Why we need intermediate turbine and Low pressure turbine???

[baasi]

As we supply steam to High-pressure turbine so we can achieve design RPM than why we need intermediate and low-pressure turbine???

 

[MikeHalloran]

The exhaust steam from the HP turbine drives the IP turbine, and the exhaust steam from the IP turbine drives the LP turbine. Each expansion stage recovers energy from the steam, resulting in better efficiency for the plant.


Mike Halloran
Pembroke Pines, FL, USA

 

[byrdj]

I like these types of questions as they cause one to review the history of design to really explain to someone new to the industry.

one needs to explain expansion ratios and effeciency.
then explain the expansion areas and volumes.
and then the practicallity in manufacturing a turbine.

It is therotical possible design a single rotor that could have the many stages, but the praticality in designing to withinstand the high pressure at the front end and the vacuum at the back would be interesting.

by having the design requirements for the three levels of pressure use for three seperate turbines makes the manufacturing possible

Not the best answer since I don't have a feel for what level I am explaining to

 

[MikeHalloran]

See also Steam Yacht "Turbinia".

Mike Halloran
Pembroke Pines, FL, USA

 

[georgeverghese]

If you look at things from an entropy basis, it is thermodymanically more efficient to extract thermal and / or a mechanical energy stream at as high a temperature as possible. Hence the smaller the dp drop across a turbine, the more the total work extracted will be for the total dp for all intermediate turbines.

Similarly, "cold" energy recovery is better done in small temp intervals for max refrigeration cycle efficency.

An interesting example of such an exergy analysis is given in Perry 6th edition.

Obviously there are practical and cost limitations to how small each dp or dT step can be.

 

[crshears]

I'm glad all the responders understood the question posed in the OP; it certainly wasn't very clear to me...

The OP doesn't state whether the HP turbine is provided for topping purposes ahead of a process or for building heating in a cold climate or...

If either of the foregoing apply there may not in fact be a need for an IP or LP turbine at all.

More information = better answers...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[baasi]

Thanks brothers for reply
georgeverghese as you said that by reducing dp more energy can be extracted but i think its not true concept as you know that more dp means more expansion and more expansion means maximum extraction of energy from steam.

 

[baasi]

Still i am confused about it , I had not cleared answer yet.
byrdj (Mechanical) your approach is somehow understandable to me but plz explain more clearly that what exactly will happen if we design turbine such that all expansion will occur in one cylinder and at the end we will get pressure equal to cylinder pressure.??

 

[byrdj]

here is a single cylinder turbine. However the most it could generate is 5 MWs and it is not very effecient with HP steam
HP stage 1
IP stages 2-5
LP stages 6 & 7

Does your turbine have one or two LPs. to pass the steam flow needed for your turbine in a single cylinder, the area of the exhuast will need to be the same as your current combined. thus if you have a single double flow LP, it will be twice as big and if 2 LPs, it will be 4 times as big. the first problem to make such large LP stages would be the centrifical force would tear them from the rotor.

Look at the thickness of the HP casing and figure out how to tranistion the casing thickness to that of the LP hood in one single casing.

So make big MWs you need BIG turbine, a BIG turbine would be impossible to make so multiple turbines are used. Being able to compound their torque on a single shaft tends to comply with the cascading pressure design instead of putting 100 turbines like above on a common gear drive

 

[racookpe1978]

On a 800 - 900 Megawatt power plant turbine, the MASS of the three rotors represents the single largest load that the plant is sized to lift. You cannot reasonably and economically lift a 150 ton load with the installed crane - else you're paying millions for a too-big crane and rails and girders and structural steel for the plant, but can work with three 50 ton loads and three 50 ton rated bearing sets and 50 ton rated turbine casings, all getting lifted on smaller crane rails and building structural.

Who could make a 200 foot long lathe to turn a 200 foot long rotor accurately enough to ship a 200 foot long rotor on a ??? to carry a 200 foot long rotor cross-country?

The real world requires the "ideal" thermodynamic design of perfect metals and infinite budgets to be adjusted.

 

[ScottyUK]

racookpe1978,

Are cranes in US plants not sized for the generator rotors and potentially the generator stator too? Most of the older UK plants with an indoor turbine hall were designed with cranes capable of lifting any of the machine components.

 

[baasi]

Thanks everyone
It was nice discussion

 

[racookpe1978]

racookpe1978,

Are cranes in US plants not sized for the generator rotors and potentially the generator stator too? Most of the older UK plants with an indoor turbine hall were designed with cranes capable of lifting any of the machine components.

Yes, yes they are rated. But they are rated for the HP rotor, gen rotor, IP rotor, LP rotor(s), etc. Building everything in one rotor, or trying to put all of the multiple rotors and their flanges and their bearings into one very long turbine casing is not reasonably nor economically practical. Making additonal split lines (vertical bolted joints) to break up the turbine casings is very, very hard - and does no real good.

Split them up by pressure (HP, IP, LP, LP) and then locate the lube oil bearings in the open air between each of pressurized steam casings.