hello all

i am a plant operator. in our plant we have MS5002B (Starting means with turbine starting) and MS5002C (STRTING means motor 5.5 kv).

i want to ask how the starting means carry the GT from zero speed to purge speed to rated speed. we know that the motor run in the constant speed.

how the hydraulic mechanism, reversing gear, self synchronizing system and torque converter work?
why we use hydraulic and lubricant oil?

thanks you very much in advanced

 

zaidhrm,

Every control room and I&C shop (Instrumentation & Control) should have a laminated (plastic-sheathed) A3 set of gas turbines P&IDs in the control room for operators, their supervisors and plant management to use and refer to during operation and troubleshooting.

Every operator, technician and maintenance person should have their own A3 set of P&IDs on which they can make--and have made--notes to help them understand and remember things about systems and components as they learn.

If you don't have your own set of P&IDs, do your best to obtain one--and study them. If you have questions about systems or components you can ask them here. They are among the simplest and easiest of P&IDs any company produces, and there is even a document produced by GE and usually provided Operation and Service Manuals called the 'Piping Symbols' drawing which is the 'legend' to describe the various symbols (which really aren't that difficult to decipher). As an operator, the P&IDs will help you tremendously in operating and troubleshooting and interpreting Process Alarms--and the knowledge will help you greatly in your career.

The turbine starter is commonly referred to as an "expander turbine" and because it can operate at variable speeds--which induction electric motors cannot do (without a soft starter (VFD--Variable Frequency Drive). So, it doesn't require a "hydraulic" torque converter like an induction electric motor does.

The fluid in most "hydraulic" torque converters on GE-design heavy duty gas turbines is lubricating oil--the same used for bearings and the high-pressure hydraulic system. It is a device that allows the electric motor to run at a constant speed as the turbine shaft accelerates from zero speed to a self-sustaining speed.

The clutch--it can be a jaw-type clutch or a SSS clutch (which means Shifting, Self-Synchronizing)--couples the starting mean (the expander turbine, or the induction electric motor and torque converter) to the turbine shaft (usually through an Accessory Gear box). The two halves of a jaw clutch must be mechanically closed against its internal opening spring before the starting means can begin to transmit torque to the turbine shaft. Some units use small hydraulic cylinders to close (engage) the jaws; others use pneumatic cylinders. This has to be done when the turbine shaft and starting means are at zero speed--which means after a failed start or turbine trip the HP shaft (of a two-shaft turbine) must coast all the way down to zero speed before it can be re-started. The design of the teeth of a jaw clutch prevent it from being safely engaged while the HP turbine shaft is spinning, and it can be catastrophically damaged if attempted.

As the HP shaft accelerates during starting the speed of the turbine side of the jaw clutch will exceed the speed of the starting means side and the internal opening spring will disengage the the two sides of the jaw clutch and the starting means can be stopped while the HP shaft continues to accelerate from the power being extracted from the hot gases flowing through the HP turbine section.

A SSS clutch is automatically engaged any time the starting means speed is greater than the HP turbine shaft speed, and it disengages automatically any time the HP turbine shaft speed is greater than the starting means speed. The primary advantage of the SSS clutch is that during a failed start attempt or after a turbine trip the shaft doesn't have to go all the way to zero speed for the clutch to be safely engaged. It is also much smaller than a jaw clutch, has fewer parts that require periodic maintenance, and is extremely reliable. (Most GE-design heavy duty gas turbines with jaw clutches can be easily retrofitted with SSS clutches.)

A "hydraulic" torque converter is like an automatic transmission in a car--without the different "gears"/speeds. It is a method of transferring torque from a prime mover (electric motor; diesel engine) to a turbine shaft that allows the two to run at independent speeds. With the exception of some expander turbines (but not all of them), most starting means prime movers (induction electric motors; diesel engines) develop most of their power at higher speeds, so it's necessary to have some "variable transmission" between the prime mover and turbine shaft in order to break the shaft away from zero speed with the starting means and accelerate it during purging and firing and then up to a self-sustaining speed.

A "hydraulic" torque converter is a very convenient and reliable means of doing so. It is really nothing more than a high-pressure oil pump and "hydraulic" motor; the pump raises the pressure of the "hydraulic" oil and the pump converts the pressure and flow back onto torque that is applied through the clutch to the HP shaft at speeds less than the prime mover operates at. (One can't just admit fuel into a gas turbine and energize the ignitors ("spark plugs") when the shaft is at zero speed.

First, the turbine is not efficient at low RPMs; and, second, there's no air "flowing" into the combustors to mix with the fuel at zero speed. Neither can one spin a gas turbine up to rated speed with some "prime mover" and then admit fuel and energize the ignitors and establish flame. Flame is best established at lower air flows, which occurs at low turbine speeds. And, even when there is flame at low speeds the turbine can't develop enough torque from the hot gases to sustain speed without an assist from an external source--the starting means.)

The "reversing gear" I'm presuming is the "hydraulic" ratchet mechanism, which, again, uses lubricating oil as the "hydraulic" medium (fluid). It's purpose is to periodically turn the HP shaft approximately 1/8th of a rotation (approximately 45 degrees) when the unit has been shut down and the HP shaft (axial compressor and HP turbine) are warm. Because, if the shaft is not turned when it's warm the weight of the axial compressor section will cause the shaft to bend, or "warp", and this can lead to damage and high vibrations during a subsequent start-up.

So, the "hydraulic" ratchet uses high-pressure lubricating oil from a dedicated pump to move a couple of racks to turn the shaft the ~45 degrees, and when the mechanism reaches the end of the forward stroke (the turning of the shaft) then it goes into what's called a retraction stroke, to pull the racks back to re-engage them on the next forward stroke. (Actually, at the end of the retraction stroke, the system starts a forward stroke--but only for a couple of seconds. This is done to help keep the jaw clutch, if used, engaged and to ready the mechanism for the next operation in approximately 3 minutes.)

All during the time the "hydraulic" ratchet is in operation, called Cooldown operation, the AC motor-driven Auxiliary L.O Pump is operated, this to keep oil flowing to the bearings to prevent them from being damaged by the heat of the rotor; the L.O. serves as a cooling fluid to the bearings during Cooldown between ratchet operations.

zaidhrm, the GE-design heavy duty gas turbine controls community here on control.com has been active for more than ten (10) years. In that time, there have been a lot of posts, many of them about the same topics, and there is a lot of very good information available in the control.com Archives--which are accessed using the 'Search' function.

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Hope this helps!