NO334793B1 - High frequency liquid driven drill hammer for percussion drilling in hard formations - Google Patents

Abstract

A fluid-pressure, high-frequency percussion hammer is shown for drilling in hard formations. The hammer piston (20) of the percussion hammer has a longitudinal relatively large bore (41) which provides minimal flow resistance for a drilling fluid through the bore (41) during the return stroke of the hammer piston (20). The bore (41) is closable in the upstream direction by a valve plug (23) which follows the hammer piston (20) during impact. The valve plug (23) is guided by a relatively long and slender valve stem (49) which is mechanically capable of stopping the valve plug (23) at about 75% of the full stroke of the hammer piston (20) and separates it from a seat seal (40). Thus, the bore opens so that the drilling fluid can flow through, and the inherent tension spring characteristics of the valve stem (49) return the valve plug (23) so rapidly that there is good flow during the return of the hammer piston (20).

Description

High-frequency fluid-driven hammer drill for percussion drilling in hard formations.

The present invention relates to a liquid-pressure-driven, high-frequency percussion hammer for drilling in hard formations, where the percussion hammer comprises a housing that has a drill bit mounted on one end that must act directly against the hard formation, which percussion hammer comprises a hammer piston movably engaged in the housing and acts against the drill bit , where the hammer piston has a longitudinal bore with predetermined flow capacity and the bore can be closed in the upstream direction by a valve plug which partially follows the hammer piston during the stroke.

Percussion hammers of this type are known, among other things, from US 3327790, US 3403739, SE 444127 and US 4450920.

Hydraulically operated percussion hammer for drilling in rock has been in commercial use for over 30 years. These are used with jointable drill rods where the drilling depth is limited by the impact energy being lost through the joints, as well as the mass of the drill rod being too large so that little energy ultimately reaches the drill bit.

Downhole hammer drills, i.e. hammer drills mounted directly above the drill bit, are much more efficient and are widely used for drilling wells down to a depth of 2-300 m. These are powered by compressed air and with a pressure of up to approx. 22 bar, which then limits the drilling depth to approx. 220 m if there is water intrusion in the well. High-pressure water-powered hammer drills have been commercially available for approx. 10 years, but these are of limited size, up to approx. 130 mm hole diameter. They are also known to have a limited lifespan and are sensitive to impurities in the water. They are widely used in the mining industry as they drill very efficiently and drill very straight holes. They are used to a limited extent for vertical hole drilling down to a depth of 1000 - 1500 m, and then without direction control equipment.

It is desired to manufacture downhole, drilling fluid powered drill hammers that can be used together with directional control equipment, which have a high degree of efficiency, can be used with water as drilling fluid, can also be used with water-based drilling fluid with additives and have an economical lifetime. It is expected to be widely used both for deep well drilling for geothermal energy and for hard-to-reach oil and gas resources.

In percussion drilling, drill bits with inserted hard metal knobs, so-called "indenters", are used. These are made of tungsten carbide and are typically from 8 to 14 mm in diameter and with a spherical or conical end. Ideally, each "indenter" should be struck with optimal impact energy in relation to the rock's hardness, compressive strength, so that it creates a small crater in the rock. The drill bit is rotated so that the next blow ideally forms a new crater connected to the previous one. Bore diameter and geometry determine the number of "indents".

Optimal impact energy is determined by the rock's compressive strength, drilling is done in rocks with a compressive strength above 300 MPa. Supply of impact energy beyond the optimum is lost energy as it does not destroy the rock, but propagates as energy waves. Too little impact energy does not create a crater at all. When the impact energy per "indenter" is known and the number of "indents" is determined, then the optimal impact energy for the drill bit is given. Penetration (ROP - rate of penetration) can then only be increased by increasing the stroke frequency.

The amount of drilling fluid that is pumped is determined by the minimum required return velocity (annular velocity) in the annulus between the drill string and the hole wall. This should be over 1 m/s, preferably 2 m/s, in order for the drilled mass, cuttings, to be transported to the surface. The harder and more brittle the rock is, and the higher the impact frequency, the finer the drill cuttings will be, and the slower the return speed can be accepted. Hard rock and high frequency will produce cuttings that appear as dust or fine sand.

The hydraulic power supplied to the hammer drill is determined by the pressure drop multiplied by the pumped quantity per unit of time.

The impact energy per impact multiplied by the frequency gives the effect. If we look at an imaginary example where granite with a compressive strength of 260 MPa is to be drilled and the drill diameter is 190 mm, 750 L/min (12.5 L/s) of water is pumped from the surface. It is calculated with approx. 900 J as optimal impact energy.

With reference to known data for similar drilling, but with a smaller diameter, one can expect a drill sink of 22 m/h with an impact frequency of 60 Hz. We expect here to increase the stroke frequency to 95 Hz, consequently the ROP will then be 35 m/h. The required net power on the drill bit then becomes: 0.9 kJ x 95 = 86 kW. We calculate the current hammer construction to have a mechanical-hydraulic efficiency of 0.89, which then gives 7.7 MPa necessary pressure drop across the hammer.

This hammer drill will then drill 60% faster and with 60% less energy consumption than known available water-powered hammer drills.

This is achieved with a percussion hammer of the type mentioned at the outset, which is characterized by the fact that the valve plug is controlled by an associated valve stem which is slidably engaged in a valve stem sleeve, where the valve stem comprises stop devices which abruptly stop the valve plug at a predetermined percentage of the full stroke of the hammer piston and separate the valve plug from a seat seal on the hammer piston, as the bore thus becomes open and allows the drilling fluid to flow through the bore, and that the valve stem's inherent tension spring properties return the valve plug at high speed by means of mechanical recoil.

Advantageously, the stop devices comprise a stop plate on the upstream end of the valve stem and a cooperating internal stop surface in the valve stem sleeve.

In one embodiment, the predetermined percentage of the full stroke of the hammer piston may be on the order of 75%.

Appropriately, it is the valve stem's inherent tension spring properties that return the valve plug, as the valve stem is long and slender.

Advantageously, the percussion hammer can also be equipped with an inlet valve assembly which does not open for operation of the hammer piston until the pressure has built up to approximately 95% of full working pressure, the inlet valve assembly being adapted to close a main barrel and a side barrel in the hammer housing pressurizing an annulus between the hammer piston and the housing that lifts the hammer piston to seal against the valve plug.

Conveniently, the hammer piston and valve assembly return with recoil, where both the hammer piston and valve assembly are equipped with hydraulic damping that controls the deceleration of the return stroke to a stop.

Appropriately, the hydraulic damping takes place with a ring piston which is pressed into a corresponding ring cylinder with controlled clearances, and thus throttles the evacuation of the trapped liquid.

Furthermore, an opening can be arranged at the top of the valve stem sleeve, into which opening the valve stem's stopper plate is able to enter, as the radial part of the stopper plate seals, with a relatively narrow radial clearance, against the inside of the opening.

Furthermore, an annular non-return valve can be arranged in an annular groove below the opening, which non-return valve is able to open and replenish liquid through bores in the valve stem sleeve.

The percussion hammer housing can be divided into an inlet valve housing, a valve housing and a hammer housing.

The hammer construction according to the present invention is of the so-called Direct Acting Hammer type, i.e. there is a closing valve on the hammer piston which in the closed position causes the pressure to propel it forward, and in the open position causes the hammer piston to be exposed to recoil. The earlier version of a hydraulic hammer has a valve system that pressure drives the hammer piston in both directions. This gives a lower degree of efficiency, but more precise control of the piston.

The key to good efficiency and high stroke frequency lies in the valve construction. It must operate at a high frequency and have good flow characteristics in the open position.

The hammer drill construction can also be used with great advantage as a surface-mounted hydraulically driven hammer for drilling with drill rods, but it will be used as a downhole hammer drill, which is described in detail here.

Other and additional purposes, distinctive features and advantages will be apparent from the following description of preferred embodiments of the invention, which is given for description purposes and given in connection with the attached drawings, where: Fig. 1 schematically shows a typical hydraulic hammer drill according to the invention,

Fig. 2A shows an elevation of a downhole hammer drill with drill bit,

Fig. 2B shows the hammer drill in fig. 2A turned about 90°,

Fig. 2C shows a view in the direction of the arrows A-A in fig. 2A,

Fig. 2D shows a view in the direction of arrows B-B in fig. 2A,

Fig. 3A shows a longitudinal section of the hammer drill shown in fig. 2A where the main internal parts are shown,

Fig. 3B shows a cross section along the line A-A in fig. 3A,

Fig. 3C shows a cross-section along the line B-B in fig. 3A,

Fig. 3D shows a cross-section along the line C-C in fig. 3A,

Fig. 3E shows a cross-section along the line D-D in fig. 3A,

Fig. 3F shows a 2 times enlarged, circled detail section H in fig. 3A,

Fig. 3G shows a 2 times enlarged, circled detail section H in fig. 3A,

Fig. 3H shows a 5 times enlarged, circled detail section F in fig. 3A,

Fig. 31 shows a 5 times enlarged, circled detail section G in fig. 3A,

Fig. 4A shows similar to what is shown in Fig. 3A, but at the end of an acceleration phase,

Fig. 4B shows an elevation of the valve assembly shown in section in fig. 4A,

Fig. 4C shows a cross-section along the line B-B in fig. 4A,

Fig. 4D shows a 5 times enlarged, circled detail section A in fig. 4A,

Fig. 4E shows a 5 times enlarged, circled detail section C in fig. 4A,

Fig. 5A shows the equivalent of what is shown in fig. 3A and 4A, but at the moment when the hammer piston strikes the impact surface in the drill bit,

Fig. 5B shows a 5 times enlarged, circled detail section A in fig. 5A,

Fig. 5C shows a 4 times enlarged, circled detail section B in fig. 5A,

Fig. 6A shows the equivalent of what is shown in fig. 3A, 4A and 5A, but when the hammer piston is in full return,

Fig. 6B shows a section along the line E-E in fig. 6C,

Fig. 6C shows a 5 times enlarged, circled detail section A in fig. 6A,

Fig. 6C shows a 20 times enlarged, circled detail section D in fig. 6C,

Fig. 6D shows a 20 times enlarged, circled detail section C in fig. 6E,

Fig. 6E shows a 4 times enlarged, circled detail section B in fig. 6A,

Fig. 7A shows correspondingly what is shown in fig. 3A, 4A, 5A and 6A, but when the hammer piston is in the last part of the return,

Fig. 7B shows a 20 times enlarged, circled detail section B in fig. 7C,

Fig. 7C shows a 4 times enlarged, circled detail section A in fig. 7A,

Fig. 8 shows curves illustrating the duty cycle of the hammer piston and the valve, Fig. 9A shows the curve illustrating the steep closing characteristic of the valve with respect to pressure drop, and

Fig. 9B illustrates flow and pressure drop across the gradually closing valve.

Fig. 1 shows a typical hydraulic hammer drill for mounting on top of jointable drill rods where the hammer mechanism is built into a housing 1 built in several housing sections, where a rotary motor 2 rotates a drill rod via a transmission 3 which rotates a shaft with a threaded part 4 which is screwed to the drill rod and a drill bit (not shown). The hammer machine is usually equipped with a mounting plate 5 for mounting to a feeding device on a drilling rig (not shown). Supply of hydraulic fluid takes place via pipe and coupling 6 and hydraulic return via pipe with coupling 7. Fig. 2 A and 2B show a downhole hammer drill with drill bit. This will be used in the following detailed description. The housing 1 shown has a first housing section 8 which houses what will later be described as the inlet valve, while a second housing section 9 houses a valve, a third housing section 10 houses a hammer piston and reference numeral 11 denotes the drill bit. Through an opening or main run 12, the drilling fluid is pumped in and a threaded part 13 connects the hammer to the drill string (not shown). A flat part 14 is for using a torque wrench to turn the hammer on/off the drill string. A drainage hole 15 is necessary for the function of the later explained inlet valve, outlet hole 16 is for the return of the drilling fluid in the annulus between the drill hole and the drill hammer housing (not shown) back to the surface. Carbide knobs 17 are the elements that crush the rock in which the drill is being done. Fig. 2C shows a view seen in the direction of the arrows A-A in fig. 2A, and Fig. 2D shows a view seen towards the drill bit 11 in the direction of the arrows B-B in fig. 2A. Fig. 3 A shows a longitudinal section of the drill hammer where the main internal parts are: an inlet valve assembly 18, a valve assembly 19 and a hammer piston 20. The drilling fluid is pumped in through the inlet 12, the inlet valve 18 passes in the open position through bores 21 shown on section A-A in fig. 3B, further through bores 22 in section B-B in fig. 3C to a valve plug 23 which is shown in the closed position in section C-C in fig. 3D against the hammer piston 20 and drives this to abut against the bottom part of the drill bit 24. Section D-D in fig. 3E shows a longitudinal groove part 25 in the drill bit 11 and the lower part of the hammer housing 10 which transmits the torque while the drill bit 11 can move axially within permitted clearances determined by a locking ring mechanism 26. This is because when the hammer piston 20 hits the drill bit 11, only the mass of this which is moved in step with the 17 penetration of the hard metal studs into the rock. It is a prerequisite that the drill string exerts a certain feed force so that the hammer housing 10 is pressed against the drill bit 11 so that the clearance against the locking ring mechanism 26 allows axial movement of only the drill bit 11 in step with the penetration into the rock. This is so that as much of the impact energy as possible is transferred to crushing the rock and as little as possible is lost to mass movement of the relatively light drill bit 11.

The detailed section in fig. 3F, which shows the inlet valve 18 in the closed position is taken from H in fig. 3A. When the hammer function is to be initiated, the pumping of the drilling fluid in the inlet 12 is started. A side bore 27 through the wall of the valve housing 8 has hydraulic communication with a pilot bore 28 in the inlet valve 18's mounting plate 29. The mounting plate 29 is stationary in the valve housing 8 and contains a pilot valve 30 which is held in open position of a spring 31. The drilling fluid flows freely to a first pilot chamber above a first pilot piston 32 whose diameter and area is greater than the area of the inlet 12. During pressure build-up, a limited movable valve plug 33 will be pressed to close against a seat 34 in the housing 8 During pressure build-up against the closed inlet valve 18, an annular space 35 between the housing 10 and the hammer piston 20 is pressurized through the side bore 27, which via longitudinal bores 36 in the valve housing 9 feeds an inlet 37, see detail section F.

The detail sections in fig. 3H and fig. 31 is taken from F and G in fig. 3A and shows the contact of the hammer piston 20 against the inner wall of the hammer housings 9, 10. The diameter of a piston 38 is slightly larger than the diameter of a second piston 39. When using the hammer drill for drilling vertically downwards, the hammer piston 20 in a depressurized state, due to the force of gravity, will obviously against the abutment surface 24 in the drill bit 11.1 in this condition, there will be clearance between the valve plug 23 and its seat 40 (see detail section F) in the hammer piston 20. Consequently, the drilling fluid will flow freely through the valve at the plug 23, through a bore 41 in the hammer piston 20 and the bores 16 ( see fig. 2A), and therefore we get too little pressure build-up for the hammer to start.

In detail fig. 3F shown arrangement with closed inlet valve 18 and pressure build-up in the annulus 35 lifts the hammer piston 20 to seal against the valve plug 23. Due to the necessary clearance between the face of the piston 38 and the inner wall of the housing 9, drilling fluid leaks into the space above the valve plug 23 through lubrication channels 42 and a bore 43 as an arrow shows in detail section F. To prevent this leakage volume from causing a build-up of pressure in the space above the valve plug 23, this is drained through a bore 44 in the valve mounting plate 29 and an opening 45 that the pilot valve 30 in this position allows, and further out through the drainage hole 15 When the pressure has risen to over 90% of the working pressure for which the hammer is designed, the piston force in a second pilot chamber 46 overcomes the closing force of the spring 31 and the pilot valve 30 changes position as shown in fig. 3G.

The first pilot chamber above the pilot piston 32 is drained and the inlet valve 18 opens. At the same time, the opening 45 is closed so that drainage through the bore 44 becomes tight so that no pressure is lost through this in operating mode. The pressure in the space above the hammer piston 20 and the closed valve plug 23 leads to the start of the work cycle with immediate full power. The arrangement with a non-return valve 47 and a nozzle 48 is there to have a reduced draining time of the second pilot chamber 46 in order to achieve a relatively slow closing of the inlet valve 18. This is so that the inlet valve 18 remains fully open and will not interfere in the operating mode when the pressure fluctuates with the beat frequency.

Fig. 4A shows the hammer drill at the end of an acceleration phase. The hammer piston 20 has at this point reached full speed, typically approximately 6 m/s. This is a result of available pressure, for example as here just under 8 MPa, the hydraulic area of the hammer piston, for example as here with a diameter of 130 mm and the weight of the hammer piston, for example as here is 49 kg. The valve plug 23 is kept closed against the hammer piston's seat opening as the hydraulic area of the valve plug 23, for example as here with a diameter of 95 mm, is slightly larger, approx. 4%, than the ring area for the hammer piston shown in section B-B in fig. 4C with 23 and 24 respectively. At this point the hammer piston has traveled approx. 75% of full stroke length, approx. 9 mm. The clearance between the hammer piston 20 and the impact surface 24 of the drill bit is approx. 3 mm, shown on enlarged detail section C in fig. 4E.

A movable valve stem 49 with a stop plate 50 now lands against the stop in a stationary valve stem sleeve 51 in the housing 9 and stops the valve stem 49 from further travel, as shown in enlarged detail section A in fig. 4D, whereupon the valve plug 23 is separated from the seat 40 in the hammer piston 20 and thus opened. The movable valve unit 23, 49, 50 is shown in elevation in fig. 4B.

The kinetic energy in the mass velocity of the valve plug 23 will, at its sudden stop, stretch the relatively long and slender valve stem 49 marginally and thus transfer to a relatively large spring force which very quickly accelerates the valve in return. The marginal stretching of the valve stem 49, calculated here for example at approx. 0.8 mm, must be below the utilization rate for the material, which in this case is high-speed spring steel. The mass of the valve plug 23 should be as small as possible, here, for example, made of aluminium, combined with the length, diameter and material properties of the valve stem 49, determine the self-oscillation of the valve assembly.

For practical use, this should be at least 8-10 times the frequency for which it is to be used. The natural oscillation frequency is determined by the formulas:

The mass and the spring constant matter the most. The natural oscillation frequency for the construction shown is approx. 1100 - 1200 Hz and therefore applicable for a working frequency of over 100Hz.

In this example, the construction shown here has a recoil speed that is 93% of the impact speed.

Fig. 5A shows the position and time when the hammer piston 20 hits the abutment surface 24 in the drill bit 11. The valve plug 23 with the stem 49 and the stop plate 50 is at full speed in return, see detail section A in fig. 5B, so that relatively quickly a large opening is formed between the valve plug 23 and the valve seat 40 of the hammer piston 20 so that the drilling fluid now flows with relatively little resistance through the longitudinal bore 41 of the hammer piston 20, see detail section B in fig. 5C.

The kinetic energy in the mass velocity of the hammer piston 20 is partly converted into a spring force in the hammer piston 20 as this is slightly compressed during the impact. When the energy wave from the impact has traveled through the hammer piston 20 to the opposite end and back, the hammer piston 20 accelerates in return. The return speed at the start here is calculated at approx. 3.2 m/s, approx. 53% of the impact speed, this because part of the energy has gone into the mass movement of the drill bit 11, while the rest has gone into the "indents" being impressed into the rock.

Fig. 6A shows the time when the hammer piston 20 is at full return speed. The valve plug 23 has at this point almost returned to the end stop where detail section A in fig. 6C shows the stem 49 with the stop plate 50 entering an opening 52 in the top of the valve stem sleeve 51.

Detail section D in fig. 6C shows how the radial part of the stop plate 50 seals, with a relatively narrow radial clearance, against the inside of the opening 52. A small negative pressure is formed in the chamber under the stop plate 50 when it travels the last 2 mm to stop. An annular non-return valve 58 opens and refills fluid through bores 59. The confined volume below the stopper plate 50 prevents the valve plug 23 from recoiling and remains in position until the next cycle starts.

The non-return valve 58 of the "ring non-return valve" type, which in design is a ring-shaped leaf spring, has been chosen as it has a small mass and a relatively large spring force and can therefore work at a high frequency.

Detail section B in fig. 6E shows the relatively large opening between the valve plug 23 and the valve seat 40 in the hammer piston 20 so that the drilling fluid flows through with minimal resistance. The underside of the valve stem sleeve 51 is shaped like an annular cylinder pit 53 shown in detail section C in fig. 6D. The top of the valve plug 23 is shaped like an annular piston 54 which with relatively narrow clearances fits into the annular cylinder pit 53. The trapped liquid volume is, as the valve returns all the way to the end stop, evacuated in a controlled manner through the radial clearances between the annular piston 54 and the annular cylinder 53 plus an evacuation hole 55. This controlled evacuation acts as a dampening force and stops the return of the valve so that it does not make recoil movements. The same type of damping arrangement is on the hammer piston 20. Detail section B shows an annular piston 56 on top of the hammer piston 20, as well as an annular cylinder groove 57 in the lower part of the valve housing 9.

Fig. 7A shows the last part of the return of the hammer piston 20. The end of

the return stroke is controlled damped to a full stop at the same time as the valve seat 40 meets the valve plug 23, shown in detail section A in fig. 7C. Detail section B in fig. 7B illustrates how the trapped liquid volume in the annular cylinder pit 57 is displaced through the radial clearances between the ring piston 56 and a drainage hole 60.

The gap between the valve seat 40 and the valve plug 23 does not need to be completely closed for the pressure to build up and a new cycle to start. Calculations show that with an opening of 0.5 mm the pressure drop is almost the same as the working pressure. This means that the surface pressure on the contact surface between the valve plug 23 and the seat 40 becomes low and the components can have a long service life.

Fig. 8 shows curves illustrating the working cycle of the hammer piston 20 and the valve. Curve A shows the speed course and curve B the position course through a work cycle. For both curves, the horizontal axis is the time axis, divided into microseconds.

The vertical axis for curve A shows the speed in m/s, impact direction towards the drill bit 11 as + upwards and - downwards, here the return speed.

The vertical axis for curve B shows the distance in mm from the starting position. Curve section 61 shows the acceleration phase, where point 62 is when the valve is stopped and its return starts. Point 63 is the impact of the hammer piston 20 against the drill bit 11. Curve section 64 is the movement of the drill bit 11 when advancing in the rock, 65 is the acceleration of the recoil, 66 is the return speed without damping and 67 is the return speed with damping. Curve section 68 is the recoil shaft ration for the valve, 69 is the valve return speed without damping and 70 is the damping phase of the valve return.

Fig. 9A shows a curve 71 which illustrates the valve's steep closing characteristic with regard to pressure drop and opening between the valve plug 23 and the seat 40 in the hammer piston. This situation is shown in fig. 9B. The horizontal axis is the opening gap in mm and the vertical axis is the calculated pressure drop in bar at the nominal amount of pumped drilling fluid, which for example here is 12.5 L/sec. As shown, the closing gap must fall below 1.5 mm before significant pressure resistance is obtained.

Claims (10)

1. Fluid pressure-driven, high-frequency percussion hammer for drilling in hard formations, where the percussion hammer comprises a housing (8, 9, 10) which at one end has a drill bit (11) mounted on it which must act directly against the hard formation, which percussion hammer comprises a hammer piston (20 ) movably engaged in the housing (8, 9, 10) and acts against the drill bit (11), where the hammer piston (20) has a longitudinal bore (41) with predetermined flow capacity and the bore (41) can be closed in the upstream direction by a valve plug (23) which partially follows the hammer piston (20) during the stroke, characterized in that the valve plug (23) is controlled by an associated valve stem (49) which is slidably engaged in a valve stem sleeve (51), where the valve stem (49) includes stop devices (50, 51) which abruptly stops the valve plug (23) at a predetermined percentage of the full stroke of the hammer piston (20) and separates the valve plug (23) from a seat seal (40) on the hammer piston (20), thereby opening the bore (41) and allowing r the drilling fluid to flow through the bore (41), and that the valve stem's (49) inherent tension spring properties return the valve plug (23) at high speed by means of mechanical recoil. 2. Percussion hammer as specified in claim 1, characterized in that the stop devices (50, 51) comprise a stop plate (50) on the upstream end of the valve stem (49) and a cooperating internal stop surface in the valve stem sleeve (51). 3. Percussion hammer as stated in claim 1 or 2, characterized in that the predetermined percentage of the full stroke length of the hammer piston (20) is in the order of 75%. 4. Percussion hammer as stated in claim 1, 2 or 3, characterized in that it is the inherent tension spring properties of the valve stem (49) that return the valve plug (23), the valve stem (49) being long in relation to its diameter. 5. Percussion hammer as stated in one of claims 1-4, characterized in that the hammer is further equipped with an inlet valve assembly (18) which does not open for operation of the hammer piston (20) until the pressure has built up to approximately 95% of full working pressure, since the inlet valve assembly ( 18) is adapted to close a main barrel (12) and that a side barrel (27) in the hammer housing pressurizes an annular space (35) between the hammer piston (20) and the housing (10) which lifts the hammer piston (20) to seal against the valve plug (23) . 6. Percussion hammer as specified in claim 5, characterized in that the hammer piston (20) and the valve assembly (19) return with recoil, where both the hammer piston (20) and the valve assembly (18) are equipped with hydraulic damping which controls the deceleration of the return stroke to a stop. 7. Percussion hammer as specified in claim 6, characterized in that the hydraulic damping occurs with a ring piston (54) which is pressed into a corresponding ring cylinder (53) with controlled clearances, and thus throttles the evacuation of the trapped liquid. 8. Percussion hammer as specified in one of claims 1-7, characterized in that an opening (52) is arranged at the top of the valve stem sleeve (51), in which opening (52) the stop plate (50) of the valve stem (49) is able to enter, the radial part of the stop plate (50) seals, with relatively narrow radial clearance, against the inside of the opening (52). 9. Percussion hammer as stated in claim 8, characterized in that an annular non-return valve (58) is arranged in an annular groove below the opening (52), which non-return valve (58) is able to open and refills fluid through bores (59) in the valve stem sleeve (51). 10. Percussion hammer as specified in one of claims 1-9, characterized in that the percussion hammer housing (1) is divided into an inlet valve housing (8), a valve housing (9) and a hammer housing (10).