What do the standards recommend for specific applications?
Road Tankers / Tank Trucks:
NFPA 77, CLC/ TR: 60079-32-1 and API RP 2003 recommend that the first procedure in road tanker material transfer operations is to earth the tanker prior to any other operation being carried out by the driver. Interlocking static earthing systems, with earth status indicators, should also be specified so that if the road tanker is not protected from static discharges due to incorrect earthing, the system will not permit the flow of product thereby eliminating the generation of electrostatic charges. The static earthing system should monitor the resistance in the earthing circuit ensuring it does not rise above 10 ohms. CLC/TR: 60079-32-1 specifies 10 ohms or 100 ohms as being suitable for convenience in monitoring, however 10 ohms would be the established standard for large companies with a good track record in static control safety.
Grounding of tank truck is critical to ensuring static charges do not accumulate on the tank body.
Railcars / Tank cars:
API RP 2003 and NFPA 77 identify parts of the railcar that could become isolated from the railway tracks. Non-conductive wear pads and bearings can be located between the barrel and the chassis and it is recommended that the barrel is earthed prior to and during, the material transfer process. This will prevent the accumulation of static charges on the barrel and eliminate the risk of discharges from the barrel to the fill pipe, and discharges to people or other grounded bodies. NFPA 77 (8.8.2) states:
“Many tank cars are equipped with nonconductive bearings and nonconductive wear pads located between the car itself and the trucks (wheel assemblies)….. Therefore, bonding of the tank car body to the fill system piping is necessary to protect against charge accumulation”.
API 2219 provides guidance on the protection of vacuum trucks when they are used to suck up flammable or combustible products. Examples include waste collection during storage tank cleaning operations and the suction of combustible powders from dust collection chambers. Of the many recommendations outlined in API 2219, the most relevant instruction is to fully ground the truck by connecting it to “a designated, proven ground source”, before commencing with transfer operations. The “ground source” describes an object with a low resistance connection to earth (ground). The standard also states the importance of confirming that the connection resistance between the truck and the designated grounding point is less than 10 ohms and that this resistance should be verified with the use of an ohmmeter (or some other type of measurement device).
As highlighted earlier, people, through regular movement, can generate large potential differences on their bodies and the accumulation of static charges should be minimised so that operators do not discharge sparks in the presence of ignitable atmospheres. The advice in the standards is to ensure hazardous area floors are conductive and that operator’s working in these areas should be wearing static dissipative shoes to prevent charge accumulation on their body. CLC/TR: 60079-32-1 states the shoes should be tested prior to entry into the zoned (classified) area:
“All personnel should be earthed by means of conducting floors and footwear (see 9.2 and 9.3). A personnel resistance monitor should be installed at every entrance to any area where such footwear is required.”
When portable metal containers like IBCs and drums are being filled or dispensed from the advice in all standards is to earth and bond all of the filling system and the receiving vessels. NFPA 77 (220.127.116.11) states that:
“Bonding should be done with a clamp that has hardened steel points that will penetrate paint, corrosion products, and accumulated material using either screw force or a strong spring.”
Earthing clamp fitted with a pair of hardened tungsten carbide points and high pressure torsion spring.
There is some guidance in the standards regarding the use of plastic lined metal containers. NFPA 77 states that a metal container, with a plastic of lining of 2mm, or less, can be treated as a metal drum. CLC/TR: 60079-32-1 contains a table with the maximum recommended lining thicknesses and emphasises the importance of ensuring that their interaction with conductive objects does not promote high energy propagating brush discharges. In general, filling insulating plastic containers is not recommended. If a conductive liquid is being processed it may be possible to dissipate charges using a conductive dip tube, however, the use of dip tubes carry extra precautionary measures.
CLC/TR: 60079-32-1 provides the clearest guidance in respect of resistance values that should be maintained when filling or discharging from FIBC Type C bags. NFPA 77 states that the bag should be grounded but does not specify a maximum resistance value. CLC/TR: 60079-32-1 states:
“The conductive fabric and the conductive threads or filaments shall have a resistance of the earthing point on the FIBC of less than 1 x 10^8 Ω.”
Type D bags are designed to dissipate charges from bags through “low energy” corona discharges. The potential drawback, however, is that the bag could induce charges on surrounding conductive objects. CLC/TR: 60079-32-1 states:
“any conductive items that may not normally be earthed (e.g. drums on pallets) should either be earthed or removed from the vicinity of the FIBC Type D. In addition, conductive items (e.g. working tools, bolts and clips) shall not be placed or stored on the FIBC.”
Identifying and controlling electrostatic hazards can be a challenging process for those of us responsible for ensuring our colleagues, employees, equipment and property are fully protected from electrostatic ignition hazards. There are many factors that can contribute to the presence of a static hazard but if the examples of grounding and bonding protection outlined above can be followed, the majority of processes at risk of static discharge will be controlled and accounted for. When an audit of a process or procedure has identified an electrostatic ignition hazard, it is important to specify grounding and bonding systems that can demonstrate compliance with the standards. Where possible, static grounding instruments that can demonstrate resistance levels recommended by the standards will ensure companies are protected from this ever-present and hazardous ignition source.
Mike O’Brien, Managing Director for Newson Gale
If you have any questions relating to the topics discussed in this article,
please contact Newson Gale.