The running and maintenance of

The importance of this is practical designs the principle is enhanced by specificclear when considering the high heat fluxes found in design features such as drum internals aimed atthe f

The Running and Maintenance of Marine Machinery

The Running and Maintenance of Marine Machinery

Published by The Institute of Marine Engineers

The Memorial Building

Reprinted with corrections 1994

A catalogue record for this publication is available from the British Library.

ISBN 0-907206-42-5

All rights reserved No part of this publication may be reproduced, stored in a retrieval system, or transmitted

in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the publisher.

Printed in the United Kingdom by Unwin Brothers Ltd, Surrey; a member of the Martins Printing Group

Contents

The authors

R Beams SA, IEng, AMIMarE

Mter serving as an Engineer Cadet from 1963 to 1967, Rod Beams worked for various shipping companiesuntil 1979, at which time he was serving as Chief Engineer He then took up a lectureship at the College ofFurther Education, Plymouth, and in 1985 became Senior Lecturer at Maritime Operations Centre, Warsash,Southampton, responsible for the operation and development of the machinery space simulator In 1990 hejoined Haven Automation Ltd, Swansea, as Simulation Systems Manager, where he is responsible forsimulator projects and the development of computer based teaching and training systems, worldwide Heholds a BA(OU) Technology degree in electronics, instrumentation and computer technology

SG Christensen CEng, FIMarE, Extra First Class Engineer DoT, SSe

Stanley G Christensen, Professor Emeritus in the Department of Engineering at the US Merchant MarineAcademy, Kings Point, New York, served at sea as a Chief Engineer in steamships and motor ships He wasthe silver medallist of The Institute of Marine Engineers in 1948, and is a William Nevins' prizewinner Hehas held senior positions in shipowning organizations in the United Kingdom and the United States ofAmerica, as a company board member, technical director, chief superintendent, and senior superintendentengineer, and is now a consulting marine engineer

J Cowley CSE, SSC, PhD, FEng, HonFIMarE, FIMeehE, Extra First Class Engineer DoT

Dr Cowley, Past President of The Institute of Marine Engineers, and now an Honorary Fellow, was SurveyorGeneral in the Department of Transport from August 1981 to May 1988 He has served as a visiting Professorand a member of the Board of Governors of the World Maritime University He was awarded the Denny GoldMedal in 1982, and the IMO International Maritime Prize in 1988

P Durham SA, CEng, FIMarE

Mter serving a mechanical apprenticeship with ICI, Mr P Durham joined The British India Steam Navigation

Co as Junior Engineer, subsequently obtaining a First Class Combined Certificate of Competency Mterserving with Sir William Reardon Smi th' s Steam Navigation Co as Chief Engineer, he was appointed Lecturer

in Marine Engineering by the, then, Llandaff College of Technology He graduated through the OpenUniversity in the early 1980s and at the time of writing was Head of the Marine Engineering section of theSchool of Maritime Studies (Wales)

A W Finney SSe(Eng), CEng, MIMeehE

Mter obtaining a degree in Mechanical Engineering from Imperial College, University of London, Mr Finneyserved a graduate apprenticeship at the Fraser and Chalmers turbine works of the General Electric Company.Training included analogue computing and experience with early digital computers Following a period inthe drawing office of the Admiralty and Special Projects Division at GEC Mr Finney transferred to theDevelopment Laboratory eventually becoming Deputy Manager In 1980 Mr Finney moved to Lloyd'sRegister of Shipping, where he is currently Senior Surveyor in the Control Engineering Department

A Hodgkin CEng, MIMarE, AIED

After a shipwright apprenticeship at H.M Dockyard Chatham, Alan Hodgkin joined Babcock and Wilcox as

a project draughtsman, becoming marine project engineer and then section leader He was appointed ChiefMarine Project Engineer in 1966, and Chief Design Engineer, Industrial and Marine Division in 1980 Heretired in 1987 after 40 years service

ix

x The AUTHORS

D G Nicholas SSe, FIMarE, MIMeehE

After a graduate apprenticeship at the English Electric stearn turbine factory at Rugby, and various otherappointments, he became Chief Designer for the Industrial Steam Turbine Division in 1964, and EngineeringManager of the Industrial and Marine Steam Turbine Division formed in 1969, when GEC took over EnglishElectric He was then responsible for a range of main propulsion machinery and turbo-generators suppliedfor VLCCs, container ships, fast ferries and naval vessels He continued as Manager of the Naval Department,and then Deputy General Manager of the Medium Turbine Division, before retiring in 1991

D G Redpath MSc, CEng, FIMarE

After serving a Marine Engineering apprenticeship with Texaco Tankers, Mr Redpath obtained OND inMechanical Engineering at Stow College, Glasgow, and Second and First Gass Certificates of Competency(Steam), and rose to the rank of Second Engineer He then served as Chief and Second Engineer with BritishRail and Bums Laird, and obtained his Motor endorsement.After working as an Engineer with NorthernIreland Electricity Authority, and gaining an HNC in Naval Architecture, he joined Lloyd's Register ofShipping as Ship and Engineer Surveyor In 1978 he became Senior Lecturer in Marine Engineering at UlsterPolytechnic, and is currently Lecturer in the Department of Engineering, University of Ulster

A C Stera MSc, MlnstR

After graduating from Warsaw Technical University,and specialising in refrigeration, MrStera was awarded

a Master's Degree in Mechanical Engineering in 1960 He then joined Blue Star Line Ltd as a seagoingengineer, and sailed on 22 reefer vessels, working as Assistant Engineer, through Second RefrigerationEngineer to Chief Engineer in 1964 He joined Lloyd's Register of Shipping in 1970, where his brief centred

on the classification of newly built reefers, container ships and containers He carried out a number ofinvestigations, machinery and insulation performance measurements at sea, the results of which have, to alarge extent, been implemented in perfecting the refrigeration installation on new vessels Following a shortsojourn in Kuwait, where he looked after the refrigeration interest of Lloyd's Register in the Middle East, hewas appointed to his present position as Principal Surveyor and Manager of the Refrigeration Department

at Lloyd's Register headquarters Mr Stera is the President of the International Institute of RefrigerationCommission D2/3, dealing with refrigerated transport

F Taylor SSe, PGCE, CEng, MIMarE, AMIEE

Fred Taylor joined Shell Tankers in 1964 as Marine Engineer Cadet, and graduated from NewcastlePolytechnic in 1971 with a degree in Electrical Engineering, sponsored by C A Parsons On graduation hetransferred to Transformer and Generator Instrumentation as an Applications Engineer In 1972 he wasappointed Lecturer at South Tyneside College (then South Shields Marine and Technical College), teachingelectrical subjects, and obtained his PGCE from Huddersfield Polytechnic in 1976 He is currently SeniorLecturer, mainly involved with Marine Engineering Certificates of Competency and specialist electricalcourses for the marine and offshore industries

JTempleton ARCST (Hons), MSe, CEng, MIMeehE

James Templeton studied Mechanical Engineering at The Royal College of Science and Technology, Glasgow,followed by a year at the School of Thermodynamics, University of Birmingham In 1961 he took up a postwith Torry Research Station, MAFF, working on the development of ship board plant for chilling, freezingand refrigerated storage of fish He joined Christian Salvesen (Seafoods) Limited in 1970 with responsibilitiesfor the specifications and installation of fish freezing and refrigerated storage plant From 1981 to 1990 heworked on development projects in a number of developing countries as a management, refrigeration andtraining consultant In 1990 he joined Lloyd's Register of Shipping, working in the refrigeration department

on the appraisal and classification of marine refrigeration installations for refrigerated cargo vessels, liquefiedgas tankers and refrigerated containers

R F Thomas CEng, FIMarE, Extra First Class Engineer DoT

Robert Thomas joined BP Tanker Co Ltd in 1958 as an Engineer Cadet under the alternati ve training scheme

On completion of his cadetship he served as an engineer in the fleet before taking up a shore appointment with

BP Shipping in 1971 In 1981 he was awarded the Denny Gold Medal for his paper 'Development of MarineFuel Standards' After serving in various parts of the BP Group, he joined DNV Petroleum Services asTechnical Coordinator in 1992

The editor, authors and publisher gratefully acknowledge the help, information and drawings supplied

by the following companies, journals and publishers

Aalborg Ciserv International A/S New Sulzer Diesel Ltd

Babcock and Wilcox Co Sabroe Refrigeration A/S

British Maritime Technology Siemens plc

Brush Electrical Machines Ltd South Tyneside College

Diamond Power Specialty Ltd Telemechanique

GEC ALSTHOM Turbine Generators Ltd Weir Pumps Ltd

Hagglunds Denison Ltd Whipp &Bourne Ltd

Hamworthy Combustion Systems Ltd Woodward Governor Company

Harland &Wolff plc

Haven Automation Ltd

Hitachi Zosen Corporation

HMSO

Ishikawajima-Harima Heavy Industries Co Ltd

JKaMeWa ABM Voith GmbH Reference is made to the following British Standards,Kawasaki Heavy Industries (UK) Ltd and extracts are reproduced ~ith the permission ofLloyd's Register of Shipping BSIStandards Complete copIes of the standards canMacawber Engineering Ltd be obtained by post from BSIPublications

MAN B&W Diesel AG BS MAlOO;BS 1170; BS 1427; BS 1523; BS 2690; BSMcDermott International, inc 2917;BS3939;BS4099;BS4941;BS5345;and BS5750

Chapter 1

Marine Boilers

A Hodgkin

INTRODUCTION in the upper end of the output range would be found

in central power stations ashore Steam pressure inBoilers of varied design and working conditions are water tube boilers can vary between 7 bar andinstalled in both steam and motor vessels The most supercritical values such as 225 bar, although natu-modern steamships have boiler plant of a ralcirculationwouldonlybeapplicabletopressuressophisticated nature, and even on motorships the below about 175 bar Steam temperature could rangesteam plant can be quite extensive, providing useful from saturation to 600-650°C, depending upon theservices and enhancing the overall efficiency of the fuel and method of firing With this vast range ofvessel duties it is not surprising that the shape and detail ofThe demand for steam propulsion is currently water tube boilers should vary considerably Al-very low, being confined to specialised ships such as though the marine sphere is only a particular sectionliquid natural gas (LNG) carriers However, a number of the whole range, the number of different boiler

of steamships may still be found in service having designs available is large

boiler plant resulting from many years of develop- As with most engineering endeavour, marinement Design modifications have been made to elimi- boiler design is a compromise A balance must benate problem areas and to adjust to changing opera- sought between first cost, longevity, running costtional constraints in much the same way that the and maintenance First cost can always be reduced atdiesel engine has progressed to its present advanced the expense of the other factors by adopting mini-state Some of the incentives for and results of this mum construction standards and high forcing rates.development are touched upon in this chapter A proper compromise in any particular case de-Water tube marine boilers have been dominant, as pends upon the operating profile of the vessel For afar as steam propulsion is concerned, since the period warship, construction standards are high because ofbetween the two world wars Even the generation of such factors as shock loads which have to be with-steam for auxiliary purposes aboard ship has come stood Forcing rates are also high to enable overallinto the province of the water tube boiler, a practice bulk and weight to be kept low Reasonable longev-which grew to prominence with increasing demand ity and maintenance levels can be expected as timefor large quantities of auxiliary steam and which spent at maximum load may not be much more thanpersists today in ships such as the large motor tanker 5% of the life of the plant Running cost is not nor-Nevertheless in the field of auxiliary steam produc- mally an overriding factor For merchant ships ation many non-water tube boilers can still be found, good compromise is achieved by building to classifi-especially where steam output and pressure are not cation requirements and adopting the low forcing

Water tube boilers can be made for steam duties as levels of the other factors to be obtained Even so,low as 1.5 ton/handashighas2.5 x 103ton/h Atthe there are distinctions to be observed, such as be-lower end of the range, the water tube boiler is found tween main propulsion and auxiliary boilers Aux-

to be uneconomical and would only be considered iliary boilers, receiving possibly much less use thanfor very specialised applications where very high main propulsion boilers, may usefully employ highersteam pressure was involved Boilers having duties forcing rates

1

2 The RUNNING and MAINTENANCE of MARINE MACHINERY

WATER TU BE BOILERS preferential path through the less fouled area, locally

increasing heat transfer in this zone and elevatingThe major designers of marine water tube boilers are tube temperature as a result This too can lead toFoster Wheeler (USA, UK), Babcock (USA, UK, Ger- eventual tube failure Further external fouling meansmany), Combustion Engineering (USA) and Kawasaki that the products of combustion leave the boiler at aHeavy Industries (Japan) higher temperature, reducing efficiency, wasting fuelAll of the above have extensive intemational licen- and imposing a fire risk

see networks so that boilers to one basic design can be The object of the circulation system is to provide amanufactured in many different places Although good supply of water to all of the heated tubes in amarine boilers have been, and can still be, offered water tube boiler Heat transferred through the tubewith forced or assisted circulation, present day walls produces steam bubbles in the water within.practice is for these designers to offer main propul- Tubes in high heat transfer zones will contain moresion boilers based upon natural circulation Forced steam than tubes in lower heat zones so the density ofcirculation units will, however, be found in many steam/water mixture will be lower in the formerexhaust gas heat recovery boilers used onmotorships than in the latter If these separate zones are con-Some of these, and many auxiliary boiler designs, are nected top and bottom by collecting vessels, such asoffered by companies other than these four, but for drums or headers, then circuits are formed in whichmain propulsion, they are dominant the different densities will cause flow to occur-From an operational point of view it is essential upwards in the low density tubes, downwards in thethat the boiler be kept clean This is particularly true others The greater the difference in density, the

on the water sides as overheating, and subsequent brisker the flow will tend to be This is the essence offailure, is only prevented by a good supply of water a natural or gravity circulating system (Fig 1) and inboiling within a clean tube The importance of this is practical designs the principle is enhanced by specificclear when considering the high heat fluxes found in design features such as drum internals aimed atthe furnace zone, where a deposit scale 0.6 mm thick preventing steam inclusion into downflowing tubescan elevate the tube temperature some 215°C above

what it would be were the tube clean This is because

the scale has a very high resistance to heat flow,

requiring a large temperature difference to pass the

heat flow incident upon the tube Such an increase in

tube temperature can bring the tube material into the

range where oxidation occurs, leading to eventual

tube failure

In early designs the need for internal cleanliness

was recognised and catered for by making provision

to simplify cleaning operations with the mechanical

means then in vogue This meant using straighttubes,

or tubes with a minimum number of easy bends, to

allow passage of tube cleaning brushes, and the

provision of access to the ends of each As a result the

boiler pressure parts were perforated with numerous

access openings each of which had to have a pressure

tight closure when the boiler was operational The

making and keeping tight all of these fittings was to

prove the downfall of the straight tube boiler and

encouraged the acceptance of a greater degree of

welding in boiler pressure parts and the adoption of

chemical cleaning

External cleanliness is important, not only because

of the risk of corrosion associated with the presence

of fire side deposits, but also due to the risk of

differential fouling In a superheater, for example, if

some parts become ~ore ~ouled than others the Figure 1 Simple natural circulation circuit (diagrammatic) including

products of combustion wIll be forced to take a primary steam separator in drum.

(to obtain maximum density) or by arranging all

downflow tubes to be unheated (for the same

rea-son)

A boiler will be divided into many such circuits

with varying heat absorption rates The flow in each

is established at the heat absorption corresponding to

maximum load when a total balance flow condition

exists It is normally sufficient to make such

calcula-tions at maximum load but further analysis may be

required if the boiler is to operate at more than one

pressure level and, in the case of a warship,

investi-gation may be needed for extended operation in a

heeled damage condition when the circulating head

is reduced due to the inclination The work involved

in analysing the many circuits which make up a

modem marine boiler is tedious and time consuming

and is best achieved with the aid of a computer

BOILER TYPES

The three main classes or types of water tube boiler in

use at sea today are bi-drum convection bank boilers,

bi-drum radiant boilers and single drum radiant

boilers

Bi-drumconvection bank boilers are developments

from the integral furnace boilers introduced in the

USA during 1939-45, which were characterised by

partially water cooled and highly rated furnace zones

followed by a convection superheaterreceiving some

radiant heat from the furnace through a screen of

generating tubes and completed by a further

sub-stantial bank of smaller bore generating tubes These

units were designed to fit into the small spaces

available in the ships of the period, having limited

headroom Steam conditions were modest at around

30 bar, 400"C at the superheater outlet At these

pressure levels a large amount of latent heat has to be

provided when generating steam With the advent of

larger ships, particularly VLCCs, and advancing

steam conditions up to around 60 bar, 510°C at the

superheater outlet, it was possible to consider an

alternative design basis characterised by a large,

moderately rated furnace, fully water cooled, and

followed by a convection superheater receiving no

direct furnace radiation At these higher steam

condi-tions the amount of latent heat added is much re- difference between saturation temperature and thatduced and, in combination with the large water cooled of the inlet water, and it mayor may not generate afurnace, a steaming economiser behind the super- small amount of steam in service

heater provides adequate generating surface Figure The early versions of the bi-drum boiler were an

2 shows how the distribution of heat has changed, important advance in their time but changes in allowing elimination of the generating bank A ing methods on crude from various sources pro-steaming economiser is defined as one where the duced residual type fuel oils which began to revealwatertemperaturerisewithi:nismorethan6O%ofthe their shortcomings The furnaces, being small and

refin-4 The RUNNING and MAINTENANCE of MARINE MACHINERY

employing large amounts of refractory, operated at

very high temperature Flame impingement was not

unknown and conditions generally for the

refracto-ries were severe and resulted in high maintenance

Refractories broke down requiring replacement They

were frequently covered in glass-like deposits, and on

the furnace floor especially thick vitreous

accumula-tions often required the use of road drills for removal

In the superheater zone the products of

combus-tion were still at high temperature and deposits from

impurities in the fuel condensed out on the tubes,

reducing heat transfer and steam temperature

Even-tually, gas passages between the tubes would

be-come so badly blocked that the forced draught fans

would be unable to supply sufficient air to the

burn-ers, combustion became impaired and the fouling

conditions accelerated Sodium and vanadium

com-pounds present in the deposits proved very corrosive

to superheater tubes causing frequent repeated

fail-ure Due to the fouled conditions there was a loss of

efficiency and expensive time consuming cleaning

routines were required

There were many palliative steps introduced

be-tween that time and the early 1960s when the first

marine radiant boiler was designed Varying degrees

of success were achieved by increasing the

propor-tion of furnace wall cooling using stud tubes or

tangent tubes (Fig 3) and by artifices such as wider

superheater tube spacing or by removing the whole

superheater to a more protected zone at lower

tem-perature It was, however, the impetus provided by

the bulk transportation of crude oil that concentrated

minds sufficiently to attack all of the problem areas of

the past and to introduce features such as all welded

gas tight membrane tube or monowall furnace

enclo-sures (Fig 3) leading to boiler types which have

generally proved successful in achieving high

effi-ciency with much reduced levels of maintenance, Figure3 Watercooledfumace wall construction: a) stud tube; b)

namely the radiant boiler described in its various tangent tube; c)membrane tube panel (monowall).

guises in the following pages

heated downcomer tubes The front wall and floor of

V-tubes of the superheater are connected to verhcal

D type boiler inlet and outlet headers Baffles are fitted inside theThis is an early bi-drum design in which the two headers, requiring the steam to make several passesdrums are connected by a multi-row bank of small through the tubes, thus achieving the high steambore generating tubes, and three rows of larger bore velocity necessary to ensure safe tube metal tempera-screen tubes in front of a V-loop superheater (Fig 4) ture in service Oil burners are fitted in the refractoryThe furnace side wall tubes extend upwards from a front wall of the furnace and, on leaving the boiler,header at floor level, turn over to form the furnace combustion gases pass over further heat recoveryroof and are connected to the steam drum The fur- surfaces such as economiser (heating feed water) ornace rear wall is water cooled and the lower headers air heater (heatingcombustionair).5teamsootblowers

of this and the side wall are fed with water from the are fitted to give means of on load cleaning of boiler,lower drum The two drums are connected by un- superheater and further heat recovery tubes

Chaprer1MARWEBO~ERS 5

Figure 5 Foster Wheeler ESD I type boiler: a) sectional view; b) superheater and attemperator arrangement.

6 The RUNNING and MAINTENANCE of MARINE MACHINERY

ESD I and ESD II type boilers water cooling and burners mounted in the furnace

In an attempt to combat the problems experienced roof This increase in radiant surface reduced thewith the early 'D' types, Foster Wheeler introduced need for a large generating tube bank which, in thisthe External Superheater D type in which the basic design, reduced to eight rows in staggered forma-construction methods remained as for the D type but tion, formed from the lowest metre or so of the fourthe superheater was removed to a position behind rows of tubes separating the furnace from the su-the generating tube bank which was reduced in depth perheater The superheater was further enlarged,This resulted in a reduced steam generating surface, permitting wide gaps between the tubes Steam

an increased superheater surface and an increase in temperature control, now used because of more heat recovery surface beyond the boiler Finding vanced steam conditions, was achieved by use of aitself in a cooler gas temperature zone compared to steam-boiler water heat exchanger located in thethe D type, the superheater exhibited a much greater upper drum (Fig 7)

ad-rate of change of steam temperature with load and for Refractory was still not eliminated, but was largelythis reason steam temperature control was adopted, shielded from direct radiation by close pitched fur-even though design final steam temperature was nace wall tubes Many expanded joints also remained.only 450°C In the mark I version (Fig 5) steam tem- The superheater tubes, being arranged parallel to theperature control was by means of a steam-combus- drum axis, tended to be long, requiring intermediatetion air heat exchanger and in the mark II by damper support along their length, and this proved to becontrol of gas flow over the superheater (Fig 6) troublesome in service Further steps were taken to

address these matters and an improved version of the

ESD 11/type ESD III (Fig 8) used gas tight, all welded monowallsTheESD I and II designs still contained a good dealof in place of refractory lined casings behind tangentrefractory material in the furnace zone and very tubes for the furnace, and extended monowall con-many expanded tube joints and gaskets It was seen struction to the superheater pass The number ofthat maintenance could be reduced if these were rows of tubes between furnace and superheater wasreduced in extent or eliminated In the ESD III the reduced from four to two and the superheater was nowfurnace was much enlarged and the bi-drum radiant aligned at right angles to the drum axis, the resultingapproach appeared with the adoption of complete shorter tubes not needing intermediate support

mono-wall construction.

8 The RUNNING and MAINTENANCE of MARINE

MACHINERY-ESD IV type located relative to adjacent boiler tubes It was With final stage development of the ESD series we ther claimed that the propensity for deposits to formarrive at the single drum radiant boiler with com- would be reduced on vertical tubes and any that didplete monowall enclosure and monowall division would be more readily removed Ample access aroundbetweenfumace and superheater This further halves the superheaters was provided for this purpose Athe number of tubes between furnace and super- conventional generating bank of small bore tubesheater so that the lower ends of all the tubes forming was provided, with external unheated downcomers,the side walls and the division wall can now be and additional, external feeders supplied water fromaccommodated in a header with all welded connec- the lower drum to the bottom headers of the watertions Both refractory and steel casings are eliminated wall circuits (Fig 10)

fur-and the steaming economiser appears to compensate

for loss of generating surface elsewhere (Fig 9) ESRD type

Achieving the maximum efficiency from steam plant

DSD type at sea requires the adoption of the reheat cycle and for

To cater for those shipowners who stated a prefer- this a special boiler type is needed In the reheat cycleence for two drum boilers of more conventional steam, after passing through the superheater and HPdesign, the DSD (double superheater D type) offered turbine, is taken back to the boiler and reheatedseveral advantages over the D type or even the ESD before returning to the intermediate and low pres-I-III A fully water cooled monowall enclosure sys- sure stages of the turbine At sea, this is the sequencetern could be used with burners mounted in the followed when in the ahead mode, but when ma-furnace roof giving good distribution of hot products noeuvring astern or when steaming in harbour, re-

of combustion to the vertically aligned superheater heated steam is not required Under these conditionstubes The primary and secondary superheater sec- the reheater tubes will not receive a cooling flow oftions were behind a three row furnace exit screen and steam and so other means of protection are required.were virtually self supporting, needing only to be The ESRDisconstructed in a manner similar to the

Chapter 1MARINE BOILERS 9

Figure 10 Foster Wheeler DSD type boiler.

ESD IV exceptthatthe convection passage containing ture is controlled interstage by the use of a the superheaters is divided into two parallel paths by boiler water heat exchanger in the boiler drum

steam-a further mono wsteam-all (Fig 11).Superhesteam-ater surfsteam-aces steam-are

deployed in both paths but reheater surface is in-. B ba caek

stalled In one path only The gas flow over the two

paths is controlled by dampers at the exit from each Integral furnace type

path, so that the gas flow to the reheater, and thereby This is essentially similar to the Foster Wheeler Dthe reheat steam temperature, can be controlled type, the initial design of both being in the USA.(Fig 12) In astern or harbour operation the dampers Differences between them are confined to detail andabove the reheater path are closed Cooling air is to specific proprietary features For example, Babcockadmitted to the space between the top of the reheater boilers of that time used studded tubes in the furnaceand the closed dampers, and passes downwards over walls with the spaces between studs and betweenthe reheater, joining the combustion gases which adjacent tubes packed with plastic chrome ore Thishave crossed part of the superheater beneath the was an excellent refractory material but lacked me-reheater, and exiting through a small permanent chanical strength and so 12 rom round studs of vary-opening in the division wall It joins combustion ing length were electric resistance welded to thegases there, flowing upwards in the parallel path furnace wall tubes to reinforce and support the re-across economiser tube surfaces and out to further fractory This proved a very durable construction butheat recovery equipment, used to ensure a high boiler in time it became difficult to obtain spares worldwideefficiency at all times Superheated steam tempera- wherever ships called in for repairs Eventually bare

Chapter 1 MARINE BOILERS 11tubes on a tangent pitch were used, as in the D type sometimes Babcock burners were used When fittedAll Babcock boilers incorporate patent steam sepa- with economisers, those parts exposed to feed waterrating cyclones in the steam drum through which the at temperatures above about 1400Cwould, on Babcocksteam/ water mixture from the heated tubes is caused boilers, be of Babcock design This would be of mild

to pass Inside the cyclones a vortex is formed creat- steel construction, the tubes having oval section studs,ing a significant separating force causing steam free electric resistance welded on This design was origi-water to exit at the bottom and dry steam to leave at nally used by the US Navy during the war, when itthe top These, together with a conventional slotted was difficult to obtain tubes with aluminium fins.dry pipe, ensured dry steam to the superheater and FosterWheelerhadaneconomiserwithCIgillsshrunksteam free water to the downcomer tubes As already onto mild steel tubes and a similar arrangement wasobserved the latter ensures a brisk circulation whilst also used by Babcock for economisers where thethe former was found to be effective over a very wide water temperature was below 140°C

range of drum water level and practically eliminated

all risk of scale build up inside the primary super- Selectable superheat and M10 types

heater tubes To assist separation of steam and water Following its successful use in frigates for the Royal

in Foster Wheeler boilers, arrangements of perforated Navy, Babcock introduced into merchant service theplates were used, although on occasion a form of cy- selectable superheat boiler which was similar to theclone was adopted with a horizontal axis as opposed to integral furnace type except that the convection passthe vertical arrangement used by Babcock(Fig 13) was divided into two parallel paths by means of a

It was normal on Babcock boilers to find combus- wall of studded tubes and plastic chrome ore Thetion equipment of Babcock design and a wide range superheater was arranged on one side only of this gaswas available Foster Wheeler boilers were fitted tight division Dampers at the outlet of each pathwith equipment from other burner makers, and enabled the gas flow over the superheater to be

Figure 13 Cut-away view of Babcock marine boiler, integral furnace type.

12 The RUNNING and MAINTENANCE of MARINE MACHINERY

controlled The range of control obtained in this way enabling the oil burners to be attached nonnal to thewas wide, and admirably suited the requirements of roof and yet fire down the long vertical axis of thethe Royal Navy In merchant service less control furnace These units were, with the use of the steamrange was permissible, and so some of this was atomising burners, able to achieve complete combus-sacrificed in an attempt to overcome operating diffi- tion within the furnace with as little as 3% excess ofculties which the selectable superheat boiler (Fig 14) air, and an efficiency in excess of 90.7% on the grossshared with theintegralfurnace design Byarranging calorific value was recorded when the units werethat the division wall did not start until after the first fitted with rotary regenerative air heaters, reducing thefour rows of superheater tubes these could then pass temperature of the funnel gases to 116°C In theover the whole depth of the boiler, the additional convection passage the widely spaced superheatersurface so obtained permitting a wider tube spacing tubes were aligned at right angles to the drum axis so

in this sensitive, high temperature zone This varia- thatthe products of combustion produced by the rowtion was marketed as the MlO type of burners in the furnace roof were evenly distributed

across the whole width of the superheater This

en-Babcock MR type courages effective use of the heating surface andThe MR boiler was introduced in response to marine minimises risk of hot spots due to maldistribution,industry demands for boilers to exhibit the highest which could adversely affect tube temperature Thepossible efficiency and the lowest possible mainte- lowest possible superheater tube temperature wasnance It is a single drum radiant boiler of all welded further encouraged by arranging the primary super-construction in which all exposed refractory and all heater, containing the cooler steam, below the second-expanded joints and gaskets were eliminated The ary, both being connected so that the steam progressesmembrane tube panel enclosure walls, in which ad- upward in parallel flow with the products of combus-jacent 63 rom od tubes were joined by welding in a 12 tion Interstage attemperation and control of superheatrom wide mild steel strip, provided water cooling is achieved by a steam-boiler water heat exchanger inand gas tightness The large fully water cooled fur- the drum This single drum radiant boiler has a drumnace had a roof sloping at 5 deg to the horizontal, diameter of at least 1.5m and it was possible to accom-

Figure 14 Cut-away view of Babcock marine boiler, selectable superheat type, showing a single furnace and two sets of dampers

for adjusting the gas flow through the superheated and saturated sections of the boiler.

odate, in addition to the attemperator, a desuperheater achieved by interstage attemperation (Fig 16).for the provision of steam for auxiliary purposes

In order to simplify construction and to introduce a

M12 type degree of standardisation, the M12 was replaced by

As already mentioned, preference for two drum boil- the M21 type, a bi-drum unit giving a choice ofers was sometimes stated and there were ships that features such as:

did not provide space, notably headroom, for the A single superheater;

radiant boilers then being used extensively in VLCCs B double superheater;

and container ships It was to meet such situations C tangent tubes, double casings;

that Babcock offered the M12, a bi-drum unit with D membrane tube panel enclosures;

primary and secondary superheaters and a fully E roof mounted burners;

water cooled furnace similar to the Foster Wheeler F front wall mounted burners;

DSD type A fully water cooled furnace with mem- and these could be combined ACE, or ACF, orbrane wall tube panels or tangent tubes backed with ADE, or ADF, or BCE,or BCF,or BDE,or BDF.Eachrefractory and steel casings could be chosen, and the of these eight alternatives could be met with the sameburners could be mounted on the roof or in the basic layout of the main boiler parts with the samefurnace front wall The double superheater was ar- overall dimensions simplifying drawing and order-ranged with the primary upstream of the secondary ing requirements (Fig 17)

in the furnace exit gas stream, each being arranged

with multiple steam passes with the hottest pass in MRR type

parallel flow Ample gas side access spaces were In 1963a power plant design study instigated by theprovided and steam temperature control was Esso Petroleum Company produced a set of marine

propulsion machinery based upon the reheat cycle top, or gas outlet end of each path Primary andand which incorporated many novel features aimed secondary superheater surfaces are arranged in each

at combining high efficiency and low maintenance path The reheater is above them in one path and inOut of this work came the Babcock MRR reheat boiler the other is an economiser wi th bare tubes connectedfrom which the straight cycle MR was soon to follow so that any steam generated rises upwards with theThe MRR is similar in construction to the MR except water flow into the steam drum to be separated in thethat the convection passage is divided into two par- cyclones The division wall is completely gas tightallel paths by a membrane tube wall, the gas flow and the superheater surfaces are so proportionedover which is controlled by dampers located at the that when the reheater path dampers are closed the

small gas flow leaking through them is cooled by thesuperheater to a temperature well below the normalreheater tube operating temperature so that no dam-age to the reheater can occur when reheat steam is notflowing In normal, ahead steaming, modulation ofthe dampers controls reheat steam temperature with-out significant disturbance in main steam tempera-ture, which is, in any case, controlled by anattemperator inside the boiler drum (Fig 18)

It will be noted that a degree of similarity existsbetween the various designs of the two major Britishboiler designers and this will be seen to further applywhen reviewing the products of the other interna-tional majors Differences in detail do occur and arethe result of individual designers' attempts to over-come operational difficulties in a continuous battle tored uce the need for maintenance, improve efficiencyand increase competitiveness A significant step inthis direction was taken during the latter part of the1970s,whenStal Laval, in co-operation with Babcock,developed a very advanced propulsion system (VAP).Steam was generated at 125bar or higher dependingupon the shaft power of the set and at a temperature

Chapter 1MARINE BOILERS 15

of 500°C by a standard MR boiler and then raised to6QOOC in a separate superheater immersed in an oil

fired fluidised bed of graded sand Mter expanding

through the HP turbine the steam was to be reheated

to 6O(}°C in a second oil fired fluidised bed built inbattery with the first and then returned to the IP and

LP turbines The combustion environment of thefluidised bed was intended to permit the achieve-ment of 6O(}°C or even higher without the problemsafflicting conventional superheater and a full scaleexperimental fluidised superheater operated by StalLaval at Orebro in Sweden proved this to be so Theturbine and gearing developments were also demon-strated to the technical press By the time this was allready for the market the diesel designers had forgedfurther ahead and the demand for steam ships was indecline so that no VAP plant entered sea service

Combustion Engineering

V2M-8

The V2M-8 is a bi-drum boiler of the integral furnacetype with a vertical superheater, with all weldedfurnace walls or with tangent tubes backed withrefractory lined steel casings Advantages claimed bythe manufacturers include: the superheater is posi-tively drained at all times regardless of the attitude ofthe ship; slag accumulation on the superheater tubes

is minimised; and the general layout of the unit issuch as to avoid pockets where explosive gas mix-tures could accumulate, thereby ensuring effectivepurging prior to lighting up Provision can be madefor firing in the roof, front or side of the furnace(Fig 19)

16 The RUNNING and MAINTENANCE of MARINE MACHINERY

V2M-9

As boiler plant in general began to demonstrate

improved reliability shipowners showed increased

interest in the single main boiler ship philosophy A

single boiler, used in place of two boilers, would

require less space, but could still have the same

capacity It could have a very large furnace so as to

give a greater residence time affording the

opportu-nity for improved combustion compared to two

smaller units Better access for maintenance would be

more easily obtained and initial cost would be

re-duced The radiant boilers previously described all

exhibited these advantages and Combustion

Engi-neering responded by taking a basic D type boiler

and extending the furnace downwards and beneath

the unit This layout necessitated supporting the

boiler unit at its mid height so reducing movement of

the upper and lower extremities due to thermal

ex-pansion Stability when mounted in the moving

plat-form of a ship at sea was also improved A double

superheater and welded furnace walls were

em-ployed and the firing platform was beneath the lower

boiler drum

A modification employed a tangential firing

sys-tem, with burners mounted in each of the four

cor-ners aligned tangential to a circle at the furnace

centre This gave increased turbulence and a longer

spiral flame path before the products of combustion

impinged upon relatively cool boiler and superheater

tubes (Fig 20)

V2M-8-LTG

The boiler and superheater are as for the V2M-8 but

an additional furnace chamber is added on the side of

the boiler generating bank remote from the main

furnace and superheater This additional reheat

fur-nace is provided with oil burners and the horizontal

tube reheater is arranged above its outlet In normal

ahead mode products of combustion, from oil burned

in the main furnace in sufficient quantity to achieve

the desired degree of superheat, pass over the

super-heater and main generating bank entering the reheat

furnace, where the balance of the fuel is burned

raising the gas temperature by an amount sufficient

for the reheater duty needed In harbour, or when

manoeuvring astern, the burners in the reheat furnace

aresecured and the products of combustion then reach F~gure 20 ~) <??mbustion Engineering V2M-9; b) later version

theuncooled reheatertubes ata temperature low enough

to avoid causing them damage (Fig 21)

provided with oil burners mounted on the roof The

V2M-8-divlded furnace products of combustion from one of these furnaces

A further derivative of the V2M-8, this reheat unit pass overreheater tube surfaces arranged at one endhas the main furnace divided by a membrane wall of the boiler whilst from the other furnace the gases(Fig 22) Each of the two furnaces so formed are pass over superheater tube surfaces at the other end

of the boiler Both gas streams combine before pass- water from the lower drum The bottom headers ofing over the main bank of generating tubes Differen- the front, rear and side furnace walls are fed bytial firing in the two furnaces gives control of reheat unheated downcomers from the steam drum Steamsteamtemperaturewhilstthesuperheatiscontrolled temperature is controlled by attemperation with a

by attemperation between stages of the double su- heat exchanger in the steam drum and auxiliaryperheater All welded furnace enclosure walls are steam at a reduced temperature is provided by aused and the superheaters and reheater are all ar- de superheater in the lower drum The steam circuitranged in the near vertical position with horizontal associated with steam temperature control incorpo-inlet and outlet headers beneath rates a control valve and a fixed orifice in a bypass

line Care is needed in sizing the orifice since if the

Y inSUffIcIentsteam WIllpass to the attemperator and

SDU type the final steam temperature may exceed safety levels.This is a basic bi-drum integral furnace boiler, the Conversely, should the orifice be too small the con-Kawasaki version having a double horizontal tube trol valve will be closed in to establish the correctsuperheater, and front fired furnace constructed with steam quantity to the attemperator and drum steamtangent tubes backed with refractory lined steel cas- pressure may exceed the working level A moreings (Fig 23) The bottom ends of the furnace exit sophisticated system would utilise a second controlscreen tubes terminate in a separate header fed with valve in place of the orifice with means provided to

prevent it from being completely closed The twovalves under the influence of the steam temperaturecontroller would operate in sequence to control thesteam temperature even if operating conditionsdrifted away from design values This avoids downtime which may be required to change the orificeplate.

UF type

This is a radiant type boiler unit with fully watercooled furnace and convection passage enclosurewalls and is very similar in arrangement and con-struction to the radiant designs of the British boiler-makers, having primary and secondary superheaterswith interstage attemperation (Fig 24)

UM type

In conformity with boilermakers elsewhere Kawasakialso offered a bi-drum unit incorporating modemconstruction methods with welded connections be-tween tubes and headers wherever possible (Fig 25).The whole unit is enclosed in membrane wall tubepanels and the oil burners are arranged in the furnace

roof There is an all welded vertical V-tube

super-heater immediately behind the furnace exit screenand generally simple tube shapes are used through-out the unit The superheater construction is novel inthat the V-tubes are made up into panels by beingwelded to stub headers at their ends (Fig 26) Theseare given a prior pressure test in the factory and then

one side only As a departure from previous designsthey introduced a third convection passage betweenthe furnace and the main divided passage (Fig 28).This third or bypass passage contains economisersurface Dampers at the outlet of the three convectionpaths could be adjusted to control reheat and super-heat in the normal ahead mode As usual whenoperating astern or in harbour the dampers above the

Figure 26 a) Method of locating superheater tubes from boiler reheater are closed In this design a double damper

tubes; b) panel construction of Kawasaki superheater. arrangement is used and the space between them can

be pressurised with air to effectively seal the connected to the main headers by welded connecting ers preventing gas flow over the reheater Since sometubes As with the vertical tube superheater pro-

damp-posed by all the boilermakers offering this type of

boiler unit the main support of the tube bundle is

taken on the main headers at the bottom Location

and guidance of the superheater tubes is obtained by

means of heat resisting alloy steel castings welded to

adjacent boiler and superheater tubes The

designa-tory letters defining Kawasaki boilers are

supple-mented by an 'E' if the final heat recovery is by

economiser or by a 'G' if final heat recovery is by a gas

to air heater; the UM type thereby becoming UME or

UMG

UFR type

To provide for the adoption of the reheat cycle

Kawasaki modified their UF type by arranging for

the convection passage to have three parallel paths

(Fig 27) As other boiler makers had done they

di-vided the main convection passage into two parallel

paths by means of a membrane tube wall with

super-heater surfaces on either side but resuper-heater surface on

gas always passes through the bypass passage, less expander As pressure and temperature advanced,heat is available for superheating and reheating To difficulties were encountered with leaking at thecompensate, the reheater is brought into a slightly expanded joints and in some cases this was coun-hotter zone and additional superheater surface pro- tered by first expanding the tube and then running avided, with some primary surface above the reheater light sealing weld around the tube end inside the

header before lightly re-expanding Each of these

A more simple solution to the problems posed by sufficient access handholes in the headers to permitreheat were obtained by Kawasaki when they intro- the expanding and welding operations to be carriedduced this unit in which the bypass passage is elimi- out These handholes had to be sealed off for steam-nated (Fig 29) The resulting design, although exhib- ing and it was usual to have an oval or circular plugiting the same constructional detail as the UFR type, pulled up on the inside onto a gasket with a strongbackcontrols reheat and superheat generally in the man- and nut on the outside Making and keeping thesener adopted by the British boilermakers tight added to the maintenance load and became

problematical as pressure and temperature levelsincreased In some cases these plugs could also be

It became clear that to Improve steam cycle With modest pressures and temperatures it was usu- ciency steam pressure and temperature would rise toally found sufficient to connect superheater tubes to the highest practical values and that maintenancethe headers by expanding the tubes into tube holes in would only be reduced by adopting all welded con-the header using a revolving mandrel expander By struction The difficulty presented by this was torevolving the tapered mandrel, rollers were forced ensure that all welded arrangements provided goodagainst the tube bore, expanding it and squeezing the access for repair at sea, should it become necessary.tube material against the metal of the header The Constructing the boiler ashore meant that the se-tube holes could be plain or were sometimes ma- quence and location of the welding operations couldchined with one or more grooves The tube end was be chosen to facilitate the making of welds of 100%also belled by an additional belling roller in the quality In a repair situation the welding work neces-

effi-sary had to be accomplished in the space and timeavailable Some of the first all welded superheatersadopted a fillet weld connection between the super-heater tubes and stubs previously welded to theheaders in the factory and stress relieved prior toconstruction.

Figure 30 shows the 'melric' joint of this type Theadvantages claimed include ease of making the filletweld joints between tube and stub in the space avail-able and the opportunity of increasing this space bybifurcating two tubes to one stub thereby doublingthe pitch of the stubs

Improved welding techniques and the use of inertgas shielding led to wider use of butt welded jointsthe connection between tube and header being via astub previously welded to the header and stressrelieved (Fig 31) The stub for these butt welded typeswas merely a short length of tube of appropriatematerial These could be made of varying length andcould be either straight or bent to suit the detailarrangement

Where the design of boiler was such that an nal welded joint was not possible (e.g Fig 17) amethod was devised for making an internal pressureweld between the tube end and the inside of theheader The need for stress relieving this joint wasobviated by a factory applied weld deposit layer of alower grade material to the inside of the header local

exter-to the tube hole, exter-to which the tube end was fused with

a full pressure weld This process required specialskills and was confined to those high temperatureparts of the superheater where it was essential Atruly all welded arrangement was not practicable bythis means The Kawasaki UM design (Fig 25), in-cludes an all welded superheater in a bi-drum boilerunit If temporary plugs are fitted in the tubes con-necting the stub headers to the main headers in such

Figure 30 Melric joints applied to the superheater of a Babcock a layout failure of one tube results in the loss of a

and Wilcox selectable superheat boiler: a) arrangement of super- hi' 1 f be

heat header and element; b) detail of melric joint; c) method of woe pane 0 tu s

blanking off from outside of header in event of element failure. There are cases where butt welds can sometimes

Chapter 1 MARINE BOILERS 23

be made possible by combining two or three tubes

into one stub by the use of bifurcation or trifurcation

pieces The space between the stubs can thereby be

increased, creating better access for welding When

using this method care must be taken not to join

together tubes having significantly different

resist-ance to flow and/ or heat absorption otherwise some

tubes may receive insufficient steam flow

Steam temperature and superheater tube

tem-perature both vary throughout the steam path through

the superheater For metal parts outside the gas

pas-sage, such as the headers, the metal temperature is

the same as the steam temperature within For the

tubes inside the gas passage the metal temperature

must be assessed taking into account all possible

variations in value, and the most adverse

combina-tions of gas flow, steam flow and gas temperature

Only after a careful analysis of those factors is it FI 32 Babe k d Wil d rf

ObI hi 0 boo 0 gure oc an I cox rum type su ace attemperator.

poSSI e to ac eve maxImum economy y

mlffiffilS-i~g u~ of the most expensiv~ alloys The four mate- Attemperators and desuperheaters

nals In common use are; ffilld steel;

1;2%molybde-num; 1;2%molybdenum-1 % chromium; and 1% mo- Each of these devices is a heat exchanger designed tolybdenum-2V4% chromium Each of these has a remove heat from superheated steam In the case ofmaximum useful working temperature determined an attemperator this is usually accomplished at an

by the onset of rapid oxidation but in practical appli- intermediate stage of a superheater in order to cations the stress resisting capabilities at the working trol the final steam temperature and to protect thetemperature will determine choice of material If too secondary stage of the superheater from excessiveIowa grade is chosen the allowable stress will be low temperature A desuperheater, however, may be usedand therefore the required tube thickness will be to reduce the temperature of a quantity of steam fromhigh This tends to raise the metal temperature fur- the superheater outlet to as low as 3°e above thether and if this significantly affects the allowable saturation temperature Two types of heat exchangerstress then the time has passed for a change of mate- may be found, ie surface type or direct contact type.rial There are, of course, more exotic alloys than The former is the most common for attemperators inthose listed but it is rare to find that they are needed view of the risk in the direct contact type of introduc-

con-in marcon-ine boilers One exception might be for naval ing impurities into the superheater Since the coolingapplications where the high ratings used ina warship medium is feed water a direct contact attemperator ismay make it desirable to consider a 12% chromium only used if feed water of the highest purity can bealloy Oil fired units are subject to high temperature assured This is less important for the desuperheatercorrosion from fuel constituents, the major cause as it is situated downstream of the superheater Wherebeing the presence of vanadium and sodium in the surface type desuperheaters are used the outlet tem-fuel which form low melting point complex sodium/ perature would be about 300e above saturationvanadium compounds with oxygen and sulphur temperature

oxides from the flue gas The corrosion mechanism is The construction of attemperators and very complex and has been subjected to considerable erheaters is similar; only the duty differs Surfaceresearch The corrosive effects can be minimised by types consist of a bundle of straight or bent tubeskeeping superheater tube and gas temperatures as connected at their ends to inlet and outlet headers,low as possible, and for this reason, when fired with the whole installed below water level inside a boilerresidual type oil fuel, boilers with conventional su- drum with connections from inlet and outlet headersperheaters are limited to a final steam temperature taken through the drum shell or drum end (Fig 32).between 525°e and 535°C LNG ships must also be To avoid thermal shock of the relatively heavy drumable to bum oil and steam temperature is similarly plates due to the high temperature steam passing intolimited Experience ashore shows that when coal is or out of the heat exchanger these connections arethe fuel, steam temperature may be safely raised to made so that the steam passes through a thermal

24 The RUNNING and MAINTENANCE of MARINE MACHINERY

rapidly enter into heat exchange with the steamthereby reducing its temperature A liner is fitted so

as to prevent spray water impingement on the hotwalls of the body of the unit and a reasonable straightlength of piping downstream is arranged to permitcomplete evaporation of the water before meetingany pipe bends

Figure 33 Thermal sleeve.

outer parts of the sleeve is preferably open to air

rather than to boiler water so as to avoid the risk of

this annular space becoming concentrated with boiler

water salts leading to corrosion

AIthough similar in construction and function the

essential difference between surface type

attemp-erators and desuperheaters is that the former must be

designed so that the steam passes through with a

minimum drop in pressure so as to minimise boiler Figure 34 Spray attemperator showing thermal sleeve.

design pressure and thickness of the pressure parts

The steam from a desuperheater is usually used for

auxiliary purposes and is not needed at high pres- Economisers

sure, so that higher steam speed and pressure loss in The gas temperature leaving a boiler cannot be the tubes is acceptable Sometimes if desuperheaters duced much below 30°C above the saturation tem-are operated with a steam flow very much lower than perature and in radiant tyPes a much higher leavingthe design capacity, the leaving steam temperature gas temperature is usually found So that an accept-will approach saturation temperature and some con- able degree of efficiency can be obtained and fueldensation may form in downstream piping To avoid consumption reduced as much as possible furtherthis the pressure reducing valve commonly found in heat recovery surfaces are needed so that the gasdesuperheated steam circuits may be positioned up- temperature at the funnel may be as low as practica-stream of the desuperheater so that the saturation ble To carry out this further heat exchange, surfacestemperature within is well below that in the drum suchaseconomiserand/orairheaterarecommonlySince the steam leaving temperature cannot be below used

re-drum saturation temperature the leaving steam re- In many radiant boiler types economisers are alsomains superheated found arranged integrally within the boiler unit andSpray type units are arranged external to the boiler in this location they consist of a number of multi-loopand have been used as desuperheaters at sea for elements of plain tubes connected at their ends tomany years Their use as attemperators has only inlet and outlet headers Since they are fed with watergained support following the introduction of more leaving any external economiser fitted and since theysophisticated water treatment regimes associated with are situated in a hot gas temperature zone and aremodem high pressure marine boilers (Fig 34) A required to perform a considerable heat exchangesuitable length of pipework containing the steam duty a portion of the water pumped through themwhose temperature is to be reduced is substituted by may be converted into steam These steaming econo-the body of the unit which contains a spray nozzle misers are arranged so that water enters the lowerarranged concentrically within Feed water at feed header and the steam/ water mixture leaves from thepump pressure is supplied to the spray nozzle fr~m top header and thence to the steam drum where thewhich it issues in a fine mist of water droplets which steam and water separate

Chapter 1MARINE BOILERS 25headers The extended surface is obtained by a vari-ety of means such as resistance welded mild steelstuds or plate fins, or by shrunk or cast iron gills(Fig 35) The former is lighter and enables a greaterheat exchange for a given volume but is suitable onlyfor those parts of the economiser surfaces where thewater temperature within exceeds around 140°C.Since the coefficient of heat transfer on the water side

is very much higher than on the gas side the tubetemperature will not differ from the water tempera-ture by any great amount A tube surface tempera-ture of 140°C is necessary to minimise the formation

of weak acid due to condensation of water vapourand sulphuric acid from the products of combustion

At lower temperatures these could produce an acidconcentration likely to cause vigorous attack on mildsteel surfaces, due to the presence of sulphur in thefuel The actual dewpoint temperature is dependentupon the proportion of the sulphur in the fuel which

is oxidised to sulphur trioxide during combustion,the amount of moisture in the combustion air and thehydrogen content of the fuel It is difficult to deter-mine in service and the figure of 140°C is given as aguide which experience shows to be adequate formost situations The operator can increase his margin

of safety by attending to the quality of combustionand operating with a minimum of excess air A com-bination of all mild steel and cast iron protectedsurfaces (Fig 36), is frequently found in externaleconomiser arrangements on boilers associated withfeed cycles having no high pressure feed heaters.When high pressure feed heaters are used the higherfeed water temperature leaving them usually per-mits all mild steel economiser surfaces This is not thecase if the high pressure feed heater is arranged inseries with the economisers Then feed water fromthe de-aerator enters the economiser with cast ironprotection from which it returns to be further heated

in a high pressure feed heater going on to the all mildsteel section of the economiser This rare arrange-ment permits a high boiler efficiency without usinggas/ air heaters and retains the advantage to cycle

I efficiency of bled steam high pressure feed heating.The arrangement is, however, inferior in efficiency tothe cycle using a maximum of bled steam feed heat-ing and final heat recovery by means of a gas/airUsed externally to the boiler forfurther heat recov- heater

ery economisers are found in cooler gas zones and are Regardless of the type of surfaces used in thefed with water at temperatures around 116°C or economiser it is now common practice for these to be185°C depending upon whether the feed cycle in- constructed with all welded connections betweeneludes high pressure feed heaters after the de-aera- tubes and headers Theselatterhaveinspectionfacili-tor In either case the economiser consists ofa number ties which are also welded Sometimes these are

of sinuous multi-loop elements of extended surface arranged so that, if necessary in an emergency, thetubes connected at their ends to inlet and outlet closure can be made without welding (Fig 37)

Air heaters

An economiser can only economically reduce thefunnel gas temperature to about 20°C above the inletwater temperature and so when high pressure feedheaters are used an acceptable boiler efficiency re-quires the use of a gas/ air heater for final heat recov-ery Three types of gas/air heater have been used atsea, namely:

Chapter 1MARINE BOILERS 27

plate is insulated on the air side and the tubes areextended into the inlet air trunking by 300 nun or so

In this first length of tube, inlet turbulence locallyincreases the air side heat transfer coefficient andshould this occur within the gas passage the tubetemperature is depressed even further As an alterna-tive a short ferrule can be inserted into the inlet end

of the tubes separating the locally enhanced heattransfer coefficient from tubes in contact with fluegas Another method is to preheat the incoming air atthe expense of a reduced performance or an increase

in heating surface to counteract the reduced meantemperature difference between gas and air

Other attempts to overcome the corrosion lem in tubular air heaters include the use of glasstubes These are completely resistant to attack fromsulphuric acid but need special arrangements to sealthem into the steel tube plate which is still prone tosuffer Also glass tubes must be carefully handled at

prob-Figure 38 Cast iron plate type gas/air heater. all times and are easily damaged during any cleaning

or maintenance operations The most widespreadintegral with the plates thus increasing the effective practice was to use mild steel tubes with a coat ofheating surface The fins are required on both sides of vitreous enamel; in effect glass coated steel tubes.the plates since the coefficients of heat transfer of air These had mixed success, dependent largely on theand gas are of the same order When cast iron is used quality and completeness of the enamel Although

it is to combat the corrosive effect of weak sulphuric more robust than glass tubes any imperfection in theacid in the same way that cast iron gills are used to enamel coating was avidly sought out by the acidprotect mild steel economiser tubes Cast iron is which then rapidly perforated the tube beneath Nev-reputed to be more resistant to this form of attack ertheless there were many applications showing sig-than mild steel although there is some controversy on nificant advantage over plain mild steel tubes andthe matter it sometimes being suggested that it is the there was the bonus feature that the smooth enamelgreater mass of cast iron which confers a longer coating reduced the tendency for gas side fouling.service life However that may be, a great deal of cast If a tubular air heater is required to reduce the gasiron is used in final heat recovery heat exchangers temperature by 50% of the difference between theand generally acceptable results obtained (Fig 38) temperature of the incoming gas and incoming air itAir is normally supplied to an air heater at a would be very bulky and heavy A rotary regenera-temperature of around 38°C and therefore certain of tive air heater for the same duty would probably bethe metal parts are going to be at a temperature where less heavy and would occupy much less space and, incorrosion is a serious risk Considerable difficulties spite of having moving parts, might be preferred Inhave been experienced with tubular air heaters where modem ships with radiant boilers even larger air heatermild steel tubes were perforated after only 6 months duties are found requiring the gas temperature to be

in service The modem tubular air heater is arranged reducedbyup to 75%of the difference between incomingwith air passing through horizontal tubes, the prod- gas and air temperatures These larger duties would beuets of combustion passing upwards over them The impracticable for the recuperative tubular or plate typetubes are expanded into tube plates at either end and air heaters and in consequence the regenerative airthe air trunking arranged so that the air makes two or heater has an established place at sea

more passes through the tubes For maximum heat The regenerative air heater is either of the recovery air inlet is at the top so that air progresses ing matrix type based on the Ljungstrom design or ofthrough successive passes in a generally downwards the fixed matrix type In the former a closely packeddirection in counter flow with the flue gas stream (Fig matrix of specially corrugated plates is slowly re-39) The coolest tube temperature and the area usu- volved so as to pass through the gas stream and beally found to suffer most from acid attack is therefore heated and then through the air stream where its heat

revolv-at the inlet end of the first air pass tubes and the tube content is delivered to the air (Fig 40) In the latter,plate nearby To lessen the risk in this area the tube slowly revolving air hoods cause the gas and air

Figure 39 Horizontal tube gas/air heater.

stream to pass sequentially over the matrix giving the there is a tendency for air to leak across the seals intosame effect of transferring heat from gas to air (Fig the gas stream and to recognise this the capacity of41) The material forming the matrix cycles in tem- the forced draught fans is adjusted accordingly Anyperature, reaching a low value whilst air is passing seal leakage greater than that allowed for results in a

On then entering the gas stream there is a tendency loss of efficiency due to overloading the fan or, worse,for condensation to occur resulting in some acid a shortage of combustion air at the burners

attack In a regenerative air heater this would ulti- When final heat recovery is accomplished bymately result in a loss of performance and eventual economisers it is still possible to have heated com-partial blockage of the fluid passages with corrosion bustion air This is desirable as it confers two advan-products Serious difficulty has been avoided by the tages Hot combustion air is certainly beneficial inadoption of enamelled elements forming the matrix support of good combustion but in addition it pro-and satisfactory service life has been obtained In the vides a boost to the steam cycle efficiency by usingtype having a fixed matrix the upper or coolest por- bled steam in a steam air heater This device is a heattion of the matrix is often manufactured in a glazed exchanger in which mild steel or cupro-nickel tubesceramic honeycomb, also with very good results are connected at their ends to inlet and outlet head-Since rotation is involved with either design each will ers The tubes have closely pitched extended surfaceinclude sliding seals to separate air and gas streams £inning applied on the outside over which the com-For efficient operation these must be maintained and bustion air is passed (Fig 42)

properly adjusted at all times Since the combustion Bled steam is admitted via the inlet header andair is always at a greater pressure than the flue gas condenses within the tubes giving up its superheat

30 The RUNNING and MAINTENANCE of MARINE MACHINERY

andlatentheatto the combustion air A certain amount where the discharge of steam will impinge on the

of cooling of the condensate is also sometimes in- area to be cleaned a cam oper(!ted valve opens in thecluded The outlet header is connected via a steam head to admit steam from the sootblowers steamtrap to the drain system These compact heat ex- piping system As the lance continues to rotate,changers are normally found on steam ships operat- bringing the nozzles clear of the heating surfaces, theing with a steam cycle which excludes high pressure cam allows the steam valve to close In such afeed heaters and are also sometimes used as air sootblower the lance is permanently in the gas pas-preheaters upstream of gas/air heaters for reasons sage and, apart from a small quantity of purge airpreviously mentioned admitted to prevent combustion products from en-

tering the head, is uncooled when not in operation(Fig 43)

In early bl-drum convection bank boIlers suchCleanliness is all important in the operation of heat sootblowers fitted in the superheater zone experi-exchangers including boilers and all the ancillaries enced a very short life due to the ravages of tempera-described For boilers, economisers and gas/ air heat- ture and corrosion As a result the superheater areaers which are exposed to products of combustion suffered severely from fouling and blockage of thesome form of on load cleaning is necessary The most gas passages making it necessary to use high pres-common method involves the regular use of soot- sure water washing off load To alleviate these diffi-blowers in which superheated steam is discharged culties superheaters were arranged to accommodateonto the heating surfaces, driving off any deposits It retractable sootblowers These allow the lance to bewill be appreciated that part of the sootblower is itself withdrawn when not in operation so that the lance isexposed to the products of combustion and this must only exposed to hot gases whilst a cooling flow of

be taken into account when choosing the sootblower steam is passing through These sootblowers possesstype and materials of construction a more powerful cleaning action, better able to deal

In its simplest form a steam sootblower consists of with the deposits which the chemistry of the fuel ash

a headpiece, including a valve, mounted external to at high temperatures causes to be bonded to thethe heat exchanger Extending from this into the gas heating surfaces and is more difficult to remove thanpassage is a tube or lance fitted with nozzles through dusty, sooty, deposits found elsewhere In operationwhich the steam discharges An electric or pneumatic the lance revolves and traverses across the gas pas-motor attached to the headpiece causes the lance to sage whilst steam jets pointing sideways at the end ofrotate and when the nozzles come into a position the lance clean a spiral path When fully inserted the