High frequency fluid driven drill hammer percussion drilling in hard formations
A fluid pressure driven, high frequency percussion hammer for drilling in hard formations is presented. The hammer piston ( 20 ) of the percussion hammer has a relatively large and longitudinally extending bore ( 41 ) that provides minimal flow resistance for a drilling fluid flowing through the bore ( 41 ) during the return stroke of the hammer piston ( 20 ). The bore ( 41 ) is closeable in the upstream direction by a valve plug ( 23 ) that follows the hammer piston ( 20 ) during the stroke. The valve plug ( 23 ) is controlled by a relatively long and slender valve stem ( 49 ) that is mechanically able to stop the valve plug ( 23 ) by approximately 75% of the full stroke length of the hammer piston ( 20 ) and separates the plug ( 23 ) from a seat ring ( 40 ). Thus the bore ( 41 ) opens up such that the bore fluid can flow there trough, and the inherent tension spring properties of the valve stem ( 49 ) returns the valve plug ( 23 ) so rapid that it will be good through flow during return of the hammer piston ( 20 ).
1. A fluid pressure driven high frequency percussion hammer for drilling in hard formations, the percussion hammer comprising:
a housing which in one end thereof is provided with a drill bit designed to act directly on the hard formation;
a hammer piston moveably received in said housing and that acts on the drill bit, the hammer piston having formed therein a longitudinally extending bore having a predetermined flow capacity, the longitudinally extending bore being closeable in an upstream direction by a valve plug that partly follows the hammer piston during its stroke;
a valve stem operatively coupled to the valve plug and slideably received in a valve stem sleeve, said valve stem comprising:
a stop plate disposed at an upstream end of the valve stem, the stop plate engaging the valve stem sleeve to: 1) stop the valve plug at a predetermined percentage of a full stroke length of the hammer piston thereby opening the bore; and 2) elastically strain the valve stem such that said elastically strained valve stem is recoiled back to an uppermost position of the valve stem in the valve stem sleeve by non-helical tension spring properties of the valve stem; and
wherein said hammer piston is mechanically compressed when striking the drill bit so that said mechanically compressed hammer piston is recoiled back towards the uppermost position of the hammer piston in the housing due to pressure tension properties of the hammer piston.
2. The percussion hammer according to claim 1 , wherein the stop plate is adapted to cooperate with an internal stop surface in the valve stem sleeve.
3. The percussion hammer according to claim 1 , wherein the predetermined percentage of the full stroke length of the hammer piston is in an order of magnitude 75%.
4. The percussion hammer according to claim 1 , wherein said valve stem being long and slender.
5. The percussion hammer according to claim 1 , comprising an inlet valve assembly, which inlet valve assembly does not open for operation of the hammer piston until the pressure is built up to approximately 95% of full working pressure, said inlet valve assembly being adapted to close off a main barrel, and that a side barrel within the hammer housing pressurizes an annulus between the hammer piston and the housing elevating the hammer piston to seal against the valve plug.
6. The percussion hammer according to claim 5 , wherein the hammer piston and the inlet valve assembly return by recoil, where both the hammer piston and the inlet valve assembly are provided with hydraulic dampening controlling retardation of a return stroke until stop.
7. The percussion hammer according to claim 6 , wherein the hydraulic dampening takes place by an annular piston, which annular piston is forced into a corresponding annular cylinder having controllable clearances, and thus restricts or chokes evacuation of trapped fluid.
8. The percussion hammer according to claim 1 , wherein an opening is arranged in a top of the valve stem sleeve, into which opening a stop plate of the valve stem is able to enter, radial portions of the stop plate seals against an internal side of the opening with relatively narrow radial clearance.
9. The percussion hammer according to claim 8 , wherein an annular backup valve is arranged in a ring groove underneath the opening, wherein the backup valve is able to open and replenish fluid through bores in the valve stem sleeve.
10. The percussion hammer according to claim 1 , wherein the percussion hammer housing is divided into an inlet valve housing, a valve housing and a hammer housing.