A pump with a flow rate of about 100 gpm and a discharge pressure of between 50 and 150 psi over bottom pressure is adequate for this type of jetting operation. Commercial nozzles are available that have balancing jets that reduce or eliminate the back thrust (Figure 236). The diver using the jet will have to provide resistance to the back thrust from the jet nozzle. In the simplest form of jetting, vast quantities of mud, sand, or silt can be moved by directing a high-velocity water stream, through a jet nozzle, at the material to be moved.

A variety of jetting mandrels and nozzles can be used, depending on the work to be done and the flow characteristics desired, such as width of jet and jet velocity. Jetting is done by supplying pressurized water from a pump mounted on a surface-support platform. Jetting is useful for moving seabed soils for the burial of cables and pipelines and for installing instrument tubes and structural piles. The discharge should be positioned down current to allow the prevailing currents to carry the material away.ĬAUTION Divers operating the air lift should use caution around the inlet to avoid the effect of suction and stay clear of the area under the discharge end where heavy objects are ejected. This may result in some of the material settling back into the excavated area. The air lift has the disadvantage of discharging the lifted material relatively close to the intake point. The discharge end of the air lift should be kept as close to the surface as possible to avoid reducing efficiency. Although it can be used in shallow waters, it is better to use the underwater dredge, described in Section 2.6.3, in water depths less than 16 to 18 feet. The air lift can be from 10 to 70 feet long, but it is inefficient in lengths of less than 30 feet.

the air pressure supplied is relatively unimportant, but it must be at least greater than the pressure at the depth of water in which the excavation is being performed. Experimentation is usually required to determine the volume of air necessary for maximum eiilciency. The seabed material starts to lift almost as soon as the low-density fluid in the discharge pipe rises. Pipe will buck at start up and if inlet is plugged, diver requires communication with surface.Īnd then submerging the intake end (foot piece) into the seabed material.

How does darkblast dredge work