What provisions in the layout of a combined-cycle should be considered?
Answer:
It is important to consider the use of a bypass stack that will permit operating the gas turbo-generator in case of a forced outage on the HRSG or steam turbo-generator. However, in certain states, such as California, also to be considered are NO limits that require steam injection and loading to limit the exhaust temperature coming out of the simple-cycle gas turbine so that they do not exceed jurisdictional limits.
What remedial measures you can suggest to cope with radial axial-bore cracks?
Answer:
For new rotors, modified heat treatment process is recommended while for existing rotors de-rating the turbine or replacement of the rotor may be a solution.
What should be done if excessive vibration is due to an unbalanced turbine wheel?
Answers:
The turbine wheel is to be checked if it became unbalanced due to over speeding. The turbine wheel must be re-balanced or replaced. What should be the remedial action? Answers:
For new rotors, control cleanliness of the steel. i.e. inclusion and segregates free and more homogeneous steel shaft is required. For current rotors, replace the rotor, grind and overbore.
What should be the remedial actions for blade-groove-wall cracking?
Answers:
Modified heat treatment of new rotors is a sound and lasting remedy. For current rotors, cracks should be machined off and lighter blades should be installed. Better, retire the cracked shaft. What should you do if you lost vacuum while operating a condensing turbine plant?
Answer:
If vacuum is lost shut down immediately. The condenser cannot stand steam pressure; the condenser tubes may leak from excessive temperature. Excessive pressure will also damage the shell, the exhaust, and the low-pressure parts of the turbine.
What steps are taken to minimize damage from moisture on steam turbine blades?
Answers:
The following measures are employed at the design stage:
Stellite inserts. Hardening of the base metal. Moisture-removal devices to combat impingement corrosion due to moisture. What steps are taken to minimize damage from moisture?
Answers:
Stellite inserts. Hardening of the base metal. Moisture-removal devices to combat impingement corrosion due to moisture. What steps/modifications should be implemented to curtail the damage from moisture impingement on steam turbine blades?
Answers:
The drainage system should be redesigned. Larger drains are to be provided. More effective water-catchers are to be in-stalled. Radial seals are to be eliminated to remove water before it can chance upon the blades. Nozzle trailing edges are to be thinned to promote the formation of smaller and less harmful droplets. What steps/modifications should be implemented to curtail the damage from moisture impingement?
Answers:
The drainage system should be redesigned. Larger drains are to be provided. More effective water-catchers are to be in-stalled. Radial seals are to be eliminated to remove water before it can chance upon the blades. Nozzle trailing edges are to be thinned to promote the formation of smaller and less harmful droplets. What type of deposits are formed on steam turbine blading?
Answers:
Water-soluble deposits. - NaCl, Na2SO4, NaOH and Na3PO4
- Water-insoluble deposits.
- SiO2 (mainly).
What types of cracking occur in the LP rotor shaft?
Answers:
Radial axial-bore cracks. Circumferential cracks.
When "stall flutter" occurs?
Answers:
This problem is encountered when operating limits are exceeded i.e., when turbine exhaust pressure exceeds the value what the manufacturer has recommended. Stall flutter induces stress in the blades
When does SPE damage usually occur on steam turbine blades?
Answer:
It occurs usually during startup or abrupt load change.
When does SPE damage usually occur?
Answer:
It occurs usually during startup or abrupt load change.
When does upgrading mean modernization of utility industry?
Answer:
Upgrading is really modernization to all those units other than those facing uncertain load growth and low-capital utility system. It involves replacement of damaged parts/components by state-of-the-art components without scrapping the entire machine.
Where are velocity compounded steam turbines mostly employed?
Answers:
They are chiefly used as the prime mover for: - Centrifugal pumps.
- Centrifugal compressors.
- Low capacity turbo-generators.
- Feed pumps of high capacity power plants.
Where do water-soluble deposits prevail?
Answer:
In the high-and intermediate-pressure sections of steam turbines.
Where is pitting corrosion mostly prevalent?
Answer:
Upstream of LP stages as well as wet stages of LP cylinder.
Where would you look for a fault if the air ejector did not raise enough vacuum?
Answer:
In this case, the trouble is usually in the nozzle. You will probably find that:
the nozzle is eroded the strainer protecting the nozzle is clogged the steam pressure to the nozzle is too low Which factors affect the extent of an upgrading program?
Answers:
Age of the unit. How it has been operated. Note: Turbines less than quarter of a century old can simply be upgraded to their original design conditions.
Why are free-standing blades in the last low-pressure stage favored more, in some cases, than those that are coupled and shrouded together?
Answers:
These free-standing blades are known to provide good and adequate protection against stresses and aggressive environment. They eliminate all areas viz. shroud/tenon interface and tie-wire/hole area where corrodents can collect Why are simple impulse turbines not so common?
Answers:
Since the whole pressure drop from boiler to condenser pressure takes place in a single row of nozzles, the velocity of the steam entering the turbine is very high. If some of this velocity is used up in a single row of turbine blading, as in the de Laval turbine, the speed of the rotation of the wheel will be too high to be blades are be useful for practical purposes, as there is the danger of structural failures due to excessive centrifugal stresses. Steam exits from the turbine with a sufficiently high velocity, meaning a considerable loss of kinetic energy. Why are some groups of steam turbine blades, particularly the first or control stages more prone to fatigue failures than others?
Answer:
Blades in the first or control stages are under partial-arc admission that forces the blades to move into and out of the steam flow causing alternating high-and low-impact forces. This periodic change of impact forces imparts fatigue stress that makes such groups of blades susceptible to fatigue failure.
Why could a turbine wheel become unbalanced?
Answer:
If the turbine is kept idle for a long spell without complete drainage of exhaust casing, the solid matter can deposit in the lower half of the wheel causing unbalance.
Why do blade roots suffer from damage?
Answers:
Fatigue is the common cause to the effect of blade root damage. Also a generic type of fault often assists this factor in design or manufacturer. Moreover, the root-fillet radii are subjected to a high degree of stress concentration with the effect that they crack relatively easily. Why do electrically induced stresses occur in steam turbine rotors occur?
Answer:
They originate due to short circuits and faulty synchronization.
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:
The cause to the effect of blade-groove-wall cracking is creep with or without low cycle fatigue. Faulty heat treatment procedure results in poor creep ductility that may also contribute to this type of faults. 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:
These rotors were manufactured from forgings which were not "clean" steel and that's why cracks were initiated at the sites of inclusions, and Segregation bands in the steel. This coupled with low inherent toughness of rotor materials resulted in bore cracks. 
