Turbine Auxilaries Q & A-iii

41. What are the effects of air leakage in the condenser?
Answers:
1. It increases the backpressure on the turbine with the effect that there is less heat drop and low thermal efficiency of the plant.
2. The pressure of air in the condenser lowers the partial pressure of steam, which means steam, will condense at a lower temperature and that will require greater amount of cooling water.
3. It reduces the rate of condensation of steam, because air having poor thermal conductivity impairs the overall heat transfer from the steam-air mixture.
42. What basic governor troubles are apt to occur?
Answers:
1. Hunting-alternate speeding and slowing of the engine, which means that the governor is too sensitive to load changes.
2. Sticking-failure to control speed, allowing the engine to run away or slow down-which means that the governor is not sensitive to load changes or parts are binding or worn.
43. What is a governor safety stop?
Answers: On throttling-type governors, the safety stop is a weighted arm that needs the support of a governor belt. If the belt breaks, the idler arm drops and shuts the steam supply valve to the engine. On Corliss units, the flyballs fall to the lowest position and knock off the safety cams; the cams disengage the catch blocks on the steam intake valves so that no steam is admitted to the engine.
44. Why is condensation or excessive carryover dangerous to reciprocating engines?
Answer: Because water is non-compressible. If an excessive amount of water gets into the cylinder, it will wreck the engine.
45. Why should a steam or moisture separator be installed in the steam line next to a steam turbine?
Answer: All multistage turbines, low-pressure turbines, and turbines operating at high pressure with saturated steam should have a moisture separator in order to prevent rapid blade wear from water erosion.
46. Under what conditions may a relief valve not be required on the exhaust end of a turbine?
Answer: If the manufacturer has provided that the turbine shells are constructed for full-inlet steam pressure for the entire length of the shell. It is absolutely essential to have a relief valve to protect the shell in the event an exhaust valve is closed and high-pressure steam is admitted to the shell on the front end of the machine. Explosions have occurred when this happened.
47. What are some conditions that may prevent a turbine from developing full power?
Answers:
1. The machine is overloaded.
2. The initial steam pressure and temperature are not up to design conditions.
3. The exhaust pressure is too high.
4. The governor is set too low.
5. The steam strainer is clogged.
6. Turbine nozzles are clogged with deposits.
7. Internal wear on nozzles and blades.
48. Why is it necessary to open casing drains and drains on the steam line going to the turbine when a turbine is to be started?.
Answers: To avoid slugging nozzles and blades inside the turbine with condensate on start-up; this can break these components from impact. The blades were designed to handle steam, not water.
49. What three methods are used to restore casing surfaces that are excessively eroded?
Answers:
1. Metal-spraying.
2. Welding.
3. Insertion of filler strips or patch plates. The manufacturer should be consulted on the metallurgy involved so that the best method can be selected.
50. What is steam rate as applied to turbo-generators?
Answer: The steam rate is the pounds of steam that must be supplied per kilowatt-hour of generator output at the steam turbine inlet.
51. What is the most prevalent source of water induction into a steam turbo-generator?
Answer: Leaking water tubes in feedwater heaters, which have steam on the shell side supplied from turbine extraction lines. The water at higher pressure can flow back into the turbine because the extraction steam is at a lower pressure. Check valves are needed on the steam extraction line to prevent the back-flow of water into the turbine.
52. What is a regenerative cycle?
Answer: In the regenerative cycle, feedwater is passed through a series of feed-water heaters and is heated by steam extracted from stages of a steam turbine. This raises the feedwater to near the temperature of boiler water, thus increasing the thermal efficiency of the cycle.
53. What is the re-heating cycle?
Answer: In the re-heating cycle, superheated steam is expanded in a high-pressure turbine and then returned to the boiler's re-heater to raise the temperature of the steam to the inlet temperature, usually to around 537°C; it is then returned to the turbine to be expanded through intermediate-pressure turbines. In some cases, the steam is again returned for re-heating in the boiler and then expanded in the lower-pressure sections of the turbine. The main purpose of re-heating the steam on large turbo-generators is to avoid condensation in the lower-pressure sections of the turbine, which can rapidly cause blade erosion problems from wet steam.
54. What does the Willans line show?
Answer: The Willians line is a plot of throttle flow versus the load, usually expressed in kilowatts; generally it is a straight line except for low and high loads. The Willians line is used to show steam rates at different loads on the turbine.
55. What are the two basic types of turbines?
Answer:
1. Impulse type.
2. Reaction type.
56. What is the operating principle of an impulse turbine?
Answer: The basic idea of an impulse turbine is that a jet of steam from a fixed nozzle pushes against the rotor blades and impels them forward. The velocity of the steam is about twice as fast as the velocity of the blades. Only turbines utilizing fixed nozzles are classified as impulse turbines.
57. What is the operating principle of a reaction turbine?
Answer: A reaction turbine utilizes a jet of steam that flows from a nozzle on the rotor. Actually, the steam is directed into the moving blades by fixed blades designed to expand the steam. The result is a small increase in velocity over that of the moving blades. These blades form a wall of moving nozzles that further expand the steam. The steam flow is partially reversed by the moving blades, producing a reaction on the blades. Since the pressure drop is small across each row of nozzles (blades), the speed is comparatively low. Therefore, more rows of moving blades are needed than in an impulse turbine.
58. What are topping and superposed turbines?
Answer: Topping and superposed turbines arc high-pressure, non-condensing units that can be added to an older, moderate-pressure plant. Topping turbines receive high-pressure steam from new high-pressure boilers. The exhaust steam of the new turbine has the same pressure as the old boilers and is used to supply the old turbines.
59. What is an extraction turbine?
Answer: In an extraction turbine, steam is withdrawn from one or more stages, at one or more pressures, for heating, plant process, or feedwater heater needs. They are often called "bleeder turbines."
60. What is a radial-flow turbine?
Answer: In a radial-flow turbine, steam flows outward from the shaft to the casing. The unit is usually a reaction unit, having both fixed and moving blades. They are used for special jobs and are more common to European manufacturers, such as Sta-Laval (now ABB).