AU2008202733A1 - Method and apparatus for cooling a steam turbine - Google Patents

Description

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention title: METHOD AND APPARATUS FOR COOLING A STEAM

TURBINE

The following statement is a full description of this invention, including the best method of performing it known to us: jzlm A0110499115v2 306019831 00 1 0 METHOD AND APPARATUS FOR COOLING A STEAM TURBINE c,q Field of the Invention The invention relates to a method and apparatus for cooling a steam turbine.

More particularly, but not exclusively, the invention relates to a method for cooling a steam turbine operating in a power generation system by force cooling Sthe turbine using steam.

C, Background of the Invention 00 0In this specification, where a document, act or item of knowledge is referred to or C discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date: part of common general knowledge; or (ii) known to be relevant to an attempt to solve any problem with which this specification is concerned.

Whilst the following discussion concerns methods and apparatus for cooling high and intermediate pressure steam turbines typically used within power generation systems, for example within electrical power stations, it is to be understood that the same principles may be applied generally to the cooling of many various types of steam turbines.

High and intermediate pressure steam turbines typically comprise a cylindrical rotor with blades attached. The rotor is situated inside an inner casing containing fixed nozzles that direct the steam flow against the rotating blades. The inner casing is itself enclosed in an outer casing with the inner and outer casings constructed in upper and lower halves.

Steam turbines usually operate at temperatures of around 500 to 600C, and to allow safe access when maintenance is required they must be taken off line and cooled. The turbine components that are heated to these temperatures must be cooled in a controlled manner to prevent damage when being removed from operational service. This is particularly important for high pressure steam turbines, the inner casing is typically has thick metal walls capable of withstanding the high pressure steam environment. To prevent distortion the casing and fixed nozzles must be cooled uniformly. Similarly, the rotor must be kept rotating jzlm A0110499115v2 306019831 00 2 during the cooling process to prevent thermal sag and bending of the turbine g shaft.

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0 If the turbine is left to cool naturally a long time is required before the temperature has decreased sufficiently to allow safe access for disassembly and maintenance. This results in significant outage time for the turbine, which can C impact significantly on the efficiency of operation of a power generation plant.

Using conventional techniques the cooling time is generally between about 48 to (Ni 00 72 hours for a typical turbine plant.

An extension of natural air cooling is forced air cooling, in which ambient air is (Ni forced through the steam path to increase the rate of cooling. Conventional methods for forced cooling turbines are described in "Large Power Steam Turbines: Design and Operation", Volume II by Alexander Leyzerovich, pages 975- 993, 1997, Pennwell Corp., ISBN 0878147149.

Power generation systems may comprise multiple steam turbine sections. A typical configuration may include three steam turbines connected in sequence, consisting of an upstream high pressure steam turbine, a downstream intermediate pressure steam turbine, and further downstream low pressure steam turbine. Techniques adopted by most power plant operators for a turbine shutdown for a plant outage include a reduction in steam flow/pressure with load prior to tripping the turbine, followed by a form of forced air cooling. Commonly, air extraction pumps (vacuum pumps) are used to draw ambient air into a condenser (via a vacuum breaker), through the low pressure turbine, into the intermediate pressure turbine and out through the combined reheat valve drains.

A further airflow path is commonly employed, namely from the condenser through the ventilator valves into the high pressure turbine exhaust and out through the turbine stop valve drains. The air is cooled in a water heat exchanger before admission to the air extraction pumps.

The rate of the forced air cooling can be increased by using air compressors to increase the pressure and special valving to increase the volume of air flow.

United States Patent No. 5,388,960 describes an apparatus that allows air to be injected between the inner and outer casing as well as between the inner casing jzlm A0110499115v2 306019831 00 3 and rotor. United States Patent No. 6,145,317 describes an alternative method for Sinjecting and exhausting air from a turbine casing to accelerate the rate of cooling.

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A drawback of air cooled systems is that application of cold air to the hot turbine (-i can cause thermal distortion or high thermal stress. Moreover, with conventional forced air cooling techniques, the rotor is cooled faster than the cooling of the Cstationary inner casing, due the fact that the rotor has a significantly larger surface Sarea to mass ratio than the inner casing. The different cooling rates can cause (-i 00 differential contraction between the rotor and the casing to such an extent that the rotor blades can make contact with the stationary nozzles. The cooling rate (Ni therefore needs to be carefully controlled to ensure a relatively uniform rate of contraction between the rotating and stationary turbine components.

The use of nitrogen in place of air is proposed in United States Patent No.

6,898,935. This technique relies on the increased heat transfer capability of nitrogen over air to reduce the time it takes to lower the turbine metal temperatures.

While all known forced cooling methods achieve a cooling rate significantly faster than natural cooling, they are still generally slow, complex and expensive. In addition, such techniques tend to consume a relatively large amount of energy and require significant maintenance burdens, that can ultimately negatively impact on the power generation plant's overall efficiency. In addition, methods using gases other than air require special storage and supply systems.

Methods of warming steam turbines during start up are known. The introduction of high pressure and temperature steam to a cold turbine would result in catastrophic damage to the turbine, and turbine manufacturers therefore include a turbine warming system. When turbines are started up from a cold shutdown condition, special warming valves are used to admit small quantities of steam to the high pressure side of the turbine. This warms the inner and outer casing and rotor,; both through the direct exposure to the steam and conduction of heat through turbine components. As with forced cooling, the rate of warming is critical to maintain a uniform rate of expansion between rotating and stationary components. Warming continues until prescribed temperatures are reached for jzlm AO11O499115v2 306019831 00 4 key turbine components, at which time the turbine is ready to be accelerated and Sloaded.

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From an operational standpoint, it is desirable to complete the warming procedure in the shortest time consistent with turbine limitations such as allowable rotor stress. Turbine warming from about 150 0 C to between about 500 Cto 600 0 C is typically achieved in about 6 to 8 hours by use of known steam control

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systems. Turbine warming using steam is generally simple, controllable and 00 efficient. In contrast, turbine cooling from about 500/600 0 C to 150 0 C using known air or nitrogen cooling methods commonly takes between 2 to 3 days.

This difference in the warming to cooling rates can be explained largely by the fact that the heat transfer coefficient of superheated steam is about 20 to 30 times higher than that of air.

As discussed above, the known methods for cooling steam turbines involve significant outage times for the turbines, and reduce their availability significantly, which can impact significantly on overall operational efficiency and the revenue derived from power generation plant operation. There is therefore a need to provide improved cooling methods and systems to increase the rate of cooling of steam turbines during shutdown.

Summary of the Invention According to one aspect, the invention provides a method of cooling a steam turbine by force cooling the steam turbine using steam.

It will be appreciated that to force cool the steam turbine using steam, a temperature differential must exist between the steam provided to the steam turbine and the steam turbine. The steam used to cool the steam turbine must therefore be controlled to provide a "cooling steam" of lower temperature than internal temperature of the steam turbine.

Preferably, the pressure and temperature of the steam used to cool the steam turbine is such that the steam is substantially above its saturation level. Preferably, the steam used to cool the steam turbine is above its saturation level by at least about 10'C, more preferably by at least about 20 0 C, particularly by at least about 0

C.

jzlm AO11O499115v2 306019831 00 C In one preferred form of the invention, the method includes cooling a steam Sturbine after shutdown from an operational mode within a power generation system whereby steam is used to force cool the steam turbine.

Maintenance work on steam turbines operating in power generation systems is necessary from time to time. Maintenance work requires the steam turbines be shutdown so that the temperature can cool to acceptable levels to allow work on c- Sthe steam turbines to be undertaken. Typically, the steam turbines are cooled to 00 below about 180 0 C. Shutting down the steam turbines from an operational mode within a power generation system typically requires the turbines to be tripped and (Ni the main steam valves be closed.

The shutdown from an operational mode of a steam turbine within a power generation system would cover any mode of operation where a steam turbine has reached a level of operating temperature sufficient that a cooling steam could be generated to facilitate the cooling of the steam turbine. For example, if the pressure within the system was lower than atmospheric pressure, it would be possible to supply to the steam turbine a source of steam cooler than a 100 0

C.

This would allow a steam turbine operating at or above 100 0 C to be cooled to a temperature below 100 0 C. Typically, the temperature range over which a steam turbine would be cooled from would range between about 500 to 600 0 C to about 100 0

C.

In one preferred form of the invention, the steam contained within the power generation system is used to cool the steam turbine. For example, instead of dumping steam from the system after the steam turbine has been tripped, the residual pressure and steam contained in boilers and/or steam conduits of the system can be feed back through the steam turbine for cooling.

In one particular embodiment of the invention, output steam exiting the steam turbine is used as a source for cooling the steam turbine. The output steam may be used for directly cooling the steam turbine, or introduced into a cooling steam contained in the power generation system, such as contained in a boiler or in input steam conduits, as a means to further cool the cooling steam before the cooling steam is used to cool the steam turbine.

jzlm A0110499115v2 306019831 00 6 C-i In an alternative embodiment, a boiler independent from the power generation Ssystem used to power the steam turbine, or an auxiliary boiler, can be used to

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generate a steam for use in cooling the steam turbine.

In another preferred form of the invention, the power generation system includes a steam pre-warming system for the steam turbine that is adapted to be selectively Coperable between a pre-warming mode that uses steam to warm the steam turbine during start-up and a cooling mode that uses steam to force cool the steam c-i 00 turbine during shutdown. Shutdown may involve any situation where the steam turbine must be tripped, which covers any temporary or permanent process c-i whereby any partial or more complete cooling may be required.

Preferably, an additional fluid is controllably introduced into the steam prewarming system when operating in the cooling mode to further cool the steam in the pre-warming system used to force cool the steam turbine. Preferably, the additional fluid is a liquid. Preferably, the fluid is obtained from the output steam exiting the steam turbine. More preferably, the fluid is a condensate obtained from the output steam exiting the steam turbine. In a preferred embodiment, a condenser is used to cool the output steam exiting the steam turbine.

Preferably, the cooling mode is controllably operated to optimize the temperature differential between the temperature of the steam used to cool the steam turbine and the temperature of the steam turbine. For example, in the early stages of cooling a steam turbine during shutdown the temperature differential can be controlled to provide a rate of steam turbine temperature decrease of above about 0 C per hour. Maximum cooling rates without damaging the steam turbines would depend on the type of turbines and the level of operation but would be approximately equivalent to the reverse of the pre-warming rates specified by the manufacturers and suppliers of the turbines.

In a particularly preferred form of the invention, the cooling mode is operably controlled by adjusting the amount of the output steam exiting the steam turbine that is introduced into the cooling steam of the steam pre-warming system and/or by adjusting the pressure of the cooling steam of the steam pre-warming system.

More preferably, the amount of the output steam introduced into the cooling steam is controlled by adjusting a valve means disposed on a portion of a bypass jzlm A0110499115v2 306019831 00 7 conduit connecting the output steam to the steam pre-warming system. More Fpreferably, the pressure of the cooling steam is controlled by adjusting a valve means disposed on a portion of a steam pre-warming system conduit.

In another preferred form of the invention, the power generation system includes a steam generator for generating superheated steam from a source of feed water, Cwherein the steam generator is connected to the steam turbine by an input stream Sconduit, whereby force cooling can be maintained when shutting down a steam (-i 00 turbine operating in a power generation system by flashing feed water into the 0steam generator or the input stream conduit. For example, cooling steam can (-i continue to be generated by flashing feed water into a boiler drum of the steam generator after the system has been shut down by using the residual energy contained in the system. The pressure in the input stream can therefore be generally maintained such that a cooling steam can continue to be forced through the steam turbine to continue to cool the turbine even in circumstances where a minor pressure leakage occurs in the system. Preferably, the power generation system includes a pump system for pumping feed water into the steam generator.

The pump system can therefore be operably controlled to provide a feed water to the steam generator where the pressure of the cooling steam falls below about 250 kPa. Typically, the amount of cooling steam created by flashing feed water into the steam generator or input stream conduit may be between about 1 to 10% of the volume of feed water introduced. Typically, for a power generation system where the steam turbine is offline, the maximum temperature of feed water is about 130'C. The saturation pressure for feed water at about 130'C is about 270 kPa, therefore flashing of feed water will occur when the pressure drops below 270 kPa.

According to another aspect, the invention provides a method of cooling a steam turbine from an operational mode within a power generation system, wherein the power generation system includes a steam generator for generating an input stream, the steam generator associated with a steam conduit for providing the input stream to the steam turbine through which the input stream can flow and exit as an output stream, the method comprising the steps of: izlm AO10499115v2 306019831 00 8 CI i. shutting down the steam turbine and steam generator from Sthe operational mode; and

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ii. initiating a cooling mode to force cool the steam turbine by N introducing a flow of cooling steam through the steam turbine 5 that is cooler than the steam turbine.

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The shutting down of the steam turbine and steam generator from the operational N, mode typically includes the closing of one or more steam valves.

00 SPreferably, the cooling steam is obtained by using the residual energy stored in the power generation system. Alternatively, the cooling steam may be obtained from an external source, such as an auxiliary steam generator. More preferably, the cooling steam is generated from the residual energy stored in the steam generator and/or steam conduits.

Preferably, the flow of cooling steam introduced through the steam turbine during the cooling mode is the reverse flow to the steam flow during the operational mode of the steam turbine within a power generation system. A reverse flow of cooling steam can be achieved by use of one or more additional conduits in association with one or more control valves to reconfigure the flow direction of the steam.

In a preferred embodiment of the invention, one or more of the steam conduits of the system are used during the cooling mode for providing the cooling steam to the steam turbine. Preferably, the steam conduits include an input stream conduit for providing steam to the steam turbine and an output stream conduit for receiving an exhaust steam from the steam turbine during the operational mode, wherein the input stream conduit and/or output stream conduit can also be used during the cooling mode for providing the cooling steam to the steam turbine.

In another preferred embodiment of the invention, the cooled output steam exiting the steam turbine during the cooling mode is further used to cool the steam turbine. The cooled output steam can be feed directly back into the steam turbine. In an alternative embodiment, the cooled output steam can be controllably introduced into the cooling steam to control the temperature of the cooling steam introduced into the steam turbine.

jzlm AO0110499115v2 306019831 00 9 In another preferred embodiment of the invention, the power generation system Sincludes a steam pre-warming system for warming the steam turbine during startup, whereby in the cooling mode the cooling steam is provided to the steam C, turbine by using the steam pre-warming system.

In another preferred embodiment of the invention, the cooling steam can be C further cooled by controllably introducing at least a portion of a cooling fluid into the cooling steam. Preferably, the cooling fluid is liquid. More preferably, the 00 cooling fluid is a condensate obtained from a cooled output steam exiting the Ssteam turbine during the cooling mode. More preferably, the system includes a bypass conduit that can be used in the cooling mode to transfer condensate from an output stream conduit to an input stream conduit, wherein the input stream conduit provides a means for transferring cooling steam from the steam generator to the steam turbine, and wherein the temperature of the cooling steam is controlled by operably controlling the amount of condensate introduced into the input stream conduit via the bypass conduit. The input stream conduit includes an input stream conduit of a steam pre-warming system used to provide steam to the steam turbine during start-up.

In a particularly preferred form of the invention, the cooling mode is operably controlled by adjusting the amount of the cooled output steam introduced into the input stream conduit and/or by adjusting the pressure of the cooling steam in the input stream conduit. More preferably, the amount of the cooled output steam introduced into the input stream conduit is controlled by adjusting a valve means disposed on a portion of the bypass conduit. More preferably, the pressure of the steam in the input stream conduit is controlled by adjusting a valve means disposed on a portion of the input stream conduit.

In large scale power generation systems, typically a series of up to three steam turbines can be sequentially mounted together on a common shaft consisting of a high pressure, intermediate pressure and low pressure steam turbine. The method of the invention can therefore include force cooling a high pressure and/or an intermediate pressure steam turbine operating together in a power generation system.

jzlm A0110499115v2 306019831 00 N According to another aspect, the invention provides a power generation system Sincluding:

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a steam turbine through which an input stream can flow and exit as (Ni an output stream during an operational mode of the steam turbine within the power generation system; a steam generation means for generating steam for the steam turbine; 00 0an input stream conduit for connecting the steam generated from the steam generation means to the steam turbine; an output stream conduit for connecting the steam exiting the steam turbine to the steam generation means for use in recycling the steam through the system; a cooling steam generation means for generating a cooling steam for use in force cooling the steam turbine following shutdown from an operational mode; a cooling control means for providing a cooling mode following shutdown of the steam turbine from an operational mode, wherein the cooling control means is adapted to selectively control the flow of the cooling steam through the steam turbine such that a temperature differential between the cooling steam and the steam turbine can be maintained to cool the steam turbine.

Preferably, the steam turbine is a high or intermediate pressure steam turbine of the type requiring a pre-warming system during start-up.

Preferably, the cooling steam generation means includes the steam generator and/or input stream conduit adapted such that, in use, following shutdown of the system from the operational mode, residual energy is used to generate a cooling steam. The cooling steam generation means may also include an auxiliary steam generator.

Preferably, the system includes a bypass conduit adapted to allow the controlled transfer of steam exiting the steam turbine during the cooling mode back to the steam turbine to facilitate the cooling of the steam turbine. Preferably, the system jzlm A0110499115v2 306019831 00 11 C-i includes a bypass conduit adapted to controllably connect the output stream Sconduit to the input stream conduit such that the steam exiting the steam turbine can be used to control the temperature of the cooling steam in the input stream N' conduit during the cooling mode.

Preferably, the system includes a condenser to facilitate cooling of the steam in the Coutput stream conduit.

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In a preferred form of the invention, the power generation system includes: (Ni 0a control valve disposed on the bypass conduit adapted to control the amount of fluid in the output stream conduit introduced into the input stream conduit; a temperature monitoring means for the steam turbine; a pressure control valve disposed on a portion of the input stream conduit for controlling the pressure of the cooling steam in the input stream conduit during the cooling mode; and a control means operably connected to the bypass conduit control valve and input stream pressure control valve, and in communication with the temperature monitoring means to enable control of the temperature differential between the steam turbine and the cooling steam to facilitate cooling of the steam turbine.

In another preferred form of the invention, the power generation system includes a pump system, preferably an air extraction pump, for pumping feed water into the steam generator, preferably adapted to operate when the steam pressure is below about 270 kPa.

In another preferred form of the invention, the system and cooling steam generation means are adapted to enable flashing of feed water into the steam generator, for example the boiler drum of a steam generator, or into the input stream conduit such that cooling of the steam turbine can be maintained during later stages of cooling the steam turbine, preferably when the steam pressure falls below about 270 kPa.

jzlm A0110499115v2 306019831 00 12 N, The present invention utilises process steam with the turbine off line to increase Sthe rate of turbine cooling while minimising the level of thermal stress. Cooling times of less than about 10 hours may be achieved using this process.

For typical power generation systems employing steam turbines it is generally accepted industry practice to start a steam turbine from cold conditions using Ssteam warming, with start up times of about 6 to 8 hours necessary to achieve operating temperatures of between about 500 to 600 0 C while minimising thermal 00 distortion and stress. The above aspects of the invention cover a method of Sturbine cooling that is effectively a reversed system to turbine warming. The process can be applied for cooling down steam turbines and steam generators during the shutdown of the power generation systems using steam as a cooling medium.

The methods of the present invention can be particularly applied to power generation systems comprising a steam generator, bypass system, high- and/or medium-pressure turbines connected to the condenser via exhaust duct and different drain lines and incorporating an existing steam pre-warming system used for start ups. After the turbine is tripped for unit shutdown, the steam to the turbine glands and condenser vacuum is maintained. The steam generator pressure is preserved by closing any high and low pressure bypass valves, and any drain and ventilator valves. Steam is accumulated in the steam generator highpressure conduits pipes), reheat-pressure conduits or in the drum for drum type steam generators).

As soon as the turbine decelerates to below the rated speed, steam cooling may commence. Cooling steam to the turbine can be applied via the conduit normally used for turbine warming during start ups. To further boost the cooling rate, existing boiler temperature sprays and high-pressure and low-pressure bypass pressure control valves may be used to maintain the desirable thermal conditions cooling steam temperature and pressure). Also different drain valves around the boiler and turbine can be used to control steam flows through the areas where a preferential cooling is required. In ideal conditions, a high cooling rate of turbine and boiler metal can be maintained at an average of between about 20 to 0 C per hour for up to between about 8 to 10 hours. This cooling rate is jzlm A0110499115v2 306019831 00 13 N- achieved by the natural reduction in steam temperature as a result of the steam Spressure decaying in the boiler conduit because of irreversible steam losses through the cooling lines and steam temperature reduction due to a "throttling" l effect. Therefore the difference between cooled metal and cooling steam is automatically sustained at a constant level and both the metal of the steam turbine and steam temperatures can be ramped down at a controlled rate.

NAfter steam pressure in the steam generator conduit drops below atmospheric (Ni 00 level the cooling process may be completed. However for drum type boilers a 0significant level of cooling can be sustained for another several hours (if required, (Ni particularly to cool high pressure boiler conduit) even for steam pressures below the atmospheric level by flashing of feed water left in the boiler drum into the steam space. To boost the flashing effect the stand by air extraction pump can be put into service.

Brief Description of the Drawings A preferred embodiment of the present invention will now be further explained and illustrated, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic drawing showing a power generating mode for directing steam in a steam generation system employing a steam turbine; Figure 2 is a schematic drawing showing a pre-warming system operation of a steam turbine in a steam generation system; Figure 3 is a schematic drawing showing an embodiment of the present invention for controllably redirecting steam in a steam generation system to cool a steam turbine; Figure 4 is a schematic drawing showing an embodiment of the present invention for controllably redirecting steam in a steam generation system to cool a steam turbine by maintaining steam pressure by flashing of feed water; and Figure 5 is a schematic drawing showing an embodiment of the present invention for controllably redirecting steam in a steam generation system having more than one steam turbine.

jzlm AO110499115v2 306019831 00 14 C Detailed Description of the Drawings In the following description of the embodiments of Figures 1 to 5, corresponding O features have been given the same reference numerals.

As shown in Figure 1, a steam generation system generally comprises a steam generator 1 coupled via steam piping (conduit) to deliver steam to a high pressure C"3 steam turbine 2 that can be mechanically coupled to turn an electrical generator The steam generator 1 typically comprises a steam drum 9 in which water is 00 converted to steam by heat from a thermal energy source such as fossil fuel.

The power generating mode for a steam generation system is shown in Figure 1.

Steam from the drum 9 enters the high pressure steam piping 10 and travels through the main turbine stop valve (TSV) 11 and high pressure control valves (HPCV) 15 before entering the steam turbine 2. After passing through the turbine 2 the steam is exhausted to and condensed in the condenser 6. Water from the condenser 6 is pumped by the boiler feed pump (BFP) 7 back to the boiler 1 via the feed water pipe (FWP) 8. During standard operation in the power generation mode, the high pressure turbine bypass valve (HPBV) 13, drain valves 12,24, warming valve (WV) 18, and temperature control valve (TCV) 19 are shut.

A general turbine steam pre-warming system operation for a steam generation system is shown in Figure 2. The operation involves generating steam from the boiler drum 9 that travels through the high pressure steam pipe 10 and enters the turbine bypass 16 via HPBV 13. The main turbine stop valves (TSV) 11 are closed.

Steam parameters in the turbine bypass 16 are controlled by HPBV 13 and TCV 19.

HPBV 13 controls pressure and TCV 19 controls steam temperature by injecting (spraying) a small amount of water into the turbine bypass pipe 16. Steam temperature in the bypass pipe 16 is maintained above the saturation level by between about 20 to 30 degrees C. Heating steam to the steam turbine 2 is applied via the warming valve (WV) 18. After passing through the turbine 2, warming steam is removed to the condenser 6 via HPCV 15 and the.below seat drain valve (BSDV) 12. To avoid water accumulation inside the various conduits (pipes), all drain valves 24 are opened to the condenser 6. Any excessive steam from the bypass 16 is dumped to the condenser 6. All steam dumped to jzlm A0110499115v2 306019831 00 condenser 6 is then condensed and the water/condensate from the condenser 6 is Fpumped by the BFP 7 back to the boiler 1 via FWP 8.

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As shown in Figure 3, an embodiment of the present invention involves the use of (Ni an existing pre-warming system in a steam generation system to cool a steam C 5 turbine by controllably redirecting a cooling steam to the steam turbine. During C turbine cool down, steam pressure levels can be maintained in the system sufficient to provide a means for cooling the steam turbine. Steam pressure levels 00 are such that during the early phase of cooling, the flashing of feed water into the boiler pipe is not applicable. All steam and water flow arrangements are similar to pre-warming system operation. Important differences are that the boiler 1 is not being fired and feed water is not pumped back to the boiler. In addition, BFP 7 is only in use to supply spray water to TCV 19 for steam temperature control and drain valves 24 are shut to reduce wastage of steam.

As shown in Figure 4, an embodiment of the present invention involves the use of an existing pre-warming system in a steam generation system to cool a steam turbine by controllably redirecting steam pressure maintained by flashing of feed water. During the later stages of turbine cool down, pressure levels drop such that flashing feed water into the boiler can be used to maintain steam pressure levels to continue to facilitate cooling of the steam turbine. Therefore, to utilise heat accumulated inside the boiler 1, water from the condenser 6 is pumped back to the boiler 1 by BFP 7 via FWP 8. A portion of water flashes into steam within the boiler drum 9. Steam generated from the drum is then redirected to cool the steam turbine in a similar operation to that described in Figure 2. The important difference is that because steam is created due to flashing of feed water, and is therefore of relatively low temperature, there is no need to use TCV 19 for steam temperature control. Although the steam generated during such an operation is close to its saturation condition, there is substantially little risk of any water being created inside the turbine 2 and bypass pipe 16 because the temperature of the metal during the cool down process always has a higher temperature than the cooling steam.

As shown in Figure 5, a steam generation system or plant used in the generation of electricity commonly comprises a steam generator 1 coupled via steam piping to jzlm A0110499115v2 306019831 00 16 deliver steam to a high pressure turbine 2, reheat (intermediate pressure) turbine S3 and low pressure turbine 4 sections, which in turn are mechanically coupled to turn an electrical generator 5. Each of these turbine sections are mounted on a C common turbine shaft to drive generator The steam generator 1 typically comprises a steam drum 9 in which water is Sconverted to steam by heat from a thermal energy source such as fossil fuel. The N steam passes from the steam generator 1 through the high pressure steam piping 00 10 so as to be directed to the steam turbine sections 1, 2 and 3 in which energy in the steam is extracted. Steam that has passed through such steam turbine sections is exhausted to a condenser 6 in which the steam is condensed and the resulting water is typically fed back into the steam generator 1 by a feed pump 7 via feed water piping 8.

High pressure steam piping or conduits 10 include main turbine stop valve (TSV) 11, below seat drain valve (BSDV) 12 and a high pressure turbine bypass valve (HPBV) 13. The BSDV 12 is typically connected to the condenser 6 via flash box 14. The steam piping 10 is coupled to the high pressure turbine section 2 such that steam passing from the steam generator 1 enters turbine 2 through the high pressure control valves (HPCV) In the arrangement shown in Figure 5, the high pressure turbine section 2 is typically coupled via steam piping 10 to steam generator 1 such that steam that has passed through the high pressure section 2 is piped back to the steam generator 1 where it is reheated. Piping connecting the exhaust of high pressure section 2 to the steam generator 1 reheating area is called the cold reheat or bypass pipe (CR) 16. The cold reheat pipe 16 comprises a non return valve (CRNRV) 17, warming valve (WV) 18 and a temperature control valve (TCV) 19.

The CRNV 17 is installed for prevention of uncontrolled steam admission to the high pressure section 2 during unit's transient conditions (start ups and shutdowns).

The reheat or intermediate pressure turbine section 3 is coupled via a hot reheat pipe (HR) 20 and a combined reheat valve (CRV) 21 to the steam generator 1 such that reheated steam is directed into the inlet of the reheat section 3. The reheat turbine section 3 is further coupled to a low pressure turbine section 4 such that jzlm A0110499115v2 306019831 00 17 C-i steam exhausting from reheat turbine section 3 is directed to the low pressure Sturbine section 4. The low pressure turbine section 4 is coupled to the condenser

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6 such that steam exhausting from the low pressure turbine section 4 passes into the condenser 6. Vacuum in the condenser 6 is maintained by air extraction pumps (AEP) 22 and 23, both AEPs connected to the condenser 6. Typically, one M AEP is on duty (22) and the other is on standby A number of valves 24 are C used to drain condensed steam out of the steam pipes and away from the C turbines, which are shown across the different parts of the power plant. These 00 Svalves are normally connected to the condenser 6 via the Flash Box 14.

CN1 After the shutdown period when turbine components have cooled from their normal operating temperatures, a predetermined warm-up procedure must be followed to avoid excess stresses on turbine components. In the pre-warming procedure the heating of the high pressure turbine section 2 is controlled by the application of steam to the turbine through the control of HPBV 13 and TCV 19 via WV 18. The WV 18 is installed in such a way that it bypasses CRNRV 17 to supply warming steam to the high pressure section 2 during turbine warming up. During warming steam to the high pressure turbine section 2 comes from CR piping 16.

To maintain a favourable thermal condition during the warming process (to minimise stresses and distortion and to keep the rotor differential expansion inside acceptable boundaries) warming steam parameters in CR piping 16 are controlled by HPBV 13 (pressure) and TCV 19 (temperature). After passing through the whole flow path of the turbine 2 the warming steam is removed from the turbine 2 to the condenser 6 via opened HPCV 15 and BSDV 12.

The heating of the reheat turbine section 3 basically results from conduction along the common shaft from the high pressure turbine section. During warming up procedure the drain valves 24 must be opened to prevent water build up inside steam pipes and turbine components. Creation of water is caused by the metal temperature being cooler than steam and in the process of warming, the steam temperature may fall below the saturation level thereby forming liquid water.

In accordance with embodiments of this invention, a method for force cooling a turbine using aspects of the warming procedure is proposed. During turbine cool down process, because metal temperatures of the different plant components are jzlm A0110499115v2 306019831 00 18 higher than steam temperatures, it is safe to keep all drain valves 24 shut (as no Swater can be created during the cool down process). As steam can be preserved

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inside the different steam generator 1 areas such as the drum 9, and high pressure 10 and CR 16 pipes, this preserved steam can be used to cool down steam generator and turbine components. This process could be seen as 'reverse M warming'.

tr- As for a traditional shutdown for maintenance the steam parameters are reduced 00 as the unit load is decreased. After the power generation system is tripped all 0drain valves 12 and 24 are kept shut. Condenser vacuum is maintained by keeping in service the AEP 22 and gland steam to the turbine sections 2, 3 and 4. It is also possible to remove gland steam from the turbines 2, 3 and 4 and put into service the standby AEP 23 to maintain the vacuum in the condenser 6. Depending on the unit condition before the turbine trip, the high pressure pipe 10 and HR pipe steam temperatures may typically vary from 350 to 600'C, and the high steam pressure may vary from about 60 to 100 bar. In addition, the CR pipe 16 steam temperature may be about 350 0 C and pressure is equal to unit load before the trip, varying from 700 to 2500 kPa.

As soon as the rotational speed of the generator shaft drops below the rated speed, the forced stream cooling process can be initiated.

Due to the variations in the steam parameters mentioned above, the different control valves HPBV 13, TCV 19, WV 18, etc. can be used to adjust the steam parameters to the thermal condition of the turbine components. This is particularly important for the high pressure turbine section 2, which is the critical element of the whole plant to be cooled due to bulk metal accumulated in this area and the critical effect of thermal expansion and associated stresses.

Prior to cooling, the CR pipe 16 steam temperature is matched to the high pressure turbine 2 metal temperature by using TCV 19. Turbine cooling is commenced by opening the WV 18, HPCV 15 and BSDV 12. Cooling steam from CR pipe 16 enters the high pressure turbine section 2 via warming valve 18, flows through the steam flow path and exits turbine via HPCV 15. Cooling steam is then directed to the condenser 6 through the BSDV 12. Steam pressure in CR pipe 16 is maintained by HPBV 13 and steam temperature by TCV 19. Temperature jzlm A0110499115v2 306019831 00 19 control valve 19 is used for controlling steam temperature in CR pipe 16 in order Fto maintain the temperature difference between turbine metal and cooling steam to achieve the optimum cooling rate while keeping the thermal conditions within allowable boundaries. TCV 19 also features protection logic configured to operate to ensure that the steam temperature is at least between about 20 to 30 0 C above Cc saturation temperature for the given pressure. During the cooling process the hot C steam accumulated in high pressure pipe 10 is gradually replaced by cooler steam C accumulated in the drum 9. Therefore the cooling process involving high 00 Spressure pipe 10 is automatically commenced as soon as the first portion of cooler S 10 steam from the drum 9 enters pipe 10. As the cooler steam passes through pipe its temperature increases due to contact with the hot pipe's metal surface, thus resulting in cooling of the high pressure pipe 10. After passing the HPBV 13 and TCV 19, the steam temperature is optimised for cooling high pressure turbine section 2. Thus the cooling process can be controlled to ensure the optimum conditions due to both the natural temperature drop in the steam generator pipes and use of the control valves.

As the cooling process continues, the steam pressure in the drum 9 reduces and steam temperature gradually decreases resulting in an increased rate of metal cooling. The cooling rate can be controlled by the setting of the warming valve 18. For example, if WV 18 is closed then the pressure decay rate in boiler drum 9 decreases and the temperature drop slows down. At the same time, closing WV 18 also causes a pressure reduction inside the turbine 2, which results in a cooling rate reduction. The same effect can be achieved by changing the position of BSDV 12 which also adjusts the steam flow.

Table 1 below illustrates test results from one embodiment of the invention. The table shows the relationship between pressure and temperatures of steam exiting the steam generator drum 9 entering the high pressure pipe 10 after an initial shutdown from 6000 kPa (pressures appearing in Table 1 assist in illustrating the cooling process): jzlmA0110499115v2 306019831 Drum Exit Drum Exit Saturation Margin above Pressure, kPa(abs) Temperature OC Temperature °C Saturation Temperature OC 6000 276 276 0 3000 234 233 1 1000 182 180 2 500 167 152 200 157 129 28 100 154 100 54 During the steam cooling process there is no danger of creating water inside the steam area of the boiler, pipework and turbine because, firstly, with pressure level decaying during the cooling process steam parameters move into the zone of superheated steam and, secondly, after contact of cooler steam with hotter metal surfaces, steam temperature increases and steam conditions move even further away from the saturation condition. As shown in Table 1, as steam pressure exiting the drum 9 decreases the margin above saturation temperatures increases.

For example, for 3000 kPa pressure the margin is 1 0 C (234-233) while for 500 kPa the margin is 15 0 C (167-152). There is a further significant reduction in pressure after the steam passes through the HPBV 13 and to the CR pipe 16, therefore at this point for the steam entering the HP turbine 2 the superheating margin is even greater.

Table 1 demonstrates that by using the steam stored in the steam generator and pipes after the plant is tripped, the metal of high pressure 10 pipe can be cooled down to below 180 0 C, and as low as 150 0 C, while steam turbine sections 2 and 3, CR 16 and HR 20 pipes can be cooled down to less than 150 0 C. Lower temperatures for steam turbine and reheat system pipes can be achieved by using the TCV 19 to reduce steam temperatures to the margin of between about 20 to 30 0 C above saturation level during the final stages of unit cool when available steam pressures reach atmospheric level.

jzlm AO110499115v2 306019831 00 21 As the heat transfer coefficient for steam is at least 20 to 30 times higher than for Sair, significantly faster cooling rates are achieved. An average rate of metal

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temperature decrease of between about 20 to 25 0 C/hr is achieved, with the C cooling rate reaching between about 30 to 35 0 C/hr during initial cooling and slowing down to between about 5 to 7 0 C/hr at the end of the cooling, when the Sdifferential between metal and obtainable steam temperatures are significantly CI lower.

oO If there is no major leakage from the steam generator (eg. due to faulty drain valves) the pressure drop through WV 18 from 6000 kPa to 200 kPa can be sustained for around 12 to 14 hours. For the particular embodiments described, typically after about 8 to 10 hours the turbine metal temperature is cool enough 180 0 C level) to allow the turbine rotor to be taken off turning gear. Pressure reduction rates can be increased by using one or more of the steam pipe drain valves 24.

Where significant steam leakage occurs inside the steam generator 1 area or from steam pipe drain valves 24, such that the steam pressure cannot be sustained for a sufficiently long period (around 8 hours) to cool the metal down to the acceptable level then, for drum type boilers, the rate of cooling can be enhanced by means of flashing of feed water from the boiler drum 9 into the high pressure steam pipe 10. In this case, feed water can be pumped by feed pump 7 via feed water piping 8 to the steam generator drum 9. Depending on feed water temperature (typically not exceeding 130 0 C) and steam pressure in the drum 9, feed water starts flashing into the steam space 10 by being superheated relative to available pressure. For example, 130 0 C water will start flashing into the steam from about 250 kPa pressure and steam created by the flashing process can be used for steam pipe and turbine cooling. Even with feed water temperature below about 100 0 C, the flashing effect can be achieved by reducing the pressure in drum 9 to below atmospheric level.

The flashing effect for low feed water temperatures can be boosted by maintaining the lowest possible level of pressure inside the steam space of the steam generator 1, which can be achieved by placing the standby air extraction pump 23 into service. Depending on feed water temperature and steam pressure the amount of jzlm A 0110499115v2 306019831 00 22 C-i steam created due to water flashing can be in order of between about 1% to Sof available volume of feed water. Even with this lower level of steam flow, the metal temperature cooling rate is still significantly higher than can be achieved by N' using air.

After the desired cooling parameters are achieved, all remaining steam from the Cboiler 1, pipes 10 and 16, and drum 9 can be removed via drain valves 24 and c- N BSDV 12. Warming valve 18 and HPCV 15 can be shut.

00 While specific embodiments of the invention have been described in detail, it will Sbe appreciated by those skilled in the art that various modifications, improvements (Ni and alternatives to those details could be developed in light of the overall knowledge of the disclosure. Accordingly, the particular arrangements and embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the above description and claims contained within this application and any all equivalents thereof.

The word 'comprising' and forms of the word 'comprising' as used in this description do not limit the invention claimed to exclude any variants or additions.

jzlm A0110499115v2 306019831

Claims (28)

1. A method of cooling a steam turbine by force cooling the steam turbine ;Z O using steam, whereby a temperature differential is established between the steam N turbine and the steam used to cool the steam turbine.

2. The method of claim 1, wherein the pressure and temperature of the steam used to cool the steam turbine is such that the steam is substantially above its C saturation level, preferably by at least about 10'C, more preferably by at least 00 about 20'C, particularly by at least about 30 0 C. (Ni

3. The method of claim 1 or claim 2, wherein the method includes cooling the steam turbine following shutdown of the steam turbine from an operational mode within a power generation system.

4. The method of claim 3, wherein the steam contained within the power generation system is used to cool the steam turbine.

The method of claim 4, wherein the steam used to cool the steam turbine is an output steam exiting the steam turbine.

6. The method of any one of claims 3 to 5, wherein the power generation system includes a steam pre-warming system for the steam turbine that is adapted to be selectively operable between a pre-warming mode that uses steam to warm the steam turbine during start-up and a cooling mode that uses steam to force cool the steam turbine during shutdown.

7. The method of claim 6, wherein an additional fluid flow is controllably introduced into the steam pre-warming system when operating in the cooling mode to further cool the steam used to force cool the steam turbine.

8. The method of claim 7, wherein the additional fluid flow includes a condensate obtained from the output steam exiting the steam turbine.

9. The method of any one of claims 6 to 8, wherein the cooling mode is controllably operated such that the temperature differential between the temperature of the steam used to cool the steam turbine and the temperature of jzlm A0110499115v2 306019831 00 24 C the steam turbine is sufficient to enable a rate of steam turbine temperature Sdecrease of above about 30'C per hour. ;Z

10. The method of any one of claims 6 to 9, wherein the cooling mode is (Ni operably controlled by adjusting the flow rate of the output steam exiting the steam turbine that is introduced into the cooling steam of the steam pre-warming system and/or by adjusting the pressure of the cooling steam of the steam pre- O warming system. (Ni 00

11. The method of any one of claims 3 to 10, wherein the power generation system includes a steam generator for generating superheated steam from a source of feed water, the steam generator being connected to the steam turbine by an input stream conduit, and whereby force cooling can be maintained when shutting down a steam turbine operating in a power generation system by flashing feed water into the steam generator or the input stream conduit.

12. The method of any one of claims 1 to 11, wherein a boiler is used to generate at least a portion of the steam used to cool the steam turbine.

13. The method of cooling a steam turbine from an operational mode within a power generation system, wherein the power generation system includes a steam generator for generating an input stream, the steam generator associated with a steam conduit for providing the input stream to the steam turbine through which the input stream can flow and exit as an output stream, the method comprising the steps of: i. shutting down the steam turbine and steam generator from the operational mode; and ii. initiating a cooling mode to force cool the steam turbine by introducing a flow of cooling steam through the steam turbine that is cooler than the steam turbine.

14. The method of claim 13, wherein the cooling steam is obtained by using the residual energy stored in the power generation system. izlm AO110499115v2 306019831 00

15. The method of claim 13 or claim 14, wherein the flow of cooling steam Fintroduced through the steam turbine during the cooling mode is the reverse flow to the steam flow during the operational mode of the steam turbine.

16. The method of claim 13 to 15, wherein the power generation system includes an input steam conduit for providing the input stream to the steam turbine and an output steam conduit for receiving an exhaust stream from the Ssteam turbine during the operational mode, wherein the input steam conduit 0 and/or output steam conduit are used during the cooling mode for providing the Scooling steam to the steam turbine.

17. The method of any one of claims 13 to 16, wherein the cooled output steam exiting the steam turbine during the cooling mode is further used to cool the steam turbine.

18. The method of claim 17, wherein the cooled output steam is controllably introduced into the cooling steam to control the temperature of the cooling steam introduced into the steam turbine.

19. A power generation system including: a steam turbine through which an input stream can flow and exit as an output stream during an operational mode of the steam turbine within the power generation system; a steam generation means for generating steam for the steam turbine; an input stream conduit for connecting the steam generated from the steam generation means to the steam turbine; an output stream conduit for connecting the steam exiting the steam turbine to the steam generation means for use in recycling the steam through the system; a cooling steam generation means for generating a cooling steam for use in force cooling the steam turbine following shutdown from an operational mode; jzlm A0110499115v2 306019831 00 26 C a cooling control means for providing a cooling mode following Sshutdown of the steam turbine from an operational mode, wherein the cooling control means is adapted to selectively control the flow of the N' cooling steam through the steam turbine such that a temperature differential between the cooling steam and the steam turbine can be Cc maintained to cool the steam turbine.

The power generation system of claim 19, wherein the system includes a 00 multi-stage turbine configuration having a high pressure steam turbine and/or Sintermediate pressure steam turbine.

21. The power generation system of claim 19 or claim 20, wherein the cooling steam generation means includes: the steam generator and/or input stream conduit adapted such that, in use, following shutdown of the system from the operational mode, residual energy is used to generate a cooling steam; and/or an auxiliary steam generator.

22. The power generation system of any one of claims 19 to 21, wherein the system includes a bypass conduit adapted to allow the controlled transfer of steam exiting the steam turbine during the cooling mode back to the steam turbine to facilitate the cooling of the steam turbine.

23. The power generation system of claim 22, wherein the bypass conduit is adapted to controllably connect the output stream conduit to the input stream conduit such that, in use, during the cooling mode, the steam exiting the steam turbine is used to control the temperature of the cooling steam in the input stream conduit.

24. The power generation system of any one of claims 19 to 23, wherein the system includes a condenser to facilitate cooling of the steam exiting the steam turbine. jzlm A0110499115v2 306019831 00 27

25. The power generation system of any one of claims 22 to 24, wherein the Spower generation system includes: ;Z 0 a control valve disposed on the bypass conduit adapted to control the amount of fluid in the output stream conduit introduced into the input stream conduit; a temperature monitoring means for the steam turbine; C- a pressure control valve disposed on a portion of the input stream 00 conduit for controlling the pressure of the cooling steam in the input C stream conduit during the cooling mode; and a control means operably connected to the bypass conduit control valve and input stream pressure control valve, and in communication with the temperature monitoring means to enable control of the temperature differential between the steam turbine and the cooling steam to facilitate cooling of the steam turbine.

26. The power generation system of any one of claims 19 to 25, wherein the power generation system includes a pump system for pumping feed water into the steam generator when in operation the steam pressure of the system falls below a prescribed level.

27. The power generation system of any one of claims 19 to 26, wherein the system and cooling steam generation means are adapted to enable flashing of feed water into the steam generator or into the input stream conduit such that cooling of the steam turbine can be maintained when in operation the steam pressure of the system falls below a prescribed level.

28. The power generation system of any one of claims 19 to 27, wherein the system includes oneor more reverse flow conduits that are configured and associated with the input stream conduit and output stream conduit such that, in use, during the cooling mode, the flow of cooling steam introduced through the steam turbine is the reverse flow to the steam flow during the operational mode. jzlm A0110499115v2 306019831