Marine Auxiliary Machinery 7E Episode 13 doc

470 Safety and safety equipment doors in the installation. In an emergency the supply is taken from the ship's batteries. Local control A control is located adjacent to each door for shut/open or for intermediate positions. The control lever is operated manually from either side of the bulkhead and movement of the lever actuates a pilot valve controlling the pump motor circuit under all conditions of control. Advantage is taken of the area differential of the piston in the door operating cylinder for closing and opening 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 hydraulic lock within the cylinder, thus preventing any possible movement of the door due to the motion of the ship. Bridge control The bridge controller is designed to close (in sequence) a maximum of twenty doors within a specified time of 60 s. This period includes a 10 s audible alarm period at each door before the closing movement starts and the alarm continues to sound until each door is fully closed. Close and re-open controls energize solenoids at the door control valve to start 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 closed condition from the bridge controller will automatically reclose when the local control lever is released. Indication that power is available to close the doors is given 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 and opened by a manually operated pump and control valves at either of two positions: (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: green for shut and red for open. The appropriate lights are duplicated alongside the manually operated emergency pumps on the bulkhead deck. To ensure that Safety and safety equipment 471 power is always available all the indicator lights and alarms are connected direct to the d.c. supply and to the ship's batteries. Lifeboat davits Boat davits (Figure 15.7) vary in design to suit the load to be handled and the layout of a ship, but the principles are common to most davits. When lowering a lifeboat no mechanical assistance apart from gravity is applied The only manual function required of the operator is to release the winch handbrake and hold it at the off position during the lowering sequence. If the operator loses control of the brake lever in difficult conditions, the attached Figure 15,7 Overhead gravity davits (Weiin Davit & Engineering Co Ltd) 472 Safety and safety equipment weight will provide a positive means of application and the boat will be held at any intermediate position. This condition applies throughout the outboard movement of the boat, from its stowed position until it is waterborne. Figure 15.9 shows the hand brake arrangement. The main brake (on the left in Figure 15.8 is fitted with two shoes, pivoted at one end and coupled at the other with the weighted lever, by a link. The lever projects from the casing through a watertight seal. The shoes are Ferodo-lined and have a normal useful life of five years or more. Figure 15.8 shows in section the main brake described above and the centrifugal brake (shown on the right-hand side of the drawing). The centrifugal brake limits the rate of descent of the boat when the handbrake is not engaged. Shoes of calculated weight act on the inner surface of a stationary drum, 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 and the boat drop back towards the water in the event of a power failure when a boat is being hoisted. The brakes require regular inspection for wear and after replacement must be properly 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-cooled engines are installed in lifeboats. Because of their cold starting characteristics, simplicity and low maintenance requirements, air-cooled engines might be favoured for open lifeboats, but water cooled engines are usual. For totally enclosed lifeboats with water-cooled engines a small single-pass heat exchanger (usually just a large bore tube) may be arranged on the outside of the lifeboat bottom, for cooling of the freshwater circuit by the sea. Hydraulic cranking system Some lifeboat engines and the larger diesel-driven emergency generator sets may be fitted with a hydraulic starter motor. The Startorque system (Figure 15.10) is such a device which uses an automatically charged accumulator to provide power to the hydraulic cranking motor. The accumulator is precharged with nitrogen to a pressure of 83 bar. The system accumulator can be re-charged by a hand pump B or by an engine driven pump I. The stored energy in the accumulator is released by a hand-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 the flywheel. The valve then opens fully allowing full flow to the hydraulic Safety and safety equipment 473 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 oil returns to the reservoir where it is pumped back to the accumulator by either the hand pump or the engine driven pump. An off-loading valve H protects the system from being overcharged and spills at 20.7 bars back to the reservoir on an open circuit maintaining flow through 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 suit particular engines by modifying the size of the accumulator. 474 Safety and safety equipment 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 inform other 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. Safety and safety equipment 475 The air and steam whistles operate on much the same principle, namely the working 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. The diaphragm details can be seen in Figure 15.12 and a section of the whistle's control valve is shown in Figure 15.13. Units of this type may be found working 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) 476 Safety and safety equipment 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 the supply pressure. Alteratively an adjustable choke may be provided instead of the plug. It is important that the correct choke setting is selected to match the maximum supply pressure. If this is too high for the setting the diaphragm might break; if the pressure is too low the volume will be inadequate. Primary control of the whistle is afforded by one or more push buttons located at strategic points on the bridge. By pushing the button an electric Safety and safety equipment 477 circuit activates the solenoid pilot valve (1) which then causes piston (4) to move, allowing air to pass to the diaphragm. An automatic device is usually fitted 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 for this equipment is so arranged that, when on automatic signal, depression of the manual button overrides the preselected sequence. Secondary control of the whistle is by a lanyard which directly operates lever (3) (Figure 15.13) allowing air on to the diaphragm. Air enters the whistle valve unit via a filter (2) which requires cleaning occasionally. An additional filter is frequently fitted at the lowest point of the air 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 removed and the 12 retaining bolts should be unscrewed, it is not necessary to remove the bottom flange. The O ring, on which the diaphragm seats, should always be renewed 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) 478 Safety and safety equipment Electric whistles An Electro-Tyfon whistle is shown in Figure 15.15 from which it can be seen that the electric motor drives a reciprocating piston through a gear train and crank. 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, [...]... to touch both seats simultaneously) It is more usual to find control valve trims of the types shown in Figure 16 .13 in which a ported cage is utilized as a valve guide These can be arranged as shown in Figure 16.13d to give tight shut-off and, in the case of the type shown in Figure 16.13c without the problems of an unbalanced force acting on the plug The valve characteristic is governed by the contour... refrigerants, took over from CO2 systems Control equipment for auxiliary boilers and engine cooling circuits, followed, so eliminating other routine duties Instrumentation and alarms had been improved and then fitted more extensively to give more complete monitoring with shut down as appropriate Long before the advent of the UMS certificate, main machinery was operating with little more than routine attention,... instrumentation F!gtire 16.8 Yoxai!) 491 Model 13A d/p cell differential pressure transmitter (Foxboro The transmitter The most common method of generating the pneumatic signal feeding the controller is to use a force—balance device An example of a force—balance device used to transmit a pneumatic signal proportional to a process pressure change can be found in Foxboro's Model 13A d/p differential pressure transmitter... Control and instrumentation The periodically unmanned machinery space was made possible first by an evolutionary process which took place over a number of years and finally by the introduction of bridge control, which for diesel engines, had long been possible During the years of progress and refinement, many improvements were made to various types of machinery to enhance reliability Some types of equipment... specification and sizing of control valves, providing they are given complete information regarding the service conditions to be met Alarm systems and data loggers The first step towards centralized control of marine machinery was to extend the conventional control and instrumentation facilities to a central control console, usually housed in a special air conditioned control room Some of the consoles were very... astern turb Note that many subsidiary services not included in this list need to be included in a complete system The subject is dealt with in greater detail in Marine Steam Boilers (Milton and Leach), another publication in the Butterworth Marine Series Semi-automatic systems Semi-automatic systems operating over a narrow range have been in use for many years They are capable of maintaining automatic... governor trip lever Reset emergency stop valve Start auxiliary L.O pump Start circulating pump Apply gland steam Start extraction pump Start air ejectors, Open steam valve to run-up turbine Computers A number of ships have been fitted with computers programmed to carry out a great variety of tasks embracing navigation, ships' housekeeping, crew wages, machinery survey and overhaul, spares and stock control,... types but it is interesting to note that Foxboro has adopted the force balance systems used in its pneumatic transmitters, for Foxboro electronic systems Figure 16.11 is a schematic drawing of a Foxboro E13 series electronic differential pressure transmitter suitable for flow, liquid level and low pressure measurement applications, This instrument measures differential pressure from 0—127mm water to 0-21.5m... differential transformer (7) which serves as a detector Any position change of the ferrite disk changes the output of the differential transformer determining Figure 16.11 Schematic diagram of Foxboro E13 series electronic differential pressure transmitters (Foxboro Yoxall) For key to numbers, see text Control and instrumentation 495 the amplitude output of an oscillator (8) The oscillator output is... the amplifier proportional to the measurement and retaining the force balance system in equilibrium Control valves The reputation of the diaphragm operated pneumatic control valve as a final element in marine control systems has been established over many years The control valve consists of four basic parts: the actuator, packing box, valve body and valve trim The diaphragm actuator receives pneumatic . in Figure 15.12 and a section of the whistle's control valve is shown in Figure 15 .13. Units of this type may be found working from air pressures of 6 to 42 bar with air . Detailed arrangement of diaphragm (Kockums, Sweden) 476 Safety and safety equipment Figure 15 .13 Control valve (Kockums, Sweden) 1, Pilot valve 3. Lever 5. Housing 7, Choke plug 2. . sequence. Secondary control of the whistle is by a lanyard which directly operates lever (3) (Figure 15 .13) allowing air on to the diaphragm. Air enters the whistle valve unit via a filter (2)