Steam Boiler: Steam Turbine Generator

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Working Principle of Steam Turbine

The working principle of steam turbine is very important to be known in the power plant system. Steam turbine is the engine, where the energy of working fluid is used directly to rotate the turbine blades. In the turbine, the working fluid undergoing a process of expansion, namely the pressure drop and flow continuously. The working fluid of steam turbine is steam. Steam turbine classification can be categorized based on steam flow direction, working principle, exit steam and steam pressure.

In general, the steam turbine system consists of several components, such as: compressors, pumps, boilers, combustion chamber, condenser and turbine. Turbine is much in use for power generation, aircraft, in the industry, and others.

Steam turbine is one of machine types that use a method of external combustion engine. The heating of working fluid (steam) is done outside the system. In brief the working principle of steam turbine as follows:

Steam enters into the turbine through a nozzle. In the nozzle, heat energy from steam is converted into kinetic energy and the steam is expanding. Steam pressure at the exit of nozzle is smaller when compared with at the time of enter into nozzle, but otherwise the velocity of steam out from nozzle is greater than at the time of enter into the nozzle.
The steam gushing out of the nozzle is directed to the turbine blades with arches shaped and fitted around the wheel turbines. Steam flowing through gaps between the turbine blades is deflected towards following the curve of the turbine blades. The changes in steam velocity raise the force that encourages and then rotate the turbine wheel and shaft.
If the steam still has velocity when it leaves the turbine blades means that only some of the kinetic energy of steam is taken by the turbine blades which are running. More than one line of blade motion is installed to utilize the remaining kinetic energy when steam leaves the turbine blades.
Before entering the second line of blade motion, so between the first row and second row blades motion is mounted one line fixed blade (blade guide) that allows you to change the direction of the steam velocity, so steam can enter the second line of blade motion in the right direction.
The velocity of steam when it leaves the last blade motion should be made as small as possible, so that the available kinetic energy can be utilized as much as possible. Thus the steam turbine efficiency is higher because of energy loss is relatively small.

Therefore the working principle of steam turbine should be well designed to perform highest efficiency and produce small turbine heat rate.

Turbine Heat Rate

Turbine heat rate can be divided as two types:

Gross Turbine Heat Rate

Gross turbine heat rate can be obtained by dividing the heat energy or enthalpy added in steam boiler among the outlet of steam and the inlet of feedwater by the output of generator in kW (kilowatt). The gross turbine heat rate can be stated in kJ/kWh or Btu/kWh. Reheater can be utilized to add heat energy in the steam turbine and generate reheat cycle. The table 1 below is example of gross turbine heat rate values.

Net Turbine Heat Rate

The net turbine heat rate can be established similar to gross turbine heat rate, with the exception of the input power of boiler feed pump is taken away from the output power of generator previous to dividing towards the enthalpy (heat energy) which is added in steam boiler.

Turbine heat rate with regard to regenerative turbine is understood to be the heat energy usage of the turbine with regards to "enthalpy in steam" provided by the steam boiler, subtract the "enthalpy in the feedwater" when warmed up by the extraction in turbine, divided by the power output on the generator. This description involves electrical and mechanical failure / losses of turbine and generator process, however excludes boiler ineffectiveness and pumping deficits. The turbine heat rate is helpful for doing economical evaluations of different turbine designs and also engineering.

Table 1: Gross Turbine Heat Rate

Steam Quality Parameters

The following below are parameters in determining the quality of steam produced by boiler:

- The amount of steam which is generated by boilers in accordance with required amount to drive turbine generator. The amount of steam produced is usually in the unit form of Kg/hour or tons/hour. Therefore the heat transfer should be maintained in order to generate expected amount of steam.

- Steam produced must have required temperature and pressure so as to drive turbine generators. Setting temperature of steam can be done by desuperheater while setting pressure of steam can be performed by relieve valve or safety valve. If the steam pressure and temperature are met then it means that one of the parameters in the determination of steam quality is met.

- The steam which is produced should be free from air and condensed gases because air and moisture can inhibit heat transfer in steam boiler.

- Steam which is produced by boilers must be clean; there should be no crust (e.g., corrosion or sediment carbonate) or impurities that can increase the rate of erosion in the pipe, orifice and valve.

- Steam is produced must be dry or superheated. The presence of water droplets in the steam will reduce the actual enthalpy of evaporation, and will also lead to scaling on the pipe wall and damage to turbine blades.

Steam Turbine Zoelly/Rateu

Steam turbine Zoelly / Rateu is steam turbine which has working principle impulse action with some levels pressure. Steam pressure dropped gradually in the line of fixed blade only, while in the line of blade motion does not decrease pressure.

The power generated is high power at low speed. Furthermore steam turbine Zoelly / Rateu is suitable for use as a driver of big power generator. The advantage of steam turbine Zoelly / Rateau is high efficiency, while it’s disadvantageous are the construction is more complicated than steam turbine than steam turbine Curtis and steam turbine De Laval and high cost for construction.

Figure 1: Steam Turbine Zoelly / Rateau

Caption:

1 = space of fresh steam and exhaust steam

2 = nozzle

3 = blade motion

4 = nozzle

5 = blade motion

6 = space of fresh steam and exhaust steam

Steam Turbine De Laval

Steam turbine De-Laval is steam turbine that works with principle of impulse action with axial flow, one level pressure and velocity. Steam turbine has an array motion of blade so all of dropping steam will raise potential energy then be converted by blades propulsion. The rotation which is resulted by steam turbine is very high and maximum electric power that can be generated is 1500 kW, so this turbine is usually used for small scale generator.

The advantageous of this steam turbine are simple construction, low cost for manufacturing process and easy to install. The main disadvantageous of steam turbine De-Laval are small capacity, has low efficiency and the rotation is too high so need transmission gear to get needed rotation to drive electric generator.

Figure 1: Steam Turbine De Laval

Caption:

1 = Axis

2 = Disc

3 = Blade motion

4 = Nozzle

5 = Stator

6 = Exhaust pipe

Heat/Pressure Loss in Regulator Valve

The flow of steam through stop valve or regulator valve is accompanied by heat loss due throttling process. This heat loss is called as heat loss in regulator valve. The pressure inlet steam (P₀) to turbine generator is usually reduced so initial pressure inlet steam (P₀’) less than before. Initial pressure drop (Δ P₀) is estimated at 3% - 5 % of P₀. If the loss is assumed as 5% so the pressure inlet steam to turbine generator can be expressed as follow:

ΔP = P₀ - P₀’

ΔP = 5% P₀

The energy loss in regulator valve is determine by equation below

Δh = h₀ - h₀’

Where:

h₀ = reduction in total calorific value of turbine

Calorific value will decrease after experiencing the pressure drop due to setting through regulator valve or stop valve. The losses occurred in valve can be seen in the figure below.

Figure 1: Expansion Steam and Losses in Valve

Caption:

H_n = Losses in nozzle

H_b = loss on the blade motion

H_c = loss due to exit velocity

P₀ = pressure inlet steam to turbine

P₀’ = pressure steam before entering nozzle

P₂ = pressure outlet steam from turbine

H₀ = decrease in heat

H₀’ = theoretical decrease in heat

H_i = decrease in heat which is utilized in turbine

Heat Loss in Steam Turbine Generator

Heat loss is a process which causes useless heat in operation. Heat loss not only occurs in boiler (steam boiler heat loss) but also occur in steam turbine generator. The following below are types of heat loss in steam turbine generator.

Internal heat loss, include:

a. Heat/pressure loss in regulator valve

b. Heat loss in nozzle

c. Heat loss in rotation blades turbine

d. Heat loss because of output velocity

e. Heat loss in blade steering

f. Heat loss due to friction disc and ventilation

g. Heat loss due to free space

h. Heat loss due wetness steam

External heat loss

The external heat losses generally due to mechanical friction such as between shaft and bearing, regulatory mechanism, lubricating oil pump, and leak in the gasket.