A lifting bag is an item of diving equipment consisting of a robust and air-tight bag with straps, which is used to lift heavy objects underwater by means of the bag's buoyancy. The heavy object can either be moved horizontally underwater by the diver or sent unaccompanied to the surface. Lift bag appropriate capacity should match the task at hand. If the lift bag is grossly oversized a runaway or otherwise out of control ascent may result. Commercially available lifting bags may incorporate dump valves to allow the operator to control the buoyancy during ascent, but this is a hazardous operation with high risk of entanglement in an uncontrolled lift or sinking. If a single bag is insufficient, multiple bags may be used, and should be distributed to suit the load. There are also lifting bags used on land as short lift jacks for lifting cars or heavy loads or lifting bags which are used in machines as a type of pneumatic actuator which provides load over a large area. These lifting bags of the AS/CR type are for example used in the brake mechanism of rollercoasters.
Physics of buoyant lifting
The volume of the bag determines its lifting capacity: each litre of air inside the bag will lift a weight of 1 kilogram, or each cubic foot will lift about 62 pounds. For example, a 100-litre (3.5 cu ft) bag can lift a 100-kilogram (220 lb) underwater object. A partially filled bag will accelerate as it ascends because the air in the bag expands as the pressure reduces on the ascent, following Boyle's law, increasing the bag's buoyancy, whereas a full bag will overflow or blow off excess volume and maintain the same volume and buoyancy providing it does not descend. A bag which leaks sufficiently to start sinking will lose volume to compression and become less buoyant in a positive feedback loop until stopped by the bottom.
Breakout The force required to lift a submerged object from the bottom can be split into two main components:
Apparent weight, which is the weight of the object less the buoyancy of its displacement. Breakout forces due to embedment in the bottom, which can be negligible, or in some cases the major part of the load. Once the object has broken free of the bottom, only the apparent weight remains, and a controlled lift requires a way of managing the sudden decrease of resistance to the lifting force. There are three basic ways this can be done:
Use of mechanical or hydraulic excavation to loosen the sediments holding the load. Use of a "Dead Man Anchor" - a large heavy weight - and restraining cable to prevent the bag from moving away too far, so that the buoyancy can be corrected to more closely match the load. Use of shallow bags with long cables to the load to provide breakout, which will only lift a short distance before surfacing, after which the load can be lifted further by staged lifts or direct lift by close-coupled bags.
Stability of the load Once a load is lifted off the substrate, it will rotate until the centre of gravity is in the position of lowest potential energy. If it is suspended from a single point, the apparent centre of gravity (corrected for inherent buoyancy) will be directly below the lift point. If it is undesirable for the load to rotate by a large angle as it leaves the bottom, the lifting point must be chosen to allow for this effect, and a multi-part sling or spreader bar may be needed, and it may be necessary to secure slings so they do not slip.
Types and construction Underwater lifting bags are lifting equipment and as such may be required to comply with safety standards.
Open lift bags (parachute lift bags)
Parachute lift bags are open at the bottom. When full any extra or expanding air will spill out. The shape of an open lifting bag should distribute the volume in a vertical rather than a horizontal direction so that the open end of the bag always remains underwater. If the open end reaches the surface, air will escape from the bag and it may sink. The simplest version are two-sided bags, either joined round the edges or folded and joined along two sides. Webbing straps may be stitched to doubler patches which are then glued or welded to the bag on light duty bags, but on large and heavy duty bags there are usually strips of bag material bonded to the bags which form flat retaining tubes for the webbing which is threaded through the tubes and may be withdrawn for maintenance and inspection. heavy duty open bags are generally conical with a domed top or a reversed truncated cone top, and may have several straps from the lifting point at the bottom, through the guide tubes on the sides, to a crown ring of webbing or steel at the top, to spread the load evenly over the fabric of the bag. Parachute lift bags cannot be overfilled and are suitable for lifts where there is a large pressure change, and where it may be necessary to capsize (invert) the bag to stop a runaway lift. Some lift bags can be converted from open to closed by screwing a cover onto the bottom opening.
Propeller lifting bags Installation and removal of propellers is a specialised application where there is usually very little clearance above the load, which is usually at least partly underneath a vessel. Lift bags for this application partly enclose the propeller when in use, and are required to hold the propeller in the correct alignment for fitting to the shaft. It may be necessary to ballast the lower blade to keep the propeller upright. and details of the rigging will depend on the precise geometry of the propeller, such as blade aspect ratio, skew, and number of blades. The propeller lift bag will cover the upper part of the propeller, but cannot project below the line of the top of the shaft. It will support the propeller by at least two blades, for stability, and the propeller will generally by slung with a gap at the top, for maximum tip clearance. The bag may be shaped with a relatively horizontal top or a curve to follow the blade disc. Attachment to the blades must be on both sides for stability, and the slings must not harm the blade surfaces or damage the leading or trailing edge.
Closed lift bags (camels)
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