FORM 2
The Patent Act 1970
(39 of 1970)
&
The Patent Rules, 2005

COMPLETE SPECIFICATION
(SEE SECTION 10 AND RULE 13)

TITLE OF THE INVENTION

“PROTECTING REHEATER COIL”
APPLICANTS:

Name Nationality Address
BHARAT HEAVY ELECTRICALS LIMITED Indian BHEL HOUSE, SIRI FORT, NEW DELHI – 110049, INDIA., with one of its manufacturing units at
High Pressure Boiler Plant, Tiruchirapalli - 620014, Tamil Nadu

The following specification particularly describes and ascertains the nature of this invention and the manner in which it is to be performed:-

BACKGROUND
The present invention in general relates to steam turbine bypass system in a steam turbine generator power plant, and more particularly to a method and system for controlling turbine bypass valve.
Generally steam turbine generator power plant comprises of a boiler, High Pressure (HP) turbine, Intermediate Pressure (IP) turbine, Low Pressure (LP) turbine, Reheater (RH), Superheaters (SH), HP turbine Bypass (HPBP) valve, LP turbine Bypass (LPBP) valve, HP turbine Bypass Spray (HPBS) valve and LP turbine Bypass Spray (LPBS) valve. Part of SH and RH coils are located in Fluidised Bed Heat Exchangers and immersed in hot ash and surrounded by refractory walls. During normal operation of the plant, fossil fuel is combusted in the boiler, the combustion results in production of steam. The steam is heated in SH which are in heat transfer relation with the boiler. The steam from the SH which is called the main steam is supplied to HP turbine for generating electricity. The temperature and pressure of the steam are reduced as it passes through the HP turbine. This steam exiting the HP turbine is supplied to RH which is in heat transfer relationship with the boiler. The steam from the RH is supplied to the IP turbine followed by supply of steam to the LP turbine. The steam exiting the LP turbine is sent to a condenser where steam is converted into water, thereafter, water is sent back to the boiler through a pump. The steam that is generated as a result of combustion in the boiler may follow an alternative path in conditions such as turbine trip, boiler trip, and load shedding conditions, among others.
Conditions such as turbine trip, boiler trip, and load shedding conditions, among other conditions requires bypassing steam around HP turbine due to safety and economic reasons, such conditions will hereinafter be referred to as bypass condition. One of the main safety reasons for bypassing steam is to protect RH coils. The RH coils which are in heat transfer relationship with the boiler would fail if steam is not supplied to the RH coils due to overheating even at boiler trip conditions due to inherent heat storage of hot ash and refractory. .
When bypass conditions do not exist, HPBP valve is kept closed, as the HPBP valve is configured to open only if the main steam pressure goes beyond a set point pressure. The set point pressure is higher than the maximum desirable main steam pressure. However, during bypass condition, the HPBP valve is instantaneously opened to bypass steam around the HP turbine even if the main steam pressure is not higher than the set point pressure. The opening of the HPBP valve enables supply of steam to RH. The RH would be damaged if the steam is not supplied to the RH due to overheating of the RH. Further, it has to be noted that the steam of desirable temperature has to be supplied to the RH. Hence, the steam exiting the HPBP valve is cooled by spraying water by opening HPBS valve, thereby reducing the temperature of the steam exiting the HPBP valve. However, the existing methods of bypassing steam around the turbine do not effectively protect the RH.
In the existing methods, after the HPBP valve is opened instantaneously, the HPBP valve is made to close again as pressure sensed after the initial opening of the HPBP valve will be less than the set point pressure. This closure of the HPBP valve results in steam not being provided to RH coil, thereby starving the RH coil which damages the RH coil as the RH coils get overheated.
Further, the HPBP valve after opening instantaneously and closing, takes a long time to open, as the HPBP valve remain closed till main steam pressure soot up beyond the set point again.
Additionally, HPBP valve is made to close as the temperature of the steam being supplied to RH after spraying will be initially higher that the maximum allowable steam temperature that can be provided to the RH. Hence, to protect the RH coils the HPBP valve is closed. However, such closure of the HPBP valve results in steam not being provided to RH coil, thereby starving the RH coil which damages the RH coil.
Further, to regulate the temperature of downstream steam (steam exiting HPBP valve) when the HPBP valve is initially opened, more spray through HPBS valve is initiated, thereby causing the downstream temperature to go below a minimum temperature of the steam that can be supplied to the RH. Supply of this steam at lower temperature wherein steam becomes water in RH coil will also damage the RH coils.
In light of the foregoing discussion, there is a need for a method and a system for bypassing steam around the turbine. Further, reheaters have to be protected when the turbine is bypassed. Additionally, HPBP valve and HPBS valve have to be accordingly controlled to protect the reheaters.

STATEMENT OF INVENTION
An object is to enable bypassing steam around a turbine.
Another object is to protect reheater during bypass condition.
Yet another object is to protect reheater from being damaged by supplying steam whose temperature is not greater than the desirable temperature of steam.
Still another object is to protect reheater from being damaged by supplying steam whose temperature is not lesser than the desirable temperature of steam.
In view of the foregoing, an embodiment herein provides a method for protecting a reheater by bypassing steam around a High Pressure (HP) turbine. The method includes the step of receiving a signal indicating existence of bypass condition. After receiving the signal, High Pressure Bypass (HPBP) valve is opened, thereby bypassing the HP turbine and supplying the steam to the reheater. Additionally, a High Pressure Bypass Spray control (HPBS) valve is opened, thereby decreasing the temperature of the steam exiting the HPBP valve. Further, a set point pressure for opening the HPBP valve is changed to pressure of main steam to dynamically follow a downward gradient pressure set point, and the HPBP valve is kept open for a override duration of time even if downstream temperature of the HPBP valve is greater than a maximum allowable temperature of steam supplied to the reheater.
Another embodiment provides a High Pressure Bypass (HPBP) controller for protecting reheater coils by bypassing steam around a High Pressure (HP) turbine, the system comprising a pressure controller, a temperature controller and a master controller. The pressure controller is configured to identify pressure of main steam and send signals to operate a HPBP valve. The temperature controller is configured to identify downstream temperature of the HPBP valve and send signals to operate the HPBP valve and High Pressure Bypass Spray control (HPBS) valve. The master controller is configured to identify a bypass condition and instantaneously open the HPBP valve in case of bypass condition by overriding the signal sent by the pressure controller and keep the HPBP valve open for a override duration by overriding the signals from the temperature controller. Further, the master controller configures pressure controller to dynamically follow a downward gradient pressure set point after a preset pressure controller override duration of time, so that the HPBP valve is kept open as long as main steam exist to enable steam flow to RH during bypass conditions.
These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

BRIEF DESCRIPTION OF DRAWINGS
The embodiments herein will be better understood from the following description with reference to the drawings, in which:
FIG. 1 is a simplified block diagram of a steam turbine generator power plant, in accordance with an embodiment;
FIG. 2 is a block diagram illustrating a HPBP controller, in accordance with an embodiment; and
FIG. 3 is a flowchart illustrating a method of bypassing steam around HP turbine, in accordance with an embodiment.
DESCRIPTION OF EMBODIMENTS
The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
As mentioned, there remains a need for bypassing steam around turbine. The embodiments herein achieve this by providing a method and a High Pressure Bypass controller. Referring now to the drawings, and more particularly to FIGS. 1 through 3, where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments.
FIG. 1 is a simplified block diagram of a steam turbine generator power plant 100, in accordance with an embodiment. The plant 100 comprises a boiler 101, High Pressure (HP) turbine 102, Intermediate Pressure (IP) turbine 104, Low Pressure (LP) turbine 106, Reheater (RH) 108a and 108b, Superheaters (SH) 110a, 110b, 110c and 110d, de-superheaters (DESH) 112a, and 112b, HP turbine Bypass (HPBP) valve 114, LP turbine Bypass (LPBP) valve 116, High Pressure Bypass Spray control (HPBS) valve 118a, Low Pressure Bypass Spray control (LPBS) valve 118b, spray block valve (BD) 120a and 120b, main steam valve 122, condenser 124, condenser pump 126 and High Pressure Bypass (HPBP) controller 128.
In an embodiment, the boiler 101 is a Circulating Fluidized Bed Combustion Technology (CFBC) operated by fossil fuel such as coal to generate steam. The steam that is generated is used to operate the HP turbine 102, IP turbine 104, and LP turbine 104. Further, the SHs 110 and RHs 108 are in heat transfer relationship with the boiler 101. In an embodiment, part of RH 108 is immersed in ash in Fluidized Bed Heat Exchanges (FBHE, not indicated in the figure). Main steam valve 122 is used to supply steam from SH 112 to HP turbine 102. HPBP valve 114 enables bypassing of steam partially or completely around the HP turbine 102. DESH 112a, HPBS valve 118a and BD 120a enable cooling of steam exiting HPBP valve 114 by spraying controlled amount of water. Similarly DESH 112b, LPBS 118b and BD 120b enable cooling of steam exiting LPBP valve 116 which enables bypassing steam around LP turbine 106. HPBP controller 128 controls bypassing of steam around HP turbine 102. Similarly, a controller may be provided to control bypassing of steam around LP turbine 106.
FIG. 2 is a block diagram illustrating a HPBP controller 128, in accordance with an embodiment. The HPBP controller 128 comprises a master controller 202, pressure controller 204 and temperature controller 208. The master controller is configured to receive commands from the pressure controller 204 and temperature controller 206. The pressure controller 204 is configured to include a programmable set point pressure and is also configured to collect readings of main steam pressure and send commands/signals to master controller to operate HPBP valve 114 based on the collected data. The temperature controller is configured to collect data relating to the temperature of steam known as downstream temperature supplied to RH coils 108, and send commands/signals to the master controller to operate the HPBP valve 114, HPBS valve 118a and BD valve 120a. Further, the HPBP controller 128 is configured to collect data and send commands to system elements within an imaginary block 127.
Additionally, it will be clear to a person skilled in the art that the HPBP controller will be configured to communicate with other system elements within the plant 100 to identify the condition of the plant such as a bypass condition. Conditions such as turbine trip, boiler trip, and load shedding conditions, among other conditions requires bypassing steam around HP turbine is known as bypass condition. Further, the HPBP controller 128 will have to communicate with other elements within the plant 100, as the operation of those elements might be based on the operations controlled by the HPBP controller 128.
During normal operation of the plant, fossil fuel is combusted in the boiler 101. The combustion results in production of steam which is heated in SH 110. The steam from the SH 110 called the main steam is supplied to HP turbine 102 for generating electricity. The temperature and pressure of the steam are reduced as it passes through the HP turbine 102. This steam exiting the HP turbine 102 is supplied to RH 108 which is in heat transfer relationship with the boiler 101. The steam from the RH 108 is supplied to the IP turbine 104 followed by supplying the steam to the LP turbine 106. The steam exiting the LP turbine 106 is sent to a condenser 124 where steam is converted into water, thereafter, water is sent back to the boiler 101 through the pump 126. The condensed water may be purified before sending the water to the boiler.
During normal operation of the boiler, wherein the HP turbine 102, IP turbine 104 and LP turbine 106 are not bypassed, HPBP valve 114 and LPBP valve 116 remain closed. HPBP controller 128 ensures that HPBP valve 114 remains closed. The pressure controller 204 which has a set point pressure configured in it enables closure of the HPBP valve 114. The set point pressure is a pressure that is higher than the maximum desirable main steam pressure. During normal operation, the main steam pressure is lesser than the set point pressure. Hence, the pressure controller 204 sends a signal to master controller 202 which ensures that the HPBP valve 114 remains closed.
The path taken by the steam changes when a bypass condition is determined. In a bypass condition, main steam from SH 110 bypasses HP turbine 102 by passing through HPBP valve 114. The steam exiting the HPBP valve 114 is regulated by DESH 112a by spraying water. The steam whose temperature is regulated is supplied to RH 108.
In an embodiment, the steam from the RH 108 is made to bypass IP turbine 104 and LP turbine 106. The LP turbine 106 may be bypassed in a manner similar to the manner in which the HP turbine 102 is bypassed.
FIG. 3 is a flowchart illustrating a method of bypassing steam around HP turbine 102, in accordance with an embodiment. At step 302 a signal indicating bypass condition is received. In an embodiment, the signal is received by HPBP controller 128. The signal is sent by elements in the plant 100 which are capable of determining a bypass condition. After receiving the signal, HPBP valve 114 is instantaneously opened at step 304. In an embodiment the HPBP valve 114 is instantaneously opened and held in that position for a preset pressure controller override duration of time. In an embodiment, master controller 202 overrides signal sent by the pressure controller 204, which may be sending a signal to the master controller 202 to keep the HPBP valve 114 closed as the main steam pressure may be less than the set point pressure which is configured in the pressure controller. In addition to opening the HPBP valve 114, HPBS valve 118a is also opened. The opening of the HPBS valve 118a enables supply of water to DESH 112a. The water is sprayed to reduce the temperature of the steam exiting the HPBP valve 114. In an embodiment, the HPBS valve 118a is initiated to open as soon as a signal indicating bypass condition is received. In another embodiment, the HPBS valve 118a opening is initiated at the same time as the initiation of opening of HPBP valve 114. Alternatively, in an embodiment, the HPBS valve 118a is opened after the HPBP valve 114 is opened to a certain percentage. In an embodiment, the HPBS valve 118a opening is initiated when the HPBP valve 118a is about 2 percent open. Further, at step 308 set point pressure in the pressure controller 204 is configured to dynamically change to be same as the pressure of the main steam enabling pressure controller to dynamically follow a downward gradient pressure set point, thereby enabling the HPBP valve 114 to be open as long as the main steam exists. In an embodiment, set point pressure is configured as soon as the HPBP valve 114 is opened. The HPBP controller 128 changes the extent to which the HPBP valve 114 is opened after a preset pressure controller override duration of time, based on the signal from the pressure controller 204 which will be configured to dynamically follow a downward gradient pressure set point. Thereafter, at step 310, HPBP controller 128 ensures that the HPBP valve 114 is kept open at least for an override duration of time. In an embodiment, the override duration of time is less than 10 seconds. In an embodiment, the master controller overrides the signal provided by the temperature controller 206 to keep the HPBP valve 114 open for at least till the end of the override duration.
In an embodiment, during the override duration, the temperature controller 206 sends signal to the master controller 202 to close the HPBP valve 114, as the temperature of the steam supplied to the RH coils 108 known as the downstream temperature may be higher than the maximum allowable temperature of steam that can be provided to the RH coils 108.
In an embodiment, at step 306, HPBS valve 118a is opened till a computed position.
In an embodiment, the computed position is calculated by the following equation:
Percentage of HPBS valve 118a open = (Main steam temperature) – (340/200) * K * 100
wherein, K = adjustable constant.
In an embodiment, K is set at 0.7.
In an embodiment, the master controller 202 closes the HPBP valve 114 after the override duration of time, if the temperature controller 206 sends a signal that the downstream temperature is greater than the maximum allowable steam temperature that can be provided to the RH coils 108. Thereby, stopping the steam supply to the RH coils 108 and protecting the RH coils 108 from being damaged due to supply of steam at undesirable temperature.
In an embodiment, the HPBS valve 118a is held at the computed position for a first duration of time, and thereafter the extent of opening is modified based on the signals provided by the temperature controller 206, which considers downstream temperature to determine the extent of opening of HPBS valve 118a.
In an embodiment, the first duration of time is a time that is preset or a time till downstream temperature of HPBP valve reaches a value which is a predetermined bias value above minimum allowable temperature of steam supplied to the reheater, whichever is less. In an embodiment the bias value is about 50.
In an embodiment, if the downstream temperature reaches a value which is a predetermined bias value above minimum allowable temperature of steam supplied to the reheater within the time that is preset, then the extent of opening of HPBS valve 118a is altered. This ensures that the temperature of steam that is supplied to the RH coils 108 do not go below the minimum allowable temperature of steam supplied to the reheater. Hence, the RH coils 108a are protected from being damaged by supplying steam whose temperature is below the minimum allowable temperature.
In an embodiment, the time that is preset is about 20 seconds.
In an embodiment, the LP turbine 106 is bypassed in a manner similar to the manner in which the HP turbine 102 is bypassed. In case of bypassing the LP turbine, pre set temperature in a pressure controller associated with the LPBS valve 116 is configured to be the same as the pressure of steam from the RH coils 108.
In an embodiment, during normal operation of the plant 100, in case the RH coils 108 are getting overheated, then the flow of hot ash into FBHE is reduced or stopped.
In an embodiment, the reduction or stoppage of hot ash to FBHE is initiated when the temperature of steam at the outlet of RH coils 108 reaches a temperature of 560 degree Celsius.
[008] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.

We claim:
1. A method for protecting reheater by bypassing steam around a High Pressure (HP) turbine, the method comprising the steps of:
receiving a signal indicating existence of bypass condition;
opening High Pressure Bypass (HPBP) valve after receiving the signal, thereby bypassing the HP turbine and supplying the steam to the reheater;
opening a High Pressure Bypass Spray control (HPBS) valve, thereby decreasing the temperature of the steam exiting the HPBP valve;
changing a set point pressure for opening the HPBP valve to pressure of main steam to dynamically follow a downward gradient pressure set point; and
keeping the HPBP valve open for a override duration of time even if downstream temperature of the HPBP valve is greater than a maximum allowable temperature of steam supplied to the reheater.
2. The method according to claim 1, wherein the step of opening the HPBS valve is initiated when the signal to open the HPBP valve is received.
3. The method according to claim 1, wherein the step of opening the HPBS valve is initiated when the HPBP valve is opened to a predetermined percentage.
4. The method according to claim 1, wherein the step of opening the HPBP valve comprises opening the HPBS valve to a computed position.
5. The method according to claim 4, wherein the computed position is determined using the equation:
Percentage of HPBS valve open = (Main steam temperature) – (340/200) * K * 100
wherein, K = adjustable constant.
6. The method according to claim 4, wherein the HPBS valve is held in the computed position for a first duration of time, wherein the first duration of time is a time that is preset or a time till downstream temperature of HPBP valve reaches a value which is a predetermined bias value above minimum allowable temperature of steam supplied to the reheater, whichever is less.
7. The method according to claim 6, comprising changing the extent to which the HPBS valve is opened, based on the downstream temperature at the end of the first duration of time.
8. The method according to claim 6, wherein the time that is preset is about 20 seconds.
9. The method according to claim 6, wherein the predetermined bias value is about 50.
10. The method according the claim 1, further comprising closing the HPBP valve if the downstream temperature of HPBP valve is substantially higher than the maximum allowable temperature of steam supplied to the reheater after the completion of the override duration of time.
11. The method according to claim 1, wherein the override duration of time is 10 seconds or less.
12. The method according to claim 1, wherein the step of opening the HPBP valve comprises opening the HPBP valve instantaneously to a predetermined position for a preset pressure controller override duration of time.
13. The method according to claim 12, further comprising changing the extent to which the HPBP valve is open based on the downward gradient pressure set point after the preset pressure controller override duration of time.
14. A High Pressure Bypass (HPBP) controller for protecting reheater coils by bypassing steam around a High Pressure (HP) turbine, the system comprising:
a pressure controller configured to identify pressure of main steam and send signals to operate a HPBP valve;
a temperature controller configured to identify downstream temperature of the HPBP valve and send signals to operate the HPBP valve and High Pressure Bypass Spray control (HPBS) valve; and
a master controller configured to:
identify a bypass condition;
instantaneously open the HPBP valve in bypass condition is identified, by overriding the signal sent by the pressure controller; and
keep the HPBP valve open for a override duration by overriding the signals from the temperature controller.
15. The HPBP controller according to claim 14, wherein the master controller is further configured to operate the HPBP valve based on the signals provided by the temperature controller after the override duration.
16. The HPBP controller according to claim 14, wherein the master controller is further configured to operate the HPBS valve based on the signals from the temperature controller after a first duration of time.
17. The HPBP controller according to claim 14, wherein the master controller is further configured to change set point pressure in the pressure controller to main steam pressure to follow a downward gradient pressure set point.
18. The HPBP controller according to claim 14, wherein the master controller is further configured to operate the HPBP valve based on the signals provided by the pressure controller after a preset pressure controller override duration of time
19. A High Pressure Bypass (HPBP) controller for protecting reheater coils by bypassing steam around a High Pressure (HP) turbine substantially as herein above described in the specification with reference to the accompanying drawings.
20. A method for protecting a reheater by bypassing steam around a High Pressure (HP) turbine substantially as herein above described in the specification with reference to the accompanying drawings.

ABSTRACT
A method for protecting reheater by bypassing steam around a High Pressure turbine is provided. After receiving a signal indicating existence of bypass condition the signal, High Pressure Bypass (HPBP) valve is opened, thereby bypassing the HP turbine and supplying the steam to the reheater. Additionally, a High Pressure Bypass Spray control (HPBS) valve is opened, thereby decreasing the temperature of the steam exiting the HPBP valve. Further, a set point pressure for opening the HPBP valve is changed to the pressure of main steam to dynamically follow a downward gradient pressure set point, and the HPBP valve is kept open for a override duration of time. Further, the extent to which the HPBP valve is opened is controlled based on the downward gradient pressure set point after a preset pressure controller override duration of time.
Reference Figure: FIG. 1