Controlling Static Part II

Precautions should be taken to prevent vapors from being ignited by the discharge of static electricity. The movement of ANY process liquids during pumping, emptying, filling and spraying can cause electrostatic accumulation. Movement of other materials, such as powders or a printing web, or cleaning operations, can also create static electricity hazards. Non-conducting footwear and clothing made of synthetic fibres can cause incendive electrostatic sparks, especially if they are worn in areas with non-conducting flooring!!!

To protect against electrostatic accumulation all metal (or other conducting) components must be adequately grounded before ANY liquid flow begins. All fixed equipment used to handle flammable liquids should be electrically bonded together and adequately grounded.

• Check ground continuity regularly (at least annually).
• Grounding contacts should be maintained and kept clean.
• Portable containers MUST also be grounded before use with bonding clips connected by a wire to the fixed ground or by some other equally effective method. Bear in mind any ungrounded metal components in a transfer systems, such as in valves, dispenser heads or sections of piping: these may need additional grounding straps.
• Other ungrounded components may not be so obvious, or may result from poor maintenance. If uninsulated wire reinforcement, in damaged pipe lagging, for example, is left exposed it may build up electrostatic charges if it is subjected to steam from leaking joints.
• If hoses are used, they should be electrically continuous and bonded to both parts of the plant.

Transfer Speeds

Static electricity generated by flammable liquids should be reduced by avoiding the free fall of liquids and restricting pumping speeds. For liquids with conductivity up to and including 50 pS/m (pico Siemens/meter) the flow velocity in a pipe where a second phase may be present (a second phase could be a gas or a solid, or an immiscible liquid such as water) should not exceed 1 m/s. Water may be present even if it has not been deliberately introduced (for example, condensate) and so a flow velocity above 1 m/s should only be considered if this is not a possibility.

The major hazard involved with the transfer of flammable liquids is the build up of static due to charge separation with potential for discharge resulting in fire and subsequent loss of containment. Certain non-polar liquids can be charged (e.g. while flowing through pipe). Detectable and hazardous charges must be expected if the specific resistance of the liquid exceeds 10⁸ W.m.  The potential for accumulation of static charges may strongly increase if the liquid contains a non-miscible component or a suspended solid. Examples include:

• Crystallisation processes in toluene;
• Quantities of water in toluene.

With the presence of a second phase, velocities less than 1 m/s MUST be employed.  Measures that can be employed to reduce these hazards include:

• Ensure that the pipe transferring the liquid is completely filled to exclude the formation of explosive mixtures;
• Wherever possible ensure no contaminants / solids are present;
• Utilise inert gas blanketing/purging;
• When transferring flammable liquids by “blowing” use an inert gas;
• Avoid mechanical mixing or agitation of low conductivity liquids wherever possible;
• Use of ball valves with grounded metal balls;
• Employ low transfer velocities. For only partially filled pipes, or pipes which discharge into containers, the velocity is to be limited as follows:

• • For chargeable esters: maximum 10 m/s;
  • For mineral oil products (e.g. gasoline, petrol, kerosene, paraffin, jet fuel) and for other chargeable liquids (excluding carbon disulphide and ether):

If these velocities are adhered to, no hazardous charges will be generated within homogeneous liquids. But when suspensions of crystals in non-conductive liquids are conveyed, hazardous charges may always be generated, even at velocities below 1 m/s.

• For ether and carbon disulphide in pipes up to a diameter of 25 mm, the maximum velocity should not exceed 1m/s. Larger pipes require lower velocities;
• A general rule for all homogeneous liquids (except carbon disulphide and ether) and all pipes: at velocities below 1 m/s, no dangerous charges will be generated;
• Flanges should be bonded;
• Use sub-surface dip pipes or bottom entry filling when discharging into vessels;
• Ensure regular inspection and testing of bonding.

Splash Filling

Splash filling of containers and process vessels MUST be avoided. Anti-static additives can be used to increase the conductivity of flammable liquids with a conductivity less than 50 pS/m, but in general terms allowing the practice of splash filling should be avoid with ALL flammable and combustible liquids. 

Powder Additions
Take particular care when adding dry powders to vessels containing flammable liquids/vapors. The movement of dry powders and other solids over insulated materials such as plastics can quickly cause incendive electrostatic sparks. Dry powders should not be added to process vessels containing flammable vapors using plastic scoops or plastic chutes, nor should they be discharged from plastic bags: if the vapors ignite, flames will be emitted from the vessel top manway directly onto workers. Consider removing workers from vulnerable positions and adopting the following measures to reduce the possibility of an accumulation of static and/or preventing the occurrence of flammable atmospheres:

• mechanical transfer systems
• double-valved hopper systems
• use of conducting plastic liner with drum earthing clip or with good earthing of operator
• airlock and nitrogen purging
• charging solids before solvent addition (check for vapors from previous use)
• wet or slurry solid before addition
• addition of solvents with a high conductivity before ones with a low conductivity if both are to be used

Additional precautions include the use of anti-static footwear and anti-static clothing and floors.