JPS60195304A - Thermal stress controller for steam turbine casing - Google Patents

Abstract

PURPOSE:To reduce a degree of thermal stress, by installing a first flow passage along an outer surface on an inner casing and also a second flow passage along an inner surface on an outer casing both of a steam turbine consisting of double casings. CONSTITUTION:A first flow passage 18 is formed on an outer surface of an inner casing 5 by a cover 17, while a second flow passage is formed on an inner surface of an outer casing 6 by a cover 27. In time of transient operation in a turbine, steam is made to flow in the first flow passage along but in time of steady operation, the steam is made to flow in the second flow passage alone, thus thermal stress in both casings is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a casing thermal stress control device for a steam turbine having a double casing structure, and particularly to a casing thermal stress control device suitable for a steam turbine that frequently starts and stops. Regarding.

[Background of the invention]

In a conventional steam turbine, the main steam temperature is 538°C and the pressure is 24°C.
The maximum is 6aug, and only a few comparatively small-scale steam turbines with higher temperatures and pressures have been experimentally attempted. One of the factors that prevents higher temperatures and pressures is that there is a limit to the heat resistance strength of structural materials. The main heat-resistant material for conventional steam turbines is ferritic low-alloy steel containing small amounts of Cr, Mo, etc. as alloying elements, and its operating temperature limit is said to be 593°C. However, due to the energy situation in recent years, the 5 steam conditions have been changed to 649℃+
Attempts are being made to dramatically improve the thermal efficiency of steam turbines by increasing the temperature and pressure to 352 AUG. However, the conventionally employed ferritic heat-resistant steel cannot be used at temperatures above 593° C., and instead, the use of austenitic heat-resistant steel, which has excellent heat resistance strength, is being considered. This type of material is widely used as a structural material for high-temperature equipment in nuclear equipment, chemical equipment, etc., and has a sufficient track record. However, when using it as a structural material for ultra-high-temperature, high-pressure steam turbines, the following points should be taken into account. becomes a problem.

That is, austenitic heat-resistant steel, for example.

If 5US316 is used, its thermal conductivity is 15 caQ/m.
h'c thermal expansion coefficient is about 17XlO-'/'C, on the other hand,
Conventionally, the thermal conductivity of ferritic steel such as 2jCr1Mo steel, which is commonly used as a structural material for steam turbines, is 35KcaR/rn h, and the C1 thermal expansion coefficient is 13.
xio-'/'C7', and the austenitic laminated steel is a material with a low thermal conductivity and a large coefficient of thermal expansion, and can be said to be a disadvantageous material considering the mechanism of generation of thermal stress.

By the way, since steam turbines are under extremely high pressure compared to other high-temperature equipment, their structural members have a thick structure that has never been seen before as an austenitic heat-resistant steel. Furthermore, due to power supply and demand conditions, steam turbines for thermal power plants are started and stopped once or twice a day, or undergo load fluctuations for peak power use.

Due to the characteristics of the materials and the thick-walled structure of steam turbines, ultra-high temperature, high-pressure steam turbines are subject to significantly greater thermal stress in various parts due to start-up, shutdown, or electrical load, compared to conventional steam turbines. What happens can be easily inferred. Therefore, in order to realize an ultra-high temperature/high pressure steam turbine, controlling thermal stress, particularly suppressing thermal stress, is an important issue. In particular, in the large-capacity turbines of P&K's thermal power plants, the safety and economic efficiency of power supply depends on how to suppress the occurrence of cracks due to thermal stress that occurs in the thick walls, and how to ensure rapid startup, shutdown, and load fluctuations. This is an important issue to ensure. For this reason, crude operation control methods have been proposed. As a method to control thermal stress in a steam turbine, the steam temperature is measured, the surface temperature of the turbine rotor and the temperature of the rotor center hole are estimated from the measured value, and the thermal stress on the rotor surface and center hole is calculated. Methods are known for controlling the rate of change of steam temperature and pressure so that the values are within acceptable values. In addition, regarding thermal stress in the boiler, the steam temperature is measured, the future steam temperature is predicted from that value, the thermal stress is calculated, and the maximum temperature increase rate that keeps the thermal stress within the allowable value is determined. Attempts have also been made to control steam temperature.

It seems that the basic concept of such conventional technology can be applied as it is to the thermal stress control of ultra-high temperature and high pressure steam turbines with some modifications. However, all of the conventional techniques control the rate of change of the temperature and pressure of steam so that the thermal stress falls within an allowable value. Therefore, in an ultra-high temperature, high pressure steam turbine that uses austenitic heat-resistant steel, in order to suppress thermal stress within an allowable value, it is necessary to significantly reduce the rate of change in steam temperature and pressure. The problem arises that the required time is longer, which is contrary to recent electricity demand patterns.

Figure 1 compares the stress changes during startup of the steam manifold pipe of an ultra-high temperature/high pressure steam turbine when ferritic heat-resistant steel and austenitic heat-resistant steel are used. Austenitic heat-resistant steel has slightly higher strength against repeated thermal stress than ferritic heat-resistant steel, but at most
It is about 0 to 20% higher. Therefore, if a thermal stress control method similar to that used in conventional steam turbines is applied to an ultra-high temperature/high pressure steam turbine, as is clear from FIG. 1, the start-up time must be increased by 5 to 10 times.

[Purpose of the invention]

An object of the present invention is to provide thermal stress control for a steam turbine that can reduce the time required for starting, stopping, or changing the load of a steam turbine, properly controlling thermal stress to within a specified value, and enabling safe operation. We are in the process of providing equipment.

[Summary of the Invention] In a double casing steam turbine equipped with an inner casing and an outer casing, main steam sent from the boiler first flows into the inner casing and performs work at each stage of the turbine, increasing the temperature and temperature.
The reduced pressure exhaust steam is sent to the next turbine. A part of this exhaust steam passes through the space between the outer casing and the inner casing as cooling steam, and is led to the external piping from a through hole provided in the outer casing. This is to prevent the temperature of the outer surface of the inner casing and the inner surface of the outer casing from rising close to the main steam temperature due to exhaust steam having a lower temperature than the main steam. Since the energy of the cooling steam is not converted into work for the steam turbine, it is desirable to keep the amount as small as possible from the standpoint of thermal efficiency.

In such a double casing steam turbine, during transient periods such as startup, shutdown, or load fluctuations, the rate of temperature change on the inner surface of the inner casing into which the main steam flows is large;
Next, the rate of temperature change is large on the outer surface of the inner casing and the inner surface of the outer casing facing the cooling steam flow path, and the outer surface of the outer casing is usually insulated with a heat insulating material, so the rate of temperature change is the smallest. In both the inner casing and the outer casing, the temperature gradient from the inner surface to the outer surface increases, generating high thermal stress.

Analyzing the mechanism of thermal stress generation during transients in this way, it is found that during transients, the rate of temperature change on the outer surface of the inner casing is actively increased, while the rate of temperature change on the inner surface of the outer casing is conversely reduced. The temperature gradient in the thickness direction can be relaxed, and therefore the thermal stress during transient periods can be reduced.

The present invention was made based on the above idea, and divides the steam flow path into a portion facing the inner casing and a portion facing the outer casing.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 2 is a longitudinal sectional view of an embodiment in which the present invention is applied to an ultra-high temperature/high pressure turbine. In the figure, a rotor shaft l, disks 2a to 2C, and blades 3a to 3
A rotor 4 consisting of a rotor 6 is held in a double casing consisting of an inner casing 5 and an outer casing 6, and is rotated by the thermal energy of main steam 7 sent from a boiler (not shown). The main steam 7 enters the nozzle box 10 through the main steam inlet pipe 9 extending to the tip of the main steam pipe 8.
Here, it becomes an annular flow path and connects the nozzles lla to lig and the blade 3.
8 to 3e, the thermal energy is converted into rotational energy and becomes exhaust steam 12 with reduced temperature and pressure.

A bleed hole 13 extending in a groove shape in the radial direction is provided at the inlet of the nozzle lla of the inner casing 5, and a plurality of blow holes 14 are formed from the bleed hole 13 toward the outer surface of the inner casing. At the outlet of the blowout hole 14, pressure P is applied when the pressure increases;
Pressure switch device 15 that closes at P2 and opens at pressure P2 when the pressure drops.
is provided.

A cover 17 that covers the entire inner casing is provided on the outer surface of the inner casing 5, and a space between the cover 17 and the inner casing S serves as a steam flow path 18 for the extracted steam 16. Furthermore, a swirling vane 23 is provided inside the cover 17 in a spiral shape when twisted in the axial direction, so that the axial steam 16 flows uniformly over the entire outer surface of the inner casing 5.

Near the connection between the outer casing 6 and the main steam pipe 8, a plurality of holes 19 are formed on the circumference, and a plurality of steam outlet holes 22 are also formed in a part of the outer casing 6, Each is connected to external piping (not shown). The exhaust steam 12 is sent to the primary low pressure turbine (not shown) through the exhaust hole 20 provided in the outer casing 6 , and a part of it flows as cooling steam 21 on the inner surface of the outer casing 6 and passes through the steam outlet hole 19 . and 22 into the external piping. In the vicinity of the steam outlets 19 and 22, pressure switching devices 24 and 25 are provided which open at a pressure P when the pressure increases and close at a pressure P4 when the pressure decreases.

For pressures P and -P4, P and P are set to slightly lower pressures than the steam pressure during rated operation, and P2 and P4 are set to slightly higher pressures than the steam pressure during partial load operation. I'll keep it.

When the steam turbine configured as described above is started, when the main steam 7 flows inside the inner casing 5, the pressure switchgear 15 is opened and the pressure switchgear 24.25 is closed until the pressure reaches Pl and P3. Therefore, since the extracted steam 16 flows along the outer surface of the inner casing 5, the rate of temperature rise on the outer surface of the inner casing becomes larger than when the present invention is not applied, and therefore the temperature gradient in the thickness direction of the inner casing 5 increases. is relaxed and thermal stress is suppressed.

On the other hand, since the pressure switches [24 and 25 are closed, the exhaust steam 21 does not flow, so the outer casing 6 is closed to the cover 17.
The rate of temperature rise on the inner surface of the outer casing is smaller than that in the case where the present invention is not applied. Therefore, the temperature gradient in the thickness direction of the outer casing 6 is relaxed, and thermal stress is suppressed.

In this embodiment, since the steam flow path is automatically opened and closed based on steam pressure, a special control circuit is not required, and thermal stress control can be performed at low cost and with certainty.

FIG. 3 is a longitudinal sectional view of an ultra-high temperature/high pressure steam turbine showing another embodiment of the present invention. The difference from the previous embodiment is that
The inner casing is made longer in the axial direction than the final stage, and a nozzle-shaped valve 26 with a pressure opening/closing device is provided in the outer casing part in front of the final stage, and behind this valve there is a flow path between the inner peripheral surface of the outer casing and the cooling steam flow path. The point is that a cover 27 is provided to form a .

When this steam turbine is started, as in the previous embodiment, when the main steam 7 flows inside the inner casing 5, the pressure increases to P.
Until reaching □ and Pyo, the pressure switch 15 is open, and the pressure switch 24, 25 and the nozzle-shaped valve 26 in front of the final stage are also closed, so the oil vapor 16 flows along the outer surface of the inner casing, The temperature gradient in the thickness direction of the inner casing is relaxed and thermal stress is effectively reduced.

On the other hand, the pressure switch device 24, 25 and the nozzle-like valve 26
Since the inner surface of the outer casing is also closed, no steam flows to the inner surface of the outer casing, so the outer casing 6 is only heated by radiation from the cover 17 and the cover 27, and the temperature increase rate of the inner surface of the outer casing is determined by applying the present invention. effectively smaller than without. Therefore, the temperature gradient in the thickness direction of the outer casing 6 is relaxed, and thermal stress is suppressed.

In this embodiment, by allowing the oil vapor to flow after the final stage, the turbine efficiency is not reduced, and a nozzle-shaped valve 26 with a pressure opening/closing mechanism is provided in front of the final stage to more effectively open the outer casing. Thermal stress at startup can be reduced, and the cooling steam effectively cools the inner surface of the outer casing during steady operation.

FIG. 4 is a longitudinal sectional view of an ultra-high temperature/high pressure steam turbine according to another embodiment of the present invention. In this embodiment, the main steam sent from the boiler (not shown) flows through the main steam pipe 28 and is divided into the main steam pipe 8 and the bypass pipe 29 before entering the turbine, and the amount of steam flowing through the bypass pipe 29 is determined by the flow rate. It is controlled by a control valve 3o, and the temperature and pressure are also controlled to predetermined values by a temperature control device [31] and a pressure control device 32, respectively. Steam 3 flowing through the bypass pipe while being controlled at a predetermined temperature and pressure
8 flows into the steam passage 18 through the steam port 33 provided near the connection between the outer casing 6 and the main steam pipe 8, flows along the outer surface of the inner casing 5, and finally is mixed with the exhaust steam 12. Meanwhile, a part of the exhaust steam 12 flows along the inner surface of the outer casing 6 as cooling steam 21, flows into the external pipes 35 and 36 through the steam outlets 22 and 34, and is returned to other steam systems. Piping 3
After 5 and 36 are brought together, a flow control valve 37 is provided to control the flow rate of the cooling steam 21.

In the above embodiment, when starting the steam turbine, the flow control valve 3
7 and the flow rate control valve 30 is opened to allow a part of the main steam 7 to flow into the steam flow path 18, but not to allow the cooling steam 21 to flow. At this time, the temperature of the steam 36 is controlled to approximately the average humidity of the temperature of the main steam 7 and the temperature of the exhaust steam 12, and the pressure is controlled to a pressure slightly higher than the pressure of the exhaust steam 12. By doing this, the rate of temperature rise on the outer surface of the inner casing becomes larger than in the case where the present invention is not applied, and therefore, the temperature gradient in the thickness direction of the inner casing 5 is alleviated, and thermal stress is effectively reduced. . On the other hand, since the cooling steam 21 does not flow to the inner surface of the outer casing, the outer casing 6 is only heated by radiation from the cover 17, and the temperature increase rate of the inner surface of the outer casing is higher than that in the case where the present invention is not applied. becomes smaller. Therefore, the temperature gradient in the thickness direction of the outer casing 6 is relaxed, and thermal stress is suppressed.

Once the transient state at startup is over and a steady state has been reached,
Flow control valve 30 is throttled while flow control valve 37 is opened. As a result, the flow rate of the steam 36 flowing through the steam flow path 18 is reduced, but the temperature of the outer surface of the inner casing has risen sufficiently until then. On the other hand, the cooling steam 21 begins to flow along the inner surface of the outer casing, but until then, the temperature of the inner surface of the outer casing is gradually rising due to radiation from the cover 17, and the rate of temperature increase due to the cooling steam 21 is Can be kept low.

Next, the case of transitioning from a steady state of operation to stop or partial load operation will be described. In this case as well, by throttling the flow rate control valve 35 and opening the flow rate control valve 28 as at the time of startup, the rate of temperature drop on the outer surface of the inner casing becomes greater than when the present invention is not applied, and The rate of temperature drop on the inner surface becomes smaller. Therefore, the temperature gradient in the casing thickness direction is alleviated and thermal stress is suppressed even when the engine is stopped or when transitioning to a partial load.

In this embodiment, the flow rate, temperature, and pressure of the steam flowing along the inner and outer casing surfaces can be controlled, so that thermal stress can be precisely controlled.

Furthermore, as a modified example, as shown in FIG. 5, steam temperature detectors 40 and 41 for detecting the final stage steam temperature T8 and the outlet temperature T2 of the steam flowing on the outer surface of the inner casing are provided at corresponding positions. Calculate the difference between the steam temperature T and the inner casing outer surface steam temperature TQ with a temperature change calculation device 42,
The temperature of the steam 38 is controlled by feeding back to the temperature control device [31] so that the final stage steam temperature T and the inner casing outer surface temperature T2 are approximately equal. As a result, the temperature gradient in the wall thickness direction that occurs in the inner casing during startup and load fluctuation operation is alleviated, and thermal stress can be actively suppressed. On the other hand, regarding the outer casing, a nozzle-shaped valve 39 provided in front of the final stage is opened and closed so that the flow path can be changed so that the flow flows outside the cover 27 provided on the inner peripheral surface of the outer casing during startup or load fluctuation. It shall have a control device. Note that during steady operation, since there is little temperature change in the thickness direction of the outer casing, cooling steam is made to flow inside the cover to effectively cool the inner surface of the outer casing. According to this embodiment, the temperature gradient in the thickness direction of the casing can be more actively relaxed, so that thermal stress can be effectively suppressed.

〔Effect of the invention〕

According to the present invention, thermal stress in the inner and outer casings that occurs during transitions such as starting and stopping the steam turbine or load fluctuations is suppressed, so that the starting time of the double casing structure steam turbine or the time required for load fluctuations is suppressed. can be significantly shortened.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram supplementing the present invention, and FIGS. 2, 3, 4, and 5 are sectional views of one embodiment of the present invention.

Claims (1)

[Claims] 1. In a double casing steam turbine comprising an inner casing and an outer casing, a first steam flow path along the outer surface of the inner casing and a first steam flow path along the inner surface of the outer casing. A second steam flow path is provided, a portion of the mainstream steam is introduced from the inlets of the first and second steam flow paths, and these steams are directed to the outer surface of the inner casing or the outer casing, respectively. The steam is discharged to the outside of the outer casing through the outlets of the first and second steam passages while flowing along the inner surface of the outer casing, and each of the first and second steam passages is provided with an opening/closing control device. the second steam flow path is closed during transient operation of the steam turbine to allow steam to flow only through the first steam flow path, and the first steam flow path is closed during steady operation of the steam turbine. A thermal stress control device for a steam turbine casing, characterized in that the steam is controlled to flow only through the second steam flow path. 2. The thermal stress control of a steam turbine casing according to claim 1, wherein the opening/closing device for the steam flow path is opened and closed using force due to the pressure of steam introduced into the steam flow path. Device. 3. A thermal stress control device for a steam turbine casing according to claim 1, characterized in that travel vanes are provided inside the first and second steam flow paths. 4. The thermal stress control device for a steam turbine casing according to claim 1, further comprising a device for controlling the pressure and temperature of steam introduced into the first and second steam flow paths. 5. In claim 1 or 4, there is provided a device for calculating the steam turbine casing thermal stress by providing temperature and pressure detectors in the first to second steam flow paths; A thermal stress control device for a steam turbine casing, characterized in that the pressure and temperature of steam introduced into the steam flow path are controlled by the output of a thermal stress calculation device.