Some Comments on Providing External Insulation as Protective Measure against FIRE

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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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Should we consider JET FIRE for Pressure Relief Valve (PSV) load determination ?

PSV is a final safeguarding device to prevent equipment or system from catastrophic failure. However, this device may not protecting equipment and/or system from FIRE attack. We may only rely on other active and passive protecting system to safeguard the system.

A wet gas KO drum operate at LOW pressure & LOW temperature. External FIRE (regardless of POOL or JET fire) heating up it gas via metal wall. As gas is low in heat transfer (~ 7 - 20 kW/m2), the heat absorbed by vessel wall do not dissipate into gas fast enough and tends to stay in the metal wall. Vessel wall (flanges, gasket, etc) probably failed before the internal pressure reached it PSV set pressure.

JET FIRE

Installing PSV does not protecting the vessel from Fire attacks regardless of pool nor jet fire. Practically, we shall shift the focus to :

• 
  
  emergency depressurization system
• 
  
  high SIL instrumented protective system
• 
  
  fire proofing system
• 
  
  high firewater spray density

Jet fireis initiated from flanges leakage, cracked pipe, damaged of some fitting attached to large pipe, etc. It subject to

• 
  
  internal pressure at release point
• 
  
  flame direction
• 
  
  relative distance between release point and impinged vessel

Jet flame contain very high momentum which impacting to the impinged vessel (but reduce according as pressure depleted), etc. There are many other transient parameters evolve with time such as wind speed, wind direction, etc.

Fire impinged area may expose to very high localised heat flux (>300 kW/m2), as the flame travel around the vessel, the heat flux reduced accordingly. Many documents e.g. "Guidelines for Protection of Pressurised Systems Exposed to Fire", by Scandpower Risk Management AS, quoted jet fire heat flux of 300 kW/m2 is just an average heat flux.

As indicated in latest API Std 521, section 5.16 "...a relief device might not prevent vessel failure from jet fire impingement.", those sizing PSV protecting a vessel may not needs to consider jet fire heat flux.

POOL FIRE

In conclusion,

1. 
   
   PSV will only be sized for pool fire using the API 521 equation
2. 
   
   Focus on emergency depressurization. Use jet fire heat flux rather than pool fire heat flux. CAUTION : if you use HYSYS, the model only use API 521 equation. Some configuration is required to estimate depressurization rate based on jet fire heat flux.
3. 
   
   While conduct depressurizations study, ensure maximum allowable working pressure due to reduce wall stress (reduce according to time) is always above internal pressure exert on the vessel wall.
4. 
   
   Increase wall thickness or apply external fire proofing if necessary.
5. 
   
   Conduct Scenario and Quantitative Risk Analysis, Jet fire flame pattern analysis etc if above measures are too excessive

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