Big Boiler Feed Pump

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fredrosse
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Big Boiler Feed Pump

Post by fredrosse » Sat Mar 14, 2015 2:39 pm

Just ran across this interesting Pump Nameplate, one of two pumps needed for the plant to get up to full power on a 1,000,000 + horsepower steam turbine electric generating plant. The plant burns about 7700 Metric tons of coal per day. Each pump is driven by its own 20,000 horsepower steam turbine.
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DetroiTug
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Re: Big Boiler Feed Pump

Post by DetroiTug » Sat Mar 14, 2015 3:26 pm

Wow, how big is the economizer? :lol:

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Re: Big Boiler Feed Pump

Post by cyberbadger » Sun Mar 15, 2015 2:16 am

Fred,

You just 1,000,000 HP +.

Let's assume it's just 1,000,000 HP.

And 20,000 For 2 Feed water turbine pumps.

Assuming also that the Feed water pumps run all the time...

Very rough calculation: 40,000/1,040,000 = .0384615384

So ~4% of the total HP for feedwater.

I'm just wondering how that compares roughly to something as small as a steam launch?
(How things scale)
Is it roughly 4% of the total HP for feedwater?

Obviously this a huge gross simplification, but I'm just curious how things scale up....

-CB
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fredrosse
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Re: Big Boiler Feed Pump

Post by fredrosse » Sun Mar 15, 2015 3:17 am

A very reasonable question indeed. For a small steam plant with around 100 to 150 psi steam pressure, the theoretical feed pump work is closer to 1% of the main engine power. However the very small pumps tend to be significantly lower efficiency than those found on a big power plant, and the steam consumption of small engines, per horsepower output, is higher than for the big turbines. So the real number for our small plants is probably in the vicinity of 2 to 3% of the main engine power output is needed to drive the feed pump.

In most power cycles, the pumping power is called "back work", and it is a very small number for steam plants, because the liquid water is so much more dense than the steam. The really high pressure steam plants, running close to 4,000 psi steam pressure, have a larger fraction of "back work", because the pumped feedwater is at a high temperature (around 400f), and is not as dense as the water being pumped in a small lower pressure steam plant.

Gas turbines have a really large amount of "back work", the compressor section, which handles intake air, will typically consume about 50% of the main turbine output. Turbine output is, say 100, compressor work is 50, so 50 is remaining to drive the propeller or generator or whatever. Before turbines and compressors were built with good efficiency, the turbine would only put out, say 70, and the compressor, also being of low efficiency would consume 65, so only 70 - 65 = 5 units of power were available, a worthless machine. That is why it took technology until 1912 to make the first sucessful gas turbine run.

For a crude steam plant, the efficiency issues of "back work" are much less of a problem. Using the same example, a high efficiency steam engine gives 100 units of output power, the feed pump work is 1, so 99 units of output power. With the poor efficiency, the engine gives 70 units of power, and the feed pump requires 2, so the net output is 70 - 2 = 68. Enough power to still provide meaningful output. The advantages of gas turbines were well known from the early 19th century, but the problem of high "back work" held up their use for almost 100 years.
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Re: Big Boiler Feed Pump

Post by Lopez Mike » Sun Mar 15, 2015 4:27 pm

Of course now, the commonest turbines are on airplanes where a very significant part of the compressor output bypasses the power turbine and is used for direct thrust.

Hmm. I'm helping drift this thread way far from steam. Sorry.
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Re: Big Boiler Feed Pump

Post by cyberbadger » Mon Mar 16, 2015 1:17 am

Lopez Mike wrote:Hmm. I'm helping drift this thread way far from steam. Sorry.
It's ok, it's Anoraks Corner...
Lopez Mike wrote:Of course now, the commonest turbines are on airplanes where a very significant part of the compressor output bypasses the power turbine and is used for direct thrust.
Depending on your definition of "commonest", I'd disagree.

In terms of sheer numbers, sure.

In terms of importance to civilization right now.... If all the operational steam turbines in coal/nuclear/etc power plants disappeared right now vs all the turbines in airplanes....

I think the steam turbines disappearing would cause many deaths, panic, looting. Much of Humanity has grown too dependent on electricity. The planes - not nearly so.

But 99% + of the general public doesn't think about the steam turbines, they are out of sight - out of mind.

-CB
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