Improper handling of low conductivity liquids is a source of static energy that causes flammable vapor ignition. Low conductivity liquids will accumulate a static charge from liquid shear charging at the pipe or vessel wall. The charge in a low conductivity liquid can NOT be dissipated by grounding.
Low conductivity liquids are typically derived from distilling or fractionating hydrocarbon feedstocks. Toluene, xylene, hexane, heptane, naphtha, mineral spirits, gasoline, kerosene and jet fuel are examples of low conductivity liquids.
Low conductivity liquids will accumulate charge simply by flowing through a pipe or hose, or by being agitated.
Factors affecting the charging rate of these liquids are:
- Liquid velocity
- the amount of turbulence in the liquid
- entrained solids
- gas bubbles or an immiscible second phase
Once charged, the ONLY WAY to dissipate the charge is either by adding sufficient quantities of a compatible conductive liquid or by time-dependent dissipation or “charge relaxation time”. Charge relaxation time is a measure of the amount of time it takes for the charge to dissipate to a safe level and is a function of liquid conductivity and dielectric constant.
With conductive liquids such as ketones and alcohols, the relaxation time is in the microsecond to milliseconds range; the relaxation time for low conductivity liquids is in the range of seconds to hundreds of seconds.
Brush discharges can occur from the surface of low conductivity liquids when a rising level of this charged liquid in a container approaches a grounded metal projection into the tank, such as a thermo-well, fill pipe, level probe or a grounded metal gauging rod held by an operator.
When low conductivity liquids flow through ungrounded metal components of a transfer line, such as a hose, metal fittings on the hose can become charged. When the fitting touches a grounded surface, a spark will occur. Since charge generation is affected by the speed of separation, it follows that charge generation is closely tied to the liquid velocity. A universally accepted “safe” velocity for low conductivity liquids or liquids with an unknown conductivity is 3 ft./sec.; at this velocity, the liquid will not accumulate a significant static charge.
Splash filling (i.e. free fall) of low conductivity flammable liquids MUST be eliminated to the maximum extent practical by submerging fill pipes below the liquid level in the vessel. Submerging fill pipes in vessels with agitators may not always be possible; in this case, DIVERTERS, which direct the discharge of liquid down the side of the vessel, should be considered.
In bulk filling operations, the velocity of the incoming low conductivity liquid should not exceed 3 ft./sec. until the pipe outlet is covered; the velocity may then be increased to 15 ft./sec.