The casing is made of four main parts
Thrust bearing housing
Ford Journal bearing
Ahead casing proper
Ahead exhaust belt
(Astern casing and belt if fitted)
Aft gland housing
Aft Journal bearing
Flexible coupling housing
Ahead Nozzle box-Contains ahead nozzle, subjected to boiler pressure and temperature hence made from cast steel
LP Turbine Casing
To reduce windage losses the astern turbine exhausts in the same direction as the LP turbine. The Astern casing is located by crossed bars that are able to take the torque reaction from the fixed blading. The bar layout also allows for radial expansion as does the steam inlet which is fitted with a sliding coupling
The turbine casing distorts due to the heat differential.
The pressure within the casing distorts casing halves shape to a more cylindrical one, with the high temperature creep results
Hence when the casing cools
The flanges become warped . This can be checked by laying a straight edge across the casing, measuring with a feeler gauge and keeping a log of the results.
No action should be taken unless absolutely necessary.
The casing may leak during warming through as the bolts fail to close the inner faces of the flange. If the leakage stops when the turbine is up to temperature then this is considered satisfactory.
However, if leakage still occurs the some machining must take place. If the leakage is allowed to remain then at high power output damage can ensue.
A temporary repair is with the use of Phurmanite, this is a goo which is pumped into the flange, under pressure through a tapped hole.
The use of shouldered bolts
Long lengths of pipe work should be avoided, as should be tight bends as these can
lead to fluid friction losses in the steam and pressure loss.
Hangers and sweeping curves before inlet to casing should be employed to ensure no weight on casing.
For the cross over pipes, to avoid large curves or frictional losses the following is now employed
The pipes fitted to the casing should have large flexibly supported bends and/or bellows pieces. If not they can give side or top thrusts on the casing and lead to stressing and misalignment.
An alternative to sliding feet as shown is to use elongated holes. The holes being elongated in the direction of required expansion. The bolt is then of the loose fit design.
Care must be taken with all sliding arrangements to ensure freedom of movement. Surfaces should be kept clean, lubricated ( molybdenum disulphide ) and free of rust and paint.
Differing materials may be used for the varying components.
Allowance for expansion over the temperature range in which the turbines operate is essential to reduce thermal stress, mechanical stress and maintain proper tooth contact and blade clearance. This is achieved by securing the turbine at one end and allowing to expand. The free end is normally the hotter end of the turbine where expansion is expected to be greatest
The turbine is allowed to expand in the fore and aft direction by molybdenum disulphide lubricated sliding feet
An alternative mounting is by 'Panting plates'. This design is particularly seen in HP turbines and in Turbo-alternators where there is less weight to support
The turbine is rigidly attached to the gear casing or pedestal. The ford end is allowed to expand. The turbine movement is absorbed by the flexible coupling
Steam enters the HP turbine dry with superheated. As it passes through the stages the degree of superheat falls to a point in the final stages of the LP turbine they dryness factor is less than one and water droplets are entrained.
When the water droplets form they are very small and travel at the same velocity as the steam. As the stream passes through further stages the water droplets fail to keep in the steam stream with the changes in direction and velocity. The droplet size increases and is removed from the steam by centrifugal action and by contact with the blades.
These droplets may impact the leading face of the rotating blades abd lead to erosion and cause a retarding effect. The damage is proportional to the swirl velocity and therefore is 4 times worse at the tip than the root.
The water droplets tend to flow to the tips of the blade and from there passes to the casing or pass on to further stages. The erosion causes pitting, perforations and blade failure.
Damage to the blades may be reduced by brazing or electron beam welding on a stellite strip
However these can be undermined by erosion and be thrown off causing considerable damage.
One method of reducing this problem is by reheating the HP exhaust system by passing through the boiler. This has the added effect of increasing plant efficiency but at increase cost of pipe run. Reducing blade height and therefore speed can lessen effects as can taper twisting the final stages
These are found in impulse turbines to create the requisite number of stages. They locate with a fixed row of blades and are sealed against the rotor shaft by a gland arrangement which must remain effective throughout the working range the diaphragm operates in.
They have a large surface area and so must have sufficient strength to resist pressure drop across them without being excessively wide which would increase rotor length. Allowance must be made for rapid temperature fluctuations found during manoeuvring.
Teh take the form of a disc with a row of blades at the circumference and a hole at the centre for the rotor. A horizontal split allows for disassembly, rotation is prevent by a locking plate at the horizontal casing joint.
Methods of Diaphragm fixing
Steam pressure holds the diaphragm plate hard against the downstream face.
In the high temperature regions typically Molybdenum-vanadium steel all parts. More generally a low carbon steel for the nozzle division plates and spacer bands, mild steel for rest. In the low temperature region cast iron diaphragms may be used. Alternately chromium or Nickel alloy steel may be used
Construction of Diaphragm Nozzles (all riveted attachment)
Diaphragm is a loose fit in the slot in the casing to allow for expansion.
Construction of Diaphragm Nozzles (partial welded attachment)
The nozzle is assembled in batches by pushing the tenon of the blade through the channel hole and riveting. A spacer is fitted and the whole tack welded. The blade batch is caulked into the casing. A small allowance is made for expansion.
Some sections on the first stages may be blanked where partial admission used.
Modern turbine designs have a curtis wheel first stage which absorbs a large portion of the energy in the steam. The exhaust from this stage has a relatively high volume therefore all further stages are full admission.
Construction of Diaphragm Nozzles (welded attachment)
Modern diaphragms are all welded. Nozzle plates or guide vanes fit into slots in the inner and outer rim. The whole is welded to the centre body and peripheral guide ring. Expansion is allowed for in the casing groove. The Nozzle blades or guide vanes are commonly made from stainless iron. The centre body from Chrome Molybdenum steel in higher temperature regions, mild steel for the lower.
Included for general interest
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.
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 10o 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.
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
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
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
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
The high inertia of the spherical carrier reduces the arrangements ability to cope with distortions and imperfections. The key prevents the 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 initial 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. Hence, 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 gauges 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.
The 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 quantities 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 absorption. 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
Obturated manoeuvring valve
Steam leakage past the side of the main valve trim pressurises the top of the valve and holds it tight on to the seat. The seat and valve trim are stellite coated.
A steam strainer if fitted to remove any large particles such as scale, magnetite flakes etc. travelling on to the turbine.
The height 'H' is important and should be limited otherwise when the pilot valve spindle contacts the valve trim and starts to lift it, steam acting underneath the trim will tend to lift it increasing the valve opening quickly.
The conical seat and spherical valve trim shape ensures tightness. The seat is shaped to ensure that there is no velocity increase which is associated with the pressure drop leading to losses. The shape also means at low lift the steam stream is designed to meet in the centre and pass on without contacting the sides hence reducing erosion
The valve operates as follows; The valve is closed with closing force coming from the pilot spindle and the pressure of the steam acting on the top of the valve trim. On open signal to the motor arrangement the pilot valve spindle moves easily opening up the balance chamber to the turbine pipework so releasing the pressure. the spindle travels further to contact the valve trim and hence lift it. The advantage of this system is that the spindle motor does not have to cope with opening the valve against the pressure acting on the back of the valve and hence can be accurately positioned for low lift.
The steam path through the opening valve is designed to give a linear lift/flow characteristic. the stem external to the body often has an arrangement for allowance for thermal expansion. Should the valve be manually over tightened shut or should he arrangement fail then seriously high stresses can be generated in the spindle which can jam.
Line 1: Isentropic expansion through the turbine realising an enthalpy of 'Hs'
Line 2: True expansion through the turbine, through an open manoeuvring valve and realising an enthalpy of 'Hfo'
Line 3: Expansion through a partially open man v/v at constant enthalpy to a lower pressure but higher degree of superheat; the steam is then expanded through the turbine. It can be seen that there is an increased slope due to a drop of efficiency of the expansion through the turbine
The amount of heat that is available to do work is determined by the initial conditions i.e. boiler conditions, and the final conditions i.e. condenser temperature and pressures.
Hence, by varying the flow of steam so can the amount of work produced by the turbines also vary. This is the basis of nozzle control at full power outputs.
However, at reduced loads, even with the additional nozzle groups closed it is necessary to reduce the flow of steam by closing in the man v/v.
Closing the man v/v has other effects other than a reduction in mass flow. With the steam being throttled through the valve in an uncontrolled way and hence with no increase in velocity the steam at lower pressure but containing the same heat energy then exists at a higher degree of superheat (but lower temperature)with a certain amount of reheating due to friction occurring in the turbulent outlet stream.
The expansion through the turbine is now carried out at a lower pressure, with the turbine operating at reduced revs due to the reduction in power developed there is a loss in diagram efficiency for the steam being expanded though the turbine.
It can be clearly seen that throttling through a partially open valve incurs a certain degree of superheat at outlet of the turbine. This can lead to overheating the main condenser due to the high exhaust temperature. However, as the mass of the steam is reduced this can generally be ignored
Steam flow control
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.
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 + 3 open
2 + 3 open
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.
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 360o 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.
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