12.1 Sheeting specifications and sealing of stacks
12.2 Making stacks gas-tight
Effective sealing of gas-tight enclosures is essential for controlling both adult and juvenile stages of LGB by fumigation. Where phosphine is used in non gas-tight enclosures there are risks of insects developing resistance. Many sheet materials are available for fumigation purposes including unsupported polythene and PVC. The minimum thickness of these unsupported materials for both durability and acceptable gas permeability is 125 microns. Laminated PVC sheets with supporting centre scrim are now in common use for fumigation, and are very durable when carefully handled. PVC materials can be affected by liquid methyl bromide, and fumigation sheets of this material should be of a type that is minimally damaged when wetted with fumigant.
None of the materials used in the manufacture of fumigation sheets are completely gas-tight because such materials are all permeable to some degree to fumigants. The rate at which diffusion through the fabric takes place depends upon the type of material, its thickness and the ambient temperature. Diffusion through the sheet will reduce the concentration of fumigant to which insects are exposed in the stack. If a sheet is highly permeable to methyl bromide there may be uneven distribution of fumigant within the stack to such an extent that in some localities the concentration time product (c.t.) is insufficient to kill all the insects present. Because a longer exposure period is required for phosphine (five days or more) there is a longer period during which losses due to diffusion will occur. As the ratio of sheet area to stack volume is greater in small compared to large stacks, losses by diffusion would be expected to be proportionally greater in small stacks.
Specifications for the gas permeability of sheet materials vary, but, for good quality PVC laminated sheets, typical values quoted are:
Permeability of fumigation sheets
Methyl bromide, for a 10 mg per litre concentration: 0.0034 mg per m2 s-1Phosphine, for a 1 mg per litre concentration: 0.00003 mg per m2 s-1
These values apply only to new sheets, and permeability to fumigants may increase with usage.
Whilst sheet thicknesses are quoted in mm, at least two other units may be encountered - 'gauge' (one hundred thousandth of an inch) and 'micron' (one millionth of a metre). These may readily be related as follows:
1000 gauge = 0.01 inches = 0.254 mm = 254 microns
Many pest control operators prefer to use lightweight sheets regardless of other properties, because such sheets require less labour and are easier to handle. However, this may be dangerous and there must be a proper compromise between durability, permeability and ease of handling. Appropriate training should be given to teams so that they familiarize themselves with handling these sheets and ensure effective fumigation operations. Locally available sheets may not be as good as imported ones, but because they are relatively inexpensive and easily obtainable their use may be preferred to imported sheets. It should be noted that the characteristics used to describe the ideal sheet are mutually exclusive and not likely to occur.
Typical specifications fur three selected types of fumigation sheet
|
Type |
Material |
Special properties |
Weight of 18 x 18 m sheet |
Tear strength N/50 mm |
Remarks |
|
1. General purpose * |
460 Decitex nylon scrim fabric coated with tough and flexible PVC on both sides. |
Specially formulated plasticisers for hot climates, to resist sunlight and to use with methyl bromide. |
92 kg |
Warp 350 |
Standard colour blue/while, excellent handling. |
|
2. Medium weight ** |
1100 Decitex polyester net fabric coated with tough and flexible PVC on both sides. |
Excellent. Flame retardant. Specially formulated plasticisers for hot climates to resist sunlight and to use with methyl bromide. |
82 kg |
Warp 100 |
Translucent or standard colours available; excellent handling, very good drape. |
|
3. Light Weight + |
Tropical quality of PVC with UV-inhibitors. 300 microns quality. |
Excellent. Specially formulated plasticisers for hot climates, to resist sunlight and to use with methyl bromide. |
68 kg |
Warp 45 |
Limited life due to low strength. |
Notes:
* Sheet 1 above is an excellent sheet for phosphine fumigations, but can be damaged by liquid methyl bromide. Using correct application techniques it will last for many years.** An excellent sheet for all types of sheeted stack fumigations, and it will have a long life if correctly handled.
+ PVC sheets must be specially formulated for use with methyl bromide. All types of PVC sheets may not be suitable. In general PVC tarpaulins do not make good fumigation sheets.
(i) Sheets can be supplied in specially made transporting covers which effectively protect the folded sheet from damage.
(ii) Repair kits comprising special adhesive and patches are available and should be specified.
(iii) Fumigation sheets do not require eyelets.
Sandsnakes laid on top of the sheet margin against the stack are probably the best method of reducing gas loss to a minimum at floor level. With the heavier PVC laminated sheets, sandsnakes need to be of sufficient dimensions and weight to form a good seal. Many of the sandsnakes in common use are too narrow, and have insufficient weight. It is recommended that to obtain a satisfactory seal of fumigation sheets at the base of the stack, sandsnakes should provide a contact width of approximately 15 cm. Sandsnakes of this width should not be too long, or filled completely, to prevent bursting when transported. A length of about 1.5 metres is the maximum recommended, otherwise transport becomes a problem. Lightweight canvas is a very suitable material for constructing this type of sandsnake. Where it is necessary to use more than one sheet to cover a stack, care is essential to ensure that the join between them is gas-tight. This can be done effectively by means of a folded join, in which, after placing one sheet in the correct position on the stack, one metre is folded back, and the edge of the second sheet is brought to overlap the fold. The two sheets can now be folded together, the width of the overlap, across the stack and at ground level, by a team working together. Laminated sheets may be sufficiently heavy for the join to require no further support, but clips can be used if necessary, particularly at the top edges of the stack, and on the vertical sides of the join.
A further potential source of fumigant leakage from a sheeted stack is at the corners. Care is necessary to ensure that the fumigation sheet is folded neatly at all corners, with overlapping of folds on the floor, to minimise gas leakage. A flattened corner fold will be much easier to seal to the floor with sandsnakes than if the sheet is pulled together from the sides and loosely bunched. Where the size of a stack is such that it does not correspond well to the fumigation sheets available, there may be a considerable excess of sheet. This excess should not be left on the floor but taken onto the top of the stack, and folded flat, leaving a one-metre margin on the floor on all four sides.
Rolled joints should always run vertically on the side of a stack from top to bottom, never horizontally or at an angle.
Folding away excess sheet
If a sheet is larger than is required to cover the stack, or a multiple sheeted stack is left with a quantity of excess sheeting material, this excess must be folded correctly to leave a good seal. If there is sufficient space to operate, more than the minimum 1m gap between stack and wall, the excess sheet can be opened fully, pulled straight and, after sand snakes have hen put in position, folded back over itself.
Greater excess lengths, particularly in narrow, gaps, should be retained on top of the stack. This will require a different technique of folding the sheet. For example, on a three-sheet stack, the two sheets covering the ends should first be put in place and folded back for the rolled joint.
Figure 12.1 Folding the middle sheet of three joined sheets.
The third sheet should be folded in the following manner to ensure that when it is opened out it exactly fills the gap between the end sheets. (Figure 12.1). The sheet is first folded in half and rolled, stating from the folded edge until a measured distance (x in Figure 12.1) from the edge is reached. The doubled sheet is opened out on both sides of this folded centre and both sides are folded from the edges to the centre as usual. When the sheet is placed on top of a stack it can be opened, leaving the centre fold closed, and the edges of the sheet should then be in the correct position.
It is not sufficient to cover a gap with overlapping sheets without rolled joints. Sand snakes could be laid across the top and on the junction with the floor but this would not make a satisfactory gas-tight seal unless the vertical sides were taped.
Multiple-sheet stacks
When covering a large stack with several sheets, joints may have to be made in two directions.
Figure 12.2 Layout for a multiple sheet fumigation.
An indication of the way to do this is given in figure 12.2. The order of placing sheets and making rolled joints is as follows:
Sheet 1, 2-joint A; 3-B, 4,5-C; 6-D, E; 7,8-F; 9-G, H. When placing the sheets on the stack it is important to ensure that the rolling joints A and C (Figure 12.2) do not overlap but are separated by at least 1m to avoid the problem of rolling a joint that has two other rolled joints on top of each other.
When sheeting a large stack in this way, particularly out-of-doors, it may be necessary to weigh down the sheets with bags or similar heavy objects while the sheets are being put into place. It is easier to carry out an operation if the sheets are folded correctly initially and taken up onto the stack and laid out in the correct position.
Figure 12.3 Multiple sheeting of a large stack - Place and unfold sheets on the stack in sequence from 1 to 9
Unfolding, Placing, and Removing Fumigation Sheets:
Figure 12.4 Make roll joints in sequence from A to H. note that the joints are offset so that no four sheets are joined together
Figure 12.5 Folding a fumigation sheet (a) Mark the centre of the sheet
Figure 12.6 (b) Fold one half towards the middle in 1-metre wide folds
Figure 12.7 After half the sheet has been folded repeat this procedure for the remainder
Figure 12.8 Then fold or roll to achieve a well-folded sheet occupying the minimum of space
Figure 12.9 Unroll the sheet towards the stack - (a) Unroll the sheet towards the stack
Figure 12.9 Unroll the sheet towards the stack - (b) Carry, never drag the sheet-
Figure 12.9 Unroll the sheet towards the stack - (c) - and place it over the centre
Figure 12.12 Unroll the half dropping part of it over the sides in such a way that as to place one metre flat on the floor
Figure 12.14 Do the same with the remainder of the sheet
Figure 12.14 Ventilation of a stack - (i) Open as many doors, windows and ventilators as possible
Figure 12.15 Remove sand snakes from one comer of the stack
Figure 12.16 (a) Two men climb on top of stack, the third remaining at floor level
Figure 12.17 (b) Lower one end of a rope to the man on the floor who secures it to a corner of the sheet
Figure 12.18 The men on the stack haul up the sheet, while the other man repositions the ladder
Figure 12.19 (a) When the sheet has been pulled back far enough to uncover the stack corner all the three men vacate the store
Figure 12.20 Warning notes are removed when the ventilation period has been completed
