Marine vol 1 part 8

THE MODERN TURBINEThe design of the marine turbine over the past twenty years has been greatly influenced by economic and competitive factors, requiring reduced fuel consumption, smaller

MARINE ENGINEERING PRACTICE

Volume I

Part 8

by

R COATS, C.Eng., F.I.Mar.E., M.I.Mech.E.,

M.R.I.N.A., M.I.Weld., M.N.E.C.I.E.S.

THE INSTITUTE OF MARINE ENGINEERS

Page

3 Cleanliness of the Machinery and its Connecting

4 Avoidance of Contamination of the Working Fluids 58

It is hoped that the ground covered, and particularly the section on theoperating aspects of the machinery will be of real practical value, and that.the supplementary information given in the abstracts from important papers

on steam turbine machinery will provide a good stimulus for further study

• "The Running and Maintenance of Marine Steam Turbines." In "The Running and

Main-tenance of Marine Machinery," Fifth Edition Marine Media Management Ltd., London.

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1 EVOLUTIONARY CHANGES AND

The evolution of the marine steam turbine over the last twenty yearshas brought about significant changes, not only in physical appearance, butalso in ratio of power to weight, in steam inlet conditions, in efficiency andfuel consumption, in reliability, in the change from manual to remote control,and, very significantly, in the time taken to reach full operating power afterstarting from cold conditions

In general, the principles governing the correct maintenance andoperation of the machinery are unchanged, with differences in emphasis andtime scale arising from increased knowledge and differences in detail designs.These principles are:

1) Cleanliness of the machinery and its connecting pipework;

2) Avoidance of contamination of the working fluids, namely water,steam, fuel and lubricating oil;

3) Adherence to makers' recommendations on type of lubricating oilfor initial fill and make-up purposes, and attention to fine filteringand water removal;

4) Orderly procedures for warming through, start-up, manoeuvring,full away and closing down to avoid distortion;

5) Attention to drainage facilities during critical periods to avoid over of water into the turbines;

carry-6) Avoidance of rust or other corrosion-promoting conditions;

7) Orderly recording and analysis of instrument readings in comparisonwith trials figures; check on power and fuel consumption;

8) Attention to auxiliary machinery to ensure correct movement offluids to and from the engine;

9) Attention to boiler cleanliness and efficient combustion to ensureoptimum overall efficiency and minimum fuel rate;

10) Where automatic controls are incorporated, periodic attention andservicing to ensure reliable operation

It is important to consider these points in more detail, but before doing

so, it is necessary to review th,' nasic design principles and illustrate thepresent state of the art

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2 THE MODERN TURBINE

The design of the marine turbine over the past twenty years has been greatly influenced by economic and competitive factors, requiring reduced fuel consumption, smaller weight to power ratio, higher steam pressures and temperatures, higher rotational speeds and higher peripheral speeds Improvements in blade and nozzle have been made by application of aerodynamic theory and vast amounts of wind tunnel research have advanced the efficiency of impulse turbines to such an extent that the high pressure portion of a machine is almost always of the impulse type The reaction stages are confined to the low pressure end of the machine The boundary between impulse and reaction stages is somewhat blurred nowadays because most impulse blades operate with some degree of reaction, and many reaction stages are made of disc and diaphragm type of construction and look like impulse stages.

There are exceptions, however, such as the Westinghouse design and the Blohm and Voss design.

The major influences on the impulse design have been the higher steam conditions and the importance of reducing leakage effects, and the reduction

in axial length arising from impulse construction.

The most popular designs of turbine in present construction are the Laval, General Electric, Mitsubishi and Kawasaki types The only active British design at the moment is the GEe (formerly AEI/English Electric) type There are, of course, many British and foreign flag ships still sailing with Pametrada turbines.

Stal-Several striking features will be evident in modern turbines when compared with those in "Running and Maintenance of Marine Machinery" There is the more general use of high pressures and temperatures, e.g.

Manufacturer Standard Inlet Conditions

Stal-Laval 62 bar/51Ooe (900 Ib/in 2 g/950°F) General Electric 59 bar/510oe (850 Ib/in 2 g/950°F) General Electric Reheat 100 bar/510oe (1450 Ib/in 2 g/950°F/950°F) Pametrada 59 bar/510oe (850 Ib/in 2 g,950°F) I.H.I Reheat 86 bar, 513°Cj51Ooe

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Several firms have standard reheat designs introduced after Pametradahad publicized their "1000/1000/1000" design The main difference wasthat Pametrada proposed a three cylinder high pressure/intermediatepressure/low pressure (H.P./I.P./L.P.) scheme, whereas General Electricand Stal-Laval used a two cylinder scheme, with H.P and J.P sections onthe same rotor

In addition, there are many improvements in details, such as flexiblecouplings, bearings, casing construction, bearing supports, and greater use

of fabrication and there is the ',vf'rall aflrl""~eral use of impulse type struction

Apart from differences in detail, there are noteworthy differences inarrangements of gearing and condenser machinery It has become fashion-able to adopt the so-called "single plane" arrangement in which all bearingcentre lines lie in the same horizontal plane

In the Stal-Laval arrangement (Fig 1) (Ref 1), use is made of a mixture

of epicyclic and parallel shaft gears The H.P turbine may have a star gearfirst reduction, with a planetary epicyclic second reduction, arranged forwardand aft, respectively, of the final reduction pinion which engages with themain wheel The L.P turbine has a planetary epicyclic first reduction geararranged aft of the final reduction pinion

The principal change which allows the single plane design to be achieved

is the axially directed exhaust, forward from the L.P turbine, direct into theside of the main condenser arranged athwartships The exhaust duct sur-rounds the forward turbine bearing, to which access has to be obtained via

a vertical shaftway This is not a very attractive feature, but has not beenknown to lead to any operating difficulties The ahead exhaust stream passesover the astern casing on its way to the condenser The astern exhaust facesthe same way, thus removing any possibility of the astern steam affecting theahead blading (Fig 2)

The main attraction claimed for the arrangement is the low headroomneeded for its accommodation, which permits the boiler to be arranged overthe turbines, thus leading to a short engine room space (Fig 3)

It will be clear that there is a power limit to the axial exhaust single flowarrangement, which has not yet been reached at 29828 kW (40000 shp) inthe Stal-Laval design Eventually a double flow exhaust will be needed, withdownward flow to an underslung condenser, for higher powers It is ofinterest, however, that Jung (Ref 2) claims that a power of 93000 shp ispossible with reheat using only one flow

General Electric's MST 13 standard (Figs 4 and 5) (Ref 3) is similar inseveral respects to the Stal-Laval arrangement It is of the single plane typewith the L.P turbine exhausting axially to :: >'ld:ate condenser Access tothe forward bearing is via a vertical space between the two halves of the

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6 MARINE ENGINEERING PRACTICE

enveloping turbine exhaust The major difference is in the type of gearing,which is entirely par::lllel shaft type For claimed economy of manufacture,the primary and secondary gearboxes are separate from each other

The higher powered MST 14 standard shown in Fig 6 (Ref 4) reverts toorthodox dual tandem construction for the gearing, and has the advantage

of sharing the power amongst four pinions in the final reduction Thisleads to a smaller diameter for pinions and wheels than is general for theStal-Laval standard The L.P turbine retains the axial exhaust

The even higher powered MST 19 standard range covers powers from

33556 kW-89 484 kW (45000-120000 shp) with a selection from two H.P.turbines and three L.P turbines in both non-reheat and reheat forms It is

of interest to note that all of these have dual tandem gears and that the L.P.turbines exhaust downwards into underslung condensers

The Pametrada standards (Refs 5 and 6) retained the orthodox ment of gearing, with dual tandem above 18643 kW (25000 shp), and in allcases have the L.P turbine exhausting downwards into an underslung

arrange-3.-Stal-Laval advanced propulsion machinery in a tanker.

condenser (Figs 7 and 8) (Ref 6) The major overall dimensions and weightsfor this standard series are given in Tables I and II of Ref 6

Figure 9 shows a 14914 kW (20000 shp) set of machinery from thestandard range-later installed in s.s.British Cmifidence-erected on the teststand at John Brown Engineering Figure 10 shows a 19760 kW (26 500 shp)set with dual tandem gearing in s.s.Ottawa. Figure 11shows the PametradaPrototype 1 machinery on the test bed at Wallsend Research Station Thisoperated at 55 bar (800 Ib/in2 g) and 557°C (1035°F) and completed anextensive series of trials in 1963 Figures 12 and 13 show the H.P turbinefrom this set

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THE MODERN TURBINE 9

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d) Spring backed main gland segments and diaphragm gland segments;e) Solid forged an ld rotor~f •.hllth H.P and L.P rotors;f) Reliance on shrouding and binding wires for inhibition of bladevibration;

g) Compound H.P and L.P astern turbines which allowed acceptance

of full steam temperature for astern operation and gave high asternpower capability (over 60 per cent of the ahead power, at free-routeastern) ;

h) All welded diaphragms in H.P turbines;

i) Brazed segmental nozzles in L.P turbines

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THE MODERN TURBINE 15

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THE MODERN TURBINE 29

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THE MODERN TURBINE 31

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THE MODERN TURBINE 33

All of these features were incorporated in the turbines for Queen Elizabeth 2 (Figs 31 to 35) which have a maximum output of 41013-5 kW

(55000 shp) per shaft at 174 rev/min Although designed for normal operation

at 55·16 bar/510°C (800 Ib/in2 g/950°F), the materials and construction weresuitable for 538°C (1oo0°F), and this was demonstrated on trials

A distinguishing feature of such turbines intended for passenger linersresults from the need to operate efficiently over a wide range of powers andrevolutions This requires a Curtis wheel and a high degree of nozzle controlfor the first stage In the QE 2 there are three groups of nozzles with fourcontrol valves In effect, powers below 22371 kW (30000 shp) are obtained

by throttling on the top group of nozzles, and the remaining range up to41013-5 kW (55000 shp) by the opening progressively of the remaining twovalves (Fig 36)

Figure 37 gives a cross-section through the valve chest for numbers 3and 4 valves, and through the operating pneumatic cylinders The Number 1and Number 2 valves are located on the bulkhead away from the turbine andare operated in the same way, but with the addition of remote mechanicaloperating gear on No.1 valve for use in the event ofloss of pneumatic control.The No.1 valve acts as the main manoeuvring valve

A word of caution is appropriate here about the difficulty of matchingpneumatic operation to the valves performing the exacting duty of nozzlecontrol Forces on the valves can change very rapidly in magnitude and di-rection during the process of opening and closing and the "springy" response

of the air system can lead to rapid hunting or chattering of the valve Apartfrom giving very severe fluctuations in the steam flow, this can lead to serioushammering wear on the internals of the valve Overcoming this is a difficultdesign problem (In this respect, the hydraulically operated pistons as usedfor example in the Stal-Laval control valves (Fig 38) have proved to becompletely steady in operation.)

An example of an earlier Pametrada design is illustrated in Figs 39 and

40 This was designed for an inlet temperature of 510°C (950°F) and a power

of 9321 kW (12500 shp) The H.P turbine is of double casing construction,the outer casing being fabricated and the inner casing being cast The strength

of the assembly lies in the longitudinal beam which incorporates the half outer casing The inner casing is supported from brackets on the outercasing The bearing pedestals are mounted on transverse beams suspended

bottom-by side members from the main beam The H.P astern turbine is at theforward end, and the casing is supported on brackets from the main beam

A flexible diaphragm type seal makes the joint between the main steam inletbranch and the outer casing, and location and centralizing of the separateelements is achieved by means of keys Sixteen single-row impulse wheelsare incorporated and the diaphragms are of the brazed segmental type Theastern wheel is a two-row Curtis stage The ahead steam exhausts from theupper-half outer casing

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THE MODERN TURBINE 37

THE MODERN TURBINE 43

Main glands and diaphragm glands are of the solid type, with the fine fins turned as an integral part of the rotor shaft, which has the advantage of reducing the amount of heat transferred to the rotor surface in the event of a rub.

The L.P turbine is also of double casing construction, the ollter casing being fabricated, and inner casings of cast steel The first five ahead stages are of impulse type, and the last seven are reaction. The L.P astern turbine has two Curtis stages A unique feature is the inward turning deflector attached to the astern casing exhaust, which not only protects the ahead blading from direct impingement of hot astern steam, but also guides the ahead steam clear of the astern blading The exhaust is downwards to an underslung condenser.

It may be of interest to note that, as a result of experience in the Second World War, all of the Pametrada turbines were designed to resist the effects

of accelerations and decelerations arising from und6rwater explosions, a fact that was often overlooked when comparisons were made.

2.3.2 Stal-Laval Turbines

The Stal-Laval Advanced Propulsion series of turbines has tended more towards simplicity of construction It will be noted that both H.P and L.P turbines are single casing castings (Figs 41 and 42) Where bleed belts are required, they are simply welded to the main casting, with large holes drilled through the casting to pass the steam into the bleed belts.

This simplicity has been incorporated into the first stage nozzle belt, which forms part of the main casting, a feature which went out of favour in other designs because of casting and distortion difficulties, but has not been known to cause any trouble in Stal-Laval sets.

The design is of impulse type throughout with a large diameter first stage

or control stage Normally, three groups of nozzles are used, with one group uncontrolled and two under the control of individual local valves This can

be seen in Fig 1 .

The astern turbine stages are entirely located within the L.P turbine casing with the exhaust end pointing forward Thus the ahead exhaust steam passes over the astern casing on its way to the condenser The astern casing

is a separate casting, supported in a cantilever fashion from the end inner bell The astern discs are separate forgings to allow for the excessive thermal gradients which can occur during astern manoeuvres It is the practice to reduce steam temperature for such manoeuvres in this design.

Another distinctive feature of this design is the side entry blades with bulb roots shown in Fig 43, which date back to the very earliest De Laval designs This makes for a strong assembly which reduces blading time for the rotors, but requires very accurate manufacture and special machine tools All blades have integral shrouds with shroud connecting wires rolled into the grooves in a unique, vibration-damping construction.