Preventing a bucket-load of trouble – protecting against dust explosions in bucket elevators
27 April 2017
Mark Shannon of BS&B Safety Systems explains the value of using chemical suppression and isolation to protect bucket elevators from dust explosions.
Bucket elevator system: Image: BS&B
There can be many potential ignition sources inside a bucket elevator. Dust explosions in bucket elevators are a real risk. Whether it’s
• a misaligned slipping belt causing friction sparks;
• spark risk coming from upstream process equipment like mills or driers;
• static charging of fines accumulated in the boot or housing.
Understanding your dust
We always recommend dust testing. It may influence the level of explosion protection you need. Laboratory dust testing is designed to identify two key performance characteristics of dust:
• The first measures maximum pressure of a dust explosion (Pmax in bar)
• The second identifies the rate of the rise in explosive pressure (Kst in m/sec)
Magnesium powder for example, has a Kst of approximately 500 bar m/s and is highly reactive. In comparison wheat flour may only have a Kst value of around 110 bar m/s – a significantly slower rate of explosion pressure rise; nonetheless, it can still cause explosions in grain and flour mills.
Chemical suppression and isolation systems often work in tandem. They are designed to detect and extinguish an explosion at a very early stage (the point of ignition). These cannons can be installed in the boot, head and legs of a bucket elevator system. If ignition/ explosion does occur, a flame can propagate at high speed up or down the legs and spread to other parts of the connected system whilst building explosive pressure.
The BS&B System V is a rapid response chemical suppression system that activates at the very start of an explosion. It prevents flame propagation and isolates interconnecting process equipment. Its installation could prevent crippling equipment damage, loss on production time and possible injury to personnel. When the Cannon™ deploys, it releases sodium bicarbonate to fully suppress/ isolate a dust explosion. Cannon deployment is prompted by a low voltage electrical signal generated from sensors in a smart controller unit. The actuators release pressurised nitrogen gas towards a sodium bicarbonate canister which discharges its contents at high velocity in milliseconds. A possible deflagration travelling through ducts, piping and connected equipment is quickly and efficiently extinguished. Suppression cannons are fitted to the boot and head of the bucket elevators.
Isolation Cannons™ work similarly and can be most effective if fitted to the legs of the bucket elevators. Isolation Cannons are strategically placed on lengths of ductwork, as due to their size, shape they allow a explosion to swiftly propagate along its length. The same applies to the legs of a bucket elevator which act in the same way. Installing a Cannon stops the explosion propagation up and down the elevator legs.
To find out more: www.bsbipd.com.
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