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LIGHTNING PROTECTION AND GROUNDING SYSTEM EVALUATION FOR FIBERGLASS REINFORCED PLASTIC (FRP) STORAGE TANKS

1 INTRODUCTION

There has been a rapidly growing trend for petroleum, water/wastewater and chemical industries to utilize Fiberglass Reinforced Plastic (FRP) storage tanks. FRP storage tanks are common for these industries due to their non-corrosive properties compared to standard metal storage tanks. These FRP storage tanks are still exposed to lightning and are a potential fire hazard. The non-conductive property of FRP materials creates additional resistance to the fast lightning current impulses, creating intense heat at the point of impact. Fires starting at a single FRP tank can engulf an entire facility.

Even in the case of an indirect lightning strike, it may also be considered that an unequal ion discharge rate between an insulated FRP tank and a nearby grounded/bonded metallic structure can cause the development of a difference of electrical potential, creating a spark which could lead to a tank explosion and fire.

Available lightning strike data related to the Tank Farm Batteries is neither comprehensive nor readily available, with most reports consisting of media releases such as the four events shown below.

Midland/Martin Count Line, TX- 9/16/13 Atascocita, TX-9/7/13

Gradford, TX – 6/25/14

Watford City, ND – 9/31/14

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In June and July of 2014, lightning-sparked fires destroyed saltwater disposal facilities at three (3) locations in North Dakota. Over 440 such sites exist in North Dakota alone. Production tanks are associated with well sites used principally for enhanced oil recovery and salt water disposal. It is reported that there are 144,000 such wells in the USA.

Downtime and recovery costs from potential lightning strikes can be very expensive and have possible safety consequences for personnel, strategic infrastructure and critical storage tanks. Tank replacement costs, losses due to operational shutdowns, and safety liability issues can be minimized with the appropriate use of modern lightning protection methods. These can entail an engineered mix of structural air terminals, equipotential bonding, and grounding. This paper describes the risks associated with FRP storage tanks to direct lightning strikes and guidance to protect facilities from direct lightning strikes and/or secondary lightning transients.

2 LIGHTNING RISK ANALYSIS- ELECTRO-GEOMETRICAL MODEL

At any moment, it is estimated that 1,800 thunderstorms are in progress somewhere on Earth. About 100 lightning bolts hit the ground each second. Historical lightning data has been compiled to create an Isokeraunic map showing the statistical average number of thunderstorm days per year, for any area of the world.

A three (3) dimensional model was developed for a typical FRP storage tank facility. Using Electro-Geometric modelling applied to the 3-dimensional representation of the facility, all of the lightning attachment points are calculated and displayed visually.

Modelled 3-Dimensional views

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Lightning Strike Analysis for 15 kA Return Stroke Current The probability of a direct lightning strike to the storage tank facility was determined with consideration of its relative location. The chart below shows the total number of expected strikes per year for the modelled facility located at two different Isokeraunic values.

The standalone direct lightning strike probability should not be the only consideration for effective lightning protection design and implementation for FRP storage tank facilities, as indirect lightning strikes may also cause sparking or flashover to vapor and/or contents that are stored within the facility.

Of course, the modelled facility may have a greater or fewer number of tanks than any existing or proposed FRP storage tank facility under consideration. The relationship between lightning strike probability and the number of tanks is very linear, and for a good approximation the change in risk doubles each time the number of tanks doubles. This allows for some useful calculations. For example, if an owner/operator maintains fourteen times the number of tanks shown, in Texas, then they can expect one tank to suffer a direct strike every year. Since it cannot be known beforehand which individual tank will be struck, it is highly suggested that all tanks should be protected.

3 LIGHTNING PROTECTION AND GROUNDING SYSTEM SOLUTIONS

ALLTEC has observed that most fiberglass storage tank facilities implement bonding and grounding sufficient for electrostatic charge dissipation. It is of upmost importance to prevent a discharge of an accumulation of static electricity from a storage tank to the ground or to another charged object of different voltage which can be the cause of a fire or an explosion if it takes place in the presence of readily flammable materials or combustible vapour and air mixtures. ALLTEC recommends that existing bonding and grounding at FRP Battery/Tank Farms should be inspected and any incompliances to

State

Isokeraunic

Value

Total number of expected strikes per

year

Direct Lightning Strike probability to the Facility

Florida 100 0.124 Once every 8.0 years

Texas/Kansas/Oklahoma 60 0.0743 Once every 13.5 years

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NFPA 77 “Recommended Practice on Static Electricity” should be identified and corrected. Proper bonding guidelines should be followed as per recommended practice.

It is important to note that bonding and grounding implementations sufficient for electrostatic charge dissipation are not adequate for lightning protection grounding systems. Even a ground resistance of 1 MΩ is adequate for static grounding [Ref. section 3.2.6.2 IEEE 142].

Approx. #2 AWG jacketed wire bonding sufficient for electrostatic dissipation, but not appropriate for lightning protection. Note bond lug to interior “carbon veil”

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Existing bonding and grounding systems at the FRP storage facilities should be further enhanced by the implementation of code compliant lightning protection bonding and grounding. All metal ladders, overhead piping, and vents should be properly bonded and grounded. Properly designed and installed lightning protection is essential, as closed top metallic tanks may have flammable atmospheres at their vents, and fiberglass tanks receiving a direct lightning strike may rupture violently. The high level of lightning protection required by a fiberglass tank facility is best provided by a mix of technologies and a selection of methods drawn from a range of globally accepted standards. For instance, API-2003, “Protection against Ignitions Arising out of Static, Lightning, and Stray Current: Appendix C”, describes Charge Dissipation Terminals (CDT) as one of the lightning protection technologies used to mitigate the path of an incoming lightning stroke. In addition, NFPA 780, IEC 62305 and other standards for lightning protection actually describe minimum requirements for lightning protection system design and installation. Designs utilizing Charge Dissipation Terminals meet or exceed those standards, while offering the enhanced performance provided by CDT technology.

Fiberglass Tank Lightning Protection Design- CDT Technology – ALLTEC

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A low impedance ground system resistance of less than 10Ω is recommended for all FRP storage tank areas. The US Military Handbook for Lightning Protection (MIL-HDBK-1004/6) and IEC 62305 std. recommend having a minimum of 10Ω ground system resistance for an effective Lightning Protection System (LPS). Metallic elements, particularly those in flammable zones, should be bonded together and adequately grounded using a “single point ground” methodology.

ALLTEC can model any existing or new/proposed external ground system by creating a soil model using tested soil resistivity values to meet any low impedance ground reference solution requirement. The resulting data is entered into specialized integrated grounding software that accurately calculates soil resistivity at various depths. ALLTEC provides an array of grounding products, including TerraDyne® Electrolytic Ground Electrodes, TerraFill® Low Resistive Backfill Material, and the TerraWeld® Exothermic Welding System to supply and install effective grounding systems. An example of a typical 10Ω ground system design for effective dissipation of lightning energy is shown in the following figures. In the case study, approximately 950 feet of conductor and four (4) Electrolytic Ground Electrodes are modelled.

Grounding Plan: FRP Storage Tank Facility

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3-D Modelled Ground System Design

Approx. Soil Resistivity

Value

Ground System

Resistance

Ground System Components

≤ 76,000 Ω-cm ≤ 9.93 Ω 950’ of Ground Conductor, and Four (4) Electrolytic Ground Electrodes

≥ 80,000 Ω-cm > 10 Ω The Ground System Design of 10Ω can be achieved with consideration of Low Resistive Backfill Material using trenched Ground Conductor and additional electrodes

4 SURGE PROTECTION SOLUTIONS

In addition to proper grounding and bonding, it is crucial to install SPDs at key points throughout a facility to adequately protect today’s sophisticated microprocessor based electronic equipment. It is strongly recommended that a comprehensive network of quality Surge Protective Devices (SPDs) are installed throughout the Tank Farm’s AC power distribution to protect critical equipment loads against hardware damage and from surge and electrical noise related operational disruptions, resulting from lightning and non-lightning related surge and noise anomalies. Noise on a power line is generically defined as low amplitude (< 50 V) disturbances that are distinguishable by an identifiable frequency pattern. A transient surge, on the other hand, is a momentary burst of energy whose duration extends into the millisecond range. The typical surge waveform is characterized with rapidly rising voltage and current values that reach their peak values in the low microsecond ranges; and decay to half those levels in under a millisecond. Noise disruptions are differentiated by their frequency patterns. High frequency noise spectrums are classified as Radio Frequency Interference (RFI), while low frequency noise patterns are labeled as Electromagnetic Interference (EMI). While surge anomalies are far more likely to interrupt equipment operation and damage electronic hardware, transient induced noise interference can also disrupt sensitive equipment operations.

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Transient surges are categorized as either externally generated impulses; a lightning induced electrical energy burst, for example, and as internal and/or manmade transient irregularities. A decaying oscillatory ringing transient (noise) attributed to power factor correction activities is an example of the latter anomaly.

Lightning generated transients are, by far, the most intense surge events. Surges originating elsewhere can be at high voltage levels, as well. Though for example, lightning induced surges initiated by direct lightning strikes can produce momentary voltages up to 75 kV at their point of impact. Indirectly coupled surge impulses originating from nearby lightning activity, on the other hand, generate voltage bursts up to 25 kV. Power source generated surge activity initiated by power factor correction capacitors and from load shedding operations can produce surge voltages up to 15 kV. Internally generated capacitive load generated transient activity, ground potential differences, and the power cycling inductive loads can each produce surge voltages as high as 10kV. Therefore, it is crucial to install SPDs at key points throughout the tank farm to adequately protect the equipment loads from the aforementioned surge threats.

A cascaded network of SPDs, designed to work in tandem with each other, should be installed on 480V Main Distribution Panels and Motor Control Centers (MCC) to protect against externally generated surge activity. They should also be installed at 120/208V UPS Panels and other sub panels that supply power to critical equipment loads to protect them from surges originating from within the tank farm.

It is also advisable to individually protect equipment loads that are electrically located further than 50 feet from a protected point within the electrical distribution. These SPDs should incorporate common mode suppression components to shunt lightning induced surge current to the electrical distribution’s ground circuit. They should also employ normal mode suppression circuits to distribute internally generated surge current between power phases; and between the phase and neutral conductors on the AC power service. They should also be equipped with a first order noise filter to attenuate disruptive transient related noise voltages to equipment safe levels. Moreover, careful consideration regarding protecting equipment control and data interfaces should be deliberated wherever their operational disruptions would cause any significant downtime or work slow-downs. At the very least, any and all control and data lines entering and exiting a structure should incorporate proper SPD protection.

Likewise, any medium voltage transformer or rotating machine load that has either suffered from premature insulation breakdown, or is deemed to be vulnerable to this type of damage, should be individually protected by quality stand-alone Station Class arrestor fortified SPDs – even if they are already equipped with Normal or Heavy Duty class surge arresters.

Implementing these recommendations will bolster Tank Farm equipment surge and noise immunity levels to the highest possible thresholds to extend their operational life expectancies, increase their efficiency levels, and reduce their maintenance, repair, and replacement costs.

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5 THE ALLTEC “PROTECTION PYRAMID”

As an ISO 9001:2008 registered full-service company, ALLTEC specializes in engineered solutions which reduce the risks associated with direct and indirect lightning strikes, as well as diminishing the hidden effects of surge events. Offering decades of knowledge and experience, ALLTEC’s recommendations advise the best methods for risk mitigation, and ultimately apply these evaluations as comprehensively engineered solutions. Our global experience has yielded specialized knowledge and system expertise for a wide range of applications, and our organization maintains one of the most knowledgeable and experienced technical staffs in the world.

ALLTEC’s Oil & Gas solutions aim to:

Protect critical facilities, equipment, records, and assets.

Provide a safe working environment for personnel.

Reduce the risk of downtime and lost revenue.

Reduce vulnerability of interdependent critical infrastructures, and build a disaster resilient enterprise

Minimize liability and maintain a competitive posture.

In addition to the advantages created by ALLTEC’s lightning protection, grounding and power quality recommendations, it is important to realize the potential hazards of operating with an under-protected facility. Costly consequences that may result from lightning strikes to Oil & Gas facilities include: lost revenue due to equipment downtime, the costs associated with lightning related equipment damage, facility and equipment repair and replacement costs, environmental impact, danger to life, possibility of legal ramifications and lost productivity due to related equipment and instrumentation operational disruptions.

Both facts and experience stress the requirement of a comprehensive grounding solutions, surge suppression and lightning protection systems for facilities like yours.

ALLTEC calls the methodology used for visualizing, designing, and implementing its unique and industry leading three-tier comprehensive facility protection approach for grounding, surge suppression and lightning protection, the ALLTEC Protection Pyramid™.

It is important to realize the interrelationship and interdependence of the three tiers in protecting any facility. Without proper grounding, neither the surge suppression nor lightning protection will function correctly. Without surge suppression, equipment is exposed to the secondary effects of lightning as well as internally generated transient voltages. Direct strike lightning protection protects the physical structure and its contents. These three subsystems interlock into a very robust and stable whole.

ALLTEC proposes that its engineering team work with your designated representatives to evaluate your facilities and investigate all problems and concerns related to lightning protection, surge suppression and grounding at the selected facilities. ALLTEC personnel have the skills, experience and tools to accomplish the requirements of this project in a timely and professional manner. We look forward to working with you.