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Developing a hydraulic pulse generator.

A compressed-water hydraulic pulse generator has been developed for applications requiring a high-energy pulse of water. The pressures and loading rates generated by discharge through a nozzle fitted into a borehole are comparable to a gunpowder blast and result in multiple fractures and fragmentations of hard rock.

THE HYDRAULIC PULSE GENERATOR (HPG) was developed as a way of fragmenting and excavating hard rock [1]. This device uses the energy stored in a water-filled accumulator to generate an ultrahigh-pressure (300- to 400-MPa) water pulse through a large 10- to 25-millimeter-diameter discharge valve. The energy of this pulse can be used to fracture rock or other materials, to drive a projectile, or to generate seismic waves (see Figure 1).

WATER COMPRESSION

HPG systems built to date have used water as a working fluid. At ultrahigh pressures water is a compressible fluid. Figure 2 shows the compression of water based on data on water compression at ultrahigh pressure from Bridgman [2]. Water compression data may be fit by an equation of the following form:

(1) V/[V.sub.o]= [(1+P/[P.sub.c]).sup.c] where [V.sub.o] is the volume of a vessel filled with water at pressure P, V is the decompressed water volume, and [P.sub.c] and c are empirical constants. The best fit to Bridgman's data is given by [P.sub.c] = 370 MPa and c=0.1671. The energy stored in a water-filled accumulator with volume [V.sub.o] can be found by integrating the work of compression

(2) [Mathematical Expression Omitted] which leads to

(3) [Mathematical Expression Omitted]

In practice there is an upper limit on the operating pressure of accumulators, which is dictated by the materials used in their construction and safety considerations. As the operating pressure increases, the pressure vessel wall thickness must also increase. This means that for a given external vessel dimension, the internal volume decreases. The safety factor is defined as the ratio between operating pressure and the pressure at which yield begins on the inside surface of the vessel.

A safety factor of 2.5 has been used in the design of HPGs that will be used in manned areas (the actual safety factor is considerably higher since yield on the inner diameter of a thick-walled ductile steel pressure vessel does not lead to catastrophic failure). Lower safety factors can be used for remote applications where higher energy/weight ratios are desired.

The HPG discharges through a fast-opening valve contained within the pressure vessel. Initial tests on pulse generation used a rupture disk to release the pressure. A pilot-operated ball valve was then developed and later modified to a poppet design [3], as shown in Figure 3. During charging, an orifice maintains a positive pressure differential between the valve inlet and the pressure vessel. This causes the poppet to seat and seal the pressure vessel. For discharge, a two-way servo-valve on the inlet line to the HPG is activated to vent the inlet line. This generates a pressure imbalance that lifts the poppet from its seat, discharging the HPG.

Once the poppet starts to lift, the pressure imbalance rapidly increases to a level equal to the total charge pressure; this causes the valve to open completely in a fraction of a millisecond. The servo-operated vent allows complete control over the discharge cycle using an electrical trigger. If the valve fails to open for any reason, the internal pressure will slowly discharge though the inlet orifice and vent valve.

Impulsive energies of up to 250 kJ have been generated by the HPG systems discussed in the following sections. The low compressibility of water means little heat is generated; the compression/discharge cycle is nearly adiabatic, and efficiency is almost 100 percent. Consistent repeated pulses may be generated at a cycle rate of 1 Hz with conventional ultrahigh-pressure power. The system may also be charged using a low-cost air-driven pump if rapid cycle time is not a consideration. The discharge valve used on the HPG has only a single moving part, and the first prototype has been discharged thousands of times without significant wear.

ROCK FRAGMENTATION AND EXCAVATION

Although explosive blasting is the most efficient means of excavating hard rock, the use of explosives has a number of drawbacks. Explosive excavation is a cyclic process in which blast holes are drilled, the holes are loaded with explosive charges, the area is evacuated, explosives are detonated, the opening is ventilated, and the broken rock is removed.

Chemical explosives generate toxic gas by-products that require extensive ventilation during underground tunneling or mining. In the deep-level gold mines of South Africa, the drill and blast cycle takes 24 hours, with much of the time devoted to ventilation and personnel transport. Explosives have been banned from some urban areas because of vibration damage induced during the nearly simultaneous detonation of explosive charges required by the drill and blast operation. Explosives also present a safety hazard during storage and use. These considerations have led to an interest in developing nonexplosive continuous excavation techniques suitable for hard rock [4].

The first HPG was designed for rock fragmentation and excavation [1]. In this application, the impulse pressure is directed into a discharge nozzle that is fitted into a borehole drilled in the rock. Figure 4 shows the pressure profile that results during discharge of a 30-kJ/300-MPa impulse into a 25-millimeter-diameter test chamber. The discharge nozzle for this test had a diameter of 23 millimeters and a length of 200 millimeters. The rise time and pulse duration shown here are comparable to that of a propellant charge, such as gunpowder, in a tamped hole.

The useful energy, or blasting strength value (BSV), of high explosives is about 1500 kJ/kg [5]. A 20-liter HPG at a pressure of 300 MPa releases 300 kJ of energy or the equivalent of 0.2 kilogram of high explosive. The following relationship is used to estimate the amount of rock that may be blasted from bench using a given weight, [q.sub.b], of high explosive [6]:

(4) [q.sub.b] = 1.45[B.sup.3]([C.sub.b]+07/B) where [C.sub.b] = 0.35 kg/[m.sup.3] and B is the height and burden of a bench. The equivalent hydraulic pulse energy can be found from W = [q.sub.b] BSV. The volume of rock removed in this simple bench blast geometry is [B.sup.3]. A 250-kJ HPG system should be able to blast a rock burden of 0.6 meter, which amounts to about a metric ton of rock.

Figure 5 shows an HPG mounted on a roadheader. This system was used to excavate granite with a compressive strength of 150 MPa. These tests demonstrated the rock fragmentation and excavation capabilities of the HPG. The HPG used had a discharge energy of 150 kJ and a valve diameter of 25 millimeters. A 250-kJ system with a 25-millimeter-diameter discharge valve has now been built (Figure 1) and is undergoing testing for use in a deep-level gold mine.

DECONTAMINATION AND DECOMMISSIONING

The HPG has also found a number of applications in the decontamination and decommissioning of nuclear facilities. These applications involve situations in which a mass of contaminated material must be fragmented to allow removal from a facility.

Our first system was designed to fit into the reactor vessel at Three Mile Island in Pennsylvania. The meltdown at this facility in 1979 resulted in a mass of ceramic-like material pooled in the bottom of the reactor vessel. An HPG system was designed to enter the reactor and break up the material. A 125-millimeter diameter 40-kJ system was fabricated and tested in a simulated reactor pool.

A second system has been designed for use in fragmenting and dislodging solid masses of radioactive salts that have formed in liquid waste storage tanks at the Hanford Nuclear Reservation in Washington [7]. The salts have surprisingly high strength and must be dislodged from the single-shell wall of the tank and internal tubing without damage. The application requires deployment of the HPG from a robotic arm so that the reaction forces must be minimized. Finally, the amount of water discharged into the tank must be minimized, since this increases the volume of radioactive material that must be handled.

For this application, the HPG end effector and recoil mount are 1 meter long and weigh 50 kilograms. The HPG has a discharge energy of 22 kJ at 345 MPa through an 11-millimeter discharge valve. The reaction load during discharge was observed to be 1300 N. At 345 MPa the system was quite effective at fragmenting a salt-cake simulant.

IMPACT TESTING, INDUSTRIAL APPLICATIONS

A typical 250-kJ HPG discharge requires approximately 100 milliseconds, which corresponds to a power of 2.5 megawatts. This level of impulsive mechanical power may be applied to a variety of industrial and testing situations. A 42-kJ HPG has been used to power a mechanical impact test system for projectiles prior to launch at a hypervelocity test range at Arnold Engineering Development Test Center at the Arnold Air Force Base in Tennessee [8].

A two-stage light-gas gun (G-range) is used to launch 63.5-millimeter-diameter projectiles at velocities up to 6 km/s. During the launch, the projectiles are subjected to base pressure spikes of up to 300 MPa. Structural failure of the projectile during launch can cause significant damage to the launcher and range track.

An impact tester, shown in Figure 6, has been built to simulate the duration and magnitude of base pressure spikes that occur during launch. This allows proof testing of projectile designs before launch. The projectiles are loaded into a launch tube simulator that incorporates a water-filled cavity at the projectile base. A 0.3-kilogram aluminum impactor cylinder is accelerated using the HPG to velocities up to 200 m/s. The cylinder impacts the water cavity, generating a pressure spike with a profile controlled by the elastic properties and dimensions of the impactor. The pressure spike is monitored with a pressure transducer.

The HPG could also be used as a highly efficient pump stage in a two-stage light-gas gun for launching hypervelocity projectiles, as shown in Figure 7. The compressed water would be used to pump a volume of light gas; typically hydrogen or helium would be used because they have high acoustic velocities. A pressure-release diaphragm then releases, allowing the gas to drive a projectile to hyper velocities. Energy storage in compressed water is nearly adiabatic because of the low compressibility of water; the energy transfer in the gas is also adiabatic because of the speed at which the process takes place. It is thus possible to design a highly efficient launcher that transfers almost all of the energy stored in the water to the projectile. The energy discharged by a 500-kJ HPG is equivalent to the kinetic energy of a 10-gram projectile that is moving at a velocity of 11 km/s.

SEISMIC SOURCE

The HPG may also be used to generate impulsive pressures in boreholes for rock mechanics and seismic studies. A 42-kJ HPG has been used to generate intense pressure pulses in 38-millimeter boreholes in granite, limestone, and concrete in a study of non-linear attenuation of stress waves [9]. This work is directed toward modeling the coupling of nuclear explosions to teleseismic radiation. In the experimental setup, the HPG is discharged into a shallow borehole in rock while the borehole pressure and ground acceleration at a small standoff are observed. Impulse pressure profiles in the three materials and the ground velocity spectrum at a standoff of 0.4 meter show that the source produces a significant signal at frequencies greater than 1 kHz.

The HPG provides a compact source of mechanical energy that may be used in a borehole for crosswell seismic work. An HPG borehole source can be configured to generate compressional and shear wave energy Existing electromechanical borehole seismic sources are limited in energy output, while explosive sources are poorly characterized. The HPG source offers high energy at frequencies up to 1 kHz. The source magnitude and spectrum may be characterized with a pressure transducer. A well-characterized source may be used to log formation attenuation properties, which are an important indicator of the presence and mobility of formation fluids. In addition, a variable-amplitude source can be used to characterize the inelastic mechanical properties of the formation. In this application, the attenuation of the signal is monitored as the source amplitude increases. The onset of nonlinear attenuation can provide an indication of the elastic limit of the formation.

The development of a hydraulic pulse generator using compressed water has led to a number of applications that require a source of impulsive mechanical power. The hydraulic pulse generator has been demonstrated to be an effective means of fragmenting rock and other hard materials, including ceramics and saltcake.

The pulse generation technique is compact, efficient, safe, and reliable. Applications currently under development include nonexplosive mining in deep-level gold mines, nonexplosive tunneling in urban areas, material removal in contaminated nuclear waste tanks, nonexplosive demolition of contaminated material, generation of compression and shear wave energy in boreholes, in situ rock mechanics testing, impact testing, materials processing, and hypervelocity projectile launch.

REFERENCES

[1.] Kolle, J.J., and Fort, J.A., 1988, "Application of Dynamic Rock Fracture Mechanics to Non-Explosive Excavation," in Key Question in Rock Mechanics: Proceedings of the 29th U.S. Symposium, P.A. Cundall et al., Eds., A.A. Balkema, Rotterdam, pages 571-578. [2.] Bridgman, P.W., 1911, "Water, in the Liquid and Five Solid Forms, Under Pressure," in Proceedings of the American Academy of Arts and Sciences, Vol.47, pages 441-558. [3.] Kolle, J.J., and Monserud, D.O., 1991, "Apparatus for Rapidly Generating Pressure Pulses for Demolition of Rock Having Reduced Pressure Head Loss and Component Wear," U.S. Patent No.5,000,516. [4.] Haase, H.H., and Pickering, R.G.B., "Non-Explosive Mining: An Untapped Potential for the South African Gold-Mining Industry," Journal of the south African Institute of Mining Metal, Vol.91, pages 381-388. [5.] Lownds, C.M., 1986, "The Strength of Explosives," The Planning and Operation of Open-Pit and Strip Mines J.P. Deetlets, Ed., SAIMM, Johannesburg, South Africa, pages 151-159. [6.] Johanson, C.H., and Persson, P.A., 1970, Detonics of High Explosives, Academic Press, London. [7.] Monserud, D.O., and Lilley, R.C, 1992, "Hydraulic End Effector Inspection and Test Results," prepared for Lawrence Livermore National Laboratories under Contract No.B199069, Quest Technical Communication No.355. [8.] Kolle, J.J., 1991a, "Impact Tester for Hypervelocity Projectiles," prepared for Arnold Engineering Development Center, Arnold AFB, Tenn., under Contract No.F40600-91-C-0009, Quest Technical Report No.549. [9.] Kolle, J.J., 1991b, "Observations of Transition Level Stress Wave Attenuation Using a Hydraulic Impulse Source," prepared for Defense Advanced Projects Agency under Contract No.DAAH01-90-C-0698, Quest Technical Report No.517.

ACKNOWLEDGMENT

This article is excerpted from the Proceedings of the Seventh American Water Jet Conference, held in August 1993 in Seattle, with the permission of the Water Jet Technology Association.
COPYRIGHT 1994 American Society of Mechanical Engineers
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1994 Gale, Cengage Learning. All rights reserved.

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Title Annotation:can be used in a number of applications
Author:Kolle, Jack J.
Publication:Mechanical Engineering-CIME
Date:May 1, 1994
Words:2499
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