Passive Fire Protection measures are intended to contain a fire in the fire compartment of origin, thus limiting the spread of fire and smoke for a limited period of time, as determined the relevant fire code. Contrary to active fire protection measures, Passive Fire Protection installations do not require electric or electronic activation or a degree of motion, so the risk for malfunction is virtually eliminated.

The purpose of Passive Fire Protection:

1. To protect human lives
2. To protect assets

When materials burn, they give off energy in the form of heat. The time that a material takes to burn and either become consumed or change its physical shape or properties can vary, depending on the material. The one common factor is time – it takes a very short time for fire to destroy or heat materials to a point where they change their physical properties. When petrochemical materials such as oil and gas burn, the increase in temperature is rapid which can cause structural materials to degrade and then collapse before people have time to leave the fire zone.

Protect human lives

To protect human lives, the structural strength of steel must be kept below the core temperature for a certain length of time, as specified for each of the different fire classes. Core temperature is the temperature when the structural properties change and become weaker leading to material distortion and collapse.

Design should allow to keep flames and smoke away from a designated area for the specified time. It is smoke and flames that kill first, but structural breakdown, preventing escape is just as serious. The figures given for the different Fire Classes are the time that the material must insulate under fire conditions:

• Class H: 2 hours
• Class A: 1 hour
• Class B: ½ hour

Fire cells are made up of classified fire divisions with a purpose to keep flames and smoke away from a designated area and prevent fire from spreading outside the fire cell within a given time period.

Protect assets

The PFP should insulate the exposed walls, ceiling, floors and structural members of an asset to keep the substrate temperature under certain specific values for a specified time. The time gained by using PFP is used to allow personnel to escape the fire zone and for firefighting personnel to access the fire without having to deal with structural collapse.

Two types of fire

• Cellulosic Fires
  • Fires that involve burning of materials such as wood/timber, fabrics and textiles
  • PFP for Cellulosic fires are not suitable for Oil and Gas fires
• Hydrocarbon Fires
  • Fires that involve the burning of Hydrocarbon based materials such as oil and gas
  • Example: fires that could occur in offshore and onshore petroleum and chemical installations

Fire temperatures

Cellulosic fire (wood, paper):

• Temperature after 5 minutes: 556 ˚C
• Temperature after 1 hour:     945 ˚C 

Hydrocarbon fire (oil, paint, solvents):

• Temperature after 5 minutes: 926 ˚C 
• Temperature after 1 hour:   1145 ˚C

Considerations

• Section Factor (A/V)
• Shape of section (e.g. I-section, rectangular hollow section)
• Size of section
• Fire resistance period
• Temperatures attained by steel sections
• Critical temperature
• Orientation of section
• Thickness and density of applied protection
• Surface preparation of steel
• Reinforcement (if any)
• Primer compatibility
• Manner of application (brush/spray)
• Effects of over-coating
• Number of coats
• Physical performance and retention of protection material

Hp/A

The mass of the material will determine how slowly or quickly the material gains and loses heat. When exposed to the same temperature and for the same time, a steel column with a thinner cross section will heat up faster than a column with a thicker cross section.

The ratio between the size of the area receiving the heat (heated perimeter) and the mass that needs to be heated (cross section area) is used to determine the amount of passive fire protection (DFT) that is needed for each steel member. This can be written as:

Two types of thick film PFP coatings

A. Organic coatings

• Epoxy coatings
• Thickness 3-30 mm
• Swells and creates an insulating layer (intumescent)

B. Inorganic coatings

• Cementicious coatings
• Thickness 20-40 mm
• Liberates water which cools the substrate

Intumescent

Intumescent coatings are paint-like materials which are inert at low temperatures but which provide insulation as a result of a complex chemical reaction at temperatures typically of about 200-250°C. At these temperatures, the properties of steel will not be affected. Because of this reaction the coating swell and provide an expanded layer of low conductivity char. When exposed to fire, an intumescent coating generally expands in thickness by 15 to 30 times in a standard test, but there can be great variations from one product to another.

Intumescent coatings can be divided into two broad families: thin film and thick film. Thin film materials are either solvent based or water based and are mainly used for buildings (cellulosic fires). Thick film intumescent coatings were originally developed for the off-shore and hydrocarbon industries but have been modified for use in buildings as well. The following applies to thick film PFP.

Epoxy PFP char

1. Organic epoxy based PFP

Organic PFP coatings are intumescent and based on epoxy binders. Intumescent means that it swells as a result of heat exposure, thus increasing in volume and decreasing in density. Intumescent material thus produce a char, which is a poor conductor of heat, thus retarding heat transfer to the substrate (provides insulation).

A typical epoxy-based PFP system

Steel pins are welded to the substrate. After blast cleaning and application of an anticorrosive primer, wire mesh is mounted on the pins. The epoxy PFP is spray applied in the specified film thickness over the wire mesh in one or two coats, so the mesh forms a reinforcement inside the completed film. Finally, a suitable topcoat may be applied.

Modern PFP systems do not require pins and the fiber reinforced mesh is wrappes into the PFP material.

New developments in Epoxy PFP

Product development of Epoxy based PFP has led to new materials that do not require placement of any reinforcing mesh or fibre material between coats. This development, with no loss of fire endurance properties, has resulted in a product system that is approximately 40% quicker to apply.

2. Inorganic cement based PFP

As the name indicates, these materials are cement based and generally contain exfoliated (fired at very high heat) MICA which when it is added to the cement provides reinforcement and it traps water which is very important for fire endurance properties of this type of PFP. Cement based PFP materials are applied thicker than Epoxy PFP but the weight on the steel is usually the same as the Epoxy PFP due to epoxy having a higher specific gravity.

• Cured cement coatings retain water; even if you measure water the content to be less than 4%, which is very dry!
• Heat will force the retained water to evaporate
• The evaporation of the water reduces the heat take up by the PFP material and the steel substrate
• This action delays the temperature increase to the substrate giving time for evacuation and fire fighting
• Adding water during the fire can extend the duration of the PFP past tested fire duration

Application of inorganic (cement based) products

• Pins to support reinforcing mesh have to be stud welded to the steel which requires grinding before welding of pins
• Blast cleaning to SA 2½ of all surfaces after pins are installed
• Application of qualified corrosion protective primer. Normal DFT 200-300 µm
• Fastening of wire mesh (plastic coated) to the pins
• Application of passive fire protection materials to specified thickness (normally 20-40 mm)
• Application of qualified topcoat (if specified, not needed for performance)

The often high film thickness may cause some problem when trying to use “standard” WFT and DFT thickness gauges. The embedded wire mesh will cause additional problem for magnetic or electronic gauges. These are some possible solutions to the problem:

WFT

• Insert a spike (caliper), take it out, measure
• Make a comb with two teeth with the desired WFT
• Make sure the comb does not hit the mesh
• Application often done in two steps, first application to the mesh, the second after the mesh

DFT

• Drill a tiny hole, measure the DFT with a spike (caliper). Note! Must repair hole.
• Ultrasonic equipment

Fire Cell definition

A segregated part of an installation where a fire may develop without spreading to other areas within a given time.

Fire Class A

Class A fires consist of ordinary combustibles such as wood, paper, fabric, and most kinds of trash

• Requires divisions made of non-combustible materials and insulating materials, sufficiently reinforced
• Divisions shall prevent propagation of flames and smoke for a minimum of one (1) hour in a standardized fire test
• The divisions shall be insulated so that the average temperature on the unexposed side does not exceed 140 ˚C above the initial temperature
• Furthermore the temperature at any single point shall not exceed 180˚C above the initial temperature within the time limits given below:
  • Class A-60:  60 minutes
  • Class A-30:  30 minutes
  • Class A-15:  15 minutes
  • Class A-  0:   0 minutes

Fire Class B

Class B Fires are fuelled by flammable or combustible liquids, which include oil, gasoline, and other similar materials

• The divisions shall be made of non-combustible materials and shall prevent the propagation of flames for at least 30 minutes of standardized fire test
• The divisions shall be insulated so that the average temperature on the unexposed side does not exceed 140˚C above the initial temperature
• Furthermore, the temperature at any single point shall not exceed 225˚C above the initial temperature within the time limits given below:
  • Class B-30:  30 minutes
  • Class B-15:  15 minutes
  • Class B-  0:  0 minutes

Fire Class H

Class H fire divisions are fueled by hydrocarbons.

• The divisions shall be made of non-combustible materials, and insulation materials
• The divisions shall be sufficiently braced/reinforced and shall prevent the propagation of flames and smoke for a minimum of two (2)  hours of the standardized fire test for a hydrocarbon fire
• The divisions shall be insulated so that the average temperature on the unexposed side does not exceed 140 ˚C above the initial temperature
• Furthermore, the temperature at any single point shall not exceed 180˚C above the initial temperature within the time limits given below:
  • Class H-120:  120 minutes
  • Class H-  60:     60 minutes
  • Class H-    0:       0 minutes

Comparing fire classes

• Class A – Cellulose fire, i.e. a fire burning on materials made of paper, wood, fabrics
• Class H – Hydrocarbon fire, a fire burning on materials such as oil, solvent and paint,- i.e. oil related products

• The time that the divisions shall prevent the propagation of flame and smoke:
  • Divisions in class B – ½ hour
  • Divisions in class A – 1 hour
  • Divisions in class H – 2 hours