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Dilemma in the use of water spray system in LNG storage facility

Question asked by mfalcati on Jul 3, 2017

Greeting, I am looking for some advise in a nwew fire protection project.

I am designing the fire protection system for a LNG storage facility in which there are 24 tanks with 40 m3 water capacity each.

I have reached a dilema in the use of the tanks' spray system. NPFA 30 and NFPA 59A suggest the use of a spray system to avoid structural damage in case of fire exposure or the tanks. They also mention the need of detecting gas leaks using gas detectors. I have read in different documents that when water enters in contact with liquid LNG, the temperature difference between the liquids generate a quick evaporation of the gas and a consequent gas cloud which may represent a greater fire/explosion risk than the actual "gas liquid pond".

I have also read that the water spray may help to dissipate and "keep the cloud in ground level" which may be helpful to control the cloud. 

My question is: The water spray system must be turn on when there is a fire detection (with flame detectors) only? or it must be also turned on in presence of gas (without flames)? Using the spray system when there is no fire does not increase fire risk?

Thanks for your time and help.