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Globe valves -Ship service systems
Valves are designed to control or interrupt flow. This is done in valves by lowering, raising or rotating a disc in relation to a seating surface or by controlling the movement of a ball.
The globe valve
(Figure 1) has a bulbous body, housing a valve seat and screw down plug or disc arranged at right angles to the axis of the pipe. For the valve shown, both seat and disc faces are stellited and almost indestructible. Alternatively, the seat may be renewable and screwed into the valve chest or given a light interference fit and secured by grub screw. The seatings may be flat or more commonly mitred.
The spindle or stem may have a vee or square thread, below or above the stuffing box. If the latter it will work in a removable or an integral bridge (bonnet). The spindle may be held in the valve disc (or lid') by a nut as shown or the button may locate in a simple horseshoe. Leakage along the valve spindle is prevented by a stuffing box, packed with a suitable material and a gland. If there is a change of direction, as in a bilge suction, the valve is referred to as an angle valve. Flow is from below the valve seat, so that the gland is not subject
to higher static pressure when the valve is closed. The disc must be guided by wings or a stem on the underside for location, or by a piston as shown.
The type of valve with the disc attached to the spindle is of the screw lift type. When the disc is not attached to the spindle (inset Figure 1) it is a screw-down non-return (SDNR) valve, as used for bilge systems, to prevent back flooding. They are also used as feed check and boiler stop valves. The disc requires guide vanes or a stem to keep it concentric with the seat when open. The greatest lift required is one-quarter of the bore; guides must be of a greater length than the lift.
Figure 1: Example of a globe valve with (inset) detail of the valve arranged as a screw-down non-return (SDNR) unit (Hattersley Newman Mender Ltd.)
1. Body 6. Disc stem nut 10. Bonnet studs 2, Bonnet 7A Stem - stop type 11. Bonnet stud nuts 3. Gland flange 7B Stem - piston SDNR 12. Yoke bush 4, Gland 8
A free-lifting non-return valve (Figure 2) is fitted in the compartment served by a bilge suction line, when the pipe is nearer to the ship side than one fifth of the ship's breadth. Such valves are intended to prevent flooding of the compartment in the event of collision damage.
Figure 2:Example of a non-return valve (Hattersley Newman Hender Ltd.)
Valve actuators
A variety of valve actuators to control the opening and closing of globe, gate and butterfly valves are available. In some types an electric motor, fitted with limit switches is used to turn a threaded stem through a yoke, purely substituting the action of a handwheel.
Most remotely operated valves have pneumatic or hydraulic actuators. These give linear motion to a piston which for a globe or gate valve moves the valve stem axially up or down. The globe valve disc may be given a slight turn on landing to clean the seat. The piston actuator for a butterfly valve rotates the valve disc through 90° directly or through a scroll arrangement (Figure 3 )
Figure 3:Pneumatic butterfly valve actuator showing scroll cam arrangement
Summarized below some of the basic procedure of machinery valves & pipeline systems :
- Valves & pipeline materials corrosion & erosion
Galvanic corrosion is a major challenge for any pipes which carry sea water. Rust is a particular corrosion problem for steel pipes exposed to contact with sea water or moisture generally and air. Pipe runs along tank tops or on deck, are examples of the latter. Steel pipes in these areas require external as well as internal protection.....
- valves-&-pipelines-strength-of-materials
The strength of materials used for pipes and fittings must be adequate for the
system pressures and possible over-pressures. Pipelines and valves, for
example, used to carry and control the flow of high temperature, high pressure
steam must obviously be made to very exacting specifications by approved
manufacturers.....
- Valves & pipelines-system cleaning & draining
It is often found, in new ships, that the bilges and bilge systems have not been
thoroughly cleaned with the result that wood, nuts, bolts, rags and other debris
are found inside valves and pipes after initial bilge pumping. These choke the
valve-chests and prevent the valves from being properly closed. They also
block strainers. ....
- Expansion arrangements
Provision must be made in pipe systems to accommodate changes in length due to change of temperature, and so prevent undue stress or distortion as pipes expand or contract.....
- Valves & cocks
Cocks and valves are designed to control or interrupt flow. This is done in cocks by rotating the plug, and in valves by lowering, raising or rotating a disc in relation to a seating surface or by controlling the movement of a ball. ...
- Butterfly valves
A butterfly valve consists basically of a disc pivoted across the bore of a ring body having the same radial dimensions as the pipe in which it is fitted.....
- Gate valves
Unlike the globe valve, gate (or sluice) valves give full bore flow without change of direction. The valve disc known appropriately as a gate,....
- Globe valves
The globe valve has a bulbous body, housing a valve seat and screw down plug or disc arranged at right angles to the axis of the pipe....
- Relief valves
Excess pressure is eased by a relief valve . This consists of a disc held closed by a spring loaded stem. The compression on the spring can be adjusted so that the valve opens at the desired pressure. ....
- Valves traps
A steam trap is a special type of valve which prevents the passage of steam but allows condensate through. It works automatically and is used in steam heating lines to drain condensate without passing any steam. ....
- Flap valves & valve chest
Scupper pipes from accommodation spaces are fitted with non-return valves. Those scuppers from spaces below the bulkhead deck, are required to be fitted with non-return valves which can be positively closed from above the bulkhead deck or, if this is not practical, with two non-return valves.....
- Quick closingvalves
Fuel oil service and some other tanks must be fitted with valves that can be closed rapidly and remotely in the event of an emergency such as fire. Wire operated valves are commonly fitted,....
- Strainers & filters
The term strainer is sometimes used specifically for a simple device made up with a single layer of coarse gauze, a very coarse wire mesh or a drilled or perforated plate. ...
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