TECHNICAL FIELD

[0001] The present invention relates to steam turbine performance, and particularly relates to keeping warm of the turbine rotor and casings in order to to limit the life cycle stress during steam turbine outages.

BACKGROUND INFORMATION

[0002] A major factor influencing steam turbines life is Low Cycle Fatigue. Steam turbines used to generate power typically have large diameter rotors and casings that, due to their size, are prone to thermal stress damage in the form of low-cycle fatigue. This damage is most prevalent during loading and unloading cycles, in particularly during start/ stop cycles due to thermal expansion and large rates of change of metal temperature that occur during these events. When accumulated low-cycle fatigue damage reaches material limits of the components cracks may form necessitating replacement. As a result it is desirable to avoid or at least minimise events that result in low-cycle fatigue.

[0003] European Patent no. EP 2 351 912 Al discusses a solution to the problem of overall working time required for a solar power station to be heated to operating temperature from a power-off state and/ or a start-up phase. The discussed solution comprises a heating device connectable to an internal hole of the shaft that enables heating of the shaft of the turbine.

[0004] US patent No. 4,282,708 discloses an alternate method that involves warm keeping of a steam turbine by supplying of heated steam to gland sections.

[0005] Patent application CA1245059Aldiscusses a further restart method that uses a plurality of electric heating blankets over an air gap to maintain the temperature of the turbine rotor at a desired initial start-up temperature. SUMMARY

[0006] A steam turbine warm keeping arrangement is disclosed that can provide a simple system in which the benefits to rotor Low Cycle Fatigue life through warm keeping during a steam turbine outage can be realised.

[0007] One general aspect includes a steam turbine arrangement comprising: a main steam line for feeding a working steam into the steam turbine in order to extract work from the working steam; a stop valve, in the main steam line, for isolating the main steam line; a control valve, in the main steam line downstream of the stop valve, for controlling the working steam flow through the main steam line; a steam turbine outer casing, having a first end and a distal second end, wherein the main steam line is connected to the steam turbine casing towards the first end; and a steam turbine exhaust column, connected to the turbine outer casing towards the second end, for discharging the working steam. The aspect further includes a warm keeping steam line connected to the main steam line downstream of the stop valve. This aspect makes it possible to pass warming steam through the entire steam turbine working fluid flow path when the steam turbine is offline. By keeping the steam turbine at elevated temperature by this means while the steam turbine is offline reduces temperature variation thus reducing Low Cycle Fatigue.

[0008] Further aspects include one or more of the following features, the warm steam line is connected to the main steam line between the stop valve and the control valve, a block valve in the warm keeping steam line, a heating device for heating a warm keeping steam as it passes through the warm keeping steam line, an electrical heating device, a mixing device in the warm steam line at or in the vicinity of the heating device configured to create turbulence flow in the vicinity of the heating device thereby increasing heat transfer efficiency, a control system for controlling the flowrate of the warming keeping steam, and a control system configured to operate the heating device only when each of the following conditions are met: the stop valve is closed, the control valve is open, and a pressure within the outer casing is equal to or less than ambient pressure.

[0009] Other aspects and advantages of the present disclosure will become apparent from the following description, taken in connection with the accompanying drawings which by way of example illustrate exemplary embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] By way of example, an embodiment of the present disclosure is described more fully hereinafter with reference to the accompanying drawings, in which:

[0011] Figure 1 is a sectional view of a steam turbine of the prior art to which exemplary embodiments may be applied;

[0012] Figure 2 is the steam turbine of Fig. 1 to which an exemplary embodiment has been applied; and

[0013] Figure 3 shows an alternate warming steam line to the main steam line connection point that may be applied to the steam turbine of Fig. 1.

DETAILED DESCRIPTION

[0014] Exemplary embodiments of the present disclosure are now described with references to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the disclosure. However, the present disclosure may be practiced without these specific details, and is not limited to the exemplary embodiment disclosed herein.

[0015] Fig. 1 shows a prior art steam turbine 10 comprising a main steam line 12 feeding a steam turbine body 20 towards a first end of the steam turbine body 20. Steam passing through the main steam line 12 via a control valve 14 passes through the steam turbine body 20 and exists the steam turbine body 20 by means of an exhaust column 40 typically connected to, in the case where the steam turbine 10 is an intermediate pressure steam turbine 10, a (not shown) low pressure steam turbine. The purpose of the control valve 14 is to modulate steam flow through the steam turbine body 20 and thus control the loading of the steam turbine 10.

[0016] The steam turbine body 20 comprises an outer casing 22 and an inlet spiral 26 for introducing essentially radially fed feed steam into an essentially axial orientated steam expanding section defined by an inner casing 24 and a rotor 30, wherein the expanding steam is used to drive the rotor 30. Expanded steam that exits the steam expanding section enters a turbine cavity 28 within the outer casing 22 before exiting the steam turbine body 20 at a second axially distal end via the exhaust column 40. The outer casing 22 may be enclosed by thermal insulation 23

[0017] Glands 34 are used to seal the outer casing 22 against the rotor 30, while a bearing 32 is further used to support the rotor 30.

[0018] Fig. 2 shows an exemplary embodiment of a steam turbine 10 in which a warm keeping steam line 50 joins the main steam line 12 between the control valve 14 and the outer casing 22. The purpose of the warm keeping steam line 50 is to enable a flow of warming steam through the working flow path of the steam turbine 10, including through the inlet spiral 26, between the inner casing 24 and the rotor 30, into the turbine cavity 28 and out through the exhaust column 40. As long as the warming keeping steam line 50 runs when the fluid pressure in the turbine cavity 28 is equal or less than the ambient pressure, the need for gland steam during standby operation is not necessary. In an exemplary embodiment shown in Fig. 2, the control valve 14 includes a stop valve 16.

[0019] In a modified exemplary embodiment shown in Fig. 3, the main steam line 12 includes a separate stop valve 16, upstream of the control valve 14, that is configured to be either in a fully open position or fully closed position. In this exemplary embodiment, the warm keeping steam line 50 joins the main steam line 12 between the stop valve 16 and the control 14. In this arrangement it is possible to modulate the warm keeping steam flow rate using a valve in the warm keeping steam line 50. This is preferred to using the control valve 14 to modulate the warm keeping steam flow rate as the warm keeping steam flow rate is typically significantly smaller than nominal operating steam rates and so typically stable control using the control valve 14 alone is not possible.

[0020] In an exemplary embodiment, the warm keeping steam line 50 includes a block valve 52 and optionally a heater 54 and turbulator 56.

[0021] The block valve 52 provides a means to isolate warm keeping steam from the steam turbine 10 during, for example, normal operation. The block valve 52 is an automatically operated valve connected to a not shown control system.

[0022] An exemplary embodiment further includes a heater 54. A purpose of the heater 54 is to provide a means to heat warm keeping steam up to a temperature that enables warm keeping steam passing through the steam turbine 10 to maintain temperature of casings 22, 24 and, in particularly, the rotor 30 thereby compensating for ambient heat loss through the outer casing 22 and associate thermal insulation as well as losses of heat carried by warm keeping steam passing through the steam turbine 10 and out through the exhaust column 40. The heater 54 may be any known device capable of imparting thermal energy into warming steam passing through the warm keeping steam line 50, including electrical devices. In an exemplary embodiment the heater 54 is located externally to the warm keeping steam line 50.

[0023] In an exemplary embodiment, warm keeping steam is provided to the warm keeping steam line 50 at a temperature ranging from 100°C up to nominal live steam temperature, wherein an electrical element wound around an outer section of the steam keeping steam line 50, downstream of the block valve 52, acts as a heater 54.

[0024] In an exemplary embodiment, the heater 54 heats warm keeping steam to greater than 550°C. The temperature to be heated by the heater 54 is dependent on the design and operation of the steam turbine 10.

[0025] A further exemplary embodiment includes a turbulator 56 in the warm keeping steam line 50 in the vicinity or at the location of the heater 54 where the heater 54 increases the temperature of the warm keeping steam. The purpose of the turbulator 56 is to generate turbulent flow thereby increasing heat transfer rates between the heater 54 and the warm keeping steam thus making it possible to reduce the heat transfer area of the heater 54.

[0026] As shown in Fig.4, in an exemplary embodiment, each turbulator 56 comprises a hollowed circular plate with a number of axially aligned holes 57 located on turbulator' s circumference. When the block valve 52 is opened, the steam flowing along the turbulators accelerates and expands successively increasing the turbulence and hence improving the heat transfer from the electrical heater 54.

[0027] An exemplary embodiment involves activating the heater 54 and then, when the steam temperature in the electrical heater reaches >550°C, i.e. nominal live steam temperature, bypassing the steam turbine by closing the stop valve 16, and/or control valvel4. The next step is to shut off the gland system and open the block valve 52 in the warm keeping steam line 50. In this way a warming steam flows through the steam turbine 10 and out through the exhaust column 40. The block valve 52 then remains open until the next start. [0028] An exemplary embodiment includes a control system (not shown) typically use with a steam turbine 10. The control system is additionally configured to control the flowrate of the warming keeping steam. In an exemplary embodiment, the control system is configured to operate the heating device 54 only when each of the following conditions are met: the stop valve 16 is closed, the control valve 14 is at least partially open, and a pressure within the outer casing 22 is equal to or less than ambient pressure.

[0029] Although the disclosure has been herein shown and described in what is conceived to be the most practical exemplary embodiment the present disclosure can be embodied in other specific. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the disclosure is indicated by the appended claims rather that the foregoing description and all changes that come within the meaning and range of equivalences thereof are intended to be embraced therein.