Purified hydrocarbon liquids have 100 000 000 times greater ability to retain a static charge than does distilled water.

One method of measuring static accumulation is the "half-value time" which is the time required, in seconds, for the original charge in a substance to drop to one-half of the original value. The following table lists the half-value times of some substances:

The time required for a static charge to decrease to an acceptable level in purified hydrocarbons is very long.

Control Of Static Electricity

Many of the problems associated with static electricity can be removed, or at least reduced, by simply "draining off" the static charge as fast as it is produced. Bonding and grounding are two methods used. Bonding and grounding are two different procedures, and are not always inter-changeable when removing static charges.

Bonding

An electrical bond attached to both conducting bodies can prevent sparking. This bond will prevent a difference in potential across the gap by providing a conductive path through which the static charges can re-combine. Thus, a charge cannot accumulate, and a spark cannot occur.

Because of mechanical considerations, static bond wires are usually large, and the resistance of the wire is low. An 8 gauge wire with competent connectors should be used to bond all equipment being used in the blending and pumping process. The connector attachment sites should be non-painted surfaces, and free from rust.

Grounding

Two sources of grounding should be used on all high flash hazard sites.

The wellhead may be used for grounding provided it is properly connected to the pumping or blending equipment. Another grounding source must be a supplementary one, such as a rig anchor driven into the ground to a depth of not less than 2.5 - 3 m or a 0.65 m by 0.65 m (minimum) plate buried not less than .3 m in the ground.

Belts

Belts made of rubber, leather, or other types of insulating materials, running at moderate or high speeds, can generate considerable quantities of static electricity. Generation occurs when the belt separates from the pulley, and charges will occur on the pulley (regardless of whether it is conducting or non-conducting) as well as on the belt.

If the pulley is made of conducting material, the charge will normally be dissipated through the shaft and bearing to ground, and will not create an ignition hazard. In some cases, however, where the machinery frame is insulated, or the bearings are composed of insulating materials, such as nylon, bonding and grounding may be required.

To prevent static charges when using belts and rollers, consider the following precautions:

• Use conductive rubber belting
• Regularly apply a conductive belt dressing
• Install static brushes or combs
• Install a continuous strip of copper foil to the pulley side of slow moving belts (this will ground the belt to the pulley)
• Bond metal bins to the conveyor frame
• Ground all rolls and pulleys not electrically connected, by using spring-loaded brass brushes.

Wearing Apparel

Under the right conditions, a great many fabrics can generate static electricity. This can occur when the fabric is brought into contact with electricity, when the fabric is brought into contact with other materials and then separated, or when the fabric is rubbed on other surfaces.

Most synthetic fabrics (nylon, orlon, dacron and rayon), are more active generators of static electricity than natural fabrics.

Rubber and leather-soled shoes can generate static when the wearer walks on dry carpeting or other non-conductive surfaces during periods of low humidity.

The potential for a fabric to generate electricity should be considered, and appropriate steps taken to minimize the potential.

The reported incidence (in the petroleum industry), of synthetic fabrics being the cause of ignition of static electricity, suggests that this hazard is not significant.