Marine Auxiliary Machinery 7E Episode 4 doc

110 Ship service systems bacteria is generated. These bacteria are said to be anaerobic. Whilst they are equally capable of breaking down sludge, in so doing they generate gases such as hydrogen sulphide and methane. Continuing use of a biological sewage system after a failure of the air supply, could result in propagation of anaerobic bacteria and processes. The gases produced by anaerobic activity are dangerous, 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 drum which acts both as a filter and a cutter) or simply passed through a bar screen from where it passes into the first chamber. Air is supplied to this chamber via a diffuser which breaks the air up into fine bubbles. The air is forced through the diffuser by a compressor. After a while a biological sludge is formed and this is dispersed throughout the tank by the agitation caused by the rising air bubbles. The liquid from the aeration tank passes to a settling tank where under quiescent conditions, the activated sludge, as it is known, settles and leaves a clear effluent. The activated sludge cannot be allowed to remain in the settling tank since there is no oxygen supplied to this area and in a very short time the collected sludge would become anaerobic and give off offensive odours. The sludge is therefore continuously recycled to the aeration tank where it mixes with the incoming waste to assist in the treatment process. Over a period of time the quantity of sludge in an aeration tank increases due to the collection of inert residues resulting from the digestion process, this Figure 3.17 Biological sewage treatment plant (Hamworthy) Ship service systems 111 build up in sludge is measured in ppm or mg/litre, the rate of increase being a function of the tank size. Most marine biological waste treatment plants are designed to be desludged at intervals of about three months. The desludging operation entails pumping out about three quarters of the aeration tank contents and refilling with clean water. The clear effluent discharged from a settling tank must be disinfected to reduce the number of coliforms to an acceptable level. Disinfection is achieved by treating the clean effluent with a solution of calcium or sodium hypochlorite, this is usually carried out in a tank or compartment on the end of the sewage treatment unit. The chlorinator shown in Figure 3.17 uses tablets of calcium hypochlorite retained in perforated plastic tubes around which the clean effluent flows dissolving some of the tablet material as it does so. The treated effluent is then held in the collection tank for 60 minutes to enable the process of disinfection to be completed. In some plants the disinfection is carried 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 any applicable rules of the responsible government department and those of the designated classification society. Guidance is provided in government and classification society publications and it is required that plans for principal systems are submitted for approval. The safety and reliability of critical individual fittings is ensured by a requirement that they are made to specification by an approved manufacturer. Materials are tested, welds are inspected, 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 pipe systems must be of the same standard and obtained if necessary, from an approved maker. Some accidents have been the result of replacement valves and 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 with sea 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 as internal protection. Sea water is an electrolyte and therefore a conductor of electricity, because the molecules of its dissolved salts split into positive and negative ions which are available as current carriers. Electrolytic action can result if there are different metals or even differences in the same metal in a pipeline. Galvanic corrosion can occur if the different metals are connected electrically and mutually in contact with the sea water. A corrosion cell formed between steel and brass in contact with sea water results in wastage of the less noble steel. A list is given in the galvanic series, in which the more noble metals are placed in order after the less noble thus: zinc, aluminium, carbon steels, cast iron, lead—tin alloys, 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 noble sacrificial anodes can give protection. Valves and pipelines 113 Steel 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 before galvanizing or application of a lining, so that weld spatter and deposits from manufacture can be removed. The mild steel, electric resistance welded (ERW) or hot rolled pipes are galvanized by hot dipping. Inadequate protection of steel, results if there are pinholes or discontinuities in protective linings. Linings should always be carried over the flange faces. Mild steel welded fabrications, similarly lined, are also used for large ship side fittings. Seamless mild steel is used for steam, high pressure air, feed discharges and all oil fuel pressure piping. Its strength reduces however, at about 460°C and above this figure, steels require small additions of alloying materials such as molybdenum and chromium. Flanges are secured to steel pipes by fusion welding or by screwing and expanding. Cast iron Cast iron has poor corrosion resistance in sea water, being especially vulnerable to graphitization. This form of attack gradually removes the iron from the surface in contact with sea water to leave soft, black graphite. The weakness of ordinary grey cast iron in tension and under shock loading limits its use to low pressure applications, and the brittle nature of ordinary grey cast iron 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 the production of fittings and fortunately techniques for improving strength have been developed. Spheroidal graphite cast iron (SG iron) and meehanite are examples of high strength versions of the material. These are suitable for use in ship side valves if made to specification by an approved manufacturer. SG iron may be used for high pressure services and for steam below 461°C. Cast iron with its high carbon content and consequent low melting temperature is ideal for the production of fittings by casting. Copper Copper pipes are suitable for moderate pressures and temperatures. Flanges are secured 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' and the practice has prevailed for so long as to be accepted. Aluminium brass and other non-ferrous pipelines, are considered very resistant to corrosion in sea water, but concentrated galvanic corrosion can occur if some part of the pipe system has a different make up. A localized corrosion cell can be set up when a fitting, such as a thermometer pocket, is of a brass, bronze or other material which is different to the parent material. Pipe systems are ideally of the same material throughout but non-ferrous alloys are protected against corrosion by the deposition of iron ions so that use of iron or steel fittings is beneficial Iron ion protection can alternatively be supplied from sacrificial or driven iron anodes or by dosing with ferrous sulphate. Dezincification of brasses is a particular type of corrosion that occurs in the presence of sea water. The attack removes zinc from the alloy, leaving porous copper which is soft. The problem is marked by a patch of copper colour in the brass. Dezincification is inhibited in brasses which are intended for sea-water contact 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 a phenomenon associated chiefly with brass tube which has been stressed by expanding or by being worked in the unsoftened condition and which is also in contact with corrosive fluids, such as sea water. Splitting can occur suddenly, or even violently as a result of stress corrosion cracking. Stainless steel A different problem is presented by corrosive liquids and those that contain hard particles and are therefore likely to cause erosion. These can cause differing rates of wastage in conventional metal pipes or cargo tanks. With some corrosive liquids wastage is slow enough, lasting over a period of years, to permit the use of common metals. Expensive stainless steel is widely used for the cargo pipes of chemical tankers intended for carriage of very corrosive cargoes. 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 by protuberances, 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 metal surfaces. The exposed metal surfaces can form galvanic corrosion cells with adjacent areas where oxide film is still present. Erosion is reduced by limiting speed of flow, avoiding sharp bends, changes of section and impediments to flow such as incorrectly cut jointing or weld deposits. Speed of liquid flow should be no greater than 1 m/s for copper; 3 m/s for galvanised steel and aluminium brass; 3.5 m/s for 90/10 cupro-nickel: 4 m/s for 70/30 cupro-nickel. Valves and pipelines 115 Strength of materials The strength of materials used for pipes and fittings must be adequate for the system pressures and possible over-pressures. Pipelines and valves, for example, used to carry and control the flow of high temperature, high pressure steam must obviously be made to very exacting specifications by approved manufacturers. Various and often varying pressures and temperatures pose problems. Temperatures of about 450°C can cause recrystallization and creep in iron and steels. Very low temperatures as with liquefied natural gas, can result in brittle failure. Varying temperatures give problems with stress due to expansion and contraction. The term fittings covers valves, cocks, branch and bulkhead pieces, reducers, strainers and filters, separators and expansion pieces, in short, everything in a system which is not a pipe. Couplings and unions are used only in small bore pipes. Cast iron and gunmetal fittings are used freely in small sizes at moderate pressures. Large fittings, those for high pressure and temperature and for oil fuel 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 the resulting creep. Pipe installation Vibration is the frequent cause of eventual pipe failure but supports and clips to prevent this problem must permit free expansion and contraction. A pipe which has to be twisted or bowed when being connected, has inbuilt stress which can lead to ultimate failure. Pipes should be accurately made (particularly replacement sections) and installed with simple supports before being permanently clipped. If pumps are designed so that the driving motor or turbine is mounted upon an extension to the pump casing proper, the tendency for mal-alignment, due to pipeline stresses, is practically eliminated. Nevertheless, it is essential that the pipe systems and heavy valve chests, are separately supported and stayed during installation, the flanged connection to the pumps being the last to be coupled after the faces are correctly aligned. This can 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 alignment and with their faces parallel. This alignment should be checked after tightening the 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 by bands 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 are incorrectly coloured. Before working on or using a pipe system, it should be traced and verified. Cleaning the system It is often found, in new ships, that the bilges and bilge systems have not been thoroughly cleaned with the result that wood, nuts, bolts, rags and other debris are found inside valves and pipes after initial bilge pumping. These choke the valve-chests and prevent the valves from being properly closed. They also block strainers. It is vital to clean before the bilge system is tested to ensure that all 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 has sometimes been found due to failure to cut apertures in metal or joints. Obviously with hydraulic or pneumatic pipe systems, foreign objects or residues 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 has been caused by 'water hammer' when steam has been admitted to pipes containing water, especially when a slight inclination of the pipe from the horizontal 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 moves along the pipe at great speed. The impact of this water at a bend or valve, can cause fracture of the pipe. Water hammer is indicated by severe and often repeated 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 opened before 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 thoroughly wanned. Only then should the valve be opened fully. Expansion arrangements Provision must be made in pipe systems to accommodate changes in length due to change of temperature, and so prevent undue stress or distortion as pipes expand or contract. One type of expansion joint (Figure 4.3) has an anchored sleeve with a stuffing box and gland in which an extension of the joining pipe can slide freely within imposed limits. Simpler schemes (Figure 4.2a 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 pipe diameter and thickness other methods may be more appropriate. Valves and pipelines 117 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 used since they will absorb some movement or vibration in several planes, eliminate maintenance, reduce friction and heat losses. Maximum and minimum working temperatures must be considered when choosing a bellows piece, which must be so installed that it is neither over-compressed nor over-extended. Its length must be correct for the temperature change. Stainless steel is the usual material for temperatures up to 500°C. Beyond that and for severe corrosive conditions, other materials are required. Normally the bellows has an internal sleeve, to give smooth flow, to act as a heat shield and to prevent erosion. If exposed to the possibility of external damage, it should have a cover. In usual marine applications, bellows joints are designed and fitted to accommodate straight-line axial movement only and the associated 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 and extension, 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 the unit be installed at its correct length as extension or compression outside its specified 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 the fitting, covers the necessary clearance in the bulkhead. Joints Joints between flanges should be impervious to damage from the fluids carried and a variety of materials are available to suit the different requirements. Rubber for example, with or without cotton insertion, is suitable for water but not for oil. High pressure can force a joint out of a flange so that the thinnest joints are used for the highest pressures. Some jointing fabrics are sheathed with 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 Valves and pipelines 119 area adjacent to the pipe bore. Most materials deteriorate with time and temperature so that periodic replacement may be necessary. Graphite compounds assist flexibility. Mating flanges should be parallel and accurately machined. Bolts should fit reasonably well and have good threads. Cocks and valves Cocks and valves are designed to control or interrupt flow. This is done in cocks 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. These fittings 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 with tightness 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 oil tanks, must be fitted with cocks having parallel as opposed to tapered plugs. This, together with the requirement for weighted handles which will automatically close the cock when released, is for safety. Tapered plugs, when tightened to hold the cock open for sounding and then forgotten, have contributed to fires when tanks have overflowed. Boiler blowdown cocks on the ship's shell, are constructed so that the handle can be removed only when the cock is closed. Globe valves The globe valve (Figure 4.6) has a bulbous body, housing a valve seat and screw down plug or disc arranged at right angles to the axis of the pipe. For the valve shown, both seat and disc faces are stellited and almost indestructible. Alternatively, the seat may be renewable and screwed into the valve chest or given a light interference fit and secured by grub screw. The seatings may be flat or more commonly mitred. The spindle or stem may have a vee or square thread, 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 the button may locate in a simple horseshoe. Leakage along the valve spindle is prevented by a stuffing box, packed with a suitable material and a gland. If there is a change of direction, as in a bilge suction, the valve is referred to as an angle valve. Flow is from below the valve seat, so that the gland is not subject [...]... 6 .4 8.5 10.7 Vel head for bell-mouthed entry 0.82 1.31 1.8 Bend Foot valve 0.76 1.13 1.52 1.95 2 .44 0. 24 0.36 0.52 0. 64 0.79 1.10 1 .43 1.76 2.31 2.86 3.96 3.3 5.2 6 .4 4.27 5.27 Non-return valve 0.305 0 .49 0.7 0.85 1.06 1 .43 1.86 2.31 Delivery valve full open 0. 24 0.36 0.52 0. 64 0.79 1.10 1 .43 1.76 Strainer less and elbows 0.091 0.152 0.2 14 0.83 1.31 0.275 0.305 0 .42 7 2.31 2.87 3.96 0.58 0.70 5.2 6 .4. .. installed with ball floats (Figure 4. 19) or open floats (Figure 4. 20) for control of a needle valve to release condensate Thermostatic traps Thermostatic traps (Figures 4. 2la, 4. 2ib and 4. 2Ic) use the expansion of an oil-filled element, a bimetallic strip or flexible bellows to actuate a valve As the condensate temperature rises in the oil filled element type (Figure 4. 2la) element A expands to close... Magnetic filters (Figure 4. 27) provide extra protection for engines and gearboxes where iron or steel wear particles are likely to be present Further reading Milton, J H and Leach, R M (1980) Marine Steam Boilers, 4th edn Butterworfhs 138 Valves and pipelines Figure 4. 27 Magnetic filter 5 Pumps and pumping The centrifugal pump is now used for most applications and systems on ships In the machinery space it... despite any changes 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. 14 Pneumatic butterfly valve actuator showing scroll cam ~rr~~g~~~n~ Figure 4. 15 Relief valve 128 Valves and pipelines Figure 4. 16 Pressure reducing valve controlling flexible diaphragm and the piston... trap would be either waterlogged or passing steam With the bimetallic strip type (Figure 4. 2ib) deflection of the bimetallic strip when temperature increases, closes the valve The device will work over a Figure 4. 19 Ball float type mechanical trap Figure 4. 20 Open float mechanical trap 132 Valves and pipelines Figure 4. 21 (a) Oil filled thermostatic steam trap; (b) Bi-metallic steam trap; (c) Bellows type... or wire basket is suspended (shown left Figure 4. 24) Flow through these units is from the top, into the basket and out from the outside of the basket They may be installed as duplex units with three way cocks at inlet and outlet so that one or both baskets can be in use, but one can be shut down for cleaning Close to 136 Valves and pipelines Figure 4. 24 Single strainers for (left) high pressure water,... 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 Ltd.) Example of a non-return valve (Hattersley Newman Hender Figure 4. 8 Gate (or sluice) valve Butterfly valves A butterfly valve (Figure 4. 10) consists basically of a disc pivoted across the bore of a ring body having the same radial dimensions... butterfly valve rotates the valve disc through 90° directly or through a scroll arrangement (Figure 4. 14) Relief valves Excess pressure is eased by a relief valve (Figure 4. 15) This consists of a disc held 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... closed rapidly and remotely in the event of an emergency such as fire Wire operated valves (Figure 4. 17) are commonly fitted, with wire pull levers located externally to the machinery space The type shown is a Howden Instanter valve As an alternative a hydraulically operated quick-closing valve (Figure 4. 18) can be fitted Quick-closing valves are examined and tested when installed and then periodically... low pressure water or oil service sea-water suction valves, similar basket strainers (shown right in Figure 4. 24) having air release cocks, are installed Lubricating oil systems are fitted with a wide variety of strainers some of which can be cleaned in situ The knife edge strainer, (Figure 4. 25) has a series of discs ganged to a shaft Interspaced between the discs are a number of thin ringers The . 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 Ltd.) Figure 4. 8 Gate (or sluice) valve Butterfly. 90° directly or through a scroll arrangement (Figure 4. 14) . Relief valves Excess pressure is eased by a relief valve (Figure 4. 15). This consists of a disc held closed by a spring. 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. 14 Pneumatic butterfly valve