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.

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.

Storing Oilseeds

Stored Grain oilseed storage

Storing oilseeds is more difficult than storing cereal grains as they are more susceptible to quality deterioration and have limited insect control options.

The decision to store oilseeds requires a planned approach, careful management and a suitable storage system.


  • Limited chemical control options for insect pests in stored oilseeds increases the importance of careful management and planning.
  • Aeration cooling is a requirement for storing oilseeds to maintain seed and oil quality, limit insect reproduction and prevent mould.
  • Moisture content in oilseeds must be lower than cereal grains because the oil content increases the risk of moulding and quality damage.
  • Successful phosphine fumigation requires a gas-tight silo.
  • Extra care is required to prevent chemical residue contamination from structural treatments.

The rate of quality deterioration in stored oilseeds depends on the quality of grain placed in storage and management of temperature, moisture content and insects. The quality of oilseed is generally measured by testing the free fatty acids (FFA), oil colour, oil content, residues of unregistered chemicals, insects or evidence of insect damage, moulds and mycotoxins. With the exception of oil content, storage management affects all of these qualities.


Storing quality oilseed

Weather or mechanically damaged seed will deteriorate more quickly during storage than high-quality seed. When oilseeds become damaged, processes such as oxidation are in progress and difficult to slow.

A dull grey and brown seed coat colour in canola is one indicator of weather damage. Another indicator is the colour of crushed seed — weather-damaged seed will be light brown in colour compared with the normal bright yellow.

The formation of FFA and off-flavours in oil will occur more rapidly in damaged seed. Canola with FFA levels greater than one per cent is more difficult to store and may be rejected at receival sites.

Why the low-moisture content?

It is often asked, why canola has to be stored at low-moisture content of 7–8 per cent. The oil content in oilseeds is higher than that in cereal grain. If the moisture content of canola is converted to an oil-free basis, canola with 45 per cent oil content and eight per cent moisture content is equivalent to cereals at 14.5 per cent moisture content.

Image: Lining up to store oilseeds: Storing oilseeds on farm can provide benefits in harvest logistics and marketing opportunities, but requires careful management and the right storage system to maintain quality.

Cereals stored at 14.5 per cent moisture content are highly susceptible to quality loss, mould growth and insect infestation. Canola at the equivalent eight per cent moisture content requires aeration cooling. Canola stored at high moisture or oil content is also at risk of spontaneous combustion.

Temperature and moisture

When grain is cool (below 20°C), mould growth and insect development are significantly reduced and cooling below 18°C will stop most insects from breeding and mould from growing.

Unlike cereal grains (which typically contain only about two per cent oil by weight) oilseeds contain from 17 per cent in soybeans and up to 50 per cent oil content in canola. The oil fraction of the seed absorbs minimal moisture, meaning a small increase in moisture content can quickly produce self heating and seed damage.

The general recommended storage conditions for canola are below 25°C and below 7 per cent moisture content but vary according to the oil content. Figure 1 shows the safe moisture contents for canola stored at 25°C, depending on its oil content. For example, canola with an oil content of 35 per cent can be safely stored at 8.5 per cent moisture content and below 25°C. But canola with higher oil content of around 50 per cent must be stored below 6.5 per cent moisture content and 25°C to be safe. The recommended safe storage for sunflowers at 40 per cent oil content is below nine per cent moisture content, or below 7.5 per cent moisture content if oil content is higher than 50 per cent.


The first step towards insect control is done before any oilseed even goes into storage — meticulous grain hygiene. Due to the limited treatment options to control insect pests in oilseeds, the first line of attack is removing them from the storage site before harvest. Cleaning silos and storages thoroughly and removing spilt and leftover grain removes the feed source and harbour for insect pests.

For more information see the GRDC fact sheet, Hygiene and structural treatments for grain storages.


Insect control

The common insects found in stored oilseed are:

  • Rust-red flour beetle (Tribolium castaneum)
  • Saw-toothed grain beetle (Oryzaephilus surinamensis)
  • Indian meal moth (Plodia interpunctella)
  • Warehouse moths (Ephestia spp.)
  • Psocids (Liposcelis spp.)

These insects tend to favour the top of the grain stack and around silo outlets. Sample both of these sites regularly. Under optimum breeding conditions of about 30°C, insects can complete their full life cycle in as little as four weeks.

Reducing the grain temperature with aeration cooling plays a vital role in lengthening the insect breeding life cycle or in most cases stopping reproduction if cooled below 18°C.

VapormateTM and phosphine are the only registered fumigants for controlling insects in oilseeds. For oilseeds, pyrethrins and diatomaceous earth (DE) are only suitable for use as structural treatments and must never be applied directly to the grain.

If using DE as a structural treatment:

  • follow label instructions carefully,
  • do not exceed the label rates, and
  • check receival standards with your grain buyers before applying.

For more information see the GRDC fact sheet, Hygiene and structural treatments for grain storages.


Insect pests commonly found in stored oilseeds

VapormateTM can only be applied by a licensed fumigator, which leaves phosphine as the only insect treatment option for growers to use on farm.

The key to successful phosphine fumigation is to apply only in gas-tight, sealed silos. This will ensure the phosphine concentration reaches the lethal dose for the required period to kill insects at all life stages. Fumigating in a non-gas-tight silo is likely to only kill a percentage of the adult insects leaving the eggs, larvae, pupae and remaining adults to reinfest the grain.

When buying a new gas-tight sealable silo, which is covered under the Australian Standard 2628, it must pass a five-minute half-life pressure test. For existing silos, a three-minute half-life pressure test will provide gas-tight conditions adequate for a successful fumigation. For more information see the GRDC fact sheet, Pressure testing sealable silos.

Most oilseeds absorb phosphine gas during the fumigation so it is vital to use the correct label dose rates and to follow the required ventilation periods stated on the label. The phosphine fumigation exposure period must be extended to 10 days if grain temperature is between 15°C and 25°C. For more information see the GRDC booklet, Fumigating with phosphine, other fumigants and controlled atmospheres.

Aeration cooling

Aeration is an essential storage tool for oilseeds. Correct management creates uniform, cool conditions in the seed bulk and slows most quality deterioration processes.

Aeration cooling:

■ Maintains oil quality — colour, low FFA, odour and rancidity.
■ Reduces the risk of ‘hot spots’, moisture migration and mould development.
■ Slows or stops insect pests’ reproduction cycle.
■ Maintains germination and seed vigour for longer.

Aeration cooling can be achieved with air-flow rates of 2–3 litres per second per tonne. Operating an aeration fan for cooling requires a planned control program, which is best achieved with an automatic aeration controller.

A critical point to managing aeration manually is to avoid running fans for extended periods when the ambient relative humidity is above 85 per cent. Selecting air well below this ensures moisture from the air is not increasing the grains moisture content especially as oilseeds are more susceptible to quality loss.

When managing aeration cooling, the initial aim is to get maximum air-flow through the grain bulk as soon as it enters storage, to prevent it from sweating and heating. After the aeration fans have been running continuously for 2–3 days to flush out any warm, humid air, reduce run time to 9–12 hours per day during the coolest period, for the next seven days.

This initial reduction in grain temperature of 10°C ensures grain is less prone to damage and insect attack, while further cooling becomes a more precise task.

During the final phase of longer-term storage, automated aeration controllers generally run fans during the coolest periods of the day, averaging 100 hours per month. For more information see the GRDC fact sheet, Aeration cooling for pest control.


Canola restricts airflow

Air on: Aeration cooling is essential for storing oilseed and may require extra capacity to achieve 2–3 l/s/t in small oilseeds like canola.Compared to aeration cooling of cereal grains, canola being a much smaller seed adds significantly more back pressure to the aeration fan. This means that an aeration cooling system set up to produce 2–3 l/s/t in cereal grain will typically be reduced to 40–60 per cent of that when used in canola.

When setting up a storage unit to cater for cereals and canola, consider using several smaller fans rather than one big fan for cooling. When canola is stored, all the fans will be needed to achieve the 2–3 l/s/t but when cereal is stored one or some of the fans can remain turned off to save using unnecessary power.

Other factors that also affect the amount of airflow through the grain are:

■ depth of the grain in storage,
■ the amount of un-thrashed and foreign plant material in the grain, and
■ the size of the motor driving the fan.

The area and type of ducting must also be adequate to disperse the air through the storage and not be blocked by the small canola seeds. Avoid splitting airflow from one fan into multiple silos as the back pressure in each silo will vary and unfavourably portion the amount of airflow to each. This will be exacerbated if different grains are stored in each silo such as canola in one and a cereal in the other.

Aeration drying

Aeration drying can be a valuable harvest tool for oilseeds but requires purpose built equipment capable of airflow rates of at least 15–25 l/s/t and adequate ventilation. Even with these higher airflow rates, aeration drying is a much slower process than aeration cooling and requires careful management. In saying that, as an example, sunflower seed can be aeration dried successfully with ambient conditions of 30°C and 30–50 per cent relative humidity. For more information on the possibilities for aeration drying and how to manage it, refer to the GRDC booklet, Aerating stored grain, cooling or drying for quality control.

Types of storage

The ideal storage for oilseeds is a cone-bottom silo fitted with aeration and sealable for gas-tight fumigation. Generally oilseeds are higher-value grains as well as being prone to mould and insect attack, so should be given priority over other grain to the best storage facility on the farm.

The ideal oilseed storage will have:

■ Aeration cooling, with an automatic controller.
■ Aeration ducting suitable for small seeds.
■ Easy ways to inload and outload causing minimal seed damage from machinery.
■ Access to the top of the grain stack to monitor temperature, moisture and insect activity.
■ Quality sealing to meet the minimum three-minute half-life pressure test for gas-tight fumigation with phosphine.
■ An easy-to-clean structure so grain can be put into a clean storage, free of insect pests and unregistered pesticide residues.

Storing oilseeds successfully requires careful management with a planned approach and a suitable storage system. Always remember that oilseeds carry a higher risk of fire during harvest, drying and storage.