Silo protection – solving one problem but creating another!
Industry transports millions of tonnes of powders every year by road tanker.
The loads are discharged into silos by fluidising the powder and blowing it in at pressures up to 2.0 bar g.
The air carrying the powder is vented at the top of the silo through filters, to prevent dust being released into the atmosphere. If the air entering the silo is not adequately vented, this method of delivery can result in over-pressurisation and consequent rupture of the silo, ejection of the filter system or even the complete silo roof. Most silos are not pressure tested and it is generally accepted that the maximum safe pressure is less than 100 mbar.
Should the filter system become blocked because of overfilling or poor maintenance, a pressure relief valve will vent any excess pressure, preventing any damage to the silo, personnel, or property in the vicinity.
However, the release of powder and dust into the surrounding atmosphere will be treated as air pollution; preventing one problem, but creating another!
The Health & Safety Executive has worked with various trade associations in the cement, concrete and quarrying industry to produce guidance notes on how to prevent these occurrences.
The Department for Environment Food and Rural Affairs (DEFRA) has also produced guidance notes for handling bulk cement, where the primary aim to is to prevent any visible emissions of powder.
While both H&SE and DEFRA are focusing on the cement and construction industries, it is clear that this guidance is appropriate for any industry that uses pneumatic conveying from road tanker to storage silo.
The guidance notes agree that a silo protection system should be fitted, that automatically prevents over-filling or over-pressurisation.
Silo protection systems have been used within the powder and solids industries for many years, but to understand how these systems work and how current systems have now evolved, we must first break the system down into its component parts.
Every silo protection system should consist of at least, a pressure sensor, a high level probe and a pressure relief valve.
The pressure sensor is the heart of the system and its purpose is simple; to detect the pressure in the silo during fill conditions and provide an output to alarm when the pressure in the silo is reaching critical levels. This unit needs to be rugged, but also sensitive enough to detect pressures between 0 and 100 mbar g. The high level probe is to warn if the silo is becoming too full and so prevent the filters from blocking up.
If both of these elements fail, the last form of defence is the pressure relief valve whose sole purpose is to vent to atmosphere any excess pressure.
Pressure relief devices capable of relieving pressures above 50 mbar, and with peak flow rates up to 13,000 m3/hr should be fitted on all powder silos.
As before though, solving one problem can invariably lead to another. In this case, protecting the silo using pressure and level components prompts the need for them to be tested regularly and that, in the past, has involved someone climbing the silo and putting themselves at risk. Advances in technology over the last few years have seen several level probe manufacturers develop sensors that can be tested from the safety of the ground, at the base of the silo, by pressing a button; Hycontrol’s capacitance and vibration probes are examples of this. The ability to test the sensor function remotely removes the potential risk of slip, trip and fall from the top of the silo.
Until now, no manufacturer had produced a pressure sensor that could easily be tested in-situ. The FLEX 500 pressure sensor breaks new ground, by making a ground level test of the pressure sensor possible.
Using a small 1” BSP mounting, the FLEX 500 can be installed relatively easily into the silo top. Two connections to it are required: one electrical to the panel at the foot of the silo and the other pneumatic from the air supply that cleans the filters. When the ground level test (GLT) button is pressed, a miniature solenoid valve allows regulated filter air into the sensor chamber. If the sensor is healthy the output will range over the operational scale and confirmation of a true satisfactory test will be displayed. The status of the sensor can then be determined as blocked, damaged or healthy. Regular testing of the pressure sensor has an added bonus in that it clears the sensor face of any dust build up every time it is carried out. This test can be automatic as part of the silo fill procedure in the associated control panel if required.
Both of the above mentioned process guidance notes refer to the need to be able to test sensors prior to receiving a delivery, or at the very least on a scheduled basis.
Fitting sensors that can be tested remotely removes the need to climb the silo to test them, reducing the risks, improving safety and satisfying the long term aim of reducing emissions.
If you would like more information on silo protection or silo level measuring systems please contact Hycontrol at:
Silo Protection by Hycontrol
Silo protection – solving one problem but creating another!
Industry transports millions of tonnes of powders every year by road tanker.
The loads are discharged into silos by fluidising the powder and blowing it in at pressures up to 2.0 bar g.
The air carrying the powder is vented at the top of the silo through filters, to prevent dust being released into the atmosphere. If the air entering the silo is not adequately vented, this method of delivery can result in over-pressurisation and consequent rupture of the silo, ejection of the filter system or even the complete silo roof. Most silos are not pressure tested and it is generally accepted that the maximum safe pressure is less than 100 mbar.
Should the filter system become blocked because of overfilling or poor maintenance, a pressure relief valve will vent any excess pressure, preventing any damage to the silo, personnel, or property in the vicinity.
However, the release of powder and dust into the surrounding atmosphere will be treated as air pollution; preventing one problem, but creating another!
The Health & Safety Executive has worked with various trade associations in the cement, concrete and quarrying industry to produce guidance notes on how to prevent these occurrences.
The Department for Environment Food and Rural Affairs (DEFRA) has also produced guidance notes for handling bulk cement, where the primary aim to is to prevent any visible emissions of powder.
While both H&SE and DEFRA are focusing on the cement and construction industries, it is clear that this guidance is appropriate for any industry that uses pneumatic conveying from road tanker to storage silo.
The guidance notes agree that a silo protection system should be fitted, that automatically prevents over-filling or over-pressurisation.
Silo protection systems have been used within the powder and solids industries for many years, but to understand how these systems work and how current systems have now evolved, we must first break the system down into its component parts.
Every silo protection system should consist of at least, a pressure sensor, a high level probe and a pressure relief valve.
The pressure sensor is the heart of the system and its purpose is simple; to detect the pressure in the silo during fill conditions and provide an output to alarm when the pressure in the silo is reaching critical levels. This unit needs to be rugged, but also sensitive enough to detect pressures between 0 and 100 mbar g. The high level probe is to warn if the silo is becoming too full and so prevent the filters from blocking up.
If both of these elements fail, the last form of defence is the pressure relief valve whose sole purpose is to vent to atmosphere any excess pressure.
Pressure relief devices capable of relieving pressures above 50 mbar, and with peak flow rates up to 13,000 m3/hr should be fitted on all powder silos.
As before though, solving one problem can invariably lead to another. In this case, protecting the silo using pressure and level components prompts the need for them to be tested regularly and that, in the past, has involved someone climbing the silo and putting themselves at risk. Advances in technology over the last few years have seen several level probe manufacturers develop sensors that can be tested from the safety of the ground, at the base of the silo, by pressing a button; Hycontrol’s capacitance and vibration probes are examples of this. The ability to test the sensor function remotely removes the potential risk of slip, trip and fall from the top of the silo.
Until now, no manufacturer had produced a pressure sensor that could easily be tested in-situ. The FLEX 500 pressure sensor breaks new ground, by making a ground level test of the pressure sensor possible.
Using a small 1” BSP mounting, the FLEX 500 can be installed relatively easily into the silo top. Two connections to it are required: one electrical to the panel at the foot of the silo and the other pneumatic from the air supply that cleans the filters. When the ground level test (GLT) button is pressed, a miniature solenoid valve allows regulated filter air into the sensor chamber. If the sensor is healthy the output will range over the operational scale and confirmation of a true satisfactory test will be displayed. The status of the sensor can then be determined as blocked, damaged or healthy. Regular testing of the pressure sensor has an added bonus in that it clears the sensor face of any dust build up every time it is carried out. This test can be automatic as part of the silo fill procedure in the associated control panel if required.
Both of the above mentioned process guidance notes refer to the need to be able to test sensors prior to receiving a delivery, or at the very least on a scheduled basis.
Fitting sensors that can be tested remotely removes the need to climb the silo to test them, reducing the risks, improving safety and satisfying the long term aim of reducing emissions.
If you would like more information on silo protection or silo level measuring systems please contact Hycontrol at:
https://edir.bulk-online.com/profile/9471-hycontrol.htm
Attachments
hycontrol_news_27.9.05 (JPG)
■