Generally, there are three common types of fire that PFP systems are tested for:
- Cellulosic fires: These are fires characterized by the burning of cellulosic products such as wood, paper, furnishings, etc. They have a relatively slow rate of temperature rise with heat fluxes of typically 100 kW/m2. There are little or no erosive effects in this test.
- Hydrocarbon fires: These are fires characterized by burning hydrocarbon fuel, typically from a pool of burning fuel or burning gas cloud. Heat fluxes typically range from 150 to 250 kW/m2. Again, there are little or no erosive effects in this test.
Jet fires: The most recently developed fire test standard is the jet fire test. A jet fire is characterized by the ignition of a discharge of flammable material under pressure. Heat fluxes are dependent on type of fuel, fuel/oxygen mixture, flame size, etc, but are typically around 250 to 300 kW/m2 with potential peaks up to 350 kW/m2. The major difference is that the jet fire produces large erosive forces that can be very destructive to some PFP systems. See comparative graphs of fire test curves below.
PFP physical properties
Cryogenic Spill Resistance – Thermal Shock Protection:
Where there is the risk of LNG spillage the PFP system should be capable of withstanding the thermal shock of being exposed to temperatures as low as -196°C (liquid nitrogen storage). In addition, the PFP system may be required to offer thermal shock protection to the underlying substrate to prevent brittle fracture or cracking.
Intumescent fire protection systems offer a high degree of resistance to the loads imposed by blast overpressures. Additionally, their low film thickness ensures that they do not significantly add to the surface area exposed to the blast and hence minimize the load that is subsequently transferred back into the structure.
It is essential to identify the normal operating temperature range of the system being protected. This is to ensure that the correct heat rise is identified and also to ensure that the intumescent PFP materials, which contain heat reactive chemicals, are correctly selected and specified for the project.
Environmental Resistance and Corrosion Protection:
The PFP should be designed and tested to demonstrate that its constituent parts and the final assembly are capable of withstanding the effects of the operating environment, high humidity, salt-water environment, deluge testing and thermal cycling. When so designed, the system will minimize the risk of corrosion to the underlying substrate/equipment. Test protocols (such as the NORSOK M-501 standard) will typically include cyclic testing against exposure to salt spray, drying in air and UV exposure, immersion, freeze/thaw testing and subsequent fire testing to demonstrate that the fire performance is not impaired.
Resistance to Mechanical Damage:
The system should be designed to withstand a high degree of mechanical impact from items such as dropped objects (hand tools, scaffold clips, etc.) as well as offering resistance to damage from personnel traffic in congested areas.
Life Cycle Performance and Maintenance:
The PFP system should be designed using materials and construction that will maintain its performance throughout the design life of the installation or facility. This is typically in the region of 20 years for new builds, but may be only 5 to 10 years on existing installations.
Critical core temperature
The purpose for establishing critical core temperatures (CCT) is to find a point at which the item being protected will maintain its designed properties:
For example, structural steel typically maintains its full load bearing strength up to temperatures of 400°C, after which its strength reduces with temperature to the point where it buckles and collapses.
Fire resisting divisions, where they are used top separate areas on an oil platform or FPSO, are typically fire protected to ensure they retain their structural integrity by restricting temperature rise to 400°C; this prevents the passage of smoke and fire from one area to another. However, where they are used to protect normally manned areas, such as temporary refuges or control rooms, they may also be fire protected to maintain a back face temperature rise of less than 140°C to protect any personnel located behind the division.
On items such as vessels, pipework, valve and actuators the fire protection may be designed to maintain temperatures below the point where there is the risk of failure of pressure containment or where the item stops functioning as designed. This may be in the region of 200 to 300°C for valves, pipework and vessels, or as low as 80°C for items such as actuators.
The required thickness of fire protection is dependent on four main factors:
- Type of fire: Cellulosic, hydrocarbon, jet fire.
- Duration of protection: Typically for 15 minutes up to 2 or more hours.
- Critical core temperature: The temperature that the item should be kept below during the protection period to perform as required.
- The configuration of the item being protected: The amount of fire protection required is based how much heat it is exposed to in proportion to how much heat it can absorb before the critical core temperature is reached.
This last factor is often calculated by means of an Hp/A calculation where the amount of PFP is dependent on the ratio of the Heated or exposed perimeter (Hp) of the item to the cross-sectional area (A) (this being proportional to the mass of steel) of the item. From this ratio, tables of tested thicknesses can be used to establish how much PFP is required. See diagram below:
The section factor dictates the rate at which a steel section will heat up when exposed to a fire. It therefore follows that it also dictates the amount of intumescent fire protection that is required to effectively insulate and slow down the heating rate. The higher the dry film thickness (DFT) of intumescent coating, the greater the insulating properties. However, that is not to say that you just put on as much as is required. The aim is to have an intumescent coating which can provide the intended protection at the lowest dry film thickness, thus keeping material costs and added weight as low as possible.