(55ay) Water As an Ignition Source

In December 2016, a fire broke out in a storage tank which contained Isomerate and C9 (the main constitutes of gasoline). The fire started as liquid accumulated above the floating roof. An attempt to direct a water jet from an oscillating monitor, which was aimed to float the hydrocarbons towards the tank's drainage, resulted with an ignition of the tan's content and with a major fire. It took about 10 hours to extinguish that fire.

Flow of water jets may create an ignition source in a mechanism called "Splash filling". The amount of charge that would be generated during the flow depends on the velocity of the water in the air as well as on the flow distance. The higher the amount of charge that would accumulate on the water during the flow, the higher the likelihood that an ignition would occur.

Several theories try to explain that ignition mechanism: According to one theory, double layers of charge form at the boundaries between liquid and container, as dipole liquid molecules accumulates at the interface, leaving the possibility of one polarity emerging dominant on droplets as they are formed on flow and breakup. According to a second theory, the ‘ion fluctuation theory’, The electrostatic charge is formed as water breaks up into droplets, due to the fluctuation in the concentration of the positive and negative ions in the water.

A third theory suggests that smaller water droplets are usually negatively charged and the larger one positive. The different buoyancy and opposite polarity of large and smaller droplets leads to charge separation and an ignition.

Analysis of the above fire suggest that even if a low expansion foam was used, there is a good chance that the tank fire would still occur (this should not be a surprise as a low expansion foam consists of 97% water).

The process safety literature is filled with fires and explosions caused due to splash filling:

*Top filling of tanks with explosion atmospheres: in few instances water was added to tanks with an explosive atmosphere to purge the explosive gasses. The likelihood of such accidents can be minimized by using a dip pipe / flowing the liquids towards the tank walls and by limiting the water velocity to less than 1 meter/second.

*Cleaning a tank / reactor with an explosive atmosphere using a spray ball: the static electricity generated during the discharge from the spray ball can easily ignite the explosive atmosphere.

*Cleaning a tank with an explosive atmosphere using a high pressure water jet : such a cleaning of oil cargo ship had caused there explosion. With spray washing of vessels, droplets are created at the spray nozzle – and as the jet impacts the vessel wall. The high input energy that creates the spray is usually high, creating a high likelihood of an ignition due to highly charged mist inside the vessel.

It is important to note that grounding and bonding will usually not provide an adequate protection as the ignition occur inside the tank.

Studies had demonstrated that liquids in amounts up to a ten liters, if not precharged by any previous action, will not generate any dangerous discharges when falling down up to three meters by means of gravity. But it is essential to verify that the container is made of conductive material and reliably earthed.

Summary of the main precautions:

(1) Use dip pipe or bottom feeding in lieu of splash filling.

(2) For uncontaminated single-phase water, restrict the inlet flow velocity to 1 m/s until the fill pipe has been submerged to a depth of twice the inlet pipe diameter. Fill rate may then be increased up to 7 m/s.

(3) inert the tank prior to using a spray ball for cleaning.

(4) Verify there is no potential for an explosive atmosphere prior to using high pressure water jet.