Hi,
We are having BHEL supplied GE Frame VI Gas Turbine with Speedtronic Mark VI control system and I am new to it so can anyone please post or send writeup or any links of websites for better understanding about Frame VI GE Gas Turbine Startup/ shutdown sequence for a beginner. Eagerly waiting to learn more and more in depth about system, logics etc.

Thank you in advance.

Mail Id: [email protected]

 

This topic has been covered several times before in the last 20+ years on Control.com on other threads--all of them accessible using the Search feature (the magnifying glass) at the top of every Forum webpage. You may have to use several different search terms, but as you get closer and closer to what you're looking for you will see how to better configure your search requests. AND, in the process, you will also learn a few things. There's almost 30 years of SpeedTronic- and Mark*-related information on Control.com, as well as some pretty good descriptions of start-up, shutdown, and cooldown sequences. It should be possible to fill in most gaps, and if you still have questions (not doubts--but questions or clarifications) we can try to help.

Right now your need is large, too large really to try to answer. Try thinking about it manageable chunks--like, finding out about the Start-Check permissives, and then how the L4 rungs (and L4Y and L4Z rungs) work. If you aren't familiary with GE's device numbering system, that's also an excellent place to start your learning--because that device numbering system is used for many of the application code signal names.

Get yourself your own copies--as large as possible (A3 or B)--of the P&IDs for the machine(s) and start learning the symbols and the Device Numbers. READ THE NOTES SECTION OF EVERY P&ID; lots of good information there, too. MAKE NOTES (with a pencil and possibly a highlighter) on your copies of the drawings. Go looking for the devices, especially when there's a maintenance outage and the machine is not running and is accessible.

That will get you started--and also get yourself a copies of the Device Summary and Control Specification provided with every machine. The Device Summary will help you get started learning and understanding GE's device numbering system, and the Control Specification will help with some of the control and protection schemes.

DON'T BELIEVE EVERYTHING YOU READ!!! There are some serious mistakes in the descriptions of the application code signal names which can be very misleading, and the ONLY documents that you can believe are the application code drawings (the relay ladder representations of the control and protection configuration and programming) and the P&IDs. But, even when looking at the application code, the comments and LONGNAME descriptions of some of the signal names are just flat wrong. Use your critical thinking skills to decide if this description/comment seems correct or not. (It's a shame the comments and signal name descriptions (LONGNAMES) can't be fully trusted, but unfortunately they can't. That doesn't mean you can't learn this stuff--you CAN. AND, I believe it will make you a better technician and operator and supervisor by having to think some of this stuff through and sort out the wheat from the chaff.

Remember--if you have questions (more focused questions than the ones you're asking in this thread) or require clarifications, ask here.

By the way, several years back one of the prolific contributors to these GT-related threads went through a LOT of start-check permissive rungs, contact by contact, and the L4 stuff, too, if I recall correctly. You should look for those and read them, and maybe even re-read them. They were pretty good, I think. At least they garnered, and continue to collect, lots of views.

Go forth and conquer! Start with your own set of P&IDs, and make notes on them. You will need the Device Summary and Control Specification to help with that--but it's really the best way to get this stuff in your head. And, the good news is--once you learn this stuff for one Frame size of GE heavy duty gas turbine, it's pretty similar for the rest of the fleet. (Some of the newer machines have some pretty complicated code but they still have fuel control valves, compressor bleed valves, IGVs L.O. pumps, Hydraulic pumps, etc.) So, when you want to move on to another site with GE equipment you will be well-prepared. (At least you will now how to read the drawings and how to troubleshoot!)

 

This topic has been covered several times before in the last 20+ years on Control.com on other threads--all of them accessible using the Search feature (the magnifying glass) at the top of every Forum webpage. You may have to use several different search terms, but as you get closer and closer to what you're looking for you will see how to better configure your search requests. AND, in the process, you will also learn a few things. There's almost 30 years of SpeedTronic- and Mark*-related information on Control.com, as well as some pretty good descriptions of start-up, shutdown, and cooldown sequences. It should be possible to fill in most gaps, and if you still have questions (not doubts--but questions or clarifications) we can try to help.

Right now your need is large, too large really to try to answer. Try thinking about it manageable chunks--like, finding out about the Start-Check permissives, and then how the L4 rungs (and L4Y and L4Z rungs) work. If you aren't familiary with GE's device numbering system, that's also an excellent place to start your learning--because that device numbering system is used for many of the application code signal names.

Get yourself your own copies--as large as possible (A3 or B)--of the P&IDs for the machine(s) and start learning the symbols and the Device Numbers. READ THE NOTES SECTION OF EVERY P&ID; lots of good information there, too. MAKE NOTES (with a pencil and possibly a highlighter) on your copies of the drawings. Go looking for the devices, especially when there's a maintenance outage and the machine is not running and is accessible.

That will get you started--and also get yourself a copies of the Device Summary and Control Specification provided with every machine. The Device Summary will help you get started learning and understanding GE's device numbering system, and the Control Specification will help with some of the control and protection schemes.

DON'T BELIEVE EVERYTHING YOU READ!!! There are some serious mistakes in the descriptions of the application code signal names which can be very misleading, and the ONLY documents that you can believe are the application code drawings (the relay ladder representations of the control and protection configuration and programming) and the P&IDs. But, even when looking at the application code, the comments and LONGNAME descriptions of some of the signal names are just flat wrong. Use your critical thinking skills to decide if this description/comment seems correct or not. (It's a shame the comments and signal name descriptions (LONGNAMES) can't be fully trusted, but unfortunately they can't. That doesn't mean you can't learn this stuff--you CAN. AND, I believe it will make you a better technician and operator and supervisor by having to think some of this stuff through and sort out the wheat from the chaff.

Remember--if you have questions (more focused questions than the ones you're asking in this thread) or require clarifications, ask here.

By the way, several years back one of the prolific contributors to these GT-related threads went through a LOT of start-check permissive rungs, contact by contact, and the L4 stuff, too, if I recall correctly. You should look for those and read them, and maybe even re-read them. They were pretty good, I think. At least they garnered, and continue to collect, lots of views.

Go forth and conquer! Start with your own set of P&IDs, and make notes on them. You will need the Device Summary and Control Specification to help with that--but it's really the best way to get this stuff in your head. And, the good news is--once you learn this stuff for one Frame size of GE heavy duty gas turbine, it's pretty similar for the rest of the fleet. (Some of the newer machines have some pretty complicated code but they still have fuel control valves, compressor bleed valves, IGVs L.O. pumps, Hydraulic pumps, etc.) So, when you want to move on to another site with GE equipment you will be well-prepared. (At least you will now how to read the drawings and how to troubleshoot!)

Thanks for the reply,
As you suggested, will go step by step. Firstly I will see overview of startup and shutdown sequence and then after startup checks, interlocks, trip logic etc.
Thanks for helping me out, and if I needed any clarifications regarding this, then will surely post here.

 

@Tapan,

Here are some very basic facts/steps about starting a GE-design Frame 6 heavy duty gas turbine with a Mark* VI turbine control system.

Starting with the terms involved, when speaking about a GE-design heavy duty gas turbine there are some words/terms which are very important to know. First, a machine can be shut down (stopped) either by an operator initiating a STOP from the HMI or by some condition which the Mark* determines requires an orderly shutdown to protect the machine. A normal operator-initiated STOP or an automatically initiated STOP starts an orderly reduction in load (MW) by ramping down the fuel flow-rate until the generator breaker is automatically opened (sometimes called "tripping the generator breaker"--NOT to be confused with a machine trip because of some dangerous condition), and then the fuel flow is again reduced in an orderly fashion to cause the machine speed to decrease, until the machine reaches the speed at which the fuel flow is stopped (by closing the fuel stop valve) and then the machine continues to slow until it reaches zero speed (when the machine is said to be "at rest") and the Cooldown sequence automatically begins.

The orderly process of shutting down (stopping) a GE-design heavy duty gas turbine after the generator breaker is opened is called a "fired shutdown"--because fuel flow is maintained, although at reducing rates as the machine speed is decreasing, again, until the fuel flow is finally stopped completely and the machine continues to slow down (sometimes called "coasting down") to zero speed, at which time the Cooldown sequence is automatically initiated.

A machine can also be shutdown automatically by the Mark* turbine control system when a serious problem is detected which could severely damage the machine--such as low-low L.O. pressure, or high-high vibration, or high-high gas turbine exhaust temperature or excessive gas turbine exhaust thermocouple differential temperatures (called "spreads"). This is technically an emergency shutdown and is very often referred to as a machine "trip." [Such a trip is NOT to be confused with a generator breaker trip, which just refers to the opening of the breaker for some reason. It's important to know that a machine trip (which includes a generator breaker trip) is VERY DIFFERENT from a normal shutdown.]

When a machine trip occurs there is almost always an alarm (such as low L.O. pressure, or high vibration or high exhaust temperature, etc.) to alert the operator to a serious problem which may get worse if not dealt with in a timely manner and may result in an emergency shutdown of the machine. [NOTE: A low L.O. pressure condition produces an alarm, a Process Alarm, alerting the operator to the condition, which if not resolved may eventually result in a low-low L.O. pressure which will cause a machine trip (emergency shutdown).] EVERY condition which results in a turbine (or machine) trip has a Process Alarm associated with it. EVERY condition. Look at the 'Trip Display' on the HMI to see every condition which will result in a machine (turbine) trip. After a trip has been initiated, one of the green bars will change to red to help indicate which condition actually caused the trip.

By the way, when a machine is tripped by the Mark* turbine control system or by an operator pressing the Emergency Stop push-button, when the machine reaches zero speed it will also automatically start the Cooldown operation (hydraulic ratchet).

When describing a machine being stopped by an operator or even an automatic shutdown (sometimes, very dirty turbine inlet air filters will result in an automatic shutdown of the machine--an orderly reduction of load, followed by a generator breaker trip (opening), followed by a fired shutdown, followed by fuel being shut off automatically at some low speed, and finally a coast-down of the machine to zero speed and Cooldown operation) as a trip. A shutdown IS NOT a trip. Especially when describing a shutdown to GE or one of its affiliates. A shutdown is an orderly, controlled reduction of fuel until the machine finally reaches zero speed and automatically "goes on" Cooldown (until an operator manually stops Cooldown after the minimum Cooldown period has expired).

A machine trip is an emergency shutdown, usually initiated by the Mark* turbine control system--but an operator pressing an Emergency Stop push-button will also trip the machine. The fuel flow will be immediately stopped, the generator breaker will open (trip--not a machine trip, a generator breaker trip!), the machine will coast down to zero speed and Cooldown operation will automatically be initiated.

[I wish to alert you to the fact that MOST sites don't have a procedure for how to handle things like a low L.O. pressure alarm (which will be annunciated prior to a low-low L.O. pressure trip)--meaning the operators won't know what to do when the alarm 'L.O. Pressure Low' is annunciated. MANY GE-design heavy duty gas turbines DO NOT have a L.O. pressure transmitter to display the bearing header pressure on the HMI for an operator to check, so someone has to physically go out to the machine's gage cabinet to look at the Bearing Header Pressure gage to see the bearing header pressure. During which time the L.O. pressure might continue to decrease until the machine eventually trips on low-low L.O. pressure. Same for high vibration. These and several other "warning" alarms need to be documented and some kind of operating procedure developed to deal with the conditions in order to prevent a machine trip--or worse, severe damage to the machine--because the operator(s) and their supervisor(s) don't know how to respond to the alarms. Just a warning about this is all I'm alerting you to....]

So, don't confuse stopping (shutting down) the machine with tripping the machine--especially when communicating with GE or one of its affiliates. If you say trip when you should have said shutdown GE gets pretty nervous (usually) and a serious miscommunication will result in wasted time and emails and discussions and investigations. Be clear and concise--especially about these two seemingly similar situations. A shutdown IS NOT a trip, and a trip is almost always the result of some serious condition (including pressing the Emergency Stop push-button).

These distinctions are very important to know and to make--everyone operating the machine(s) needs to know and understand these important differences. People, particularly GE people, can be mistakenly led to believe some serious condition shut off fuel to the machine, when in fact an operator just clicked on the STOP button on the HMI if someone makes the mistake of saying the machine tripped--when it fact, it was shut down by an operator. NOW, it is entirely possible that some serious condition occurred shortly after the operator initiated a normal STOP resulting in a machine trip. but, the two are NOT the same thing.

For the majority of GE-design Frame 6 machines Cooldown is accomplished using a hydraulic ratchet mechanism which turns the turbine shaft approximately 45 degrees every three minutes, pausing between the partial rotations. During this time the Aux. L.O. Pump (driven by an AC motor) runs continuously providing lubrication oil to the bearings of the turbine, reduction gear (also called a "load gear") and generator to cool them to prevent damage caused by heat from the shafts softening the bearing material and which is also required for the partial rotations. When a machine is shut down (STOPped) or tripped (emergency trip) while running at load the machine shaft (axial compressor and turbine shaft) is said to be hot. There is a long distance between the #1- and #2 Bearings and the axial compressor portion of the shaft is actually made up of individual discs and is MUCH longer than the turbine section of the shaft and is held together by long throughbolts. If the machine shaft is not rotated when it reaches zero speed the weight of the axial compressor will cause the bolts to stretch unevenly (because the metal of the bolts is said to be in its elastic range). This will result in a downward bowing, or "warping" of the axial compressor portion of the machine shaft. As it cools, it will straighten itself slightly--but NOT completely, and if the machine is STARTed while the shaft is bowed (warped) the machine will vibrate excessively, and the axial compressor blades (stationary and rotating) can be severely damaged. The Cooldown sequence cannot be stopped by an operator until the machine has cooled sufficiently to prevent any damage from being at zero speed after the shaft was hot (the axial compressor section is very long--much longer than the turbine section) and can bow or warp when it's hot and not rotated at least partially or slowly. Typically, this isn't defined by any specific temperature, but by a timer--which for many GE-design Frame 6 machines is usually 24 hours, after which time the machine should be "cool" enough to stop the Cooldown sequence (accomplished by an operator initiating a STOP from the HMI), which will cease the ratcheting sequence and stop the Aux. L.O. Pump motor.

This is a very brief description of the shutdown/trip sequence. If you want to know most of the conditions which will result in a machine trip make a copy of the Trip Display on the HMI and start working through the descriptions (all of which are usually directly related to the text message which is displayed when the Process Alarm is annunciated). I say "most" of the conditions because there are usually one or two Process Alarms on that page that are actually the result of two or more conditions which are very similar and/or related, and instead of having several more Process Alarms. (And, as you will learn--if you haven't already--the Mark* is usually considered by most operators and site supervision/management to be primarily an alarm-producing machine (it doesn't have to be--but the Mark* and its inputs and outputs are usually not well-maintained and/or the commissioning personnel didn't properly sort out all of the nuances of the machine, results in LOTS of nuisance Process Alarms (which CAN BE fixed, but it requires time, which requires money...). Most operators get so numb to all the Process- (AND Diagnostic) Alarms annunciated by the Mark* they just ignore most alarms unless the machine trips and then they don't know how to scroll through the Alarm Display and determine precisely which condition actually tripped the machine. Sad, but unfortunately, very, Very, VERY true.)

 

STARTing a GE-design Frame 6B heavy duty gas turbine is slightly more straightforward.

In order to initiate a machine START for synchronized operation (electrical power production), one has to select AUTO or REMOTE from the Master Control Select section of the HMI Main Display (sometimes called the 'Start-up Display'). And once AUTO or REMOTE is selected, one of the fields in the Status section of the HMI Main Display will show either READY TO START or NOT READY TO START. It's VERY important to be in either AUTO or REMOTE (for normal electrical power production operation) to see EITHER READY TO START or NOT READY TO START!!! The machine can't be in OFF (in the Master Control Select section of the HMI Main Display) to get the ready- or not ready indication. The Mark* turbine control system has to be in some mode of operation (CRANK, FIRE, AUTO or REMOTE) to get the READY TO START or NOT READY TO START indication. (OFF means the machine is not in an operating mode, so it won't display the starting status.)

If the Status field indicates READY TO START the operator can then click on START and confirm the action and the start-up sequence will begin. If the Status field indicates NOT READY TO START and the operator doesn't understand the reason for the indication change to the Start Check HMI display and any field with a red background will be the reason for the NOT READY TO START indication. AND, every condition--but for one--will also be indicated with a Process Alarm on the Alarm Display to alert a conscious operator to the condition(s) preventing a START. And the one exception is--that's right, the Master Control Select status.

If the machine is at zero speed and is "cool" or "cold" (meaning its been at least 24 hours since it was shut down or tripped and the internal machine temperatures are at or near ambient temperature) AND it hasn't been more than a couple of days since the machine was shut down or tripped a START can be initiated from zero speed without being on Cooldown. If it has been more than four or five days since the machine was shut down or tripped and the machine has not been on Cooldown (hydraulic ratchet) during all that time it's possible that the axial compressor might have begun to bow or warp just due to its own weight, and that could cause high or even high-high vibration during starting and acceleration to rated speed. So, the OEM advises if it's been more than four or five days since the machine was operated and if the machine hasn't been on Cooldown for most of, or all, of that period (after the Cooldown period expired) that it be placed on Cooldown for approximately four hours (longer if its been many days or weeks since the machine was last operated) in order to rotate the shaft and help to correct any bowing or warping which might have occurred since the last time the shaft was rotated. NOTE: It IS NOT required to put the machine on Cooldown prior to ANY START for any period of time, contrary to wildly popular (and false) belief. It it were, the Mark* turbine control system would prevent a START from being initiated until an appropriate amount of time on Cooldown had expired--and it doesn't. Depending on conditions (including L.O. temperature!) the vibration during starting might be higher than normal (which should be below the alarm setpoint) but if it's absolutely necessary to get the machine started and producing electrical power quickly it is possible to START the machine if it hasn't been on Cooldown for a few days.

If the machine was recently shut down or tripped and is on Cooldown (hydraulic ratchet) and its desired or necessary to START it and produce electrical power it can be started when all the start-check permissives are met (in other words, a READY TO START indication can be seen in the Status field section of the HMI Main Display).

When a START is initialized and begins from zero speed, the Auxiliary L.O. Pump is started. And usually, the Auxiliary Hydraulic Pump is also started shortly after that and operate all the way to near 100% rated speed. In fact, the very last permissive for the START sequence to fully begin is when L.O. pressure is detected at the collector end of the generator--at that point the Starting Means is energized, the hydraulic ratchet mechanism begins turning and shortly after that the torque converter may be pressurized (there are different types of torque converters and slightly different sequencing required; some are continually pressurized whenever the Aux. L.O. Pump is running; others are only pressurized when the Starting Means is running; others are only pressurized when a solenoid is energized--the equipment depicted on the Starting Means P&ID will dictate the required sequencing).

Many older machines require a GE-design Frame 6B heavy duty gas turbine to be a zero speed to initiate a START. This is solely because of the type of coupling between the output of the Starting Means (induction electric motor or diesel engine) and the turbine shaft (through the Accessory Gear). Older machines, and some new ones, use a jaw clutch which SHOULD NOT be closed when the turbine shaft half of the clutch is spinning (in other words, when the turbine shaft is still spinning and NOT at zero speed). Usually the Mark* turbine control system will not indicate READY TO START until the shaft is at zero speed--to protect the jaw clutch from serious damage.

Many newer machines, and some older machines which have had an upgrade to the clutch mechanism between the Starting Means and the turbine shaft, use a SSS clutch--a self-synchronizing, shifting clutch mechanism which can be closed while the turbine is still turning--but only below a set speed (usually around 100 RPM or so). It's important to understand the type of clutch mechanism in use on the machine (this can be determined from looking at the Starting Means P&ID) to understand what limitations may be applicable to starting the machine.

If the shaft is at zero speed when a START is initiated the Cooldown system (hydraulic ratchet) will be started. This is because the shaft of the machine (axial compressor and turbine, and the Load Gear and generator shafts (since they are coupled together)) has a very high inertia and the Starting Means by itself does not have the ability to start the shaft turning (to "break the shaft away from zero speed") when it's at zero speed. So, the "bumping" action of the hydraulic ratchet gives the Starting Means just enough additional force to help break the shaft away from zero speed and accelerate it to purge speed. (Of course, if the machine has a SSS clutch and it is being started above zero speed the assist from the hydraulic ratchet is not necessary.)

Once the machine speed reaches what's called "purge speed"--which is the speed at which the timer starts for calculating the amount of air being moved through the axial compressor, turbine, exhaust and HRSG (boiler, if present) and the exhaust stack. This is called purging--using the axial compressor to push out any combustible gases from the machine and exhaust (including the HRSG if present) to prevent a possible explosion when the ignitors are energized. Once the purge timer has expired, the Mark* turbine control system begins the process of establishing flame in the combustors by energizing the ignitors (sometimes called "spark plugs") for approximately 30- to 60 seconds and admitting fuel into the combustors. There are two ignitors, for redundancy (only one ignitor is required to establish flame in all the combustors; the second one is redundant but they are both energized at the same time and for the same amount of time), and again, they are only necessary to help establish flame in the combustors and they are de-energized when the firing period expires AND flame has been detected (usually in more than one combustor--there are NOT flame detectors in all ten combustors, but there are usually redundant flame detectors, either two or four combustors). They fuel admitted during firing is very slightly more than required to maintain flame and its done to help establish flame in all the combustors. Once flame is established in some combustors the amount of fuel is reduced slightly during what's called the "warm-up" period. (Reducing the fuel after firing is done to reduce the thermal stresses on the combustors, turbine and exhaust areas to prolong the life of the components in the combustors and turbine and exhaust.) Once the warm-up period is complete the ignitors are de-energized (they are left energized to help maintain flame when the fuel is reduced from firing to warm-up), and fuel is slowly increased to help accelerate the machine. It should be noted the amount of fuel being burned at this point (firing, warm-up and initial acceleration (up to about 60% of rated speed) is not sufficient to keep the machine spinning without the assist of the Starting Means--which is still being used to provide torque to help the machine accelerate to self-sustaining speed somewhere between 50-60% of rated speed, usually. The IGVs (Inlet guide Vanes) are kept "closed" (at minimum angle) to reduce the amount of work being done by the compressor during starting and acceleration and to reduce the amount of fuel required to establish and maintain flame during firing, warm-up and initial acceleration to self-sustaining speed.

Once the machine shaft speed is high enough to allow the clutch mechanism halves to separate the machine is said to be self-sustaining--meaning enough work can be extracted from the amount of fuel being burned to keep the machine spinning and to accelerate it all the way to rated speed. At approximately 70% of rated speed the IGVs are ramped open to the minimum operating angle (usually somewhere around 57 DGA (DeGrees Angle) and the unit continues to accelerate to rated speed.

Once the machine accelerates to 95% of rated speed it is said to be "at synchronous speed"--even though it's still 5% below rated speed. The philosophy behind this control scheme is that if the machine reaches 95% speed it's highly it will reach 100% speed (actual synchronous speed), AND there are several things which are done when the machine reaches this speed level which can be done slightly below synchronous speed without affecting machine operation--those are the Auxiliary L.O. and Auxiliary Hydraulic Pumps are stopped (L.O. and hydraulic pressure is provided by the main pumps (driven by the Accessory Gear), the compressor bleed valves are closed (they are open during starting (and shutdown) to reduce the amount of energy required to accelerate the machine AND to prevent axial compressor stall/surge during starting (and shutdown), and the Mark* transitions from acceleration control to Droop Speed Control also as it reaches 100% speed.

At that point the machine is ready for synchronization; if the machine is operating in AUTO mode (of the Master Control Select) the operator will (usually) have to initiate synchronization, either manual- or automatic synchronization. If the machine is in REMTOE mode it's customary for the machine to immediately transition to automatic synchronization mode for closing the generator breaker. (Sometimes there are other permissives which must be met, such as utility tie breakers being in a certain position, etc., or some DCS or external control system must allow synchronization to take place; these are all site-specific conditions not typical OEM conditions.)

Once synchronization is complete, if the machine is operating in parallel with (synchronized to) a grid with other prime movers and their generators, the machine is almost always operating in Droop Speed Control, and it can be loaded to the desired value as required. Or, the operator can select Base Load--which will load the machine to its maximum allowable limit for the conditions at that time.

That's the basics. A little more basics than I gave in the shut down/trip description, but then a lot more goes on during starting and acceleration and synchronization than during stopping/tripping. The things I didn't mention in the shut down/tripping description were the compressor bleed valves which are opened either at some very low load (less than 5 MW, usually) or when the generator breaker is opened (tripped) or when the machine decelerates below 94% speed. Also, the Auxiliary L.O. and Auxiliary Hydraulic Pumps are started when the machine decelerates through 94% speed. To be honest, I don't recall precisely when the IGVs are reduced to their mechanical minimum position during shutdown.... At least not at this time, anyway. Someone will hopefully write to provide that piece of information.

 

STARTing a GE-design Frame 6B heavy duty gas turbine is slightly more straightforward.

In order to initiate a machine START for synchronized operation (electrical power production), one has to select AUTO or REMOTE from the Master Control Select section of the HMI Main Display (sometimes called the 'Start-up Display'). And once AUTO or REMOTE is selected, one of the fields in the Status section of the HMI Main Display will show either READY TO START or NOT READY TO START. It's VERY important to be in either AUTO or REMOTE (for normal electrical power production operation) to see EITHER READY TO START or NOT READY TO START!!! The machine can't be in OFF (in the Master Control Select section of the HMI Main Display) to get the ready- or not ready indication. The Mark* turbine control system has to be in some mode of operation (CRANK, FIRE, AUTO or REMOTE) to get the READY TO START or NOT READY TO START indication. (OFF means the machine is not in an operating mode, so it won't display the starting status.)

If the Status field indicates READY TO START the operator can then click on START and confirm the action and the start-up sequence will begin. If the Status field indicates NOT READY TO START and the operator doesn't understand the reason for the indication change to the Start Check HMI display and any field with a red background will be the reason for the NOT READY TO START indication. AND, every condition--but for one--will also be indicated with a Process Alarm on the Alarm Display to alert a conscious operator to the condition(s) preventing a START. And the one exception is--that's right, the Master Control Select status.

If the machine is at zero speed and is "cool" or "cold" (meaning its been at least 24 hours since it was shut down or tripped and the internal machine temperatures are at or near ambient temperature) AND it hasn't been more than a couple of days since the machine was shut down or tripped a START can be initiated from zero speed without being on Cooldown. If it has been more than four or five days since the machine was shut down or tripped and the machine has not been on Cooldown (hydraulic ratchet) during all that time it's possible that the axial compressor might have begun to bow or warp just due to its own weight, and that could cause high or even high-high vibration during starting and acceleration to rated speed. So, the OEM advises if it's been more than four or five days since the machine was operated and if the machine hasn't been on Cooldown for most of, or all, of that period (after the Cooldown period expired) that it be placed on Cooldown for approximately four hours (longer if its been many days or weeks since the machine was last operated) in order to rotate the shaft and help to correct any bowing or warping which might have occurred since the last time the shaft was rotated. NOTE: It IS NOT required to put the machine on Cooldown prior to ANY START for any period of time, contrary to wildly popular (and false) belief. It it were, the Mark* turbine control system would prevent a START from being initiated until an appropriate amount of time on Cooldown had expired--and it doesn't. Depending on conditions (including L.O. temperature!) the vibration during starting might be higher than normal (which should be below the alarm setpoint) but if it's absolutely necessary to get the machine started and producing electrical power quickly it is possible to START the machine if it hasn't been on Cooldown for a few days.

If the machine was recently shut down or tripped and is on Cooldown (hydraulic ratchet) and its desired or necessary to START it and produce electrical power it can be started when all the start-check permissives are met (in other words, a READY TO START indication can be seen in the Status field section of the HMI Main Display).

When a START is initialized and begins from zero speed, the Auxiliary L.O. Pump is started. And usually, the Auxiliary Hydraulic Pump is also started shortly after that and operate all the way to near 100% rated speed. In fact, the very last permissive for the START sequence to fully begin is when L.O. pressure is detected at the collector end of the generator--at that point the Starting Means is energized, the hydraulic ratchet mechanism begins turning and shortly after that the torque converter may be pressurized (there are different types of torque converters and slightly different sequencing required; some are continually pressurized whenever the Aux. L.O. Pump is running; others are only pressurized when the Starting Means is running; others are only pressurized when a solenoid is energized--the equipment depicted on the Starting Means P&ID will dictate the required sequencing).

Many older machines require a GE-design Frame 6B heavy duty gas turbine to be a zero speed to initiate a START. This is solely because of the type of coupling between the output of the Starting Means (induction electric motor or diesel engine) and the turbine shaft (through the Accessory Gear). Older machines, and some new ones, use a jaw clutch which SHOULD NOT be closed when the turbine shaft half of the clutch is spinning (in other words, when the turbine shaft is still spinning and NOT at zero speed). Usually the Mark* turbine control system will not indicate READY TO START until the shaft is at zero speed--to protect the jaw clutch from serious damage.

Many newer machines, and some older machines which have had an upgrade to the clutch mechanism between the Starting Means and the turbine shaft, use a SSS clutch--a self-synchronizing, shifting clutch mechanism which can be closed while the turbine is still turning--but only below a set speed (usually around 100 RPM or so). It's important to understand the type of clutch mechanism in use on the machine (this can be determined from looking at the Starting Means P&ID) to understand what limitations may be applicable to starting the machine.

If the shaft is at zero speed when a START is initiated the Cooldown system (hydraulic ratchet) will be started. This is because the shaft of the machine (axial compressor and turbine, and the Load Gear and generator shafts (since they are coupled together)) has a very high inertia and the Starting Means by itself does not have the ability to start the shaft turning (to "break the shaft away from zero speed") when it's at zero speed. So, the "bumping" action of the hydraulic ratchet gives the Starting Means just enough additional force to help break the shaft away from zero speed and accelerate it to purge speed. (Of course, if the machine has a SSS clutch and it is being started above zero speed the assist from the hydraulic ratchet is not necessary.)

Once the machine speed reaches what's called "purge speed"--which is the speed at which the timer starts for calculating the amount of air being moved through the axial compressor, turbine, exhaust and HRSG (boiler, if present) and the exhaust stack. This is called purging--using the axial compressor to push out any combustible gases from the machine and exhaust (including the HRSG if present) to prevent a possible explosion when the ignitors are energized. Once the purge timer has expired, the Mark* turbine control system begins the process of establishing flame in the combustors by energizing the ignitors (sometimes called "spark plugs") for approximately 30- to 60 seconds and admitting fuel into the combustors. There are two ignitors, for redundancy (only one ignitor is required to establish flame in all the combustors; the second one is redundant but they are both energized at the same time and for the same amount of time), and again, they are only necessary to help establish flame in the combustors and they are de-energized when the firing period expires AND flame has been detected (usually in more than one combustor--there are NOT flame detectors in all ten combustors, but there are usually redundant flame detectors, either two or four combustors). They fuel admitted during firing is very slightly more than required to maintain flame and its done to help establish flame in all the combustors. Once flame is established in some combustors the amount of fuel is reduced slightly during what's called the "warm-up" period. (Reducing the fuel after firing is done to reduce the thermal stresses on the combustors, turbine and exhaust areas to prolong the life of the components in the combustors and turbine and exhaust.) Once the warm-up period is complete the ignitors are de-energized (they are left energized to help maintain flame when the fuel is reduced from firing to warm-up), and fuel is slowly increased to help accelerate the machine. It should be noted the amount of fuel being burned at this point (firing, warm-up and initial acceleration (up to about 60% of rated speed) is not sufficient to keep the machine spinning without the assist of the Starting Means--which is still being used to provide torque to help the machine accelerate to self-sustaining speed somewhere between 50-60% of rated speed, usually. The IGVs (Inlet guide Vanes) are kept "closed" (at minimum angle) to reduce the amount of work being done by the compressor during starting and acceleration and to reduce the amount of fuel required to establish and maintain flame during firing, warm-up and initial acceleration to self-sustaining speed.

Once the machine shaft speed is high enough to allow the clutch mechanism halves to separate the machine is said to be self-sustaining--meaning enough work can be extracted from the amount of fuel being burned to keep the machine spinning and to accelerate it all the way to rated speed. At approximately 70% of rated speed the IGVs are ramped open to the minimum operating angle (usually somewhere around 57 DGA (DeGrees Angle) and the unit continues to accelerate to rated speed.

Once the machine accelerates to 95% of rated speed it is said to be "at synchronous speed"--even though it's still 5% below rated speed. The philosophy behind this control scheme is that if the machine reaches 95% speed it's highly it will reach 100% speed (actual synchronous speed), AND there are several things which are done when the machine reaches this speed level which can be done slightly below synchronous speed without affecting machine operation--those are the Auxiliary L.O. and Auxiliary Hydraulic Pumps are stopped (L.O. and hydraulic pressure is provided by the main pumps (driven by the Accessory Gear), the compressor bleed valves are closed (they are open during starting (and shutdown) to reduce the amount of energy required to accelerate the machine AND to prevent axial compressor stall/surge during starting (and shutdown), and the Mark* transitions from acceleration control to Droop Speed Control also as it reaches 100% speed.

At that point the machine is ready for synchronization; if the machine is operating in AUTO mode (of the Master Control Select) the operator will (usually) have to initiate synchronization, either manual- or automatic synchronization. If the machine is in REMTOE mode it's customary for the machine to immediately transition to automatic synchronization mode for closing the generator breaker. (Sometimes there are other permissives which must be met, such as utility tie breakers being in a certain position, etc., or some DCS or external control system must allow synchronization to take place; these are all site-specific conditions not typical OEM conditions.)

Once synchronization is complete, if the machine is operating in parallel with (synchronized to) a grid with other prime movers and their generators, the machine is almost always operating in Droop Speed Control, and it can be loaded to the desired value as required. Or, the operator can select Base Load--which will load the machine to its maximum allowable limit for the conditions at that time.

That's the basics. A little more basics than I gave in the shut down/trip description, but then a lot more goes on during starting and acceleration and synchronization than during stopping/tripping. The things I didn't mention in the shut down/tripping description were the compressor bleed valves which are opened either at some very low load (less than 5 MW, usually) or when the generator breaker is opened (tripped) or when the machine decelerates below 94% speed. Also, the Auxiliary L.O. and Auxiliary Hydraulic Pumps are started when the machine decelerates through 94% speed. To be honest, I don't recall precisely when the IGVs are reduced to their mechanical minimum position during shutdown.... At least not at this time, anyway. Someone will hopefully write to provide that piece of information.

Thanks @WTF? for the detailed explanation.
Sorry for the delayed response. I have gone through your reply and other couples of threads for understanding startup sequence of the GT, Now I have pretty much basic idea startup sequence. Also I have found basic sequential block diagram for startup sequence attaching herewith, might be helpful for others.




If possible can anyone please share about oil pumps or documents related to it, like I have basic idea that during start command pumps gets start command and when the turbine reaches FSNL, they get their OFF command.

 

@Tapan,

You have the correct idea about the Aux. L.O. and Aux. Hyd. Pump motors; they are very simple control circuits, just on and off. They usually get turned of at speed level 14HS (Synchronous Speed)--which is usually set for 95% of rated (so not quite FSNL). The philosophy is that if the turbine reaches 95% of rated speed it's probably going to reach rated speed (synchronous speed for the generator--since the turbine drives the generator to produce electricity) they do some "switching" of auxiliaries at just before synchronous speed (FSNL) so that a LOT of things aren't all happening at exactly the same time.

The Operation & Service Manuals provided with the equipment by the turbine packager usually have individual sections for each of the major systems of the turbine and auxiliaries. There is some very basic descriptions of the components of the system in each of the sections, for example, Lube Oil and Hydraulic Oil. (It should be noted that L.O. and Hydraulic Oil (and sometimes what's referred to as Control Oil) are all the same fluid: lubricating oil, just at different pressures. The L.O. pumps feed the suctions of the Hydraulic pumps and the Hydraulic pumps raise the pressure of the fluid to that required for proper operation of the hydraulic devices and actuators. Be careful, though--those system descriptions ARE NOT site-specific. They are meant to be a basic description of system operation and components and NOT for any specific or particular machine. But, still they are very good places to begin learning about the machine and its auxiliaries.

I learned a little "rhyme" from a former colleague and contributor to Control.com for starting a GE-design heavy duty gas turbine: Fuel, Mode, START, Load. When starting a GE-design heavy duty gas turbine that can start on more than one fuel it's important to check to see that the desired fuel for this particular START is selected--because when the machine reaches speed level 14HM (Minimum Firing Speed) it's not possible to change fuels if the wrong was not set prior to the START initiation. AND, fuel can't be changed until the unit reaches speed level 14HS--so if the wrong fuel is selected one has to STOP the machine, let it drop below speed level 14HM, change the fuel and then re-start the machine when the READY TO START status is annunciated. OR wait until the machine gets to FSNL and then select the desired fuel and wait a couple of minutes for the fuels to be switched (transferred) and then continuing on with operations. Many machines can also change fuels (transfer fuels) while loaded (with the generator breaker closed and producing electric power).

Once the desired fuel selection is confirmed or made, then one selects the mode from the Master Control Selection area--usually AUTO or REMOTE for normal operation. Then the READY TO START status will be annunciated when all permissives are verified to be correct and the operator can then initiate a machine START. Once the machine START is selected the desired load for the operation can be set--even before flame is established in the machine from the Load Selection area--there is usually Pre-Selected Load or Base Load available (Peak Load is for "emergency" situations and not normally selected unless requested by the utility or host facility). And when the machine reaches FSNL and synchronization is complete (usually Automatic synchronization) the machine will automatically go to the selected load. So, Fuel, Mode, START, Load is a good little rhyme to remember the most basic steps in starting and operating a GE-design heavy duty gas turbine.

 

Hi,
We are having BHEL supplied GE Frame VI Gas Turbine with Speedtronic Mark VI control system and I am new to it so can anyone please post or send writeup or any links of websites for better understanding about Frame VI GE Gas Turbine Startup/ shutdown sequence for a beginner. Eagerly waiting to learn more and more in depth about system, logics etc.

Thank you in advance.

Mail Id: [email protected]

Hi

Better have to find out kind of following document it is very helpful for getting a clear overiview of GT start up and shutdown operations process ..:




It is a GEEPE document but BHEL may have one also