Deck Machinery

• Windlass• Mooring winches• Hatch cover openers (pull wire or hydraulic type)• Winches and derricks or cranes• Gangways and motors• Cargo pumps for LPG/LNG or chemical carriers• Whistle/Horn• Life boat winch and safety equip. drives…

Anchor Handling Efficient working of the anchor windlass is essential to the safety of the ship.

It’s design and performance is subjected to strict classification society rules.

Basically they require that

• Cable lifter brake shall be capable of controlling the cable and anchor when disconnected from the gearing at letting go. The Av. Speed of cable shall be 5-7 m/s.

• The heaving capacity shall be 4-6 times the weight of one anchor at speeds between 9 and 15 mts/minute. The lifting wt shall be between 20-70 tonnes.

• The braking effort obtained at the lifter shall at least 40% of the breaking strength of the cable.

• The windlass must be capable of pulling the anchor from a depth of 25% of the total cable carried, i.e. 50% of the length of chain on one side.

• It should be capable of lifting the anchor from 82.5m to 27.5m at 9m/min.

Normal anchor handling equipment incorporates warp ends for mooring purposes with light line speed of up to 1m/sec.

Drives 1

Electric or Electro hydraulic drives are used for dry cargo deck machines

Electric drives • Should be totally enclosed • DC drives are still used because they got good torque range

over the full speed though they need regular attn..• Control of contactor operated armature resistance is fully

replaced with Ward Leonard system for good regulation ; especially at lowering loads ( The present day Ward Leonard generator is driven by an AC motor)

• DC motors may also be controlled by thyristors which converts AC to variable DC voltage

• AC Induction motors can be wound rotor or cage type. Speed control being pole changing or rotor resistance change type. Another form of AC motor control being VFD drives which controls the applied frequency and voltage.

Drives 2

Hydraulic Systems provide a good means of distribution of power obtained from pump driven by a constant direction/speed AC motor. This oil can be made to drive thro’ hydraulic motors to power the actuating devices. Both constant delivery and variable delivery type pumps and motors are commonly used

The fixed output pumps can be of the Woodward hydraulic engine governor type which maintain reserve oil at pressure to cater to demands

Variable displacement pumps can be of axial or radial piston types where operational valves can be avoided.

Hydraulic System Design

• Careful design, selection, layout, and installation of components essential for the trouble free operation

It is very essential for all hydraulic systems be provided with It is very essential for all hydraulic systems be provided with interlocking arrangements for pump and motors so that control levers interlocking arrangements for pump and motors so that control levers remain automatically in neutral to avoid inadvertent start ups.remain automatically in neutral to avoid inadvertent start ups.

Overload protection thro’ relief valves to safeguard system at 30-40% over pressureAtmospheric contamination isolation, oil compatibility, system cleanliness, regular routine maintenance etc. can see thro’ long periods of trouble free operation

Conventional equipments • Conventional type of equipments are

1. Mooring windlass. Normally either an electric or Hydraulic motor drives 2 cable lifter and 2 warp ends. There are many designs but due to slow speed of cable lifter(3-5rpm) a slow speed worm gear and a single step spur gear between cable lifter and warp end is used

2. Anchor Capstans. Vertical capstans use a vertical shaft, with the motor and gearbox situated below the winch unit (usually below decks. With larger cables the capstan barrels is mounted separately on another shaft

3. Winch windlasses. This arrangement uses a mooring winch to drive the windlass. Both port and starboard units are interconnected to facilitate standby and additional power should the situation arise

Control of WindlassControl of Windlass

• As the location is very vulnerable, the equipment shall demand less maintenance and the design and layout shall reflect this.

• Design on adequate margin of the strength rather than on life is the main criteria while on the planning stage. Slipping clutches safe guard against shocks. Enclosed oil lubricated and open gears are common depending on sizes

• Normally these are controlled locally like starting and manual application of brake while letting go the anchor etc.

• But remote controls are getting popular in the recent times

Anchoring equipment

The anchoring equipment fitted to the majority of vessels consists of two matched units, offering a degree of redundancy. These units consists of an anchor, chain (or for smaller vessels wire), a gypsum or chain lifter wheel, brake, lift motor and various chain stopper arrangements. When not in he use the chain is stowed in a chain locker.Systems fitted with wire are stowed on a drum in the same way as winches.

1. The windlass must be capable of pulling the anchor from a depth of 25% of the total cable carried, i.e. 50% of the length of chain on one side

2. It should be capable of lifting the anchor from 82.5m to 27.5m at 9m/min.

Chain locker

A false bottom is fitted to the chain locker consisting of a perforated plate. This allows water and mud to be removed from the space. The end of the chain is attached to the hull by a quick release mechanism known as the 'bitter end'. The strength of this will not be sufficient to prevent a run away unbraked chain. The arrangement must be easily accessible.

Hawser

The chain is led overboard by a strengthened and reinforced pipe called a Hawser. One of the reasons for bow flare is to allow the anchor and chain to lie well clear of the hull when in use, preventing damage.

Chain stopperFor anchoring operations the stopper bar is locked upright. When it is required to fix the position of the chain the stopper is lowered into the position shown. This allows the brake to be released and is typically used for stowing the anchor. chain stopper arrangements are not designed to stop a runaway chain.

Alternately an arrangement known as the 'devil's claw' may be used which has a forked locking piece. For smaller vessels, and where extra security is required bottle jacks with wire straps passed though the chain may be used.

ChainEnd pull will cause the link to collapse in. This repeated many times will lead to fatigue failure. Hence, stud linked chain is insisted upon

Here a stud is welded on one side in the link to brace it against deformation. An alternative to this albeit in limited use is shown below.

Chain sizingEach vessel is given an equipment number which is calculated with use of a formula and takes into account the vessels size, underwater area and sail area. From this a 'look-up' table may be used to give an appropriate size of cable. The diameter of the chain may be read from this table and differs depending on the grade of steel. This grade of steel varies from U1 ( mild steel), U2 (Special Steel) to U3 (extra special steel).

Chain

The size of cable that is to be used is found by the use of a formula which is Equipment number = D2/3 + 2Bh +A whereD = DisplacementB = beamh = Freeboard + height of deckhouses over B/4 wideA = Transverse area including deckhouses over B/4 wide

Ranging Anchor Chain

Ranging Anchor ChainDuring docking the anchor chain is lowered from the chain locker to the dock bottom and laid out for inspection.

This allows the inspection of the chain for broken or lost chain studs. A random set of links are measured from each shackle length ( Shackle refers to a standard length- nominally 27.5m), of chain joined to other shackle lengths by a splitable link. There is an allowable wear limit allowed nominally 12%.

Anchor designsAnchor shown below is of the 'flipper' type. Regulations allows these to be smaller than standard types used in many small to medium sized tankers

Mooring equipment• Duties of warping capstans and mooring winches vary between 3-30

tonnes @ .3-.6 m/s and twice the speed for recovering light lines. Steel rope up to a max. circumference of 140 mm is used

• Mooring winches tightens the wire up to the stalling capacity of the winch (normally 1.5 times full load) then the load is held by the motor brake

• Auto mooring winches incorporates controls which let off or overhaul at preset tension. There is a certain range of tension associated with each action. This is to limit the hauling capacity of the winch, safe guard against rope breakage, and slackness etc. Spring loaded gear wheels, torsion bars and fluid pressure sensing are common as sensing device in the auto system monitors

• Normally locally controlled however remote control too is popular

• To facilitate easy reversing spur gears are used however worm gearing is also not uncommon

Cargo Handling• Lift load at suitable speed• Hold the load from running back.• Lower the load under strict control.• Smoothly take up of slackness of sling.• Dropping the load as reqd.• Allow the winch to stall on o’load and restart when

the stress is relieved.• Have good acceleration and retardation.Also when electrically driven• Lowering speed shall be safe for the motor armature• Stop running back in the event of power failure • Prevent the winch from restarting on power return

w/o manually starting up.

Cargo Handling Drives

• Electric and hydraulic systems quite common for the cargo winches

• Electro-hydraulic cranes are self contained units with all machinery enclosed in the crane house. This protects it from the weather, corrosion and damage. The standard range covers lifting capacities from 25 to 90 tonnes, with outreaches up to 32 m. Each crane is normally tested electrically, hydraulically and mechanically before installation on board.

Derricks

• For the conventional Union Purchase arrangement or the slewing derrick systems, standard cargo winches are used for all the activities like hoist, luff and slew.

• Cargo winch nos. and capacity are decided in advance keeping in mind the no of hatches and the size to work.

• The speed varies from 0.45 m/s at full load to 1.75m/s at light load with 40 kw at full load of 7 T and 20 kw for 3 T

• Advantage of the derrick system is that only 2 winches are reqd. and has a faster cycle time. But safe working load is less and takes quite some time to rig up the system prior to cargo work.

• Slewing derrick system was an exception to the above and which could be rigged up and change in set up was faster.

Deck Cranes

Deck Cranes• Presently large no. of ships are fitted with the cranes which can be

operational faster and spot the cargo easily.• Pole changing motors are being replaced with Ward Leonard

system or Electro hydraulic system are popular.• Most crane makers incorporate a rope system for luffing and this is

commonly rove to give a level luff. The cable geometry is so arranged that the Jib and luffing motor need not be designed to lift the load. However different heel angles can put a strain on the winch and shall be included while designing.

• Some crane manufacturers use a hydraulic ram for the luff. Pilot operated leak valves ensure safety in the event of loss of pressure. Auto limiting devices are built-in to safeguard against operation beyond permissible jib radius. Some cranes are provided with varying speed depending on the load.

Crane Mounted Load ComputerTo carry out a safe lifting operation a set of variables must be known; these consist of the following

The weight of the lift. The height of the lift The Radius of the lift Obstructions within the lift area The Sea State

The newer version of Cranes have a load computer which measures the load weight, Boom Extension and Boom angle. Form this it can compare computed load against a model stored within its memory. As the load approaches overload, alarms are sounded. The computer has an extra mode which takes into account operation with the fly boom. This load computer is there as a safety factor and in no way should be considered to replace proper planning.

Weight Of lift

This may be either a known weight i.e. a weight which is certified and clearly marked, or an unknown estimated weight- in which case the weight is estimated and a factor of safety applied To be added to the lift weight is the weight of the hook and lifting accessories before calculations are carried out. For the hook this is given as a test weight of

0.20 tonne for 10t hook and headache ball 0.65 tonne for 50t 3 sheave block

Note that unless the lift weight is certified it is always classed as estimated in all circumstances.

Height of the Lift.

This is measured form the boom pivot point and notnot the deck

Radius of Lift

In a similar fashion the radius is measured from the pivot point and not the centerline of the crane. The distance from the pivot to the centerline

Special instructions

Obstructions within lift area The area not only where the load will be lifted and

put down, but also the area covered whilst the crane is slewing. Should this be of particular concern a lifting plan should be created and discussed with the crane driver highlighting areas of concern and how best the Crane drive may avoid them. It should be understood that the crane driver may be unsighted of some of these obstructions therefore where this is considered to be a high risk a lift supervisor should be designated to guide the crane driver at all times.

Special consideration has to be given to lifts of unusual shape or where spreader bars are in use.

Special instructionsThe Sea State Vessel lift operations differ from shore based

operations in that dynamic load forces have to be taken into consideration. The worst sea state condition considered to occur during the whole operation should be used and lift calculations based on that The Dynamic Loading factor stated in QGPS Lifting Equipment Regulations is 2.4 times for routine loading/unloading. A factor of 1.35 may be applied after written consent. maximum wind speed is given as 25knots and maximum wave height of 2m

Lifting tackle InspectionsA lifting tackle inspection by a competent person is required

on all lifting accessories every 6 months. However, it is also required that all lifting accessories are examined for defects before use and this includes all crane operations. Appendix C gives a listing of the failure parameters applicable to typical lifting accessories

Calculation of Boom extension The easiest way to do

the following is with graph paper with suitable scaling

However it is possible to calculate the required boom length.

Checking the Cranes Capability

Checking the Cranes Capability

Here we can see that at 20m radius/28.04m boom extension the lift capability is 11.9 tonne. For a 22m radius with same boom extension the lift capability is 10.5 tonne. As are lift is 6.4 tonne the crane is suitable.

• We therefore instruct the crane driver to Gib Up and Boom out to 27.5 m placing the Gib 2 metres above the lift

• These instructions may also be used for shore crane operations. On the capability chart a darkline denotes the limit of stability and refers to lifting weights with the boom at right angles to the bed rather than over the cab. For shore operations the capability chart refers to full outrigger extension only and a separate chart must be in place if half outrigger extension is to be used

The Effects of Dynamic Loading

• For this document 'Dynamic loading refers only to the effects of movement of the vessel due to rolling only. The effects of Pitching, lift and lower acceleration and deceleration, relative movement between vessel and platform from which weight is being lifted or lowered to is not considered.

•Effects of Heel Angle

Sling Angles

The above shows the loading in slings depending on the included angle. It can be seen that fitting too short a pair of slings and thereby creating too great an included angle can substantially increase the loading in the sling and cause it to fail . Hence should not consider any lifting operation with an included angle greater than 90 degrees and then should give a 1.5 factor for the slings i.e. the slings should be at least 0.75 tonne each to lift the 1 tonne weight

Hatch Covers • State-of-the-art hatch covers can be divided into three

basic types: • Lift-away hatch covers.

• Rolling hatch covers • Hydraulic folding hatch covers

All these types share

• Weather tightness • durability • optimized weight/strength ratio

Lift-away hatch covers.

Single-opening & Multi-opening. Can be operated by the ship’s crane or external help. Sealing between hatch covers and coaming is generally achieved by sliding rubber packing

Hydraulic Folding Types

The folding pair is operated by hydraulic cylinders acting directly on the end hinge arms which are connected at stools on the deck. When the cylinders push the end panel up from the closed position, the cover is folded and the second panel, fitted with wheels, rolls on the rails to the stowage position

Rolling types for combination/dry bulk carriers

• Side-rolling hatch covers stow in a transverse direction while end-rolling types stow longitudinally. The traditional side-rolling cover consists of two panels per hatch, each panel rolling sideways on a pair of transverse ramps, thus presenting a minimum obstacle when loading. In some cases both panels can be stowed together on one side to further enhance access when loading and unloading. This alternative reduces daylight opening by approximately 50%.

• Rack and pinion drive 

• Chain drive 

• Roll-up-Roll are normally used for operation

Variable Frequency Drives

A variable-frequency drive (VFD)

• Since the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives.

• RPM =120f/pVariable frequency drive controllers are solid state electronic power conversion devices. The usual design first converts AC input power to DC intermediate power using a rectifier bridge. The DC intermediate power is then converted to quasi-sinusoidal AC power using an inverter switching circuit. The rectifier is usually a three-phase diode bridge, but controlled rectifier circuits are also used. Since incoming power is converted to DC, many units will accept single-phase as well as three-phase input power (acting as a phase converter as well as a speed controller); however the unit must be derated when using single phase input as only part of the rectifier bridge is carrying the connected load.

Variable Frequency Drives• AC motor characteristics require the applied voltage to be

proportionally adjusted whenever the frequency is changed in order to deliver the rated torque.

For example, if a motor is designed to operate at 400 volts at 50 Hz, the applied voltage must be reduced to 240 volts when the frequency is reduced to 30 Hz.

Thus the ratio of volts per hertz must be regulated to a constant value (400/50 = 8 V/Hz in this case).

For optimum performance, some further voltage adjustment may be necessary, but nominally constant volts per hertz is the general rule.

This ratio can be changed in order to change the torque delivered by the motor.

VFD

• The latest method used for adjusting the motor voltage is called pulse width modulation PWM. With PWM voltage control, the inverter switches are used to divide the quasi-sinusoidal output waveform into a series of narrow voltage pulses and modulate the width of the pulses.

Life Boat

Schematic

Of L

aunch

Life boat.When lowering no mechanical assistance except gravity shall be applied. The only physical work needed being release of winch hand brake hold at the off position during the lowering sequence. The centrifugal brake provides controlled speed (36m/minute) to the lowering when hand brake is released. If the operator looses balance and fall off, the brake gets engaged due to the weight in the handle & the life boat shall remain stationary at the place of stop. A ratchet mechanism in the hoisting arrangement ensures that the drum will not reverse and the boat fall back into the water to provide safety in the event of power failure while lifting.

Gravity davits

Skate

Totally Enclosed Machinery Propelled Survival Craft TEMPSC

Main Features of TEMPSC

Sirens & whistlesSignal indicates the presence of the ship in poor visibility or the vessels intension of movement. Steam, air and electric whistles are commonly used. Some have audible range of 9 nautical miles.Air and steam operate on the same principle viz. the working fluid to cause the diaphragm to vibrate and the consequent sound waves to be amplified in a horn. They operate with pressures in the range of 6 – 40 bar with air/steam consumption in the range of 25 35-lts/sec.The types of electrically operated ones work on the principle of an electric motor drives a reciprocating piston thro’ a gear train and crank. This generate an air pressure which operates a diaphragmIn all the cases clean dry medium is essential for the trouble free performance of the whistle.

Water tight doorsAdequate water tight subdivisions of the ship is effected by steel watertight bulkheads from double bottom tank top to freeboard deck of the ship. It may be necessary to provide doors in some of this bulkheads, and these doors must be properly watertight and be able to close and open from both sides and from remote in the event of emergency. Where a local hand operated pinion is provided for opening or closing the door and an extended spindle above the water line is provided for remote operation. A vessel which have so many water tight bulkheads pierced by water tight doors below water line, a powered remote system is essential for its operation. Hydraulic or electric drive are common. Local control also shall be available. In bridge control, maxm. of 20 doors shall be closed with in 60 secs. This include a 10 sec audible alarm at each door prior to closing. The alarm shall sound till each door is fully closed. In these event too each door shall be operated locally and on release of local handle the door shall fall back closed. There shall be indications of the status of the doors at the bridge and at the manually operated emergency pump at the bulk head deck.

Gas carriersLiquefied gas carriers are classified as suitable for transport of LPG and ammonia or LNG or both if appropriately equipped. LPG term for gasses such as propane, butane, propylene, butylenes, C4- isomers. These can be liquefied at modest pressures.LNG Methane and mixtures containing ethane and traces of other gasses.Liquefied chemical gas ammonia, vinyl chloride, chlorine. The pressures are important as upper critical pressure and temperature plays an important role in deciding the tank design These are the limiting feature.Critical pressureThe minimum pressure which would suffice to liquefy a substance at itscritical temperature. Above the critical pressure, increasing the temperaturewill not cause a fluid to vaporize to give a two-phase system.

Similarly every gas has a critical temperature above which a gas cannot be liquefied irrespective of the pressure

Gas carriers principle

• For temp down to -55’C special carbon manganese low carbon steel for tanks and hull (sec. barrier)

• Nickel alloyed high tensile steel is only allowed for temp below -55’C.

• Lower the temp. higher the Ni content• Ni as high as 9% is reqd. for -165’C LNG transport or

Austenitic steel is generally used for membrane type of tanks.

• Same stringent regulation applies to insulation , supports etc. Special wood and foam is used for this purpose.

Gas carriers operation

For LNG the boil off is used as fuel for the engine.Pressurized vessel need simple / lesser equipment for cargo discharge. However low temperature vessels need complicated equipment such as compressors, heat exchangers and lots of secondary equipments and control gear. Most of these are situated in deck house on the main deck divided into 2 compartments namely compressor room with liquefaction plant and a separate motor room. The cargo pipe system consists of liquid, vapor, condensate, drain, purge and vent lines. Valves for remote closing are fitted at pipe entry into tanks and at cross over manifold positions. 2 or more compressors are fitted which can cool down more than the estimated boil off gas. Capacity to heat up the cargo while discharging is important as usually the shore pipes are designed to accept at -10’C. Usually submersible pumps are used in LNG tankers while deep well pumps are common in LPG tanks. The pumps being submerged do not allow hydraulic fluid drives because of temperature. Submerged pumps do not normally allow any NPSH complications. The deep well pumps, if fitted with electric motors shall be of type Ex e (enhanced safety requirement compliant) or hydraulicSubmerged pumps are usually induction motor driven and normally are harmless in that environment .

Gas carriers operation

Gas freeing is essential before change of cargo. This is done by1)Tanks heated to atmospheric temperature. Special vaporizer/ heater is used or compressor heat is used.2) Purging with inert gas from an independent inert gas generator. IG at 0.4 bar is sent via gas freeing line as gasses are vented thro’ vent mast till the vented gasses shows below explosive limitThe tanks are then gas freed usually using the inert gas blower with fresh air. This is continued till vent shows out CO2. The tanks are inspected for dirt and polymers.

TankersTankersBallast tanks are inerted to protect those in the event of tank leaks. Capacity Ballast tanks are inerted to protect those in the event of tank leaks. Capacity 125% of the cargo discharge rate.125% of the cargo discharge rate.

Gas carriers operation

Fire prevention SystemDeck water spray system to avoid for cooling in case of fire and protect the

ship structures against brittle failures if the cargo leaks. Water sprays are operated by heat sensing devices. Cargo deck house, manifold area etc. are having dry powder and CO2 fire extinguishers are provided to be operated remotely.

The gas carriers are if classified for carrying chemicals, are to be provided with foam fighting system also.

Potentially the most serious situation occurs when the tank is almost empty, Chances of mixing with air causing explosive mixture formation.

Hence 2% of the cargo is left in the tank to maintain an atmosphere entirely of cargo vapor. With no air present and the atmosphere entirely of hydro carbon, the tank is safe. Only when the tank needs repair or cargo change the tank is inerted and inspected. The cargo left over are used to maintain the tank temperature so that undue thermal stresses are not developed on the structures.

0.25 -0.3 % of the cargo is some times allowed as boil of to be consumed for boiler /engine use as reqd.

Maintenance

of

deck machinery

General

• Objective of the maintenance schedule is to keep the equipment to its original condition as possible. The equipment manufacturer will provide maintenance schedule. But conditions very drastically between type of ships, cargo carried, ports of call, environment etc. and the schedule too shall vary accordingly.

• A few minutes spent on operating and greasing the working parts when the lubricant has been washed out by rain or spray, can save many hours at a later date.

• At suitable intervals inspection shall be carried out for checking any change in condition of the working parts and made good any gaps.

• Elementary precautions shall see the equipment thro’ many trouble free years of service

Routines• Bearings. “2 stroke” of the grease gun every six months is

all what is normally needed. Once the routine is being done the problem arise while replacing the bearing due improper handling and other repairs.

• Induction motors. Apart from bearing, attn is needed towards insulation resistance. If the value is close to 1 M ohm indicates moisture and the cause should be rectified, and insulation can be brought up by heating. If this value falls close to .25 m. ohm the motor should never be started unless insulation can be brought to many m. ohms. At times the winding is virtually short circuited and if no improvement is observed on drying up, winding need to be redone.

• In the case of wound rotor type motors insulation of the rotor winding too need to be checked as before. The brush, guiding system with the spring and the ring should be checked. Should the brush needs replacement, new brush shall after proper bedding need be used. Care be taken to avoid ingress of carbon dust into vulnerable motor parts.

Routines contd.• DC motor needs regular inspection and cleaning to ensure

commutator performance. Brushes are softer than that for the slip ring and a variety of graphite combinations are available . The commutator should be dark coppery brown, the gap between bars shall be clean. Due to prolonged use the copper shall wear out and whenever the mica starts protruding out it should be cut to effect a 1mm clearance. Quality of the carbon material is of paramount importance.

• Insulation of the armature and field winding should be checked at proper intervals and very low ohmic values indicate major damage of the concerned windings.

Routines contd.• In the Ward Leonard set, on starting, if the generator runs

on the reverse it can seriously affect the motor driving it. Drive couplings need no constant attn. However they need to be in perfect alignment.

• Safety slipping clutches need proper friction pads at the correct pressure. Slippage due to presence of oil, grease and dirt can lead to over tightening and the concerned safety failures.

• Normally fail safe braking system is provided where springs close the brake and power releases it. Needs cleanliness and gap adjustment. Whenever a friction pad needs replacement the whole set shall be changed. Where brake shows signs of over heating or excessive wear is noticed the same shall be investigated and corrected.

Routines contd.• Cables and terminations need to be inspected

periodically. Cable terminations, bends and joints should be checked for signs of heat. Lugs and cable shall be correctly matched while fitting and the crimping tool shall be proper.

• Control of the machinery are normally situated on deck locally where adverse effect of weather is predominant. Salt water corrosion and condensation is common. Operating gear shall be well maintained properly lubricated. Any sign of the effect of environment should be rectified. This applies to limit switches trip bars, emergency stop stations etc.

• Extra care should be taken when working on live panels.• Anti condensation heaters whenever provided should be

checked for the correct functioning.

Routines contd.

• Open contactors should be maintained clean and any silver plating should never be ground. The magnet coils should be checked for looseness and vibration.

• Block contactors seldom need maintenance if operated properly and cleanliness is maintained.

• Relays and smaller contact comes in the form of sealed units. They seldom need replacement as the load is very small. Should they need replacement the specification need be strictly adhered to.

• Thermal overload relays are made use of the distortion of a bimetallic strip to trip a circuit. In recent time a resistance changing semiconductor –” thermistors” are used. Magnetic overloads work on the principle of over current attracts an armature tripping the circuits,

Routines contd.

• Power regulation resistances may need attn when signs are visible for color, insulation damaged etc. If fan cooled power regulators are used the same may need normal; attn.

• Rectifiers, thyristors, etc need no spaecial maintenance other than cleanliness and inspection

Care to be taken with hydraulic system

• Filtration and system cleanliness One of the most difficult and controversial feature of

hydraulic technology and remains the single factor in “user education” towards this.Basic filter construction varies from coarse metal mesh (100-150 microns) to high level filtration of (1 micron).

Generally 2 types filter body construction available.

LP working pressure up to 20 bar and

HP type with working pressure up to 400 bar.

Filtration terminology

• Many methods indicating the filtering characteristics of the element exists. But these two are quite common.

• Absolute rating based on 99% efficiency• Nominal rating based on an arbitrary efficiency,

from a test curve showing the percentage of known size particles transmitted by the media. This shall be typically 95% and the particles stopped defines the nominal efficiency.

• Pressure drop

Source of contamination

• With proper procedures hydraulic system reliability is achieved in filthy environments viz. earth moving, mining and marine environments.

• In any circuit debris may be present due to; a. Inadequate preparation like welding or accidental damages of pipe runs or parts

b. Ingress due to mishandling c. Self generated by the machines

Of the above, a and b can aggravate the problems arising at cEquipment which are of an efficient design and well laid out components ease the task of maintenance.

Basic

Commissioning procedures, tank, piping equipment design, cooling, material selection, choice of the fluid etc add immensely to the behavior of the system.

This is highly true with hydraulic mineral oils containing special additives to cover lubricity, anti foaming, corrosion resistance, VI improver etc. when subjected To severe duty conditions as in marine appln., if not maintained proper , can lead to extensive service difficulties.

Deterioration of hydraulic fluid• Water was used earlier , even now like in lock gate or moving bridge

operation water is used as the hydraulic fluid. Due to inherent problems associated with lubrication ,rusting, operating temperature range do not find favor with.

• Present day practice is to use straight mineral oils with additives to enhance properties of oxidation stability, film strength, rust prevention, foam resistance demulsibility, pour point depressant anti wear property, VI improver, lubricity etc.

• Mineral oils degenerate very slowly but rigorous marine duty conditions make this oil susceptible to decay in presence of products of corrosion or metal wear. Oxidation products tend to increase the viscosity and cause sludge deposit. Also tend to encourage formation of emulsion when traces of water is present.

• Water can promote rusting which can cause immense damage to the system. Condensation, leaky shaft seals, system coolers etc are the source which need periodic attn or when ever defects are detected.

• Fine metal wear is inevitable which are abrasive is removed by fine filters along with rust and any grits which find its way into the system

Fault Finding

• Ship’s deck equipment like windlass and mooring winches employs relatively simple control schemes and a logical method of elimination is the quickest method.

• Electrical and electronic equipments are usually provided with manufacturer’s control charts which details the logical steps for the maintenance.

• The simplest of suspects like a jammed limit switch, a blown control fuse, weak relay coil, a loose or broken wire etc should never be overlooked.

• Mechanical or pneumatic timing devices can be checked with power off however electrical timing circuits or encoders needs to be energized.

Fault Finding contd.

• A detailed knowledge and operating experience of a control system is essential for speedy faulty finding, a calm orderly and logical approach will definitely produce results.

• Disorganized “check and try” shortcuts can produce some additional defects and make the task more difficult and time consuming and must be avoided at any cost.