Pages From Handbook Of Fire And Explosion Protection Engineering Principles 6u4o5r

152

Handbook of Fire and Explosion Protection Engineering Principles

If a gas contains liquid, water vapor or solid particles such as rust particles or dirt, a static charge can be generated. Generally static electricity can be overcome or controlled by several basic approaches: bonding, grounding or controlled generation. Bonding tries to achieve a common electrical potential on all equipment so that a charge does not have to opportunity to accumulate. Grounding is the process of electrically connecting one or more conducting objects to a ground potential to dissipate the charge buildup in a safe manner. Most process facilities are provided with a grounding grid. The primary purpose of the grounding grid is to limit the effects of corrosion induced by charges, but it also serves as a means to dissipate electrical charges that could be a source of ignition.

14.11

Lightning

Lightning is generally considered a form of static electricity that is discharged from particles in the atmosphere. Many instances of lightning-induced hydrocarbon fires have been recorded, especially at atmospheric storage tanks. NFPA requirements state that if equipment, process vessels or columns, and tanks are suitably constructed of substantial steel construction that are adequately grounded, and do not give off combustible vapors, no other mechanism of lightning protection is required. This is also true of flares, vent stacks, and metal chimneys by nature of their construction and grounding facilities. As most storage tanks release combustible vapors as seals and vents, they are susceptible to lightning-induced fires. Common European practice is to provide lightning rods on the highest vessel at a facility to provide a cone of protection. NFPA 780 provides additional guidance for the provision of lightning protection measures. Direct lightning strikes can ignite combustible contents of cone roof storage tanks unless the roof is provided with bonding for the structural . Floating roof tanks with seal hangers in the vapor space may be ignited indirectly when charges on the roof are released by a nearby lightning strike. Floating roof tanks are commonly protected against lightning ignition by bonding the floating roof to the seal shoes at not less than 3 meters (10 ft) intervals, use of insulating sections in the hanging linkages, covering sharp points on hangers with insulating materials, and installation of electrical bond straps across each pinned hanger t. Buildings that are more than 15.2 meters (50 ft) high, that contain combustible liquids in large amounts or store explosive materials, should be provided with lightning protection measures in accordance with the requirements of NFPA 780. Ships with steel hulls or masts have suffered little or no damage from lightning and no special protection measures are considered necessary. During loading or unloading of vessels it is common practice to suspend operations and close all openings into tanks during the appearance of lightning storms.