Gases Neutralization

Subject: Expert Opinion on the Safe Implementation of "GREEN UP FOG" in Neutralizing Explosive Environment within Large Fuel Tanks 

References:

1)IEC 60079-2011: Explosive Atmospheres: General requirements (Also Israel Institute Standard 60079-2011)

2)NFPA 497-2004: Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas

3)Atex 137: Explosive Atmospheres

1. General

   To cleanup fuel sludge in large fuel tanks it is imperative to begin the operation in neutralizing the existing explosive environment within the tank by utilizing the agent "GREEN UP FOG" that increases considerably the minimum ignition energy and the temperature flesh point of the explosive gas and also diminishes the strong odor of the gas.

   Following the completion of the neutralizing process the sludge cleaning can be safely commenced.

   Neutralizing of the explosive atmosphere and diminishing of the fuel stench is obtained by spraying the agent known as "GREEN UP FOG" (See safety related details in Appendix B) as depicted in photo 1.

 

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Photo 1: "GREEN UP FOG" spraying in a fuel tank

 

Method

• Between six to eight spraying units (as shown in photo 1) are placed inside the tank through the floating tank's manholes located on the floating roof.

• All units are well interconnected to earth ground.

• The neutralizing process requires continuous fogging operation between 24-36 hours.

• Each unit is connected to high pressure delivery pump. The pumps are deployed outside the tank structure and are grounded.

• Each unit spraying nuzzle emits between 10-20 liter per hour of neutralizing agent.

• Each unit containing 5 spraying nuzzles emits between 50-100 liter/hour of neutralizing agent.

• The number of spraying units is selected in accordance with the dimensions of the tank and the available manholes on the roof.

• There are several types of spraying nuzzles in accordance with the required spraying range.

Fogging process outcomes

• Gas temperature flash point is increased to 80-90 ˚C

• Strong fuel odor is neutralized

• Tank manholes can be opened for tank cleaning to commence.

During the fuel sludge cleanup the fogging process must continue to ensure efficient neutralizing of the potentially explosive gas emission from the sludge.

Figure 1 shows a fuel tank with a floating roof, explosive gas within the tank ullage and the sludge in the bottom of the tank. 

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Figure 1: Floating Roof Tank layout

 

 

1. Purpose

   The purpose of this discussion is to determine that the fuel tank fogging operation does not create a lightning-like threat due to static generation of the agent spraying and the charged up mist with static electricity. Also to define that the fogging operation is reducing the flammable zone from Zone 0 to unclassified zone.

   Note that according to Ref. 1-3, flammable gas and vapors are defined as potentially creating "flammable/explosive environment" if the flash point temperature is less than 55oC and minimum ignition energy is less than 5mJ.

2. Lightning like ESD

   In principle, lightning like discharges can occur within dust/mist clouds or from dust clouds to earth when the field strength due to the charged particles is high enough.  Such lightning like discharges have been observed in large ash clouds during the eruption of volcanoes.  They are obviously capable of igniting flammable dusts/mists, but they have never been observed in dust/mist clouds of the size encountered in industrial operations.

   According to experimental investigations such discharges are unlikely to occur in silos and tanks of volume less than 60 m3 or in silos of diameter less than 3 m and of any height. Those dimensions are not necessarily the upper safe limits; they are based solely on the size of the equipment in the above mentioned investigations.

   Such discharges are unlikely to occur in larger silos or tanks providing that field strengths remain below 500 kV/m.

   The movement of pure gases or of a mixture of gases generates little, if any, static electricity but if the gases contain solid or liquid particles these can become charged. In industrial processes such particles are common. They can be due to contamination, such as dust or water droplets, they can be a condensed phase of the gas itself, such as carbon dioxide snow, or the droplets in wet steam, or they can be deliberately introduced, e.g. spraying "GREEN UP FOG".

   Examples of processes where particle charging can give rise to significant amounts of electrostatic charging include: pneumatic transfer of materials; the escape or release of any compressed gas containing particles; the release of liquefied carbon dioxide; the use of industrial vacuum cleaners; and spray painting and fogging.

   Charged particles produced by these mechanisms can give rise to a number of types of incendive discharges: spark discharges can occur when charge builds up on insulated conductors as a result of impingement or collection of particles; brush discharges can occur when charged clouds or jets of charged particles are close to earthed metal projections; propagating brush discharges can occur due to charged particles impinging on thin layers of non-conductive material; and cone discharges can occur when charged particles collect and form a cone, as in a silo.  There is no evidence to suggest that lightning-like discharges can occur in industrial scale equipment.

   It is not possible to prevent the electrostatic charging of particles but ignition can be avoided either by ensuring that the atmosphere is not flammable or by preventing incendive discharges. The precautions that can be taken to avoid incendive discharges include the following:

   • Ensuring that all metal and other conducting objects are earthed;

   • Avoiding the use of highly non-conductive materials;

   • Reducing charge densities by restricting flow velocities or by suitable nozzle design;

   • Removing the particles.

      

1. "GREEN UP FOG" Spraying Safety

   The spraying operation within the tank begins within Zone 0 explosive atmosphere. It gradually diminishes the flammability nature of the fuel mist and gas. At the process end the flammable environment classification of the tank ullage per references 1-3 becomes "nonhazardous, unclassified zone". The mist generated by the spraying of the "Green up Fog" might become charged up with static electricity. Nevertheless it is incapable of creating an incendive discharge as discussed above and herein:

   • All metal and other conducting objects are earthed;

   • Highly non-conductive materials do not exist inside the tank;

   • Reducing charge densities by restricting flow velocities or by suitable nozzle design – Not relevant for the fogging process (relevant for transfer of liquids and bulk powder). The spraying nuzzle design reduces the amount of static charge generation.

   • Removing the particles – Not relevant for the fogging process (relevant for solid particles)

 

1. Conclusions

   At the end of the fogging process the gas temperature flash point is increased to 80-90 ˚C and MIE becomes much greater than 5mJ (no available information of the actual MIE). Therefore, the gas environment mixed with the "Green up Fog" agent becomes non-flammable in accordance with References 1-3.

   Since the only theoretical ESD threat is lightning like discharge from the charged up mist particles it is certain that such discharge can't occur in the volume and characteristics of the Fuel Tank (see Section 3 above).

   The conclusion is therefore obvious – the use of "Green up Fog" is safe for the purpose of neutralizing the odor and gas flammability prior and during cleanup of the fuel sludge in large fuel tanks with floating roofs.

    

    

   Cordially,

    

   Moshe Z. Netzer

   ESD Reliability and Safety Expert

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