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The use of lay-flat polythene tubing in the fumigation of stacks of bagged maize
7. stored Prod. Res., 1969, Vol. 5, pp. 365-369.
Pergamon Press. Printed in Great Britain.
The Use of Lay-flat Polythene Tubing in the Fumigation of Stacks of Bagged Maize J.
D. PEARSON, M. J. HARDCASTLE and T. J. PIPKIN Grain Marketing Board, P.O. Box 8014, Causeway, Rhodesia (First received 4 September, 1967, and injinal form 13 May,
1969)
small-bore distribution systems normally employed in methyl bromide fumigations of bagged grain have certain disadvantages, especially where large stacks are concerned, and an alternative method, based on lay-flat polythene tubing, is described. The results of a comparative trial, in which the two methods were applied under almost identical conditions, show that improved distribution of fumigant within the stack can be achieved with the ‘lay-flat system’. Abstract-The
IN THE fumigation of stacks of bagged produce under gas-proof sheets, the fumigant most commonly used, methyl bromide, is usually introduced into the enclosed space above the stack through an arrangement of spray nozzles incorporated in a highpressure, narrow-bore piping system (BROWN, 1959). Although treatments conducted in this way have been generally successful in dealing with infestations of grain pests in Grain Marketing Board storage, the use of small-bore piping systems entails certain disadvantages which reduce the overall efficiency of fumigation operations, particularly when these involve large stacks such as are built at Board depots. Since the fumigant is released from the distribution piping as a liquid, subsequent evaporation and expansion into the surrounding space tends to produce a fall in temperature which may be considerable where the treatment requires the use of a large quantity of methyl bromide. As a result, a substantial proportion of the dose applied will not vaporize immediately and will tend to sink through the stack as a cold liquid-vapour mixture. This leads to a situation where excessively high concentrations may be established in the lower regions of the stack, while the concentrations achieved at the top of the stack may be minimal for complete control of infestation. It has been suggested that distribution of fumigant may be improved by employing a heating device to vaporize the liquid before it is discharged into the stack (HAMMER and AMSTUTZ, 1955)) but unless a powerful heat source is used, an inordinate amount of time may be spent in achieving complete vaporization of large amounts of methyl bromide. In treating Board storage we have sought to achieve the same objective
365 G
366
J.
D. PEARSON, M. J. HARDCASTLE and T. J. PIPKIN
by delivering the fumigant at a slow rate, so that the heat absorbed will, to some extent, be replaced by solar heat. However, this procedure is also wasteful of time and does not completely prevent ‘layering’ of the gas. Another disadvantage of the conventional distribution system is that outlet orifices may become blocked by small particles of solid matter present in ‘the fumigant, or as a result of freezing of moisture present in the system. Obviously, this will also produce an uneven distribution of the fumigant. In considering ways in which the efficiency of the Board’s fumigation operations might be improved, it was decided to investigate the possibility of using a distribution system based on lay-flat polythene tubing. Although this material will not withstand any appreciable pressure, it seemed probable that the risk of bursting could be obviated by making suitable provision for the release of gaseous fumigant from the upper surface of the tubing, while at the same time the number and size of the gas outlets could be chosen so that the tubing would remain inflated while liquid methyl bromide was present in the system. This would permit rapid delivery of suitable quantities of the fumigant into lengths of tubing positioned on top of the stack and, providing that the tubing was reasonably level from end to end, there would be little risk that any fumigant would escape in liquid form. Evaporation would then take place at a rate which depended on the rate of heat absorption from the surrounds and, although complete vaporization of the dose might be a relatively slow process, the larger scale temperature effects observed in the release of liquid methyl bromide from small-bore piping systems would be avoided. There would also be no risk of outlets becoming blocked and the fumigant would be evenly distributed from the outlets despite any tendency for the liquid to collect unevenly within the tubing Preliminary trials were conducted to test the proposed technique and it was found that 10 in. (254 mm) wide, 0 - 01 in. (0 - 254 mm) gauge lay-flat tubing, punched with O-25 in. (6.4 mm) dia. holes at 5 ft (l-525 m) intervals, withstood the natural vapour pressure exerted by methyl bromide under local temperature conditions. In delivering the fumigant, however, the cylinder valve was opened slowly to avoid a sudden build-up of pressure within the tubing. Once liquid methyl bromide entered the tubing, heat absorption resulting from evaporation produced a pressure drop and the tubing collapsed slightly. Thereafter, the appropriate quantity of fumigant could be rapidly delivered without fully distending the tubing. A full-scale fumigation trial was subsequently carried out to compare the results obtained with the lay-flat distribution system and with the standard polythene piping arrangement employed in Board storage. In preparation, gas-sampling lines were incorporated in a 35,000 bag maize stack (46 ft (14 m) wide, 196 ft (59.75 m) long, 14 ft (4 - 27 m) to eaves and 2 1 ft (6 *4 m) to apex) built within an open-sided steel shed. The arrangement of sampling points is shown in Fig. I. In the first fumigation, two 80 ft (24 - 4 m) lengths of lay-flat tubing (Fig. 2) were laid in line along a shallow channel formed by separating the two longitudinal rows of bags which constituted the apex of the stack. The inner ends of the tubes were roughly 10 ft (3.05 m) apart, the outer ends being similar distances from the ends of the stack. The stack was covered with PVC sheets which were joined together and weighted to the ground in the usual manner. A panel of clear
Use of Lay-flat Polythene Tubing in Fumigation of Stacks of Bagged Maize
7,
6
367
.8 i
(b)
,j 17
~ I8
19,,20
FIG. I. Layout of gas-sampling points. A, Cross-section half-way along stack. B, End of stack. Lay-fiat with
025in
tube punched holes
at 511
Intervals
:IIc!~~;;~
FIG. 2.
Layout of 80 ft lay-flat distribution system.
polythene sheeting was inserted immediately above one of the lay-flat tubes so that the behaviour of the fumigant and tubing could be observed directly. At a dosage rate of 2 lb of methyl bromide/1000 ft3 (32 g/ma) the total weight required was calculated to be 308 lb (140 kg). A half of this quantity was delivered, concurrently, to each of the tubes, dosing being completed within a period of 30 min. Visual inspection indicated, however, that the fumigant remained within the tubing for at least 8 hr and possibly for a considerably longer period. The second fumigation of the stack was carried out some 4 months later, but under very similar climatic conditions, i.e. overcast, with daytime ambient temperatures in the range 23-28°C. The two 80 ft (24.4 m) polythene pipes (Fig. 3) were laid in the channel in the positions previously occupied by the lay-flat tubes and the stack was prepared for fumigation as before, except that the polythene panel was omitted. Treatment was carried out at the dosage rate previously employed, 154 lb (70 kg) of methyl bromide being introduced, simultaneously, into each of the delivery pipes at a maximum rate of 2 lb (0.91 kg)/min per pipe.
\ “:
J. D. PEARSON, M. J. HARDCASTLE and T. J. PIPKIN
368
40fl
I-
-_
’
FIG. 3. Diagram
20fi
---
--
Dlstribut’lon pipe drIlled wcth O.lin. holes at 5ft intervals
of standard O-5 in. polythene
piping system.
The gas concentrations present at the various sampling points were determined, at intervals during each fumigation, by a thermal conductivity method (HESELTINE et al., 1958). The readings obtained at each sampling point were plotted against time, and concentration-time products were derived by measurement of the areas enclosed by the resulting curves at 24-hr and 50-hr exposure periods, These concentration-time products are given in Table 1. TABLE 1. METHYL BROMIDE CONCENTRATION-TIME PRODUCTSOBTAINED IN A MAIZE
Lay-flat Sampling point
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
STACK
BY TWO
FUMIGATING
METHODS
0.5 in. Bore piping
tubing
Concentration-time products (mg/hr/l) 24 hr
50 hr
325 290 330 365 330 350 380 385 320 310 380 375 410 345 810 475 310 345 345 350 330 350 360 350 335 350 355 350
710 670 710 765 720 740 780 790 705 675 760 750 850 735 1310 820 660 715 720 730 685 730 745 720 705 710 745 715
Sampling point
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Concentration-time products (mg/hr/l) 24 hr
50 hr
1580 1165 1205 1060 640 380 375 410 460 275 285 335 365 350 315 295 1130 835 850 1055 345 345 350 345 290 315 225 310
2390 1810 1825 1550 1090 705 725 750 835 585 555 645 705 495 685 595 1470 1180 1180 1400 700 670 675 670 650 610 595 645
-
369
Use of Lay-flat Polythene Tubing in Fumigation of Stacks of Bagged Maize
In Fig. 4 the concentration-time products are presented in a form suggested by Miss E. Reynolds (W. B. Brown, 1961, personal communication). Each-group of concentration-time products has.been arranged in order of magnitude and plotted .
0 . . . . .
s
_=
..
0°C
c?
0 ox 200
. 400
x
Lay-flat
tubing
A
Lay-fiat
tubing
0
0.5in
piplng
l
0.5in.
piping
: 24hr : 50hr : 24hr : 50hr
A
,2
600 Methyl
,
I
I
I
I
I
I
I
I
600
1000
1200
1400
1600
1600
2000
2200
2400
bromide
concentration-time
products,
mg /hr
per
L.
FIG. 4. Graph showing ranges of concentration-time products (at 24 and 50 hr) obtained during the two experimental treatments.
at equal intervals between 0 and 1 on the vertical axis. The highest reading in each group is assigned the value of 1 and all other readings in the group ranked in numerical order below it. As can be seen, a better distribution of concentrationtime products between sampling points was obtained in the fumigation employing lay-flat tubing. There would also appear to be a smaller risk in this treatment that there might have been unsampled positions in the stack where concentration-time products were appreciably lower than the minimum actually observed. Other tests of the lay-flat distribution method were conducted in Board storage during 1966. Although these did not involve direct comparisons with the more conventional system, the results of gas-concentration measurements indicate that, under local conditions at least, the experimental technique will consistently give a better distribution of fumigant in large-scale treatments. REFERENCES W. B. (1959) Fumigation with methyl bromide under gas-proof sheets. Bull. Pest In&t. Res. Bd No. 1, 2nd ed. HAMMER,0. H. and AMSTUTZ,F. C. (1955) Apparatus for more rapid vaporisation of methyl bromide. Down to Earth 11, NO. 2, 11-13. HESELTINE,H. K., PEARSON,J. D. and WAINMAN,H. (1958) A simple thermal conductivity meter for gas analysis with special reference to fumigation problems. Chemy Znd. 1958, 1287-1288.
BROWN,
Pergamon Press. Printed in Great Britain.
The Use of Lay-flat Polythene Tubing in the Fumigation of Stacks of Bagged Maize J.
D. PEARSON, M. J. HARDCASTLE and T. J. PIPKIN Grain Marketing Board, P.O. Box 8014, Causeway, Rhodesia (First received 4 September, 1967, and injinal form 13 May,
1969)
small-bore distribution systems normally employed in methyl bromide fumigations of bagged grain have certain disadvantages, especially where large stacks are concerned, and an alternative method, based on lay-flat polythene tubing, is described. The results of a comparative trial, in which the two methods were applied under almost identical conditions, show that improved distribution of fumigant within the stack can be achieved with the ‘lay-flat system’. Abstract-The
IN THE fumigation of stacks of bagged produce under gas-proof sheets, the fumigant most commonly used, methyl bromide, is usually introduced into the enclosed space above the stack through an arrangement of spray nozzles incorporated in a highpressure, narrow-bore piping system (BROWN, 1959). Although treatments conducted in this way have been generally successful in dealing with infestations of grain pests in Grain Marketing Board storage, the use of small-bore piping systems entails certain disadvantages which reduce the overall efficiency of fumigation operations, particularly when these involve large stacks such as are built at Board depots. Since the fumigant is released from the distribution piping as a liquid, subsequent evaporation and expansion into the surrounding space tends to produce a fall in temperature which may be considerable where the treatment requires the use of a large quantity of methyl bromide. As a result, a substantial proportion of the dose applied will not vaporize immediately and will tend to sink through the stack as a cold liquid-vapour mixture. This leads to a situation where excessively high concentrations may be established in the lower regions of the stack, while the concentrations achieved at the top of the stack may be minimal for complete control of infestation. It has been suggested that distribution of fumigant may be improved by employing a heating device to vaporize the liquid before it is discharged into the stack (HAMMER and AMSTUTZ, 1955)) but unless a powerful heat source is used, an inordinate amount of time may be spent in achieving complete vaporization of large amounts of methyl bromide. In treating Board storage we have sought to achieve the same objective
365 G
366
J.
D. PEARSON, M. J. HARDCASTLE and T. J. PIPKIN
by delivering the fumigant at a slow rate, so that the heat absorbed will, to some extent, be replaced by solar heat. However, this procedure is also wasteful of time and does not completely prevent ‘layering’ of the gas. Another disadvantage of the conventional distribution system is that outlet orifices may become blocked by small particles of solid matter present in ‘the fumigant, or as a result of freezing of moisture present in the system. Obviously, this will also produce an uneven distribution of the fumigant. In considering ways in which the efficiency of the Board’s fumigation operations might be improved, it was decided to investigate the possibility of using a distribution system based on lay-flat polythene tubing. Although this material will not withstand any appreciable pressure, it seemed probable that the risk of bursting could be obviated by making suitable provision for the release of gaseous fumigant from the upper surface of the tubing, while at the same time the number and size of the gas outlets could be chosen so that the tubing would remain inflated while liquid methyl bromide was present in the system. This would permit rapid delivery of suitable quantities of the fumigant into lengths of tubing positioned on top of the stack and, providing that the tubing was reasonably level from end to end, there would be little risk that any fumigant would escape in liquid form. Evaporation would then take place at a rate which depended on the rate of heat absorption from the surrounds and, although complete vaporization of the dose might be a relatively slow process, the larger scale temperature effects observed in the release of liquid methyl bromide from small-bore piping systems would be avoided. There would also be no risk of outlets becoming blocked and the fumigant would be evenly distributed from the outlets despite any tendency for the liquid to collect unevenly within the tubing Preliminary trials were conducted to test the proposed technique and it was found that 10 in. (254 mm) wide, 0 - 01 in. (0 - 254 mm) gauge lay-flat tubing, punched with O-25 in. (6.4 mm) dia. holes at 5 ft (l-525 m) intervals, withstood the natural vapour pressure exerted by methyl bromide under local temperature conditions. In delivering the fumigant, however, the cylinder valve was opened slowly to avoid a sudden build-up of pressure within the tubing. Once liquid methyl bromide entered the tubing, heat absorption resulting from evaporation produced a pressure drop and the tubing collapsed slightly. Thereafter, the appropriate quantity of fumigant could be rapidly delivered without fully distending the tubing. A full-scale fumigation trial was subsequently carried out to compare the results obtained with the lay-flat distribution system and with the standard polythene piping arrangement employed in Board storage. In preparation, gas-sampling lines were incorporated in a 35,000 bag maize stack (46 ft (14 m) wide, 196 ft (59.75 m) long, 14 ft (4 - 27 m) to eaves and 2 1 ft (6 *4 m) to apex) built within an open-sided steel shed. The arrangement of sampling points is shown in Fig. I. In the first fumigation, two 80 ft (24 - 4 m) lengths of lay-flat tubing (Fig. 2) were laid in line along a shallow channel formed by separating the two longitudinal rows of bags which constituted the apex of the stack. The inner ends of the tubes were roughly 10 ft (3.05 m) apart, the outer ends being similar distances from the ends of the stack. The stack was covered with PVC sheets which were joined together and weighted to the ground in the usual manner. A panel of clear
Use of Lay-flat Polythene Tubing in Fumigation of Stacks of Bagged Maize
7,
6
367
.8 i
(b)
,j 17
~ I8
19,,20
FIG. I. Layout of gas-sampling points. A, Cross-section half-way along stack. B, End of stack. Lay-fiat with
025in
tube punched holes
at 511
Intervals
:IIc!~~;;~
FIG. 2.
Layout of 80 ft lay-flat distribution system.
polythene sheeting was inserted immediately above one of the lay-flat tubes so that the behaviour of the fumigant and tubing could be observed directly. At a dosage rate of 2 lb of methyl bromide/1000 ft3 (32 g/ma) the total weight required was calculated to be 308 lb (140 kg). A half of this quantity was delivered, concurrently, to each of the tubes, dosing being completed within a period of 30 min. Visual inspection indicated, however, that the fumigant remained within the tubing for at least 8 hr and possibly for a considerably longer period. The second fumigation of the stack was carried out some 4 months later, but under very similar climatic conditions, i.e. overcast, with daytime ambient temperatures in the range 23-28°C. The two 80 ft (24.4 m) polythene pipes (Fig. 3) were laid in the channel in the positions previously occupied by the lay-flat tubes and the stack was prepared for fumigation as before, except that the polythene panel was omitted. Treatment was carried out at the dosage rate previously employed, 154 lb (70 kg) of methyl bromide being introduced, simultaneously, into each of the delivery pipes at a maximum rate of 2 lb (0.91 kg)/min per pipe.
\ “:
J. D. PEARSON, M. J. HARDCASTLE and T. J. PIPKIN
368
40fl
I-
-_
’
FIG. 3. Diagram
20fi
---
--
Dlstribut’lon pipe drIlled wcth O.lin. holes at 5ft intervals
of standard O-5 in. polythene
piping system.
The gas concentrations present at the various sampling points were determined, at intervals during each fumigation, by a thermal conductivity method (HESELTINE et al., 1958). The readings obtained at each sampling point were plotted against time, and concentration-time products were derived by measurement of the areas enclosed by the resulting curves at 24-hr and 50-hr exposure periods, These concentration-time products are given in Table 1. TABLE 1. METHYL BROMIDE CONCENTRATION-TIME PRODUCTSOBTAINED IN A MAIZE
Lay-flat Sampling point
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
STACK
BY TWO
FUMIGATING
METHODS
0.5 in. Bore piping
tubing
Concentration-time products (mg/hr/l) 24 hr
50 hr
325 290 330 365 330 350 380 385 320 310 380 375 410 345 810 475 310 345 345 350 330 350 360 350 335 350 355 350
710 670 710 765 720 740 780 790 705 675 760 750 850 735 1310 820 660 715 720 730 685 730 745 720 705 710 745 715
Sampling point
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Concentration-time products (mg/hr/l) 24 hr
50 hr
1580 1165 1205 1060 640 380 375 410 460 275 285 335 365 350 315 295 1130 835 850 1055 345 345 350 345 290 315 225 310
2390 1810 1825 1550 1090 705 725 750 835 585 555 645 705 495 685 595 1470 1180 1180 1400 700 670 675 670 650 610 595 645
-
369
Use of Lay-flat Polythene Tubing in Fumigation of Stacks of Bagged Maize
In Fig. 4 the concentration-time products are presented in a form suggested by Miss E. Reynolds (W. B. Brown, 1961, personal communication). Each-group of concentration-time products has.been arranged in order of magnitude and plotted .
0 . . . . .
s
_=
..
0°C
c?
0 ox 200
. 400
x
Lay-flat
tubing
A
Lay-fiat
tubing
0
0.5in
piplng
l
0.5in.
piping
: 24hr : 50hr : 24hr : 50hr
A
,2
600 Methyl
,
I
I
I
I
I
I
I
I
600
1000
1200
1400
1600
1600
2000
2200
2400
bromide
concentration-time
products,
mg /hr
per
L.
FIG. 4. Graph showing ranges of concentration-time products (at 24 and 50 hr) obtained during the two experimental treatments.
at equal intervals between 0 and 1 on the vertical axis. The highest reading in each group is assigned the value of 1 and all other readings in the group ranked in numerical order below it. As can be seen, a better distribution of concentrationtime products between sampling points was obtained in the fumigation employing lay-flat tubing. There would also appear to be a smaller risk in this treatment that there might have been unsampled positions in the stack where concentration-time products were appreciably lower than the minimum actually observed. Other tests of the lay-flat distribution method were conducted in Board storage during 1966. Although these did not involve direct comparisons with the more conventional system, the results of gas-concentration measurements indicate that, under local conditions at least, the experimental technique will consistently give a better distribution of fumigant in large-scale treatments. REFERENCES W. B. (1959) Fumigation with methyl bromide under gas-proof sheets. Bull. Pest In&t. Res. Bd No. 1, 2nd ed. HAMMER,0. H. and AMSTUTZ,F. C. (1955) Apparatus for more rapid vaporisation of methyl bromide. Down to Earth 11, NO. 2, 11-13. HESELTINE,H. K., PEARSON,J. D. and WAINMAN,H. (1958) A simple thermal conductivity meter for gas analysis with special reference to fumigation problems. Chemy Znd. 1958, 1287-1288.
BROWN,