Patent Application
20130111921
The present invention relates to a method and apparatus for warming a gas turbine engine rotor and, in particular, to a method for controlling the gas turbine rotor temperature during periods of extended downtime using steam to heat air extracted from a gas turbine enclosure which is then fed directly to the rotor. In an alternative embodiment, the method utilizes auxiliary boiler steam for purposes of heating the air fed to selected rotor passages during extended periods of downtime.
Gas turbine engines typically include a compressor section, a combustor section and at least one turbine that rotates in order to generate electrical power. The compressor discharge feeds directly into the combustor section where hydrocarbon fuel is injected, mixed and burned. The combustion gases are then channeled into and through one or more stages of the turbine which extracts rotational energy from the combustion gases. The temperature of gas turbine rotor blades rises very quickly when a gas turbine is started because the blades are exposed to very high-temperature exhaust gases. The temperature of the outer peripheral parts of a turbine also increase very quickly due to heat conduction from the blade as compared to inner peripheral rotor components. The rate of increase in temperature thus tends to be slower on the inner side of the rotor than on the outer side. The difference in conductivity of components can also cause a temperature differential between the inner and outer peripheries of rotor components, creating additional thermal stresses during startup. A separate centrifugal stress also exists during startup due to rotation of the engine.
Thus, the combination of thermal and centrifugal stresses on the rotor are much higher when the engine has been sitting idle during, for example, periodic maintenance. As a result, during startup following extended periods of downtime, the rotor disks can undergo significant thermal and mechanical stresses and are vulnerable to premature failure due to the shock occurring during startup, particularly at or near the rotor disks.
An example of a conventional rotor warming structure for a combined cycle plant includes a central gas flow passage with gas from a compressor fed into the central passage in the rotor. Normally, a portion of the compressed gas is introduced into the gas turbine blades through branches emanating from a central passage. Another known method for warming the rotor prior to startup relies on an electrical heating system surrounding the rotor. However, such systems can be prohibitively expensive and often do not sufficiently protect against temperature differentials during startup. Both air and electrical systems also do not take advantage of the potential heating and cost-saving benefits using on-site steam available within the same power generating plant.
A primary object of the present invention is to provide a method and apparatus for keeping a gas turbine rotor warm during periods of extended downtime by using a portion of the flow from a steam turbine or, alternatively, from an outside steam source, in order to heat air originating from the gas turbine enclosure. The higher temperature air in turn serves as a more effective primary heat source for the rotor cavity and blades.
As detailed below, a new method for warming the rotor of a gas turbine engine comprises the steps of feeding an ambient air stream to an air blower to increase the air pressure; extracting a portion of compressed air from the discharge of the air blower while feeding a partial air stream to one side of a heat exchanger (e.g., shell and tube type); feeding steam (typically saturated) to the other side of the same heat exchanger; passing the resulting heated air stream from the heat exchanger into and through defined flow channels inside the rotor; continuously monitoring the air temperature inside the rotor during the warming operation; and controlling the amount of air and steam fed to the heat exchanger based on the temperature detected inside the rotor using a feedback control loop. The feedback control data can also be used to monitor and adjust the flow rate into the rotor of a heated air stream.
The invention also includes a related structure for warming a gas turbine rotor during periods of downtime comprising an air blower, a heat exchanger for heating compressed air from the air blower using heat from an internal steam source, a plurality of air passages into and out of the rotor sufficient in size and number to carry a prescribed amount of heated air through the rotor to heat the turbine blades and rotor cavities to a uniform temperature, steam fluid flow passages into and out of the heat exchanger, and a feedback control loop for controlling the amount of air and steam fed to the heat exchanger.
The warming structure described herein is particularly useful in cold weather conditions and specifically intended to replace conventional electric heaters used to warm a gas turbine rotor during extended periods of downtime. In an exemplary embodiment, steam from another part of the plant is used as the principal heating medium and results in a more cost effective and reliable heating system without using conventional electric heaters. A portion of sealing flow from the steam turbine or gland leakage steam transfers heat to an inlet air feed using a combination heat exchanger and air blower. An alternative embodiment uses a similar configuration but with auxiliary boiler steam as the primary source of heat.
The invention offers particular advantages to multi gas turbine plant configurations where saturated steam is readily available for warming up one or more gas turbine rotors during periods of downtime. As the principal heating source, the steam can be extracted from auxiliary boiler/gland steam/sealing sources. A shell and tube heat exchanger transfers heat from the steam to air taken from a gas turbine enclosure that has been compressed using a blower. The heated air is then fed to the gas turbine inlet plenum through a control valve and piping network and the spent steam is fed back into the gas turbine engine bottoming cycle. As such, the invention provides a much more cost effective method for keeping the gas turbine rotor warm while the system is out of service.
Turning to the figures,
A typical gas turbine rotor such as that shown in
Heated air from the heat exchanger system generally described above and in more detail in
In order to ensure that warming of the gas turbine engine rotor occurs at a prescribed rate without creating potential damage to the rotor disks,
An exemplary heat exchanger design useful in achieving the objectives of the invention is summarized below in Table 1. The shell and tube heat exchanger for the rotor uses one of two alternative streams, namely a portion of the sealing flow from a steam turbine or, in the alternative, a portion of auxiliary boiler steam form an outside source. In the example of Table 1, the amount of air to be heated on the tube side and corresponding steam flow requirements are identified for an exchanger having the specific tube sizes, dimensions and pitch configuration as shown. Table 1 also includes exemplary tube bundle design criteria, as well as inlet and outlet design temperatures for the air and steam as they enter and exit the exchanger. The resulting heated air is used in connection with the control system as described above in order to bring the rotor disks to the desired minimum temperature and thereafter maintain the same internal temperature during startup of the engine.
While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
