Fumigation of multiple grain storages from a single source of fumigant

Fumigation of grain in a storage facility having a plurality of grain silos or bins is effected from a single source of a gaseous fumigant. The single source of gaseous fumigant is connected to a gas supply duct through which a carrier gas flows. The gas inlet ports of the silos are also connected to the gas supply duct. The connection between each gas inlet port and the duct is provided with a respective valve and a respective orifice plate, the orifice or aperture of which is sized so that the pressure drop across the orifice plate into the silo is substantially greater than the pressure drop across any other component between the gas source and the silo. With this arrangement, the flow rate of gaseous fumigant through the grain-containing silos in the facility suffers only a minor perturbation when a valve between the duct and a silo is opened or closed. This minor perturbation can be readily compensated by maintaining the gas pressure in the duct at a constant value. The exit gases of the silos in the facility may be recycled through the silos, with the addition of gaseous fumigant periodically or as necessary to maintain a predetermined fumigant concentration.

TECHNICAL FIELD 
This invention concerns the fumigation of stored, particulate commodities, 
such as grain and pulses. More particularly, it concerns a method and 
arrangement for the simultaneous fumigation, from a single source of a 
gaseous fumigant, of a number of silos in a commodity storage facility 
having a plurality of silos. 
BACKGROUND 
In this specification, for convenience, the term "grain" will be used in 
the sense that it encompasses not only grain but other particulate 
foodstuffs or commodities that are commonly stored in bulk, such as 
peanuts, lentils, peas and other pulses. This list is not intended to be 
exhaustive. Also, in this specification, the term "grain pests" will 
encompass the pests usually found in stored "grain" and well known to 
persons who work in grain storages, being predominantly beetles and some 
species of moths. 
For many years, various chemical pesticides have been applied to stored 
grain to kill the grain pests that may be present. Many of such chemical 
pesticides leave residues which can be harmful, and care has to be 
exercised to ensure that the relevant maximum residue limit is not 
exceeded. In addition, as noted in the specification of U.S. Pat. No. 
3,614,841 to G W Query, there have been "industry . . . accepted practices 
which are directed to periodic overdoses of insecticide and result in 
wastage of food products in hopes that such periodic overkill would 
provide . . . residual protection . . . before the infestation of the food 
product became sufficiently advanced to require another drastic 
treatment". 
To avoid this last-mentioned problem, Query treats growing crops in 
greenhouses and the exterior surfaces of multiple storage bins located 
within essentially sealed enclosures by periodically administering a 
predetermined quantity of insecticide through spray nozzles in overhead 
conduits which are located within the greenhouses or enclosures. To ensure 
that the appropriate quantity of insecticide is used, Query has suggested 
two techniques. One of those techniques is the discharge into an enclosure 
of the entire contents of a single canister containing a liquid 
insecticide and an aerosol propellant, by puncturing the canister with a 
"puncturing opener" which is connected to a flexible hose that leads to 
the conduit which contains the overhead spray nozzles. Query's second 
technique requires periodic injection, into the overhead conduit of each 
greenhouse or bin enclosure, of a pre-measured quantity of a liquid 
insecticide, which has been stored with a propellant gas in a respective 
container mounted outside each greenhouse or bin enclosure. After each 
administration of insecticide, the respective external container is 
re-charged with a mixture of the liquid insecticide and propellant gas 
from a source of such a mixture. 
The aforementioned problem with residues has led to the preference for 
using gaseous fumigants instead of the pesticidal chemicals. And among the 
gaseous fumigants, phosphine has been preferred because any residue that 
might be left in the grain will be lost or oxidized to harmless phosphate 
when the grain is processed to produce a food. 
The main problem with applying a gaseous fumigant such as phosphine to a 
grain storage (such as a silo) concerns dosage rates and maintaining an 
environment within the grain which ensures proper elimination of the 
pests. As noted by the present inventor in his paper entitled 
"Flow-through phosphine fumigation--a new technique" which was delivered 
to the Stored Grain Protection Conference, 1983, if the so-called 
"one-shot" or "one-pass" technique is used with phosphine from a gas 
source (for example, from the reaction of moisture with a solid 
formulation containing aluminium phosphide) being applied to a leaking 
silo, the concentration of phosphine within the grain in the silo is 
likely to decay to zero in about 4 or 5 days. Thus the fumigant is 
ineffective after about 5 days. Even when a fumigant is applied to grain 
in a completely sealed silo (for example using the techniques described in 
the specification of U.S. Pat. No. 4,200,657 to J S Cook), there is a 
decay of the effective concentration of phosphine in the grain as it is 
absorbed by the grain and used to exterminate grain pests. Thus, whether 
the fumigant is applied to the grain utilising Cook's one-pass technique 
or his recirculating technique (each technique, Cook claims, results in a 
uniform distribution of phosphine or other fumigant within the grain mass 
before the forced gas flow through the grain mass is discontinued), there 
will be a significant fall-off in effectiveness of the technique after a 
relatively short time period. A similar significant reduction of 
effectiveness is experienced with the one-shot fumigation technique 
described in the specification of South African patent No 86/4806, which 
corresponds to Australian patent No 589,646, in the name of The 
Commonwealth Industrial Gases Limited. That technique simply requires the 
release of a phosphine-containing gas into the grain mass stored within a 
silo. 
For complete elimination of grain pests, it is essential that a 
sufficiently high dosage of phosphine remains in the grain mass long 
enough to ensure that the more tolerant stages (eggs and pupae) in the 
development of an insect pest mature into a less tolerant stage (larvae or 
adults) and are killed by the phosphine. In this way, resistant strains of 
the pests cannot develop. 
In a substantially gas-tight silo, the phosphine concentration decays to 
zero in about 16 days. The work required to make old grain storages 
gas-tight is costly and is not always successful. An improved fumigation 
technique, which ensures that an adequate phosphine dosage is always 
achieved within the grain, is the subject of Australian patent No 640,669. 
In many grain storage establishments, a number of silos of different shapes 
and sizes are built in close proximity to each other and grain is 
deposited into an empty silo or removed from a silo containing grain, as 
required. In such systems, as in any multi-silo storage facility, it would 
be advantageous to provide a single fumigation arrangement, utilizing a 
single source of phosphine (or other suitable gaseous fumigant) and 
carrier gas, which ensures that any number of the silos in the facility 
can be fumigated simultaneously, using fumigant from the single source of 
gaseous fumigant, without the need to re-design the fumigation system each 
time a silo is brought into use for grain storage, or is removed from the 
system because it is about to be emptied of grain. 
DISCLOSURE OF THE PRESENT INVENTION 
It is an objective of the present invention to provide an arrangement which 
can be used to implement the simultaneous fumigation of any number of 
silos in a grain storage facility having a plurality of silos--possibly of 
different sizes and types--with variation of the number of silos in use at 
any time, utilizing a single source of a gaseous fumigant. 
To achieve this objective, a single source of a gaseous fumigant is 
connected to a single gas supply duct or inlet manifold, through which a 
carrier gas (typically air) flows. The gas supply duct is connected to the 
gas input port of each silo in the grain storage facility. Between the gas 
supply duct and each silo gas input port, there is a respective on/off 
valve (which is opened when grain in the silo is to be fumigated and is 
closed when the silo is empty or is about to be emptied) and a respective 
orifice plate. Each gas input port should be connected to a respective gas 
distribution arrangement within its associated silo to ensure that the 
input gas moves throughout the grain mass in the silo when fumigation is 
occurring. The orifice or aperture of each orifice plate is scaled or 
sized to ensure that the maximum pressure drop in the entire fumigation 
system is a substantial pressure drop across each orifice plate. 
This arrangement effectively renders the fumigation system as a whole 
insensitive to variations in the system downstream of an orifice plate. 
Thus if, after commissioning an installation and establishing steady state 
conditions, different commodities are stored in the bins, or a bin has 
less or more of a commodity in it (that is, there is a change in the 
bin-fill ratio), the orifice plates do not have to be adjusted or modified 
to allow for the different back pressures downstream of the orifice 
plates. In addition, if a silo containing grain is to be emptied of grain 
and the valve in the connection between that silo and the gas supply duct 
is closed, there will be only a minor perturbation of the gas supply to 
the input ports of the remaining silos in the system, and the minor 
correction in gas flow that is required to compensate for the removal of a 
silo from, or the addition of a silo to the system can be effected by 
bringing the pressure within the gas supply duct back to its steady state 
value by adjustment of a suitable control mechanism for the supply of gas 
to the gas supply duct. 
Thus, according to the present invention, there is provided a method of 
simultaneously fumigating particulate commodities stored in a plurality of 
silos of a storage facility, using a gaseous fumigant supplied by a single 
source and a carrier gas, each silo having a gas inlet port at or near the 
base thereof, said method comprising 
(a) establishing a flow of a mixture of said gaseous fumigant and carrier 
gas through a single gas supply duct; 
(b) connecting the inlet port of each silo directly to said single gas 
supply duct; 
(c) providing each inlet port of each silo with an orifice plate having an 
aperture, the size of the aperture being such that the pressure drop 
between said gas supply duct and the associated silo is substantially 
greater than the pressure drop across any other component between said 
single source of gaseous fumigant and the silo with which the orifice 
plate is associated; and 
(d) maintaining a substantially uniform gas pressure within said gas supply 
duct. 
Also according to the present invention, there is provided an arrangement 
for effecting the simultaneous fumigation of any number of silos in a 
grain storage facility having a plurality of silos or bins, said 
arrangement comprising: 
(a) a single gas supply duct through which a carrier gas flows; 
(b) a single source of a gaseous fumigant connected to said gas supply 
duct; 
(c) control means for varying the supply of said gaseous fumigant from said 
source to said gas supply duct; and 
(d) a respective direct connection between said gas supply duct and a gas 
inlet port at or near the base of each silo in the facility, each said 
connection including a valve and an orifice plate, each orifice plate 
having an aperture which is sized to provide a pressure drop across the 
orifice plate which is substantially greater than the pressure drop across 
any other component between said single source of gaseous fumigant and the 
silo with which the orifice plate is associated. 
The carrier gas is conveniently air, with the flow through the gas supply 
duct being established by a blower or fan. The control means of feature 
(c) of the apparatus of the present invention may comprise a control valve 
between the source of gaseous fumigant and the gas supply duct, or it may 
comprise any suitable arrangement for controlling the rate of supply of 
carrier gas to the gas supply duct (for example, means to vary the speed 
of the fan mentioned above. 
Once such a system has been set up to establish the required flow rate of 
the mixture of gaseous fumigant and carrier gas through the silos or bins, 
the flow rate through the grain in each silo or bin containing grain can 
be maintained at the required level when an on/off valve in a connection 
between the gas supply duct and a silo is opened or closed, simply by 
varying the operating conditions of the system to maintain a substantially 
constant pressure of the gas within the gas supply duct. 
The fumigation arrangement of the present invention was designed for use 
with a plurality of vertical silos (that is, silos having a height to 
width ratio of at least 1.5 to 1). However, the system is equally 
applicable to grain storage facilities comprising silos or bins of 
different types and capacities. 
Preferably, the fumigation method practised with the present invention will 
be that described in the specification of Australian patent No 640,669, 
the contents of which are incorporated into this specification by this 
reference to Australian patent No 640,669. 
It will be appreciated that if the silos or bins of the multi-silo storage 
facility are vented to the atmosphere, gaseous fumigant will be constantly 
lost from the fumigation system. However, the present invention can be 
adapted to form a recirculating arrangement in which the loss of gaseous 
fumigant is minimised. In such an implementation of the present invention, 
the outlet at the top of each silo or bin is connected to a second duct 
(an outlet manifold) which is joined, via a gas conduit which contains a 
recirculating fan (also called a "blower"), to the gas supply duct (the 
inlet manifold). The recirculation circuit thus established is provided 
with a valve (which has been termed a system valve) which is adjusted 
whenever a bin is added to or removed from the recirculation circuit, to 
ensure that the static pressure in the gas supply duct or inlet manifold 
is maintained at a value which provides the required flow of fumigating 
gas through each operational silo or bin in the storage facility. That is, 
the setting of this system valve is adjusted to compensate for the 
perturbation of the static pressure in the gas supply duct when there is a 
change in the number of bins being fumigated, which occurs (a) when the 
connection to the inlet port of a silo or bin is "closed" (for example, 
when a bin is about to be emptied), and (b) when a previously closed 
connection to the inlet port of a bin in the storage facility is "opened" 
to the fumigating gas. 
Thus, according to this modified form of the present invention, there is 
provided a method of simultaneously fumigating particulate commodities 
stored in a plurality of silos or bins of a storage facility using a 
single source of gaseous fumigant, each silo or bin having a gas inlet 
port and a gas outlet port, said method comprising: 
(a) connecting the inlet port of each silo or bin to a single gas supply 
duct; 
(b) connecting the outlet port of each silo or bin to a single gas outlet 
duct; 
(c) forming said gas supply duct, said gas outlet duct, a gas movement 
means (for example, a blower or fan), and said silos or bins into a gas 
recirculation circuit, said silos or bins being included in parallel in 
said recirculation circuit; 
(d) providing a system control valve in said recirculation circuit 
downstream of said gas movement means, said system control valve being 
adjustable to maintain the static pressure within said gas supply duct at 
a predetermined value or within a predetermined range of values; 
(e) providing each inlet port of each silo or bin with an orifice plate 
having an aperture which is sized so that there is a substantial pressure 
drop between said gas supply duct and the interior of the associated silo 
or bin; and 
(f) connecting a source of gaseous fumigant to said recirculation circuit 
for the controlled supply of gaseous fumigant thereto. 
Also according to this modified form of the present invention, there is 
provided a fumigation arrangement for the simultaneous fumigation of a 
number of silos or bins in a storage facility comprising a plurality of 
silos or bins, each silo or bin having a gas inlet port and a gas outlet 
port, said fumigation arrangement comprising: 
(a) a single gas supply duct to which the gas inlet port of each silo or 
bin is connected; 
(b) a single gas outlet duct to which the gas outlet port of each silo or 
bin is connected; 
(c) a gas conduit between said gas outlet duct and said gas supply duct, 
connected to form a gas recirculation circuit comprising said gas supply 
duct, said gas outlet duct, said gas conduit and said silos or bins; said 
silos or bins being included in parallel in said recirculation circuit; 
(d) gas movement means (for example, a blower or fan) included in said gas 
recirculation circuit; 
(e) a system control valve in said recirculation circuit downstream of said 
gas movement means, said system control valve being adjustable to maintain 
the static pressure within said gas supply duct at a predetermined value 
or within a predetermined range of values; 
(f) a respective orifice plate included in each of said gas inlet ports, 
each orifice plate having an aperture which is sized so that there is a 
substantial pressure drop between said gas supply duct and the interior of 
the associated silo or bin; and 
(g) a single source of gaseous fumigant connected to said gas recirculation 
circuit, for the controlled supply of gaseous fumigant thereto. 
Any suitable source of fumigant gas may be used with this recirculating 
fumigant method and arrangement (including a packaged source of phosphine 
of the type described in the specification of International patent 
application No PCT/AU93/00270, which is WIPO Publication No WO 93/25075). 
If the fumigant source is a cylinder of fumigant gas, the fumigation 
arrangement will preferably (unless a "one-shot" fumigation technique is 
contemplated) include fumigant gas injection apparatus which ensures that 
the concentration of fumigant in the recirculating gas is increased when 
it reaches or falls below a predetermined minimum value. The fumigant gas 
injection apparatus may comprise a fumigant gas sensor in the gas supply 
duct, the output signal of this sensor being monitored by a microprocessor 
which is programmed to cause additional fumigant gas to be supplied to the 
recirculation circuit whenever the concentration of fumigant gas falls 
below a pre-determined level. Alternatively, this apparatus may comprise a 
mechanism which provides a periodic injection into the recirculating gas 
of a quantity of fumigant, the frequency of this injection and the amount 
of fumigant added being based on the observed leakage rates of the bins. 
These and other features of the present invention and its modified form 
will be exemplified in the following description of realisations of the 
present invention. In the following description, which is provided by way 
of example only, reference will be made to the accompanying drawings.

FURTHER DISCUSSION OF THE PRESENT INVENTION 
The diagram of FIG. 1 has appeared previously (i) in the paper by Dr R G 
Winks entitled "The effect of phosphine on resistant insects", which is 
included in the Proceedings of the CASGA Seminar on Fumigation Technology 
in Developing Countries, 1986, pages 105 to 118, and (ii) in the article 
entitled "A new way of fumigating with phosphine", which appeared in Rural 
Research, No 140, Spring 1988, pages 15 to 18. This diagram shows how 
insect eggs and the pupal stage of the development of a beetle or other 
grain pest is significantly more resistive to fumigation than the larval 
and adult stages. Thus if a grain storage is fumigated at a constant 
concentration of phosphine, the concentration must be sufficiently high to 
destroy all eggs and pupae, or it must be maintained at a value which is 
adequate to kill all adult pests and their larvae for a time sufficient 
for the more tolerant eggs to become the less tolerant larvae and for the 
pupae to become adults. 
By monitoring the carbon dioxide production (from insect respiration) in a 
sample of grain which was infested with Sitophilus granarius, to which 
phosphine was supplied at a constant concentration, the results displayed 
in graph form in FIG. 2 were obtained. These results show that even with 
phosphine concentrations as low as 4 micrograms per liter at a temperature 
of 25.degree. C., all stages of the pest had been destroyed after a period 
of about 6.5 weeks. These results also show that the time for all insect 
stages to be destroyed decreases as the phosphine concentration is 
increased. Similar results have been obtained using other concentrations 
of phosphine and with infestations of other species of grain pests 
(including Tribolium castaneum, Rhyzopertha dominica, Sitophilus oryzae, 
Sitophilus zeamais, Bruchus pisorum, Oryzaephilus surinamensis and 
Ephestia cautella). From the experimental data obtained, it has been found 
that all insect pests are killed in a reasonable time with a constant 
phosphine concentration of at least 4 micrograms per liter. With phosphine 
concentrations of 2 micrograms per liter, the insect population of the 
grain increased with time when the pest was Sitophilus granarius and 
Sitophilus oryzae. Thus concentrations of phosphine lower than 4 
micrograms per liter do not effectively fumigate a grain mass. 
Although it may appear, at first sight, that increasing the concentration 
of phosphine in the carrier gas also increases the efficacy of the 
fumigation technique, this conclusion is incorrect. There are also 
economic factors to consider. A detailed assessment of economic factors 
has shown that in a "flow through" fumigation arrangement (that is, an 
arrangement in which the mixture of gaseous fumigant and carrier gas is 
not recycled), at concentrations in excess of 200 micrograms of phosphine 
per liter, fumigant is being wasted. Thus the phosphine concentration in 
the carrier gas should be kept within the range of from 4 to 200 
micrograms per liter. For an effective yet economical fumigation 
technique, the concentration of phosphine should be held at a constant 
value selected from within the concentration range of from 4 to 100 
micrograms per liter, preferably in the range from 4 to 50 micrograms per 
liter, with due consideration being given to the times to insect 
population extinction for the concentration chosen. In an arrangement in 
which the mixture of fumigant and carrier gas is recycled, however, 
cocentrations of fumigant in excess of 200 micrograms per liter may not be 
uneconomical, particularly in a well-sealed system, because the gaseous 
fumigant is not all lost to the atmosphere. 
Referring now to FIG. 3, the schematically illustrated embodiment of the 
present invention comprises a source 17 of a fumigant-containing gas which 
is supplied under a pressure of about 500 to 700 Pa (that is, a pressure 
of about 2 inches water gauge). This pressure is usually established by a 
fan (not shown in FIG. 3) which provides a flow of the carrier gas, into 
which the fumigant gas (preferably phosphine) is introduced by 
conventional techniques. The source 17 of gas is connected to a gas supply 
duct or inlet manifold 13 by a control valve 8. The gas supply duct 13 is 
connected to a gas entry port 11 of each silo 10 in the system by a 
respective connection comprising an on/off valve 19 and an orifice plate 
20. Preferably a distributor 9 (for example, a louvre arrangement) is 
provided in each silo to ensure that the carrier gas and its fumigant are 
distributed within the grain mass in such a manner that a uniform flow of 
gas is established within each grain mass. 
The orifices or apertures in the orifice plates 20 are set so that the 
maximum pressure drop in the system (when fumigation of the grain in the 
silos is in progress) is across the orifice plates. As explained earlier 
in this specification, a steady state fumigation system is set up with the 
required gas flow through the masses of grain being fumigated. This steady 
state operating condition will be perturbed to only a small degree if an 
additional silo or bin is brought into the system, or if one of the valves 
19 is closed because fumigation of the contents of a silo is no longer 
required (for example, when the silo is to be emptied of grain). Such a 
minor perturbation can be readily corrected by adjusting the setting of 
the control valve 8 so that the pressure of gas in the gas supply duct or 
inlet manifold 13 is returned to its steady state value, whereupon the 
system will again be operating in its required manner. In addition, and 
perhaps more importantly, when the present invention is used, only very 
minor changes in pressure in the gas supply duct are experienced if a bin 
should be only partly filled, or if some of the bins of the storage 
facility contain different commodities. 
If phosphine is the gaseous fumigant, the phosphine may be obtained from a 
cylinder of a pressurised mixture of phosphine and carbon dioxide. 
However, as noted earlier in this specification, an on-site generator or a 
packaged phosphide formulation may be the preferred source of phosphine in 
areas where a regular supply of gas cylinders cannot be guaranteed. 
Variations to the arrangement shown in FIG. 3 are possible. As already 
noted, the control valve 8 may be replaced with an alternative arrangement 
for controlling the supply of fumigant-containing gas to the duct 13. For 
example, the control of the gas supply could be by variation of the speed 
of a fan that is used to provide carrier gas to the gas mixture supply 17, 
or by the use of multiple fans which are brought into the system, with 
their speeds varied, as required. 
In FIGS. 4 and 5, the same reference numerals have been used to identify 
components which are common to these illustrated fumigation arrangements 
and the arrangement of FIG. 3. 
FIGS. 4 and 5 each illustrate an arrangement of three silos or bins 10, 
each having an inlet port 11 and an outlet port 12, which are included, in 
parallel, in a gas recirculation circuit which also includes a gas supply 
duct or manifold 13, a gas outlet duct or manifold 14, a gas connecting 
duct or conduit 15 and a blower or fan 16. A supply of a fumigant gas 
(which is shown in the drawings as a cylinder 17 containing a pressurised 
mixture of carbon dioxide and phosphine, but which in practice may be any 
suitable source of a gaseous fumigant) is connected to the duct 13, to 
inject fumigant gas, as required, through a nozzle 18. The silos or bins 
10 each contain a quantity of a particulate commodity. Each inlet port of 
the bins 10 is supplied with an on/off valve 19. 
In the known FIG. 4 arrangement, to fumigate the contents of the bins 10, 
the valve 19 in the gas inlet port 11 of one of the silos or bins is 
opened and the valves 19 in the gas inlet ports of the other bins are 
closed. Fumigation of the bin having its associated valve 19 open is then 
effected using the conventional recirculating fumigant technique. When 
fumigation of the contents of that bin is completed, the opened valve 19 
is closed and one of the other valves 19 is opened. Fumigation of the 
contents of the bin which then has its associated valve 19 open is then 
effected. This procedure is repeated until the stored commodity in each 
bin of the storage facility has been fumigated. 
Clearly, the fumigation of a number of silos, one at a time, using the 
known arrangement illustrated in FIG. 4, takes considerable time when 
phosphine is the fumigant, and there is the problem of potential 
re-infestation of the fumigated contents of a bin while the commodity 
stored in another bin is being fumigated. 
In the modified arrangement of the present invention that is illustrated in 
FIG. 5, an adjustable system valve 25 is included in the gas recirculation 
circuit. The system valve 25 is used to maintain the static pressure in 
the duct 13 at a predetermined value or within a predetermined range of 
values. 
Normally a manometer 24 will be used to set the static pressure in (and 
hence the flow rate of gas through) the gas supply duct 13. Control of the 
setting of the system valve 25 to establish the required static pressure 
in the duct 13 is normally effected by an operator at the start of the 
fumigation, when (i) whenever the fumigation of a silo has ceased and that 
silo is removed from the recirculation circuit, or (ii) a new silo is 
added to the recirculation circuit. Alternatively, control of the setting 
of the system valve 25 may be effected automatically, using an electrical 
signal generated by the manometer and input to a microprocessor 26 which 
is programmed to generate a control signal to drive a motor (preferably a 
stepping motor) 27 which mechanically varies the setting of the system 
valve 25. Such servo-systems for maintenance of a predetermined pressure 
in a chamber, duct or the like are known per se. 
As in the arrangement shown in FIG. 3, each bin or silo 10 of the FIG. 5 
arrangement is provided with an orifice plate 20 at its inlet port. The 
aperture or orifice of each orifice plate 20 is sized so that gas flows 
through its associated silo at a required rate when the gas pressure in 
the gas supply duct 13 is at its required predetermined value. The 
commodities stored in any number of bins in the storage facility may then 
be fumigated, simultaneously, by opening the respective valves 19 and 
adjusting the system valve 25 to establish the required static pressure in 
the duct or manifold 13. 
When a fresh fumigation of silos is established, the operating conditions 
of the fumigation system are re-established. The perturbation to the flow 
of fumigant-containing gas that is caused by opening a hitherto closed 
valve 19 to enable the fumigation of the commodity stored in another silo 
10, or by closing one of the opened valves 19 to remove one of the bins 10 
from the fumigation process (for example, when that bin is to be emptied 
of its commodity), is corrected by adjusting the setting of the system 
valve 25 to return the pressure in the gas supply duct 13 to its required 
value. When such an adjustment of the valve 25 has been made, the 
fumigant-containing gas will again be passed simultaneously through the 
"opened" bins or silos 10, in parallel, at the respective required rates. 
The addition of fumigant to the recirculating gas, to compensate for losses 
due to leakage and sorption by a commodity in the bins being fumigated, 
may be effected manually. For manual control of the addition of fumigant, 
earlier monitoring of the operation of the storage system is required to 
establish, empirically, for the conditions under which the storage bins 
are being used, (i) when fumigant has to be added to the recirculating 
gas, and (ii) how much fumigant needs to be added, to maintain at least 
the required minimum concentration of fumigant in the recirculating gas. 
Such periodic addition of fumigant to the recirculating gas may be effected 
automatically, using a known form of metering device, which is connected 
to a cylinder that contains the gaseous fumigant under pressure and which 
is operated by a timer-controlled solenoid. 
However, the preferred arrangement for the addition of fumigant to the 
recirculating gas, to compensate for leakage and sorption, is shown in 
FIG. 5. The concentration of fumigant in the gas in the gas supply duct 13 
is monitored by a fumigant gas sensor 21. The output signal of the sensor 
21 is connected to a microprocessor 22, which may be integrated with the 
microprocessor 26. The output of the microprocessor 22 is adapted to 
control the supply of gaseous fumigant from the cylinder 17 to the gas 
supply duct 13. Whenever the output signal from the fumigant sensor 21 
indicates that the concentration of phosphine (or other gaseous fumigant) 
in the duct 13 has fallen below a predetermined value, the microprocessor 
22 causes the release of further fumigant from the cylinder 17 until at 
least the predetermined value of the fumigant concentration in the 
recirculating gas has been established. Using this arrangement, the 
concentration of fumigant in the recirculating gas is automatically 
increased to maintain a required minimum value and compensate for 
different rates of gas loss from the bins 10. 
The cylinder 17 may be mounted on scales 23, which are used to monitor the 
quantity of liquefied gas remaining in the cylinder 17. If desired, a 
known form of apparatus which generates an alarm signal if the liquefied 
gas in the cylinder 17 should reach a low level may be included in the 
fumigation arrangement. 
An advantage of the fumigation arrangement illustrated in FIG. 5 and 
described above is that, by adjusting the apertures in the orifice plates 
20 so that the flow of gas through each silo or bin has substantially the 
same linear velocity, and each silo has substantially the same inlet 
pressure, leaks in the tops or bases of the silos have minimal effect. 
If the bins 10 are all sealed to a gas-tightness standard and there is 
little difference in the very low leakage rates of, or the distribution of 
leaks between, the bins, it is a simple matter to establish a satisfactory 
distribution of fumigant in each bin of the system. Under these 
conditions, there will be a very low decay rate of the fumigant 
concentration in the recirculating gas. The only significant loss of 
fumigant will be by sorption by a stored commodity in the bins. Thus the 
"one shot" fumigation technique may be practised, provided the single 
injection of fumigant gas into the recirculation circuit is such that the 
concentration of fumigant in the recirculating gas at the end of a 
specified period of fumigation is not lower than the minimum required 
concentration. Alternatively, and preferably, a "slow release" source of 
fumigant gas (such as one of the packaged formulations described in the 
specification of International patent application No PCT/AU93/00270-WIPO 
Publication No WO 93/25075) may be included in the recirculation circuit, 
in the knowledge that the generation of fresh fumigant will compensate for 
the low leakage from the bins and the sorption of the fumigant by the 
stored commodity, and thus a low (but acceptable) level of fumigant will 
be available for the entirety of the fumigation process. 
The control of the supply of fumigant using a microprocessor, as 
illustrated in FIG. 5, is particularly useful when there are larger--and 
possibly different--leakages from the bins 10. If there are holes in the 
tops and bottoms of the bins, leakage of gas from the lower holes and 
ingress of outside air through the higher holes can occur, with a 
consequential rapid dilution of the concentration of the gaseous fumigant. 
In this situation, the sensor 21 and its associated microprocessor 22 will 
ensure that a predetermined concentration of fumigant gas is maintained in 
the gas supplied to the bins 10 at all times. 
It will be appreciated that the pressure conditions within the 
recirculation circuit are such that if the bottoms of the bins 10 and the 
walls of the bins 10 are essentially sealed and free of leaks, diluent air 
will not be drawn into the recirculation circuit through holes in the tops 
of the bins, even with different sized holes in the various bins. 
Similarly, if the tops of the bins and the walls of the bins are 
essentially sealed, diluent air will not be drawn into the recirculation 
circuit through holes in the bottoms of the bins. 
Although reference has been made, above, to the fumigation process (which 
has been called the "SIROFLO" process) which is described in the 
specification of Australian patent No 640,699 (granted on the Australian 
patent application derived from International patent application No 
PCT/AU90/00268), and to the phosphine sources described in the 
specification of International patent application No PCT/AU93/00270, it is 
not necessary for the fumigant gas to be phosphine. Any suitable gaseous 
fumigant--including carbonyl sulphide and cyanogen--may be used in the 
present invention. 
Trials of the present invention have been conducted at the Black Mountain 
site of the Commonwealth Scientific and Industrial Research Organisation, 
in the Australian Capital Territory, Australia, where a storage facility 
comprising three silos, each of 50 tonnes storage capacity, has been 
established. The top of each of these silos is provided with a removable 
circular manhole of diameter 148 mm. The bottom or base of each silo is 
provided with a removable circular manhole of diameter 100 mm. As each of 
these silos is constructed to be gas tight, the manholes are being used to 
simulate leaks from the tops and bottoms of the silos. The silos or bins 
have been configured, at different times, as shown in FIGS. 3 and 5. 
The trials of the present invention have shown that it is effective in 
maintaining an efficacious fumigant concentration in the bins, 
irrespective of whether one, two or three bins have been included in the 
gas recirculation circuit. In particular, the trials have shown that 
(a) with leaks in the tops (only) of the bins (up to and including the 
removal of all three manhole covers), the decay rate of phosphine is 
exponentially related to the total area of the "leaks", varying from 
y=32.979e.sup.-0.178x (where y is the leakage rate and x is the total area 
of the leaks) with no leak, to y=185.9e.sup.-2.7163x with a 148 mm 
diameter top leak and all three bins in the recirculation circuit, to 
y=3203e.sup.-5.1911x when all three bins are in the recirculation circuit 
and all three top manhole covers are removed; 
(b) with leaks in the bases (only) of the silos, the effect of the leaks is 
similar to, but less significant than, the effect of leaks in the tops of 
the silos, ranging from y=41.359e.sup.-0.2759x with a 100 mm diameter leak 
in the base of one bin only, to y=51.529e.sup.-0.3361x with a 100 mm 
diameter base leak and all three bins in the recirculation circuit; 
(c) with openings in both the tops and bases (bottoms) of the silos, the 
decay rate of phosphine is similar to that observed with top leaks only; 
and 
(d) in all configurations of the multiple-bin storage facility, it is 
possible to maintain an efficacious minimum concentration of phosphine in 
the fumigation system by a regular periodic injection of phosphine, and 
the intervals between the additions of phosphine are such that the 
multiple-bin fumigation with recirculation of fumigant-containing gas is 
very economical, less fumigant gas being required than the quantity of 
gaseous fumigant that would be required for a flow-through system, using 
the "SIROFLO" technique that is described in the specification of 
Australian patent No 640,699. 
It should be noted that although exemplary embodiments of the present 
invention have been illustrated in this specification, and described 
above, variations to, and modifications of, those embodiments may be made 
without departing from the present inventive concept.