Every once in awhile I come across something that is worthwhile repeating and sharing with others.  usually some lesson regarding safety or how to improve a process, measurement or material sensing.  In this case I ran across a white paper written by Michael O'Brien, Product Manager for Newson Gale, Ltd. with headquarters located in Nottingham, England (I wonder who the sheriff is? - I just couldn't resist that commentary).  They have a location in Jackson, NJ as well.  This was of interest to me, not just because of the safety aspect to protect against dust explosions (see a dust explosion story by clicking here) but because of the impact static discharge can have on level sensors.

Not all dust explosions are started by static electricity discharges, but it does happen and sometimes the source of ignition is not a certainty.  The previously mentioned white paper indicates that "walking across a carpet can generate 35,000 volts on a person".  When you combine this type of voltage potential on a highly conductive metal object the energy discharge can be above that which is known to create explosions in the presence of flammable vapors.  This has even happened at gas pumps:



Static discharge can occur often during the processing, conveying, handling and storage of bulk solids.  Consider a pneumatic conveying system which is used to move or transfer plastic pellet or resin from storage silos to machine hoppers.  Since most pipe or tubing, and vessels are made from aluminum or steel the resistance to electric charge transfer is very low (aluminum is 2.82 x 10 to the -8 power; dang how do you make the -8 superscript in TypePad LOL).  This means that static will discharge from these metals (your piping, chutes and bins) very easily!

Static discharge not only can create a hazardous explosion, it can also be the source of damage to sensors in contact with the material or the material bins and piping.  Those sensors commonly affected are RF Admittance/capacitance units and the Capacitive Proximity Switch, both used often.  The later tends to be more sensitive to the effects from static discharge because the former are usually manufactured with adequate protection against some level of static discharge.

So how do we address the problems associated with static discharge in an industrial bulk solid processing operation?  The standards all seem to agree, as the aforementioned white paper points out, proper grounding of processing, conveying and storage equipment!  The white paper points out that this is the single most important thing you can do to prevent static discharge caused explosions.  By the same action you remove a possible threat to your sensors.  How good does the ground have to be?  Good.  Less than 10ohms resistance between the metal components and ground.  Static electricity must discharge quickly to the earth ground plane within the facility to protect your facility against hazardous dust explosions and to protect your sensors attached to this equipment.

Having spent the last 35 years of my life immersed in a wide variety of aspects in the process measurement and control industry I was recently asked for a recommendation regarding a level control device that would be able to control a bin filling operation and be "safe".  What a soapbox I was handed. 

The powder and bulk solids industry seems as though it lags in adopting some technology, at least from an instrumentation standpoint.  However, it isn't just the risk averse nature of most powder and bulk processors, but it is that much instrumentation technology developed for use with liquids (80% of the overall measurement business in level measurement) does not cross over directly or seamlessly to powder and granular material level measurement.  It takes time to adapt technology for the challenges of bulk solids.

The same exists in the area of instrument "safety".  Driven by large legal and financial ramifications in the chemical and petrochemical area, as well as standards writing bodies such as the global IEC (International Electrotechnical Commission), most "safety" related approaches are quickly developed and adopted in liquid oriented industries, such as chemical, petrochemical, pharmaceutical etc.  So when asked, I summarize the four levels of basic safety regarding the function or output of a level measurement or monitoring device.

I recently posted to the Just Level Expert blog the following list.  You might want to setup an RSS feed from that blog so you don't miss anything.

SAFEST: Material detection sensors certified to SIS and SIL according to IEC 61508 and 61511.  Refer to this White Paper to explain more about this safety level.  There are level indicators certified at this level, but primarily technology for liquid applications.  It's an interesting subject if you are involved with process safety.  Check it out.

SAFER: Self-validating level sensor technology that verifies its health and communicates it to you.  This is only available with rotary paddle technology for solids and doesn't eliminate moving parts, the drive motor, and comes at quite a hefty price tag.  Fail-Safe rotary paddle, RF and vibrating element devices are just as reliable and protect against many sensor failures, those that occur due to power disruptions.

SAFE: Material detection sensors that are FAIL-SAFE so the sensor output indicates an alarm if the power supply to the sensor fails.  Rotary paddle units offer the highest value with this capability, but make sure you ask specifically for a unit that has a fail-safe output on power failure.  RF and vibrating element sensors commonly provide this capability on a standard basis.

TAKING YOUR CHANCES: This is any material detection sensor that does not offer at least the SAFE level or above.  This is common with some brands of rotary paddle bin level indicators, proximity sensors and also with tilt switches and diaphragm switches.  Consider switching these out for the "SAFE" kind.

One final point, thinking about relying on your inventory level sensor for control?  Don't!  And don't take my word, listen to what the accident investigation report said regarding their findings and recommendations regarding the fuel depot explosion and fire late in 2005 that was estimated to cost 1 billion pounds (that's pounds sterling British currency).  You can read about that in an editorial column from September 2009.


Got a question?  Give us a call at 888-61LEVEL or 815-625-2224.  Email us at bluelevel@blueleveltechnologies.com.

Many of us work in companies that process bulk solid materials.  Often this processing produces dust., lots of dust.  That's why there are dust collection systems and many companies that specialize in that area.  Unfortunately sometimes there are hazards we don't know about, or at least we may not be aware of them.

In a recent review of the reports coming from the investigation of the accidents in Buncefieeld England and at the BP facility in Texas City where failure of level detections systems contributed to major accidents, I was made aware of a CSB video that is really pretty cool, though scary and horrifying at the same time.

The USA Chemical Safety Board investigates accidents after they occur.  They have spent considerable time investigating numerous accidents involving dust explosions and fires.  This is well worth the viewing of this 30 minute video. 

CLICK HERE TO WATCH VIDEO

Remember, even the seemingly most benign dust can be hazardous.  The reason dust hazards could be more serious than those as a result of vapors is because awareness and focus on dust related hazards is relatively low by comparison. Dust can be dangerous, be very respectful of it.

Joe Lewis