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Saturday, February 21, 2015

Steam turbine designs

I've posted a few times before about steam turbines, here, here, and here.  One of the things I find fascinating about steam turbines is how vast the variety is among them, depending upon the application.
  • Steam turbines that are used for marine propulsion will have stages to turn them in the reverse direction, and a separate astern steam throttle valve.
  • Steam turbines that are used in power plants for generation of electrical power come in a stunning variety of shapes, sizes and designs.
  • Steam turbines that are used provide cogeneration for other processes are often very compex and unique.
  • Turbines can be designed for various operating pressures and outputs, making each design slightly different and interesting in its own way.
Below: An early steam turbine diagram from 1905 for shipboard use.  The astern steam throttle is at top left. These two stages turn the shaft in the opposite direction than the seven forward stages.  Only two stages are used for astern movement, because it is not used often and so efficiency is not necessary.
I've previously only discussed the different blade designs on steam turbines, which is how energy is extracted from expanding steam in the turbine.  To recap, there are two types:  Impulse and Reaction.

In the Impulse turbine design, steam is directed at a set of blades on a shaft and bounced off of them.  This "impulse" or ricochet, causes the blades to move.  Below is a simple diagram showing the difference between the two basic designs.

Reaction turbines achieve movement by squirting steam through nozzles,  The steam squirts out, and the reaction force causes the blades to spin in the opposite direction from the steam jet.

There are other ways to configure (and classify) a steam turbine though, besides the blade design.  Let's have a look.

Steam turbines are also classified by steam inlet and/or exhaust conditions.  In a previous post on the steam cycle, all of the steam produced by the boiler passed through the steam turbine and was condensed back to water in the main condenser.  This is called a "condensing" turbine, because all of the steam enters the turbine, passes through, and is condensed.  Not all steam turbine installations work this way.

Non-Condensing Turbines
Another type of steam turbine is called a non-condensing turbine, which is sometimes referred to as a back-pressure turbine.  These are most often used in an industrial setting that requires a steady supply of low pressure steam, which is called "process steam".

A typical installation for a non-condensing turbine might be in a refinery, desalination plant, a lumber mill or a paper mill.  Let's use the example of a lumber mill.  Bits of trees that are not useful for making boards can be burned in a furnace, and produce steam at 500-1000 psig.  This steam is used in a steam turbine-generator to make electricity for the lumber mill's large electric motors.

However the lumber mill also needs to dry the boards that have been cut from the trees.  To dry out this wood, it is placed in a kiln to drive the moisture from the green cut lumber.  Low pressure steam is ideal for heating a kiln, and so lumber mills use the exhaust from the steam turbine for this.  The steam is shipped at a few psi of pressure to large radiators inside the lumber kilns, rather than to a condenser.

Below is a diagram of a non-condensing steam turbine.

The turbine exhaust steam condenses in the kiln's radiators instead of a condenser.  Air fans outside the kiln bring in a fresh supply of dry air, and this air is heated by passing it through the steam-filled radiators.  This hot, dry air removes moisture from the green lumber, is vented from stacks as hot, humid air.  It also smells pretty nice, if they are drying out pine, fir or cedar ;)

Below: A wood kiln leaking humid air.

Re-heat turbines
Another type of steam turbine is the reheat steam turbine.  This is a design where steam passes through part of the steam turbine, then returned to the boiler and re-heated.  After reheating, the steam is sent back to the steam turbine for additional expansion.  This adds a level of complexity, but it increases the efficiency of a fossil-fueled plant.  To my knowledge, reheating is not used in geothermal or solar applications.  It has been tried in in nuclear applications, but it's tricky and somewhat unsafe.  In the nuclear application it did boost efficiency by 5%.   

Below, the diagram of the entire steam/water cycle of a reheat turbine.  HP or High Pressure Steam (3) exits the boiler, and passes through the HP turbine.  The HP turbine has extracted a lot of work out of the steam and reduced its pressure and temperature.  The steam exiting the HP turbine (4) is called "Cold Reheat Steam".  After passing through the boiler's reheat section, the steam is called "Hot Reheat Steam" (5).  This reheated steam is sent to the LP or Low Pressure turbine where additional work is extracted.  The spent steam then is exhausted into the condenser (6).  The process repeats continuously as the condensed steam is pumped back to the boiler for conversion to HP steam. 

Extraction Turbines
Extraction turbines are quite common in large power plants, but in my entire career I have only worked with one!   All the rest have been condensing or reheat turbines.  Extraction turbines come in two varieties, that are sometimes combined:  Uncontrolled extraction and Controlled extraction.  Those terms mean about what you would think.  Part of the steam is let out of the turbine after a certain pressure reduction, and you either control the amount of steam let out, or you don't.

Uncontrolled Extraction Turbines
Below is a diagram of an uncontrolled extraction turbine.  Backpressure on the process steam header dictates how much steam exits the turbine and how much flows all the way through it.  If process steam demand drops off, the steam will simply flow through the remainder of the turbine instead.   
Uncontrolled extraction turbines are most often found in coal-fired power plants.  There might be 3-7 extraction points used to preheat condensate, supply steam to the de-aerator, or to preheat feedwater before it enters the boiler.  This is done to increase thermal efficiency.

Below:  A feedwater heater.

Below is a small steam turbine with extraction points at the 2nd, 5th, 8th and 10th stages, to provide steam at various pressures and temperatures.  The extraction points will typically be at the bottom of the turbine, and have non-return valves that go shut if the turbine trips.

Controlled Extraction Turbines
Controlled extraction turbines have two sets of control valves, and with this design it is possible to force steam to leave the turbine from an extraction point.  

On the diagram below, the left steam inlet valve controls the flow of high pressure steam into the turbine.  The first extraction point (at the bottom) after 3 stages, is a controlled extraction.  If the second steam control valve in the middle of the turbine is shut, all steam exiting the 3rd stage of the turbine will be forced to exit the turbine.  If the second steam control valve is throttled, only the desired portion of the steam is allowed to flow through the second half of the turbine.  (Yellow parts are within the lube oil system)

Controlled extraction is useful when a specific amount of process steam is required, or when a lot of process steam is needed in a very short period of time.  From experience, controlled extraction turbines can be quite a handful to operate in conjunction with their associated boilers and other equipment during upset conditions :)

Below: A double controlled extraction turbine.  Do NOT want to operate!  A single controlled extraction machine was stressful enough.

Induction Turbines
Induction turbines are similar to extraction turbines, but steam flows into the turbine from a second, lower pressure source, rather than flowing out of the turbine.  See below:
I am not aware of any induction turbine installations.  I would think that an induction turbine would require very strict steam conditions for the second inlet, because if the steam temperature was much different than the steam it was merging with, that would cause a great deal of thermal stress inside the turbine.

OK, we have discussed blade configurations and steam conditions.  Let's finish classifying steam turbines by flow and physical arrangement, and then look at a few cool pictures of installations.

Here's the simplest arrangement:  The single casing, condensing steam turbine.  100% of the steam runs through, spins the turbine, and gets condensed.  

Below is a high pressure (HP) turbine and an intermediate pressure (IP) turbine inside a single casing. It is also a reheat turbine, because the IP has been reheated in the boiler.

 The HP turbine is at the top and HP steam flows from the center to the top of the picture.  The IP steam turbine is 5 stages at the bottom.  Steam also enters from the center and flows to the bottom of the photo.  This is called "Opposed flow", and reduces the amount of thrust the rotor would experience if the steam all flowed in the same direction.

Now let's look at a tandem compounded machine, where two casings sit on the same shaft.  On the machine below, there are two turbines in separate casings, each driving the same shaft.  This is a tandem compound arrangement.

Lastly there is the double-flow turbine, which is typically used with low-pressure (LP) steam. Similar to the Opposed Flow arrangement, the desire is to minimize thrust.  Low pressure steam is sent to the center of the turbine and expands outward to either end.  The difference is that in the Opposed flow arrangement, the steam supplies are at different pressures, while the double flow steam is all the same pressure.

Below, a low pressure double flow steam turbine rotor.  Steam enters at center and expands outward to either end, and then enters the condenser.

Below is a pretty common arrangement for mid-size power plants.  Notice how it uses both Opposed flow on the HP/IP turbine (left), and Double flow on the LP turbine (right) to minimize thrust.  The thrust bearing (far left) is pretty tiny for such a large machine.

The last configuration is cross-compounding.  In this arrangement, spent steam from a HP/IP turbine enters a turbine on a separate shaft that drives a completely different generator.  Talk about complex!

My head hurts now...  Time to look at cool pictures of turbine installations.

I want one of these...

Below, a single-stage turbine that is ofthen used to operate pumps in a facility that has a lot of process steam available (refineries, paper mills, etc).

Below, a large tandem compound, three casing machine.  Single flow HP, double flow IP, and three double flow LP turbines.

Below, the same arrangement, however this one is assembled.

Nuclear plants use saturated steam, which is at a low enough temperature that moisture has to be removed from it between different steam turbines.

This is how you control the steam supply into a medium size steam turbine.  The seven valves hanging down underneath are lifted up against spring pressure by cams rolling under each arm.  The cams open each valve sequentially for greater control of flow.

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