Why turbine has no. of stages

1. Apr 26, 2007

Hi,
i m elect. engg and i wanna 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?

2. Apr 26, 2007

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.

Last edited: Apr 26, 2007
3. Apr 26, 2007

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 isnt very good. =)

4. Apr 26, 2007

clmohn

opps

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 im not for sure.

Chris

5. Apr 27, 2007

FredGarvin

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.

6. Apr 27, 2007