Marine Auxiliary Machinery 7E Episode 13 doc

The control lever is operated manually from either side of thebulkhead and movement of the lever actuates a pilot valve controlling thepump motor circuit under all conditions of control.

doors in the installation In an emergency the supply is taken from the ship'sbatteries.

Local control

A control is located adjacent to each door for shut/open or for intermediatepositions The control lever is operated manually from either side of thebulkhead and movement of the lever actuates a pilot valve controlling thepump motor circuit under all conditions of control Advantage is taken of thearea differential of the piston in the door operating cylinder for closing andopening the door A slightly greater force is available in unsealing (opening)the door

The control lever is spring loaded to the mid-position to ensure a hydrauliclock within the cylinder, thus preventing any possible movement of the doordue to the motion of the ship

Bridge control

The bridge controller is designed to close (in sequence) a maximum of twentydoors within a specified time of 60 s This period includes a 10 s audible alarmperiod at each door before the closing movement starts and the alarmcontinues to sound until each door is fully closed

Close and re-open controls energize solenoids at the door control valve tostart the pump motor Limit switches, actuated by the movement of each door,control the electric circuit

Any door which is re-opened locally while the system is under the closedcondition from the bridge controller will automatically reclose when the localcontrol lever is released Indication that power is available to close the doors isgiven by a 'power on' amber light, with a test button provided

Emergency control

Under conditions when no power is available, the doors may be closed andopened by a manually operated pump and control valves at either of twopositions:

(a) Adjacent to each door from either side of the bulkhead, and

(b) From a position on the bulkhead deck

Light indication

A coloured light indicator on the bridge shows the position of each door: greenfor shut and red for open The appropriate lights are duplicated alongside themanually operated emergency pumps on the bulkhead deck To ensure that

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power is always available all the indicator lights and alarms are connecteddirect to the d.c supply and to the ship's batteries

Figure 15,7 Overhead gravity davits (Weiin Davit & Engineering Co Ltd)

weight will provide a positive means of application and the boat will be held atany intermediate position This condition applies throughout the outboardmovement of the boat, from its stowed position until it is waterborne Figure15.9 shows the hand brake arrangement The main brake (on the left in Figure15.8 is fitted with two shoes, pivoted at one end and coupled at the other withthe weighted lever, by a link The lever projects from the casing through awatertight seal The shoes are Ferodo-lined and have a normal useful life of fiveyears or more.

Figure 15.8 shows in section the main brake described above and thecentrifugal brake (shown on the right-hand side of the drawing) Thecentrifugal brake limits the rate of descent of the boat when the handbrake isnot engaged Shoes of calculated weight act on the inner surface of a stationarydrum, being thrown out by centrifugal effect against the restraining springs.The lowering speed of the boat can be kept within the predesigned limit of

36 m/min A ratchet arrangement ensures that the drums will not reverse andthe boat drop back towards the water in the event of a power failure when aboat is being hoisted

The brakes require regular inspection for wear and after replacement must beproperly tested The handbrake must obviously be able to hold the boat and to

arrest downward movement after a limited free run to test the limiting effect of

the centrifugal brake

Lifeboat engines

The SOLAS 1974 Convention requires that lifeboat engines, where fitted,should be compression ignition engines Both water-cooled and air-cooledengines are installed in lifeboats Because of their cold starting characteristics,simplicity and low maintenance requirements, air-cooled engines might befavoured for open lifeboats, but water cooled engines are usual For totallyenclosed lifeboats with water-cooled engines a small single-pass heatexchanger (usually just a large bore tube) may be arranged on the outside of thelifeboat bottom, for cooling of the freshwater circuit by the sea

Hydraulic cranking system

Some lifeboat engines and the larger diesel-driven emergency generator setsmay be fitted with a hydraulic starter motor The Startorque system (Figure15.10) is such a device which uses an automatically charged accumulator toprovide power to the hydraulic cranking motor The accumulator is prechargedwith nitrogen to a pressure of 83 bar

The system accumulator can be re-charged by a hand pump B or by anengine driven pump I The stored energy in the accumulator is released by ahand-operated valve F, assisted by a check valve which allows a small quantity

of oil to pass, enabling full engagement of the starter motor pinion with theflywheel The valve then opens fully allowing full flow to the hydraulic

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Figure 15.8 Welin davit winch Section of brakes showing centrifugal

brake (Welin Davit & Engineering Co Ltd)

cranking motor which generates enough torque to start the engine The oilreturns to the reservoir where it is pumped back to the accumulator by eitherthe hand pump or the engine driven pump

An off-loading valve H protects the system from being overcharged andspills at 20.7 bars back to the reservoir on an open circuit maintaining flowthrough the re-charging pump I at all times The unit cranks the engine at about

375 rev/min for nine revolutions although these figures may be varied to suitparticular engines by modifying the size of the accumulator

Figure 15.9 Section of main brake (Weiin Davit & Engineering Co Ltd

Figure 15,10 Schematic of Startorque system showing principles of

operation

A Oil reservoir and filter F Starter operating valve

B Hand pump G Hydraulic cranking motor

C Non-return valves H Off loading value

D Hydraulic accumulator I Mechanical recharging pump

E Hand shut-off valve

Whistles and sirens

Audible signals, to indicate the presence of a ship in poor visibility or to informother vessels of the ship's intended movements, have long been used at sea.Steam, air and electric whistles have all been fitted for this duty Some have

audible ranges of as much 9 nautical miles.

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The air and steam whistles operate on much the same principle, namely theworking fluid causes a diaphragm to vibrate and the sound waves generated

are amplified in a horn.

The arrangement of a Super Tyfon air whistle is shown in Figure 15,11 Thediaphragm details can be seen in Figure 15.12 and a section of the whistle'scontrol valve is shown in Figure 15.13 Units of this type may be foundworking from air pressures of 6 to 42 bar with air consumptions in the range

Figure 15.11 Super Tyfon air whistle (Kockums, Sweden)

Figure 15.12 Detailed arrangement of diaphragm (Kockums, Sweden)

Figure 15.13 Control valve (Kockums, Sweden)

1, Pilot valve 3 Lever 5 Housing 7, Choke plug

2 Filter 4 Piston 6 Spindle

25—35 litres/s Variously sized choke plugs (7) are fitted depending on thesupply pressure Alteratively an adjustable choke may be provided instead ofthe plug It is important that the correct choke setting is selected to match themaximum supply pressure If this is too high for the setting the diaphragmmight break; if the pressure is too low the volume will be inadequate.Primary control of the whistle is afforded by one or more push buttonslocated at strategic points on the bridge By pushing the button an electric

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circuit activates the solenoid pilot valve (1) which then causes piston (4) tomove, allowing air to pass to the diaphragm An automatic device is usuallyfitted which permits the selection of one or more automatic periodic signals.The Tyfon auto-control (Figure 15.14) allows automatic selection of either a 1

or 2 minute cycle The normal duration of signal is 5 s The electric circuit forthis equipment is so arranged that, when on automatic signal, depression of themanual button overrides the preselected sequence

Secondary control of the whistle is by a lanyard which directly operateslever (3) (Figure 15.13) allowing air on to the diaphragm

Air enters the whistle valve unit via a filter (2) which requires cleaningoccasionally An additional filter is frequently fitted at the lowest point of theair supply line and this will require draining periodically A routine inspection

of this filter at monthly intervals is recommended

Should the diaphragm require changing, the dirt cover should be removedand the 12 retaining bolts should be unscrewed, it is not necessary to removethe bottom flange The O ring, on which the diaphragm seats, should always berenewed and it is important to tighten the 12 retaining screws evenly.Whenever work is to be done on the whistle the air and the electrical supply

to the unit must be isolated

Figure 15.14 Wiring diagram for Tyfon auto-control whistle (Kockums,

Sweden)

Electric whistles

An Electro-Tyfon whistle is shown in Figure 15.15 from which it can be seenthat the electric motor drives a reciprocating piston through a gear train andcrank This generates an air pressure which vibrates the diaphragm

Further reading

International Conference on Safety of Life at Sea, IMCO (1974).

Fire Prevention and Detection, Marine Engineers' Review, 1980.

The Merchant Shipping (Fire Appliances) Rules.

Survey of Fire Appliances; Instructions for the Guidance of Surveyors HMSO,

Abbott, H (1987) Safer by Design, The Design Council.

Figure 15.15 Sectional arrangement of electro-Tyfon whistle (Kockums

Ltd)

Safety and safety equipment 479

Bignell, V., Peters, G and Pyrn, C (1977) Catastrophic Failures, The Open University

Press,

Kletz, T HAZOP & HAZAN, The Institution of Chemical Engineers.

Kletz, T (1988) Learning from Accidents in Industry, Butterworth-Heinemann,

Control and instrumentation

The periodically unmanned machinery space was made possible first by anevolutionary process which took place over a number of years and finally bythe introduction of bridge control, which for diesel engines, had long beenpossible During the years of progress and refinement, many improvementswere made to various types of machinery to enhance reliability Some types ofequipment such as steam reciprocating pumps for engine room services andcargo work, were discarded in favour of centrifugal pumps which are simpleand reliable Automatic operation had been introduced for refrigerationequipment as the then modern CFC type refrigerants, took over from CO2systems Control equipment for auxiliary boilers and engine cooling circuits,followed, so eliminating other routine duties Instrumentation and alarms hadbeen improved and then fitted more extensively to give more completemonitoring with shut down as appropriate Long before the advent of the UMScertificate, main machinery was operating with little more than routineattention, as the engine room staff carried out maintenance

Monitoring and control equipment together with the various alarms, mustcontinue to operate when any or all machinery and systems being monitored,have failed The power supply for electrical alarms and control equipment istherefore independent of the main supply and may be based on a 24 V directcurrent (d.c.) system The application of a moderate voltage, direct currentsupply to control means that the equipment is simple and safe, with batteriesincorporated for independent operation should there be a main electrical powerfailure The 24V d.c supply may be provided through a transformer andrectifier arrangement with batteries in float or kept fully charged foremergencies or power can be supplied from batteries which are alternately onload or on charge

Low pressure d.c system

A ship's 440 V alternating current (a.c.) electrical power system is, of course,highly dangerous and not easily adaptable for control purposes Alternatingcurrent supplies operating at 55 V or more are considered as potentially lethalpartly because of the frequency of 60 Hz (or 50 Hz) which can cause muscularcontraction in shock victims Additionally the alternating nature of the supplycauses generation of nuisance stray currents and problems with solenoids.Direct current electrical systems operating at battery voltage, when used for

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control and emergency systems, are safe, ideal for operation of solenoids,require minimum size of both wire and insulation, can be driven from mainsthrough transformers and rectifiers with stand-by batteries to provideemergency power

The mains a.e, transformed down to 30 V (necessarily greater than the 24 Vbatteries for charging) and rectified, supplies the control system and maintainsthe emergency battery charge In the event of mains failure, the de-energizedsolenoid activates a connection between batteries and the system while alsoisolating it from the mains

Instrument and control air

The derivation of quality air for control systems is dealt with in Chapter 2

Control system

The simple control loop has three elements, the measuring element, thecomparator element and the controlling element The loop may be effectedpneumatically, electronically or hydraulically In some instances the controlloop will be a hybrid system perhaps utilizing electronic sensors, a pneumaticrelay system and hydraulic or electric valve actuators Each system has itsstrengths and weaknesses:

Pneumatics — require a source of clean dry air — can freeze in low temperature,

exposed conditions, but equipment is well-proven and widely used

Electronics — good response speeds with little or no transmission losses over

long distances, easily integrated with data logging system, required to beintrinsically safe in hazardous zones

Hydraulics - require a power pack, may require accumulator for fail-safe action

— compact and powerful and particularly beneficial in exposed conditions.

Measurement of process conditions

The range of parameters to be measured in merchant ships includestemperatures, pressures, liquid levels, speed of rotation, flow, electricalquantities and chemical qualities Instrumentation used for remote informationgathering purposes invariably converts the measured parameter to an electricalsignal which may be used to indicate the measured value on a suitablycalibrated scale, provide input information to a data logger or computer,initiate an alarm or provide a signal for a process controller

As stated earlier, however, the more favoured means of providing processcontrol information (as opposed to information display only) is to use apneumatic system

Electrical transducers

Any device used to measure one parameter in terms of another, such as change

of temperature by change in electrical resistance, is called a transducer Thefollowing are examples of electrical transducers used in shipboard instrumenta-tion systems

Temperature measurement

Liquid in glass, mercury in steel and vapour pressure type thermometers havebeen used on ships for many years The three main types employing electricalproperties are the resistance thermometer, the thermocouple and thethermistor These are ideal for the provision of input to local or panel mounteddisplays, data loggers or control systems

Essentially, a resistance thermometer is a precision resistor with a knowntemperature coefficient of resistance (i.e change of resistance with temperature).The majority of resistance thermometers used in marine systems have as theiractive element a coil of fine platinum wire mounted on a ceramic former The

common standard of calibration is 100 Q, at 0°C, increasing by approx 0.385 O

per °C up to 100°C For more accurate calibration, the manufacturer'stemperature/resistance tables should be consulted Resistance thermometerelements may be housed in many configurations to suit particular applications.The most widely used housing is a stainless steel tube surmounted by athreaded portion and connecting head for mounting in pipelines or tanks.Another type comprises a length of mineral-insulated, stainless steel or coppercovered cable, with the resistance thermometer element built into one end.This design is useful to measure temperatures in difficult locations such as thesterntube outboard bearings

Thermocouples are formed by the junction of two wires of dissimilar metal.When the free ends of these wires are connected to a measuring circuit, avoltage will appear across the instrument terminals which is a function of thedifference in temperature between the junction of the two thermocouple wires(hot junction) and the instrument terminals (cold junction) This voltage isknown as a thermo-electric e.m.f., and is different for the various thermo-couplematerials used Typical thermocouple combinations using specially developedalloys are copper/constantan, iron/constantan and chromel/constantan, thelatter producing the largest signal (approx 53 mV at 700°C) In many cases, it

is not practicable to run the thermocouple leads back to the measuringinstrument without some break point This may be achieved by:

(a) Extension leads These are simply cables of the same materials as thethermocouple element

(b) Compensating cables These cables may be made of a cheaper alloyhaving the same thermo-electric properties as the thermocouple, butbeing incapable of withstanding the same environmental conditions.(c) Copper cables If copper wires are introduced at an intermediate point in

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the cable run, then the thermocouple will measure the difference intemperature between the hot junction and the point at which the change

to copper wires is introduced,

The main advantage of thermocouples over resistance thermometers, aremechanical strength and when necessary small dimensions The disadvantagesare a small working signal, the problem of controlling or compensating for thecold junction temperature and lower accuracy

The thermistor has many of the advantages of both thermocouples andresistance thermometers Common types take the form of a small bead ofsemi-conducting material, from which two measuring leads are led away to aterminal arrangement, with mechanical protection in the various formssuggested for resistance thermometers The thermistor element of one type,exhibits an extremely large negative temperature coefficient in some casesthousands of ohms for a temperature shift of 100°C These devices can be madevery small, very rugged and with extremely accurate resistance/temperaturecharacteristics

Pressures

The majority of pressure transducers operate by first producing a mechanicalmotion proportional to applied pressure, from which is derived an electricalsignal by some secondary mechanism

The types most common are the bourdon tube/potentiometer mechanism,

in which the motion of the free end of tube is used to move the slide of apotentiometer, and the diaphragm/strain gauge type There are two generictypes of strain gauge, known as bonded and unbonded gauges Bonded gaugesare cemented to the diaphragm, and consist essentially of a grid of conductingmaterial which exhibits a small change in electrical resistance when its shape ischanged due to the flexing of the diaphragm In unbonded types the activeelement is generally a wire stretched between rigid supports which aredisplaced by the motion of the diaphragm (Figure 16.1)

As strain gauges only produce a very small change in resistance, they arenormally used in Wheatstone bridge circuits

Measurement of liquid levels

The majority of engine room service tanks need only be monitored betweenfixed high and low limits, so that float type level switches are adequate Forbunker tanks, a widely used system is the 'bubbler' type level gauge, in whichtubes are led to the tanks The pressure necessary to pass a small volume of airthrough the tubes is reflected by manometer tubes calibrated in terms of tanklevel

Capacitative type sensors may also be used, in which the level in a tank ismeasured by the change in capacitance in a circuit comprising two concentric

Figure 16.1 Pressure transducer (Bell & Howell Consolidated

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conjunction with differential pressure transducers, flow being proportional tothe square root of the pressure drop across the constructive element

The Torductor torque transducer

The ring type Torductor torque transducer consists of three identicalpole-rings with poles mounted on them The number of poles fitted is chosen as

a multiple of four to reduce the influence of the ring joints necessary for easymounting of the torque transducer around the shaft The poles are fitted withcoils having alternately reversed winding directions The middle ring isdisplaced half the pole-pitch relative to the outer rings and the distancebetween the rings is approximately equivalent to half the pole-pitch Themiddle ring is normally used as primary and excited with 50 or 60 Hz The twoouter rings are used as secondaries and connected in series, with mutuallyreversed winding directions, as indicated by the letters A and B in Figure 16.2,which shows the development of the shaft surface under the ring typeTorductor torque transducer and the projection of the poles The primary polesare marked N and S depicting a certain instant in the magnetizing cycle

If the shaft is unloaded and without internal stresses, the magnetic fieldsbetween the different N- and S-poles will be symmetrical so that the zeroequipotential lines will be situated symmetrically under the secondary poles Aand B The secondary flux and hence the secondary voltage is thus zero at zerostress

When torque is applied to the shaft, the principal stresses + 0 indicated inFigure 16.2 are obtained The permeability in the direction of tension, i.e.between the poles B and S and between A and N, is then increased, while thepermeability in the direction of compression, i.e between the poles B and Nand between A and S is decreased Thus all A-poles come magnetically nearer

to the N-poles and all the B-poles magnetically nearer to the S-poles The result

is magnetically the same as if the secondary rings had been tangentiallydisplaced in mutually opposite directions and opposite to the torsion of theshaft The resulting fluxes through the poles co-operate in inducing an outputvoltage in the series-coupled windings The output is normally of the order of

10 V and 1 mA, i.e large enough for an instrument without any amplification

As the ring type Torductor torque transducer measures almost uniformlyaround the shaft, the 45° stresses are virtually integrated around the

Figure 16.2 Principle of ring type Torductor torque transducer

(a) and (b) Physical arrangement ± Principal stresses

(c) Development of shaft surface under the N, S Primary poles

Torductor poles A, B Secondary poles

circumference and the modulation of the output voltage is thus reduced to avery small value

The effective response time of the Torductor is mainly determined by theexciting frequency and the desired degree of filtering of the output signal For50-60 Hz excitation it can be of the order of 10—30ms, dependent on thecircuitry chosen

The ring type Torductor torque transducer can be statically calibrated withgood accuracy The calibration has to be performed with the torque transducermounted around the shaft as the sensitivity is dependent on its compositionand heat treatment This is however, a drawback which the Torductor torquetransducer in principle shares with all other torque transducers

The Torductor tachometer

The tachometer (Figure 16.3) is a three-phase a.c generator The frequency,and not the voltage, is used as a measure of the rate of rotation This avoids theproblem of brushes, non-linearity in the amplitude of the output voltage andconductor resistances The electronic equipment consists of both analogue anddigital circuits It should be mentioned that a very stable oscillator, common to

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Figure 16.3 Torductor power meter

1 Torque transducer 4 Integrator N Total number of revolutions

2 Tacho-generator T Torque P Shaft horsepower

3 Multiplier n Rate of rotation W, Total power output

the entire equipment, is used as a reference for generating pulses having apredetermined duration The operation of the electronic equipment is briefly asfollows

The tachometer signal is converted into a square wave having a constantpulse length and with a frequency proportional to the tachometer frequency.This square wave controls an electronic contact with a constant input voltage.The mean current through this will then be proportional to the tachometerfrequency The current is fed into an a.c amplifier, whose output voltage is thusdirectly proportional to the rate of rotation The direction of rotation and thusalso the polarity of the output signal, is determined through sensing of thephase sequence The total number of revolutions is indicated on a counter onthe engine-room console A digital circuit imparts to the counter one pulse forevery tenth revolution of the shaft

The shaft (delivered) horsepower is obtained according to the same principle

as the rate of rotation If the constant input voltage of the electronic contact isreplaced by a voltage proportional to the torque, the mean current through thecontact will be proportional to the shaft horsepower The current is fed aspreviously to an a.c amplifier, which in its turn drives a pointer instrument.The total power output is obtained through integration of the power signal,which is therefore fed into an amplifier with capacitance feedback serving as anelectronic integrator A level discriminator senses when the output voltagefrom the amplifier exceeds a certain value The integrator then receives aresetting pulse via an electronic contact Each of these pulses represents acertain amount of energy and the total number of pulses the total output of themachinery Indication is accomplished with an electro-mechanical counter

Pneumatic control systems

For this discussion, measurement is the value of a process quantity or qualityobtained by some suitable measuring device It may be recorded or indicated

by a pen or pointer on a scale The terms 'pen' and 'measurement' willsometimes be used synonymously For example a 'change in pen positionupscale' means an increase in measurement

A simple control loop is shown in Figure 16.4 The measuring elementdetermines the pressure downstream of the control valve and transmits this tothe comparator element, which is a pneumatic controller The pneumaticcontroller compares the measured value with a manually set desired value andgenerates a correcting signal which causes a control valve to open or close AFoxboro Model 40 Controller is shown schematically in Figure 16.5 whichserves to illustrate the way in which pneumatic controllers work The desiredvalue sought in the controlled system is set by turning the control settingindex knob, thus positioning the index A system of levers and links is arranged

so that whatever position the index is placed in any deviation by the pen fromthe pre-set index will result in a proportional movement of the horizontal linkattached to the proportioning lever

The particular controller shown in Figure 16.5 is known as a proportional orsingle-term controller This type of instrument produces a correctingpneumatic signal in the range 0.2—1 bar which is proportional to the deviation

of the measured value (pen position) from the desired value (index position).Like most proportional controllers the Model 40 incorporates a device foraltering the ratio of actuating signal change to feedback position change; thusthe relationship between pen change and output change, called proportionalband, is adjustable to suit the process The effects of proportional bandadjustment are illustrated in Figure 16.6

The basic device used to modify the signal pressure is the flapper nozzle unit

A regulated air supply is passed through a reducing valve of fine capillary bore

Figure 16.4 Simple pressure control loop

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Figure 16.5 Model 40 controller - proportional (Foxboro Yoxall)

Figure 16,6 Effects of successive increments of proportional bandwidth

and thence to a nozzle of much greater diameter If the nozzle is unobstructedthe pressure in the intervening tubing will be low If the flapper is re-positionedcloser to the nozzle, the pressure will rise in the section between the restrictionand nozzle By connecting the input of an amplifying relay (the control relay ofFigure 16.5) between the restriction and the nozzle, a flapper movement in theregion of 0.015 mm can be made to generate a signal change of 0.83 bar Thethree graphs in Figure 16,7 show the behaviour of the process variable after asudden load change A wide proportional band brings the variable back to astable value quickly but at a new value substantially below the original setpoint This effect is known as offset The moderate and narrow proportionalband widths show successively longer stabilization times and smaller offsets.The proportional band adjustment is therefore very important, as it enablesthe control system to be matched to the characteristics of the process undercontrol It is obvious from Figure 16.6 that the essence of band width control isfinding the best compromise to prevent excessive cycling after a load change,which is an undesirable effect