reducing the risk of floating roof tank fires

Lightning Eliminators & Consultants, Inc. 303-447-2828 • www.LECglobal.com

Copyright 2008 LEC, Inc. • Rev 05/08 of 5

REDUCING THE RISK OF FLOATING ROOF TANK FIRES A New Solution for Bonding the Roof & Shell

Joseph A. Lanzoni – Lightning Eliminators & Consultants, Inc.

May 2008

Floating roof tanks are highly susceptible to fires caused by lightning currents. Yet traditional measures fail to prevent numerous tank fires every year. A new method prevents ignition of the stored product, offering a real solution for the storage industry.

The Role of Lightning in Tank Fires Tank fires caused by lightning are much more common than might be imagined. A review of petroleum storage tank fires between 1951 and 2003 (Persson and Lönnermark, 2004) found:

• 15 to 20 tank fires reported each year, ranging from rim seal fires to fires engulfing entire tank farms.

• 1/3 of the 480 tank fires reviewed were attributed to lightning. Another study (Rasmussen, 1995) revealed that:

• 16 of 20 accidents involving petroleum storage facilities in North America were due to lightning strikes.

• Of all natural causes, lightning initiated 61% of accidents at storage and processing sites. The Unique Problem with Floating Roof Tanks Floating roof tanks are widely used to store flammable liquids such as crude, gasoline, and diesel since their design helps minimize evaporation of the stored product. But at the same time, this design introduces a new problem due to the likelihood of combustible vapors at the roof-shell interface. As a result, the area above the roof to the top of the shell is classified as a Class I, Division 1 area. Seals intended to prevent these vapors from escaping become worn, cracked and damaged over time. And the tank shell itself is often found to be out-of-round due to environmental factors and repeated cycling. The shell’s inner surface can also become uneven from corrosion or petroleum residues such as paraffin and tar. With oxygen present around the tank and flammable vapors at the seal, only a heat source is needed to complete the elements of the fire triangle. How Does Lightning Cause Tank Fires? A lightning strike can be characterized by an initial fast-rise current followed by a slower, long-duration current. The fast-rise current can rise from 0 to 200kA in 100 μseconds (IEC, 2006). Independent testing has shown the fast-rise component rarely possesses the necessary duration and energy to initiate a tank fire;



therefore it is not the focus of this analysis. Instead, the key concern is discharging the long-duration current before it can ignite flammable vapors at the roof-shell interface. Whether lightning terminates on or near the tank, currents flow across the roof-shell interface in proportion to the paths of lowest impedance. But the tank’s construction causes the roof and shell to discharge at different rates, creating a dangerous voltage between the two. If not discharged quickly enough, an arc will form at the shunt, and be sustained by the long-duration current. The long duration current can last up to 0.5 seconds and transfer an average of 200C or 400A (IEC, 2006). Due to its magnitude, duration, and relatively large charge transfers, this current acts like an arc welder and will ignite materials between the roof-shell interface. This creates a shower of heavy sparks that fall into flammable vapors above or behind the seal, completing the fire triangle. Therefore, it is imperative that the floating roof be electrically bonded to the tank shell to prevent a voltage buildup between the two. This bond must have a low-impedance across a wide range of frequencies, in order to safely handle the long-duration current. The ideal bond must be also easy to install on new tanks or retrofit onto existing tanks, and must be easy to inspect, maintain, and replace if necessary. Unfortunately, none of the traditional methods meet these requirements. Drawbacks of Traditional Bonding Methods 1. Shunts NFPA 780 requires that stainless steel shunts be spaced no more than every ten feet around the roof perimeter. These shunts are bolted to the edge of the floating roof and press against the inside of the shell, creating a connection with the shell. Unfortunately, shunts do not provide a reliable bond for several reasons:

1. Heavy crude oil components, such as wax, tar, and paraffin tend to coat the inside of the tank wall, forming an isolative barrier between the shell and shunts.

2. Rust on the inside of the shell creates a high-resistance connection between the shell and shunts. 3. 10 to 25% of tanks are painted on the inside, typically with an epoxy-based paint which insulates the

shell from the shunts. 4. Large tanks can be out-of-round by several inches causing the shunts to be pulled away from the shell.

Another method uses shunts submerged in the stored product. These submerged shunts may provide some benefit when arcing occurs, since no air would be present (and therefore no flammable fuel-air mixture). However, the submerged shunts still rely on a pressure contact and are subject to all the conditions outlined above. In addition, submerged shunts are exceedingly difficult to inspect and maintain. 2. Walkway Another attempt to create a roof-shell bond relies on the tank’s walkway. Nearly all tanks have a walkway or ladder with the upper end attached to the rim of the tank, and the lower end resting on the floating roof. The quality of the electrical connection via the ladder is questionable. The upper is a bolted hinge subject to looseness, corrosion, and paint. The lower end is a pressure connection with only two wheels resting on rails, also subject to corrosion and paint.



3. Roof-Shell Bonding Cable Another method is to install a cable from the top of the shell to the middle of the roof, typically on the order of 250 to 500MCM. The cable is connected to the top of the rim near the top of the internal ladder, suspended along the bottom of the ladder, and bonded to the center of the roof. The cable must be long enough to accommodate the roof in its lowest position. For example, on a 200 foot diameter tank 50 feet high, the cable must be about 120 feet long to reach the center of the roof when the tank is empty. Although at 60Hz this cable has low impedance, at lightning frequencies it has very high impedance. For example, at 100 kHz, the impedance of 100 feet of 250MCM cable is estimated at over 32 ohms. Therefore, when thousands of amps of electricity flow across the tank, the impedance of the roof-shell bonding cable is too high to prevent sustained arcing at the shunts. The Solution As discussed earlier, the ideal solution to bonding the tank roof and shell must meet several requirements to effectively prevent tank fires caused by lightning currents. Lightning Eliminators & Consultants, Inc. (LEC) has developed a patented solution that fulfills all of these requirements. LEC’s Retractable Grounding Assembly™ (RGA™) creates a permanent electrical bond between the roof and shell. When properly applied, multiple RGAs provide low-impedance pathways to safely discharge the long-duration current responsible for many tanks fires. The RGA can easily be installed on new tanks or retrofitted to existing tanks, even while online. The RGA’s design allows for easy inspection, testing, and maintenance. The RGA attaches between the roof and shell with a wide, spring-loaded cable constructed from 864 strands of #30AWG tinned copper wire braided to form a wide, flat strap 1.625 inches wide by 0.11 thick.

LEC’s Retractable Grounding Assembly (RGA)



The cable automatically retracts and extends with the roof level, always creating the shortest electrical connection between roof and shell. Because the housing attaches to the top of the shell, and the cable attaches to the roof, it is independent of the condition of the tank wall and any shunts. The stainless steel housing provides excellent durability and corrosion protection. The RGA has been awarded the CE Mark of Conformity as well as ATEX Certification (ITSO8ATEX15806). Since floating roof tanks tend to have a very large diameter, the optimal low-impedance connection is created by installing multiple RGAs. The recommended quantity of RGAs is shown in the following graph, using the example of crude oil:

Required RGAs - Crude Oil

0

2

4

6

8

10

12

20 30 40 50 60 70 80 90 100 110

Tank Diameter, m

RG

A U

nits

23m tall18m tall13m tall8m tall

Maximum applicable tank height = 23m

Assumptions: Rground ≤ 5 Ω, 250kA strike current, 95% strike

Multiple RGAs will have lower overall impedance than any of the other options. For example, LEC recommends installing five RGAs on a 200 foot diameter (61 meter) tank storing crude oil. If this tank is 50 feet high and half full, the RGAs will have a combined impedance of approximately 0.9 ohms at 100 kHz. This compares to over 32 ohms for a roof-shell bonding cable on an identical tank.



Summary There are five possible ways to electrically bond the roof and shell of a floating roof tank. The five bonding methods compare to one another and to the ideal practice as follows:

Bonding Method Impedance at

Lightning Frequencies

Likelihoodof Arcing

at Seal

Easy to

Inspect

Independent of Wall/Roof

Condition

Number of

Bonds Ideal Practice Low Low Y Y Multiple

Shunts – Above the Seal High High Y N Multiple Shunts – Submerged High Low N N Multiple Walkway / Ladder High High Y N Two

Roof-Shell Bonding Cable High High Y Y One Multiple RGAs Low Low Y Y Multiple

As shown above, in comparison to the other methods, multiple RGAs have the best overall roof-shell bonding characteristics. Therefore, installing RGAs will virtually eliminate the possibility for sustained arcing at the roof-shell seal interface, and thereby reduce the risk of lightning-related tank fires. Contact LEC to learn more about the RGA for floating roof tanks or to explore a particular application:

Phone: 303-447-2828 Email: [email protected]

Bibliography

1. Henry Persson and Anders Lönnermark. 2004. Tank Fires, Review of fire incidents 1951-2003,

Brandforsk Project 513-021 2. Kirsten Rasmussen. 1995. Natural events and accidents with hazardous materials. Journal of

Hazardous Materials, Volume 40, Number 1. 3. International Electrotechnical Commission. 2006. International Standard IEC 62305-1. 4. National Fire Protection Association. 2004. NFPA 780: Standard for the Installation of Lightning

Protection Systems, Chapter 7, Paragraph 7.4.1.2.

reducing the risk of floating roof tank fires

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