(a) How does erosion of turbine blades occur ? State the methods of preventing erosion of turbine blades. (b) What do you mean by TTD of a feed water heater ? Draw temperature-path-line diagram of a closed feed water heater used in regenerative feed heating cycle. (c) In a 15 MW steam power plant

Question

(a) How does erosion of turbine blades occur ? State the methods of preventing erosion of turbine blades.

(b) What do you mean by TTD of a feed water heater ? Draw temperature-path-line diagram of a closed feed water heater used in regenerative feed heating cycle.

(c) In a 15 MW steam power plant operating on ideal reheat cycle, steam enters the H.P. turbine at 150 bar and 600°C. The condenser is maintained at a pressure of 0.1 bar. If the moisture content at the exit of the L.P. turbine is 10.4%, determine :

(i) Reheat pressure ; (ii) Thermal efficiency ; (iii) Specific steam consumption ; and (iv) Rate of pump work in kW. Assume steam to be reheated to the initial temperature.

Accepted Answer

(a) The erosion of the moving blades is caused by the presence of water particles in (wet) steam in the L.P. stages. The water particles strike the leading surface of the blades. Such impact, if sufficiently heavy, produces severe local stresses in the blade material causing the surface metal to fail and flake off.

The erosion, if any, is more likely to occur in the region where the steam is wettest, i.e., in the last one or two stages of the turbine. Moreover, the water droplets are concentrated in the outer parts of the flow annuals where the velocity of impact is highest.

Erosion difficulties due to moisture in the steam may be avoided by reheating (see Fig. 12.31). The whole of steam is taken from the turbine at a suitable point 2, and a further supply of heat is given to it along 2-3 after which the steam is readmitted to the turbine and expanded along 3-4 to condenser pressure.

Erosion may also be reduced by using steam traps in between the stages to separate moisture from the steam.

(b) TTD means “Terminal temperature difference”. It is the difference between temperatures of bled steam/condensate and the feed water at the two ends of the feed water heater.

The required temperature-path-line diagram of a closed feed water heater is shown in Fig. 12.32.

(c) The cycle is shown on T-s and h-s diagrams in Figs. 12.33 and 12.34 respectively. The following values are read from the Mollier diagram :

[latex]h_1=3580[/latex] kJ/kg, [latex]h_2=3140[/latex] kJ/kg, [latex]h_3=3675[/latex] kJ/kg, and [latex]h_4=2335[/latex] kJ/kg

Moisture contents in exit from L.P. turbine = 10.4%

[latex]x_4=1-0.104=0.896[/latex]

(i) Reheat pressure : From the Mollier diagram, the reheat pressure is 40 bar.

(ii) Thermal efficiency, [latex]\eta_{ ext {th }}[/latex] :

Turbine work [latex]=\left(h_1-h_2 ight)+\left(h_3-h_4 ight)[/latex]

= (3580 – 3140) + (3675 – 2335) = 1780 kJ/kg.

Assuming specific volume of water = [latex]10^{-3}[/latex] m³/kg, the pump work = [latex]10^{-3}(150-0.1)[/latex] = 0.15 kJ/kg, i.e., may be neglected in computing of [latex]\eta_{ ext {th }}, h_5=h_4=191.8[/latex] kJ/kg, ([latex]h_f[/latex] at 0.1 bar) from steam tables,

[latex]Q_{ ext {input }}=\left(h_1-h_5 ight)+\left(h_3-h_2 ight)[/latex]

= (3580 – 191.8) + (3675 – 3140) = 3923.2 kJ/kg

[latex]\% \eta_{ ext {th }}=\frac{1780}{3923.2} imes 100=45.37 \%[/latex].

(iii) Specific steam consumption :

Steam consumption

[latex]=\frac{15 imes 10^3}{1780}=8.427[/latex] kg/s

Specific steam consumption [latex]=\frac{8.427 imes 3600}{15 imes 10^3}= 2 . 0 2 2 5[/latex] kg/kWh.

(iv) Rate of pump work :

Rate of pump work = 8.427 × 0.15 = 1.26 kW.

Question 12.21: (a) How does erosion of turbine blades occur ? State the met...

Related Answered Questions

Steam at a pressure of 20 bar and 250°C enters a turbine and leaves it finally at a pressure of 0.05 bar. Steam is bled off at pressures of 5.0, 1.5 and 0.3 bar. Assuming (i) that the condensate is heated in each heater upto the saturation temperature of the steam in that heater, (ii) that the ...

Verified Answer:

Verified Answer:

A steam power plant operates on ideal Rankine cycle using reheater and regenerative feed water heaters. It has one open feed heater. Steam is supplied at 150 bar and 600°C. The condenser pressure is 0.1 bar. Some steam is extracted from the turbine at 40 bar for closed feed water heater and ...

Verified Answer:

Verified Answer:

A steam power plant of 110 MW capacity is equipped with regenerative as well as reheat arrangement. The steam is supplied at 80 bar and 55°C of superheat. The steam is extracted at 7 bar for feed heating and remaining steam is reheated to 350°C, and then expanded to 0.4 bar in the L.P. stage. ...

Verified Answer:

Verified Answer:

A binary vapour cycle operates on mercury and steam. Standard mercury vapour at 4.5 bar is supplied to the mercury turbine, from which it exhausts at 0.04 bar. The mercury condenser generates saturated steam at 15 bar which is expanded in a steam turbine to 0.04 bar. (i) Determine the overall ...

Verified Answer:

Verified Answer:

Verified Answer:

Verified Answer: