The present invention relates to control of percussive hammer devices, such as pneumatic percussion drills and rock breakers.
A downhole pneumatic hammer is, in principle, a simple device consisting of a ported air feed conduit, more commonly known as a feed tube, check valve assembly above the feed tube to preventingress of wellbore fluids into the drill, a reciprocating piston, a case, a drill bit, and associated retaining hardware. The typical valveless device, for example, possesses on the order of 15 components. The reciprocation of the piston is accomplished by sequentially feeding high-pressure air to either the power chamber of the case (the volume that when pressurized moves the piston towards the bit shank) or return chamber of the case. The regulation of the air flow can be accomplished either by use of passages (e.g., slots, grooves, ports) machined into the feed tube, piston body, or hammer case; or a combination of active valving and porting through either the piston, the case, or an additional sleeve.
However, existing designs do not provide the most efficient use of the total air energy available because they have built-in inherent inefficiencies. The present invention greatly reduces these inefficiencies.
Impact applications, such as percussive drilling, that utilize sliding valves to control fluid flow (usually a gas) within the device are subject to control difficulties if the valve is not properly located relative to port positions during a cycle. Misalignments and mis-positionings of the valve can result in poor regulation of the device pressure chambers. Standard valve materials, such as steels or high strength plastics, are stiff and have very little internal damping, leading to predominantly elastic impact collisions in which almost all of the impact velocity of the component is preserved in rebound.
A typical configuration consists of an air feed conduit (tube), a reciprocating piston, and a spool valve within the piston. During operation, the air feed conduit is stationary, the piston reciprocates bi-directionally along the feed conduit axis, and the valve moves within the piston covering radial ports in the piston at different points in the cycle to regulate air flow that is used to control the piston's motion. In applications where rapid velocity reversals of the piston occur (e.g., hammer drilling), the valve within the piston tends to recoil elastically off the position-limiting surfaces of the piston. This recoil often causes the valve to unintentionally cover, or expose, the incorrect ports, leading to control or performance problems.
Against this background, the present invention was developed.