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zbang said:
And to further muddy the waters and drift away from the OP's question- because a mach number relates to the speed of sound in a medium, it will vary. In dry air at 70 deg(f), it's about 1125 ft/sec but in water at 70 deg(f) it's about 4870 ft/sec. In super-heated steam (300 psi, 500deg(f) superheat), the speed of sound is about 1775 ft/sec :cool:.
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and to go completely gonzo, not all turbines spin at subsonic speeds. Some, the tips go supersonic by design.
 
Besoeker said:
If the thing is 6ft across (daimeter) then the radius isn't 3 feet times two pie. It's 3 feet. At 100 revolutions per second that would 1885 fps.
In any case, I don't know where you got the 100 revolutions per second. Synchronous speed for a two pole machine, the fewest number of poles you can have, is 3600 rpm on your 60 Hz supply. Which is 60 revolutions per second.

Also, note iwire's point about the speed of light.

You seem to have got into a total muddle with the numbers.
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I went to the fastest claim about 60 hertz. Being a large turbine. Spinning at 60 rps. I don't know how big a large turbine could be but if it could spin 60 rps and if it was 6ft across then you would take the radius to find the diameter which would be 3 x pie x 2 which would be almost 20ft diameter x 60 rps which would be 1200 fps. Isn't that approaching the speed of sound? What if the turbine were a little bigger?
Thank you everyone.
Russell


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Russell Ensslin said:
if it was 6ft across then you would take the radius to find the diameter which would be 3 x pie x 2 which would be almost 20ft diameter x 60 rps which would be 1200 fps.
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You keep using the word diameter where circumference is correct.

circumference = 2 * radius * pi
or = diameter * pi

At 3600 rpm a 6' diameter round object has an edge velocity of 1131 ft/sec. That is just over the speed of sound in dry air at 70 deg, but water content and temperature make a difference. Note that large turbogenerators are often cooled with hydrogen, not air. Also, to quote wikipeida "To control the centrifugal forces at high rotational speeds, the rotor is mounted horizontally and its diameter typically does not exceed 1.25 meter; the required large size of the coils is achieved by their length." This comes out to an edge velocity of about 773 ft/sec. (https://en.wikipedia.org/wiki/Hydrogen-cooled_turbo_generator)
 
Interesting https://en.m.wikipedia.org/wiki/Mean_piston_speed
my old RS4 had a mean piston speed 25 m/s or ~84 ft/s

It is a good indicator of the class and performance of an engine relative to its competitors. Honda S2000 had the highest piston speed for any production car (25.2 m/s) until the B7 Audi RS4 (2006-2008) debuted. This Audi was powered by a 4.2-liter naturally aspirated V8 (92.7mm stroke; 8250 RPM redline), resulting in a mean piston speed of 25.5 m/s.

The new Shelby is 25.6 m/s

that car rev'ed like a mofo
 
JFletcher said:
The slowest genset Ive personally seen is a 20cyl GE 710 that idled at 286RPM and pumped out about 2MW at 900RPM (4500 HP engine). Yeah, it's hard to think of that much copper and steel spinning around, but really, 900RPM is slow.
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while they weren't AC gensets, at texaco refinery, in wilmington,
there are two air compressors for plant air, piston type, that are
driven by large diesels running off fuel oil. just under 300 rpm.
it was a 12 cylinder diesel driving a 12 cylinder compressor.

the pistons are 12" bore by 24" stroke. the thing that's amazing
is that these things have been running since the 1920's, and never
shut off. they have a dry sump lube system that just switches between
55 gallon barrels of engine lube. the put an o-scope and transducer
on them to see if there is any vibration or wear. they shut them off
when there is a rebuild warranted. they have two of them, and switch
them off, month on, month off. i was told they'd each been rebuilt once in 50 years of service... but that was in the late 1970's. hell, they probably are still running.

the thing that was most impressive is the low frequency vibration that
permeated everything nearby, including the ground.
 
Fulthrotl said:
while they weren't AC gensets, at texaco refinery, in wilmington,
there are two air compressors for plant air, piston type, that are
driven by large diesels running off fuel oil. just under 300 rpm.
it was a 12 cylinder diesel driving a 12 cylinder compressor.

the pistons are 12" bore by 24" stroke. the thing that's amazing
is that these things have been running since the 1920's, and never
shut off. they have a dry sump lube system that just switches between
55 gallon barrels of engine lube. the put an o-scope and transducer
on them to see if there is any vibration or wear. they shut them off
when there is a rebuild warranted. they have two of them, and switch
them off, month on, month off. i was told they'd each been rebuilt once in 50 years of service... but that was in the late 1970's. hell, they probably are still running.

the thing that was most impressive is the low frequency vibration that
permeated everything nearby, including the ground.
Click to expand...
You said they have never been shut off then later said they cycle them one month on, one month off:huh:
 
Fulthrotl said:
while they weren't AC gensets, at texaco refinery, in wilmington,
there are two air compressors for plant air, piston type, that are
driven by large diesels running off fuel oil. just under 300 rpm.
it was a 12 cylinder diesel driving a 12 cylinder compressor.

the pistons are 12" bore by 24" stroke. the thing that's amazing
is that these things have been running since the 1920's, and never
shut off. they have a dry sump lube system that just switches between
55 gallon barrels of engine lube. the put an o-scope and transducer
on them to see if there is any vibration or wear. they shut them off
when there is a rebuild warranted. they have two of them, and switch
them off, month on, month off. i was told they'd each been rebuilt once in 50 years of service... but that was in the late 1970's. hell, they probably are still running.

the thing that was most impressive is the low frequency vibration that
permeated everything nearby, including the ground.
Click to expand...

Nifty. 5Hz is probably close to an earthquake's frequency. cue "good vibrations" heh.

The genset was a GM/EMD not GE. Between the quad turbo-compressors shrieking high frequency noise and those huge pistons (one cylinder on this thing is about the same displacement as 2 entire small block Chevy's) making low end rumble, it was the only place in the plant that required double hearing protection. Even taking readings on it every hour with proper protection, you'd have a headache by evening's end (typically ran 2-9pm). Two stroke engine.

Funny you mention the dry sump. You'd think oiling one of these was total loss or premix by how much oil it consumed; upward of a complete 55 gallon barrel/day. I guess that's not bad when you're burning almost 5k a day of #2 diesel.
 
zbang said:
Note that large turbogenerators are often cooled with hydrogen, not air.
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The GENERATOR is often (I'd go with always for utility operated steam turbine driven) hydrogen cooled. Its diameter, IME, is much less than that of the low pressure turbine. I'd not expect it to have peripheral speed that was supersonic, even in air.

It's been many years since I've seen the pressure distribution across a steam turbine, but I __BELIEVE__ the entire low pressure section operates in a vacuum at normal loads. A little Google research gives reputable sources confirming my 40 year old memory that tip speeds are definitely supersonic unless the vacuum has a major impact ... see http://www.hitachi.com/rev/pdf/2013/r2013_01_103.pdf which says 786 meters/sec.
 
GeorgeB said:
The GENERATOR is often (I'd go with always for utility operated steam turbine driven) hydrogen cooled. Its diameter, IME, is much less than that of the low pressure turbine. I'd not expect it to have peripheral speed that was supersonic, even in air.

It's been many years since I've seen the pressure distribution across a steam turbine, but I __BELIEVE__ the entire low pressure section operates in a vacuum at normal loads. A little Google research gives reputable sources confirming my 40 year old memory that tip speeds are definitely supersonic unless the vacuum has a major impact ... see http://www.hitachi.com/rev/pdf/2013/r2013_01_103.pdf which says 786 meters/sec.
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What is the speed of sound in a vacuum? :D
 
Steam-powered devices are all about pressure and temperature differentials. When operating either a steam turbine or engine you want a strong vacuum in the condenser. That pulls the spent exhaust steam out of the motor and indicates that there isn't any un-condensed gas in the condenser. There are a couple of interesting graphs about exhaust vacuum at http://www.nwfpa.org/nwfpa.info/com.../365-improve-steam-turbine-efficiency?start=3

(Not pulling out my marine turbine operations manuals at the moment; no time for that now.)
 
ggunn said:
What is the speed of sound in a vacuum? :D
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How much vacuum? The cooling water temperature determines it, and it isn't by any standard a hard vacuum ... at 80F, about .5 psia, saturated steam. What I was trying, without luck, to find is what the inlet pressure is to the low pressure turbine. I want to say that we, with river water in winter, got below 0.5" Hg ... but that's a long time ago, like 1973.

I'm sure all are aware that a "large" turbine has (most of the time), a HP section, maybe 2000-3600 psi and maybe 1000F inlet. When the steam exits that, it is reheated, similar temperature but I don't remember pressure, then it exits that section and may or may not be reheated again before going into the LP turbine. Sometime all turbines are on one shaft, sometimes the LP is separate; if separate, the ones I remember ran at 1800 (4 pole); HP/IP "always" were 3600 rpm.

The engineers at (then mostly GE and Westinghouse) did much calculation, extracting steam at various points to heat feedwater in the goal of maximizing efficiency ... which at best was in the 30% range with fossil fuel, lower for nuclear (lower temperatures). My point was that vacuum indeed plays a point, but the authoritative link confirmed supersonic.

I wish I had my power plant reference materials from then, but they were lost in a move.
 
JFletcher said:
I'll put it this way: every space movie fight scene (that happens in space) would be much more realistic if you muted the volume. Boring, but realistic.
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You mean lasers don't go "pedew, pedew, pedew"? :D
 
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