Steam Turbine shaft speed reference 1

[stevenz]

I am trying to find out how the speed reference is set for an steam turbine system of a power plant. All the governors are of mechanical/Hydraulic type. Speed governor is a centrifugal one. I know that speed changer,or Speeder gear is adjusted for speed set point (reference). A motor drives the speeder gear. The speeder gear permits adjustment of the no-load running speed over a range of plus six percent to minus six percent rated speed (3400-3800RPM). My questions are: 1) Can I control shaft to maintain a speed at any points from 0 to 3600RPM or shaft can be controlled at a speed from 3400 RPM to 3800 RPM?
2) How the reference is set? Say if I want a 1800 RPM, how I
should run the motor to drive the speeder gear.

I appreciate any inputs.

 

[byrdj]

Your range of "CONTROLLED" speed will be limited by the mechanical governor design.

I'm familiar with GE's flyweight types, so that will be the discussion.

the design of the flyweights for an generation unit are such that the weights are agaisnt a mechanical stop until the speed exceeds minimun. the IEEE standards for mechanicl governors recomened the range for allowing off frequency synchronization was sometheing like -5% to +7%, thus the flyweight minumn speed could be like -10%.

They did design and make some flyweights that had a lower speed operating range, like for feed pump drives that allow controlled speed as low as 25%.

This narrow range of speed control is just due to the governors design. An operator can "control" and hold the speed any where he wishes. ie, he becomes the governor, and makes feedback corrections. SOME of the electronic governors can allow the referance (desired speed) be adjusted (or selected) from 0 to overspeed.

now how is referance set....
for the GE flyweight, as the flyweighs go out with increasing speed, the connecting center point will go down. the GE governor uses an adjustable bushing pilot valve, with the valve connected to the weights and the bushing to the speed referance adjuster.

starting at rated speed, off line lets call the flyweight position here 0, the speed referance bushing must be adjusted to bring the lap of the pilot valve to null or the oil flow thru the pilot valve will stroke the control valves. So the speed referance is a "position" adjustment.

To increase the speed 5%, the referance adjuster positions the bushing lower about 0.15", thus the lap of the pilot is no longer at null and oil flows to a piston that opens the control valves. as the control valves open, the turbine speeds up, the flyweights go out, the center pivot connected to the valve goes down. with an increase of 5% speed the valve will go down about 0.15" and brings the lap back to null, stopping the opening of the control valves BUT at a greater speed.

so to lower the speed referance, the bushing is raised. but besides the mechanical limitations of the acme thread that positions the bushing, there will be a corresponding minimun flywiegh speed where the weights are all the way in agaisnt a stop. so if you raised the bushing to -20%, but the flyweights are limited to -10%, the control valves would just close.

that's all the time I have now, hopethat helped

 

[stevenz]

Thanks a lot! The mechanical governors is exactly the GE's flyweight type. I still have some thing not clear.
When running the turbine off a barring to 3600 RPM, the following procedures may be followed:
-0 to 1600 RPM in minutes
-Warming period at 1600 RPM up to 60 mintues
--etc

Since the governor can only controls for a limited range of RPM, How can I maintain a low speed ,say 1600 RPM up to 60 minutes?

I appreciate your help.

 

[byrdj]

The operator has to perform the task of acceleration control and speed holds.

when speed is below LSS, the governor will want to open the CVs 100%, thus VERY fast acceleration. The operator will roll the unit either:

1)by using a device called Load Limit, which is a direct CV positioner below governor demand. thus on barring, the governor wants CV wideopen, but the operator closes the LL before resetting and opening the MSVs. The operator will pulse the LL to crack the CVs, watch his rpm counts or accel meter to establish the 60rpm/m. at hold speed (I've never heard of a 1600rpm hold, 1000 and 3000 are the norm) the operator will pulse the LL to bring accel to zero, and have to occasioanly bump LL to maintain speed since steam flow will also vary with boiler pressure. the varance of the hold rpms is a function of how good an operator. besides you DO NOT want to hold at a constant speed below rated due to bucket resonance (not to be confussed with rotor criticals)


the Once the unit is accelarated past the LSS (95%) the governor will start controlling the CVs and the operator can now use the speed load changer SLC to bring speed up to 3600. However, most plants preffer to have the SLC at HSS (107%) and let the operator use the LL to control the speed to synch and applied intial load. then after boiler is stable, bring SLC down to take control of the CVs and then run LL to 100%

2) the other way is to allow the CVs to go wide open and there be a stop valve bypass mechanism, the SVBP uses a small intergal valve inside the MSV to control steam flow. the operator will use the SVBP to control acceleration, and speed holds. then depending on where the governor (SLC) is set the CVs will either take control at LSS or the preffered is to synch on the MSV and apply intial load. then bring the CVs down to transfer either with LL or SLC

The SVBP roll is called full arc, since all CVs are open, all 360 degrees of the nozzle arc will be passing steam and provide uniform heating of the cassing and no impluse loading on the buckets. Once the maximun flow on the SVBP is reached, the CVs take control and the unit is now transferred to partial arc.

some machines have the CVs cammed as single admission, those all CVs open together to provide FA rolling