Safety First - or Is It?

“Sure!” everyone might say. It makes sense right? When it comes to controlling your manufacturing process – including the storage and handling of powder and granular materials - you want to make sure that the process is safe for employees and the public, i.e. local community. So why is cost, arguably, a leading reason for many decisions regarding the control of powder/granular material storage?
    
What happens when the high level indicator on a storage vessel fails? The unknown failure of a level sensor used to stop the filling process of a storage vessel can result in disastrous consequences. Take as one example from liquid storage applications, the explosion of the Buncefield fuel depot on December 11, 2005. Located in southeast England, the sleepy community of Hertfordshire was suddenly and viciously awakened at 6:01am by the first explosion at the facility. By the time the accident was over and flames extinguished, 20 fuel storage tanks and a majority of the facility was destroyed, 43 people were injured and 2000 more evacuated from their homes. The cause of the explosion? A failed level sensor used for inventory measurement and fill control, which had no high level control backup.
    
While dust explosions are rarely caused by bin or silo overfilling, dust explosions - no matter the cause - can be deadly and costly. Overfilling a silo of plastic pellet or cement powder will not typically result in an explosion, but can be costly. The results of an overfilled silo? 1) Loss of material, 2) Cost to clean up the spill, 3) Cost of production shutdown due to spill, 4) Cost of bad PR when powder is spewed into the environment and settles on nearby private property and employee vehicles, 5) EPA problems for some materials and situations, and 6) Damage to dust collectors, bin vents, and other equipment.
    
Often I see bulk solids storage silos with no level instrumentation at all, for inventory and/or fill control. I still see companies relying upon manual checks of material level and refilling when these manual checks indicate a full truck of material can fit in the silo, or the company relying on the driver of the truck to know when to stop filling. It is often incorrectly assumed that this is a safe and inexpensive method. Companies often do not effectively consider employee safety. In addition, this manual method is an inconsistent, and perhaps ineffective, way of controlling an important process function.
    
Many companies have instrumented their silo inventory with measuring devices to provide either the weight of the silo or to measure the changing level of the material within the silo. This is a good step to eliminate manual labor, enhance safety, and perhaps even improve on insurance rates because of elimination of the routine silo climbing for inventory measurement. Often the same level sensor used for inventory monitoring is also used for fill control as well. This can lead to reliance on a single device rather than implementation of a fill control system specifically designed for that purpose in order to provide a consistent and safe process of filling the silo.
    
Using a level sensor for fill control is not uncommon. But often this level sensor is of the low cost rotary paddle variety and only used to indicate an alarm when the high level is detected. Rotary paddle units may not be the most reliable given their moving parts and their propensity for failure of drive motors without warning, possibly leading to the same results: overfilling and all of its safety and cost issues.

So what should be done? My recommendations are simple:

1. Do not store material in a silo without a fill control system.
2. Do not use your inventory monitoring system to control filling without a backup.
3. Consider the self-validating type of rotary paddle level control for filling control purposes. This unit can detect a sensor failure and provide a warning of this condition.
4. USE the failure warning of the aforementioned level control device, many people do not.
5. Consider a solid-state level sensor that has a longer life; RF admittance, capacitance, vibrating rods, or tuning forks.
6. Close the loop! Use a fill shutoff system with visual/audible alarm, timer and valves. This provides emergency closing of the fill lines to prevent overfilling.

    Joe Lewis is responsible for directing and leading Aplus Finetek Sensor Inc. USA operations. He has more than 35 years in the process measurement and control industry, a BSEE from Roger Williams University, and an MBA from Bryant University, both in Rhode Island. He is a senior member of the ISA and has spent the last 20+ years in various business, marketing, and production roles for powder and bulk solids instrumentation companies. He can be reached at 815-632-3132 or [email protected]. For more information on Aplus Finetek Sensor Inc., visit www.aplusfine.com.

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