Improved method for estimating and setting exhaust purge time in a combustion system, and combustion system thereof

Abstract

The present disclosure relates to a method for estimating and setting an exhaust purge time in a combustion system including a gas turbine, the method being configured to estimate whether a purge cycle is necessary based on values ​​collected by a pressure detector located upstream of the gas turbine, specifically within a fuel gas compartment feeding the gas turbine, and also to a combustion system for implementing such a method.

Description

[Technical Field]

[0001] The present disclosure relates to a method for estimating and setting exhaust purge time in a combustion system, particularly in a combustion system comprising a gas turbine. Also disclosed herein is a combustion system for performing such a method. [Background technology]

[0002] Combustion systems that include gas turbines and their exhaust ducts must be purged of residual gases before starting or restarting the turbine, as residual gases or vapors can create an explosive atmosphere within the system.

[0003] There are safety standards that specify how to perform purge cycles for such gas turbine exhaust systems, namely API 616, ISO 21789, ISO 3977-3, NFPA 37, and NFPA 85. Specifically, all of the aforementioned standards require that a purge cycle be generally included and performed before starting the gas turbine. Typically, at least three complete volume changes of the gas turbine and downstream exhaust duct are performed during such a purge cycle.

[0004] Both ISO 21789 and NFPA 85 standards also introduce the possibility that such a complete purge cycle may be avoided if alternative safety measures are implemented. Specifically, the NFPA 85 standard introduces the concept of purge credits.

[0005] However, the risk assessment allowed by the ISO2 I789 and NFPA 85 standards, or the concept of purge credits introduced by the NFPA 85 standard, have never been thoroughly analyzed. In fact, the current approach taken by most manufacturers is conservative, and the purge cycle cannot be less than five times the exhaust volume (the maximum value defined by the NFPA 85 standard). The purged volume is considered to be greatest at the outlet of the exhaust duct (including the vertical section), and a purge cycle is always performed at each start-up or restart of the gas turbine.

[0006] However, such a conservative approach, while effective in terms of safety, can take a long time to implement, which can result in a long start-up of the gas turbine and therefore affect the availability of the combustion system.

[0007] This can lead to delays in turbine networking (in the case of turbogenerators) and overloading of some auxiliary systems such as starters, requiring special designs for long operating times, which means increased production costs.

[0008] As an example, for a combustion system with a gas turbine having a high exhaust volume and a recovery boiler installed in series, the purge time to complete five volume changes in the downstream duct can last approximately 96 minutes, depending on the size of the exhaust duct. Summary of the Invention [Problem to be solved by the invention]

[0009] Based on the above, an improved method for setting exhaust purge time in a combustion system that can reduce the operating time during turbine start-up would be welcomed by the industry, not only increasing gas turbine availability but also reducing associated operating costs.

[0010] In one aspect, the subject matter disclosed herein is directed to a method for estimating and setting an exhaust purge time in a combustion system. The method includes a turbine, specifically a gas turbine, to which fuel gas is delivered by a fuel gas compartment provided with a shut-off valve. Specifically, the fuel gas compartment includes a fuel gas inlet through which the fuel gas enters the combustion system, a first internal shut-off valve disposed downstream of the fuel gas inlet with respect to the transfer of the fuel gas to the turbine during use, and a second internal shut-off valve disposed downstream of the first internal shut-off valve. At least one pressure detector is disposed between the first and second internal shut-off valves. The method according to this aspect includes starting the turbine and checking the first and second internal shut-off valves by receiving pressure detected by the at least one pressure detector and confirming that no leaks are detected. When the check is completed and when the turbine has been shut down at least once, at least one variable is read. Specifically, the at least one variable is associated with a previous turbine shut-down and indicates whether purge credits are available. Thus, if at least one variable indicates that purge credit is available, the exhaust purge time is set to zero, meaning the next purge cycle is skipped, otherwise, if at least one variable indicates that purge credit is not available, the exhaust purge time is set to a predetermined purge time value.

[0011] Specifically, if purge credits are available, the purge cycle can be skipped a finite number of times. In other words, the above method allows for evaluating whether purge credits remain in the system, and therefore allows for starting the turbine without a purge cycle.

[0012] According to one aspect, if the at least one variable indicates that purge credits are not available and that the previous shutdown was a normal shutdown, the predetermined purge time value can be set to a first time for reducing residual exhaust gases within the combustion system below a safety threshold, where the residual exhaust gases are assumed to be equal to a first predetermined gas volume. Conversely, if the at least one variable indicates that the previous shutdown was not a normal shutdown, the predetermined purge time can be set to a second time for reducing residual exhaust gases within the combustion system below a safety threshold, where the residual exhaust gases are assumed to be equal to a second predetermined gas volume.

[0013] According to one aspect, the checking can be performed by pressurizing a first volume of the fuel gas compartment between the fuel gas inlet and the first shut-off valve with fuel gas entering the fuel gas compartment through the fuel gas inlet while the first shut-off valve is closed, and verifying that the pressure detected by the at least one pressure detector does not rise above a first pressure limit. After such pressurization of the first volume, the first shut-off valve can be opened while the second shut-off valve can be closed. Thereafter, the first shut-off valve can be closed again, and a predetermined time can be waited to verify that the pressure detected by the at least one pressure detector does not fall below a predetermined leak threshold.

[0014] According to one aspect, the at least one variable can be calculated during a previous shutdown of the turbine. Specifically, when the turbine is shut down, the first and second shut-off valves can be closed, and a signal related to a loss of flame can be received before or after closing the first and second shut-off valves. If the signal related to a loss of flame arrives after closing the first and second shut-off valves, the residual fuel gas in the second volume of the fuel gas compartment is checked. This second volume is downstream of the first shut-off valve and upstream of the second shut-off valve. Specifically, if the checked residual fuel gas is less than a predetermined value, the at least one variable can be assigned a value indicating that purge credits are available. On the other hand, if the checked residual fuel gas is equal to or greater than the predetermined value, the at least one variable can be assigned a value indicating that purge credits are not available and that the shutdown was normal. Finally, if a signal related to a loss of flame arrives before the first and second shutoff valves are closed, at least one variable can be given a value indicating that purge credits are not available and that the shutdown was not normal.

[0015] According to one aspect, if a predetermined time has passed without purging, the method may assign a value to at least one variable indicating that no purge credits will be available for the next turbine startup.

[0016] According to one aspect, the subject matter disclosed herein is also directed to a combustion system including a control unit configured to perform the method, a turbine, particularly a gas turbine, connected to the control unit, and a fuel gas compartment coupled to the turbine to deliver fuel gas to the turbine and connected to the control unit. Specifically, the fuel gas compartment includes a fuel gas inlet for fuel gas entering the combustion system, a first internal isolation valve disposed downstream of the fuel gas inlet with respect to transfer of fuel gas to the turbine and connected to the control unit, a second internal isolation valve disposed downstream of the first isolation valve and connected to the control unit, and at least one pressure detector disposed between the first and second internal isolation valves. [Brief explanation of the drawings]

[0017] A complete understanding of the disclosed embodiments of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings and examples. [Figure 1] FIG. 1 illustrates a front view of a first embodiment of a combustion system according to one aspect of the present invention, the combustion system including a gas turbine and a fuel gas section for delivering fuel gas to the gas turbine. [Figure 2] FIG. 2 illustrates a front view of a second embodiment of a combustion system according to an aspect of the present invention, the combustion system including a gas turbine and a fuel gas section for delivering fuel gas to the gas turbine. [Figure 3] FIG. 3 shows a block diagram of a fuel gas section of a combustion system according to one embodiment of the present invention. [Figure 4] FIG. 4 shows a block diagram of a control unit of a combustion system according to one embodiment of the present invention. [Figure 5] FIG. 5 shows a flowchart of a method according to an embodiment of the present invention, which includes a turbine startup step in a combustion system, a valve checking step, and a purge time estimation step, according to an embodiment of the present invention. [Figure 6] FIG. 6 shows a flow chart that examines the valve check step of the flow chart of FIG. [Figure 7] FIG. 7 shows a flow chart of a method according to one aspect of the invention, the flow chart including steps for checking for stall of a turbine in a combustion system. DETAILED DESCRIPTION OF THE INVENTION

[0018] A gas turbine is a rotating machine that can convert chemical energy, i.e., chemical fuel, into mechanical energy for operating a load or for generating electrical energy. Gas turbines are implemented for several services, from driving mechanical loads to generating electrical energy. Because combustion occurs within the gas turbine, the pipes connected to the gas turbine need to be purged periodically for safety reasons, more specifically to prevent possible explosions. However, this maintenance causes some downtime and interruption of the service provided by the gas turbine. Therefore, it is beneficial to reduce the required purge time.

[0019] A new combustion system is disclosed that operates to estimate whether a purge cycle is necessary based on pressure values ​​collected by a pressure detector located upstream of the combustion stage of the gas turbine, specifically in the fuel gas compartment that feeds the turbine itself. With this solution, the purge step is performed only when necessary, and not necessarily at each gas turbine start-up, maintaining the same level of safety.

[0020] Referring to Figures 1, 2, 3, 4, 5, 6, and 7, a combustion system 1 (shown specifically in Figures 1 and 2) configured to perform the method 100 shown in Figures 5, 6, and 7 is described herein. The method 100 estimates and sets an exhaust purge time in the combustion system 1 itself.

[0021] The combustion system 1 includes a gas turbine 10 and a fuel gas compartment 11 coupled to the turbine 10. In some embodiments, the gas turbine 10 is fastened to the fuel gas compartment 11. In some other embodiments, the gas turbine 10 is integrally formed with the fuel gas compartment 11. In use, the fuel gas compartment delivers fuel gas to the turbine 10.

[0022] The combustion system 1 also includes an exhaust section 13 coupled to the gas turbine 10. In some embodiments, the exhaust section 13 may be fastened to the turbine 10. In use, the exhaust section 13 receives exhaust gases of the turbine 10 and extracts such exhaust gases produced by combustion within the gas turbine 10.

[0023] Finally, the combustion system 1 includes a control unit 14 connected to the turbine 10 and to the fuel gas section 11. The control unit 14 may be located within the casing near the gas turbine 10, or may be located at a remote location connected to the gas turbine 10 through a communication channel such as the internet, an Ethernet cable, or the like, as shown in Figures 1 and 2.

[0024] Referring to FIG. 4 , an embodiment of the control unit 14 is illustrated comprising a bus 141, a processor 142 connected to the bus 141, a computer-readable memory 143 connected to the bus 141 for storing data and any possible required programs, and an input / output port 144 configured to connect to and control the electrical interface of the gas turbine 10 of the combustion system 1.

[0025] Also, as will be better detailed below, the computer-readable memory 143 stores computer-readable code that, when accessed by the processor 142, causes the processor 142 to execute a program for performing the method 100.

[0026] In some embodiments, the processor 142 can be a safety PLC. One example is the ADAPT ESD 3701 / 55 by Baker Hughes. In addition, the processor 142 can also be of a different programmable type.

[0027] The fuel gas compartment 11 can be isolated from any other compartment of the combustion system 1. In fact, the combustion system 1 includes a filter house 12 for feeding fresh air to the turbine 10. The filter house 12 is located in a safe area, meaning that it is located in an area where fuel gas cannot enter. Due to this layout of the combustion system 1, the only way for fuel gas to enter the turbine 10 is through the fuel gas compartment 11, which is isolated during all commissioning activities.

[0028] Specifically, the fuel gas compartment 11 (shown in FIG. 3 ) includes a fuel gas inlet 7 through which fuel gas may enter the combustion system 1. The fuel gas compartment 11 also includes a first internal isolation valve 3 located downstream of the fuel gas inlet 7 with respect to the transfer of fuel gas to the turbine 10 in use, and a second internal isolation valve 4 located downstream of the first internal isolation valve 3.

[0029] Furthermore, a first pressure detector 5 is disposed upstream of the first internal shut-off valve 3 , and a second pressure detector 6 is disposed between the first internal shut-off valve 3 and the second internal shut-off valve 4 .

[0030] The second pressure detector 6 is preferably Safety Integrity Level (SIL) certified and is connected to the control unit 14, e.g. hard-wired to a safety PLC, so as to ensure higher safety of the combustion system 1. However, other connection systems for connecting the pressure detector 6 and the control unit 14 can be envisaged.

[0031] 3 is an external shutoff valve 7. Thus, the fuel gas compartment 11 can be divided into a first volume 15 between the external shutoff valve 7 and the first internal shutoff valve 3, and a second volume 16 between the first internal shutoff valve 3 and the second internal shutoff valve 4.

[0032] Finally, the fuel gas compartment 11 includes an external vent valve 8 disposed in the first volume 15 and an internal vent valve 9 disposed in the second volume 16. As better explained below, the external vent valve 8 and the internal vent valve 9 allow for depressurization of the second volume 16, if necessary. The fuel gas compartment 11 also includes a warm-up valve 90 to also allow for depressurization of the first volume 15.

[0033] There is also a metering valve 40 located downstream of the second internal shut-off valve 4 and upstream of the gas turbine 10 for measuring the flow rate of gas towards the turbine 10. To improve the safety of the combustion system 1, the feedback of the metering valve 40 is connected to the control unit 14, preferably hard-wired to a safety PLC. Additional hardware that may be installed downstream of the second internal shut-off valve 4 is not considered for the purposes of this disclosure.

[0034] As will be better disclosed below, thanks to the second pressure detector 6, a computer method 100 executed by the control unit 14 makes it possible to evaluate whether purging is necessary before starting the turbine 10 and the time required for such purging.

[0035] The exhaust compartment 13 includes a vertical duct (as shown in Figures 1 and 2) and one or more horizontal ducts. Due to the isolation of the fuel gas compartment 11 during commissioning activities, there is no certain scenario in which fuel gas could enter the exhaust compartment 13. For this reason, it is reasonably assumed that there will be no fuel gas in the exhaust compartment 13 when the fuel gas compartment 11 is first placed into service.

[0036] As mentioned above, the method 100 for estimating and setting purge time values ​​RPT1, RPT2, RPT3 in a combustion system 1 includes the steps shown in FIG.

[0037] In step 101, the gas turbine 10 is started, and then step 102 is performed to check the first internal shut-off valve 3 and the second internal shut-off valve 4. The check step 102 is performed by receiving the pressure detected by the second pressure detector 6 and confirming that no leakage is detected.

[0038] Thereafter, if the check step 102 is completed, and if the gas turbine 10 has been shut down at least once, the method 100 includes a step 103 of reading at least one variable V associated with the previous shut down and indicating whether purge credit is available. If the at least one variable V indicates that purge credit is available, a step 104 of setting the exhaust purge time to zero is executed; otherwise, if the at least one variable V indicates that purge credit is not available, the method 100 includes steps of setting the exhaust purge time to predetermined purge time values ​​RPT1, RPT2, RPT3 (see reference numerals 105, 106, 108).

[0039] The at least one variable V may also be a purge time value RPT1, RPT2, RPT3 to be set, estimated during a recent gas turbine 10 shutdown.

[0040] The method 100 is also designed to check for additional permissions ("start OK") before proceeding to crank.

[0041] In practice, during check 102, a full stroke test of the metering valves 40 is performed to check their functionality. A first test is performed by moving the valve from fully closed to fully open and vice versa at a speed of 5% strokes per second. This valve check can be performed simultaneously for all metering valves 40. During this step, the valve drivers can generate trips according to their appropriate parameterization (i.e., manufacturer settings). As part of this test, the basic process safety control system (BPCS, part of the control unit 14) commands the metering valves 40, while the safety PLC performs a "demand vs. feedback" check on the metering valves 40. Valve feedback is wired directly to the safety PLC. Demand is shared from the BPCS to the safety PLC. If the test is passed, the method 100 continues with its further steps. Otherwise, if there is a discrepancy exceeding a predetermined threshold, the control unit 14 issues a specific alarm and the start-up 101 of the gas turbine 10 is aborted. As an example, if there is a discrepancy higher than 3% for more than 0.5 seconds, the start-up 101 of the gas turbine 10 may be aborted.

[0042] Additionally, during check 102, a bottle test (shown in FIG. 6) is performed on the first internal shut-off valve 3 and the second internal shut-off valve 4, as will be explained below.

[0043] To perform such a bottle test, before starting up 101 the gas turbine 10, i.e. while the gas turbine 10 is stationary (stopped), the external vent valve 8 and the internal vent valve 9 or the warm-up valve 90 and the internal vent valve 9 are moved to an open state, while the isolation valves (external isolation valve 7, first internal isolation valve 3, and second internal isolation valve 4) are moved to a closed state. No other checks are foreseen during this stage / state, and no safety checks are performed. This state is before the gas turbine start-up.

[0044] After start-up 101 of the gas turbine 10, a valve check step 102 is initiated, opening the external isolation valve 7 to pressurize the first volume 15, while the external vent valve 8 and internal vent 9 remain closed (step 1020 in FIG. 6). All other valves 3, 4 remain in the same state as before. This is before cranking.

[0045] In this step, the pressure measured by the first pressure sensor 5 is checked to see if it exceeds the threshold P1 that allows cranking. クランク When the same threshold value P2 is reached, a check is made on the second pressure sensor 6 instrument and the detected second pressure value P2 is equal to or exceeds the pressure limit P2 th1 Such monitoring is performed for a predetermined amount of time.

[0046] As shown in step 1022 of Figure 6, if the second pressure sensor 6 detects a high pressure, the control unit 14 will issue a specific alarm and the start-up of the gas turbine will be aborted. th1 means higher than

[0047] Alternatively, if the internal vent valve 9 is opened (first part of step 1021 in FIG. 6), the standard sequence for warming up the fuel gas can proceed.

[0048] Specifically, after the above-mentioned steps 1020 and 1021 are successfully completed, the external isolation valve 7 is closed when the gas turbine 10 is no longer at "zero speed" during acceleration to cranking speed. At the same time, the first pressure detector 5 detects a first pressure value P1 inside the first volume 15 that is equal to the predetermined pressure P1. th A depressurization step 1021 can be performed to depressurize the first volume 15 by opening the warm-up valve 90 until detecting

[0049] The control unit 14 is provided with a maximum duration control for depressurizing 1021 the first volume 15 to achieve a predetermined pressure P1 thIt can be verified that the predetermined amount of time has elapsed.

[0050] The given pressure P1 th If this is not achieved (step 1024 in FIG. 6), the control unit 14 issues a specific alarm and the start-up 101 of the gas turbine 10 is aborted.

[0051] After the depressurization 1021 of the first volume 15 is successfully completed and the gas turbine 10 is at purge speed, the first shutoff valve 3 is commanded to open for a period of time to pressurize 1023 the volume of the second volume 16 to check for on-skid double block and bleed leaks. After resetting the "open command" 1023 of the first shutoff valve 3, a time delay t 遅延 After this time delay t, preferably after 1 second, the leak test can be started. 遅延 is intended to enable confirmation of a leak when the pressure of the second pressure detector 6 is detected.

[0052] Specifically, after the second pressure value P2 is detected at the second pressure detector 6, further testing can be performed, including monitoring the second pressure value P2 and comparing the actual detected pressure reduction with the theoretical leak curve. This verification continues for a predetermined amount of time. If the actual pressure reduction falls below a predetermined leakage threshold of the theoretical leak curve, the valve check 102 fails (step 1027 in FIG. 6 ). In such a case, the control unit 14 issues a specific alarm to initiate a post-purge sequence if the on-skid double block-and-bleed valve detects a leak, and the start-up 101 of the gas turbine 10 is aborted. Alternatively, upon completing the valve check 102 (step 1026 in FIG. 6 ), the method 100 proceeds to further steps 103-105 to estimate and set the purge time. Other valves not mentioned in this paragraph remain in the same initial state.

[0053] In summary, the valve check step 102 described above involves at least pressurizing 1020 the first volume 15 with fuel gas entering the fuel gas compartment 11 from the fuel gas inlet 7 while the first shutoff valve 3 is closed, and determining whether the pressure P2 detected by the second pressure detector 6 is equal to or greater than the pressure limit P2. th1 , including the substep of verifying that the temperature does not rise above

[0054] After pressurization substep 1020, the first shutoff valve 3 is opened (substep 1023) while closing the second shutoff valve 4. After the opening (substep 1023) is completed, the first shutoff valve 3 is closed 1025, and a predetermined time period is allowed to elapse, after which it can be confirmed that the pressure P2 detected by the second pressure detector 6 does not fall below a predetermined leak threshold, specifically a theoretical leak curve.

[0055] Specifically, before the closing sub-step 1025 and after the opening sub-step 1023, in the decompression step 1021, the first pressure detector 5 detects a predetermined intermediate pressure P1 th The first volume 15 can be depressurized by opening the external vent valve 8 or the warm-up valve 90 until a pressure drop is detected.

[0056] Therefore, check step 102 determines whether the pressure detected in the fuel gas compartment 11 is within the operating pressure range, i.e., P1 th and P2 th1 Check whether it is between

[0057] If the first internal shut-off valve 3 is detected to be leaking, e.g., a Class IV leak, a purge will be required for the next turbine start-up. In this case, the purge time can be estimated as a function of the residual exhaust gas inside the combustion system 1, where the residual exhaust gas is assumed to be equal to a first predetermined gas volume. Specifically, the predetermined gas volume can be estimated by assuming a worst-case scenario occurs, i.e., the first internal shut-off valve 3 leaks while the second internal shut-off valve 4 and the internal vent valve 9 are fully open. This function of the estimated residual gas can estimate the time required to clean the volume of the combustion system 1 and reach an estimated residual gas level below 25% of the lower explosive limit (LEL).

[0058] If the second internal shut-off valve 4 is detected to be leaking, the next purge time may also be estimated in a similar manner. However, in such a scenario, the predetermined gas volume may be estimated by assuming that both internal shut-off valves 3, 4 are open during the trip.

[0059] Furthermore, after the valve check step 102, the method 100 is configured to estimate the purge time values ​​RPT1, RPT2, RPT3 to be set for the purge by the following steps.

[0060] If this is the first start-up of the gas turbine 10, the purge time values ​​RPT1, RPT2, RPT3 are set 109 to a value equal to the maximum between the minimum time value and the time required to insulate a volume of air equal to the estimated volume of the combustion system 1. The first start-up of the gas turbine 10 refers to the first start-up 101 of the gas turbine 10 after the combustion system 1 is installed on-site, with the gas turbine 10 coupled to the exhaust section 13. In this case, no previous shutdown data is available.

[0061] As discussed below, the minimum time value setting 109 during the initial gas turbine start-up is preferably equal to two minutes, while the estimated volume of the combustion system 1 is at least equal to the volume of the exhaust section 13. The rationale for this first estimated purge time is that during commissioning, the exhaust duct of the exhaust section 13 is filled with air (fuel gas cannot enter it), the filter house 12 is installed in a safe area, and the fuel gas system can be mechanically blocked (by a spectacle blind or manual valve until the first ignition). If the previous assumptions do not apply, a complete purge is performed by blowing in an air volume equal to five times the estimated volume of the combustion system 1. After the initial exhaust of the gas turbine 10 is purged, the following purge time values ​​RPT1, RPT2, RPT3 are calculated as follows: In all cases, the estimated purge time values ​​RPT1, RPT2, RPT3 are preferably equal to or greater than two minutes to ensure the safety of the combustion system 1.

[0062] As described above, the method 100 sets the purge time values ​​RPT1, RPT2, and RPT3 based on information related to a previous shutdown. Specifically, the control unit 14 reads 103 at least one variable V associated with the previous shutdown that indicates whether purge credits are available. Purge credits are available only if the previous shutdown of the gas turbine 10 was a normal shutdown. In accordance with the definition of a normal turbine shutdown given by the NFP85 standard, this is a normal sequence of events that automatically provides a normal shutdown of the gas turbine 10 without any abnormal conditions in the combustion system 1. Thus, two possible scenarios exist. In the first scenario, a normal shutdown 200 occurs, which can be a shutdown or ES / ESN when a flame loss occurs after closure 201 of the first and second internal shut-off valves 3 and 4 and no abnormalities are detected in the fuel gas compartment (see FIG. 7 ). In the second scenario, an abnormal shutdown 200 occurs, which may be due to a flame extinction prior to the closure 201 of the first internal shut-off valve 3 and the second internal shut-off valve 4, or another abnormality in the fuel gas compartment 11 being detected.

[0063] As already mentioned, a normal shutdown 200 of the gas turbine 10 occurs when a signal of extinction of the flame arrives after the first and second internal shut-off valves 3, 4 have been switched closed 201. Such a signal is evaluated by the control unit 14, preferably by a safety PLC. When a normal shutdown is detected, it is assumed that no unburned fuel gas has entered the interior of the exhaust section 13, in particular the exhaust duct.

[0064] Therefore, the following sequence 203 and checks can be performed to achieve purge credits.

[0065] After the flame has been extinguished, with the two internal shut-off valves 3, 4 closed (as confirmed by the two limit switches), the internal vent valve 9 is opened 2030 and the second volume 16 is depressurized to a predetermined pressure, preferably equal to 0.3 bar gauge. At this value, the internal vent valve 9 is closed again and a second pressure P2 is read by the second pressure detector. If the pressure is equal to the second pressure limit P2, th2 The first shut-off valve 3 can be checked for leaks by checking 2031 that the second pressure P2 does not exceed the second pressure limit P2. Such monitoring is performed for a predetermined amount of time. th2 If it is higher than 200 kJ / s, the control unit 14 will issue a specific alarm 205 and the credit purge will be lost. In such an event, the combustion system 1 can be checked to ensure that the time required to depressurize the volume is shorter than the time required to bring the crank speed below that of shutting down the gas turbine 10. If this is not the case, the time can be reduced with the assistance of a user, in particular an engineer.

[0066] After completing step 2031 of checking the pressure inside the second volume 16, the internal vent valve 9 is opened.

[0067] The following data can be monitored by the control unit 14, specifically the safety PLC 14, two hours after the flame has been extinguished: the closing limit switches of the first shut-off valve 3 and the second shut-off valve 4 are reached and maintained (or virtual limit switches); - the open limit switch of the internal vent valve 9 is reached and maintained (virtual limit switch), -P2, second pressure limit P2 th2 Specifically, below 0.3 bar gauge. The gas pressure GPI at the fuel gas inlet 7 is less than 0.3 bar gauge (this data can be checked by the unit control panel which is connected to the control unit 14 and is not in the safety PLC).

[0068] If all of the above conditions are met, the control unit 14 sets 204 a predetermined variable V to a value indicating that the last shutdown was a normal shutdown and that a credit purge is available. Such a credit purge may be set to last up to a predetermined time limit. To optimize the availability of the gas turbine 10 while ensuring the safety of the combustion system 1, the optimal time limit has been found to be eight days. This means that if the gas turbine 10 is started up within eight days of the previous shutdown, the exhaust will not be purged.

[0069] Thus, the at least one variable V may comprise a time counter that is set to a first predetermined value after a shutdown of the turbine 10 indicates that purge credits are available if the shutdown of the turbine 10 follows a purge cycle, the time counter varying over time, and when the time counter equals a predetermined time limit, the control unit 14 gives the at least one variable V 208 a first value indicating that no purge credits will be available upon the next startup of the turbine 10. Such a time counter may be configured to progress from 8 days to 0 days.

[0070] Therefore, it is important that the timer memory is preserved. In the event of timer memory loss (DC loss), a complete purge should be performed, for example, by blowing in a volume of air equal to five times the volume of the exhaust compartment 13. In other words, if the combustion system 1 is affected by memory loss, at least one variable V is set to a value indicating that no purge credits are available, and the exhaust purge time is estimated as a predetermined maximum purge time value, specifically the time required to complete the exchange of at least five times the volume of the exhaust compartment 13.

[0071] If at least one of the conditions monitored by the control unit 14 is not met during the gas turbine shutdown 200, the control unit 14 sets 205 a predetermined variable V to a value indicating that the last shutdown was a normal shutdown but that credit purge is not available due to an abnormality detected in the fuel gas compartment 11.

[0072] In both cases where purge credits are no longer available because a predetermined time limit has elapsed, and in cases where at least one of the conditions by the control unit 14 was not satisfied during the gas turbine shutdown 200, the purge time is set 105 to be equal to a predetermined first value RPT1 that is a function of the estimated residual exhaust gas inside the combustion system 1, the residual exhaust gas being assumed to be equal to a predetermined gas volume. Specifically, the value RPT1 may be calculated as the time required to reduce the amount of estimated residual exhaust gas inside the combustion system 1 below a predetermined safety threshold, preferably equal to 25% of the lower explosive limit (LEL).

[0073] Therefore, the enhanced purge time philosophy implemented by the method 100 detailed above ensures that the maximum amount of fuel gas potentially present in the exhaust section 13 is always below the safety threshold, preferably equal to 25%, as required by the NFPA 69 standard.

[0074] This condition monitors the pressure P2 detected at the second pressure detector 6 and determines whether the pressure exceeds a certain second pressure limit P2 during the cranking phase. th2 This second pressure limit P2 can be monitored and guaranteed by checking that it is less than th2 is not valid during valve check 102 and for steps involving layer for protection analysis (LOPA).

[0075] Therefore, if the pressure measured by the second pressure detector 6 during the cranking time is greater than the second pressure limit P2 th2 If it remains below this, the sequence can proceed to ignition, otherwise the sequence is aborted, the external shut-off valve 7, the first internal shut-off valve 3 and the second internal shut-off valve 4 are closed, the external vent valve 8 (or warm-up valve 90) and the internal vent valve 9 are opened and the control unit 14 issues a specific alarm.

[0076] In this case, given the purge time values ​​RPT1, RPT2, the RPT3 required for the next purge can be calculated as the time required to reduce the amount of estimated residual exhaust gas inside the combustion system 1 below a predetermined safety threshold, and such estimated residual exhaust gas can be calculated by assuming the worst case situation, i.e., the first internal shut-off valve 3 is open while the second internal shut-off valve 4 and the internal vent valve 9 are fully open.

[0077] In the case of an abnormal shutdown of the gas turbine 10, i.e., in the case of an abnormal shutdown of the gas turbine 10 in which a signal of extinction of flame arrives before the first internal shut-off valve 3 and the second internal shut-off valve 4 are switched closed, the control unit 14 sets 106 at least one variable V to a second value V2 indicating that an abnormal shutdown has occurred. In this case, the purge time value RPT2 is estimated by assuming that unburned fuel gas has entered the interior of the exhaust section 13, specifically, into the exhaust plenum and the exhaust duct. Therefore, in this scenario, no credit purge is available, and the purge time value RPT2 may be calculated as the time required to reduce the fuel gas in the exhaust section 13 to a value below 25% LEL.

[0078] 5, the method 100 is also configured to determine whether a flame is not detected (a non-ignition condition) during an ignition phase of the gas turbine 10. In this case, at least one variable V is set to a value indicating that no purge credit is available, and the exhaust purge time values ​​RPT1-RPT3 are set 107 equal to a third value RPT3 that is a function of residual exhaust gases within the combustion system 1, which residual exhaust gases are assumed to be equal to a predetermined gas volume.

[0079] In fact, if ignition fails, unburned fuel gas may enter the exhaust section 13, specifically the exhaust plenum. In this case, the control unit 14, specifically the safety PLC, may command the metering valve 40 to open to its maximum extent to avoid excessive fuel being trapped in the combustion system 11. The maximum opening value may be evaluated as a function dependent on a maximum value associated with the fuel gas pressure, a minimum value associated with the fuel gas temperature, a maximum value associated with the fuel gas molecular weight, the maximum opening capacity of the metering valve 40, and the total fuel ignition time. The purge time value RPT3 may be determined as the time required to reduce the fuel gas concentration within the combustion system 1, specifically in the exhaust section 13, to below 25% LEL.

[0080] Finally, if during the ignition phase the metering valve 40 opening is greater than the previously identified threshold, the start-up sequence should be aborted and the purge time can be calculated in the same manner as in the ignition failure case. The metering valve 40 opening is used to calculate the amount of fuel gas injected.

[0081] In this case, the amount of fuel gas injected into the exhaust section 13 can be estimated by multiplying the excess fuel flow rate by the time for ignition plus the time required for the system to react to a failure to ignite.

[0082] As already mentioned above, the purge time values ​​RPT1, RPT2, RPT3 set in different scenarios may be a function of the volume of the combustion system 1. Furthermore, the first value RPT1, the second value RPT2, and the third value RPT3 may all be calculated using the same parameters and therefore may be the same (as shown in Table 1 below).

[0083] Specifically, purge volume may be defined as the total exhaust volume of exhaust section 13 that is purged in each job, taking into account the final placement and geometric dimensions of exhaust section 13 itself.

[0084] The estimation of the total volume of the exhaust section 13 may consider all partial volumes of the exhaust section 13 downstream of the exhaust section of the turbine 10. For a combustion system 1 operating on a fuel gas lighter than air, the purged volume may be: In the case of a simple cycle without any waste heat recovery unit (WHRU) or heat recovery steam generator (HRSG) and with a horizontal exhaust duct and a silencer installed in the horizontal duct, the volume up to the chimney, excluding the vertical parts, since such gases necessarily go upwards and therefore do not require purging of the vertical parts; - for a simple cycle with silencers installed in the horizontal exhaust duct and vertical duct (without any WHRU or HRSG), the volume up to the silencer included, - For simple cycles with vertical exhaust ducts (without any WHRU or HRSG), the volume up to the silencer included, or - For combined cycles with horizontal and vertical exhaust ducts (with WHRU or HRSG), the volume to the outlet of the last evaporator section in the HRSG or the volume to the outlet of the silencer, the choice depends on which element is the last: if both elements are installed in horizontal ducts, the volume considered is up to the chimney, excluding the vertical parts.

[0085] The exhaust section of the gas turbine 10 may be defined as the plane located below. - exhaust casing flange for axially exhausting gas turbine models (as shown in Figure 1), in which case the volume of the gas turbine 10 should not be included in the definition of the purge volume; -Exhaust casing flange upstream of the exhaust plenum for gas turbine 10 models with radial exhaust ducts (vertical / horizontal, as shown in Figure 2), in this case the volume of the exhaust plenum of the gas turbine 10 should be included in the definition of the total purge volume.

[0086] For fuel gases that are heavier than air, the purge volume should also take into account the vertical ducts of the exhaust section 13.

[0087] Furthermore, to estimate the total purged volume, the exhaust section 13 may be divided into suitable partial sections to facilitate calculation of the geometric volume. The volume calculation may consider only the empty volume of each section and ignore the volume occupied by internal components such as noise baffles, deflectors, or piping. In a typical exhaust system configuration with an HRSG for a gas turbine 10 with radial exhaust ducts, the geometric volumes to be considered may be the volume of the simple chimney and the volume of the chimney with the HRSG. In this case, the total purged exhaust volume is given by the sum of the geometric volumes associated with all considered sections.

[0088] Finally, as mentioned above, the purge time values ​​RPT1, RPT2, RPT3 can always be calculated as the time required to reduce the estimated unburned fuel gas inside the combustion system 1, specifically inside the exhaust section 13, to below a predetermined safety threshold, for example 25% of the LEL.

[0089] In other words, the purge time value may be defined as the minimum duration to reduce the concentration of fuel gases / flammable vapors / hydrocarbons to a safe limit by purging the GT exhaust system with fresh air.

[0090]

number

[0091] The exchange rate refers to the minimum number of times the total exhaust volume must be discharged to ensure a complete purge cycle. The total exhaust volume, as detailed above, represents the sum of the exhaust geometric volumes of the section of the gas turbine 10 considered and the section of the exhaust compartment 13. The site purge air flow rate is given by the flow rate of fresh air delivered by the axial compressor of the gas turbine 10 rotating at a purge speed, e.g., 1700 rpm or 2200 rpm. In practice, the purge air flow rate represents the fresh air flow rate delivered by the axial compressor and therefore available in the exhaust discharge section when the gas turbine 10 is operated at the purge speed. Considering that the axial compressor can be assimilated to a volumetric machine, the air flow delivered by the axial compressor can be calculated as a function of its speed, which consequently affects the purge cycle duration (a higher purge speed means a higher purge flow rate, which means a shorter purge cycle duration).

[0092] Typically, the exhaust purge sequence may be performed at a crank speed = nominal purge speed of the gas turbine 10. Another aspect that may affect the air flow rate available at the turbine exhaust discharge section is the presence of air extractions from the axial compressor (i.e., anti-surge, overbore bleeds, bleeds for external auxiliary systems, etc.). The presence of air extractions directly to atmosphere may be taken into account to correct for the actual flow rate available for the purge sequence.

[0093] If for a given installation the purge rate needs to be adjusted outside the range defined above, a thorough examination of the rotational dynamics and its effect on the purge flow rate, as well as confirmation of the capabilities of the starting system, should be evaluated on a case-by-case basis and adjusted by a predetermined correction factor.

[0094] In practice, the purge time values ​​RPT1, RPT2, RPT3 can be estimated by considering different gas turbine 10 exhaust configurations (axial, vertical / horizontal with plenum), as well as other options in exhaust system design (simple chimney, CHP with heat recovery system, etc.) and application (new unit or replacement / upgrade of gas turbine 10 in an existing plant). The duration of the purge cycle can also be defined for each job, specifically considering the final configuration, the characteristics of the gas turbine 10 and its exhaust section 13, and the composition of the fuel gas.

[0095] Example 1 This disclosure discloses, by way of example only, a table comparing the purge times required to start a gas turbine 10 using standard techniques and using the method according to the present invention, the gas turbine 10 having a high exhaust mass flow rate.

[0096] [Table 1]

[0097] The advantage of using the method 100 according to the present invention is evident in increasing the availability of the gas turbine 10 .

[0098] While aspects of the present invention have been described in terms of various specific embodiments, it will be apparent to those skilled in the art that many modifications, changes, and omissions are possible without departing from the spirit and scope of the claims. Additionally, unless otherwise specified herein, the order or sequence of any process or method steps may be varied or rearranged according to alternative embodiments.

[0099] Reference will now be made in detail to the embodiments of the present disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the disclosure, not as a limitation of the disclosure. Indeed, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. References throughout this specification to "an embodiment" or "one embodiment" or "some embodiments" mean that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the disclosed subject matter. Thus, the appearances of the phrases "in one embodiment," "in one embodiment," or "in some embodiments" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0100] When presenting elements of various embodiments, the articles "a," "an," "the," and "said" are intended to mean that there are one or more of the elements. The terms "comprising," "including," and "having" are intended to be non-exclusive and mean that there may be additional elements other than the listed elements.

Claims

1. A method, performed by a control unit operatively connected to a turbine and a fuel gas compartment, for estimating and setting an exhaust purge time for a combustion system having the turbine and the fuel gas compartment, the method comprising: starting up the turbine of the combustion system; a first step of checking pressure in the fuel gas compartment of the combustion system with at least one pressure detector, If, in the first step of checking, the pressure detected in the fuel gas compartment is within an operating pressure range and the turbine has been shut down at least once, reading at least one variable associated with a previous shutdown of the turbine that indicates whether purge credit is available; and setting the exhaust purge time to zero if the at least one variable indicates that purge credit is available; if the at least one variable indicates that purge credit is not available and the previous shutdown of the turbine was a normal shutdown; The exhaust purge time is set to a first time for reducing residual exhaust gases within the combustion system below a safety threshold, the residual exhaust gases being assumed to be equal to a first predetermined gas volume; if the at least one variable indicates that the purge credit is not available and that the previous shutdown of the turbine was not an orderly shutdown; The method, wherein the exhaust purge time is set to a second time different from the first time, the second time being a time for reducing residual exhaust gases inside the combustion system below a safety threshold, and the residual exhaust gases are assumed to be equal to a second predetermined gas volume different from the first predetermined gas volume.

2. 2. The method of claim 1, wherein the first step of checking includes the substeps of checking, in use, by the at least one pressure detector, a first shut-off valve located downstream of a fuel gas inlet for the transfer of fuel gas to the turbine, checking a second shut-off valve located downstream of the first shut-off valve, and verifying that no leaks are detected.

3. The first step of checking a substep of pressurizing a first volume of the fuel gas compartment between the fuel gas inlet and the first shut-off valve using fuel gas entering the fuel gas compartment through the fuel gas inlet while a first shut-off valve disposed downstream of the fuel gas inlet is closed; and a substep of verifying that the pressure detected by the at least one pressure detector does not rise above a first pressure limit; a substep of, after the pressurizing step, opening the first shut-off valve while closing a second shut-off valve disposed downstream of the first shut-off valve; 2. The method of claim 1, further comprising, after the step of opening the first shut-off valve, the substep of closing the first shut-off valve and waiting a predetermined time to verify that the pressure detected by the at least one pressure detector does not fall below a predetermined leak threshold.

4. 4. The method of claim 3, wherein before the step of opening the first isolation valve and after the step of pressurizing, there is a step of depressurizing the first volume performed by opening a warm-up valve until a first pressure detector located in the first volume detects a predetermined intermediate pressure.

5. 2. The method of claim 1, wherein the first step of checking includes checking the fuel gas flow rate through a metering valve located downstream of a second shutoff valve located downstream of a first shutoff valve located downstream of the fuel gas inlet.

6. The at least one variable was determined to be: shutting down the turbine and closing a first shut-off valve located downstream of a fuel gas inlet and a second shut-off valve located downstream of the first shut-off valve; receiving a signal related to the extinction of the flame; If the signal related to the extinction of the flame received in the receiving step arrives after the first shut-off valve and the second shut-off valve are closed, a second step of checking that the residual fuel gas within a second volume of the fuel gas compartment downstream of the first shut-off valve and upstream of the second shut-off valve is less than a predetermined value; if the residual fuel gas checked in the second checking step is less than the predetermined value, assigning a value to the at least one variable indicating that a purge credit is available; otherwise, if the residual fuel gas checked in the second checking step is not less than the predetermined value, assigning a value to the at least one variable indicating that no purge credit is available and that the shutdown was normal; Otherwise, if the signal related to the extinction of the flame received in the receiving step arrives before the first shut-off valve and the second shut-off valve are closed, 2. The method of claim 1, wherein the step of assigning a value to the at least one variable indicating that purge credits are not available and that the shutdown was abnormal is performed.

7. The second step of checking and opening an internal vent valve disposed within the second volume of the fuel gas compartment while closing the first isolation valve and the second isolation valve to depressurize the second volume of the fuel gas compartment to a predetermined second pressure.

7. The method of claim 6, further comprising the substep of, after the opening substep, checking a pressure detected by a second pressure detector disposed between the first shutoff valve and the second shutoff valve as an indirect measurement of a residual fuel gas value, and verifying that the pressure is less than a second pressure limit.

8. 2. The method of claim 1, including the step of assigning a value to the at least one variable indicating that no purge credits will be available at the next turbine start-up after a predetermined time has passed without purging.

9. 2. The method of claim 1, wherein if the turbine cannot be started, the at least one variable indicates that no purge credit is available, the exhaust purge time is set to a third time value for reducing residual exhaust gas within the combustion system below a safety threshold, and the residual exhaust gas is assumed to be equal to a third predetermined gas volume.

10. 2. The method of claim 1, wherein if the combustion system is affected by a memory loss, the at least one variable indicates that no purge credits are available and the exhaust purge time is set to a predetermined maximum purge time value.

11. 2. The method of claim 1, wherein when the turbine is started for the first time, the exhaust purge time is estimated to be equal to the maximum value between a time value set for initial turbine start-up and the time required to blow a volume of air equal to an estimated volume of the combustion system.

12. 1. A combustion system comprising: The turbine and a fuel gas compartment coupled to the turbine for delivering fuel gas to the turbine, the fuel gas compartment comprising: a fuel gas inlet for the fuel gas entering the combustion system; a first shut-off valve positioned downstream of the fuel gas inlet with respect to the transfer of the fuel gas to the turbine; a second shut-off valve disposed downstream of the first shut-off valve; a fuel gas compartment comprising at least one pressure detector disposed between the first shut-off valve and the second shut-off valve; a control unit operably connected to the turbine and the fuel gas compartment, the control unit configured to execute a method for estimating and setting an exhaust purge time for the combustion system, the method comprising: starting up the turbine; checking the pressure in the fuel gas compartment with the at least one pressure detector; If the detected pressure in the fuel gas compartment in the checking step is within an operating pressure range and the turbine has been shut down at least once, reading at least one variable associated with a previous shutdown of the turbine that indicates whether purge credit is available, and if the at least one variable indicates that purge credit is available, setting the exhaust purge time to zero; if the at least one variable indicates that purge credit is not available and the previous shutdown of the turbine was a normal shutdown; The exhaust purge time is set to a first time for reducing residual exhaust gases within the combustion system below a safety threshold, the residual exhaust gases being assumed to be equal to a first predetermined gas volume; if the at least one variable indicates that the purge credit is not available and that the previous shutdown of the turbine was not an orderly shutdown; A combustion system, wherein the exhaust purge time is set to a second time different from the first time, the second time being a time for reducing residual exhaust gas inside the combustion system to below a safety threshold, and the residual exhaust gas is assumed to be equal to a second predetermined gas volume different from the first predetermined gas volume.

13. the fuel gas compartment: a first pressure detector disposed upstream of the first shut-off valve; a second pressure detector disposed between the first shutoff valve and the second shutoff valve; a warm-up valve disposed upstream of the first shut-off valve and connected to a control unit; The control unit pressurizing a first volume of the fuel gas compartment between the fuel gas inlet and the first shut-off valve using fuel gas entering the fuel gas compartment through the fuel gas inlet while the first shut-off valve is closed, and verifying that the pressure detected by the first pressure detector or the second pressure detector does not rise above a first pressure limit; After the pressurizing step, depressurizing the first volume by opening the warm-up valve until a first pressure detector disposed within the first volume detects a predetermined intermediate pressure; After the depressurizing step, opening the first shutoff valve while the second shutoff valve is closed; 13. The combustion system of claim 12, further comprising: after the step of opening the first shut-off valve, closing the first shut-off valve and waiting a predetermined time to verify that the pressure detected by the second pressure detector does not fall below a predetermined leakage threshold.

14. the fuel gas compartment includes an internal vent valve between the first and second shutoff valves and connected to the control unit; The combustion system of claim 12 , wherein the internal vent valve is open with the first and second isolation valves closed.

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