General Electric Turbines - MS7001E/EA

Is there a maximum speed at which a turbine should not be "lit off"? If the unit is in the "crank" mode, is there any danger in going into the "fire" mode? Typical light off speed is 10% - 15% rated speed (3,600 RPM - {360/540}). If the unit was in the crank mode, at 20% rated speed, would it damage the equipment if it was placed directly intot he fire mode?

 

I believe if you move from "crank" to "fire" the Speedtronic will automatically reduce the speed to firing speed.

The reason is that the turbine will likely not light off at the higher speed. The reason for cranking at 20% is to reduce the time necessary to purge the exhaust prior to firing. This reduces the overall startup time of the unit.

 

Junebug, huh? Extremely interesting choice of pseudonym; extremely interesting.

The discussion below presumes a 7E/EA unit with a Voith torque converter with torque adjuster mechanism and describes the way the sequencing is normally configured in a Speedtronic turbine control system and the way the torque adjuster mechanism is to be adjusted per the Control Specification drawing.A Frame 7E/EA unit is typically not to be fired at 20% speed. The control Specification usually says that firing speed is 10% TNH.

When a 7E/EA with a Voith torque converter with a variable torque adjuster mechanism is STARTed in CRANK, FIRE, AUTO or REMOTE mode, the unit goes to <b>**purge**</b> speed with the torque converter at purge torque position (which is usually 150-160% of starting motor nameplate torque) which is usually approximately 20% TNH. When the unit passes through TNK14HM1 (usually 10% TNH) the purge timer starts timing.

When the purge timer expires, the unit coasts back to just below minimum firing speed to TNK14HM2 (usually 9.5% TNH)and the torque adjuster is set to <b>**firing**</b> torque position (which is supposed to be 10% TNH, -0% TNH, +2% TNH,, which is usually about 60% of starting motor nameplate rated current). Once the speed drops below TNK14HM2, the torque converter is re-pressurized to start slowly accelerating the unit back through TNK14HM1 to start firing.

If the unit is in CRANK mode, when the purge timer expires and the unit decelerates to TNK14HM2 and starts slowly accelerating again at the firing torque setting, it will just accelerate to about 12% TNH and just sit there, <b>CRANKing</b> until the operator initiates a STOP or selects FIRE, AUTO or REMOTE, at which time the firing sequence will start.

So, approximately 20% TNH is <b>PURGE</b> speed, and approximately 12% TNH is <b>CRANK</b> speed.

If the unit is in CRANK mode, and at approximately 20% TNH the purge timer should not be expired. And if the operator selects FIRE, AUTO or REMOTE mode while the purge timer is still timing, nothing will happen until the purge timer expires, the unit coasts down to below minimum firing speed and the starts accelerating slowly back through minimum firing speed at which time the firing sequence will begin.

Crank speed should NOT be 20% TNH, because at 20% TNH the starting motor is drawing approximately 150-160% of rated current, which is quite high--normal, but high, so it's only to be done for short periods. Cranking is defined to be continuous rotation at low speed with the starting means, and that should occur after the purge time has expired and only at or less than rated nameplate current. Cranking might be done for hours, for cooldown purposes or for off-line water wash rinsing. So, cranking should not be done above rated nameplate current for any extended period of time, or the starting motor's lifespan will be shortened.

Most of the starting motors used for GE Frame 7E/EA units are built with Class F insulation and because they are only operated for "short" periods above rated current and they are supposed to go through a cooldown after acceleration to cool the motor, its rotor and windings, they can last for many years.

But, the answer to your question should be that even if you are <b>purging</b> at approximately 20% TNH in CRANK mode and the operator selects FIRE, AUTO or REMOTE mode the unit should not immediately start firing. The firing timer should continue until it expires, the unit should coast down, then slowly start accelerating to <b>firing</b> speed and then firing should begin.

If firing were to occur at 20% TNH, and flame was actually established the internal temperatures would likely be very high, high enough to shorten hot gas path parts life.

The reality is that most torque converters have been misadjusted over the years, or have gone out of adjustment. And worse, some sequencing has been improperly modified by people, including increasing firing FSR to get the unit to fire above desired speed.

 

Please, can you send me a dray of the MS7001E/EA? I need the dimensions.
Thank you!

<b>Moderator's note:</b> I think he means drawing.