Walking through process areas in a petrochemical facility, whether new or old, one is immediately struck by the sheer amount of infrastructure required to support process equipment, vessels and pipelines. Structural systems have been developed over the years to support petrochemical processes and can include:
- Cast-in-place reinforced concrete
- Pre-cast reinforced concrete
- Structural steel
The first three of the four structural systems noted above have inherent fire resistance when impinged during a fire event. In petrochemical facilities, fires when they occur, develop quickly into “pool fires”, rapidly reaching 2,000ºF (1,100ºC) in a matter of minutes. Unfortunately, unprotected structural steel members lose about ½ their strength at 1,000ºF (538ºC) and rapidly loses more strength as the temperature rises. Unprotected structural steel frame collapse happens quickly as compared to cast-in-place/pre-cast reinforced concrete or masonry structures.
As a construction material, structural steel compares very favorably economically to reinforced concrete members and can be easily modified and/or strengthened at a later date should process upgrades and/or additional imposed loads be applied to the structure. However, its lack of fire resistivity requires fireproofing material application to exposed surfaces that may be subject to a fire event. Many products are commercially available for this purpose and a selection process is necessary in evaluating function, form and cost.
The physical properties that govern selection of fireproofing types:
- Resistance to thermal diffusivity.
- Specific Weight (Density).
- Bonding strength.
- Weatherability and chemical tolerance.
- Protection from corrosion.
- Hardness and impact resistance.
Important fireproofing behavior during fire exposure:
- Vibration resistance: compressive, tensile and flexural product strength.
- Vapor permeability and porosity.
- Surface temperature of substrate.
- Resistance to hydraulic erosion and thermal shock.
Types of fireproofing materials and systems available for structural steel frames include:
- Cementitious concrete encapsulation.
- Fixed Water Spray Systems.
- Intumescent epoxy coatings.
- Spray-applied Fire Resistive Materials (SFRM).
- Subliming, intumescent and ablative mastics.
- Lightweight castable cementitious products.
- Pre-formed inorganic panels or masonry.
- Endothermic wraps.
Probably the most widely used passive fireproofing system for structural steel members at-risk in a petrochemical fire is cementitious concrete encapsulation with the other products used in specialty applications (LNG vessels, Tanks, Spheres, etc.). General characteristics associated with fireproofing concrete types are as follows:
- Made with portland or modified fire-resistive cements.
- Specific weights.
- Dense Concrete 140 to 150 lb/ft3 (2200 to 2400 kg/m3).
- Lightweight 25 to 80 lbs/ft3 (400 to 1300 kg/m3).
- Lightweight concrete materials have better fire protection properties than dense concrete.
- Thermal conductivity tends to be inversely proportional to specific weight.
- Capable of withstanding direct flame impingement of up to 2000ºF (1100ºC).
- High alkalinity within concrete materials provide a corrosion passivity to embedded metal materials (structural steel and/or reinforcing steel bars).
During a fire event, high temperature gradients form within concrete and as a result, the hot surface layers tend to separate and spall from the cooler interior of the body. The formation of cracks begin to propagate at joints, in poorly consolidated parts of the concrete, or in planes of the embedded metal items such as structural steel members and/or reinforcing steel bars. Once exposed, the steel will conduct heat and accelerate the action from high temperature exposure. During fire suppression efforts, although a priority at the time of the fire, typically does more damage to the concrete than allowing the concrete to cool slowly. Specifically, the application of water to hot concrete surfaces during fire fighting activities is like “quenching” the concrete. This practice causes a large reduction in concrete material strength because severe temperature gradients are formed within the affected concrete member. When initially exposed to high temperatures, such as in the case of a fire, concrete absorbs heat through an endothermic heat of reaction when chemically bound water is released from the crystalline structure and is reduced to lime by high heats.
Essentially, concrete is a “hard sponge” with a network of small conduits or capillaries allowing directional passage of water from interior to exterior regions to cool hot contact surfaces. However, as moisture moves within the concrete, small amounts of soluble salts are in solution which deposit within the pores of the concrete as the water quickly evaporates during the fire event. As the salt deposit fills-in the voided areas, the concrete pores become blocked and do not allow further transfer of water to cool high temperature contact surfaces. Therefore, the water accumulates behind this barrier and is phase changed from a liquid into a gas. This gas or steam expands in volume and pressurizes the concrete within unblocked pores. The steam pressure ultimately exceeds the tensile capacity of the concrete creating a surface spall. This mechanism is why partial depth repairs to concrete fireproofing do not work for thin section repairs as the partial-depth patch will fail explosively during the next fire event. Steam builds along the repair material/concrete bonding interface and essentially “hurls” the patch along with a thin layer of parent concrete that has failed in tension.
While there’s no disputing concrete’s inherent fire endurance, unseen deterioration in the form of embedded metal corrosion can be hidden from view and not readily visible, in some cases, until it;s too late. During embedded steel corrosion activities, the steel metallurgy changes, with corrosion products requiring and occupying more space than the parent material. As such, significant tensile stresses are exerted on the concrete in the immediate proximity of the corroding steel member. Although inherently strong in compression, concrete is relatively weak in tension. Therefore, unrestrained portions of the concrete mass (i.e. protective concrete cover overtop of the steel embeddment) will crack at the corroding member interface. The progression from crack-to-delamination-to-open spall accelerates the electrochemical processes of corrosion.
Typically, fireproofing concrete placed overtop structural steel members will also incorporate clips and welded-wire mesh detailing to maintain fireproofing integrity during a fire event. However, the clips and mesh are also subject to corrosion and can initiate concrete cracking during the ongoing corrosion activity of these items.
When owner/operators wish to assess the state of fire protection, an innovative approach to detect, document and organize the data obtained in the field is necessary when effective decisions are required regarding funding appropriation for maintenance of fixed assets. Many times during this process however, other conditions that may be even more serious are revealed such as loss in structural steel member/connection integrity due to corrosion and competent steel section-losses. In the USA, many of the petrochemical facilities are approaching the century-mark in age and without a regular inspection/maintenance program, Owner/Operators can expect significant expenditures based on hidden defects detected during infrastructure condition assessments.
Using the LQQ assessment method, an evaluator first (L) locates the areas of distress within a member or structure. Next the distress is (Q) qualified within industry guidelines and prioritized according to safety, structural integrity and aesthetics. Distressed areas are then (Q) quantified for repair construction cost-estimating purposes. An important facet of this process is that it provides an initial baseline of fireproofing system conditions at the time of inspection. Essentially, Condition Survey Documents are developed with corresponding notations as to the fireproofing Distress Priority. Recognizing that with time, distress advances and over extended periods, Distress Priority designations will change to reflect additional deterioration. Owner/Operators have found Condition Survey Documentation to be an extremely useful tool when making enlightened maintenance funding decisions. Beside assistance in the decision making process, Condition Survey Documentation provides a tangible product for repair construction cost-estimating purposes. Estimated costs, based on accurate documentation, provide Owner/Operators with a clear picture of their infrastructure needs so they can develop real-world maintenance budgets.
Many Owner/Operators have turned to Risk Based Inspection (RBI) Programs as well as Best Practices Industry guidelines to get their “hands around” the total breadth of their fireproofing needs and provide a systematic approach at addressing inconsistencies and below-standard conditions. One such approach is the implementation of a Plant Condition Management System (PCMS). The PCMS allows the Owner/Operators to assess concrete fireproofing structure by structure or plant-wide. Essentially this device provides a tool to employ Risk-Based Inspection Techniques and effectively identifies:
- Type of Construction, Item
- Ambient Temperature
- Exposure to Moisture and Chemicals
- Other Design and Operating Conditions
- Deterioration/Damage Description, Characterization
- Possible Causes
- Condition Assessment
- Deterioration/Damage Severity, Location
Owners/Operators face increasing competition on a global basis and the need for focused business strategies at developing effective maintenance budgets for process support infrastructure, is in many cases, critical for continued operation of a facility. Efforts to trim bottom-line costs and maximize profits continues to provide business challenges requiring the use of timely and effective decision-making business tools without sacrificing safety.