Tank filling


Tank fillig with non-conductive liquids always generates static electricity. With slow filling of pure liquid hazard may be acceptable even in non-inert conditions, but with faster filling (liquid velocity in the pipe above few m/s) or with suspended impurities in the liquid (more than 10 ppm m/m of solid particles or more than 0.5% v/v of immiscible liquid) hazardous levels of electrostatc charging may be unavoidable even at low filling velocity. The simulation below (using coupled Poisson, Navier-Stokes and Nernst-Planck equations) shows electrostatic field evolution in the 1 m diameter earthed metal tank that is being filled with highly charged liquid of conductivity of the order 10-13 S/m (e.g. hydrocarbons, ethers with impurities) where charging occurs in the pipe flow prior to entry of the liquid in the tank.

The strength of electrostatic field and liquid velocity field (arrows) at cross-section of the tank. The strength of electrostatic field is represented with colours where white means negligibly weak electrostatic field (in the sense of explosion protection) while red colour means that above the liquid level spontaneous (brush) discharges are expected to occur. It can be seen that exposed narrow and/or sharp earthed conductive objects (like level probes) tend to dramatically intensify the strength of the electrostatic field and therefore such exposed parts promote discharges. The video is time-lapsed.

Voltage
Voltage and electrostatic field arrows (logarithmically scaled) at cross-section of the tank. White colour indicates areas with voltage less than few kV with respect to ground while red indicates area with approx. 100 kV with respect to the ground.

Because contamination of non-conductive liquids usually can't be reliably prevented (e.g. contamination in delivery tanker), the only safe way to handle non-conductive flammable liquids is to perform operations with such liquids in inert conditions.

Similar charging can occur also during stirring operations in chemical reactors where the electrode for the promotion of electrostatic discharge may be sampling probe used by the operator.