Protective coatings for high-risk structures
Published: 01 June, 2007
Protective coatings are designed to provide a degree of tolerance of tolerance to the steelwork of industrial structures whilst a fire is brought under control.
Two common scenarios of fire may occur in a hydrocarbon-processing complex. The first is a pool fire and this occurs when a flammable liquid leaks from a vessel or pipeline to form a fluid reservoir, which then ignites.
The second and potentially more dangerous type is a jet fire, which can happen following the rupture of a pressurized vessel and or gas pipeline.
Comments Ian Stewart of PPG Protective & Marine Coatings: “A hydrocarbon fire will generate temperatures of more than 1,000°C within ten minutes of ignition with heat fluxes of around 150 KW/m2. A jet fire will exhibit the same temperature rise, but the heat flux could be double that of the pool fire.”
There are two types of passive fire protection normally used in hydrocarbon processing complexes, namely cementitious and epoxy intumescent.
The cementitious approach
Cementitious products are generally based on Portland cement with exfoliated vermiculite and lightweight aggregates. These products are delivered to the jobsite in sacks or one-ton bags and are mixed with water before being spray or trowel-applied.
Cementitious materials protect the steel in two ways. Firstly, they contain trapped water of crystallisation, which in a fire situation will be released and keep the steel around 100°C until all the water has been released. The product then acts secondly as an insulator.
Epoxy intumescent coatings involve two component, usually solvent-free materials, which are delivered to the jobsite in drums. These are mixed together before being spray and trowel-applied. In the event of fire these coatings intumesce (i.e. they swell to 4 or 6 times their original thickness) to form a tough insulating carbonaceous char.
Typically, the areas which must be fire protected involve land- based oil and gas-processing facilities where pipelines, vessels, structural steel and storage tanks are at risk.
Test & standards
It is important that passive fire protection products are fire tested to the appropriate standard. In the past hydrocarbon fire testing was carried out to BS476 Part 20 and 21 :1987 following the hydrocarbon time/temperature curve.
In the US testing is carried out according to the Underwriters Laboratory UL 1709 hydrocarbon fire test. Recently, fire testing has been carried out to the European Standard ENV 13381-4:2002 following the hydrocarbon time/temperature curve using the new plate thermocouple to measure the furnace temperature.
Jet fire testing is carried out in accordance with OTI 95-634 Offshore Technology Report - Jet Fire resistance test. In France, a GASAFE test is carried out for the passive fire protection of LPG and LNG Storage tanks.
The passive fire protection material is applied at a thickness to maintain the steel, in the case of structures, below the critical temperature of 550°C. The thickness depends on the steel section size.
Typically, passive fire protection materials would be expected to delay the collapse of a loaded steel section for 2 hours in a hydrocarbon fire. The higher the mass-per-linear-metre of the steel section, the lower the thickness of passive fire protection will be required for the same time period.
In the case of steel vessels, pipelines and storage tanks consideration has to be given to vessel wall thickness and the likely temperature at which the steel would rupture. The thickness of passive fire protection applied to the surface of a vessel would be related to critical temperature, which is typically 400° C. The process engineer would define the actual critical temperature at the design stage.
Improving your protection
The life of the passive fire protection system will always be dependent on using the best primer system to give optimum corrosion-protection of the underlying steel. In addition, weather protection and chemical-resistance can be improved by using the latest advanced protective coating topcoats e.g. engineered epoxy polysiloxane topcoats.
By comparison, passive fire protection products such as cementitious and epoxy intumescent offer many benefits. The primary benefit being that the fire protection is not reliant on a system that requires activation either automatically or manually.
When properly-applied by experienced installers, passive fire protection products require little maintenance and thus their whole life cost is low.
“Whether it’s a pool fire or jet fire, there is a passive fire protection solution to a hydrocarbon fire exposure. These solutions in many cases have been in use since the 1970s and now have the track record to complement our knowledge of their long term performance,” concludes PPG’s Ian Stewart.