In some operating states, supplemental additional sealing steam must be fed to the sealing-steam system of a steam turbine. The feeding of supplemental sealing steam from the live-steam line involves considerable problems. According to the invention, therefore, the sealing-steam system is connected via a feeder line to a bleed point of the superheater of the boiler, so that the temperature of the supplemental sealing steam is well below the live-steam data and, in particular, is compatible with the material temperature in the region of the high-pressure shaft seal.

FIELD OF THE INVENTION
 The present invention relates to, a system for feeding sealing steam into
 shaft seals of a steam turbine, therefor a period.
 BACKGROUND OF THE INVENTION
 At the points of steam-turbine casings where the shaft of the machine
 passes through the casing, devices which prevent the ingress of air into
 the low-pressure turbine stages and also prevent the escape of steam into
 the atmosphere from casing sections of higher pressure must be provided.
 In this case, the only suitable seals are essentially non-contact seals,
 which of course exhibit residual leakage quantities.
 Systems in which a barrier sealing system prevents the escape of steam from
 the high-pressure shaft seal are therefore normally implemented. The
 leakage steam is thus drawn off into a separate system instead of flowing
 into the atmosphere. This steam may expediently be directed to the
 low-pressure shaft seal, where it flows out as sealing steam and displaces
 air from the shaft seal.
 The high-pressure leakage quantity and the sealing-steam quantity for the
 low-pressure shaft seal are ideally in equilibrium; however, systems via
 which excess leakage steam is drawn off, for example, into the condenser
 or, conversely, supplemental sealing steam is supplied are normally
 provided. In this case, the supplemental-sealing-steam feed, in particular
 in transient operating states, is of importance if the pressure in the
 high-pressure casings, for example during start-up of the machine, is not
 yet sufficient in order to deliver a sufficiently large amount of sealing
 steam. In the event of highly throttled or closed control valves, sealing
 steam must even be supplied to the high-pressure shaft seals.
 The supplemental sealing steam is normally fed from the live-steam line.
 Thus sealing steam having the high thermodynamic data of the live steam is
 available at the turbine inlet, and this sealing steam is successively
 reduced, for example by water injection, to states which are adapted in
 particular to the material temperature of the shaft and casing of the
 steam turbine at the respective sealing point.
 In transient operation of a turbine, the feeding of live steam into the
 sealing-steam system, in particular during start-up or an emergency trip,
 results in inadmissibly large jumps in temperature, which affect the
 casing and the shaft journals. Such sudden changes in the live-steam data
 can be corrected by water injection. However, the reduction in the steam
 temperature by means of water injection is not without its problems
 especially at high-pressure shaft seals. There is the risk of unevaporated
 water encountering the hot shaft, a factor which in turn leads to
 undesirable thermal shocks.
 The problems which occur during start-up of a plant can in the meantime be
 coped with by a sufficiently slow increase in the temperature at the
 outlet of the superheater of the boiler--that is to say the boiler firing
 temperature. In particular during the operation of combined-cycle plants,
 this means that the gas turbines have to be operated at a very low load
 over a longer period. Apart from the poor efficiency of such an operation,
 such an operating mode of the plant results in operational difficulties
 which are not to be underestimated.
 At this point, EP 0 605 156 B1 proposes to provide means of feeding the
 live-steam line from different temperature levels of the boiler. During
 the start-up of the turbine, the feed of the steam superheated to the
 maximum extent to the live-steam line is completely or partly interrupted.
 In this case, the live steam is supplied from an intermediate stage of the
 superheater at reduced temperature level. Admixing of saturated steam from
 the boiler drum provides a further means of controlling the live-steam
 temperature.
 Apart from the very high equipment cost, which is necessary to realize the
 method disclosed by EP 605 156 B1, the thermodynamic state of the steam,
 which is directed as additional sealing steam to the sealing-steam system,
 is still linked to the live-steam state. The result of this is that, even
 when the circuit specified in this publication is implemented, measures
 for conditioning the sealing steam in certain operating states are
 necessary. In particular during an emergency trip of a steam turbine, even
 when the circuit and the method according to EP 0 605 156 B1 are used,
 there is therefore a high risk of subjecting the shaft and the casing to
 inadmissibly large jumps in temperature, in particular in the region of
 the high-pressure shaft seal. This is because, during operation of the
 machine, the steam from the high-pressure shaft seal expands at least over
 some labyrinth tips and is therefore already considerably cooler than the
 live steam. However, during an emergency trip of the turbine, markedly
 hotter live steam is suddenly fed to the high-pressure shaft seal. As
 mentioned above, even water injection cannot compensate for this jump in
 temperature and also leads to the risk of applying water droplets to the
 surface of the hot shaft.
 SUMMARY OF THE INVENTION
 The invention intends to provide a remedy here. The aim of the invention,
 in a circuit for feeding steam into the sealing-steam system of a steam
 turbine, the steam turbine being supplied with live steam from a boiler
 via a live-steam line, which boiler comprises at least one evaporator and
 a superheater, and there being a bleed point at least at one point of the
 superheater below the live-steam temperature, is to design the circuit in
 such a way that the sealing-steam temperature is adapted to the material
 temperatures in the region of the shaft seals.
 According to the invention, this is achieved in that the bleed point is
 selected in such a way that the steam temperature at this bleed point is
 adapted to the material temperature in the region of a high-pressure shaft
 seal, in that a feeder line for the sealing-steam system is connected to
 this bleed point, and in that the feeder line and the sealing-steam system
 are completely isolated from the live-steam line.
 The essence of the invention is therefore to uncouple the supply of the
 steam turbine with the requisite supplemental sealing steam from the
 live-steam supply of the machine. To this end, a bleed point, according to
 the invention, is provided at the superheater of the boiler, and
 superheated steam below the live-steam temperature is bled at this bleed
 point, the superheated steam being utilized as supplemental sealing steam.
 The advantages of this circuit come into effect, in particular, during an
 emergency trip of the steam turbine, if the steam turbine has already been
 in operation for a certain time, and if partly expanded high-pressure
 steam from the turbine casing has been admitted for quite some time to the
 components of the machine, for example in the region of the high-pressure
 shaft seal. During an emergency trip of the turbine, additional sealing
 steam must be supplied to the shaft seals very quickly. In a circuit which
 corresponds to the prior art cited, supplemental steam at live-steam
 temperature is fed to the sealing-steam system, and this supplemental
 steam is first successively cooled down to a temperature compatible with
 the material, for example by water injection. This temperature control is
 of course sluggish, for which reason the shaft and casing in transient
 operating states are subjected to detrimental jumps in temperature.
 According to the invention, the feeder line for the supplemental sealing
 steam is therefore connected to a bleed point of the superheater, at which
 bleed point the steam temperature is below the live-steam temperature and
 is compatible with the material temperature of shaft and casing, in
 particular in the region of the high-pressure shaft seal. In this case, a
 bleed point of the superheater at which the steam is present at a
 temperature of around 400.degree. C. is preferably to be selected.
 To control the supplemental-sealing-steam flow into the sealing-steam
 system, there is advantageously a control valve in the feeder line. This
 control valve controls the pressure in the sealing-steam system and
 releases the supplemental sealing steam if the pressure falls below a
 certain minimum value. In this case, it is also appropriate to provide the
 feeder line with a drain device, where possible directly upstream of this
 control valve, or to lay the feeder line in such a way that it drains
 automatically: since steam does not flow continuously through the feeder
 line, condensate can form here, the ingress of which into the
 sealing-steam system would be detrimental.
 In a simple case, the feeder line may lead directly from the bleed point of
 the superheater to the sealing-steam system; however, given an appropriate
 design of the water/steam cycle, the bleed point of the superheater may
 also be connected to an auxiliary-steam rail, in which, for example, steam
 is present at a pressure of 20 bar and a temperature of around 400.degree.
 C. In addition to the sealing-steam system, further subsystems, such as
 the evacuating ejectors for example, may then be supplied with steam from
 this auxiliary-steam rail. Furthermore, an auxiliary-steam rail is of
 advantage if a plurality of steam sources, such as, for example, the
 waste-heat boilers of a plurality of gas turbines or an additional
 auxiliary boiler, are to be connected to the subsystems of the steam
 turbine.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 shows a first embodiment of the invention. Feedwater is put under
 pressure by a pump 12 and evaporated in an evaporator 1. The saturated
 steam is superheated to the live-steam state in a superheater 2 and fed
 via a live-steam line 3, the emergency-trip valve 44 and the turbine
 control valve 43 to the steam turbine 5 and expanded. The expanded steam
 is condensed in the condenser 6 and is again available as feedwater.
 The pressure in the sealing-steam system 9 is controlled in coordination
 with the valves 8 and 45. In normal operation of the steam turbine 5, a
 steam quantity flows out of the high-pressure part of the turbine 5 to the
 high-pressure shaft seal 55 and from there into the sealing-steam system.
 This steam quantity is cooled in an injection cooler 10 by the injection
 of water into the steam and is directed as sealing steam to the
 low-pressure shaft seal 56, the water injection branching off from the
 feedwater line, and the injection quantity being set by an injection
 control valve 46. If the entire steam quantity flowing in at the
 high-pressure shaft seal 55 cannot be utilized at the low-pressure shaft
 seal, some of the steam is drawn off via the pressure-relief valve 45, for
 example into the condenser.
 However, a sufficient sealing-steam quantity, that is a minimum pressure in
 the sealing-steam system 9, must also be ensured when the turbine control
 valve or the turbine emergency-trip valve is closed or throttled to a very
 high degree, and thus the pressure at the inlet into the turbine 5 is low,
 so that no steam can flow from there to the high-pressure shaft seal. Such
 operating states occur in particular during start-up and during an
 emergency trip. In this case, the supplemental-sealing-steam control valve
 8 opens when the pressure in the sealing-steam system 9 is too low. Via
 the supplemental-sealing-steam control valve 8 and a feeder line 7, the
 supplemental-sealing-steam system 9 is connected to the superheater 2 at a
 suitable bleed point 24, at which there is superheated steam at a
 temperature which is compatible with the material temperature which is
 present during steady operation at the high-pressure shaft seal. Here, a
 feeding-steam temperature of about 400.degree. C. will prove to be
 suitable in most cases.
 Since superheated steam does not flow permanently through the feeder line
 7, it is expedient to provide a drain in this line, where possible
 directly upstream of the supplemental-sealing-steam control valve 8.
 Otherwise there is the risk of steam condensing out in the feeder line 7
 and of this condensate being carried along into the sealing-steam system 9
 when the supplemental-sealing-steam control valve is opened. The drain 11
 also avoids a situation in which, under certain circumstances, condensate
 droplets impinge on hot material in the region of the high-pressure shaft
 seal and cause jumps in temperature.
 The mode of operation of the invention is as follows: the directing of the
 steam in the sealingsteam system during normal operation has already been
 described. The invention now comes into effect in particular when
 supplemental sealing steam has to be directed to the high-pressure shaft
 seal, that is in particular, as mentioned above, during start-up and
 during an emergency trip of the turbine. According to the prior art, the
 feeder line 7 is connected to the live-steam line 3 upstream of the
 emergency-trip valve 44. Especially during an emergency trip, this would
 mean that live steam flows directly to the high-pressure shaft seal 55, to
 which partly expanded steam from the high-pressure part of the turbine 5
 is admitted during normal operation. This results in a thermal shock to
 the material of the shaft and the casing in the region of the
 high-pressure shaft seal 55. In order to avoid local overheating phenomena
 of the material, water injection may also be present in such a case in the
 sealing-steam feed line to the high-pressure shaft seal. However, even
 this water injection can ultimately only regulate the sealing-steam
 temperature in a rapid manner, and in particular the impingement of water
 droplets on the hot components has to be avoided. The sealing-steam system
 is likewise fed with live steam during start-up, as a result of which the
 shaft and casing at the high-pressure shaft seal 55 are subjected to very
 large temperature differences. The cumulative effect of these actions
 results in a potential reduction in the service life on the one hand, but
 also in unfavorable relative expansions, for which reason operating
 clearances have to be dimensioned to be larger than necessary and so as to
 be conducive to a high efficiency.
 In an supplemental-sealing-steam feed according to the invention, these
 disadvantages are avoided by virtue of the fact that the sealing-steam
 system 9, as already mentioned, is not fed with live steam but with steam
 from an intermediate bleed 24 of the superheater, the temperature of this
 steam being lower than the live-steam temperature.
 A further embodiment variant of the invention is shown in FIG. 2. The
 water/steam cycle is identical to that described above. On the other hand,
 for the feeding of the sealing-steam system, an auxiliary-steam rail 20 is
 connected between the bleed point 24 of the superheater 2 and the
 sealing-steam system 9. In the variant shown, a reducing valve 21 is
 located in the feeder line 71. The pressure of the auxiliary-steam rail is
 controlled by means of this valve. The variant is suitable in particular
 when a plurality of steam sources are connected to one or more consumers.
 In this example, in addition to the sealing-steam system 9, evacuating
 ejectors 22 are also connected to the auxiliary-steam rail. Furthermore, a
 further steam source is connected to the auxiliary-steam rail by means of
 a connecting line 23. This could be a further boiler, or even a small
 auxiliary boiler as often used in combined-cycle plants in order to
 provide auxiliary steam for an accelerated start of the plant. The
 auxiliary-steam rail is also advantageously equipped with a drain 11.
 In the example shown, the reducing valve 21 controls the pressure of the
 auxiliary-steam system. Here, 20 bar, at around 400.degree. C., will often
 prove to be expedient. As described above, the pressure in the
 auxiliary-steam system 9 is set by the additional-sealing-steam control
 valve 8 and the pressure-relief valve 45.
 Although this invention has been illustrated and described in accordance
 with certain preferred embodiments, it is recognized that the scope of
 this invention is to be determined by the following claims.