The potential for heat input from an external fire resulting in the overpressure of a pressure vessel is one of the most common overpressure scenarios identified in processing plants.  Reducing the required relief rate can help to reduce capital costs tied to the relief system; however, some mitigation measures may prove more costly than face value.  There are several common practices centered on reducing heat input to the vessel’s contents that can help reduce the required relief rate in the event of an external fire:

• Ensure adequate drainage and local firefighting are present
• Supply a means of cooling the vessel externally, such as a water spray
• Install fireproof insulation

The installation of fireproof insulation sounds like an easy fix as it can drastically reduce the required relief rate through the use of a corresponding environmental factor (F) according to API Standard 521 6^th Ed. §4.4.13.2.7, taking the conductivity and thickness of the insulation into consideration.  Correct application and installation is key as there are several physical requirements for insulation credit to be taken, and several risks associated with the installation to be considered.

In order to qualify as fireproof, the insulating material must be a non-combustible material capable of withstanding high temperatures.  Cellular PVC, for example, would not be appropriate as it would easily melt in a fire.  Additionally, the insulation must be attached to the vessel in such a way that it will remain in place during a fire and subsequent firefighting activities.  That is, the banding must be heat resistant and strong enough to withstand being knocked off by firewater systems.  According to API Std. 521, stainless steel cladding and banding is generally required and aluminum is specifically stated to be inadequate.

Identifying and installing the proper type of fireproof insulation is not the only aspect of installation requiring careful consideration.  Insulation can trap moisture against the outside of the vessel wall, which can lead to corrosion of the vessel, most commonly found hidden beneath the insulation on the underside of horizontal installations or at the bottom of vertical installations.  Corrosion under insulation (CUI) is attributed to several industry incidents, and occurs in both carbon steel (as general corrosion) and in many austenitic stainless steels (as stress corrosion).  There are several conditions that may predispose a vessel to CUI, as is outlined in Anderson, 2010 (full reference below).  The potential for CUI does not mean that fireproof insulation, or insulation in general, cannot be installed on process vessels and piping.  Insulation is routinely used, but proper selection, installation, maintenance, and inspection activities are required.

• Review operating conditions for the vessel.  Temperature cycles (heat / cool), operation below the ambient dew point, operation between 32-300°F for carbon steel, or operation between 140-300°F for austenitic stainless steels in the presence of chlorides increase the likelihood of CUI.
• Review the equipment metallurgy and corrosion tolerances to establish a baseline for the vessel’s revised Mechanical Integrity program, which will consider CUI.
• Select a heat proof insulation material (stable up to 1,660°F for 2 hours) having a thickness sufficient to provide the desired level of insulation to the system (typically > 2”).
• Select an appropriate insulation covering and cladding that will not be dislodged by firefighting activities and is fire resistant (e.g. stainless steel, not aluminum per API Std. 521).
• Minimize gaps in the insulation where moisture penetration may be possible.
• Use a risk based approach to establish and implement a mechanical integrity program for the vessel.  Such programs require periodic inspection of the vessel to confirm corrosion is within allowable limits.

For more information on corrosion under insulation, see the following references:

• API RP 581 2nd Edition (2008) – Risk-Based Inspection, §17 CUI Damage Factor
• Anderson, Stephan A., “Out of Sight, Out of Mind”, Hydrocarbon Engineering, August 2010.
• Stambaugh, Brandon, “Feeling the Burn”, LNG Industry Journal, Summer 2009.

In summary, fireproof insulation can provide significant value by reducing the required relief loads resulting from an external fire scenario, but is not without risks.  Proper specification of the insulation and installation is necessary to ensure it withstands the fire.  Additionally, it should be noted that the cost of installing fireproof insulation can be significant given the requirement for stainless steel cladding and increased inspection costs for the life of the equipment.  Whatever the need may be, if fireproof insulation is required, then careful consideration of the insulation being selected, how it is installed, and the likelihood of vessel corrosion under the insulation are a must.