Pressure testing sealable silos

Stored Grain pressure testing

Fumigating with phosphine in unsealed silos does not kill pests at all stages of their life cycle. Repeat fumigations in unsealed silos increases resistance levels and selects for insects with a higher phosphine tolerance. Pressure testing a silo ensures it can hold gas concentrations sufficient to kill all insects at all life stages.

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Key Points

  • A silo sold as a ‘sealed silo’ needs to be pressure tested to be sure it’s gas-tight.
  • It is strongly recommended that growers ask the manufacturer or reseller to quote the AS2628 on the invoice as a means of legal reference to the quality of the silo being paid for.
  • Pressure test sealed silos upon erection, annually and before fumigating with a five-minute half-life pressure test.
  • Maintenance is the key to ensuring a silo purchased as sealable can be sealed and gas-tight.

What is a sealed silo?

Just because a silo is sold as a sealed silo, does not automatically mean it’s suitable for fumigation.

To some people a sealed silo may be one that keeps rain out or one that is sold labelled as a sealed silo.

A silo is only truly sealed if it passes a five-minute half-life pressure test according to the Australian Standard AS2628. Often silos are sold as sealed but are not gas-tight — rendering them unsuitable for fumigation.

Even if a silo is sold as ‘sealed’ it is not sealed until it is proven gas-tight with a pressure test.

The term ‘sealed’ has been used loosely during the past and in fact some silos may not have been gas-tight from the day they were constructed.

However, even a silo that was gas-tight to the Australian Standard on construction will deteriorate over time so needs annual maintenance to remain gas-tight.

Why do I need to do a pressure test?

In order to kill grain pests at all stages of their life cycle (egg, larvae, pupae, adult), phosphine gas concentration levels need to reach and remain at 300 parts per million (ppm) for seven days or 200ppm for 10 days.

Trials show that these levels of gas concentration are impossible to achieve in silos that are not pressure tested and gas-tight, so insects will not be killed at all life stages. The fumigation may appear successful when the adults die but the surviving eggs and pupae will continue to develop and reinfest the grain.

A pressure test is a measure of how well a silo will seal to contain fumigation gas.

pressure testing image 1When to perform a pressure test

If silos are properly maintained pressure testing does not take long and should be done at three distinct times.

  1. When a new silo is erected on farm carry out a pressure test at a suitable time of day to make sure it’s gas-tight before paying the invoice or filling with grain.
  2. Importantly, a silo also needs to be pressure tested when full, before fumigating grain. If the silo has a slide plate outlet that has been tested empty, retest when full to make sure the pressure of the grain doesn’t compromise the seal. The weight of grain can break the seal on the slide-plate outlet where it is not well supported by cams or bolts etc. For older, poorly-designed cone-bottom silos, gentle pressure from a jack may assist the seal. If the weight of grain on the slide plate stops it from sealing, some added pressure from a jack under the silo will assist the sealability.
  3. Pressure testing silos needs to be part of the annual maintenance. It is much easier to replace seals and carry out repairs when silos are empty.

Carrying out a pressure test


If regular silo maintenance is undertaken to keep seals in working order, pressure testing is easy by following these seven simple steps.

1. Choose the right time to pressure test

pressure testing image 2Consider the ambient conditions of the day before pressure testing.

The best time to pressure test silos is in the morning within an hour of sunrise or on a cool, overcast day ­— when the ambient temperature is stable and the sun is not heating the silo.

Air inside a silo heats and expands as the daily temperature rises and the sun warms the silo walls.

If a pressure test is done when the ambient conditions are changing, air inside the silo expands and gives a false reading.

pressure testing image 4

2. Check seals

Before performing a pressure test check seals around the lid, access hatch, hopper or boot and make sure the aeration fan seal is in a sound condition.

Check to ensure all latches on lids are locked down firmly.

3. If there is no aeration fan – install an air valve

pressure testing image 3

If the silo doesn’t have an aeration fan, install a tubeless tyre valve to pressurise the silo using an air compressor. Unscrew the centre of the valve to get higher air-flow into the silo.

Alternatively for larger silos or if the air compressor is too small, install a PVC male fitting that can connect to a venturi gun (Blovac) that fits on the end of the air line.

4. Check oil levels

pressure testing image 5Some sealable silos do not have a gauge on the oil relief valve. If this is the case mark the start and finish oil levels with a pen.

Oil relief valves can be bought and fitted, or a piece of clear tube connected to a second air valve fitted to the silo will suffice.

Before pressurising the silo, check the oil levels are equal on both sides of the gauge and are at the middle indicator mark as shown below.

5. Pressurise the silo

pressure testing image 5If fitted with an aeration fan, pressurise the silo by turning the fan on for a few seconds, then sealing the inlet on the fan.

This job is easier with two people —one to operate the fan and the other to watch the oil gauge and look for leaks, see step 7.

As soon as the oil levels are more than 25mm apart, or the oil is bubbling, stop the aeration fan and close off the fan inlet immediately. Be careful — there is potential for damage if fans are left running for extended periods while the silo is sealed or with the inlet blocked off.

If the silo doesn’t have an aeration fan, use the tyre valve and an air compressor to pressurise the silo. An air-operated venturi gun, such as a Blovac, with connection fittings to the silo can also be used.

6. Time the half life

pressure testing image 7Wait until the pressure drops and the oil levels are 25mm apart (aligned with top and bottom marker).

The time taken for the oil to drop from 25mm to 12mm apart must be no less than five minutes on new silos.

For older silos three minutes is acceptable.

Whether it is three or five minutes, this process is known as the half-life pressure test.

7. Looking for leaks

pressure testing image 6If the the half-life pressure test on a new silo is less than five minutes, there is a leak that needs fixing. If an existing silo does not meet a three minute half-life pressure test, it as a leak that needs fixing.

To find leaks, pressurise the silo again and use soapy water in a spray bottle to check for air leaks around seals.

Common places for leaks are: bottom outlet, aeration inlet seal, damaged lids (caused by the auger when lining it up to fill the silo), stretched springs on latches, between the bottom cone or base and the silo wall joint, the roof and wall joint and where the lid ring joins the roof.

Cone-bottom silos with a slide plate outlet can be sealed by adding a small amount of pressure to the slide plate with a jack.

Older silos may require more extensive maintenance to achieve a gas-tight seal. When the leak has been fixed, pressurise the silo again and redo the half-life test — steps five and six.

Australian standard for sealed silos

pressure testing image 8

A benchmark for sealing grain silos has been developed to boost the effectiveness of pest control.

Standards body SAI Global published an Australian standard for gas-tight sealed silos in response to industry concerns that phosphine fumigation performed in improperly sealed storages was not killing off the full life cycle of pests.

Resistance to phosphine has increased over the past 10 years with many grain silos failing to meet the gas-tight standard required for effective fumigation.

Resistance to phosphine in target insect pests has increased in frequency and strength threatening effective control.

The standard is based on a new silo meeting a five-minute half-life pressure test. When a pressure test is undertaken, oil levels in the pressure relief valve must take a minimum of five minutes to fall from 25mm to a 12.5mm difference if the silo is sufficiently gas-tight.

The standard allows growers to refer to an industry benchmark when choosing to buy a sealable silo. It is strongly recommended that growers ask the manufacturer or reseller to quote the AS2628 on the invoice as a means of legal reference to the quality of the silo being paid for. Ultimately, this gives growers confidence they have invested in a silo that will perform in the way it is intended. That is, work as a gas-tight chamber and hold a lethal concentration of gas for the time specified on the label, for control of insects at all life stages.

This will prolong the life of phosphine rather than add to the already increased level of resistant insects.

Grain Storage Pest Control Guide – Western Region

Stored Grain Hygiene treatment
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The tolerance for live pests in grain sold off farm is nil. With growers increasing the amount of grain stored on farm, an integrated approach to pest control is crucial.

 

Caution: Research on unregistered pesticide use Any research with unregistered pesticides or of unregistered products reported in this document does not constitute a recommendation for that particular use by the authors or the authors’ organisations. All pesticide applications must accord with the currently registered label for that particular pesticide, crop, pest and region.

KEY POINTS

  • Effective grain hygiene and aeration cooling can overcome 85 per cent of pest problems.
  • When fumigation is needed it must be carried out in pressuretested, sealed silos.
  • Monitor stored grain monthly for moisture, temperature and pests.

Prevention is better than cure

The combination of meticulous grain hygiene plus well-managed aeration cooling generally overcomes 85 per cent of storage pest problems. For grain storage, three key factors provide significant gains for both grain storage pest control and grain quality – hygiene, aeration cooling and correct fumigation.

Managing Grain Hygiene and Structural TreatmentsHygiene

The first grain harvested is often at the greatest risk of early insect infestation due to contamination. One on-farm test found more than 1000 lesser grain borers in the first 40 litres of wheat passing through the harvester. Remove grain residues from empty storages and grain handling equipment, including harvesters, field bins, augers and silos to ensure an uncontaminated start for new-season grain. Clean equipment by blowing or hosing out residues and dust and then consider a structural treatment (see Table 2, page 3). Remove and discard any grain left in hoppers and bags from the grain storage site so it doesn’t provide a habitat for pests during the off season.

Aeration cooling

Freshly-harvested grain usually has a temperature around 30°C, which is an ideal breeding temperature for storage pests (see Table 1, page 2). Studies have shown that rust-red flour beetles stop breeding at 20°C, lesser grain borer at 18°C and below 15°C all storage pests stop breeding. Aim for grain temperatures of less than 23°C during summer and less than 15°C during winter. When placing grain into storage, run aeration fans continuously for the first 2-3 days to push the first cooling front through the grain and to create uniform moisture conditions. Then run the fans during the coolest 9-12 hours per day for the next 3-5 days. This will push a second cooling front through the grain bulk. Aeration cooling generally only requires air-flow rates of 2-4 litres per second per tonne. Finally the grain requires approximately 50 hours of appropriate quality air each fortnight during storage. Use an aeration controller that will perform the cooling process at the right time and continue to aerate the grain selecting the coolest air to run fans. An effective aeration controller will also ensure fans don’t operate when the relative humidity is higher than 85 per cent, which can re-wet and damage grain if operated for extended periods.

Ineffective fumigation

Fumigation with phosphine is a common component of many integrated pest control strategies. Taking fumigation shortcuts may kill enough adult insects in grain so it passes delivery standards, but the repercussions of such practices are detrimental to the grains industry. Poor fumigation techniques fail to kill pests at all life cycle stages, so while some adults may die, grain will soon be reinfested again as soon as larvae and eggs develop. What’s worse, every time a poor fumigation is carried out, insects with some resistance survive, and pass the resistance gene into their progeny making control more difficult in the future.

Monitoring Grain Pitfall TrapEffective fumigation

Using the right type of storage is the first and most important step towards an effective fumigation. Only use fumigants, like phosphine, in a pressure-tested, sealed silo. Research shows that fumigating in a storage that is anything less than pressure sealed doesn’t achieve a high enough concentration of fumigant for a long enough period to kill pests at all life cycle stages. For effective phosphine fumigation, a minimum of 300 parts per million (ppm) gas concentration for seven days or 200ppm for 10 days is required. Fumigation trials in silos with small leaks demonstrated that phosphine levels are as low as 3ppm close to the leaks. The rest of the silo also suffers from reduced gas levels. Achieve effective fumigation by placing the correct phosphine rates (as directed on the label) onto a tray and hanging it in the top of a pressuretested, sealed silo or into a ground level application system if the silo is fitted with recirculation. After fumigation, ventilate grain for a minimum of one day with aeration fans running, or five days if no fans are fitted. A minimum withholding period of two days is required after ventilation before grain can be used for human consumption or stock feed. The total time needed for fumigating is 10-17 days. As a general rule, only keep a silo sealed while carrying out the fumigation (for example, one to two weeks). If grain moisture content is low (8-12%) the silo can remain sealed after fumigating but regular monitoring is essential to check for insect infestation and moisture migration to the head space.

Table 1 Effect of Grain Temp and Moisture

Monitoring

When grain is put into storage it needs monitoring just like it does when it’s in the paddock – regularly. Check stored grain at least monthly, taking samples from the bottom, and if safe, the top of the storage.

Things to monitor:

  • Insect pests
  • Grain temperature
  • Grain moisture content
  • Grain quality and germination

Storage choices

When buying a new silo, buy a quality, sealable silo fitted with aeration and check with the manufacturer that it meets the Australian Standard for sealable silos (AS2628). Experience has shown that at least two sealable, aerated silos on farm provide the option for an effective fumigation and delivery program. Many older silos are not designed to be sealed and cannot be used for fumigation, however retrofitting aeration can reduce insect multiplication through grain cooling.

Seed held on farm (cereals — wheat, barley, oats)

Seed that is dry, cool and sound (not weather damaged) will remain viable for longer. In well-managed storage, germination percentages can be expected to reduce by only 5 per cent after six months. To achieve this, keep grain moisture content below 12%. Grain temperature also has a major impact on germination. Aim for grain temperatures of 20°C and below in seed storage by using aeration cooling (with auto control). Wheat at 12 per cent moisture content stored at 30-35°C (unaerated grain temperature) will reduce germination percentages and seedling vigour when stored over a long period. Position small seed silos in the shade or paint them reflective white to assist keeping grain cool. WA growers can treat seed with a grain protectant combined with a dyed grain fungicide in combination with aeration cooling to maximise insect control.

Pulse and oilseeds

Insect control options are limited for stored pulses and oilseeds. Aeration and phosphine fumigation are the main methods and controlled atmosphere (inert gasses such as carbon dioxide or nitrogen) may be an option. The effectiveness of phosphine fumigation on oilseeds is often reduced due to phosphine sorption during treatment. Monitoring gas concentrations with a gas monitor is essential to ensure the correct concentration is achieved for the correct length of time. Use sound grain hygiene in combination with aeration cooling to reduce insect activity. Small seed-size grains, such as canola, may need larger-capacity aeration fans to combat the greater amount of back pressure in the storage. Always store these grains at their recommended grain moisture content level.

Table 2 Resistence and Efficiancy Guide Stored Grain 2010

Fig 1 Phoshine Resistence - National Situation

Phosphine resistance is widespread – plan, monitor and control for clean grain

  • Dispose of grain residues and seed gradings. Clean empty storages and grain handling equipment, including harvesters, field bins and augers.
  • Sieve stored grain for the presence of insects at least monthly, or use pitfall traps. Also check grain temperature and moisture.
  • If grain temperature has been kept below 15°C by aeration, live insect numbers are likely to be low.
  • Sample grain three weeks before sale to allow time for any treatment.
  • For effective fumigations, pressure test sealable silos at least once a year to identify any leaks and ensure rubber seals are maintained.
  • Phosphine fumigation typically requires 7 to 10 days in a gastight sealed silo. When completed, open silo top with care, ventilate using aeration fan for one day; if not aerated, open silo top and ventilate for five days. The minimum withholding period is then two days after ventilation is completed. The total time needed for fumigation is therefore 10-17 days.
  • Sieve a half-litre sample onto a white tray. Hold tray in sunlight to warm for 20 to 30 seconds to encourage insect movement.
  • If live insects are found, identify them and fumigate in a gas-tight silo according to the label.
  • Take care when climbing silos to sample grain for insects and wear a safety harness. Sample from the base, and if safe, take a sample from the surface of the grain.