API RP 545:2009 pdf download

API RP 545:2009 pdf download.Recommended Practice for Lightning Protection of Aboveground Storage Tanks for Flammable or Combustible Liquids.
Al Phenomenon of Lightning and Secondary Effects on Tanks
All Introduction
This section summanzes the present knowledge on the lightning flash process and attachment mechanism.
A.l.2 Lightning Principles
Under lair weather conditions, there is nonnally a steady but weak vertical electric field at the earths surface with virtually no ground currents and a small d.stnbuted charge on the ground. Horizontal flat surfaces will have a very uniform charge distribution, (I.e. a similar surface charge density everywhere). The highest surface charge occurs on thin pointed objects such as church spires, tops of aerials, tips of lightning rods, etc. Where the surface charge is highest, the local electric field is the highest. Sharp-pointed, upward-facing items will tend to discharge a small current Into the air, such as aerials and lightning air terminals This will often be a silent, invisible discharge in the order of a micro amp.
Electrical storms involve the relatively slow movement of heavily charged clouds. Charging mechanisms in the storm build up an electrostatic field over a large area across the base of the storm cloud. This field induces an opposite charge on the surface of the earth beneath it. This induced ground charge flows along the surface of the earth beneath the storm cloud at a relatively slow rate. The charging current flows are relatively small and cause no damage. This charge differential is periodically neutralized almost instantaneously by a lightning stroke that collapses the field. At that time, a heavy ground current flows toward the lightnwig attachment point equalizing local ground charge distribution.
The lightning process starts in the douds, with a stepped leader descending to earth. The stepped leader often exhibits branchw’ig on its path to the ground as it attempts to find the best route to the ground. The path & the stepped leader is very irregular because of random variations in the local air conditions and other factors, When the stepped leader is within about 100 m (334 ft) or less from the tank (or ground), the electric field at ground level rises sharply, and the electric field on the highest items becomes great enough to launch an upward streamer towards the down- coming leader In fact, two or more streamers may rise almost simultaneously from ground objects (tanks, vents, trees, etc.) but only one usually is successfii in making the connection to the downward leader (see Figure Al). This is the usual mechanism by which a lightning stroke completes its path to the ground.
A1.3 Lightning Electrical Parameters
A complete lightning discharge is called a flash. Each cloud-to-ground flash is composed of one or more lightning strokes. Over 90 % of cloud-to-ground flashes are of negative polarity (Uman. The Lightning Discharge, p. 8). A typical negative cloud-to-ground flash contains three to four strokes, but may have as many as 30 (Uman, Alt About Lightning. p. 41). The currents in any one stroke can range from lust a few kilo amperes (kA) to over 200 LA.
The csrent Ni a typical negative cloud-to-ground stroke has several components, as listed below and as illustrated in Figure A.2.
— Component A—First return stroke.
— Component B—Intermediate current
— Component C—Continuing current
— Component D—Subsequent return stroke, if present, followed by additional B and C components. etc., until the completion of the discharge
A.1.5 Sparking
Sparking is the most hkely cause of tank fires from hghtning in external floating roof tanks (EFRTs) owmg to the tendency for current from any stroke on the tank or closely nearby to dnve the current across the floating roof, via the shunts or via any other metal making intentional or unintentional contact between the floating roof and the shell.
Thermal and alr-gap sparks shouki be considered as follows.
a) A thermal spark s defined as a minute piece of incandescent material which has been ejected from some sparking site, usually a place where tens or hundreds of amps or more are passing through a very poor )aint. such as the contact point from a shunt on to the Inner shell wall of an open floating roof tank, or from a pooely bolted flange joint, etc. The white-hot metal sparks falling from welding operation are examples of thermal sparks. wtNch are actually very small particles of metal, burning as they fly through the air Usually, they are less effective as incendiary sources than air-gap sparks.
b) An air-gap spark occurs In a location with a small gap between conducting items where the lightning creates a voltage Large enough to cause electrical breakdown of the aw or vaporlalr mixture in the gap.