Typically when water is injected into the air supply of a conventional backhead, the water will travel through all hammer components and exhaust out the bottom of the bit to improve hole cleaning. Water is not a concern when small volumes are used, but a drop-off of drilling performance can occur when significant amounts of water are required. Drawbacks can include slower drilling due to wet bottom conditions and a reduction in hammer/bit life due to cavitation from water.
Numa stated that its vortex backhead avoids a drop-off of drilling performance by ejecting water out the top of the backhead before it can go through internal hammer components. Accordingly, Numa said, the design allows drillers to realise the benefits of water injection without the wear and loss of hammer performance.
Numa engineers have created a spinning process (vortex) in the backhead that leverages the density difference between water and air. Fluids entering the top end of the hammer are spun, pushing heavy material (water) to the outside to be ejected out the top of the backhead. Lighter material (air) stays to the inside and travels down the feed tube to power the hammer and clean the bottom of the hole.
The amount of water ejected by the vortex backhead can be controlled by the two jets, or chokes, found at the top of the backhead. A standard hammer is supplied with 1/8in (3.2mm), 3/16in (4.8mm) and 1/4in (6.4mm) jets that can be changed to control water injection according to drilling preference and conditions.
The design is available for Numa's 12in (305mm) hammer line when drilling conditions require a significant amount of water injection.