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The art of lightning protection

Hydrocarbon Engineering,


Lightning-related petroleum storage tanks fires are more common than most people think. Of the 480 tank fire incidents reported in the media, about one third have been attributed to lightning.1 Another study, sponsored by 16 oil industry companies, found that 52 of 55 rim seal fires were caused by lightning, and concluded that ‘lightning is the most common source of ignition’.2

Figure 1 shows a still taken from a video of a lightning-related tank fire in an internal floating roof tank (FRT) located in Wynnewood, Oklahoma.3 Figure 2 shows a still taken from a video of a lightning-related tank fire at a saltwater disposal facility located in Greeley, Colorado.4 In both cases, lightning was the cause of ignition. In both cases, in addition to the huge cost of lost product and damage to the physical plant, there were also numerous large, incalculable costs. The fire at the Greeley facility was one of several lightning-related fires at injection well facilities during the past few years. Since 2013, there have been at least three lightning-related fires at injection well facilities in North Dakota, with four more in Texas and two more in Colorado.5

Ignition mechanism: FRTs

Lightning strikes are characterised by very high stroke currents arriving in a very brief amount of time. For example, an average lightning strike delivers about 30 000 amps of electricity to the ground within a few milliseconds. This current will flow across the surface of the earth until the cell between the thundercloud and earth is neutralised. The current will flow in all directions, although the amount will vary in proportion to the paths of lowest impedance.

The mostly likely strike location on an FRT is the top of the rim or the gauge pole. However, lightning may endanger an FRT if a stroke terminates on the roof, the shell, anything attached to the roof or shell, such as the gauge pole, or a grounded structure or the earth near the FRT. If lightning terminates on any of these locations, or near an FRT, a portion of the total lightning current will flow across the roof-shell interface. If lightning should terminate on the tank shell, as illustrated in Figure 3, sizable currents will flow across the roof-shell interface.

If lightning terminates near an FRT, either to the earth or to a grounded structure, as illustrated in Figure 4, smaller currents will flow across the roof-shell interface. In either case, lightning-related currents will flow across the roof-shell interface. If the impedance between the roof and shell is high, arcing will occur across the seal interface.


Figure 1: Lightning-related tank fire on internal FRT- Caribbean Petroleum, Puerto Rico 2009.


Figure 2: Lightning-related fire at injection well facility, Deanville, Texas, USA 2015.

A typical lightning stroke contains numerous components, as shown in Figure 5. The fast component, or first return stroke (Component A in the figure) is extremely brief, yet contains the peak current. The long, slow component (Component C) lasts much longer than either the fast Component A or the transitional Component B and is responsible for vapour ignition.

Ignition mechanism: non-metal tanks

Non-metal and lined tanks are often used to store corrosive byproducts, such as saltwater, from the hydraulic fracturing process, because these byproducts are highly corrosive to steel. Non-metal and lined tanks also exhibit significant lightning-related risk due to their non-conductive nature. Non-conductive tanks are being deployed with increasing frequency in the Denver-Julesburg, Permian, Marcellus and other oil shale regions. A non-conductive tank may be either a tank constructed of a non-conductive material (such as fibreglass) or a steel tank lined with epoxy or other non-conductive material. They are considered non-conductive because, unlike steel tanks, electrical charge cannot dissipate from the tanks’ contents to ground via the tank.

As a working example, a common storage tank being used in a petroleum-related field operation, such as one being used for saltwater disposal, will be examined. If the tank is partially full, the space above the fluid typically contains a combustible vapour. In a grounded conventional steel tank, the conductive steel allows for charge equalisation between the tank’s contents, the tank itself and ground. However, for a non-conductive or lined tank, there is no charge transfer and equalisation, and so a charge differential between the combustible vapour and ground could occur, as illustrated in Figure 6. A direct or nearby lightning strike will cause a rise of ground potential, and of all grounded objects, by hundreds of thousands of volts in a few milliseconds. If the potential difference between a grounded object or surface is exposed to the vapour and the vapour reaches the electrical breakdown strength of the vapour space, an arc will form and ignition will follow.


Illustration of Current Flows Resulting from Lightning Strike to Tank Shell(Note that current flows across the roof-shell interface in numerous locations.)


Illustration of Current Flows Resulting from Nearby Lightning Strike(Note that current flows across the roof-shell interface in multiple locations.)

FRT recommended practices

Historically for FRTs, shunts were used to ‘bond’ the floating roof to the shell of the tank. A shunt is a strip of string steel, affixed to the floating roof and sliding against the inside tank wall. Unfortunately, shunts do not provide a positive, low impedance bond between the floating roof and tank shell for several reasons, including:

  • Heavy crude oil components, such as wax and tar, can accumulate on the inside of the tank wall, thus increasing contact resistance.
  • Corrosion (rust) on the inside of the shell will increase the resistance of the bond between the shell and shunts.
  • Many FRTs are painted on the inside, typically with an epoxy-based paint. If the inside of the tank is painted, the paint will insulate the shell from the shunts.
  • Large tanks are typically out-of-round by several inches. If a tank is elongated for some reason, the shunts will be pulled away from the shell in the long dimension of the tank.

Independent third party testing, performed in cooperation with the API and the Energy Institute in the UK, has shown that arcing will occur at the shunt-shell interface under all conditions; it does not matter if the shunts are clean or dirty, new or old, neglected or well maintained. It also does not matter if the inner shell walls are clean, rusty, painted or coated; arcing will occur in all situations. This testing programme has been well documented in API 545-A.

API RP 545 is the ‘Recommended Practice for Lightning Protection of Aboveground Storage Tanks for Flammable or Combustible Liquids’. NFPA 780 is the ‘Standard for the Installation of Lightning Protection Systems’. Both standards recommend installing multiple roof-to-shell bypass conductors on floating roof storage tanks. During a lightning event, Component C of the strike will be conducted by the bypass conductors, thus preventing sustained arcing at the seal interface (it is Component C that causes ignition). The bypass conductors will ensure that the roof and shell stay at the same potential during thunderstorms, thus mitigating the risk of ignition of flammable vapours that may be present. There are thousands of floating roof storage tanks currently in operation, with the majority of them lacking sufficient bypass conductors, thus increasing their risk of lightning-related fires.


Figure 5: Lightning Flash Components (not to scale) [Ref. 6]

Non-metal production tank recommended practices

NFPA 77 is the ‘Recommended Practice on Static Electricity’. API RP 2003 is the ‘Recommended Practice for Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents’. Both NFPA 77 and API 2003 recommend an internal grounding conductor inside all non-conductive and lined tanks being used to store a potentially combustible fluid. This internal grounding conductor must be grounded to earth to effectively neutralise any charge differentials that may exist between the tank contents and ground. The internal grounding conductor should run the length of the tank, inside the tank, and be anchored or somehow secured to the tank bottom. In addition, all metal tank fittings, such as flanges, hatches, etc., must also be bonded and grounded. Ironically, NFPA 77 goes on to state that non-conductive tanks ‘are not permitted for storage of Class I, Class II or Class IIIA liquids’.


Figure 6: Unequal electrical potentials from lightning strike near non-conductive tank.

Conclusion

Floating roof petroleum storage tanks and non-metal and lined production tanks are vulnerable to the effects of lightning, for unique but related reasons. Several applicable industrial standards address these situations and offer practical, achievable techniques to lower lightning-related risk. Some lightning protection equipment manufacturers have also responded by designing and manufacturing equipment to prevent lightning-related ignitions on these types of tanks.


Written by Joseph A. Lanzoni, Lightning Eliminators, USA. This is an abridged version of an article taken from the Spring 2016 issue of Tanks & Terminals. Subscribers can view the issue by logging in.

References

  1. PERSSON, H., and LÖNNERMARK, A., Tank Fires, Review of Fire Incidents 1951 - 2003, Brandforsk Project 513 - 021.
  2. Large Atmospheric Tank Fires (LASTFIRE), Project Analysis of Incident Frequency Survey, June 1997.
  3. The entire video can be viewed at https://youtu.be/KGlwLC_1qOI.
  4. The entire video can be viewed at https://youtu.be/Ey-UY3RIQxo.
  5. BEERS, B., Exposure to Lightning Strikes at Injection Well Facilities, Energy Pipeline Magazine, August 2015.
  6. SAE ARP (Aerospace Recommended Practice) 5412, Aircraft Lightning Environment and Related Test Waveforms, SAE Publications, USA, 2000.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/01042016/the-art-of-lightning-protection-2885/


 

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