GB2189170A

GB2189170A - Cavitation nozzle

Info

Publication number: GB2189170A
Application number: GB08709267A
Authority: GB
Inventors: Brian Richard Ferrier
Original assignee: BRIAN FERRIER PUMP SUPPLIES LI
Current assignee: BRIAN FERRIER PUMP SUPPLIES LI
Priority date: 1986-04-16
Filing date: 1987-04-16
Publication date: 1987-10-21
Also published as: GB8709267D0; GB2189170B; GB8609289D0

Abstract

A cavitation nozzle 1 for use in eroding a solid surface with a pressurized fluid jet comprises a divergent inlet portion 10, a convergent portion 13, and an outlet portion 12. The convergent portion has a semi-apex angle of from 35 to 40 degrees, preferably from 37 to 38 degrees, and a surface substantially free of irregularities and discontinuities, preferably being finished to a tolerance of 0.025mm. A method of eroding a solid comprises the steps of forming a cavitating fluid jet by directing a flow of pressurized liquid through the nozzle 1 so as to form vapour bubbles or pockets in the liquid flow, and impinging the resulting multiphase fluid jet against the solid at a distance from the nozzle outlet portion at which the vapour bubbles or pockets collapse upon impact with the solid. The nozzle is suitable for sub- sea operation. <IMAGE>

Description

SPECIFICATION Cavitation nozzle The present invention relates to cavitation nozzles suitable for use in eroding a solid surface with a pressurized fluid jet.

The basic principle of the use of cavitation in a fluid jet has been known for some time.

In general cavitation nozzles operate on the principle that when a flow of liquid is subjected to certain dynamic changes by being passed through a suitably formed passage, small vapour pockets or bubbles are formed in the liquid flow thereby creating a multiphase flow system. As these bubbles are carried along in the liquid flow they develop until at a certain stage downstream from the point of formation they reach a destructive condition whereby upon striking a solid surface they collapse upon impact and generate large shock waves in the solid surface which result in progressive breaking away and erosion of the surface.

Previously known cavitation nozzles have offered only limited increases in erosion over conventional high pressure fluid jets and/or have suffered from internal erosion resulting in relatively rapid loss of cavitation efficiency and hence of erosion efficiency.

It is an object of the present invention to avoid or minimize one or more of the above disadvantages.

The present invention provides a cavitation nozzle for use in eroding a solid surface with a pressurized fluid jet which nozzle comprises a divergent inlet portion, a convergent portion, and an outlet portion, said convergent portion having a semi-apex angle of from 35 to 40 degrees, preferably from 37 to 38 degrees, and having a surface substantially free of irregularities and discontinuities, preferably being finished to a tolerance of 0.025 mm, desirably 0.01 mm, most preferably 0.0025 mm.

Nozzles of the present invention have been found through various tests to have substantially greater erosion efficiency than conventional high pressure water jet nozzles and previously known cavitation nozzles, often by as much as an order of magnitude or more, and to have a performance comparable with solid particle entrainment cleaning systems. Moreover by means of suitable choice of the nozzle parameters it is possible to obtain relatively broad and uniform solid removal blast patterns.

The divergent inlet portion preferably has a semi-apex angle of the order of from 5 to 20 , preferably from 10 to 170, most preferably from 12 to 14 . The inlet end internal diameter of the divergent portion is generally comparable to that of conventional high pressure water jet nozzles and high pressure water supply lines being for example in the range from 0.5 to 1 cms, e.g. about 0.7 cms.

Desirably the nozzles include an intermediate generally parallel-sides portion, which conveniently has a length of from 1 to 4 times the length of the convergent portion, preferably from ij to 3 times, most preferably from 2 to 2- times, the length of the length of the convergent portion.

The outlet portion of the nozzle is also preferably in the form of a generally parallel sided passage of finite length, generally having a diameter of from 1 to 4 mm, preferably from 2 to 3 mm, and a length of from 1 to 5 mm, preferably from 2 to 4 mm. Conveniently the outlet passage has a bore diameter to length ratio of from 0.70 to 0.75.

Advantageously the nozzle is provided with a shroud means defining a generally parallelsided passage extending from the outlet portion of the nozzle for reducing any deleterious effects of high ambient pressures such as may for example by encountered at substantial depths under water. Desirably the shroud means defines a passage having an internal diameter of from 3 to 10, preferably from 4-7 times the internal diameter of the nozzle outlet portion. Preferably the shroud means passage has a length to from 5 to 25, most preferably from 12 to 20 times the nozzle outlet portion internal diameter.

The nozzle of the present invention may be made of any suitable material which is substantially rigid, dimensionally stable and erosion resistant but will generally be of a metal or alloy. Preferably at least the convergent portion of the nozzle will be of steel, desirably of stainless steel. Advantageously there is used a free machining metal such as free machining stainless steel in order to facilitate formation of the internal surface of the nozzle to a high tolerance.

As noted above it is important that at least some parts of the nozzle configuration are formed with a high degree of precision. In addition to surface finish of the convergent portion it is also desirable that the nozzle outlet passage should be finished to a tolerance of 0.025 mm, preferably 0.01 mm, most preferably 0.0025 mm. Also the inner surfaces of the nozzle are desirably formed so as to obtain maximum concentricity, preferably to within 0.025 mm. Suitable machining processes for achieving such tolerances are known in the art and include processes such as reaming for the formation of parallel sided passages and drilling with suitably shaped bits for convergent or divergent portions.

The nozzles of the invention may be used both in a gaseous medium e.g. under atmospheric conditions or submerged in a liquid medium such as water as for example in subsea operations. Indeed in some cases performance underwater may even be enhanced.

In connection with the latter it will be appreciated that a wide range of ambient pressures may be obtained at different depths under water and optimum nozzle configurations may vary somewhat for different ambient pressures, though the effect of such differences may be reduced as noted above by the use of a nozzle shroud. The nozzles may be used with various liquid flow rates and supply pressures and again it will be appreciated that optimum nozzle configurations may vary somewhat with these. In general the nozzles of the invention are suitable for water supply pressures of the order of from 1 to 1000 bar, for example from 600 to 800 bar. Typical flow rates that may conveniently be used are from 30 to 100 litres per minute, for example 50 to 80 I.p.m.

In a further aspect the present invention provides a method of eroding a solid with a cavitating fluid jet comprising the steps of forming a fluid jet by directing a flow of pressurized liquid through a nozzle of the present invention so as to form vapour bubbles or pockets in the liquid flow, and impingeing the resulting multiphase fluid jet against the solid at a distance from the nozzle outlet portion at which the vapour bubbles or pockets collapse upon impact with the solid.

A suitable stand-off distance of the nozzle outlet portion from the solid surface will depend upon various factors such as the ambient pressure and the fluid flow pressure which in turn will depend inter alia upon the nozzle configuration. In general though a suitable stand-off distance will be in the range of from 5 to 30 times, for example 10 to 20 times, preferably about 15 times the nozzle outlet internal diameter.

It will be appreciated that the nozzles of the invention may be used for various purposes including on the one hand cutting or other machining of solids and on the other hand cleaning of solid substrates by removal of deposits of other solids more or less firmly attached thereto. A particular advantage of the nozzles and method of the invention is that such cleaning may be effected with minimum damage to the substrate and/or so as to provide a relatively smooth finish to the cleaned substrate surface.

Further preferred features and advantages of the present invention will appear from the following detailed description given by way of example of a preferred embodiment illustrated with reference to the accompanying drawings in which: Figure 1 is a longitudinal section of a nozzle of the invention; and Figure 2 is a graph illustrating the performance of a nozzle of the invention in comparison with previously known water jets.

Fig. 1 shows a nozzle 1 comprising a tail portion 2, a head potion 3, and a shroud 4 mounted on the front end 5 of the head portion 3. The tail portion 2 is screwthreadedly connected 6 in generally conventional manner to a high pressure water supply hose 7.

At its inlet end 8 the tail porition 2 has a parallel sided cylindrical inlet passage 9 which leads into a divergent frustoconical portion 10.

This in turn leads into a parallel sided cylindrical intermediate portion 11 inside the head portion 3. At its front outlet end 5 the head portion has a small bore outlet portion 12 which is connected to the intermediate portion 11 by a convergent frustoconical portion 13.

Finally the shroud 4 defines a cylindrical passage 14 extending forwardly of the outlet portion 12 and having an internal diameter substantially larger than that of the outlet portion 12 though still somewhat less than that of the intermediate portion 11.

The tail portion 2, head portion 3 and shroud 4 are interconnected via suitable screwthreaded connections 15, suitable seals 16 conveniently in the form of 'O'-rings being mounted in axially mutually engaging surfaces 17 therebetween.

Fig. 2 shows comparative rates of erosion, conveniently measured in terms of mass loss from a target solid during a given period of time for three different nozzles comprising a conventional high pressure water jet nozzle I, a known cavitation nozzle II, and a cavitation nozzle of the present invention Ill. All the nozzles had the same outlet diameter of 2.5 mm and were operated under identical conditions with a water supply pressure of 700 bar and a flow rate of 64 litres per minute.

As may be seen from the graph all three systems exhibit a significant variation in performance with stand-off distance. Moreover the known cavitation nozzle II provides only a limited improvement over the high pressure jet I. In contrast the nozzle of the present invention provides an increased rate of erosion well over an order of magnitude greater than even that of the known cavitation nozzle. With such a substantial increase in erosion efficiency, the performace of the nozzle of the present invention approaches that of grit entrainment systems and thus offers an effective alternative which moreover has the important advantage of eliminating the need for transportation and complex handling systems for the grit as well as avoiding the problems of rapid nozzle erosion by the grit which occurs in practice.

Claims

1. A cavitation nozzle for use in eroding a solid surface with a pressurized fluid jet which nozzle comprises a divergent inlet portion, a convergent portion, and an outlet portion, said convergent portion having a semi-apex angle of from 35 to 40 degrees, and having a surface substantially free of irregularities and discontinuities.

2. A nozzle as claimed in claim 1 wherein said semi-apex angle is from 37 to 38 degrees.

3. A nozzle as claimed in claim 2 wherein said semi-apex angle is about 37.5 degrees.

4. A nozzle as claimed in any one of claims 1 to 3 wherein the convergent surface is finished to a tolerance of 0.025mm.

5. A nozzle as claimed in claim 4 wherein said surface is finished to a tolerance of 0.01 mum.

6. A nozzle as claimed in claim 5 wherein said surface is finished to a tolerance of 0.0025mm.

7. A nozzle as claimed in any one of claims 1 to 6 wherein the divergent inlet portion has a semi-apex angle of from 5 to 20 degrees.

8. A nozzle as claimed in claim 7 wherein said inlet portion semi-apex angle is from 10 to 17 degrees.

9. A nozzle as claimed in claim 8 wherein said inlet portion semi-apex angle is from 12 to 14 degrees.

10. A nozzle as claimed in any one of claims 1 to 9 wherein diameter of the divergent portion is from 0.5 to 1 cms.

11. A nozzle as claimed in any one of claims 1 to 10 wherein is provided an intermediate generally parallel-sided portion, upstream of the convergent portion.

12. A nozzle as claimed in claim 11 wherein said intermediate portion is from 1 to 4 times the length of the convergent portion.

13. A nozzle as claimed in claim 12 wherein said intermediate portion is from 1} to 3 times the length of the length of the convergent portion.

14. A nozzle as claimed in any one of claims 1 to 13 wherein the outlet portion of the nozzle is in the form of a generally parallel sided passage of finite length.

15. A nozzle as claimed in claim 4 wherein the outlet passage has an internal diameter of from 1 to 4mm and a length of from 1 to 5mm.

16. A nozzle as claimed in claim 15 wherein the outlet passage has an internal diameter of from 2 to 3mm and a length of from 2 to 4mm.

17. A nozzle as claimed in claim 16 wherein the outlet passage has a bore diameter to length ratio of from 0.70 to 0.75.

18. A nozzle as claimed in any one of claims 1 to 17 which nozzle is provided with a shroud means defining a generally parallelsided passage extending from the outlet portion of the nozzle for reducing any deleterious effects of high ambient pressures such as may be encountered at substantial depths under water.

19. A nozzle as claimed in claim 18 wherein said shroud means defines a passage having an internal diameter of from 3 to 10 times the internal diameter of the nozzle outlet portion.

20. A nozzle as claimed in claim 18 or claim 19 wherein said shroud means passage has a length of from 5 to 25 times the nozzle outlet portion internal diameter.

21. A nozzle as claimed in any one of claims 1 to 20 wherein at least the convergent portion is made of stainless steel.

22. A nozzle as claimed in claim 21 wherein the stainless steel is a free machining stainless steel.

23. A nozzle as claimed in any one of claims 1 to 22 mounted in a pressurized fluid jet apparatus.

24. A nozzle as claimed in claim 23 wherein is used a pressurized fluid jet apparatus formed and arranged so as to have a liquid flow rate of from 50 to 80 Ipm at a supply pressure of from 600 to 800 bar in use of the apparatus.

25. A method of eroding a solid with a cavitating fluid jet comprising the steps of forming a fluid jet by directing a flow of pressurized Iqiuid through a nozzle according to claim 1 so as to form vapour bubbles or pockets in the liquid flow, and impinging the resulting multiphase fluid jet against the solid at a distance from the nozzle outlet portion at which the vapour bubbles or pockets collapse upon impact with the solid.

26. A method as claimed in claim 25 wherein the nozzle outlet portion is disposed at a stand-off distance from the solid surface of from 5 to 30 times the nozzle outlet internal diameter.

27. A cavitation nozzle for use in eroding a solid surface with a pressurized fluid jet which nozzle is substantially as described hereinbefore with particular reference to Fig. 1 of the accompanying drawings.

28. A method of eroding a solid with a cavitating fluid jet substantially as described hereinbefore with particular reference to Figs.

1 and 2 of the accompanying drawings.