Static electricity is all around us, and if steps aren't taken, it can pose dangerous threat to terminal operations
Aug 1, 2012 12:00 PM, By Rick Weber
You reach for a doorknob and receive a brief — but painful — shock due to static. Now imagine the potential for damage when static builds up in a terminal operation.
“Static electricity is the ‘unseen danger’ in hazardous areas,” said Graham Tyers of Newson Gale, a company specializing in the design, development, and manufacture of hazardous-area static-grounding systems, devices, and accessories. “Static is a naturally occurring phenomenon. It's all around us, whether at work or at home. Sometimes it's just a nuisance.
“But static is a big problem for many industries. It's a major financial loss in many industries where a few volts of static charge could wipe out semiconductors or communications equipment. For industries with flammable/combustible atmospheres, static discharge can have disastrous consequences. There are close to 300 serious incidents per year in the United States. And worldwide, there are more than two incidents daily that are recorded or reported.”
Tyers discussed static electricity concerns during a presentation entitled “Static Hazards for Routine and Non-Routine Operations” that he gave during the 32
He explained the process like this: Relative motion leads to charge separation. Surfaces come together and separate, causing electrons to be shed from one to another, creating an imbalance. Energy in the form of a spark will accumulate and eventually discharge in the absence of a low resistance path to the earth/ground.
At a typical operation in a terminal — for example, filling a tank truck — it looks like this.
“As liquid is flowing into a tank truck, materials are coming together and separating,” he said. “The tank truck and liquid form an opposite charge. In fact, a static charge can accumulate in this way on all kinds of isolated conductive objects. Static generation itself, when you think about any kind of activity, is going to generate static electricity. But if it's left to accumulate on an isolated body, that's when you really have a problem.”
Where can a charge accumulate?
- Isolated conductive objects
“Any type of metal object that does have a connection to ground — a drum, a tank truck, or something small like a tool in a hazardous area.”
“We as people can be viewed as isolated conductors. We have inherent capacitance in our body and, if we are insulated from the ground, can start to build up a static charge.”
- Low-conductivity liquids.
“As I walk across the floor, as my feet contact and separate, a charge is starting to build up on my body. When liquids flow through piping, especially at high velocity or with some turbulence in flow, static charge can develop in the liquid or pipe wall.
“What's happening is that as a charge is being generated, it's creating a rise in potential on an isolated body, and that's basically a function of the speed of generation and the charge and the time. The logical thing is that the longer it's left to go on, the higher the potential. When you have that potential rise and also have capacitance in the object, you start to build up potential energy.
“Most of the things we see in a terminal have these kinds of capacitance. A fairly large object like a road tanker has an electrical capacitant of around 5,000 pico farads. Even a four-inch flange has a certain amount of capacitance (10). If these objects can be raised to a fairly high potential, which actually is quite easily achieved, this could translate to a high amount of energy available for discharge. You can calculate that by using the energy equation: So with a road tanker, you can look at in excess of 2,000 mJ available for discharge. Now that may be a difficult concept, but if you compare that to a lot of solvents and flammable gases, most ignite at much less than 12 mJ. Even the smallest objects can give enough energy to create an ignition source.
“When dealing with metal objects — like a tank truck, because it's predominantly steel — it will give all that energy in a single incentive spark. And that's the most dangerous condition.”
He said most of the technical standards and industry guidelines to help solve the problem — such as NFPA 77 and API RP 2003 — point to effective grounding and bonding as the starting point to removing a static hazard from operations.
“Grounding is connecting everything to earth or ground,” he said. “So as far as any charge is generated, it's dissipated. Bonding is linking together conductive objects to all the same electrical potential. Usually they're done together.
“You also want to minimize charge generation. For instance, flow velocities of non-conductive liquids. If you reduce flow rates, you get to a point where static charge is not regenerated to such a high degree. You also want to maximize charge dissipation.”
Typical operations exposed to static hazards:
- Bulk storage tanks.
“There are some major concerns about static electricity. Rapid transfer of low-conductivity/high-resistivity products creates static-charge accumulation on liquids, loose solids, and associated plant. Hydrocarbons and solvents are among the worst liquids for charge accumulation, and dry-powdered materials are highly resistive and many can readily support combustion. Standard advice worldwide is to ensure effective earthing/grounding and bonding for tanks, pipelines, hoses, loading equipment, and tankers.
- Road tanker transfers.
“Road tankers are reasonably well-insulated by rubber tires, and therefore it is important to ensure that the vehicle is bonded to ‘earth-potential’ before starting product transfer. Typical low-grade methods may be alligator clips/welding clamps attached to copper wire. European and American guidelines recommend resistance of 10 ohms or less for tankers and associated filling equipment. The first action should be to earth the tanker before connecting hoses, loading arms, or opening any hatches or valves. There's also a recommendation to use a static ground-indicating/monitoring system, preferably interlocked with filling system. Ensure any portable metal objects in the transfer area are also bonded to earth. This means drip pans, pails, mobile platforms, etc. Grounding clamp assemblies may be used for additional bonding. Also remember people can be ‘isolated conductors’ — consider Static Dissipative (SD) footwear for all operatives in hazardous areas.”
- Rail tankers.
“International standards vary. In Europe, less importance is placed on earthing — it's assumed that railcars will be earthed by intimate contact with the steel rails. In America, NFPA guidelines also recommend railcars are earthed just the same as guidelines for road tankers. Elsewhere, companies have their own standards and procedures. In any case, it's still necessary to ensure the filling system is bonded and earthed.”
- Transloading operations.
“These are railcars used as temporary storage tanks or semi-permanent tanks in field applications (i.e. oil exploration, gathering and hydraulic fracturing operations), loading between railcar/road tanker or even smaller units (IBCs, drums), and mobile, skid-mounted systems. The need for verification of earthing and bonding is even more important.”
- Vacuum truck operations.
“This involves clearing flammable or combustible waste products from tanks, pipeline maintenance, and environmental spill-control ‘clean-up.’ They're working in remote locations without access to permanent/verified grounding equipment. Ensure that the vacuum truck is connected to a viable ground source prior to commencing vacuum transfer of flammable/combustible products. A person is a potentially isolated conductor and can accumulate substantial amounts of static charge. Correctly specified Static Dissipative (SD) footwear should be worn by personnel working in potentially flammable or combustible areas. SD Footwear should be checked regularly to make sure it is in good condition and able to safely dissipate static electricity.
- Terminal operations.
“Static electricity is a potentially serious problem for hazardous-area terminal operations. However, the problem may be controlled by following the available technical standards and guidelines and taking a three-step approach to safety: effective grounding and bonding of all metal/conductive objects and isolated conductors — including people — and minimizing charge generation and maximizing charge dissipation.”
Jim Grasty, Alltec Corp
Grasty said Alltec uses a protection pyramid methodology to deal with static.
“Rather than treating tiers as independent, Alltec regards each layer as an internal component,” he said. “When all three tiers are designed, installed, and maintained together as a total system, comprehensive safety can be achieved.
“The foundation is design installation and maintenance of a co-compliant, single-point, low-impedence grounding system. Upon this foundation, surge-protection devices are staged. Lastly, a well-designed lightning system is created to protect against direct strikes.”
He said rim-seal fires are the most common fire for floating-roof tanks, especially external floating roof tanks.
“The rim area is where the tank is most vulnerable to the existence of flammable vapors and possible sparking,” he said.
“Even internal floating-roof tanks can be vulnerable,” he said, referring to a lightning-instigated fire to an internal floating-roof tank in Greensboro, North Carolina, in 2010.
© 2013 Penton Media Inc.
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