Marine Auxiliary Machinery 7E Episode 4 doc

Mild steel weldedfabrications, similarly lined, are also used for large ship side fittings.Seamless mild steel is used for steam, high pressure air, feed discharges andall oil fuel press

110 Ship service systems

bacteria is generated These bacteria are said to be anaerobic Whilst they areequally capable of breaking down sludge, in so doing they generate gases such

as hydrogen sulphide and methane Continuing use of a biological sewagesystem after a failure of the air supply, could result in propagation of anaerobicbacteria and processes The gases produced by anaerobic activity aredangerous, being flammable and toxic

Extended aeration plants used at sea are package plants consisting basically

of three inter-connected tanks (Figure 3.17) The effluent may be comminuted(i.e passed through a device which consists of a rotating knife-edge drumwhich acts both as a filter and a cutter) or simply passed through a bar screenfrom where it passes into the first chamber Air is supplied to this chamber via adiffuser which breaks the air up into fine bubbles The air is forced through thediffuser by a compressor After a while a biological sludge is formed and this isdispersed throughout the tank by the agitation caused by the rising air bubbles.The liquid from the aeration tank passes to a settling tank where underquiescent conditions, the activated sludge, as it is known, settles and leaves aclear effluent The activated sludge cannot be allowed to remain in the settlingtank since there is no oxygen supplied to this area and in a very short time thecollected sludge would become anaerobic and give off offensive odours Thesludge is therefore continuously recycled to the aeration tank where it mixeswith the incoming waste to assist in the treatment process

Over a period of time the quantity of sludge in an aeration tank increasesdue to the collection of inert residues resulting from the digestion process, this

Figure 3.17 Biological sewage treatment plant (Hamworthy)

build up in sludge is measured in ppm or mg/litre, the rate of increase being afunction of the tank size Most marine biological waste treatment plants aredesigned to be desludged at intervals of about three months The desludgingoperation entails pumping out about three quarters of the aeration tankcontents and refilling with clean water.

The clear effluent discharged from a settling tank must be disinfected toreduce the number of coliforms to an acceptable level Disinfection is achieved

by treating the clean effluent with a solution of calcium or sodiumhypochlorite, this is usually carried out in a tank or compartment on the end ofthe sewage treatment unit The chlorinator shown in Figure 3.17 uses tablets ofcalcium hypochlorite retained in perforated plastic tubes around which theclean effluent flows dissolving some of the tablet material as it does so Thetreated effluent is then held in the collection tank for 60 minutes to enable theprocess of disinfection to be completed In some plants the disinfection iscarried out by ultra-violet radiation

Further reading

Allanson, J T and Charnley, R (1987) Drinking water from the sea: reverse osmosis,

the modern alternative, Trans I Mar E, 88.

Giichrist, A (1976) Sea Water Distillers, Trans I Mar E, 88.

Hill, E C (1987) Legionella and Ships' Water Systems, MER

Merchant Shipping Notice No M1214 Recommendations to Prevent Contamination

of Ships' Fresh Water Storage and Distribution Systems

Merchant Shipping Notice No M1401 Disinfection of Ships' Domestic Fresh Water.The Merchant Shipping (Crew Accommodation) Regulations 1978, HMSO

4 Valves and pipelines

The various pipe systems for commercial ships must comply with anyapplicable rules of the responsible government department and those of thedesignated classification society Guidance is provided in government andclassification society publications and it is required that plans for principalsystems are submitted for approval The safety and reliability of criticalindividual fittings is ensured by a requirement that they are made tospecification by an approved manufacturer Materials are tested, welds areinspected, major fittings are tested and marked, systems are pressure tested by

or in the presence of a representative of the appropriate authority Every effort

is made to ensure safety and reliability Replacement components for pipesystems must be of the same standard and obtained if necessary, from anapproved maker Some accidents have been the result of replacement valvesand other components being of inferior quality,

Materials - corrosion - erosion

Galvanic corrosion is a major challenge for any pipes which carry sea water.Rust is a particular corrosion problem for steel pipes exposed to contact withsea water or moisture generally and air Pipe runs along tank tops or on deck,are examples of the latter Steel pipes in these areas require external as well asinternal protection

Sea water is an electrolyte and therefore a conductor of electricity, becausethe molecules of its dissolved salts split into positive and negative ions whichare available as current carriers Electrolytic action can result if there aredifferent metals or even differences in the same metal in a pipeline Galvaniccorrosion can occur if the different metals are connected electrically andmutually in contact with the sea water A corrosion cell formed between steeland brass in contact with sea water results in wastage of the less noble steel Alist is given in the galvanic series, in which the more noble metals are placed inorder after the less noble thus: zinc, aluminium, carbon steels, cast iron, lead—tinalloys, lead, brass, copper, bronze, gunmetal, copper-nickel iron, monel metal

A metal in contact with one occurring later in the series, as with steel and brass,may corrode rapidly in sea water Because the action is galvanic, less noblesacrificial anodes can give protection

Steel being subject not only to galvanic corrosion but also to rusting, appears

to be a poor material to select for sea water pipes or for installation in tank top

or deck areas Mild steel pipes for sea water are protected by being galvanized

or rubber lined Welding and pipe bending should be completed beforegalvanizing or application of a lining, so that weld spatter and deposits frommanufacture can be removed The mild steel, electric resistance welded (ERW)

or hot rolled pipes are galvanized by hot dipping Inadequate protection ofsteel, results if there are pinholes or discontinuities in protective linings.Linings should always be carried over the flange faces Mild steel weldedfabrications, similarly lined, are also used for large ship side fittings.Seamless mild steel is used for steam, high pressure air, feed discharges andall oil fuel pressure piping Its strength reduces however, at about 460°C andabove this figure, steels require small additions of alloying materials such asmolybdenum and chromium Flanges are secured to steel pipes by fusionwelding or by screwing and expanding

Cast iron

Cast iron has poor corrosion resistance in sea water, being especiallyvulnerable to graphitization This form of attack gradually removes the ironfrom the surface in contact with sea water to leave soft, black graphite Theweakness of ordinary grey cast iron in tension and under shock loading limitsits use to low pressure applications, and the brittle nature of ordinary grey castiron excludes its use for side shell fittings where failure could result in flooding

of the machinery space Ease of casting makes the material ideal for theproduction of fittings and fortunately techniques for improving strength havebeen developed Spheroidal graphite cast iron (SG iron) and meehanite areexamples of high strength versions of the material These are suitable for use inship side valves if made to specification by an approved manufacturer SG ironmay be used for high pressure services and for steam below 461°C.Cast iron with its high carbon content and consequent low meltingtemperature is ideal for the production of fittings by casting

Copper

Copper pipes are suitable for moderate pressures and temperatures Flanges aresecured to copper and its alloys -by brazing or sweating

Non-ferrous alloys

Basically, brass is an alloy of copper and zinc; bronze an alloy of copper and tin

In both cases there may be additions of other metals and there is some

114 Valves and pipelines

confusion of nomenclature; some high-tensile brasses are called 'bronze' andthe practice has prevailed for so long as to be accepted

Aluminium brass and other non-ferrous pipelines, are considered veryresistant to corrosion in sea water, but concentrated galvanic corrosion canoccur if some part of the pipe system has a different make up A localizedcorrosion cell can be set up when a fitting, such as a thermometer pocket, is of abrass, bronze or other material which is different to the parent material Pipesystems are ideally of the same material throughout but non-ferrous alloys areprotected against corrosion by the deposition of iron ions so that use of iron orsteel fittings is beneficial Iron ion protection can alternatively be supplied fromsacrificial or driven iron anodes or by dosing with ferrous sulphate

Dezincification of brasses is a particular type of corrosion that occurs in thepresence of sea water The attack removes zinc from the alloy, leaving porouscopper which is soft The problem is marked by a patch of copper colour in thebrass Dezincification is inhibited in brasses which are intended for sea-watercontact by additions of a very small amount of arsenic (0.04%) or other elements.Some brasses are prone to corrosion-stress cracking but this is aphenomenon associated chiefly with brass tube which has been stressed byexpanding or by being worked in the unsoftened condition and which is also incontact with corrosive fluids, such as sea water Splitting can occur suddenly, oreven violently as a result of stress corrosion cracking

Stainless steel

A different problem is presented by corrosive liquids and those that containhard particles and are therefore likely to cause erosion These can causediffering rates of wastage in conventional metal pipes or cargo tanks Withsome corrosive liquids wastage is slow enough, lasting over a period of years,

to permit the use of common metals Expensive stainless steel is widely usedfor the cargo pipes of chemical tankers intended for carriage of very corrosivecargoes

Erosion

Erosion of metal may be the result of abrasives or of high water speeds,entrained air, turbulence and cavitation The latter are often caused byprotuberances, tight bends or an abrupt change of pipe cross sectional area.Erosion from turbulent flow and cavitation also aids corrosion (corrosion/erosion) by removing the oxide film that assists in the protection of metalsurfaces The exposed metal surfaces can form galvanic corrosion cells withadjacent areas where oxide film is still present Erosion is reduced by limitingspeed of flow, avoiding sharp bends, changes of section and impediments toflow such as incorrectly cut jointing or weld deposits Speed of liquid flowshould be no greater than 1 m/s for copper; 3 m/s for galvanised steel andaluminium brass; 3.5 m/s for 90/10 cupro-nickel: 4 m/s for 70/30 cupro-nickel

Strength of materials

The strength of materials used for pipes and fittings must be adequate for thesystem pressures and possible over-pressures Pipelines and valves, forexample, used to carry and control the flow of high temperature, high pressuresteam must obviously be made to very exacting specifications by approvedmanufacturers

Various and often varying pressures and temperatures pose problems.Temperatures of about 450°C can cause recrystallization and creep in iron andsteels Very low temperatures as with liquefied natural gas, can result in brittlefailure Varying temperatures give problems with stress due to expansion andcontraction

The term fittings covers valves, cocks, branch and bulkhead pieces, reducers,strainers and filters, separators and expansion pieces, in short, everything in asystem which is not a pipe Couplings and unions are used only in small borepipes

Cast iron and gunmetal fittings are used freely in small sizes at moderatepressures Large fittings, those for high pressure and temperature and for oilfuel under pressure, are cast or fusion welded (fabricated) mild steel or SG iron.For temperatures above 460°C they are usually of 0.5% molybdenum steel.The addition of 0.5% molybdenum, inhibits recrystallization and therefore theresulting creep

Pipe installation

Vibration is the frequent cause of eventual pipe failure but supports and clips toprevent this problem must permit free expansion and contraction A pipewhich has to be twisted or bowed when being connected, has inbuilt stresswhich can lead to ultimate failure Pipes should be accurately made (particularlyreplacement sections) and installed with simple supports before beingpermanently clipped If pumps are designed so that the driving motor orturbine is mounted upon an extension to the pump casing proper, the tendencyfor mal-alignment, due to pipeline stresses, is practically eliminated.Nevertheless, it is essential that the pipe systems and heavy valve chests, areseparately supported and stayed during installation, the flanged connection tothe pumps being the last to be coupled after the faces are correctly aligned Thiscan contribute materially to the life of the unit

Horizontal pumps should be laid down on suitable chocks, accurately fitted

to ensure that the couplings, with their bolts removed, are in correct alignmentand with their faces parallel This alignment should be checked after tighteningthe holding down bolts and again after the pipes are coupled and preferably full

of liquid

Colour coding

It is usual to identify pipes by a colour code for the individual system or bybands of paint at intervals on pipes of a common colour There are standard

116 Valves and pipelines

codes but individuals or companies may prefer variations Frequently pipes areincorrectly coloured Before working on or using a pipe system, it should betraced and verified

Cleaning the system

It is often found, in new ships, that the bilges and bilge systems have not beenthoroughly cleaned with the result that wood, nuts, bolts, rags and other debrisare found inside valves and pipes after initial bilge pumping These choke thevalve-chests and prevent the valves from being properly closed They alsoblock strainers It is vital to clean before the bilge system is tested to ensure thatall suction pipes, joints, valves and glands are free from air leaks Pipes too must

be cleaned and checked as being clear before and after assembly Blockage hassometimes been found due to failure to cut apertures in metal or joints.Obviously with hydraulic or pneumatic pipe systems, foreign objects orresidues from manufacture can cause serious malfunction

Drains

Disastrous explosions have been caused by accumulations of oil or oil vapour

in diesel engine air lines which were not regularly drained Severe damage hasbeen caused by 'water hammer' when steam has been admitted to pipescontaining water, especially when a slight inclination of the pipe from thehorizontal allowed the water to have a large free surface area

On steam being admitted, condensation occurs on the cool water surface or

in a cold section of the pipe, a partial vacuum develops and the water movesalong the pipe at great speed The impact of this water at a bend or valve, cancause fracture of the pipe Water hammer is indicated by severe and oftenrepeated banging in the pipe Steam pipes are fitted with drains which should

be left open so that water will not accumulate otherwise drains must be openedbefore admitting steam Steam master valves are first opened very slightly or'cracked open' when a line is being brought into use until the pipe is thoroughlywanned Only then should the valve be opened fully

Expansion arrangements

Provision must be made in pipe systems to accommodate changes in lengthdue to change of temperature, and so prevent undue stress or distortion aspipes expand or contract One type of expansion joint (Figure 4.3) has ananchored sleeve with a stuffing box and gland in which an extension of thejoining pipe can slide freely within imposed limits Simpler schemes (Figure4.2a and 4.2b) allow for change of length with a right angle bend arrangement

or a loop For high pressures and temperatures with associated greater pipediameter and thickness other methods may be more appropriate

Figure 4.1 Tie rod expansion joint

Figure 4.2 Steam-line expansion arrangements (a) Expansion loop

upwards Large bore drain pocket fitted before loop (b) Expansion loop horizontal, no drainage required

Stainless steel bellows expansion joints (Figure 4.3) are commonly usedsince they will absorb some movement or vibration in several planes, eliminatemaintenance, reduce friction and heat losses

Maximum and minimum working temperatures must be considered whenchoosing a bellows piece, which must be so installed that it is neitherover-compressed nor over-extended Its length must be correct for thetemperature change Stainless steel is the usual material for temperatures up to500°C Beyond that and for severe corrosive conditions, other materials arerequired

Normally the bellows has an internal sleeve, to give smooth flow, to act as aheat shield and to prevent erosion If exposed to the possibility of externaldamage, it should have a cover In usual marine applications, bellows joints aredesigned and fitted to accommodate straight-line axial movement only and theassociated piping requires adequate anchors and guides to prevent misalignment

It will be apparent that, in certain cases, the end connections will act adequately

as anchors and that well designed hangers will be effective guides

Figure 4.3 Bellows type expansion fitting

118 Valves and pipelines

An axial bellows expansion joint can accommodate compression andextension, usually stated as plus or minus X mm, i.e it will compress or extend

X mm from the free length, at which it is supplied It is most important that theunit be installed at its correct length as extension or compression outside itsspecified limits will cause premature fatigue

Watertight bulkheads

Pipes are carried through watertight bulkheads with the use of special fittings(Figure 4.4) to avoid impairment of their integrity The large flange of thefitting, covers the necessary clearance in the bulkhead

Joints

Joints between flanges should be impervious to damage from the fluids carriedand a variety of materials are available to suit the different requirements.Rubber for example, with or without cotton insertion, is suitable for water butnot for oil High pressure can force a joint out of a flange so that the thinnestjoints are used for the highest pressures Some jointing fabrics are sheathedwith copper or stainless steel, which may be grooved finely and lightly in the

Figure 4.4 Bulkhead piece for use when a pipe passes through a

watertight bulkhead

area adjacent to the pipe bore Most materials deteriorate with time andtemperature so that periodic replacement may be necessary Graphitecompounds assist flexibility.

Mating flanges should be parallel and accurately machined Bolts should fitreasonably well and have good threads

Cocks and valves

Cocks and valves are designed to control or interrupt flow This is done incocks by rotating the plug, and in valves by lowering, raising or rotating a disc

in relation to a seating surface or by controlling the movement of a ball Thesefittings have bodies furnished with flanged or screwed ends (or ends prepared

by welding) for connection to the joining pipes

Cocks

A cock may be straight-through, right-angled or open-bottomed as required

by its situation in a pipe system Its plug may be tapered or parallel withtightness achieved by lapping in or by resilient packing material (Figure 4.5)often in the form of a ready made sleeve

In machinery spaces, the short sounding pipes for fuel or lubricating oiltanks, must be fitted with cocks having parallel as opposed to tapered plugs.This, together with the requirement for weighted handles which willautomatically close the cock when released, is for safety Tapered plugs, whentightened to hold the cock open for sounding and then forgotten, havecontributed to fires when tanks have overflowed

Boiler blowdown cocks on the ship's shell, are constructed so that the handlecan be removed only when the cock is closed

Globe valves

The globe valve (Figure 4.6) has a bulbous body, housing a valve seat andscrew down plug or disc arranged at right angles to the axis of the pipe For thevalve shown, both seat and disc faces are stellited and almost indestructible.Alternatively, the seat may be renewable and screwed into the valve chest orgiven a light interference fit and secured by grub screw The seatings may beflat or more commonly mitred The spindle or stem may have a vee or squarethread, below or above the stuffing box If the latter it will work in a removable

or an integral bridge (bonnet)

The spindle may be held in the valve disc (or lid') by a nut as shown or thebutton may locate in a simple horseshoe Leakage along the valve spindle isprevented by a stuffing box, packed with a suitable material and a gland Ifthere is a change of direction, as in a bilge suction, the valve is referred to as anangle valve Flow is from below the valve seat, so that the gland is not subject

120 Valves and pipelines

Figure 4.5 Example of a sleeve-packed cock (Richard Klinger Ltd)

to higher static pressure when the valve is closed The disc must be guided bywings or a stem on the underside for location, or by a piston as shown.The type of valve with the disc attached to the spindle is of the screw lifttype When the disc is not attached to the spindle (inset Figure 4.6) it is ascrew-down non-return (SDNR) valve, as used for bilge systems, to preventback flooding They are also used as feed check and boiler stop valves The discrequires guide vanes or a stem to keep it concentric with the seat when open.The greatest lift required is one-quarter of the bore; guides must be of a greaterlength than the lift

A free-lifting non-return valve (Figure 4.7) is fitted in the compartmentserved by a bilge suction line, when the pipe is nearer to the ship side than onefifth of the ship's breadth Such valves are intended to prevent flooding of thecompartment in the event of collision damage

Figure 4.6 Example of a globe valve with (inset) detail of the valve

arranged as a screw-down non-return (SDNR) unit (Hattersley Newman Mender Ltd.)

1 Body 6 Disc stem nut 10 Bonnet studs

2, Bonnet 7A Stem - stop type 11 Bonnet stud nuts

3 Gland flange 7B Stem - piston SDNR 12 Yoke bush

4, Gland 8 Bonnet gasket 13 Handwheel

5A Disc — stop type 9 Gland packing 14 Handwheel nut

5B Disc - piston SDNR

Gate valves

Unlike the globe valve, gate (or sluice) valves (Figure 4.8} give full bore flowwithout change of direction The valve disc known appropriately as a gate, ismoved at right angles into the flow by a screwed spindle working in a nut Itrests when closed, between circular openings furnished with seats Valves andseats may be tapered or parallel on their facing sides

Such a valve is not suitable to partially open operation since wire-drawing ofthe seat will occur The bonnets of these valves are frequently of cast iron andcare should be taken when overhauling To ensure tightness, some parallelgates are fitted with twin discs, dimensioned similarly to the chest seats butpressed against the seats by a spring when closed

Where change of direction is required, a full bore angle valve (Figure 4.9)may be used

122 Valves and pipelines

Figure 4.7 Example of a non-return valve (Hattersley Newman Hender

Figure 4.0 Full bore angle valve

Figure 4.10 Type B A butterfly valve (Hindle Cockburns Ltd)

flow characteristics and low pressure drop The valve is quick-acting if required,

as only a quarter of a turn of the spindle is required to move the valve from thefully open to the fully closed position Sizes range from 6 mm to over 1000 mmbore

For fine control of cooling water temperature a special type of gangedbutterfly valve may be used to bypass coolers Known as a diverter valve itconsists of a Y or T casting with butterfly valves in two of the legs (Figure4.11), A pneumatic actuator working from a signal provided by a temperaturesensor opens one valve while closing the other This gives precise control of

124 Valves and pipelines

Figure 4.11 A diverter valve (Cockburn-Rockwell Ltd.)

the flow rate in the main and branch lines In the event of a temperaturecontroller failure, a built-in return spring opens or closes the main and branchlines (as appropriate to the system of operation) to provide maximum coolingflow Manual control is available for emergencies

Flap valves

Scupper pipes from accommodation spaces are fitted with non-return valves.Those scuppers from spaces below the bulkhead deck, are required to be fittedwith non-return valves which can be positively closed from above thebulkhead deck or, if this is not practical, with two non-return valves Acommon type of non-return valve (Figure 4.12} has a hinged flap which ispushed open by outward flow and closed by its own weight The flap preventsinward passage of sea water

Change-over valve chests

Dual purpose tanks such as those for either oil or water ballast require exclusiveconnections to separate systems Special valve chests (Figure 4.13) withinterchangeable blanks and connecting passages are installed for this duty Theexample shown has two suction valves flanked by a blank on one side and adome on the other In the position shown, the two suction valves have access

Figure 4.12 Flap check valve (Blakeborough and Sons Ltd.)

1 Cover 5 Seat ring

2 Body 6 Face ring

3 Hinge shaft 7 Door

Relief valves

Excess pressure is eased by a relief valve (Figure 4.15) This consists of a discheld closed by a spring loaded stem The compression on the spring can be

126 Valves and pipelines

Figure 4.13 2-valve change-over chest for oil and ballast suctions as

arranged when filling or discharging ballast

adjusted so that the valve opens at the desired pressure The special case of

boiler safety valves is dealt with in Marine Steam Boilers by J H Milton and

R, M Leach Selection of a valve of the correct size and loading is importantsince they have a narrow pressure range

Under normal conditions a relief valve should operate consistently withinreasonable limits of its set pressure Incorrect function may be due to thesetting, valve seat deposit or damage Relief valve seats should be checkedwhenever the pump is overhauled

Pressure reducing valves

If it is necessary to provide steam or air at a pressure less than that of the boiler

or compressed air reservoir, a reducing valve is fitted This will maintain thedownstream pressure within defined limits over a range of flow, despite anychanges in supply pressure

In the reducing valve shown (Figure 4.16) the higher inlet pressure (PJ acts

in an upward direction on the main valve and in a downward direction on the

Figure 4.15 Relief valve

128 Valves and pipelines

Figure 4.16 Pressure reducing valve

controlling flexible diaphragm and the piston beneath it These two parts are in

a state of balance The large spring pushes against the spindle, tending to openthe valve against the reduced steam pressure (P2) acting on the area y!2 at thetop of the valve Any decrease in pressure on the outlet side, will allow thevalve to be pushed open by the spring Any increase will close it It is importantthat this type of valve is installed in the vertical position

The self-regulating valve can be replaced by an automatic process controlvalve for fluid pressure control as used for control functions in unmannedmachinery spaces This permits remote control of the set-point of the valve and

by careful selection of valve trim (the control industry's term for the internalparts of the valve which come in contact with the controlled fluid and form the*actual control portion) a variety of flow characteristics can be achieved The

subject is dealt with further in Chapter 16,

Quick closing valves

Fuel oil service and some other tanks must be fitted with valves that can beclosed rapidly and remotely in the event of an emergency such as fire Wireoperated valves (Figure 4.17) are commonly fitted, with wire pull levers locatedexternally to the machinery space The type shown is a Howden Instantervalve As an alternative a hydraulically operated quick-closing valve (Figure4.18) can be fitted

Quick-closing valves are examined and tested when installed and thenperiodically when the tank is not in use, to ensure that the mechanismfunctions correctly Wires are sometimes found to be slack or hydraulicsystems empty

Figure 4.17 Howden Instanter quick closing valve (James Howden & Co.

Ltd.)