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Providing external insulation is one of the effective way to minimize the impact of the fire attack. This is pretty inline with API Std 521 recommendation.
However, there are several problems associated with insulation :
i) The insulation external protection layer shall be able to resist high temperature ( > 900 degC per API) and sustain fire for 2 hours. Involvement of safety engineer and mechanical engineer during early design stage shall be considered for safety and availability compliance.
ii) Corrosion under insulation is a complicated and tricky stuff. It is hard to avoid and corrosion protection layer shall be provided as minimum. As the corrosion protective layer will be covered by insulation layer, any damage to the corrosion protective layer and present of corrosion, operator may not aware of the occurrence of corrosion by visual inspection. A sophisticated assessment and inspection method is required. In the event, corrosion occur, there is potential of the corrosion material accumulated and damage the insulation layer without knowing. Corrosion under insulation may defect the purpose of insulation.
iii) The protection layer and insulation is subject to the risk of external mechanical damage cause by impact of fire water jet, mechanical work during maintenance, inspection, site modification works, etc. Once the layer is damage, it is very difficult to repair and maintain similar level of protection as per original requirement such as fire resistance for 2 hours at > 900 degC.
iv) Galvanic corrosion and mercury embrittlement of aluminum blind rivets may occur (in suitable environment) if aluminum cover sheet is used. Consider sheeting made of galvanized steel or stainless steel and be fastened with stainless steel or galvanized self-tapping screws, not with aluminum blind rivets.
v) Whenever consider use of insulation layer, the fire scenario shall be assessed carefully if jet fire is potential scenario. Jet fire would leads to higher heat flux, jet momentum may mechanically damage the insulation layer, etc.
Other protection measures against FIRE (discussed earlier) shall be considered concurrence with application of external insulation.
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