VARIOUS UNDERWATER VALVES IN SHIPS

Generally speaking the body, bottle, bridge and cover of all valves are made of gunmetal whereas the valve, disc, spindle, thrust collar, studs and nuts are of aluminium or bronze. Soft faces are made of vulcanized synthetic rubber, except in ball plug valves where they are of an approved low friction type material.

VARIOUS UNDERWATER VALVES FITTED INCLUDES:

SEA OR COCK VALVE

Sea valves, normally of the screw down type, metal seated, are fitted for admitting salt water into the ship for flooding compartments and to supply the various salt water pumps in the system. In way of the double bottom they are secured to the inner bottom by a flange connected to the inlet tube between inner and outer bottom. Outside the double bottom the seacock is fitted to a distance piece on the outer bottom. Gratings of galvanized mild steel are fitted to the sea inlets as near as possible to the outer bottom plating. 

Where the sea tube serves more than one valve the clear area of the opening in the grating is suitable for the total number of valves using that tube. Zinc protectors are fitted inside the tube to take the effect of galvanic action set up by the non-ferrous material of the seacock as described in the chapter on cathodic protection. To enable compartments containing explosives or inflammable material to be flooded in dry dock, flooding bonnets are provided which can be secured to the mouth of the inlet tube after removing the grating. A flooding bonnet consists of a number of hose connections welded to a base plate. As the water falls below the inlet, when docking, the grating is removed and the bonnet secured in place, hoses being led from the hose connections to the shore hydrant. The bonnet is removed and the grating replaced when the vessel is being undocked. Pump suctions connected to a seacock used for flooding magazines or other danger spaces must be taken from the sea side of the master valve of the seacock.

SCREW-DOWN VALVE  

Screw-down valves fitted with soft face and renewable seats are fitted for isolation and control purposes, e.g. on branches from the salt and fresh water mains, and on filling pipes discharging into fresh water filling funnels. Alternatively diaphragm valves are used. Where it is desired to run pipes at right angles and space is limited, screw-down right-angle valves are fitted.

SCREW-DOWN NON-RETURN VALVE

Screw-down non-return valves fitted with soft face and renewable seats are fitted to all suction leads to ensure that the pumps are kept flooded. Their design is similar to the SDV except that the' bottle' has a longer neck with an expansion chamber on the valve. When the spindle is in the open position, the valve slides up the neck of the bottle when the pump is drawing, and falls back on the seating when suction is lost.

SCREW-DOWN NON-RETURN AND FLOOD VALVE

Screw-down non-return flood valves fitted with soft face and renewable seats are fitted to all suction leads where it is required to flood the compartment via the suction pipe. This valve is similar in action to SDNRV for pumping, but has a still longer neck to allow the valve to be lifted clear of the seating for flooding purposes. The inlet to the pump is inclined in order to facilitate the flow of water when pumping. Such valves are fitted at the feet of  branches to bottom compartments.

SCREW-DOWN DIAPHRAGM VALVE

Screw-down diaphragm valves are fitted in main suction systems where a high vacuum is essential in preference to screw-down valves. In this type of valve a flexible diaphragm isolates the fluid controlled by the valve from the valve operating mechanism.

PLUG VALVE

This is a form of shut-off valve, used where a full on or full off control is required, having a 'plug' which may be tapered, parallel or spherical in shape. It incorporates design features which reduce friction between the plug face and the body seat during operation and seals them against leakage. This type of valve has the advantage that it offers low resistance to flow.

BUTTERFLY VALVE

The flow regulator in this type of valve is a metal disc rotating in axial trunnion bearings. The valve has a resilient lining in the body incorporating the seat and may be used to control the flow from full open to full off according to the service required. These valves are now fitted in preference to double faced sluice valves in fluid systems in sizes above 4-in nominal bore. For smaller sizes valves of the full bore plug type are preferable.

SLUICE VALVE

Sluice valves are used for local connection between two or more compartments, e.g. one double bottom compartment draining into another. In shape they may be square or circular and consist of a casting with machined faces over which slides the valve casting operated by gearing.

SCREW-DOWN AND NON-RETURN DRAIN VALVE

These are fitted in flats and auxiliary machinery spaces for draining into compartments below. The non-return is fitted in the pipe below the deck leading to the compartment below, and is necessary to prevent water entering the compartment above should the one below be flooded.

STORM VALVE

Storm valves are simple non-return valves and are fitted to all drain and soil pipes discharging overboard, to prevent flooding when the sea rises above the outlet. Positive closing type storm valves are fitted when required. All storm valves are faced with leather to minimize noise when the vessel is rolling.

NON-RETURN VALVE

Non-return valves are fitted to prevent reversal in direction of flow, e.g. on the return piping in hot water circulating systems. They are also fitted on the discharge side of pumps to prevent back pressure.

REDUCING VALVE

Reducing valves are fitted where a low pressure service is supplied from another of higher working pressure. They are provided with renewable seats and discs of erosion resistant material. Where suitable, pressure reduction is achieved by the comparatively simple process of fitting an orifice plate of nylon or aluminium bronze. A diagrammatic sketch is given to illustrate the action of a reducing valve. When no pressure is on the spring and the inlet is open to water pressure, the valve A and piston B are in equilibrium, so that no water passes through the valve. When pressure is put on the spring to the required indicator setting, the valve A opens to let water through, until the pressure above A just exceeds that to which the spring is set: the valve A then closes against the spring. This action is repeated as the water is used.