Transferring non-conductive flammable liquids

A major hazard involved with the transfer of non-conductive flammable liquids is the build-up of static electricity with the potential for discharge resulting in fire and subsequent loss of primary containment, an LOPC event. Certain non-polar liquids can be charged (e.g., while flowing through pipes and hoses). Detectable and hazardous charges must be expected if the specific resistance of the liquid exceeds 10⁸ ohms (Ω.m).

The potential for accumulation of static charges can vastly increase if the liquid contains a non-miscible component or a suspended solid. For example:

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

With the presence of a second phase, velocities less than one (1) m/s should be built into the design of the process.

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;
• Utilize inert gas blanketing;
• When transferring flammable liquids by ‘blowing across, use an inert gas;
• Avoid mechanical mixing or agitation of low conductivity liquids wherever possible;
• Use of ball valves with earthed metal spheres;
• Employ low transfer velocities. For only partially filled pipes, or pipes that 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 homogenous 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 pipelines 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 pipelines: at velocities below 1 m/s, no dangerous charges will be generated;
• Flanges MUST be bonded;
• Use sub-surface dip pipes or bottom entry filling when discharging into vessels;
• Ensure regular inspection and testing of bonding and grounding

Powder Transfers

Powder transfers can be carried out by several different methods:

• Screw conveying;
• Vacuum transfer;
• Pneumatic conveying;

Two distinct types of pneumatic conveying are used for powder transfer, namely low pressure / dilute phase or high pressure / dense phase. Low pressure / dilute phase systems tend to employ high system velocities ranging from 10 to 25 m/s, whereas high pressure / dense phase systems tend to employ low system velocities ranging from 0.25 to 2.5 m/s.

Intensive charging of the conveyed material and pipeline is possible during pneumatic powder transfer potentially resulting in:

• Electrostatic discharge between conductive parts (e.g. between metal flanges and a part of the steel structure of the building);
• Entrainment of considerable charges into receiving containers.

Powders can be divided into three (3) groups depending upon the volume resistivity of the material composed of the particles. These groups are:

• Low resistivity powders (e.g., metals having volume resistivities up to about 10⁶ Ω.m);
• Medium resistivity powders (e.g., many organic powders, such as flour, having volume resistivities in the approximate range 10⁶ Ω.m to 10⁹ Ω.m);
• High resistivity powders, (e.g., certain organic powders, many synthetic polymers and some minerals, such as quartz, having volume resistivities above about 10⁹ Ω.m.)

Measures that reduce these hazards include:

• Ensure piping used for pneumatic conveying are made from metal with good grounding/bonding. Resistance to ground for all conductive components should be < 10 ohms;
• Ground all operators loading powder so that their resistance to ground is < 1 x10⁸ ohms;
• Avoid the use of insulating coatings on the inner surfaces of metal containers and pipes;
• Use plastic flanges with plastic transfer lines;
• Avoid the use of coating or sheathing on pipes constructed of insulating material;
• Use antistatic plastic or paper bags in or around flammable gases, vapors, or dusts having minimum ignition energies of < 4 mJ;
• Discharge powder into the container or silo via intermediate loading equipment (e.g., a cyclone fabricated from a conductive material to reduce velocities and earth charge. (Alternatively, rotary valves, bag dump hoppers, or scroll feeder systems can be employed).

Source:  UK’s Health and Safety Executive