Power plants – Combustion products used as motive fluid
Reexamination Certificate
1999-09-03
2001-07-03
Casaregola, Louis J. (Department: 3746)
Power plants
Combustion products used as motive fluid
C060S039091
Reexamination Certificate
active
06253537
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a rotational speed control method in a process for stopping a gas turbine, and specifically to an operational method for controlling a rotational speed in the stopping process so as to avoid excessive stress to be caused in a moving blade.
2. Description of the Prior Art
FIG. 3
is a perspective view of a gas turbine moving blade, wherein numeral
11
designates a moving blade, numeral
12
designates a platform thereof and the moving blade
11
is rotated by a high temperature combustion gas G in a direction R. In such a gas turbine moving blade, operated at a rated rotational speed, when a load decreases to no load and fuel is shut off for a stop of the operation, then excessive thermal stress and centrifugal force arise in the stopping process, as described later, and a crack may occur in the blade.
FIGS. 2
are explanatory views of the transition of the occurrence of stress in the blade in the above mentioned gas turbine stopping process. FIG.
2
(
a
) shows a load state, FIG.
2
(
b
) shows a rotational speed state, FIG.
2
(
c
) shows a metal temperature state and FIG.
2
(
d
) shows the state of stress at the point A of the moving blade
11
of FIG.
3
. In FIG.
2
(
a
), a gas turbine is operated with 4/4 load (full load) until time t
1
, on the time axis. Fuel is throttled starting from the time t
1
to time t
2
, when the load decreases to 0/4 load (no load). A gas turbine rotor is kept rotated in a state of no load until time t
3
when the fuel is shut off, and then the load comes to a zero state rapidly.
In FIG.
2
(
b
), corresponding to the load transition of FIG.
2
(
a
), the gas turbine is usually kept operated at a rated rotational speed from the time t
2
, when the load becomes 0/4, to the time t
3
, when the fuel is shut off. When the fuel is shut off at the time t
3
, the rotational speed then decreases rapidly to come to a stop.
In FIG.
2
(
c
), the metal temperature is shown with respect to point A of the moving blade
11
and point B of the platform
12
, both shown in FIG.
3
. As the high temperature combustion gas flows at a constant rate until the time t
1
and likewise the load is the 4/4 load and the rotational speed is the rated speed until this time t
1
, the metal temperature is kept at a high temperature level. When the load starts to decrease at the time t
1
, the fuel is then throttled starting from the time t
1
, and the metal temperature goes down until the time t
2
of the 0/4 load to then be kept constant until the time t
3
while the state of the 0/4 load continues. As the thermal capacity is larger in the platform
12
than in the moving blade
11
, the metal temperature is kept higher at the point B than at the point A until the time t
3
.
When the fuel is shut off to zero at the time t
3
, the metal temperature decreases rapidly at both the points A, B. In this process, while the rotational speed also decreases gradually, at time t
4
, when the rotational speed has not yet decreased sufficiently, the differential temperature between the point A and the point B reaches a maximum, and thereafter the temperatures at the respective points decrease gradually to come to the same final temperature.
FIG.
2
(
d
) shows a state of stress at the point A of the moving blade
11
. The stress is constant until the time t
1
and then decreases slightly as the load decreases to the time t
2
when the load becomes the 0/4 load. Thereafter, even in the state of no load from the time t
2
to the time t
3
, the stress decreases slightly further. However, at the time t
4
, when the largest differential temperature &Dgr;T occurs as shown in FIG.
2
(
c
), an excessive thermal stress is generated. In addition to this thermal stress, as the rotational speed still exists to some extent, a centrifugal force in proportion to the rotational speed squared acts. Hence a large force is added to the point A, and a crack may occur, as the case may be, to break the blade.
As mentioned above, in the process oft the gas turbine operated with a full load being decreased in loads so as to be operated with no load, the fuel is shut off and the rotational speed decreases. As the rotational speed does not sufficiently decrease, there occurs a large differential temperature between the moving blade and the platform, and thereby a large thermal stress occurs in the moving blade. Further, in addition to this thermal stress, a centrifugal force in proportion to the rotational speed squared acts. Thus, if such process is repeated, the blade may be broken. There had been no countermeasure in the prior art for preventing the large force acting in the moving blade in this process, and an appropriate countermeasure has been long desired for safety purposes as well.
SUMMARY OF THE INVENTION
In view of the problem in the prior art, it is an object of the present invention to provide a rotational speed control method in a gas turbine stopping process, wherein, in a process that a gas turbine operated at a rated rotational speed is decreased in load to be operated with no load and is then shut off from fuel for stopping operation, a gas turbine rotational speed is controlled to be decreased so that a centrifugal force caused thereby is decreased as well as fuel being throttled so that the differential temperature in a moving blade is made smaller. Thereby the total stress caused by the centrifugal force and the differential temperature is thereby decreased, and breakage of the moving blade can be prevented.
In order to achieve the object, the present invention provides the following.
A rotational speed control method in a gas turbine stopping process comprises a gas turbine operated at a rated rotational speed with a full load being decreased in load gradually so as to be operated with no load. It is then shut off from fuel for stopping of the operation. The rotational speed of the gas turbine is controlled to be decreased at a predetermined rate starting from the time when a power supply from the gas turbine becomes zero while the gas turbine is decreased in load gradually so as to be operated with no load. The rotational speed is further controlled to be decreased to about 60% of the rated rotational speed at the time when the fuel is shut off; then the rotational speed is decreased freely.
In a gas turbine operated at a rated rotational speed with a full load, when fuel is throttled, with the load being decreased gradually to no load, and the fuel is shut off in a no load operation, then, at a point where the rotational speed has not yet been sufficiently decreased in the stopping process, the differential temperature between the moving blade and the platform becomes large, by which an excessive thermal stress occurs. In addition to this thermal stress, the centrifugal force in proportion to the rotational speed squared acts on the moving blade. Thus, a large force is added to the moving blade, and breakage of the moving blade may occur.
In the present invention, therefore, the load is decreased gradually to a no load operation, the gas turbine rotational speed is decreased at a predetermined rate from the rated rotational speed starting from the time when power generation is stopped, for example, when a power supply from the gas turbine becomes zero, or when a generator ceases power generation, and further control is done to decrease the rotational speed to about 60% of the rated rotational speed at the time when the fuel is shut off completely. The level of about 60% of the rated rotational speed is decided in consideration of restrictions of a critical rotational speed in terms of shaft or blade vibration and compressor surge. Thus, the rotational speed is decreased to about 60% of the rated rotational speed at the time when the fuel is so shut off, and thereafter the rotational speed is also lower than that of the prior art case. Hence the centrifugal force, which is proportional to the rotational speed squared, can be decreased greatly as compared with the prior art. Also, as the
Hashimoto Yukihiro
Suenaga Kiyoshi
Tomita Yasuoki
Casaregola Louis J.
Mitsubishi Heavy Industries Ltd.
Wenderoth , Lind & Ponack, L.L.P.
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