Turbines

Question 1

With respect to steam turbine blades explain the differences between the inserted and straddle methods of attaching turbine blades to the turbine rotor.

Answer 1

The turbine blades can be fitted by either inserted (fir tree) or by straddle.

Inserted – This has a male fitting on the blade and a female cut out within the turbine.

Straddle – This has the male fitting on the turbine and the blade has the female cut out.

Question 2

Sketch a typical side entry blade method of attachment, indicating where an allowance for blade expansion is provided, explain how the blade is entered into the turbine rotor and how it is secured in place.

Answer 2

The bulb is designed slightly smaller to allow for the bulb root to expand when the turbine is running at full temperature. This will the hold in place allowing no movement.

The blades are fitted in an axial direction and crimped either side.

Question 3

Explain the function of lacing wires or shrouding that may be fitted to the blades of turbine rotors.

Answer 3

Lacing wires and shrouding are fitted to reduce vibration and increase radial strength.

Question 4

Explain why it may be necessary to protect the blades of an LP ahead or astern turbine from erosion. Clearly identify which blades may require protection, where the protection is fitted, how the protection is fitted and the material used top provide the protection.

Answer 4

The blades that require protection are the later blades due to water droplets being formed on the earlier blades therefore causing erosion. The protection fitted is Stellite shields welded onto the back of the leading edge of the blade.

Question 5

Name the various types of Turbine and state which are commonly used in combination.

Answer 5

Parsons –Reaction
De Laval – Impulse
Rateau - Impulse
Curtis - Impulse

Question 6

Describe operating principal of the following:

i)	a reaction turbine;

Answer 6

Steam passes through fixed nozzles which pressure energy is converted into velocity. The high velocity steam is redirected at moving blades. This then causes the blades to rotate. Any left over steam velocity the goes into the next set of nozzles and the process is repeated.

Question 7

Describe operating principal of the following:

an impulse turbine.

Answer 7

Steam can travel across a uniform path between the blades without a pressure drop. The change in direction creates a change in velocity. In theory the steam entry will leave at the same speed but due to friction speed will be loss over time.

Question 8

Sketch a cross section showing the tube arrangements, explaining why the direction of steam flow is important;

Answer 8

it is important to keep the feed water heated otherwise additional energy is going to have to be used to reheat.

Question 9

Describe the arrangements that permit the condenser shell to expand and contract.

Answer 9

Sliding feet allow the system to expand with an 0.25mm gap. Panting plates can be used by when heating up they are aligned but when cold they sit slightly offset.

Tubes have one fixed end and the other end is able to expand with the heat.
Tube end plate is able to expand with the casing due to the tubes and casing being different metals.

Question 10

Explain, with reasons, where the sealing medium is supplied from when the turbine is at rest and running at full load in the case of:-

i)	the HP turbine rotor glands;

Answer 10

The pressure supplied is about 1.4 bar and goes to a chamber in the middle of the seal box. The pressurised steam escapes back into the glad steam system.

Question 11

Explain, with reasons, where the sealing medium is supplied from when the turbine is at rest and running at full load in the case of:-

iii) the LP turbine rotor Steam inlet end gland.

Answer 11

The pressure supplied is about 1.4 bar and goes to a chamber in the middle of the seal box. Passes into the seal box and stops air being drawn into the turbine.

Question 12

Explain the differences between the inserted and straddle methods of attaching turbine blades to the turbine rotor.

Answer 12

The turbine blades can be fitted by either inserted (fir tree) or by straddle.

Inserted – This has a male fitting on the blade and a female cut out within the turbine.

Straddle – This has the male fitting on the turbine and the blade has the female cut out.

Question 13

Sketch a typical side entry blade method of attachment, indicating where an allowance for blade expansion is provided, explain how the blade is entered into the turbine rotor and how it is secured in place.

Answer 13

The bulb is designed slightly smaller to allow for the bulb root to expand when the turbine is running at full temperature. This will the hold in place allowing no movement.

The blades are fitted in an axial direction and crimped either side.

Question 14

Identify, with reasons, the key properties required of the oil to be used in the lubrication system.

Answer 14

• Provides an oil film between to reduce friction between moving parts
• Removes heat from bearings or generated heat along the shaft.

Question 15

Explain how the following problems with the oil may arise and how they may be prevented:-

i)		Oxidation;

Answer 15

Caused by water vapour condensing in the head space of tanks causing corrosion. This can be reduced by adding oxidation inhibitors to the oil.

Question 16

Explain how the following problems with the oil may arise and how they may be prevented:-

Foaming;

Answer 16

Leakage of air or water causing bubbles on the surface. This acts as a blanket causing the oil to increase in temperature and cause a rise in pressure. This will lead to oxidation. This can be reduced with anti foaming inhibitors.

Question 17

Explain how the following problems with the oil may arise and how they may be prevented:-

Emulsification;

Answer 17

Water contamination causing the oil incompressible and result in a loss of hydrodynamic lubrication. Carry regular lubrication tests and purifier the oil often.

Question 18

Explain how the following problems with the oil may arise and how they may be prevented:-

iii) Microbial degradation.

Answer 18

Water contamination or organic material like carbon. Its supplied from by turbine oil hydrocarbon drains. Test for water below less than 500ppm. Also check the separator sight glass.

Question 19

List the steps to be taken in warming through a main turbine plant from cold prior to proceeding to sea, clearly identifying the reasons for each step taken and the minimum acceptable duration of the overall process. It may be assumed that the main boilers are on line and up to operating temperature and pressure.

Answer 19

• Warm through steam lines and open all drain lines to remove any condensate.
• Open all drains on the turbine
• Start main condenser sea water circulating pumps at slow speeds
• Start turbine lubricating system circulation pumps, checking bearings and gearbox sprayers for good oil supply.
• Apply gland steam to the turbines. Using air ejectors, draw a vacuum of 10 inches of mercury in the condenser.
• Start condenser extraction pump on recirculation
• Warm through turbines for 2hrs while turning on the turning gear.
• Remove turning gear
• Open bulkhead stop valves
• Ask bridge from permission
• Ease main throttle. Alternate ahead and astern every 10 to 15mins with short burst of steam.
• Note turbine clearances
• Open stern guardian valve to warm through astern turbine
• 30mins before standby by increase condenser vacuum to 65%, 20 inches mercury
• 10min before standby increase condenser vacuum to 90%, 24 inches mercury
• at standby increase to full condenser vacuum and follow telegraph instructions

Question 20

Explain why it essential to warm the turbine plant through thoroughly prior to operating it under load.

Answer 20

Otherwise irregular expansion or distortion will occur, causing excessive condensation, thermal shocks and excessive stresses. This will increase the rate of wear or even damage the main propulsion system.

Question 21

Name the various types of Turbine and state which are commonly used in combination.

Answer 21

Parsons –Reaction
De Laval – Impulse
Rateau - Impulse
Curtis - Impulse

Question 22

Describe operating principal of the following:

i) a reaction turbine;

Answer 22

Steam passes through fixed nozzles which pressure energy is converted into velocity. The high velocity steam is redirected at moving blades. This then causes the blades to rotate. Any left over steam velocity the goes into the next set of nozzles and the process is repeated.

Question 23

Describe operating principal of the following:

an impulse turbine.

Answer 23

Steam can travel across a uniform path between the blades without a pressure drop. The change in direction creates a change in velocity. In theory the steam entry will leave at the same speed but due to friction speed will be loss over time.

Question 24

Explain the function of shrouding that may be fitted to the blades of turbine rotors and describe the methods for attaching the Shrouding to the turbine blades.

Answer 24

Shrouding is fitted to reduce vibration at the blade ends and increase radial strength. The shrouds are either a flat strip riveted to a blade tenon or a rolled shroud wire

Question 25

d) Explain why it may be necessary to protect the blades of an LP ahead or astern turbine from erosion. Clearly identify which blades may require protection, where the protection is fitted, how the protection is fitted and the material used to provide the protection.

Answer 25

The blades that require protection are the later blades due to water droplets being formed on the earlier blades therefore causing erosion. The protection fitted is Stellite shields welded onto the back of the leading edge of the blade.