One option for mitigating the static hazards associated with low conductivity liquids is to raise the conductivity of the liquid. This can be accomplished by either blending the low conductivity liquid with a conductive liquid (e.g., alcohol, ketone) or by adding a static dissipating additive into the low conductivity liquid.
When blending a low conductivity liquid with a conductive one, the “rule of thumb”, which works mostof the time, is that the amount of conductive liquid should be at least 15-20 percent of the volume of the poorly conductive liquid. A conductivity measurement is needed to assure that the conductivity of the blend is adequate.
The other method of increasing liquid conductivity is to add a small amount of commercially available “conductivity enhancer” or “antistatic additive.” These additives are used in very low concentrations, typically in the parts per million range, and are usually added to the liquid by the supplier. The users of these products should be aware that the functionality of these additives will be reduced over time (they plate out on tank and pipe walls and can be leached out by water). Some filter systems, especially clay filters, may remove these compounds.
When handling low conductivity solvents, the following precautions should be taken:
- Ground ALL conductive (metal) objects that come in contact with the low conductivity liquid, such as funnels, scoops, hose fittings, pails and other metal containers.
- Minimize the free fall of low conductivity liquids. Ideally, these liquids would be transferred using a fill pipe that discharges below the surface of the liquid with the flow velocity limited to about 3 ft./sec. until the pipe outlet is fully submerged. Where this is not possible, direct the liquid to the wall of the receiving container to ensure that the stream does not break up. Always remember that “slower is better” to minimize static charge generation.
- When agitating low conductivity liquids, keep the rotational velocity of the liquid as low as the process will allow.
- minimize splashing in the vessel or air entrainment (vortexing) and avoid the use of high speed, high shear agitators for these liquids.
- leave the closed and grounded containers and vessels of recently transferred low conductivity solvents undisturbed so the charge can be dissipated through relaxation. Several minutes will usually be sufficient for charge dissipation; however, keep in mind that longer relaxation time means less retained charge.
- It may be necessary to inert equipment where low conductivity flammable liquids are being processed. Inerting is a procedure whereby the oxygen in the equipment (e.g., mixing tank, filter, centrifuge) is replaced with an inert gas such as nitrogen in order to eliminate the possibility of a fire or explosion. NFPA 69, Explosion Prevention Systems gives details on inerting.