Safer silo protection from Hycontrol
The quarrying industry transports hundreds of thousands of tonnes of powders every year by road tanker. These loads are discharged into silos by fluidizing the powder and blowing it in at pressures up to 2 bar. The air carrying the powder is vented at the top of the silo, through filters, to prevent dust being released to the atmosphere. If the air entering the silo is not adequately vented, this method of delivery can result in over-pressurization 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 damage to the silo, personnel, or property in the vicinity. However, the resulting release of powder and dust into the atmosphere will be treated as air pollution; thus one problem may be prevented but another will be created.
The HSE has produced guidance notes on how to prevent such occurrences. DEFRA has also produced guidance notes for handling bulk cement, where the primary aim to is to prevent any visible emissions of powder. These guidance notes agree that a silo protection system that automatically prevents overfilling or over-pressurization should be fitted.
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, detecting the pressure in the silo during fill conditions and providing an output to an alarm when the pressure starts to reach critical levels. This unit needs to be rugged but also sensitive enough to detect pressures between 0–100 mbar.The high level probe is to warn if the silo is becoming too full, thereby preventing the filters from becoming blocked. 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 13,000m3/h should be fitted on all powder silos.
However, as before, solving one problem invariably leads to another; in this case, protecting the silo using pressure and level components prompts the need for them to be tested regularly. In the past this 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 in safety at the base of the silo, all at the press of a button; Hycontrol’s capacitance and vibration probes are examples of this. This ability to test the sensor function remotely helps reduce the risk of slips, trips and falls at the top of the silo.
Hycontrol say that up to now it has not been easy to test pressure sensors in situ. Their Flex 500 pressure sensor, however, is said to break new ground by making ground-level testing of the pressure sensor possible. Using a small 1in BSP mounting, the Flex 500 can be installed relatively easily into the silo top. Two connections are required: one electrical to the panel at the base of the silo and the other pneumatic from the air supply that cleans the filters. When the ground-level test 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 satisfactory test will be displayed. The status of the sensor can 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. The test can be carried out automatically as part of the silo fill procedure if required.
Both of the guidance notes mentioned earlier refer to the need to be able to test sensors prior to receiving a delivery, or at 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.
Hycontrol Ltd, Larchwood House, Orchard Street, Redditch, Worcs B98 7DP; tel: (01527) 406800; fax: (01527) 406810
Should the filter system become blocked because of overfilling or poor maintenance, a pressure-relief valve will vent any excess pressure, preventing damage to the silo, personnel, or property in the vicinity. However, the resulting release of powder and dust into the atmosphere will be treated as air pollution; thus one problem may be prevented but another will be created.
The HSE has produced guidance notes on how to prevent such occurrences. DEFRA has also produced guidance notes for handling bulk cement, where the primary aim to is to prevent any visible emissions of powder. These guidance notes agree that a silo protection system that automatically prevents overfilling or over-pressurization should be fitted.
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, detecting the pressure in the silo during fill conditions and providing an output to an alarm when the pressure starts to reach critical levels. This unit needs to be rugged but also sensitive enough to detect pressures between 0–100 mbar.The high level probe is to warn if the silo is becoming too full, thereby preventing the filters from becoming blocked. 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 13,000m3/h should be fitted on all powder silos.
However, as before, solving one problem invariably leads to another; in this case, protecting the silo using pressure and level components prompts the need for them to be tested regularly. In the past this 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 in safety at the base of the silo, all at the press of a button; Hycontrol’s capacitance and vibration probes are examples of this. This ability to test the sensor function remotely helps reduce the risk of slips, trips and falls at the top of the silo.
Hycontrol say that up to now it has not been easy to test pressure sensors in situ. Their Flex 500 pressure sensor, however, is said to break new ground by making ground-level testing of the pressure sensor possible. Using a small 1in BSP mounting, the Flex 500 can be installed relatively easily into the silo top. Two connections are required: one electrical to the panel at the base of the silo and the other pneumatic from the air supply that cleans the filters. When the ground-level test 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 satisfactory test will be displayed. The status of the sensor can 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. The test can be carried out automatically as part of the silo fill procedure if required.
Both of the guidance notes mentioned earlier refer to the need to be able to test sensors prior to receiving a delivery, or at 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.
Hycontrol Ltd, Larchwood House, Orchard Street, Redditch, Worcs B98 7DP; tel: (01527) 406800; fax: (01527) 406810
