I have a question about turbo-generator jacking systems.
Generators typically have a jacking system but don't have an emergency or backup jacking system.

Assume the generator and turbine trip or shut down for any reason, but the jacking pump fails to start (due to a malfunction). What happens? The shaft speed is decreasing, but the generator's jacking pump isn't running.

 

@doremami.

The main purpose of jacking oil on a heavy piece of industrial equipment is to protect the bearings at low speed. That's all jacking oil does--keep the weight of the shaft (the turbine shaft and/or the generator shaft) from damaging the bearings at low RPMs. The high-pressure jacking oil flow into the bearings serves to lift (or "jack up") the shaft by increasing the height of the oil wedge between the lower portion of the bearing and the shaft. That's the purpose of jacking oil. It also helps when starting the turbo-generator because it makes the turbo-generator shafts easier to turn because of the increase oil flow and oil wedge height.

You are correct--if the jacking oil pump fails to start during coast down (after a shutdown or emergency trip) it can lead to bearing and shaft damage, especially if the shaft and/or bearing is very hot and there isn't sufficient lubricating oil flow of a normal "cool" lubricating oil temperature. A bearing made using Babbitt material can be very soft if the bearing temperature nears approximately 300 deg F (approximately 150 deg C).

Many machines have embedded bearing metal temperature sensors (thermocouples or RTDs) and the bearing oil drains can also have temperature sensors (again, thermocouples or RTDs). Along with the bearing oil temperature these readings can be used to monitor bearing conditions during coastdown when the jacking oil system is inoperative.

I think most turbo-generator manufacturers have a small margin of safety designed into the bearing lubrication- and jacking oil systems for the--hopefully--infrequent lack of jacking oil pressure-flow during coastdown from a running condition. In my experience a vigilant check of the three temperatures mentioned (bearing metal, bearing oil drain, and bearing lubrication oil) during and after such an occurrence is important for determining if the bearing/shaft may have suffered any damage. Also, if the bearing(s) is(are) equipped with proximity sensors the clearance indicated during and after low speed shutdown without jacking oil as well as after reaching zero speed can help determine if the bearings suffered and serious damage. But, as long as the lack of jacking oil pressure during coastdown is not a common occurrence most equipments seem to be able to safely get to zero speed without serious damage to the bearings/shafts once is a while--PRESUMING the lubricating oil temperature, pressure and flow are normal (temperature and flow-rate) during and after coastdown to zero speed. (If the jacking oil system is not reliable, that's a serious problem.)

However, starting a turbo-generator without jacking oil is not recommended--and, in fact, it may not even be possible for the starting means to turn the turbo-generator shaft without the assist jacking oil provides.

So, there are recommended procedures for monitoring the coastdown of a turbo-generator when jacking oil pressure/flow is not sufficient or available to see if damage may have occurred, and it is extremely important to maintain the flow of lube oil to the bearings after reaching zero speed until it is felt the machine has sufficiently cooled (the turbine part of the machine--which is the part that gets hottest and can transmit heat from the interior of the machine to the bearing through the shaft). (That time is usually about 24 hours for many machine, but could be more for some machines--check with the manufacturer/packager of the equipment to be sure.)

And, if until there is normal jacking oil pressure/flow it is probably not recommended to put the machine on "cooldown" (barring; hydraulic ratchet; gear; turning gear; jacking gear; etc.) as turning the shaft at low speeds without sufficient jacking oil pressure/flow could most likely cause damage whether the bearings are hot or cold. There are procedures for dealing with a hot machine which couldn't be put on cooldown/gear after reaching zero speed--again, check with the manufacturer or packager of the equipment to be sure.

Remember, while you or your colleagues or your supervisors may think a DC jacking oil pump or redundant jacking oil pumps should have been provided with the turbo-generator the turbo-generator manufacturer/packager usually warrants the equipment for some period after commissioning and it's not likely that with the potential cost of damage from lack of jacking oil pressure/flow they would not have considered such mitigating measures--but didn't deem it necessary to do so. AND, even while the machine is under warranty, and certainly after the warranty period has expired the operator has a responsibility to maintain the equipment in a normal manner--which means including proper checks and monitoring of the jacking oil system (in this case).

Finally, in my personal opinion jacking oil is most necessary for starting and during cooldown/gear/barring operation. Yes; under certain unusual conditions (problems with lube oil temperature and flow--and remember, also, that jacking is usually lubricating oil so if the lubricating oil temperature or flow is not normal the jacking oil flow may not be either) it's important to have jacking oil for coastdown operation--but it's most critical for starting and initial low-speed operation.

I'm NOT a turbine designer; just an old (meaning experienced) commissioning engineer who's seen many turbo-generators over several decades.

 

@doremami.

The main purpose of jacking oil on a heavy piece of industrial equipment is to protect the bearings at low speed. That's all jacking oil does--keep the weight of the shaft (the turbine shaft and/or the generator shaft) from damaging the bearings at low RPMs. The high-pressure jacking oil flow into the bearings serves to lift (or "jack up") the shaft by increasing the height of the oil wedge between the lower portion of the bearing and the shaft. That's the purpose of jacking oil. It also helps when starting the turbo-generator because it makes the turbo-generator shafts easier to turn because of the increase oil flow and oil wedge height.

You are correct--if the jacking oil pump fails to start during coast down (after a shutdown or emergency trip) it can lead to bearing and shaft damage, especially if the shaft and/or bearing is very hot and there isn't sufficient lubricating oil flow of a normal "cool" lubricating oil temperature. A bearing made using Babbitt material can be very soft if the bearing temperature nears approximately 300 deg F (approximately 150 deg C).

Many machines have embedded bearing metal temperature sensors (thermocouples or RTDs) and the bearing oil drains can also have temperature sensors (again, thermocouples or RTDs). Along with the bearing oil temperature these readings can be used to monitor bearing conditions during coastdown when the jacking oil system is inoperative.

I think most turbo-generator manufacturers have a small margin of safety designed into the bearing lubrication- and jacking oil systems for the--hopefully--infrequent lack of jacking oil pressure-flow during coastdown from a running condition. In my experience a vigilant check of the three temperatures mentioned (bearing metal, bearing oil drain, and bearing lubrication oil) during and after such an occurrence is important for determining if the bearing/shaft may have suffered any damage. Also, if the bearing(s) is(are) equipped with proximity sensors the clearance indicated during and after low speed shutdown without jacking oil as well as after reaching zero speed can help determine if the bearings suffered and serious damage. But, as long as the lack of jacking oil pressure during coastdown is not a common occurrence most equipments seem to be able to safely get to zero speed without serious damage to the bearings/shafts once is a while--PRESUMING the lubricating oil temperature, pressure and flow are normal (temperature and flow-rate) during and after coastdown to zero speed. (If the jacking oil system is not reliable, that's a serious problem.)

However, starting a turbo-generator without jacking oil is not recommended--and, in fact, it may not even be possible for the starting means to turn the turbo-generator shaft without the assist jacking oil provides.

So, there are recommended procedures for monitoring the coastdown of a turbo-generator when jacking oil pressure/flow is not sufficient or available to see if damage may have occurred, and it is extremely important to maintain the flow of lube oil to the bearings after reaching zero speed until it is felt the machine has sufficiently cooled (the turbine part of the machine--which is the part that gets hottest and can transmit heat from the interior of the machine to the bearing through the shaft). (That time is usually about 24 hours for many machine, but could be more for some machines--check with the manufacturer/packager of the equipment to be sure.)

And, if until there is normal jacking oil pressure/flow it is probably not recommended to put the machine on "cooldown" (barring; hydraulic ratchet; gear; turning gear; jacking gear; etc.) as turning the shaft at low speeds without sufficient jacking oil pressure/flow could most likely cause damage whether the bearings are hot or cold. There are procedures for dealing with a hot machine which couldn't be put on cooldown/gear after reaching zero speed--again, check with the manufacturer or packager of the equipment to be sure.

Remember, while you or your colleagues or your supervisors may think a DC jacking oil pump or redundant jacking oil pumps should have been provided with the turbo-generator the turbo-generator manufacturer/packager usually warrants the equipment for some period after commissioning and it's not likely that with the potential cost of damage from lack of jacking oil pressure/flow they would not have considered such mitigating measures--but didn't deem it necessary to do so. AND, even while the machine is under warranty, and certainly after the warranty period has expired the operator has a responsibility to maintain the equipment in a normal manner--which means including proper checks and monitoring of the jacking oil system (in this case).

Finally, in my personal opinion jacking oil is most necessary for starting and during cooldown/gear/barring operation. Yes; under certain unusual conditions (problems with lube oil temperature and flow--and remember, also, that jacking is usually lubricating oil so if the lubricating oil temperature or flow is not normal the jacking oil flow may not be either) it's important to have jacking oil for coastdown operation--but it's most critical for starting and initial low-speed operation.

I'm NOT a turbine designer; just an old (meaning experienced) commissioning engineer who's seen many turbo-generators over several decades.

Thank you. Your response was extremely detailed and accurate.

 

@doremami.

The main purpose of jacking oil on a heavy piece of industrial equipment is to protect the bearings at low speed. That's all jacking oil does--keep the weight of the shaft (the turbine shaft and/or the generator shaft) from damaging the bearings at low RPMs. The high-pressure jacking oil flow into the bearings serves to lift (or "jack up") the shaft by increasing the height of the oil wedge between the lower portion of the bearing and the shaft. That's the purpose of jacking oil. It also helps when starting the turbo-generator because it makes the turbo-generator shafts easier to turn because of the increase oil flow and oil wedge height.

You are correct--if the jacking oil pump fails to start during coast down (after a shutdown or emergency trip) it can lead to bearing and shaft damage, especially if the shaft and/or bearing is very hot and there isn't sufficient lubricating oil flow of a normal "cool" lubricating oil temperature. A bearing made using Babbitt material can be very soft if the bearing temperature nears approximately 300 deg F (approximately 150 deg C).

Many machines have embedded bearing metal temperature sensors (thermocouples or RTDs) and the bearing oil drains can also have temperature sensors (again, thermocouples or RTDs). Along with the bearing oil temperature these readings can be used to monitor bearing conditions during coastdown when the jacking oil system is inoperative.

I think most turbo-generator manufacturers have a small margin of safety designed into the bearing lubrication- and jacking oil systems for the--hopefully--infrequent lack of jacking oil pressure-flow during coastdown from a running condition. In my experience a vigilant check of the three temperatures mentioned (bearing metal, bearing oil drain, and bearing lubrication oil) during and after such an occurrence is important for determining if the bearing/shaft may have suffered any damage. Also, if the bearing(s) is(are) equipped with proximity sensors the clearance indicated during and after low speed shutdown without jacking oil as well as after reaching zero speed can help determine if the bearings suffered and serious damage. But, as long as the lack of jacking oil pressure during coastdown is not a common occurrence most equipments seem to be able to safely get to zero speed without serious damage to the bearings/shafts once is a while--PRESUMING the lubricating oil temperature, pressure and flow are normal (temperature and flow-rate) during and after coastdown to zero speed. (If the jacking oil system is not reliable, that's a serious problem.)

However, starting a turbo-generator without jacking oil is not recommended--and, in fact, it may not even be possible for the starting means to turn the turbo-generator shaft without the assist jacking oil provides.

So, there are recommended procedures for monitoring the coastdown of a turbo-generator when jacking oil pressure/flow is not sufficient or available to see if damage may have occurred, and it is extremely important to maintain the flow of lube oil to the bearings after reaching zero speed until it is felt the machine has sufficiently cooled (the turbine part of the machine--which is the part that gets hottest and can transmit heat from the interior of the machine to the bearing through the shaft). (That time is usually about 24 hours for many machine, but could be more for some machines--check with the manufacturer/packager of the equipment to be sure.)

And, if until there is normal jacking oil pressure/flow it is probably not recommended to put the machine on "cooldown" (barring; hydraulic ratchet; gear; turning gear; jacking gear; etc.) as turning the shaft at low speeds without sufficient jacking oil pressure/flow could most likely cause damage whether the bearings are hot or cold. There are procedures for dealing with a hot machine which couldn't be put on cooldown/gear after reaching zero speed--again, check with the manufacturer or packager of the equipment to be sure.

Remember, while you or your colleagues or your supervisors may think a DC jacking oil pump or redundant jacking oil pumps should have been provided with the turbo-generator the turbo-generator manufacturer/packager usually warrants the equipment for some period after commissioning and it's not likely that with the potential cost of damage from lack of jacking oil pressure/flow they would not have considered such mitigating measures--but didn't deem it necessary to do so. AND, even while the machine is under warranty, and certainly after the warranty period has expired the operator has a responsibility to maintain the equipment in a normal manner--which means including proper checks and monitoring of the jacking oil system (in this case).

Finally, in my personal opinion jacking oil is most necessary for starting and during cooldown/gear/barring operation. Yes; under certain unusual conditions (problems with lube oil temperature and flow--and remember, also, that jacking is usually lubricating oil so if the lubricating oil temperature or flow is not normal the jacking oil flow may not be either) it's important to have jacking oil for coastdown operation--but it's most critical for starting and initial low-speed operation.

I'm NOT a turbine designer; just an old (meaning experienced) commissioning engineer who's seen many turbo-generators over several decades.

The remaining question I have is this scenario: Suppose the shaft speed is decreasing due to a turbine trip. Meanwhile, our logic is programmed to engage the jacking oil pump when shaft speed falls below 320 rpm. But if the jacking pump fails to start, two critical questions arise:

What immediate actions should we take?

What are the potential consequences? Specifically, could this lead to shaft-to-babbitt contact?

 

@doremami,

If I were operating the machine (or training the machine operators) I would pay close attention to the three parameters I mentioned. If the jacking oil system failed to start after being commanded to do so there should be some kind of alarm to indicate what the problem might be--even if it's just that the pump failed to start. I would also be very curious about why the jacking oil pump failed to start and I would be going to the location of the jacking oil pump motor starter(s) to examine them--was it because the motor starter was not energized or the main breaker tripped or there was an overload detected. If there is a second, redundant, jacking oil pump I would be checking to see why it hasn't started (I presume if there is a second, redundant, jacking oil pump it will start on low/no pressure/flow from the lead jacking oil pump--and there would also be an alarm to alert the operator to the failure of the standby/back-up jacking oil pump to start).

BUT if the jacking oil pump (and the standby jacking oil pump if present) can't be started very quickly then there's nothing really to be done except wait and hope for the best. I would also make sure that the main lubricating oil flow to the bearings is at it's specified temperature--and if it's higher, especially if it's much higher, than specification I would be working on getting more cooling water flow to drop that temperature. Cold lubricating oil is thicker than hot lubricating oil and that might help with protecting the bearings--both during coastdown and while at rest.

I would also be working to make sure that when the machine reached zero speed that if the jacking oil pump wasn't running that the barring/turning gear or ratchet mechanism wouldn't come on. Just leave the machine to cool naturally--don't turn on any turbine compartment vent fans or open any turbine compartment doors or turn on the exhaust frame blowers (if present).

PATIENCE IS A VIRTUE IF THE MACHINE CAN'T BE PUT ON COOLDOWN (gear; turning gear; barring; ratchet). Yes, the axial compressor shaft is going to deform because it's hot and the metal of the shaft and axial compressor assembly are in their elastic range (but not their plastic range!). As the machine cools it will actually start to straighten again with time (approximately 24 hours depending on the machine and its configuration) and there are procedures for straightening the shaft further before attempting full speed operation again.

Usually the axial compressor shaft is the longest shaft of the machine train (the axial compressor shaft; the turbine shaft; the generator shaft)--AND the heaviest. Yes, the generator shaft is very heavy but it's not usually as hot as the axial compressor (or turbine) shaft during shutdown/emergency trip conditions. Some axial compressors are made by bolting many wheels together with long through-bolts and that somewhat adds to the complexity of the situation because there are multiple types of metal with different coefficients of expansion, etc. BUT JUST BE PATIENT and DON'T FORCE THE MACHINE TO TURN UNTIL IT'S FULLY COOLED--NATURALLY COOLED. Believe me, I have seen some serious damage done to axial compressors and axial compressor cases using hydraulic jacks/rams to try to get the machine rolling again, which led to several months of lost production waiting for parts to repair the damage--which wouldn't have occurred had the machine been left alone for at least 24 hours.

Again--I have to believe (because the manufacturers or packagers warrant these machines for a period after commissioning) that the one-off occurrence of shutting down without jacking oil pressure/flow with all other factors being normal will not generally cause damage to the machine. There IS some oil wedge between the shaft and bearing as long as the main/auxiliary oil pump is running and supplying oil flow--cooling oil flow--to the bearings. And, the oil pumps are left running during the period immediately after the machine reaches zero speed for a period of approximately 24 hours while the machine cools naturally. So while no jacking oil flow could lead to some shaft-to-bearing contact it shouldn't be too serious. (I'm speaking of newer bearings with normal bearing material thickness.)

I know of some machines that use the machine's hydraulic oil pumps to supply jacking oil to the bearings--but there are usually two, redundant, hydraulic oil pumps and two solenoids controlling the oil jacking pressure/flow to the bearings. The two hydraulic pump motors get their mains power from to different sources/auxiliary transformers--they are as redundant as they can be. We don't know anything about the machine you are or might be working on--so it's impossible for us to say what could be done in the event the jacking oil pressure/flow to the bearings is unavailable. But, again, the one-off occurrence on a well-maintained and operated machine probably won't result in much damage to the bearings/shaft if any.

And, by using the proximity vibration sensor--recording the readings as the machine coasts down and when and after it reaching zero speed it would be possible to detect any deflection of the bearing material caused by excessive heat which would result from the shaft rubbing on the bearing material. (Again, I'm presuming journal bearings with Babbitt material for the bearings.)

By recording the parameters I have suggested during coastdown and after the machine reaches zero speed for approximately 24 hours you will have the best data to review to make some determination if damage might have occurred. And have a good chance of knowing whether or not the machine needs to come apart because jacking oil pressure/flow was unavailable during coastdown after a trip.

The biggest issue with the bearings of a machine that was tripped from a loaded condition is the internal temperatures of the machine--specifically the temperature of the axial compressor and turbine shaft sections of the machine. (It's always been my understanding that jacking oil is used primarily for helping to get the generator turning during starting and while the generator rotor gets warm (so the generator shaft journals will also get warm) they don't get as hot as the axial compressor and turbine shafts. The turbine shaft gets hottest and will transfer that heat to the bearings if the lubricating oil flow doesn't remove the heat from the shaft/bearings.

One of the largest manufacturers of large turbo-generators had a philosophy that auxiliary motors are "expendable"--meaning that if they experience an overload the motor starter wouldn't trip and it would run until it fails. That philosophy was prominent when auxiliary motors were only used to get the machine started as the main devices (lube oil pump; hydraulic pump) were driven by the turbine shaft and weren't used during normal running operation--unless there was a problem with the main pump(s). This was also during a period when many turbo-generators were used for emergency power generation, and to help support grids when loads were high (the function was called "peaking" or "peak shaving)"so getting the machines running was secondary to saving an auxiliary motor or pump. Also, back then, the machines weren't heavy enough and the bearings were engineered not to need any jacking oil pressure/flow. To my way of thinking, burning up a jacking oil pump motor to keep jacking oil flow during a coastdown is cheap. But getting the jacking oil pump motor running and keeping it running under duress (it can be pretty stressful when struggling to start and keep a "failed" jacking oil pump motor pump running!) might take longer than it takes for the machine to coastdown.

I have thrown a lot of information at you--just mostly food for thought. Again, we don't know anything about the machine you are or might be working on/operating. We don't know if there's been a failure at your plant or some anecdotal information about a similar failure at another plant. We don't know if you're trying to write SOPs (Standard Operating Procedures) or just curious. There's a LOT we don't know. And finally, it's every plant's decision to decide what and how they will run and maintain their machines. Manufacturers and consultants (paid or unpaid) can make suggestions or recommendations, but in the end it's the owner's operators who should be able to decide what to do and when to do it. (I have seen some plant owners ignore advice they paid for--and spend a LOT of money repairing their machine, all the while complaining, "You didn't tell us the consequences were going to be this bad!!!" (And I've heard that more than three or four times in my career.)

Seriously, the best source of information is the usually the original equipment manufacturer or the packager of the equipment (depending on who has or had the warranty on the machine after commissioning).

 

@doremami,

If I were operating the machine (or training the machine operators) I would pay close attention to the three parameters I mentioned. If the jacking oil system failed to start after being commanded to do so there should be some kind of alarm to indicate what the problem might be--even if it's just that the pump failed to start. I would also be very curious about why the jacking oil pump failed to start and I would be going to the location of the jacking oil pump motor starter(s) to examine them--was it because the motor starter was not energized or the main breaker tripped or there was an overload detected. If there is a second, redundant, jacking oil pump I would be checking to see why it hasn't started (I presume if there is a second, redundant, jacking oil pump it will start on low/no pressure/flow from the lead jacking oil pump--and there would also be an alarm to alert the operator to the failure of the standby/back-up jacking oil pump to start).

BUT if the jacking oil pump (and the standby jacking oil pump if present) can't be started very quickly then there's nothing really to be done except wait and hope for the best. I would also make sure that the main lubricating oil flow to the bearings is at it's specified temperature--and if it's higher, especially if it's much higher, than specification I would be working on getting more cooling water flow to drop that temperature. Cold lubricating oil is thicker than hot lubricating oil and that might help with protecting the bearings--both during coastdown and while at rest.

I would also be working to make sure that when the machine reached zero speed that if the jacking oil pump wasn't running that the barring/turning gear or ratchet mechanism wouldn't come on. Just leave the machine to cool naturally--don't turn on any turbine compartment vent fans or open any turbine compartment doors or turn on the exhaust frame blowers (if present).

PATIENCE IS A VIRTUE IF THE MACHINE CAN'T BE PUT ON COOLDOWN (gear; turning gear; barring; ratchet). Yes, the axial compressor shaft is going to deform because it's hot and the metal of the shaft and axial compressor assembly are in their elastic range (but not their plastic range!). As the machine cools it will actually start to straighten again with time (approximately 24 hours depending on the machine and its configuration) and there are procedures for straightening the shaft further before attempting full speed operation again.

Usually the axial compressor shaft is the longest shaft of the machine train (the axial compressor shaft; the turbine shaft; the generator shaft)--AND the heaviest. Yes, the generator shaft is very heavy but it's not usually as hot as the axial compressor (or turbine) shaft during shutdown/emergency trip conditions. Some axial compressors are made by bolting many wheels together with long through-bolts and that somewhat adds to the complexity of the situation because there are multiple types of metal with different coefficients of expansion, etc. BUT JUST BE PATIENT and DON'T FORCE THE MACHINE TO TURN UNTIL IT'S FULLY COOLED--NATURALLY COOLED. Believe me, I have seen some serious damage done to axial compressors and axial compressor cases using hydraulic jacks/rams to try to get the machine rolling again, which led to several months of lost production waiting for parts to repair the damage--which wouldn't have occurred had the machine been left alone for at least 24 hours.

Again--I have to believe (because the manufacturers or packagers warrant these machines for a period after commissioning) that the one-off occurrence of shutting down without jacking oil pressure/flow with all other factors being normal will not generally cause damage to the machine. There IS some oil wedge between the shaft and bearing as long as the main/auxiliary oil pump is running and supplying oil flow--cooling oil flow--to the bearings. And, the oil pumps are left running during the period immediately after the machine reaches zero speed for a period of approximately 24 hours while the machine cools naturally. So while no jacking oil flow could lead to some shaft-to-bearing contact it shouldn't be too serious. (I'm speaking of newer bearings with normal bearing material thickness.)

I know of some machines that use the machine's hydraulic oil pumps to supply jacking oil to the bearings--but there are usually two, redundant, hydraulic oil pumps and two solenoids controlling the oil jacking pressure/flow to the bearings. The two hydraulic pump motors get their mains power from to different sources/auxiliary transformers--they are as redundant as they can be. We don't know anything about the machine you are or might be working on--so it's impossible for us to say what could be done in the event the jacking oil pressure/flow to the bearings is unavailable. But, again, the one-off occurrence on a well-maintained and operated machine probably won't result in much damage to the bearings/shaft if any.

And, by using the proximity vibration sensor--recording the readings as the machine coasts down and when and after it reaching zero speed it would be possible to detect any deflection of the bearing material caused by excessive heat which would result from the shaft rubbing on the bearing material. (Again, I'm presuming journal bearings with Babbitt material for the bearings.)

By recording the parameters I have suggested during coastdown and after the machine reaches zero speed for approximately 24 hours you will have the best data to review to make some determination if damage might have occurred. And have a good chance of knowing whether or not the machine needs to come apart because jacking oil pressure/flow was unavailable during coastdown after a trip.

The biggest issue with the bearings of a machine that was tripped from a loaded condition is the internal temperatures of the machine--specifically the temperature of the axial compressor and turbine shaft sections of the machine. (It's always been my understanding that jacking oil is used primarily for helping to get the generator turning during starting and while the generator rotor gets warm (so the generator shaft journals will also get warm) they don't get as hot as the axial compressor and turbine shafts. The turbine shaft gets hottest and will transfer that heat to the bearings if the lubricating oil flow doesn't remove the heat from the shaft/bearings.

One of the largest manufacturers of large turbo-generators had a philosophy that auxiliary motors are "expendable"--meaning that if they experience an overload the motor starter wouldn't trip and it would run until it fails. That philosophy was prominent when auxiliary motors were only used to get the machine started as the main devices (lube oil pump; hydraulic pump) were driven by the turbine shaft and weren't used during normal running operation--unless there was a problem with the main pump(s). This was also during a period when many turbo-generators were used for emergency power generation, and to help support grids when loads were high (the function was called "peaking" or "peak shaving)"so getting the machines running was secondary to saving an auxiliary motor or pump. Also, back then, the machines weren't heavy enough and the bearings were engineered not to need any jacking oil pressure/flow. To my way of thinking, burning up a jacking oil pump motor to keep jacking oil flow during a coastdown is cheap. But getting the jacking oil pump motor running and keeping it running under duress (it can be pretty stressful when struggling to start and keep a "failed" jacking oil pump motor pump running!) might take longer than it takes for the machine to coastdown.

I have thrown a lot of information at you--just mostly food for thought. Again, we don't know anything about the machine you are or might be working on/operating. We don't know if there's been a failure at your plant or some anecdotal information about a similar failure at another plant. We don't know if you're trying to write SOPs (Standard Operating Procedures) or just curious. There's a LOT we don't know. And finally, it's every plant's decision to decide what and how they will run and maintain their machines. Manufacturers and consultants (paid or unpaid) can make suggestions or recommendations, but in the end it's the owner's operators who should be able to decide what to do and when to do it. (I have seen some plant owners ignore advice they paid for--and spend a LOT of money repairing their machine, all the while complaining, "You didn't tell us the consequences were going to be this bad!!!" (And I've heard that more than three or four times in my career.)

Seriously, the best source of information is the usually the original equipment manufacturer or the packager of the equipment (depending on who has or had the warranty on the machine after commissioning).

Thank you for your time and the detailed answers you've given.
I am a member of the logic design team at a generator manufacturing company. The generators we build, like other generators, only have one jacking or lifting pump. In our logic, when the shaft speed is between 0 and 320 RPM, the command to turn on the jacking pump is issued. There are two scenarios here:

- Startup Mode: When the speed increases from 0 to 320 RPM and beyond.

- Trip or Shutdown Mode: When the speed decreases from 3000 RPM to 320 RPM or lower.

In the first scenario, we don’t have any issues. We initiate startup only when we are sure the jacking pump’s operation is free of malfunctions. However, the problem lies in the second scenario. We have no control over shutdown or trip events, and we don’t know when they will occur.

This made me wonder: In the second scenario, if the jacking pump fails to turn on, what happens?

 

Okay; things are clearer now. However, it would seem that someone in the company's mechanical department should be able to answer the question of the possibility of bearing/shaft damage in the event of coastdown. This isn't really a controls issue. And when the machine is coasting down after a shutdown or a trip the machine can't generally be restarted before reaching 320 RPM nor can it be stopped "before" reaching 320 RPM (it still had to decrease below 320 RPM even if there was a giant brake).

It would seem that there would be some margin of "safety" or "protection" presuming the lubricating oil flow and the lubricating oil flowing to the generator bearings was normal in the event of a lack of jacking oil pressure/flow during a coastdown. What does the group responsible for bearings say about this? And, if your company is providing the jacking oil pump for the generator how are the generator bearings lubricated--with the turbine's lubricating oil pumps or does your company also provide the lubricating oil flow to the generator bearings?

But, again, it doesn't seem (to me) that your questions in Response #4 are controls-related--they are machine-related, and machine-specific. While it's likely the turbine control system controls when the generator jacking oil pumps are to be running, the consequences on the generator are not controlled by the turbine control system. If the jacking oil pumps don't start and run when they should be running (below 320 RPM) it's up to the site operations staff to take appropriate action as quickly as possible.

Also, it's likely the generator bearings and generator bearing drains have temperature sensors, and it's also probably likely that in this day and age the generator bearing also have proximity vibration sensors. While they are probably directly connected to the turbine control system, along with the generator bearing lubricating oil supply temperature sensor having that data in the event of a problem with the jacking oil on a coastdown can help make some very important decisions about troubleshooting and possible repairs after such an event.

It sounds like you need to have some very specific requirements about the starting of the generator jacking oil pump that you should be expressing to the turbine control system provider/programmer during a shutdown/coastdown to try to make sure the jacking oil pump is working properly. Can the jacking oil pump be started earlier than 320 RPM (decreasing) to test for pressure/flow and alarm if pressure/flow isn't sufficient? This might give the operations staff more time to try to get the generator jacking oil pump/system working before the speed decreases to a critical value in your company's opinion--because waiting to alarm until the machine speed decreases below 320 RPM to alarm if there isn't sufficient jacking oil pressure/flow doesn't leave much time for the operations staff to respond.

Digital turbine control systems are pretty reliable--if maintained properly. This includes motor starters. Electric motor reliability is as high as it's ever been, and probably pump reliability as well. Making sure the turbine control system supplier/programmer understands all of the requirements of the generator oil system and working with them to try to ensure reliability will be key to the protection of the generator bearings.