Introduction

Static electricity is a major cause of fires and explosions in many industries. The hazard of electrostatic spark ignition of flammable vapor can be minimized by taking actions to limit the accumulation of electrostatic charges to safe values. Of primary importance is the proper bonding and grounding of equipment and containers. In addi­tion, charge accumulation in liquids must be limited, in many instances, by controlling the rate of charge generation and/or the rate of charge dissipation. Occasionally, such methods cannot be applied, and the use of inert gas in vapor spaces must be considered.

This bulletin addresses the hazards of static electricity, created by the handling and processing of flammable liquids.

Sources of Static Generation

The most common generators of static electricity are processes involving flammable liquids. Static electricity is generated by liquids flowing through pipes, and in mixing, pouring, pumping, filtering, or agitating liquids. The rate of generation is influenced by the conductive of the liquids, the amount of turbulence in the liquid, the interfacial surface area between the liquids and other surfaces, liquid velocity, and the presence of impurities.

Some specific locations where static electricity is generated include:

• Piping Systems - In piping systems the generation rate and the subsequent accumulation of static charge are a function of the flow rate, liquid velocity, pipe diameter, and pipe length.
• Filling Operations - The turbulence experienced in filling operations, caused by large flow rates, splashing or free-falling liquids, greatly increases the charge accumulation above the level generated in piping systems.
• Filtration - Filters, because of their large surface area, can generate as much as 200 times the electrostatic charge generated in the same piping system without filtration.
• Dispersing Operations - Of all operations in the coatings industry, dispersing operations can be particularly hazardous in view of the extremely high rate of charge generation when particulates are present. With poorly-conductive liquids the charge accumulation can cause hazardous sparking in the vapor space, such as to an exposed agitator blade in a mixer or to a conductive fill pipe. High charge generation rates can also occur when liquids are mixed, thinned, tinted or agitated.

Methods of Static Control

In addition to being dependent on the charge generation rate, charge accumulation is a function of the resistance of the path by which charges dissipate. Within a liquid, the dissipation of static electricity is dependent on a property of the liquid known as “conductivity”. Some flammable liquids have very low conductivities and tend to accumulate static charges. Toluol, an example of such liquid, has a long history of causing fires. (See Table 1 for conductivity data on some pure liquids from Lange’s Handbook)

Although the generation of static electricity cannot be eliminated, its rate of generation and accumulation can be reduced by the following procedures:  

• Piping Systems - The most effective method of reducing the accumulation of static charges in piping systems is through proper pipe sizing to keep liquid velocities low. A recommended maximum velocity in piping systems is 15 feet per second. Table II lists the flow rates for various pipe sizes for a velocity of 15 feet/sec.
• Filling Operations - Splash filling and free-fall of flammable liquids should be eliminated to the maximum extent practical by lowering fill velocities, by providing diverters to direct the discharge of liquid down the side of the grounded vessel being filled, or by submerging fill pipes below the liquid level in the vessel. Submerging of fill pipes in paint manufacturing vessels may not always be practical. In bulk-filling operations the velocity of the incoming liquid should not exceed 3 feet per second until the pipeoutlet is covered; the velocity may then be increased to the 15 feet per second maximum mentioned previously. Table II also lists the flow rates for various pipe size for the velocity of 3 feet per second.
• Filtration - Experience has shown that this hazard maybe controlled by installing filters far enough upstream of discharge points to provide a 30 second liquid relaxation time prior to discharge. The required relaxation time depends upon the conductivity, the liquid velocity, and the type of filter. For example, the 30 second relaxation time may not be necessary with a conductive liquid.
• Dispersing Operations - For dispersing operations, the conductivity of the liquid should be raised1, if necessary, to above 2000 conductivity units (C.U.) (2 x 10-5micromho/cm) before particulates are added. If possible, polar solvents should be added before non-polar solvents or particulates are added. Polar solvents are more conductive than non-polar solvents. In some instances, proprietary anti-static agents, developed for use with fuels, can be used as additives to reduce the charge accumulation. Typically, only a few parts per million of the additive are required. Tests should be run to ensure that the conductivity additive does not cause formulation problems. The additive may not be suitable for use in coatings for food containers.

If the conductivity cannot be raised to the recommended value, the vessel should be inserted. Pebble mills present an additional hazard because their porcelain lining is an insulator that will prevent the flow of static charges from liquid to ground, even if the mill is grounded. This hazard is best controlled by inserting the mill. The rate of charge dissipation on most solid surfaces can be increased by raising the humidity. It should be noted that the static accumulation in liquids cannot be controlled by raising the ambient humidity.