ABRASIVE WATER SUSPENSION JET TECHNOLOGY – FUNDAMENTALS, APPLICATION AND DEVELOPMENTS –

1 Introduction

Water jets have been used as a flexible tool for the cutting of soft materials like plastics, rubber, wood and textile materials since the 70s. However the addition of abrasive material to the water jet in the 80s extended the range of machinable materials to practically all technical materials including metallic and even ceramic materials. This is due to the fact, that the actual cutting process no more is carried out by the water itself, but by the abrasive particles, each particle contributing to the cut by a micro-chipping process. The only function of the water is the acceleration of the particles [1].

2 Generation of Abrasive Water Suspension Jets

Basically two systems of abrasive water jets, differing in their generation, their properties and their application fields have been developed. Their fundamental difference is the point of time of the abrasive addition leading to the specific jet properties. Abrasive Water Injection Jets (AWIJs) are generated by a water jet, passing through a mixing chamber and re-entering a focussing tube. This creates a low pressure in the chamber, which is used for the pneumatic transport into it. There the abrasive material is accelerated by the water jet and focussed in the secondary focussing tube.

Abrasive Water Suspension Jets (AWSJs) however are characterised by the fact of the admixture of abrasive material and water takes place before the nozzle. This has the effect that contrary to AWIJs the jet only consists of water and abrasive material [1].

During the development of AWSJs three different types of admixture of water and abrasive were investigated and applied (figure 1). One method is the admixture of the suspension at ambient pressure and the pumping of this suspension through a nozzle. Although this method is very simple to apply, it has never reached economic importance, mainly because of the high abrasive wear in the pump during the pressurisation of the suspension (figure 1, left) [2].

Figure 1: Generation mechanisms of abrasive suspension jets

The second possibility of generating AWSJs is the indirect pressurisation of a premixed suspension (figure 1, middle). Water is pressurised and drives a piston in a pressure vessel. This pushes the premixed suspension out of a high pressure storage vessel and accelerates the suspension in the nozzle. In order to stabilise the suspension, a highly concentrated polymer solution of a viscosity about four orders of magnitude higher than water is used as the liquid component of the cutting medium. With this method AWSJs of high pressure and coherence have been generated. On the other hand however the volume of the vessel and therefore the cutting time is limited. Additionally the wear of the nozzle, the piston and valves cause technological problems. This has limited the technological and economic success of this development [3]. The third method of admixture is the bypass principle (figure 1, right). In this case water is pressurised. A part of the flow is lead through a bypass branch passing a high pressure storage vessel, which is filled with abrasive material. By impulse exchange an abrasive slurry is driven out of the vessel and recombines with the main flow in the mixing cell, where the cutting suspension is formed. This then is transported through a hose to the work piece, where it is accelerated in the suspension nozzle. This generation method is also characterised by a limited capacity of abrasive material. Due to the fact however, that the abrasive storage vessel is completely filled with abrasive material, the cutting time is significantly longer than that of AWSJs generated by indirect pumping. Additionally neither a separating piston exists, nor valves are charged by abrasive material, so that systems of AWSJ cutting systems generated by the bypass principle are less wear sensitive [3].

3 Commercial AWSJ Cutting Systems

Due to the fact of minimum wear sensitivity and long cutting times, only AWSJs according to the bypass principle have been used for industrial applications. Another advantage over the indirect pumping is the possibility of adjusting the abrasive mass flow rate in the suspension during the cutting process, which can be done by the regulation of the bypass flow rate. For this a regulation valve as well as a restriction valve is in the bypass branch. The main parameters like flow rates in the main line and the bypass are as well as the pressure are used for the control of the system. The abra

Figure 2: AWSJ cutting machine

Depending on the technological advances and the specific demands on the cutting systems two different developments could be observed in AWSJ cutting technology. First of all the working pressure of the systems was shifted to higher levels. Starting from 35 MPa and lower at the beginning today’s commercial systems reach pressure levels up to 200 MPa. At the same time the flow rates of water and abrasive were reduced, which allows a cutting process using less energy and abrasive material. In the field of research abrasive suspension jets of 400 MPa pressure level at flow rates up to 4 l/min have been developed for machining purposes. The high constructive expenses and the difficult handling of the small abrasive material however have prevented the commercial implementation [4] (figure 3).

Figure 3: Hydraulic ranges of AWSJ cutting systems

On the other hand lower-cost cutting systems have been developed using moderate pressure levels (up to 50 MPa) at low flow rates. These systems have been constructed with a light-weight abrasive storage vessel and are designed specifically the mobile dismantling of structures of limited thickness (figure 4).

maximum pressure: Pressure vessel: Total weight: size (L x W x H): 50

Figure 4: Mobile AWSJ cutting system (Mini-MACE by ANT AG)

4 Influence of Main Parameters

The cutting process of abrasive water jets is determined by a micro-chipping process of the abrasive particles. Their removal potential directly depends on their velocity. Due to the fact, that the abrasive is accelerated by the water, the pressure is an important parameter for the cutting efficiency. Above a certain pressure the cutting efficiency increases proportionally to the pressure (figure 5).

Another important parameter on the cutting efficiency is the abrasive flow rate, which refers of the number of micro-chipping processes taking place. The more abrasive particles participate in the cutting process, the more efficient is the jet. The gradient however decreases so that the cutting efficiency reaches a maximum (figure 4). This is due to two facts. On one hand the ratio of accelerating water and particles to be accelerated decreases so that the velocity of the jet is lower (figure 5). On the other hand damping effects in the kerf gain influence.

The third main parameter is the influence of the nozzle diameter. An increase of the nozzle diameter has several effects. First of all it leads to an increase of the hydraulic power. This allows accelerating the abrasive particles in a more efficient way. Secondly an increased nozzle diameter leads to higher jet stability, which is important for cutting at large standoff distances as well as for cutting complex structures. On the other hand the width of cut also is increased, so that at a certain degree an increase of nozzle diameter leads to a decrease of cutting efficiency (figure 5).

The parameter however mostly used to control the cutting depth is the traverse rate. An increase of the traverse rate leads to a reduction of the hydraulic and abrasive load on a given traverse increment. This leads to a dependency, which is about reciprocally proportional to the traverse rate (figure 5) [5].

Figure 5: Influence of main parameters

5 Comparison between AWIJs and AWSJs

The different generation mechanisms of the two abrasive water jet cutting systems lead to specific jet properties. In AWIJs the abrasive material is added at atmospheric pressure, in AWSJ systems at working pressure. This leads to higher constructive expenses at AWSJs and a lack of continuous cutting as well as to long switching times. On the other hand the jet pump effect of AWIJs leads to a jet composition of about 95% air, 4% water and 1% abrasive, whereas AWSJs consist of about 95% water and 5% abrasive. Therefore AWSJs are characterised by a more efficient particle acceleration process and by a higher jet stability, which gives them a higher cutting efficiency. Due to the fact that AWSJs are generated by only one nozzle, where the also the abrasive passes through, they are worn. This has the effect of a continuous diameter increase and consequently of an increase of hydraulic power of the jet and its efficiency. This extra cutting efficiency has to be taken into account especially at long cutting tasks of several hours f.e. for nuclear dismantling.

The different properties of both types of abrasive water jets have lead to their core application fields. On one hand the high flexibility of AWIJs has qualified them for machining purposes. On the other hand the high efficiency of AWSJs has lead to different application fields in dismantling and decommissioning [1].

6 Application Fields of AWSJs

The fields of application of AWSJs have mainly been driven by the technological advances, but also by the demands of the market.

The low pressure level of AWSJ cutting systems until the mid 90s limited the application of this technology to fields, where the use of high flow rates of water and abrasive were not of disadvantage. On the other hand AWSJs allow long distances between the jet generation and the place of cutting activity. Therefore the first successful applications of AWSJs were in the repair and de-

Courtesy of ANT AG

commissioning of offshore structures like oil rigs or onshore structures like casings (figure 6).

Figure 6: Cutting of casings

The second large application field for AWSJs is demilitarisation. For the dismantling of bombs and missiles of WWII a safe cutting technology was demanded. Abrasive water jets are a cutting technology, being able to cut practically all technical materials without thermally influencing the work piece. In the case of cutting explosives this is of specific importance. AWSJs additionally do not contain air, which gives an extra security to beware the dismantling process from sparking.

These properties give the optimal conditions for the successful application of AWSJs in this field (figure 7).

Figure 7: AWSJ cut through a TNT antipersonnel H.E. shell (100mm)

The increase of working pressure in the second half of the 90s allowed a significant decrease of the flow rates of the jet. This is of specific importance for application fields, where secondary waste is to be limited, f. e. when cutting in contaminated areas like nuclear power plants. In these applications the amount of water and abrasive brought into the area of radiation is an important factor of costs. Therefore this application field only was opened, when the pressure level was increased above 140 MPa. In 1999 AWSJs were used for the first time for the dismantling of a nuclear power plant in Kahl, Germany. The parameters of the cutting of the thermal shied and the lower core shrowd are given in table 1. This will be one major application field for this technology in future [4]

Table 1: Parameters of the dismantling of the nuclear power plant in Kahl with AWSJ

7 Development of Polymer supported Abrasive Suspension Jets

In abrasive suspension cutting systems using the principle of indirect pumping the suspension is stabilised by polymeric additives leading to a viscosity 4 magnitudes higher than water (figure 1, middle). These systems showed a highly efficient jet generation process, however never reached technological relevance. Additionally polymers had been successfully used to increase the stability of pure waterjets in the 70s. Therefore the implementation of polymers into the technological relevant types of abrasive suspension jet cutting systems has been realised. These are the abrasive suspension jet cutting systems working on the bypass principle (figure 1, right). The polymers used are long-chained molecules leading not only to an increase of viscosity but also to a viscoelastic fluid behaviour like polyacrylamides or polyethyleneoxides. Generally there are two possibilities of polymer implementation into the cutting system, to replace the water feeding the high pressure pump by polymer solutions or to inject highly concentrated polymer solutions under working pressure into the suspension system. The first possibility is easy to realise; the second has the advantage that the polymers do go through the plunger pump and are not exposed to the high shear stresses with the risk of mechanical degradation [6]. The use of polymers positively affects the sedimentation resistance of the particles in the suspension and therefore contributes to the stability of the cutting process. During the jet generation process in the suspension nozzle the increased viscosity shifts the Reynolds number of the flow to lower values leading to an intensified particle acceleration process. Thus the process of material removal is enhanced. However not only the process of particle acceleration is improved but also the jet stability is increased by the use of polymers as has been observed in pure waterjets. Both effects positively influence the cutting potential of abrasive suspension jets, which is presented in figure 8.

Figure 8: Cutting efficiency of polymer supported abrasive suspension jets [6]

For both nozzles used an increase of cutting efficiency could be observed. However the increase at the short nozzle exceeds the increase of the long nozzle. These results prove the intensified acceleration process as well as the increase of jet stability by the use of polymers. Especially the jet stability can be taken advantage of by the use of abrasive suspension jets for dismantling activities, where often small standoff distances cannot be realised and complex structures have to be separated. In these hollow distances appear, in which the jet looses stability and efficiency. Especially for these complex dismantling tasks a significant improvement of cutting potential could be shown by the use of polymers in abrasive suspension jets [7].

8 Summary and Conclusions

AWSJs are a complementary development to AWIJs in water jet technology. Whereas AWIJs easily are generated by the adaptation of a cutting head on the water nozzle, the abrasive addition at AWSJs takes place at working pressure. This demands higher constructive expenses and leads to longer switching times. On the other hand the jet generated only consists of the phases water and abrasive and is more efficient and stable. These differences lead to the fact that AWSJs are not used for machining purposes, but are very efficient for cutting tasks under difficult conditions like the following:

-long distances to the cutting place

-thick and complex structures

-submerged cutting operations

-cutting of explosives These demands are typical for dismantling activities of offshore structures, nuclear dismantling or of military structures. This shows that the development of AWSJs will not replace AWIJs, but complement with it and lead to further application fields.

By selection of the suitable parameters (pressure, flow rates, traverse rate) the properties of AWSJs can be fitted to the specific application optimised towards the desired target parameter. Although the development tends to the increase of pressure and the reduction of flow rates and energy, a level of 200 MPa seems to be a reasonable limit. This is due to the difficult handling of the fine abrasive particles and the over-proportionally increasing costs of the high pressure components.

New developments of the implementation of polymers in abrasive water suspension technology will allow enhancing the cutting potential. This will be not only in terms of cutting velocity and cutting depth, but also in terms of independency of the standoff distance as well as the complexity of structures to be cut.

9 Acknowledgement

The authors are scientists of the Water jet Laboratory Hannover (WLH) in the Institute of Materials Science, University Hannover and members of the German Working Group of Water jet Technology (AWT).