Steam Turbine Classification

Steam turbines have numerous configurations and means of classification. A steam turbine is generally classified as being either high-pressure or low-pressure, condensing or non-condensing, single-stage or multi-stage, single-valve or multi-valve, extraction or non extraction, direct drive or gear drive, and for either electric generator, mechanical drive, or propulsion turbine.

Classification of steam turbines: - 

According to the type of steam flow:

Axial turbines: In these turbines, Steam enters radially to rotor shaft & flows in a direction parallel to the axis of the turbine rotor (SHAFT).

In today’s power industry majority of turbines are Axial flow.

Axial-flow, non-reheat combined-cycle steam turbine

Radial turbines: In these turbines,steam / Gas flow path is outwards from the shaft to casing (enters the turbine in the direction of its radius & leaves in the direction parallel of the shaft)

See sketch below of Radial Steam Turbine used in Ship Turbinia in 1894....

Below is the cut section of Three-stage axial-flow direct-acting Parsons steam turbine ...

According to way of energy conversion or type of Blading –

I detailed in my previous article about Impulse & Reaction turbine in 2 basic designs of team turbines.

There are different designs of blades in steam turbine which is, how energy is extracted by expanding steam in the turbine & they are Impulse and Reaction. See below, images will tell you how it looks like.

<---Impulse Turbine

Impulse -Reaction Turbine ---->

According to the number of shafts:

Single shaft turbines 

Tandem shaft Turbines   

Multi-shaft turbine – HP, IP, LP turbine rotors placed face to face

Cross Compound – Two parallel shafts

Will look on this in separate article... its more fascinating.

According to the heat drop process (exhausting condition):

Condensing turbines : These are the primary types of steam turbine & used for large drive applications above a certain power rating ( roughly > 8 MW). These turbines exhaust directly to condensers that maintains vacuum condition at discharge of steam turbine.

In these turbines, all of the steam produced by boiler passed through the steam turbine, pressure drop occurred, spine the rotor than the low pressure & temp steam is directed to the condenser.

Here is the simple arrangement of single casing condensing steam turbine. 100% of the steam pass through, spins the turbine, and gets condensed.

Extraction turbines: An extraction turbine has one or more openings in its casing for extraction of portion of steam at some intermediate pressure. The extracted steam steam may be used for process purpose.

In these turbines, the steam is withdraw (extracted) from intermediate stages at a certain pressure for plant processing like process heating purposes.

Simple arrangement of uncontrolled extraction turbine in beside image....

Extraction turbines come in two types, that are sometimes combined: Uncontrolled extraction and Controlled extraction. 

Above two terms mean, 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 : -

Beside simple sketch is of an uncontrolled extraction turbine. Back pressure 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.

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 : - 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.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.

Back pressure turbines:This type of steam turbine also called a non-condensing turbine, Back-pressure turbines can be used when a large quantity of process steam is required. The turbine exhaust steam is supplied to the process and the electric output is dependent on the demand for the process steam.

These turbines are most suitable for mechanical drive application like drivers of compressors or pumps.

The terms back pressure refers to steam turbine that exhaust steam above atmospheric pressure. The discharge pressure is usually established by specific application of steam in plant. Lower pressures are frequently used in small & large low pressure application of heating system & higher pressures are often used when supplying steam to industrial purposes.

A typical installation for a non-condensing turbine might be in a refinery, desalination plant

Animated sketch of Extraction Back Pressure Steam Turbine

Topping turbines: Also called as Reheat steam turbine. This is a technique 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.

Below is GE's D- Series Reheat Steam Turbine

According to the steam conditions at inlet to turbine:

Low pressure turbines: These turbines use steam at a pressure of 1.2 to 2 Kg /cm2.

Medium pressure turbines: These turbines use steam up to a pressure of 40 Kg /cm2.

High pressure turbines: These turbines use steam at a pressure above 40 Kg /cm2.

Very high pressure turbines: These turbines use steam at a pressure of 170 Kg /cm2 and higher and temperatures of 550°C and higher.

Super critical pressure turbines: These turbines use steam at a pressure of 225 Kg / cm2 and higher.

See my previous post on the basic design (working principle) of steam turbine.

Will write separate article on further classification of Turbine based on Staging (Group of Blades), Compounding, Group of Stages, Number of Shaft, Division of Steam Flow...