Why Do Turbines Have Multiple Stages?

[anveshjadav]

Hi,
i m elect. engg and i want to help frm mech ppl here,
Why the turbine has stages like hp, ip and lp? Why not a single long turbine? I also want to know that if we want to trip a large turbine used at power plant, how much time it takes to stop the steam supply. I mean can steam valve be closed immediately or it takes some time?
Thanks in advance.

 

[turbo]

Turbines have stages because each stage only extracts a bit of the work from the incoming steam. The stages are designed to work with one another so that each stage can extract a bit of work from the steam exhausted from the stage upstream of it. In order to extend the range of pressure-drops that work can be extracted from, the exhaust steam is fed to a condenser where it rapidly loses volume as it is cooled, pulling a vacuum on the final stages of the turbine.

Each stage consists of a set of stationary vanes and a rotating set of buckets on a wheel attached to the turbine rotor. Each stage is designed so that the incoming steam experiences a pressure drop while transiting from from the inlet side to the outlet side. The stationary vanes direct the steam at the rotating buckets and the force of the steam impinging on the buckets causes the wheel/rotor to rotate. The stages get larger and larger in diameter as you go from the first stage to the last, because the steam expands a little at each stage as it undergoes the pressure-drops.

 

[clmohn]

stages

Also, the number of stages is correlated to fit the application. For example, if you have a very large steam pressure upstream that has massive amounts of energy you want to remove every last drop of energy from the steam and convert it to mechanical work (shaft). You do this through many stages (depending on upstream conditions) as mentioned before, each stage has a certain pressure drop. the pressure drop represents thermo energy converted to mechanical energy (the shaft the stages are attached to). Ideally you want the last stage in the configuration to have an exit pressure ambient (1 atm), at zero velocity. This means that you have successfully extracted all the energy within the steam and converted it to shaft work which I assume you are talking about power generation.
Hope this helps a little.

chris

ps. use a different english converter, yours isn't very good. =)

 

[clmohn]

opps

I forgot about your other question about the steam valve.

I am not sure about that one, but intuitively I would think you can not just shut off a steam supply line without the cooperation of other systems, or the steam emergency blow out valves will be activated... steam pressure regulators as well as the steam source need to be compensated if the feed line to turbine is shut down. But I am not for sure.

Chris

 

[FredGarvin]

Just to add to what has already been mentioned.

Why not one long turbine? Mostly because in an axial turbine, the length is very much dictated by rotor dynamics. The longer you make the turbine stage, the more risk you stand of running through various modes in the shaft and rotor.

Also, turbine blades have stress limitations on them just like anything else. One has to balance the rotating and pressure stresses versus things like flow angles and the work extraction. You simply can't do it in on shot.

Also, in terms of power generation, the use of different turbine rotors allows for the use of things like intercoolers on compressors and reheaters for turbines.

 

[anveshjadav]

Thanks to all

 

[cliff]

If your still interested, I'll add a comment. In looking at a large electrical generating steam turbine rotor, figure about 450 hp output per blade. Usually the steam is reheated, (sent back to the boiler) between the HP and IP turbine stages to replace the heat removed in the steam in generating the horsepower in the HP. (The pressure is not increased, only the lost heat is replaced. This is done to insure that the steam is, and remains dry, with no water droplets able to form as it expands through the turbine. Moisture in the steam errodes the blading as quickly as sandblasting.)
To answer your question about stopping the steam quickly, yes it is done. A large steam turbine driving a 2 pole generator at 60 cycle electrical output operates at 3600 rpm. If the generator is tripped, (disconnected) and the steam is not instantly removed, the turbine will over speed and destory itself in "10 cycles of electricity", or 1/6th of a second. In the case of a 800 megwatt machine, this destuction is equivilant to a 747 flying crashing into something at 500 mph. Consequently as soon as any overspeed beyond 3600 rpm (3610 rpm) is detected by the equipment, various steam valves start reacting immediately. This action increases at a faster pace as the overspeed condition increases. There are several valves, one at the boiler to shut off the steam going to the turbine. Another at the boiler to shutoff the steam that has been reheated from the HP and is being returned to the IP turbine. And another set at the turbine steam inlet. These systems are designed to be extremely fail safe, and are.
When a large steam generator trips, the impact to the steam system is very dramatic, as the steam traveling from the boiler to the turbine has considerable mass, in the range of 3300 psi, at 1005*F, and a flow of about 5,000,000 lbs. per hour. All of which is traveling at several hundred mph. The piping from the boiler to the turbine is suspended about every 20 feet or so by large springs, (larger than on a big truck) to support the piping and absorb the shocks. Even so, 40 tons of piping may instantly move from one to two feet in the bat of an eye lash.