Question & Answers on Steam Turbines-ii

Why do shroud bands suffer from damage?

Answer: Steam borne solid particles and moisture strike the shroud band continually and in that process they remove material from rivet heads until the rivet becomes too weak to exercise its clamping effect whereupon it fails to hold the band in place.

Why do thermal stress occur in the steam turbine rotors?

Answer: Transient operating phases i.e. startup and shutdown the genesis of thermal stress induced to the turbine shaft.

Why do these two types of cracking take place?

Answers:

1. The cause to the effect of blade-groove-wall cracking is creep with or without low cycle fatigue.
2. Faulty heat treatment procedure results in poor creep ductility that may also contribute to this type of faults.
3. Whereas thermal fatigue have been identified as the single cause to rotor-surface cracking.

Why does mechanical stress occur in turbine rotors?

Answer: The factors that contribute to mechanical stress in the shaft are the centrifugal forces and torque’s generated due to revolving motion of the shaft as well as bending arising during steady-state operation.

Why does SCC occur at the intermediate pressure stage in the steam turbine?

Answer: Steam turbines of nuclear power plants usually operate on wetter steam, than those of thermal power plants. So even at the intermediate pressure stage, steam becomes wet and it precipitates the impurities i.e. corrodents dissolved in it These corrodents deposit and build up on rotor shaft causing stress-corrosion-cracking.

Why is it poor practice to allow turbine oil to become too cool?

Answer: If the turbine oil is allowed to become too cool, condensation of atmospheric moisture takes place in the oil and starts rust on the polished surfaces of the journals and bearings. Condensed moisture may also interfere with lubrication.

Why is there a relief valve on a turbine casing?

Answer: The turbine casing is fitted with spring-loaded relief valves to prevent damage by excessive steam pressure at the low-pressure end if the exhaust valve is closed accidentally. Some casings on smaller turbines are fitted with a sentinel valve, which serves only to warn the operator of overpressure on the exhaust end. A spring-loaded relief valve is needed to relieve high pressure.

Why must condensate be subjected to salinity tests where brackish cooling water is used?

Answer: Condensate may leak from the cooling-water side to the steam side of the condenser and contaminate the feedwater, thus causing scale to form in the boilers. Or brackish cooling water may leak into the steam space from cracked or porous lubes or from around the joints at the end of the tube ends, etc. By taking salinity readings of the condensate, leaks may be found before they can do any harm.

Why must steam turbines be warmed up gradually?

Answer: Although it is probable that a turbine can, if its shaft is straight, be started from a cold condition without warming up, such operation does not contribute to continued successful operation of the unit. The temperature strains set up in the casings and rotors by such rapid heating have a harmful effect. The turbine, in larger units especially, should be warmed slowly by recommended warm-up ramp rates because of close clearances.

Why were cracks at the bore common for high-pressure and intermediate-pressure rotors of the early sixties in steam turbines?

Answers:

1. These rotors were manufactured from forgings which were not "clean" steel and that's why cracks were initiated at the sites of inclusions, and
2. Segregation bands in the steel. This coupled with low inherent toughness of rotor materials resulted in bore cracks.