"Regulations for Design and Construction of Marine Boilers"
5 Design and Construction
5.1 Access Arrangements
- Boilers are to be provided with openings in sufficient number and size to permit internal examination, cleaning and maintenance operations are to be provided with access manholes.
- Manholes are to be provided in suitable locations in the shells, headers, domes, and steam and water drums, as applicable.
- The "net" (actual hole) dimension of elliptical or similar manholes is to be not less than 300 mm x 400 mm. The "net" diameter of circular manholes (actual hole) cannot be less than 400 mm. The edges of manholes are to be adequately strengthened to provide compensation for vessel openings.
- In pressure vessels which are part of a boiler and are not covered by the requirement in a) above, or where an access manhole cannot be fitted, at least the following openings are to be provided, as far as practicable: Head holes, Hand holes, Sight holes.Sight holes may only be provided when the arrangement of manholes, head holes, or hand holes is impracticable.
- Covers for manholes and other openings are to be made of ductile steel, dished or welded steel plates or other approved design.They are to be of self-closing internal type.
- Closing devices of internal type covers, may be fitted with a single fastening bolt or stud. Larger closing devices are to be fitted with at least two bolts or studs.
- Covers are to be designed so as to prevent the dislocation of the required gasket by the internal pressure. Only continuous ring gaskets may be used for packing.
- In general, cocks and valves are to be designed in accordance with the requirements.Cocks, valves and other fittings are to be connected directly or as close as possible to the boiler shell.
- Cocks and valves for boilers are to be arranged in such a way that it can be easily seen when they are open or closed and so that their closing is obtained by a clockwise rotation of the actuating mechanism.
5.3 Boiler burners
Burners are to be arranged so that they cannot be withdrawn unless the fuel supply to the burners is cut off.
5.4 Allowable water levels
- For water tube boilers the lowest permissible water level is just above the top row of tubes when the water is cold.
- For fire tube boilers with combustion chamber integral with the boiler, the minimum allowable level is to be at least 50 mm above the highest part of the combustion chamber.
- For vertical fire tube boilers the minimum allowable level is 1/2 of the length of the tubes above the lower tube sheet.
5.5 Steam outlets
- Each boiler steam outlet, if not serving safety valves, is to be fitted with an isolating valve secured either directly to the boiler shell or to a standpipe of substantial thickness, as short as possible, and secured directly to the boiler shell.
- The number of auxiliary steam outlets is to be reduced to a minimum for each boiler.
- Where several boilers supply steam to common mains, the arrangement of valves is to be such that it is possible to positively isolate each boiler for inspection and maintenance.
- Where steam is used for essential auxiliaries (such as whistles, steam operated steering gears, steam operated electric generators, etc.) and when several boilers are fitted on board, it is to be possible to supply steam to these auxiliaries with any one of these boilers out of operation.
- Each steam stop valve exceeding 150 mm nominal diameter is to be fitted with a bypass valve.
5.6 Feed check valves
- Each fired boiler supplying steam to essential services is to be fitted with at least two feed check valves connected to two separate feed lines.
- Feed check valves are to be secured directly to the boiler or to an integral economiser.
- Where the economisers may be bypassed and cut off from the boiler, they are to be fitted with pressure-limiting type valves.
- For water tube boilers, at least one of the feed check valves is to be arranged so as to permit automatic control of the water level in the boiler.
- Provision is to be made to prevent the feed water from getting in direct contact with the heated surfaces inside the boiler and to reduce the thermal stresses in the walls.
5.7 Blow-down devices
- Each boiler is to be fitted with at least one bottom blow-down valve or cock and, where necessary, with a similar valve or cock for scumming from the surface. These valves or cocks are to be secured directly to the boiler shell and are to be connected to overboard discharge pipes.
- The diameter of valves or cocks and of the connected piping is not to be less than 20 mm and need not be more than 40 mm.
- Where the bottom blow-down valve may not be directly connected to the boiler shell in water tube boilers, the valve may be placed immediately outside the boiler casing with a pipe of substantial thickness suitably supported and protected from the heat of the combustion chamber.
- Where two or more boilers have the bottom blow-down and surface scumming-off valves connected to the same discharge, the relevant valves and cocks are to be of the non-return type to prevent the possibility of the contents of one boiler passing to another.
Each superheater, whether or not integral with the boiler, is to be fitted with cocks or valves so arranged that it is possible to drain it completely.
5.9 Water sample
- Every boiler shall be provided with means to supervise and control the quality of the feed water.For this purpose, boilers are to be fitted with at least one water sample cock or valve.
- Suitable inlets for water additives are to be provided in each boiler.
5.10 Marking of boilers
Each boiler is to be fitted with a permanently attached plate made of non-corrosive metal, with indication of the following information:
- Name of manufacturer
- Year of manufactur
- Serial number
- Design pressure
- Design temperature
- Test pressure
- Date of the test
For lagged vessels, these markings are also to appear on a similar plate fitted above the lagging.
5.11 Safety valve arrangement
- Every steam boiler with a total heating surface of 50 m2 and above is to be provided with not less than two spring loaded safety valves of adequate capacity. For steam boilers having heating surface less than 50 m2, only one safety valve need be fitted.
- Where a superheater is an integral part of the boiler, at least one safety valve is to be located on the steam drum and at least one at the superheater outlet.
- Where fitted, superheaters which may be shut-off from the boiler, are to be provided with at least one safety valve.
- In the case of boilers fitted with a separate steam accumulator, safety valves may be fitted on the accumulator if no shut-off is provided between it and the boiler without increasing the boiler pressure more than 10% above the design pressure.
5.12 Safety valve setting (Important)
- Safety valves are to be set under steam in the presence of the Surveyor to a pressure not higher than 1.03 times the design pressure.
- Safety valves are to be so constructed that their setting may not be increased in service and their spring may not be expelled in the event of failure.
- Where safety valves are provided with means for regulating their relieving capacity, they are to be so fitted that their setting cannot be modified when the valves are removed for surveys.
5.13 Safety valve fitting on boiler
- The safety valves of a boiler are to be directly connected to the boiler and separated from other valve bodies.
- Where it is not possible to fit the safety valves directly on the superheater headers, they are to be mounted on a strong nozzle fitted as close as practicable to the superheater outlet.
- The cross-sectional area for passage of steam through restricted orifices of the nozzles is not to be less than 1/2 the aggregate area of the valves.
- Safety valve bodies are to be fitted with drain pipes of a diameter not less than 20 mm for double valves, and not less than 12 mm for single valves, leading to the bilge or to the hot well. Valves or cocks are not to be fitted on drain pipes.
- Each air pressure vessel is to be fitted with a drainage device allowing the evacuation of any oil or water accumulated in the vessel.
- Drainage devices are also to be fitted on other vessels, in particular steam vessels, in which condensation water is likely to accumulate.
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