Main Bearing

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Teh length/diameter ratio is 2/3 to 3/2, the smaller figure is more relative to modern designs and can help reduce oil whirl. The top clearance is 0.5mm, this is sufficiently large to allow for large quantites of oil flow to aid cooling. No oil ways are provided other than a small amount of metal washed away at the inlt oil ports.

Maximum oil temperature is 83'C

White metal thickness 0.25 to 0.5mm, the thicker this layer the greater the ability to cope with dirt absorbtion. Thicker white metal is required for gun metal backed bearings due to the possibility of copper pick up should the white metal run. The white metal adheres better to the steel and provides better rigidity.

Typical white metal 85% tin, 8.5% Antimony, 6% copper.

An antisiphon device prevents all the oil leaving bearing if there is a failure of oil supply

Dummy bearings must be introduced to allow removal of lower bearing for inspection

Thrust Bearing

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The thrust bearing is placed at the inlet end of the turbine casing as this is the hottest end and hence the most effected by differential turbine/casing expansion. This helps to prevent damage to the glands and also allows the use of reduced clearances, necessary as the specific volume of the steam is at its highest

Standard

A half set of pads are fitted in the aft thrust direction as these are mainly for location only and do not carry any axial loading caused by the passage of the steam.
Oil enters the lower portion of the bearing and passes up via a restriction to ensure the assembly remains flooded with oil

Self Aligning

The high inertia of the spheical carrier reduces the arragements ability to cope with distortions and imperfections. The key prevent sthe rotation of the carrier.

Modern Self Aligning

This design has less inertia and hence is more effective with dealing with distortion.

Mitchell tilting pad bearings are commonly used due to their self aligning properties. The length of the pads is limited due to lubrication problems at the thin end of the oil wedge.
The pads are formed initially as a single ring then machined to requirements. This is why all pads must be changed following failure

Thrust Bearing Clearance

For inital setting up the rotor is centralised by jacking for'd and aft, and the clearance on the ahead side measured. A complete set of pads with carrier are made up to exactly the correct size. The astern size is measured, a running lube oil clearance subtracted and the astern set built up. The whole lot is fitted and final clearances measured. The retaining ring is split at the horizontal axis. Stop plates at the joints prevent movement of the mitchell pads, one of these stop plates is extended and prevents the retaining ring moving
On the HP turbine normally only a half set of thrust pads are fitted. For the LP turbine with an astern turbine a full set is used. Shils and liners are fitted to set clearance.
There is a tendency for oil to be flung to the periphery under centrifugal action. ence, the orifice is fitted to ensure flooding, also metering flow from main system.

Total clearance = 0.25mm

This may be checked by attaching a finger plate to the casing and jacking the rotor for'd and aft. Poker guages may be used when the turbine is running.
Gun metal or mild steel is used for backing plate. Babbit metal (87% Tin, 8-9% Antimony, 3-4% Copper) for bearing face
An independent thrust collar may be case hardened and fitted using a combination of interference fit, longitudinal key and circumferential retaining ring.

Nozzles

Convergent-divergent nozzles

Steam leaving the boiler has high heat energy, low kinetic energy. The amount of heat energy or enthalpy is dependent on the pressure and condition of the steam ( dryness fraction, degree of superheat )
If the pressure is then dropped, then some heat energy must then be released. This heat may be used to perform work or be allowed to manifest itself as an increase in velocity.

Assuming the mass of steam must pass a point at any time, then;

C.S.A is proportional to specific volume of steam/ velocity

At inlet to nozzle the specific volume of the steam is relatively low, and rate of increase is low

Velocity increases at a greater rate

C.S.A is proportional to specific volume/ velocity

Therefore, area required for flow contracts As expansion proceeds, rate of change of specific volume increases to a point where it overtakes the rate of change of velocity and an increase in C.S.A is required

The point immediately prior to this is the min C.S.A and is called the throat.
If the remainder of the path is then kept constant then this nozzle is called convergent and the steam will leave the nozzle with no discontinuity of flow
The amount of steam discharged will depend upon inlet/exhaust pressure ratio.
limit :-

Exhaust pressure = 0.55 inlet pressure ( suphtd )

Exhaust pressure = 0.58 inlet pressure ( sat )

This is called the critical pressure as no drop in exhaust pressure will increase the flow.
If the steam flow enters a pressure less then the critical then the expansion becomes uncontrollable and there is a rapid dissipation of energy, scattering the stem and causing turbulence in the steady flow. If a divergent section is attached then expansion is controlled by gradually increasing the area making the discharge pressure equal to the back pressure.

Steam leaves the nozzle without discontinuity of flow.

Divergent section has an angle of divergence of 8 to 10^o to centre line

Converging section made as short a possible as rapid contraction to damp turblence and help stream line for laminar flow.

Expansion theoretically adiabatic.

Wear, erosion, deposits create turbulence and reconvert some k.e. back to heat energy.

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Nozzle plate and Boxes

The nozzles may be formed by machining of the nozzle plate, or by casting in steel partition plates. Alternately, nozzles may be fabricated of vanadium-molybdenum steel and welded into segments. These may be fitted into the nozzle box which is welded to the turbine casing.

Different nozzle designs

In this instance the T-section nozzle plate is manufactured as a continuous ring, fully stressed relieved, then cut into three sections with gaps to allow for expansion.
The nozzle box is made of a similar material to the nozzle ring and is welded into the casing, followed by stress relieving

The T-Section segments are entered circumferentially into the T-Slots in the nozzle box casting
Copper end seals let into radial recesses in the T-Slot cut down circumferential leakage
Continuous 360' nozzle plate minimises tip leakage over the blades.

Modern Nozzle Plate

Loss of efficiency due to recirculation

Balance holes may be drilled in the blade wheel to reduce the loading caused by this effect. This has the effect of increaseing the amount of recirculation, introduces a stress raiser and increases windage losses. To try to elleviate soem of this the bore is carefully radiused and polished
A certain amount of reaction is put on the blade by casuing a pressure drop across the blade to equal that caused by the eduction effect-approxiamtely 10% ( Degree of reaction = Enthalpy drop over blade/enthalpy drop over stage)

Negative Reaction

The degree of reaction R is defined as the ratio of the heat drop in the moving blades to the sum of the heat drop in the nozzles and the moving blades i.e.

R = h_b/h_n + h_b

The heat drop across the moving blades is manifest as an expansion of the steam during ites passage through the moving blades and thus as increase in steam velocity.
If a compression takes place at the same section along the blade length instead of an expansion thus being equivalent to work done then the term becomes negative, and provided h_n > h_b the expression becomes negative at the section considered.
The actual mechanism where by this occurs is linked to the vortex flow theory.
Simplified this states that because of the oblique angle of the steam flow out of the nozzle the flow path in the gap between the nozzle outlet and moving blade inlet follows a line of flow something like a spiral and that there must be therefore inertial forces set up which cause a variation in steam pressure in the radial direction to the gap.
Where the nozzle height ratio (ratio radial height L of the nozzles to the mean diameter D) is small the effect is limited, but in those stages where the nozzle height ratio is large it has a profound effect on the distribution of heat drop in the nozzles and blades.
Calculation of steam conditions at mean blade height (as be used in the preceding stages) is no longer indicative of flow characteristics.

Shown is a section of nozzle and blade. It is assumed the pressure is sensibly constant in a radial direction i.e. the flow lines are entirely axial in direction relative to the casing. However,there is a pressure gradient in the radial direction in the gap between the nozzles and moving blades so that if the blade profile were calculated on the conditions prevailing at the mean height of the nozzles and blades, based on a pressure drop through the moving blades of P2 - P3, the pressure in the gap near the tip (P2T) would be greater than the mean height inlet pressure (P2) and the pressure near the root (P2R) would be less than the mean height inlet pressure (P2).
If the degree of reaction at the moving blade height were small so that the expansion in the moving blades were small, then P2 would be only slightly greater than P3 and the inlet pressure at the root P2R could in fact be less than P3. This would lead to an apparent increase in pressure through a part of the moving blades or negative reaction. Also, the pressure difference P2T-P3 at the tip could be greater than at the mean height. So the degree of reaction would be positive but larger at the mean height.
Thus, the degree of reaction may increase from negative to positive from root to tip.
In reality, there is not necessarily a flow reversal at the section where negative reactions occur as would expect but simply an over-expansion of the steam at exit from the nozzles.
Such a blade would be highly inefficient, not only due to the high losses associated with negative reaction but also due to shock losses at entry to the moving blades.
Modern designs ensure a degree of positive reaction at the root of every moving blade and design conditions to avoid negative reaction at all other off design conditions.

Taper-Twisting of blades

Reasons for taper-twisting of the final stages of LP turbines
• Due to the change in centrifugal velocity with the increase diameter towards the exhaust end the true vector velocity of the steam varies over the length of the nozzle. The blade must be twisted to ensure the steam enters at the correct angle
• The tip has an aerofoil section to increase the reaction to equalise the flow of steam across it which would otherwise be non-uniform due to the pressure difference between the tip and base due to centrifugal action
• The tapered blade design allows an increased distance between the blade and nozzles. This gives water droplets more time to increase in speed driven by the steam flow.
  In addition the tangential velocity is much greater than that of the axial velocity and hence the rotating disc of steam tends to centrifuge out the water droplets
• When viewed as a cantilever beam the tapered design is ideal from a mechanical point of view to resist bending
• The tip aerofoil section increases the reaction to equalise the flow of steam across the bladewhich would otherwise be non-uniform due to the pressure difference between tip and base caused by the centrifuged steam

Steam flow control

Throttling

Throttling of the manoeuvring valve leads to an unacceptable drop in efficiency This is caused by the constant enthalpy expansion of the steam passing through the partially open valve; this reduces the pressure and increases the superheat of the steam. The reduced pressure means that less energy is available for conversion to work, and the less efficient expansion through the turbine.

Hence, alternatives are provided whereby the mass of steam passing to the engine, and so power produced, can be altered.

Manual selective

For commercial ships the above system is quite satisfactory, it can be seen that there is no control over the main group and hence losses will occur at lower loads.However, for ships which spend the bulk of their time at high loads between ports this is no a problem. Throttle losses are still incurred at loads between the opening and closing of nozzle groups but is reduced by shutting off the nozzles - opening the man v/v fully and controlling load on the boiler pressure.

Selective Nozzle control

This system works by sequentially opening and closing man v/vs to allow steam to pass to the turbine. The spiral groove cut in the wheels does not simply open on valve then the next. Rather by using the different nozzle numbers contained in the group, it can give variations in the number of nozzles in use by opening and closing groups as the wheel rotates in the same direction. This system would not have the controlled man v/v of the system above.
This system, due to inherent unreliability's, does not lend itself to bridge control.

1. All shut
2. 1 open
3. 2 open
4. 3 open
5. 1 + 3 open
6. 2 + 3 open
7. 1 + 2 + 3 open

Sequential control - Bar lift type

This system has found much use on Turbo-alternator generator sets and is similar to Selective Nozzle control, but has much increased reliability. It consists of a series of nozzle groups which are brought into line by the opening of their respective valve. The valves are operated by the lifting of a beam or bar, which is connected via a servo to the governor. Sequential control is gained by the adjusting of the height the bar must lift before contacting with the valve spindle nut; each valve, by adjustment of its spindle nut will start to open at varying bar lift.
Steam flow to the nozzles is at a maximum with little throttling effect.


Some oil is allowed to leak past the servo and pilot valve, this improves the action and gives a cooling effect to these parts which are by necessity located close to the hot parts of the turbine.

All round and partial admission

This refers to the steam flow around the circumference exiting the nozzles and entering the first stage of the turbine.

All round admission- This normally refers to Parsons reaction turbines with no impulse stages.

Steam is led to an inlet belt containing a complete 360^o covering of fixed blades. Power variation is by closing of the manoeuvring valves.

Partial admission- Normally found on impulse turbines or reaction turbines having a curtis wheel first stage.

Due to the low specific volume of the steam at inlet conditions the requisite size of nozzles for full admission would be impracticably small.,p> Hence, the steam enters in sections, those area on the circumference not covered by nozzles are hooded to reduce windage and overheating.
For manoeuvring it is recommended that all the nozzle groups are opened. This reduces the blade loading otherwise all the steam passing through the man v/v would be acting on a small number of blades. Maximum efficiency is achieved with the man v/v full open and hence sets of nozzles are shut off at full away. However some manufactures recommend that all of the nozzles are opened up to reduce the blade passing vibration caused by the flexing of the blades as they pass the steam jet.

Overload

For overload conditions in excess of normal a bypass v/v may be fitted which admits steam a number of stages down from the HP inlet.By introducing the low specific volume steam further down where the nozzle area are greater allows more steam flow. In this condition the main stop is closed and the first few stages idle.
Modern practice however is to leave the man v/v open so a small amount of power is produced over the first stages.