Passive Fire Protection

Today modern buildings and structures have some degree of fire protection in order to:

• Protect lives
• Delay possible structural collapse allowing for evacuation
• Provide areas of temporary refuge in the case of fire
• Ensure the integrity of escape routes by preventing or delaying the escalation of a fire
• Protect high-value assets

Types of Fire Protection

There are two basic types of fire protection: active and passive.

Active fire protection includes alarms and detection systems, sprinklers and water deluge systems, firefighting equipment and foam and powder extinguishers.

Passive fire protection (PFP) involves components of structural methods and materials such as concrete, low-density cement, mineral fibre boards, vermiculite cement and ablative and intumescent coatings.

Within this training unit, we will be reviewing various forms of passive fire protection only and not active fire protection.

PFP Materials and Systems

Many types of PFP materials exist and their performance in real fires will vary due to the way in which each material performs its fire protection function.

During the selection of a suitable material, the particular risk must be taken into account and some or all of the following points should be considered:

• Strength
• Durability
• Low added weight
• System integrity
• Non-corrosive
• Non-hazardous
• Ease of installation
• Cost-effectiveness
• Fire performance

Passive Fire Protection – Organic Coatings

Organic coatings are durable and resistant to general mechanical wear and fall into the following groups: intumescent coatings, ablative coatings and subliming compounds.

Intumescent Coatings

All intumescent coatings are normally epoxy-based materials which, when subjected to fire exposure, expand to form an insulating char with a low thermal conductivity acting as a thermal barrier between the fire and the substrate.

Typically, during this reaction, toxic fumes and smoke are released from the coating at temperatures above 300°C (570^oF) which makes them unsuitable for use in enclosed areas.

Intumescent coatings have excellent adhesion to steel substrates and high impact resistance.

They can be prefabricated into panels to form fire rated divisions and cast onto pre-formed metal chassis in order to provide protection to equipment

Ablative Coatings

This type of organic coating gradually erodes under fire exposure due to the absorbed heat energy input that changes the virgin solid coating into a gas composite.

This action prevents heat absorption into the substrate to which it is applied.

Like intumescent coatings, they are resistant to mechanical damage but the application procedure is complex which contributes to relatively high application costs.

Subliming Compounds

The active ingredient in this type of coating absorbs heat as it changes directly from the solid to a gas phase (sublimation). As in the case of ablative coatings, intumescents are incorporated to provide an additional insulating layer.

The degree of protection provided by subliming compounds is a function of the temperature of sublimation for each particular compound, the thickness of the coating material, the heat capacity of the substrate and the degree and time of fire exposure.

Intumescent Coatings

This training will describe how intumescent coatings can achieve passive fire protection in many structure types including offshore constructions, ships and commercial buildings.

We will review the requirement for fire protection, the types of fire protection, testing and methods of application along with survey and maintenance.

We will look at the role of the fireproofing inspector.

Intumescent Coatings

Intumescent coatings have been used to protect the steelwork in buildings and other structures from fire for approximately 40 years.

These coatings work by swelling up in the event of a fire and physically creating a barrier between the steel and the fire for up to four hours.

Steel typically starts to lose its structural strength at temperatures above about 400^oC (750^oF) and these coatings can delay the time it takes to reach this temperature, allowing time for personnel to escape and fire crews to tackle the fire, if practical.

Intumescent Coatings

Intumescents are often referred to as thin-film or thick-film coatings.

Thin-film coatings can be solvent or water-based products and have dry film thicknesses (DFTs) of less than 5,000 microns (200 mils) or 5 millimetres (0.2 inch).

Thick-film coatings are typically solvent-free, epoxy-based with DFTs of up to 25,000 microns (1000 mils) or 25 millimetres (1 inch).

Intumescent Coatings

The acceptance and use of intumescent coatings increased dramatically across the globe in the 1970s as the major oil companies learned of their ability to protect structural steel from the extreme heat caused by hydrocarbon fires, including jet fires caused by leaking hydrocarbons.

In 1988 an explosion and subsequent oil and gas fires at the Piper Alpha, a North Sea (UK) oil production platform, resulted in the deaths of 167 people and $3.0 billion in damages.

Intumescent Coatings

The severity of this disaster, considered the worst offshore oil disaster at the time, prompted increased development and use of intumescent coatings for protection against hydrocarbon fires. The coatings developed tended to be thick-film coatings, often with mesh reinforcement.

Also, in the 1980s, exposed steel was used more prevalently in the design of commercial structures and high-rise buildings, increasing the use of thin-film intumescents which looked more like conventional paint and therefore could meet the aesthetic requirements of architects.

How Do Intumescent Coatings Work?

Intumescent coatings react to fire by expanding to form a carbon “char” with low thermal conductivity, which essentially forms an insulating layer reducing the rate of heat transfer and extending the time necessary to reach the critical failure temperature of the underlying steel.

It’s a complex chemistry incorporating the organic (coating) binder resin and an acid catalyst, for example, ammonium polyphosphate, which decomposes to yield a mineral acid.

How Do Intumescent Coatings Work? (cont)

This acid reacts with a carbon source, for example, pentaerythritol, to produce a carbon char.

A spumific (foam producing) agent, such as melamine, reacts with the acid source and decomposes, evolving into an inert gas which then expands the char.

These are the basic reactions taking place, although more complex interactions also occur. For example, filler particles are incorporated into the formulation to act as nucleating sites or “bubble growth” sites and the resin binder plays a large part in softening and charring. A reinforcing mesh can be used to support the formed char.

Cellulosic vs. Hydrocarbon Fires

A cellulosic fire has a fuel source composed mainly of cellulose — for example, wood, cardboard or paper.

A temperature curve for a cellulosic fire would look like this between 5 minutes and 60 minutes:

1. At the end of the first 5 minutes 576^o C (1070^oF)
2. At the end of the first 10 minutes 679^o C (1250^oF)
3. At the end of the first 15 minutes 738^o C (1360^oF)
4. At the end of the first 30 minutes 841^o C (1545^oF)
5. At the end of the first 60 minutes 945^o C (1739^oF)

Cellulosic fires are slower to reach maximum temperature but may eventually reach or surpass the temperature of a hydrocarbon fire.

Cellulosic vs. Hydrocarbon Fires

Hydrocarbon fires can reach temperatures around 1,000^oC (1830^oF) in less than five minutes.

A hydrocarbon pool fire typically has a temperature curve as follows:

1. At the end of the first 3 minutes 880^oC (1616^oF)
2. At the end of the first 5 minutes 945^oC (1735^oF)
3. At the end of the first 10 minutes 1032^oC (1890^oF)
4. At the end of the first 15 minutes 1071^oC (1960^oF)
5. At the end of the first 30 minutes 1098^oC (2010^oF)
6. At the end of the first 60 minutes 1100^oC (2015^oF)

A pool fire is defined as a turbulent diffusion fire burning above a horizontal pool of vaporizing hydrocarbon fuel where the fuel has zero or low initial momentum.

A jet fire is a turbulent diffusion fire resulting from the combustion of a fuel continuously released with high pressure.

Testing Intumescent Coatings

No two fires are the same. The conditions depend on the type and quantity of fuel, the availability of oxygen and ambient conditions. For reproducible product testing. “standard” fires have been defined. UL 263 “Standard for Fire Tests of Building Construction and Materials”,

ASTM E119 “Standard Test Methods for Fire Tests of Building Construction and Materials” British Standards BS 476 (parts 20 and 21) “Fire tests on building materials and structures” and EN 13381 (part 8), “Test methods for determining the contribution to the fire resistance of structural members” describe how intumescent coatings are tested with cellulosic fire exposure.

Performance depends on coating thicknesses, the types of steel section, I sections, hollow sections and the section orientation, i.e. beam or column.

Testing Intumescent Coatings

Thermocouples are used to measure furnace temperature and core steel temperature. Other test standards include MIL-STD-3020 “Fire Resistance of U.S. Naval Surface Ships, UL 1709, “Rapid Rise Fire Tests of Protection Materials for Structural Steel” for hydrocarbon fire exposure, ISO 22899-1, “Determination of the resistance to jet fires of passive fire protection materials” and IMO Resolution A.754 (18), “Recommendation on Fire Resistance Tests for ‘A,’ ‘B’ and ‘F’ Class Divisions” for fire protection of decks, bulkheads and doors on marine vessels.

It is not possible to test every variation, so the test results are analyzed to produce an assessment of performance.

Ensuring Durability

To protect steel in a fire a coating must be resistant to the environment and be intact at the time of the fire. Poor durability can lead to ineffective fire protection resulting in structural failure during a fire and expensive restoration afterwards.

Poor durability can also lead to corrosion of the substrate, compromising structural integrity. To ensure the durability of intumescent coatings the key ingredients — ammonium polyphosphate, melamine and pentaerythritol — are all sensitive to moisture and must be formulated carefully.

Different resins are used to formulate intumescent coatings for different applications. Water-based acrylic materials are formulated for use in mainly dry, internal locations.

Ensuring Durability

Solvent-based acrylic materials are used to formulate intumescent coatings for use in internal or sheltered external locations.

Solvent-based or solvent-free epoxy materials are used to formulate intumescents that can be used in any location. These resins have different weathering performance, and therefore, protection capabilities.

To test the durability of an intumescent coating, standard coating test procedures are used such as:

• ASTM D638 “Tensile modulus, strength, and strain”
• ASTM D790 “Flexural modulus, strength, and strain”
• ASTM D695 “Compressive modulus, strength and strain”
• ASTM D2794 “Impact resistance”
• ASTM D2240 “Shore D hardness”
• ASTM C177 “Thermal conductivity”
• ASTM E1269 “Specific heat”

and NORSOK M 501, “Surface preparation and protective coating,” Underwriters Laboratory, UL 1709, “Rapid Rise Fire Tests of Protection Materials for Structural Steel” and European Technical Approval Guidance, ETAG 18-2, “Reactive Coatings for Fire Protection of Steel Elements. ”In addition, the intumescent coating should not spall or crack in use, be resistant to atmospheric and chemical attack and be recoatable with itself — even after prolonged curing.

There should also be excellent bonding between substrate, primers and the intumescent to combat the problems of under-film corrosion.

Specifying Fire Protection

Firstly, the item to be protected must be identified, whether it is structural steel, vessels or divisions such as fire-resistant bulkheads or decks on ships. The general rule is, the thicker the coating, the longer the protection – up to a limit.

The thickness of the intumescent used will depend on the weight and type of the steel member being protected.

As the weight of steel decreases, the thickness of the intumescent should increase.

Lightweight steel sections will heat up faster than heavier sections and will, therefore, need more protection for a given time.

Specifying Fire Protection

Rather than just figuring the weight of the steel, specific calculations must be made in order to determine the appropriate thickness of the coating, taking into consideration the shape or shapes of the steel and accounting for any cutouts or irregularities in the beams.

The critical steel temperature which must be protected against should be defined — for example, structural steel between 200 and 750^oC (390 and 1380°F), vessels between 200 and 350^oC (390 and 660°F), or an averaged 140^oC (285°F) temperature rise or 180^oC (355°F) point temperature rise for divisions where the critical temperature requirement is much lower to protect personnel on the other side of the division or in a safety refuge.

Specifying Fire Protection

Next, the section factor must be considered, as well as the fire protection period of between 30 minutes and four hours. The section factor (Hp/A and W/D) is the ratio of the fire exposed perimeter to the cross-sectional area of the steel.

Most intumescent coating suppliers provide guidance in calculating the thickness of the coating required for a specific use and some have dedicated departments staffed with trained fire engineers who will do the calculations for you.

Consideration must also be given to the service environment the structure or vessel will be exposed to as well as any special requirements such as blast resistance, high or low substrate temperature or cryogenic spill protection.

Specifying Fire Protection

Section Factor W/D

The W/D section factor relates to the relative surface area that can be directly heated (exposed to the fire) and the mass (or weight) of a piece of steel.

W = Weight of section (in pounds/foot)

D = Heated perimeter (in inches)

This diagram illustrates that the thicker steel beam has a higher W/D and will require thinner PFP while the thinner steel beam has a lower W/D and will require thicker PFP. It is important to recognise that this is the opposite of Hp/A.

Summary

Within this training unit, we have given an overview of passive fire protection, specifically with the use of intumescent coatings.

In addition to offering fire protection for up to a number of hours, intumescent coatings offer speed of application, shop or field application, aesthetic appearance and ease of inspection and maintenance.

Intumescent coatings can protect a variety of steel surfaces from structural columns and cellular beams, to building components, vessels and complex shapes.

They can be formulated to protect against cellulosic and hydrocarbon fires including jet fires and fires resulting from explosions.