Dangers of Delta P (Differential Pressure) in Diving
Diving hazards caused by differential pressure happen when water moves from an area of high pressure to an area of lower pressure. There is no risk when there is no water flow, but once the flow starts, the forces exerted are usually very substantial.
There are numerous situations where water flow occurs, this can be due to the opening of a valve, cutting into a void, or a pump starting. When the flow passes through an opening, any diver approaching the high-pressure side may be drawn in and trapped. Serious or lethal injuries often occur in such conditions. The difference in height between two water bodies at different levels generates a suction force through any hole or opening in the barrier between them. The greater the difference in height or the bigger the opening in the barrier, the larger and more dangerous is the suction force. However, very substantial suction forces can also be created when there is a small difference in water levels and there is a relatively large opening, this can be lethal even at low depths of 3 meters or less.
Adequate risk assessment procedures must be carried out before diving operations start. Prior to any diving operation where a potential Delta P hazard is identified isolations and lock out/tag out procedures must be put in place. It is always preferable to have a double isolation where possible.
Extra care should always be taken around:
• Pumps, intakes, valves and drains.
• Dams, locks, gates and weirs.
• Cracks/ voids in pipes or hoses.
• Propellers, thrusters, on and around vessels.
• Large underwater moving equipment.
• Risk of failure of plant or machinery.
• Submerged pipes or empty structures sealed at atmospheric pressure or pressures that are higher or lower than the surrounding water pressure.
Calculating force due to differential pressure
Force = Pressure x Area
Force (kilogram force) = Difference in levels x Water Density x Area
Water Density = 1025 kg/m3 for sea water
or = 1000 kg/m3 for fresh water
Area = π x (Diameter/2)2
Force = 3.5 x 1025 x π x (0.3/2) = 254 kgf
Water velocity relationship to distance from hazard
Divers are at risk from water flow, suction or turbulence. The area of fast-moving water around these inlets (whether produced by the operation or failure of plant and machinery) is called the Differential Pressure Danger Zone (DPDZ). Divers usually cannot detect the presence of differential pressure hazards around them when submerged in the water and once they do, it’s already too late.
As shown in the figure above, a diver who is moving from point a to b will not feel the change in the water flow rate but when moving the same distance, 300mm, from b to c the diver would experience an exponentially large increase in the flow rate velocity and be pulled in.
Divers should not operate in water currents exceeding 0.5 m/s (1 knot).
The below tables are a reference of force and flow through varying pipe diameters at varying depths.