Question Set 60

1. Why pistons are usually dished at top ?

Ans: Pistons are usually hollowed at top to (i) provide greater space for combustion, (ii) increase surface for flue gases to act upon, and (iii) better distribution of stresses.

2. What is the function of thermostat in cooling system of an engine ?

Ans: Thermostat ensures optimum cooling because excessive cooling decreases the overall efficiency. It allows cooling water to go to radiator beyond a predetermined temperature.

3. What are the causes of failure of boiler tubes ?

Ans: Boiler tubes, usually are made from carbon steel and are subject to (a) high rates of heat transfer,( b ). bending stresses due to uneven heating, especially at expanded or welded joints into headers or drums, © external erosion from burners and flue gas, (d) possible corrosion on the boiler side, and (e) occasional manufacturing defects.

Failure may occur due to following reasons :

(a) High thermal ratings may lead to rapid failure if the internal fluid flow is reduced for any reason. The resultant overheating leads to a failure by creep, characterised by the bulging of the tube with the eventual development of a longitudinal split.

(b ) Fatigue cracking due to bending stresses occur. These are associated with change of section and/or weld undercut, where tubes are expanded or welded into headers.

© Failure may arise due to overstressing of a reduced section of metal.

(d) Sudden failure of the boiler tube due to corrosion arises from embrittlement of the carbon steel due to interaction between atomic hydrogen from the corrosion process and the iron carbide present in the steel.

(e) Defects in tube manufacture, although far from being a regular occurrence, can be a cause of serious trouble. Lamination in boiler tubes or score marks arising from the cold drawing of tubes, give rise to premature failure and may promote corrosion at these regions.

4. What are the causes of failure of superheater tubes ?

Ans: Superheater tubes are subjected to the most severe combination of stress, temperature and corrosive environment. In addition to high-temperature strength, resistance to corrosion is also important. For example, low-alloy ferritic steel such as -1/% Cr, 1% Mo would not be used at metal temperatures above 580°C because of inadequate resistance to corrosion and oxidation over a full service life of 100,000/150,000 hr.

Failures in superheater tubes may arise from :

(a) Prior fabrication history (b ) Faulty heat treatment

© Consequences of welding (d) Overheating of the tube metal

(e) Gas-side corrosion (f) Stress corrosion (austenitic steels).

5. Why supercritical boilers use less amount of steel compared to non-supercritical boilers ?

Ans: Supercritical boilers do not head heavy drum for separation of steam from mixture of water and steam.

6. Out of electric heater and heat pump, which is economical in operation ?

Ans: Heat pump.

7. Which furnace burns low-ash fusion coal and retains most of the coal ash in the slag?

Ans: Cyclone furnace.

8. How the thickness of thermal boundary layer and thickness of hydrodynamic boundary layer related ?

Ans: Ratio of their thickness = (Prandtl number)-1/3.

9. What is the effect of friction on flow of steam through a nozzle ?

Ans: To decrease both mass flow rate and wetness of steam.

10. Why gas turbine power plant needs efficient compressor ?

Ans: Because a large portion of turbine work is eaten away by compressor and its inefficiency will affect net power output and cost of generation.

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