Power Engineering - 2B1 (Chapter 2) UPDATED ACTUAL Questions and
CORRECT Answers
Question: What is bowing and what causes it?
Correct Answer: When the turbine is still and left cold the mass of the rotor can cause it to sag
Question: What is hogging and what causes it?
Correct Answer: When the turbine is still and left hot. The bottom half will cool quicker than the upper
half causing the rotor to bend upwards
Question: What are turning gears? What are they also called?
Correct Answer: Barring gears. Turn the turbine at slow speed to reduce hogging and bowing
Question: What speed do turning gears typically turn a turbine?
Correct Answer: Between 20 and 40 RPM but some as low as 3 RPM
Question: How does running turning gears cool? What does it cool?
Correct Answer: The fan action of the turbine cools the turbine and bearings
Question: How long do turning gears operate before starting up a steam turbine?
Correct Answer: 3 hours
Question: How long do turning gears operate after shutting down a steam turbine?
Correct Answer: 24 hours
Question: What can be said about hydrogen cooled generators and turning gears?
Correct Answer: When shutdown the turning gears keep the turbine turning all the time. This is so as to
prevent hydrogen from escaping through the shaft seals
Question: What type of lubrication is needed when turning gears are operating?
Correct Answer: When turning gears are operating the mechanical oil pump will NOT provide enough
lubrication. Therefor the auxiliary pump is used.
Question: When turning gear is used, what lubrication is provided in addition to the auxiliary oil pump?
Why?
Correct Answer: A separate motor driven oil pump to provide oil to the bearings
Question: Blade Clearances - What can be said of tight blade clearances?
Correct Answer: The tighter the clearances the more efficient the turbine
Question: When the blade clearances are excessive, what can happen?
Correct Answer: Steam will leak around the blades reducing the overall efficiency
Question: What could happen if the blades clearances are too tight?
Correct Answer: Rubbing might occur
Question: What is done with regard to blade clearances during construction? Why?
, Correct Answer: Blades clearances are recorded. That way during maintenance the clearances can be
checked against the record
Question: What are three reasons why all rotors are out of balance to some degree?
Correct Answer: 1. Differences in density of material 2. Machining errors 3. Differences in blade masses
Question: What are two rotor states where balancing is carried out?
Correct Answer: Statically and dynamically
Question: What is looked for when static balancing is carried out?
Correct Answer: The tendency of the rotor to roll is measured
Question: What is done when dynamic balancing is carried out?
Correct Answer: The rotor is sped up by an electric motor and the vibrations are measured
Question: What is done to balance the rotor when dynamic balancing is carried out?
Correct Answer: Mass is added or removed until the vibration is eliminated as much as possible
Question: When balancing a rotor, what should the target be?
Correct Answer: No more than 0.025 to 0.05 mm eccentricity
Question: Everything, including rotors, has a natural frequency. What is the relationship between rotor
speed and natural frequency?
Correct Answer: The natural frequency will deflect the shaft as speed increases.
Question: What is critical speed and why is it so important?
Correct Answer: Critical speed is when deflection is so rapid that it can bend the shaft. It's important to get
through critical speed as quickly as possible so as not to damage the shaft.
Question: What are three things that critical speed depends on?
Correct Answer: 1. Length of shaft between supports 2. Shaft diameter 3. Shaft stiffness
Question: What happens once you pass through critical speed?
Correct Answer: Deflection decreases until the unit runs smooth again
Question: How may critical speeds can a rotor have?
Correct Answer: It can have one or several
Question: Lubricating oil system complexity varies with the size of the turbine. What type of lubricating oil
system do small (less than 150 kW) turbines have?
Correct Answer: Often only ring oiled bearings
Question: Lubricating oil system complexity varies with the size of the turbine. What type of lubricating oil
system do medium turbines have? Explain it in detail.-
Correct Answer: Ring oiled bearings and a circulating system. The circulation system sprays oil to the
gears and also provides oil to the bearings
Question: Lubricating oil system complexity varies with the size of the turbine. What type of lubricating oil
system do large turbines have? Explain in detail.
CORRECT Answers
Question: What is bowing and what causes it?
Correct Answer: When the turbine is still and left cold the mass of the rotor can cause it to sag
Question: What is hogging and what causes it?
Correct Answer: When the turbine is still and left hot. The bottom half will cool quicker than the upper
half causing the rotor to bend upwards
Question: What are turning gears? What are they also called?
Correct Answer: Barring gears. Turn the turbine at slow speed to reduce hogging and bowing
Question: What speed do turning gears typically turn a turbine?
Correct Answer: Between 20 and 40 RPM but some as low as 3 RPM
Question: How does running turning gears cool? What does it cool?
Correct Answer: The fan action of the turbine cools the turbine and bearings
Question: How long do turning gears operate before starting up a steam turbine?
Correct Answer: 3 hours
Question: How long do turning gears operate after shutting down a steam turbine?
Correct Answer: 24 hours
Question: What can be said about hydrogen cooled generators and turning gears?
Correct Answer: When shutdown the turning gears keep the turbine turning all the time. This is so as to
prevent hydrogen from escaping through the shaft seals
Question: What type of lubrication is needed when turning gears are operating?
Correct Answer: When turning gears are operating the mechanical oil pump will NOT provide enough
lubrication. Therefor the auxiliary pump is used.
Question: When turning gear is used, what lubrication is provided in addition to the auxiliary oil pump?
Why?
Correct Answer: A separate motor driven oil pump to provide oil to the bearings
Question: Blade Clearances - What can be said of tight blade clearances?
Correct Answer: The tighter the clearances the more efficient the turbine
Question: When the blade clearances are excessive, what can happen?
Correct Answer: Steam will leak around the blades reducing the overall efficiency
Question: What could happen if the blades clearances are too tight?
Correct Answer: Rubbing might occur
Question: What is done with regard to blade clearances during construction? Why?
, Correct Answer: Blades clearances are recorded. That way during maintenance the clearances can be
checked against the record
Question: What are three reasons why all rotors are out of balance to some degree?
Correct Answer: 1. Differences in density of material 2. Machining errors 3. Differences in blade masses
Question: What are two rotor states where balancing is carried out?
Correct Answer: Statically and dynamically
Question: What is looked for when static balancing is carried out?
Correct Answer: The tendency of the rotor to roll is measured
Question: What is done when dynamic balancing is carried out?
Correct Answer: The rotor is sped up by an electric motor and the vibrations are measured
Question: What is done to balance the rotor when dynamic balancing is carried out?
Correct Answer: Mass is added or removed until the vibration is eliminated as much as possible
Question: When balancing a rotor, what should the target be?
Correct Answer: No more than 0.025 to 0.05 mm eccentricity
Question: Everything, including rotors, has a natural frequency. What is the relationship between rotor
speed and natural frequency?
Correct Answer: The natural frequency will deflect the shaft as speed increases.
Question: What is critical speed and why is it so important?
Correct Answer: Critical speed is when deflection is so rapid that it can bend the shaft. It's important to get
through critical speed as quickly as possible so as not to damage the shaft.
Question: What are three things that critical speed depends on?
Correct Answer: 1. Length of shaft between supports 2. Shaft diameter 3. Shaft stiffness
Question: What happens once you pass through critical speed?
Correct Answer: Deflection decreases until the unit runs smooth again
Question: How may critical speeds can a rotor have?
Correct Answer: It can have one or several
Question: Lubricating oil system complexity varies with the size of the turbine. What type of lubricating oil
system do small (less than 150 kW) turbines have?
Correct Answer: Often only ring oiled bearings
Question: Lubricating oil system complexity varies with the size of the turbine. What type of lubricating oil
system do medium turbines have? Explain it in detail.-
Correct Answer: Ring oiled bearings and a circulating system. The circulation system sprays oil to the
gears and also provides oil to the bearings
Question: Lubricating oil system complexity varies with the size of the turbine. What type of lubricating oil
system do large turbines have? Explain in detail.
