Thursday September 9 2021
Damage to silos occurs most commonly during the refilling process. Without silo filling protection, the risk of overfilling is significant and can lead to a number of problems.
Such problems include the release of product into the atmosphere. Not only does this represent a financial loss for the silo owner, but there is also the potential to release hazardous materials into the environment causing danger to workers and other people on site. In this case, the operator could end up facing more than just lost product. Legal claims and potential fines might be incurred too, simply because the right tools weren’t in place.
Overfilling may also cause internal issues or damage that prevents the silo being able to fulfil its function. This includes problems such as blocked pipes, meaning the product will not evacuate properly and will need to be dug out before the problem can be rectified, or damage to the discharge mechanism, again requiring product to be dug out.
With the probable need for external contractors to help repair the problem, not to mention the loss of product, this can be a costly mistake for silo operators and one that can mean the silo is out of commission for days. However, there is a far more dangerous problem that can result from overfilling a silo, and one that has the potential to cause major damage and even injury.
Although silos have a degree of strength, they’re not designed to cope with excessive pressures and therefore, tactics must be employed to ensure they don’t become over-pressurised.
Over-pressurisation occurs when fluidised powders such as cement are blown from the delivery tanker into the silo by air. The air used to carry the powder from the tanker to the silo is vented via filters at the top of the silo.
These filters have a fully operational life of 2,500 hours and then deteriorate preventing the dispelled air from escaping, which will then start to lift the pressure relief valve so excess air can escape. If the filters are not changed the relief valve will open every time a tanker discharges product into the silo and release fugitive emissions to air and eventually excess residue will build around the relief valve making it faulty.
Also, if the silo has been overfilled and cannot vent adequately, over-pressurisation may result. The consequences of this can be severe and include ejection of the filter system or silo lid, or even a complete rupturing of the silo, which along with being extremely dangerous, will also be very costly.
There are a number of essential operations to carry out to ensure over-pressurisation doesn’t occur. These include:
The filter systems at the top of a silo serve two purposes. They enable the air used to carry the powder into the silo to escape, to prevent over-pressurisation, and they filter out any product that hasn’t settled out of the suspension and prevent it from leaving the silo.
However, as a result of this, they can soon become blocked, stopping the air from being able to escape and causing pressure to build up inside the drum.
Therefore, it is essential to regularly clean these filters as part of a silo maintenance plan. This is usually done by reverse jet cleaning which involves firing short blasts of air against the normal direction of flow of the filter, and by mechanical shaking to release the powders.
The length of time the shaking takes place for will depend on the type of product stored in the silo.
As the filters are designed to have an operational life of 2,500 hours, keeping a log of those hours is essential to ensuring they are changed before they malfunction.
There are two basic types of press relief valve employed by silos. These are a deadweight valve and a spring-loaded valve.
With a deadweight valve, the weight of the valve lid keeps the pressure relief system closed until the pressure in the silo exceeds the lid weight. Once this point is met, the pressure relief system is opened, and the air is discharged into the atmosphere.
With a spring-loaded valve, the spring provides the resistance but once pressure builds up to beyond a set threshold, the valve is released and again the air is discharged.
Keeping the valves in full working condition is essential to avoid the over-pressurising a silo. Areas to pay particular attention to are the lid hinges where dirt and old product might build up and cause them to become stiff. Ensuring lids are not tight or stuck where they meet the silo is also essential, as anything that makes them difficult to open could lead to pressure building up.
Many silos employ a self-cleaning system that ensure it is functioning at its optimum performance and blockages and build ups are prevented. Such systems also ensure the filters are kept clear and prevent the build-up of excess residue around the pressure relief valve.
Therefore, it is vital to ensure the system is operational. Self-cleaning systems should be regularly checked and any issues discovered dealt with immediately.
Managing the rate of product discharge from a tanker is essential when dealing with fluidised powders. Sometimes, demand for product is such, or the tanker driver might be on a deadline, that there will be pressure to rush the discharge. But this is where mistakes can happen.
Rushing the discharge can easily lead to overfilling of the silo, blocking the vents and then the inevitable build up in pressure.
Discharge rate is controlled by the tanker driver who does this by regulating the flow of air and of powder down the delivery line.
If this is done without adequate monitoring, product can build up quickly until the silo is overfilled, blocking air vents and filters, and stopping air from escaping.
However, this is not the only way a silo can become over-pressurised during the tanker discharge process. Surges in air can also lead to this problem.
Surges in air can result from inadequate monitoring of powder levels in the tanker and at the end of the discharging procedure when the tanker contains residual air pressures significantly higher than atmospheric pressure. If this happens, the airflow into the silo will be high until the pressure in the tanker returns to atmospheric levels. However, this might not happen quickly enough to avoid over-pressurisation.
Ensuring that the mechanical aspects of the silo are working, such as vents and valves, and that the tanker driver discharges product in a safe manner are essential to avoiding overfilling your silo, and the dangerous build up of pressure. However, the final piece of the puzzle which is equally important is a monitoring and warning system.
Having a reliable system in place that can warn of product overfill and pressure build up will ensure these problems are far less likely to arise.
Such a system also helps prevent overflow spills, control surge bins, detect plugged chutes and automatically switch off control pumps and conveyors when a problem has been detected to ensure processes don’t get out of sync.
There are two types of sensors employed in silo warning systems. These are point level sensors and continuous level sensors.
Point level sensors indicate the presence of material at a particular point in a silo bin. Many operators favour these because they require little or no maintenance and are very durable, meaning they rarely need to be replaced.
If considering point level sensors, we would recommend the Bindicator Roto-Bin-Dicator® PRO paddlewheel along with the Bindicator VRFII® Series capacitance probe, an on/off device with relay or switch outputs.
Continuous level sensors monitor the level of product in a silo as it rises and falls. This can be done via guided wave radar, through-air radar and by weight.
Although these sensors are more sophisticated than point level monitors, they’re designed to be robust and be able to take consistent measurements in steam, dust, and other challenging conditions to give an accurate measure of the amount of product in a silo at any one time.
Our top picks for continuous level sensors are Bindicator TDR2000 guided wave radar, the MP Series through-air radar, the Yo-YoTM Series cable and weight sensor, and, specifically for dry bulk and powder inventory management, the F78MP series radar, also by Bindicator.
Both point level and continuous level sensors are compatible with MySilo, an online portal that allows you to monitor the amount of material in one or multiple silos remotely to prevent overfill and better track when they need to be refilled.
MySilo brings significant efficiencies to a business by enabling silo operators and business managers to be in complete control of their silo levels, making for better forecasting, less deliveries, and less vehicle movements on site.
Silo operators, other relevant staff and even suppliers can sign into the online portal to check levels and ensure there is enough material to keep all projects ongoing and it can also be set to send customisable notifications by text or email and online when re-ordering needs to take place.
As well as preventing dangerous overfills and the resulting over-pressurisation, MySilo enables companies to safely record data and measure silo contents without having to check it manually. It also allows for silos with windows to be tracked accurately with the sensors in case of blockages near the glass, showing a false representation of the contents amount.
The inventory data recorded by the sensors enables operators and managers to make more informed and accurate decisions, as they are able to plan the projects easily through real time monitoring.
To find out more about Bindicator sensors or MySilo, get in touch.
For further reading on the topic and more specifically the prevention of storage silo overpressurisation during road tanker deliveries of non-explosive powders, check out this great guide published by the Mineral Products Association.