Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF PREPARING A SENSITISED EXPLOSIVE
Technical Field
This invention relates to a method of sensitising an explosive by providing
expanded microspheres therein.
Background Art
In most fluid explosives or slurry explosives, voids are introduced into the
explosives in order to reduce their natural densities and thereby increase
their
sensitivity to a required level for detonation. This process is known as
sensitisation.
Sensitisation is an important part of the art of manufacturing fluid
explosives or
slurry explosives. Known methods of sensitisation include the use of chemical
gassing, whereby gas bubbles are generated at the required rafie and to the
required extent -to effectively sensitise the explosive. Another well-known
method is to add so-called mechanical voids"to an unsensitised explosive.
Mechanical voids that are well known include expanded perlite and
microspheres. Microspheres may include organic spheres such as polystyrene,
or microballoons including glass microballoons or polymeric microballoons.
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A microballoon comprises a hollow body or shell (usually made of glass or a
suitable polymeric material) and a gas contained in the hollow body. In the
case
of polymeric microballoons the gas is known as a. blowing agent. Explosive
manufacturers usually buy pre-expanded microballoons which they then mix
with an e~eplosive to produce a sensitised product. Pre-expanded microballoons
are prepared by subjecting unexpended microballoons to heat and other
treatments causing them to expand and thus having a reduced density
compared to the unexpended microballoons. Almost all kinds of expansion are
too complex for the manufacturer of explosives to carry out with the result
that
they purchase pre-expanded microballoons. Microspheres such as polystyrene
can also be pre-expanded in the same manner to reduce their density.
Several different methods of thermal expansion of polymeric microspheres are
known. Known methods of dry expansion involve relatively high capital cost, in
particular because of the precautions that must be taken against fire or
explosion of the blowing agent (usually a~ hydrocarbon gas) that is liberated
during the expansion. On the other hand steam expansion does not have this
hazard because of all the water present in both droplet and vapour form during
the entire time when the hydrocarbon gas is liberated. One known method of
steam expansion comprises preparing a slurry of microspheres which is fed to a
pipe together with steam thereby causing the microspheres to expand. US
patent 4,513,106 teaches that a disadvantage of this method is that the
expanded particles leaving the pipe have to be cooled directly with water in
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order not fio agglomerate which results in a final product having
unsatisfactory
high water contenfi. It is staffed that if cooling water is not used or is
replaced by
cooling air or a cooled apparatus manfile, a product of fused particles will
be
obtained. US 4,513,106 discloses a method of overcoming this problem
whereby a slurry of unexpanded microspheres is introduced into a pressure
zone to which steam is fed fio partly expand the microspheres. The partially
expanded microspheres fihen leave fihe pressure zone under a pressure drop
whereby the spheres are further expanded and accelerated into steam with a
velocity of at least 1 meter per second. The accelerated stream is injected
infio
a gas volume whereby the sfiream is disinfiegrafied and cooled. The expanded
non-agglomerated spheres are fihen separated from fihe gas. It will be
appreciated that fihis process is a fairly complicated one for avoiding
agglomeration of expanded spheres, agglomeration being a common problem
where microspheres are thermally expanded.
A further problem with especially pre-expanded microspheres is fihat they are
bulky due to their low density. This results in high transportation costs and
high
storage costs of the spheres. These spheres, especially glass microballoons,
are also difficult to handle. They are prone to fluidise or settle in an
unpredictable manner, and are prone to float in the air causing inhalation
hazard for the operators handling them.
US Patent 6,113,715 discloses a method whereby unexpanded microspheres
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are expanded in situ in an emulsion explosive during the formation of the
emulsion explosive i.e. during the emulsification process itself. Although
this
solves some of the above problems, the process introduces its own problems,
amongst others difficulty in controlling the end-point of the expansion, since
rapid cooling of the emulsion or emulsion component containing the expanded '
microspheres to quench the expansion is difFicult both because of the large
volume of the emulsion or component compared to~that of the microspheres
and because of the high viscosity of the emulsion or component containing the
expanded microspheres. This process is also, potentially hazardous since the
high shear emulsification process involves the introduction of significant
energy
to the emulsion. The emulsion is at least an energetic material and can be
,explosive. Simultaneous sensitisation of the emulsion while the emulsion is
being subjected to high shear can lead to hazardous conditions.
It is accordingly an object of the present invention to provide an alternative
method or process of preparing an explosive with expanded microspheres
therein.
Disclosure of the Invention
According to the present invention there is provided a method of preparing a
sensitised explosive comprising -
- providing thermally expandable microspheres which, when expanded, are
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suitable to reduce the density of an explosive;
- mixing the microspheres with steam to cause thermal expansion of the
microspheres and to provide a resulting stream of wet expanded
microspheres; and
5 - introducing the resulting stream of wet expanded microspheres, into an
explosive to provide a sensitised explosive.
The term "explosive" as used in this specification includes, in addition to
its
normal meaning a non-sensitised explosive or explosive pre-cursor which
becomes a sensitised explosive when sensitised by the addition of expanded
microspheres.
It will be appreciated that the stream of wet expanded microspheres comprises
a mixture of expanded microspheres and water in gaseous andlor liquid form.
This stream may contain other substances such as air or oil in liquid or
vapour
form. The volume of air is preferably of the same order as the volume of the
expanded microspheres to ensure that pneumatic conveyance is maintained
and no blockages occur. The air is also very useful in cleaning unexpanded
rnicrosphere slurry out of the system at shutdown, again to avoid blockages
due
to expansion of the microspheres by residual heat in the metal of the system.
The thermally expandable microspheres may be made of a polymeric material.
The polymeric material may comprise a polymeric foam, for example
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polystyrene. Preferably the microspheres comprise microballoons, preferably
microballoons made of a polymeric material. The microballoons may comprise
the product sold under the trade name of Expancel.
It will be appreciated that the microspheres are suitable to expand when
contacted with steam.
Prior to expansion the microspheres are preferably provided in a suitable
carrier
fluid, preferably a liquid to provide a slurry, which slurry is then
introduced into
the steam for expansion. The carrier liquid preferably comprises water. The
carrier liquid may also contain oil such as mineral oil or fuel oil. The oil
preferably reduces the viscosity of the final product and thereby enables it
to
remain fluid with volume fractions of microballoons high enough to otherwise
impart a paste-like consistency to the sensitised explosive. The carrier
liquid
may comprise a mixture of oil and water. The carrier liquid may also contain
additives which may affect the properties of the explosive; for example, the
carrier liquid may contain a cross-linker for a watergel explosive. The volume
to
volume ratio of unexpanded microspheres to carrier liquid may be any practical
ratio, but preferably it is from 1:1 to 1:10, preferably it is from 1:1 to
1:2. The
mass % of dry microspheres in the carrier liquid may be from 10% to 50%, but
preferably ifi is from about 40% to about 45%.
A motive fluid may also be used to propel the microspheres (including a slurry
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thereof) thereby to introduce the microspheres as a stream into the steam. The
motive fluid may comprise a gas, preferably air. In one embodiment, a slurry
of
microspheres is both atomised and injected into the steam using compressed
air.
Preferably the steam is a stream of steam and preferably it is at a
temperature
of above 105°C, preferably between 110 and 130°C.
Where the microspheres are provided as a slurry the mass ratio of steam to
slurry may be any suitable ratio. In one embodiment it may be from 1:5 to 5:1,
preferably it is about 1:2.
It will be appreciated that the stream of steam may include one or more other
fluids therein, preferably a gas such as air or the like. The one or more
other
fluids may be introduced into the stream of steam prior to introducing the
microspheres into the stream of steam.
The STP volume ratio of airto dry steam may be form 0 to 5:1, preferably
aboufi,
1:1.
The microspheres and stream of steam may be mixed in any suitable manner
but preferably they are mixed by introducing both the microspheres and the
steam into a tubular member. An eductor may be used to introduce the
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microspheres and steam into the tubular member or other mixing device,
Alternatively, the microspheres (preferably in the form of a slurry and mixed
with
a motive fluid) may be sprayed into the stream of steam and preferably the
resulting mixture is introduced into a tubular member where further mixing of
the
microspheres and steam take place.
The stream of expanding microspheres and water is preferably-conveyed at
very high velocity down the tubular member. The high velocity usually ensures
high turbulence in the tubular member, thus ensuring good heat transfer from
the steam to the microspheres, which may result in almost instantaneous
expansion of the microspheres.
The rate of steam supply and the physical arrangement of the tubular member
into which the microspheres and the steam and optionally the air are injected
determines the pressure in the resulting stream of wet expanded microspheres.
This mixture preferably contains safiurated steam and its temperature is
therefore determined by the pressure. The temperature profile to which the
expanding microspheres are subjected can be controlled by controlling the
pressure profile.
The residence time of the microspheres in the steam and the maximum
temperature of the stream of wet expanded microspheres is in use controlled to
achieve the desired extent of expansion.
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The explosive may comprise an explosive which normally includes water
therein.
In one embodiment of the invention the explosive may comprise a fluid
explosive (preferably a liquid based explosive) such as a gel explosive or an
emulsion explosive. The explosive may comprise a watergel or a water-in-oil
emulsion.
Alternatively the explosive may comprise a slurry explosive, preferably a
pumpable composition. The slurry explosive may comprise a mixture of a fluid
explosive and a granular material. The granular material may comprise an
ammonium nitrate product, preferably ammonium nitrate fuel oil (ANFO) but it
may also comprise other products such as granular nitrate, a granular
propellant
or even a granular or particulate explosive such as TNT.
The explosive is preferably provided at a temperature which quenches the
expansion of the expanded microspheres when received therein.
Accordingly the explosive is preferably provided at a temperature belov~ the
temperature at which expansion of the microspheres will take place (the
expansion temperature). Preferably the explosive is at a temperature below
80°C. In the case of a watergel the temperature of the explosive may be
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at ambient temperature. In the case of an emulsion the temperature of the
explosive may be from ambient temperature to about 60 to 75°C. Last
mentioned temperature range is the normal manufacturing temperature of an
emulsion. It will be appreciated that the explosive may preferably also serve
5 to prevent or reduce agglomeration of the expanded microspheres.
The mass % of expanded microspheres in the sensitised explosive may be
from 0.03 to 0.4%, but is preferably from 0.06 to 0.1 %. The volume of the
stream of wet expanded microspheres and optionally air may comprise
10 about 5% to about 100% of the volume of the explosive, but will preferably
be about 10 to about 30% by volume of the explosive.
The stream of wet expanded microspheres is preferably introduced directly into
the explosive without intermediate processing, especially infiermediate
processing such as drying or filtering to separate fluid, especially water,
from
the expanded microspheres.
The explosive and expanded microspheres are preferably thoroughly mixed and
any mixing means suitable for safe use with explosives may be considered.
Known mixing means include single-screw augers, twin-screw mixers, ribbon
blenders, bowl mixers and the like. The inventor has found that convenient and
effective mixing between the explosive and stream of expanded microspheres
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can be achieved by introducing the stream of expanded microspheres and a
stream of the explosive into a tubular member. Barriers may be provided in the
tubular member to improve the mixing process. Preferably however, the tubular
member is non-linearto cause a change of direction of the streams as they flow
through fihe tubular member. The tubular member may comprise a curved
member, such a hose or pipe. Preferably the tubular member comprises a
straight pipe with one 90 degree bend therein, preferably a short radius
elbow.
Preferably the bend is located towards the exit end of the tubular member.
Preferably both the stream of wet expanded microspheres and the explosive
are introduced in substantially the same direction into the tubular member.
Preferably the microspheres and the stream of steam are introduced co-linearly
or co-axially into the tubular member. In one embodiment of the invention thus
is achieved by means of a co-axial eductor.
The process preferably comprises a continuous process.
The process may be used in a fixed explosives manufacturing plant to sensitise
an explosive in the manufacturing process. Such explosive may be one suitable
for use in bulk or packaged form. The process may also be used at a site
remote from manufacture of the explosive, for example at a storage site, to
sensitise an as yet non-sensitised explosive. For example a non-sensitised
explosive may be sensitised as it is pumped from a delivery vehicle into a
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storage tank. Alternatively, the process may be used on an explosives blending
and delivery vehicle in order to sensitise the explosive before it is pumped
to a
delivery point, e.g. a borehole. This method may be used in addition to but
preferably in place of the more common current method of chemical gassing for
sensitisation.
The invention also relates to a sensitised explosive prepared according to the
method substantially as set out above.
The invention will now be further described by means of the following non-
limiting examples and drawings wherein:
Figure 1 is a diagrammatic drawing of an.apparatus suitable for carrying out
a method of sensitising an explosive according to the present
invention;
Figure 2 is a diagrammatic drawing of another apparatus for carrying out a
method of sensitising an explosive according to the present
invention; and
Figure 3 is a diagrammatic drawing of yet another apparatus for carrying out
a method of sensitising an explosive according to the present
invention.
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Referring now to Figure 1, an apparatus 10 suitable for preparing a sensitised
explosive according to the invention comprises an eductor 11 defining an inlet
12 an outlet 13 and a suction port 14. A tubular member in the form of an
expansion pipe 15 is connected to the outlet 13 of the eductor 11 and to a
suction port, 16 of a second eductor 17. The pipe 15 has a length of
approximately 1 meter and a diameter of 75mm. The eductor 17 also defines
an inlefi 18 and an outlet 19. A hose 20 is connected to the outlet 19. The
hose
20 is curved to have a general S-shape, and the inlet 21 of the hose 20 is not
in
line with the outlet 23 of the hose 20. The hose is 1.6m long with a 100mm
diameter.
Referring now to Figure 2, an alternative apparatus100 suitable for carrying
out
a method of sensitising an eXplosive according to the invention comprises a
container 101 for an aqueous slurry of unexpended microballoons and a pump
102 for pumping the slurry of unexpended microballoons into an orifice sprayer
103 through an inlet 104. Air is introduced into the orifice sprayer 103
through
the inlet 105 and this air serves as a motive fluid to propel the microspheres
into
a mixing tee 106. Air and a stream of steam are introduced into the mixing tee
through the inlet,107. The mixing tee has a diameter of 25mm. The resulting
mixture of steam, air and microballoons then flow from,the mixing tee 106 into
a
tubular member in the form of an expansion pipe 108 with a length of 1,5m and
a diameter as shown in Table 2. The microballoons expand in the expansion
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pipe 108 in the presence of the steam to form a sfiream of wet expanded
microballoons. The expansion pipe 108 also serves as a suction port of an in-
line and co-axial Penberthy eductor 109. In use an explosive is introduced
info
the eductor 109 through the inlet 110 which explosive serves as the motive
fluid
for the eductor 109. The eductor 109 is coupled to a tubular member 111 in the
form of a pipe and the stream of expanded wet microbalioons from the
expansion pipe 108 and explosive introduced through the inlet 110 are
introduced co-axially to flow in the same direction into the tubular member
111
wherein they are mixed thoroughly. The tubular member 111 comprises a pipe
(length of 1,5m and diameter of 76mm with a 90°C short radius elbow
towards
the exit end 113 of the member 111. A hose 114 is coupled to the tubular
member 111 for delivering the mixture of sensitised explosive to a suitable
point
e.g. a truck, storage flank or borehole etc.
Referring now to Figure 3, an alternative apparatus 200 suitable for carrying
out
a method of sensitising an explosive according to the invention is shown. The
apparatus 200 is very similar to the apparatus 100 of Figure 2 and the same
reference numbers are used to denote corresponding parts. However, in this
case the orifice sprayer 104 (of Figure 2) is replaced with an eductor 201.
The
eductor 201 defines an inlet 202 through which air (as the motive fluid is
introduced). The slurry of unexpanded microballoons is sucked into the eductor
201 through the suction port 203. The eductor 201 is coupled to the mixing tee
106. In this case only steam (not steam and air) enters the mixing tee 106
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through the inlet 107.
Example 1
5 Utilising the apparatus of Figure 1
A slurry of unexpended, thermally expandable polymeric microballoons was
prepared by mixing (on a mass basis) 10 parts unexpended microballoons of
the type sold under the trade name of Expancel 551 WU with 5.7 parts of a
10 carrier liquid in the form of water. This mixture comprised 45% dry weight
Expancel 551 WU and 55% by weight water. The volume ratio of Expancel
551 WU: water was 1:1.33. The Expancel 551 WU was in the form of a wet
cake.containing about 29% water.
15 The slurry of microballoons was pumped at a rate of 3.2kg/minute into the
suction port 14 of the eductor 11 of the apparatus 10 of Figure 1. The motive
fluid of the eductor 11 was a stream of steam at a temperature of about
115°C
and a nominal pressure of 0.8bar gauge pressure flowing at l.Okglminute
through the inlet 12 of the eductor 11. The steam was nominally dry steam with
little if any liquid water. An unknown amount of air, estimated fio be
approximately 0.45kg/minute, was sucked into the eductor 11 through the
suction port 14.
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The slurry of unexpended microballoons was thus introduced infio a stream of
steam causing thermal expansion of the microballoons thereby providing a
resulting stream of water wet expanded microspheres flowing through the
expansion pipe 15. The stream of water wet expanded microspheres
comprised expanded microspheres, steam, liquid water in droplet form and air.
The stream of wet expanded microspheres was sucked into the eductor 17
through the suction port 16. A non-sensitised emulsion explosive in the form
of
a water-in-oil,emulsion explosive entering the eductor 17 through the inlet 18
at
a rate of 1000kg/minute provided the motive fluid for the eductor 17. The
resulting sensitisation by microspheres turned the non-sensitised emulsion
explosive into a sensitised emulsion explosive.
The non-sensitised emulsion explosive comprised 8.5% (mass by mass)
continuous fuel phase of a suitable mixture of mineral oil (obtained under the
name of MODEF from Continental Nitrogen & Resources) and an emulsifier of
the PIBSAtype (obtained under the name NBX2000Afrom Nelson Bros.) The
ratio of these in the fuel phase was 7:1 mass oil: mass PIBSA-type emulsifier,
The discontinuous aqueous phase comprised an 82% (mass by mass)
ammonium nitrate solution. The emulsion explosive had a viscosity of about 20
000 cp at 10rpm at a temperature of 75°C. The temperature of the
emulsion
explosive was about 75°C when introduced into the eductor 17.
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In this arrangement where the explosive is used as the motive fluid in the
eductor 17, the pressure at the exit of the expansion pipe (15) is less than
atmospheric pressure. Because the stream issuing from the pipe (15) contains
a mixture of steam and water in equilibrium, the drop in pressure to below
atmospheric pressure is believed to cause evaporative cooling of the mixture
to
below the boiling point of water at atmospheric pressure i.e. to below
100°C.
This provides an important safety feature for the process, although it is not
essential. The relatively huge mass of explosive compared to the mass of the
hot stream of steam and microballoons will cause substantially immediate
cooling. It can easily be shown, as one skilled in the art will realise, that
the
temperature rise of the final product of this process can easily be limited to
a
few degrees. If is thus relatively easy to control temperatures at all points
in the
explosive to safe limits.
In this manner the stream of wet expanded microspheres was introduced into
the emulsion explosive and they mixed thoroughly in the hose 20. The resultant
mixture flowing from the outlet 23 may be received in any suitable vessel or
rnay
be introduced directly into a borehole.
The density of the emulsion explosive prior to introduction of the expanded
microspheres was 1,3 glee and after the introduction of the expanded
microspheres the emulsion explosive had a density of 1,24 glee.
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Examples 2 to 10
Utilising the apparatus of Figures 2 and 3 '
The apparatus of Figures 2 and 3 respectively were used to conduct further
experiments.
The slurry of unexpanded thermally expandable polymeric microballoons
contained 40% (by mass) microballoons and 60% (by mass) water with a
volume ratio of about 1:1.5. Microballoons of the type sold under the trade
name Expancel 007WU was used. [This wet unexpanded material is a kind of
filter cake. It contained about 30% (by weight) water, varying from batch to
batch and printed on the container; that figure is used to calculate what wet
weight ratios to use to get 40% dry weight].
The emulsion explosive was the same as used in example 1.
The common test parameters used in examples 2 to 10 are provided in table 1.
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Table 1: Common Tests Parameters
Initial emulsion density1.30g/cc
Emulsion feed flow 1000 kg/min
rate
Steam feed flow rate 1 kglmin
(estimated)
Steam pressure at boiler500,000
N/mz
Emulsion temperature 67 - 77
deg C
Slurry pump output ~ 0.0037
When the apparatus of Figure 2 was utilised the rate of air supply through
the orifice sprayer 103 was estimated to be 0.7kg/min and the rate of air
supply to the inlet 107 of the mixing tee 106 was about the same. When
the apparatus of Figure 3 was used the rate of air supply to the inlet port
202 of the educator 201 was estimated to be about 0.7kg/min.
The results obfiained are provided in Table 2
Table 2
Microballoon ExpansionMeasured
ExperimentApparatusSlurry addition Microbailoonpipe density
(108) of
(referringpump rate, slurry diameter,sensitised
to dry wt
relevant (102) as % pressure,(mm) explosive,
rpm
figure) emulsion psi g/cc
2 2 . 660 0.098% ? 31.75 1.249
3 2 660 0.098% ? 31.75 1.256
4 2 1000 0.149% ? 31.75 1.235
5 3 1000 0.149% 13 31.75 1.229
6 3 1000 0.149% 14 31.75 1.224
7 3 1000 0.149% 35 19.05 1.166
8 3 700 0.104% 30 19.05 1.182
9 3 500 0.075% 28 19.05 1.217
10 3 1400 0.209% 42 19.05 1.120
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The quality of dispersion of the microballoons in the sensitised emulsion
explosive of examples 2 to 10 was extremely uniform in all cases. No broken
5 balloons were visible under the microscope, nor were any agglomerates
visiblep
the microballoons were spherical and separate.
It will be appreciated that by introducing the expanded microspheres directly
into the explosive the explosive quenches the expansion process and also
10 prevents agglomeration of the expanded microspheres. it is accordingly not
required to use expensive cooling equipment. It is also not necessary to
introduce cooling liquids which would dilute the expanded microspheres. The
introduction of such cooling liquids could reduce the dry content of the
expanded microspheres below levels acceptable for use in explosives. Drying
15 equipment would then be required to reduce the cooling liquid to an
acceptable
level.
According to the present invention the wet or aqueous expanded microspheres
can be introduced directly info the explosive without the necessity of
20 intermediate processing such as filtering or drying, intermediate storage,
metering of expanded microspheres, etc.
It will also be appreciated that the water level in the sensitised explosive
ca n be
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controlled by controlling the amount of water in the expanded microspheres and
the amount of water provided in the emulsion explosive prior to introduction
of
the expanded microspheres, Since the expanded microspheres are introduced
directly into the explosive, less water is required to prevent excessive
agglomeration of the expanded microspheres and thus rendering it easier to
control and limit the amount of water infiroduced into the explosive.
The process also has the advantage that low density expanded microspheres
do not have to be transported, stored or handled. Accordingly the difficulties
associated therewith are avoided.
It will be appreciated that many variations in detail are possible without
thereby
departing from the scope and spirit of the invention.
20
