Drilling  in Water

DTH hammers are equipped with a check valve to assist when drilling under water. The check valve closes when the air supply is turned off, this maintains the air pressure in the hammer and prevents water from entering the hammer. Drilling under water increase the back pressure which decreases the efficiency of the hammer thus slowing the penetration rate. A point can be reached when the up hole velocity of the air is insufficient to overcome the water head and piston will stop, to overcome this extra air must be utilized.

Back pressure in water is determined by the amount of head of water above the hammer. The back pressure can be calculated by using the following.

1 metre Head = 0.1kg/cm2
1 kg/cm2 = 10 metres of Head
1 ft head = 0.434 psi
1 psi = 2.3ft of Head

After a pipe change in a hole with 200m of water the operator will need 200m x 0.1kg/cm2 (20 kg/cm2 ) of air supply to unload the water from the hole. Once the water starts to flow from the hole the pressure required will drop.

Salt water

During many drilling operations salt water is encountered, this can cause premature corrosion and wear. When drilling in high salinity levels extra oil should be added to the drill string between shifts, with the hammer being left under the water level to avoid exposure to air between shifts.