turbine speed

[ccjc81]

Hello,I work at a large fossil fuel plant an had a question on turbine speed.we have 4 steam turbines.2 cross compound and 2 tandem compound.they all operate at 3600 rpms.my question was how do you know when there at 3600 rpms?Looking for something more in depth than a basic answer.any input would be greatly appreciated. I'm new to this field and looking for some solid advice.thank you all.

 

[waross]

Is the floor shaking?
Are large circuit breakers tripping out?
Are there loud, bad sounding noises coming from the turbine hall?
Are the pressure safety valves blowing off a lot of steam?
Are people running around in an excited state saying things like; "Holly Gadzooks!", "Jumpin' Jupiter!", "Pole Slip!" and "Never did that before!"
If none of these things are happening than all of the turbines are probably running at the proper speed.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Er, umm, perhaps a digital tachometer of some kind in a control room? Could be calibrated in electrical Hz, in which case you'd have to multiply by 60 to get RPM, ASSuming the turbines are direct coupled to the generators and not geared to them, and that your grid operates at 60 Hz, none of which you have clearly stated...

When I worked in a fossil/steam generating plant I could generally tell by the relative pitch of the sound of the turbines against the power frequency hum of the transformers in the plant roughly how fast the turbines were spinning; try it sometime.

If the generators are directly coupled to the turbines and are 60 Hz synchronous machines synchronized to the grid, the speed of the turbine will be locked to that of the grid at very close to 3600 RPM.

Won't anyone on site answer your questions?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ScottyUK]

A common technique is to use an inductive pickup on the barring gear bullwheel, or a similar pickup on a toothed wheel provided for the purpose at the pedestal end. The device receiving the signal is programmed or calibrated to give an output proportional to speed, with enough headroom to allow an overspeed event to be recorded accurately. A modern DEH controller will take the raw signal directly into the controller and use it as the feedback of the speed control loop when the machine is not synchronised onto the grid. Once synchronised the signal can be utilised as a high accuracy frequency meter.

 

[davidbeach]

The first question would be whether 3600 rpm or 60 Hz is the more important value. Other than the mechanical overspeed trip, 60 Hz is probably the more important of the two.

 

[ScottyUK]

I'm waiting with bated breath to find out how you might achieve one condition without the other. ;-)

Joking aside though, turbine controllers are generally programmed to regulate shaft speed rather than frequency, so from the generating site's perspective speed is probably of greater interest. After a while you get quite adept at mentally converting speed into frequency. That's when you know you have been doing it too long.

 

[SlideRuleEra]

I'm waiting with bated breath to find out how you might achieve one condition without the other.

About 20 years ago, we did... sort of, for a few seconds. Operator error caused a 280 megawatt steam turbine driven generator on turning gear to be motorized by direct connection to the 60 Hz grid. Needless to say, there was sudden acceleration that was soon halted by a self-destructing combination of overheating, lubricating oil fire, cooling hydrogen explosion, and mechanical vibration breaking the machine's anchor bolts out of the turbine / generator pedestal. Never made it to 3600 rpm... but the machine tried its best. Repairs / rebuild, that I was involved with, took 18 months.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[wayne440]

I'll bet that resulted in interesting sounds- from both the machine and the operator.

 

[davidbeach]

If you're working with the electrical system, it would seem that 60 Hz is the more important of the two (or whatever other frequency the system happens to have at the instant). If you're strictly looking at the mechanical system then it seems that 3600 rpm is the more relevant value. Yeah, there's an easy mental conversion between the two, but each end has a different preferred set of units.

 

[ScottyUK]

SlideRuleEra,

That sounds like a very messy incident.

 

[SlideRuleEra]

ScottyUK - It was very bad, I was not present when the incident happened (late night on New Years Eve). Damage to the turbine building floor surrounding the pedestal indicated that the massive pedestal was rocking back and forth a few inches.

The stator had to be shipped, by special rail car, back to GE for a complete rebuild. The steam turbine was rebuilt on site. I handled the on site lifting / transport of the stator and the pedestal rebuild - anchor bolts 4" diameter x 6 feet long had broken out of the concrete. The entire effort was a corporate high priority project, we were able to get good techical and contractor support.

If there was an upside to the incident, the then 20 year old steam turbine was upgraded with more efficient modern turbine blades to get several more megawatts out of the unit.

wayne440 - The operator was not publicly "blamed", but I don't know what happened behind closed doors - glad I was not there.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[crshears]

Before our company was split up, the opposite happened to one of 'our' generating units, namely the turning gear was inadvertently engaged onto a steam turbine @ full operating speed, something that should have been impossible but proved not to be.

Little damage to the unit itself, but the entire turning gear casting was ripped clean off of its mounting studs and tossed out into the adjacent shipping channel 1000 or so feet away, exiting the plant through a hole that it tore in the roof in its haste to depart and arcing cleanly through the air well above the intervening 115 kV switchyard. Thankfully no vessel was present at the time...

Turning gear was fished out of the shipping channel, rehabilitated and re-installed. Never heard the discipline story on that one, either...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ScottyUK]

Makes us running down a 365MVA set on liquified white metal after the emergency L/O pump tripped to protect itself seem quite tame. I didn't see it first hand but I imagine that sparks pouring out of the bearings of a hydrogen-cooled generator is worrying.

Looking back there were a few days where we had a guardian angel watching over us - the manual synchronisation about 60° out of phase, the inadvertent admission of steam to a stationary machine with everything wide open... it's easy to forget just how much power is unleashed.

 

[spciesla]

Since you describe the plant as a large fossil fuel plant, I assume that each of the turbine generator units have a permanent vibration monitoring system. These systems should have a once per turn tachometer (Keyphasor) to provide for making vibration phase measurements. There is likely a speed measurement/display associated with this as well.

I also concur with the other posters about the sound / feel of the units when they are running at full speed. You may not be able to explain what it sounds like, but you know when it sounds different.

 

[SlideRuleEra]

Spciesla's post reminded me of the "Syncroscope", used to compare the frequency and phase of the spinning generator to the frequency and phase of the grid. When the needle is stationary at the position shown on the photo the two match and the generator can be connected to the grid.

I suppose there is automated equipment to do this now.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[waross]

That looks a lot like the synchroscope that I found with a wiring error of 30 electrical degrees. The tiny plant had 2 600 KW and 3 350 KW diesels. The operators got a lot of breaker trips trying to synchronize and the occasional sheared key in a coupling. They had been running that way for several years before I discovered the problem.
When the incoming unit was connected correctly the needle would jump to the 1 o-clock position.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

There's automatic gear to do it, and the lack of experience of manual sync activities and over-reliance on automation was the cause of the crash sync I mentioned above. The auto-synchroniser had failed and we were doing a manual sync, a very unusual activity at our station. I'd asked for the close command as the needle was at the 11 o'clock position and rotating clockwise but the guy doing it was a bit hesitant and closed it at about 2 o'clock.

 

[bohdand]

ccjc81 - If you can you be more specific in your question we can provide a targeted answer. Are you looking for offline speed, online speed, synchronizing, electronic governor, mechanical governor, etc. In general there are three ways that I am familar with on large machines to "measure" speed for governing purposes (I'm sure there are many more):

1) A shaft mounted permanent magnet generator that drives flyballs which control speed/load via hydraulics
2) Toothed gear on shaft using a proximity probe to count the teeth as they pass by used by a digital governor
3) looking at the frequency of the generator voltage signal - used for synchronization and digital governor

 

[crshears]

Hello again, ccjc81, wherever you are...

What's up with the post-and-ghost?

Numerous individuals have supplied what apppear to be legitimate, well-considered, and relevant responses to your query, yet we have never heard from you again...

Have any of the responses been of value to you? If so, please let the readership know; it's the considerate thing to do.

I'm not trying to scare you off, in fact far from it; I for one continue to learn from participating in these fora, and I'd encourage you to keep up the contact. And remember, as others have stated: providing more information leads to better answers.

I hope we hear from you!



CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[HamburgerHelper]

I can't find the calc but for my dynamics and stability class at University of Idaho, the Mohave generator had the equivalent in stored mechanical energy as something like 140 cars traveling at 60 mph. There is a pick I think someplace that I wish I could find that shows a large metal part that was thrown out of the building and landed on the otherside of a nearby pond when it was damaged during sub synchronous resonance.