Gut Rumbles
 

August 16, 2003

p.f.m.

When I worked in the power plant, I enjoyed training new operators. I could blow their minds by demonstrating how a turbine generator worked.

The turbine is turning 10,800 RPM. Thanks to a reduction gearbox, the generator is turning 3,600 RPM. We are producing one megawatt of electricity and consuming 20,000 pounds of steam per hour.

Now, let open a couple of hand valves on the turbine and get some more steam in here. Whoh! We're consuming 35,000 pounds of steam per hour now, and producing 1.5 megawatts of electricity.

But the turbine still spins at 10,800 RPM and the generator still turns at 3,600 RPM. None of that has changed. Where did the extra power come from?

When they shook their heads in amazement, I explained it as "PFM."

Pure. Fucking. Magic.

I went into resistance later, but it surely was fun to play with their heads at the beginning.

Comments

Did you ever send one of your newbies off to find you the water hammer??

Posted by: G-MAN on August 16, 2003 10:30 AM

Yeah. P F M. I like that. In EE, I aced the theory part, but dam near flunked the lab. I kept making circuits that were identical to the diagrams. Prof would come by and throw all my wires on the floor. Far as I'm concerned, it IS P F M.

Posted by: Larry on August 16, 2003 10:46 AM

Crank up the excitation a bit, apply more torque at the shaft. Torque x speed = power.

On the generator end, torque = current. More steam in, you're either going to get more speed, or more torque. Since the generator is line-synched and voltage-controlled, you can't get more speed, so you must make more current.

Let's see...a 1.5MW steam turbine generator. Output was 13.8kV? Generator was hydrogen cooled?

Turbine houses are very cool.

Until something goes amiss.

Posted by: Kevin Baker on August 16, 2003 07:17 PM

The generator was water cooled from a tower recirc loop.

Posted by: Acidman on August 17, 2003 05:27 AM

I fly a C-130. The power source, a jet engine, runs at a constant 98-102% RPM. Just like you said, it took me a while to grasp how an engine running at a notional 100% constantly could actually produce more power.

Here's how it works, I push up the throttles, pouring more gas into the combustion chamber. The gas mixes with air, combusts and flows into a turbine. The turbine extracts this thermal energy and converts it into mechanical energy (makes a shaft turn). This shaft transmits the energy to a reduction gearbox which converts the high RPM / low torque energy into low RPM / high torque energy.

When I increased the amount of fuel, the turbine began to spin faster, threatening to exceed 100% engine RPM. The propeller control senses the increasing speed using a fly weight and spring. The fly weight shifts a valve to port hydraulic fluid to the front of the propeller, moving the propeller so the blade takes a larger "bite" of the air, thereby slowing the engine almost instantaneously back to 100%.

From what you said, it sounds like power generators work about the same.

Posted by: David on August 17, 2003 11:35 AM

Another way to think of it is this:
Generators and AC motors are the same thing. The grid links them all together, and they all spin at the exact same speed (not true, but useful for visualization).
The difference between generators and motors is that generators have an external torque driving them, while motors drive the external torque. From the perspective of the grid, motors are slightly behind phase, while generators are slightly ahead of phase.
When a steamer increases power, it is attempting to push the entire system faster. It's pushing against a very large system, though. Typically there are hundreds of other generators and millions of motors. It DOES move the system slightly, just not very much.
All this is before taking into consideration the regulation on the grid. The grid attempts to operate at 60.0000 Hz, or 3600 RPM. The regulation is done by comparing wall clocks against atomic clocks (WWV), and it's accurate within a few cycles of 5,184,000 per day or 1,892,160,000 cycles per non-leap year.

It's common to use turbines at hydro plants as 'spinning reserve'. They're left with just enough water on the turbines to balance internal friction. In this mode, the unit can be either a generator or motor, working against the momentum of the turbine.

Posted by: Dishman on August 17, 2003 01:28 PM

1.5 MW? I'm guessing that that sucker is about 6' in diameter and maybe 4' deep max. Do they hydrogen-cool generators that size?

Another favorite stunt - one site I worked at had a plant air cross-connection to the generator hydrogen to permit purging. There was another (possibly temporary) cross-connection between the plant air and a service water system....

Yep, they managed to put service water into the generator, which kinda busted the outage schedule. I'm thinking some personnel were, uh, corrected too.

Posted by: J Bowen on August 17, 2003 03:26 PM

PFM was the explanation I learned for how the lithium bromide plant on the submarine worked. I swear, it took me a week each time I wanted to truly understand how the damned thing operated. Much simpler to just say PFM.

Posted by: wheels on August 17, 2003 04:08 PM

If you have a grid that has a nominal load of 10,000 megwatts and all the plants online supplying that grid are capable of at full loads creating 11,500 megwatts then you have a spinning reserve of 1,500 megawatts as a precaution for any plants tripping offline.
You have to remember that all powerplant are not created equally.
Some plants are designed to run at nothing but full load all the time, these are called baseload unit and are generally the biggest around. Then you have what are called cycling or load following plants that are capable of ramping down and up as system demands dictate, these are generally your medium to small plants.

Posted by: G-MAN on August 17, 2003 06:49 PM

I ran one base-loaded turbine (back-pressure off the boilers) and two "swing" turbines that ramped up and down according to load. The base-loaded one ran full-tilt, 24-7. The others could idle if they needed to.

Posted by: Acidman on August 17, 2003 08:00 PM

You do a good work, keep it going

Posted by: Walker David on May 3, 2004 01:12 PM
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