steam turbines in nuclear power plants are evaporation, elliciency of residual expansion, <<tc., can be particularly prone to overspeeding in the event of a corrected to more realistic values.

shutdown, because the large amounts of steam trapped in Now that it is possible to carry out accurate calculations, piping and vessels (water separators/rcheaters) still con- the overspeed situation to be expected can now be tain a considerable amount of energy and because the film predicted reasonably weil for new turbines still in the of condensate on the walls of all casings and pipes flashes design stage.

due to the sudden drop in pressure and, when it subse- If the calculation indicates that thc machine is liable to quently expands, also produces a large amount of energy. reach a dangerous overspeed (without valves), two valves However, in contrast to earlier assumptions. this energy is in series are provided, controlled by independent systems released relatively slowly. Therefore, the rise in speed is (the one by the stcam regulating system, the other by the fairly steep to begin with, i.e. at the customary rate, and tripping system). In this way the turbine and generator are then after about onc second. it reaches the level known for reliably protected, even if one of the systems should fail.

conventional plants. Due to the residual energy in the For the "Cook" turbine described, the calculation proved slowly evaporating films and deposits of moisture, there the necessity of providing two butterfly valves, clearly follows a second, though slower rise in speed, which does recognizable in Fig. 4 and 5.

not reach its considerably higher culminating point for Ifthe spccd rise werc suflicient to cause the ovcrspeed trip about 30 to 40 s. This "creeping" rise in speed is countered to operate, though still belo~ the danger limit (assuming by incorporating valves in the crossover pipes. This is no valves), it would be sullicient to incorporate only one shown by the test results reproduced in Fig. 26. valve. If the overspeed trip were not actuated, the By comparing the results of measurements and calcula- interceptor valves can be dispensed with altogether.

Fig. 26- Shutdown tests on turbines in a PWR plant n ~ Relative increase in speed (%)

~ Time (s)

Interceptor valves blocked in open position B Interceptor valve in operation Fig. 25 - Protection of the wall of the h.p. casing by rings ol'tainless ~ Test results steel (A) ~ Extrapolated values 20 100%

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Safety Analysis Bibliography The special conditions under which turbines in nuclear [1] 11'. Hossll, K Krickr Morc large turbosets for the USA power plants operate necessitates a special safety analysis Brown Boveri Rev. 1968 SS (I/2) 3-14.

being undertaken, in which the most improbable incidents have to be taken into account. The Atomic Energy [2] A. LQrhyr Some advantages of welding turbine rotors.

Commission (USA) therefore stipulates a twofold analysis Weld. J. 1968 47 (6) 461-474.

of the plant. All components are examined, firstly in respect of their own inherent safety and secondly with [3], H. Miihlhauserr Modern turbines for feed pumps.

regard to thc results of a possible incident. Brown Boveri Rev. 1971 S8 (10) 436-451.

The corresponding examination of the turbine covers the, design, production and testing, in so far as the safety is concerned (control and safety systems, safety factors, design principles, manufacturing procedures, and so on).

Finally the conditions applying in the event of runaway were investigated. This situation is extremely unlikely, though, as it can only arise when all safeguards fail. Above a certain speed which is only slightly below the theoretical runaway speed of 200% of rated, isolated components begin to fail: plastic deformation of blade fixings and at the generator end-bells, fouling of the rotor, explosion of the rotor (especially disc sections, which are deliberately designed to burst first).

Next it is necessary to investigate whether and with what residual energy debris can be expelled from the machine.

If debris does penetrate the turbine casing. the residual energy must be so small that there is no risk of the containment being penetrated if it is struck.

Owing to the high degree of safety inherent in Brown Boveri turbine designs, the construction permit for "Cook" plant was obtained on presentation of the results of the various investigations. The operating permit review is in progress at thc time of writing.

Prospects The extensive preparations enabled us to present a mature design to AEP at the end of 1967, which can be used for the most part in the execution of other orders.

Certain problems in production and in thc procurement of materials were recognized in good time and the appro-priate conclusions drawn for future machines of this class.

With all the data worked out, we are now in a position to proceed to still larger unit capacities (over 2000MW) without difficulty, provided certain prerequisite conditions arc fulfilled.

18

BBC BROWN BOVERl BBC Brown, Boveri 4 Company, Ltd.

CH-5401 Baden/Switzerland Printed in Sveitterlend (780l 500.0)

Cteeerticetron ltn. 01010