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An article by Graham Boaler, Technical Manager
Why Fire Protect Steel?
Steel begins to lose strength as its temperature rises in a fire and this loss of strength can lead to structural collapse. By insulating the steel, the rate of heat transfer can be reduced, extending the time taken to reach the structural failure temperature and ‘buying time’ for the evacuation of personnel. Similarly, fire protection may also be used to prevent escalation, which may result from the failure of process piping and vessels.
Fire Protection Classifications.
The fire protection industry conveniently groups the various methods of fire protection into two broad categories:
| Active Fire Protection |
Passive Fire Protection |
| The term ‘active’ is generally used to mean actively attacking the fire itself and includes such systems as deluge, sprinklers, inert gases, etc. It also covers detection and alarms. |
The term ‘passive’ is generally used to mean protection against fire by insulation. Such products do not attack the fire itself but insulate the substrate from the heat effects of the fire. |
Passive Fire Protection (PFP) itself can be split into 'reactive' systems and 'passive' systems.
| Reactive Systems |
Passive Systems |
| Reactive systems are those that change their physical and/or chemical nature when exposed to fire. Insulation is only provided when in the changed state. Examples include intumescent, ablative & subliming systems. |
Passive systems, when exposed to fire, do not change chemically and any physical changes are minimal. Insulation is inherent in the ‘normal’ state. Examples include, cast concrete, vermiculite board, sprayed lightweight vermiculite cement, etc. |
The range of passive fire protection products available is extensive and includes glazing, fire stopping, flame retardant coatings, partitions and structural fire protection products.
It is common practice to find active and passive systems used together, e.g. detection, alarm and deluge/sprinklers in conjunction with passive fire protection on critical structural elements and process vessels.
This article is concerned only with the passive fire protection of structural elements and process vessels and piping.
Fire Types
There are many different fire test curves available but two broad distinctions are commonly used when considering structural elements in the oil, gas and petrochemical industries.
Fire test curves designed to replicate the time/temperature profile of a ‘typical building fire’ fuelled by items such as wood and paper (commonly called the ‘cellulosic curve’ and defined in standards such as BS476 Part 20 to 22 and ASTM E119), are not really suitable for fires resulting from hydrocarbon-based fuels as these have a much faster rate of temperature rise.
There are two commonly used ‘hydrocarbon’ fire test curves; one is defined by several standards, including BS476 Appendix D, ISO 834 (hydrocarbon curve) and the Norwegian Petroleum Directorate (NPD); the other by Underwriters Laboratory in their standard UL1709.
The following diagram compares these curves:
Hydrocarbon fuel burning under atmospheric pressure is classed as a ‘pool fire’ and fires that result from pressurised inventory streams are referred to as jet fires.
Products tested to a hydrocarbon fire curve are required in both onshore and offshore oil and gas facilities, and may be required in other types of chemical plant depending on the hazardous nature of the process.
Jet Fire
Jet fires occur when hydrocarbon fuel is released under pressure through a relatively small opening such as a crack or hole. Such fires are particularly fierce with greater heat flux and high levels of turbulence, which has the potential to ‘erode’ the fire protection product. The current standard for jet fire testing is OTI 95 634 although ISO 22899-1 is under development.
Jet fire resistance is frequently required on offshore oil and gas platforms but less often required ‘onshore’. However, facilities such as gas processing, LNG plants and other process areas where flammable hydrocarbon products are under pressure, may require PFP products with jet fire resistance.
Blast Resistance
Major fires involving hydrocarbon products often start following an explosion due to the ignition of a flammable leakage. For PFP products to be effective in a fire it is essential that they remain ‘in place’ after an explosion and blast testing is designed to evaluate this capability. Currently there is no industry standard and so blast testing tends to be ‘ad hoc’ in nature.
Blast resistance is rarely specified for the PFP used by onshore facilities. Perhaps this should be reviewed when one considers the potential for explosion to occur at many hydrocarbon processing operations.
Epoxy PFP is very resistant to high levels of ‘blast over pressure’ and this is one of its major advantages.
How do Intumescent coatings work?
Intumescent fire protection products consist of various chemical compounds dispersed in a suitable ‘coating resin’ binder. These coatings are ‘inert’ at normal ambient temperatures but react when exposed to temperatures above about 200ºC. The reacted coating forms a thick carbon based ‘char’ that has a much lower thermal conductivity than in the un-reacted state. This ‘char’ effectively insulates the steel from the heat of the fire, reducing the rate of temperature rise and extending the time taken to reach the critical failure temperature.
Testing and ‘Approval’.
For most projects an owner will have to carry out a risk assessment and then satisfy an appropriate organisation that the necessary fire safety measures have been implemented. These ‘appropriate organisations’ may be regulatory bodies or they may be the Insurers of the asset. Often there may be more than one ‘interested body’ and both the relevant authority and the Insurers may need to be satisfied with the appropriate level of fire protection.
Fire protection products are tested against National or International Standards normally at independent fire testing laboratories, and may additionally require ‘certification’. Certification can take several different forms and may be by Governmental bodies in some countries, Marine Classification Societies (such as Lloyd’s Register, Det Norske Veritas, etc.) for offshore projects, or Insurance Underwriters.
Fire testing of structural elements
The shape of the structural element, ‘H-section’ or ‘hollow section’, affects the quantity of intumescent fire protection required, as does the mass per linear metre of the steel. A thicker cross-section beam for example has inherently more ‘fire resistance’ than does a lighter cross-section beam, i.e. the thicker section will take longer to reach critical failure temperature and hence will require less fire protection for a given fire resistance time period.
The relationship between the mass of the steel and the area exposed to the fire is known as the ‘section factor’ or Hp/A (heated perimeter (exposed to fire) divided by the cross-sectional area of the steel section). A heavy steel section will have a low Hp/A and a light steel section a high Hp/A - the higher the Hp/A, the greater the required thickness of fire protection.
In the offshore industry typical failure temperatures are conservative and 400ºC is used. In the onshore sector it is more likely that one will find either 538ºC (1000ºF) or 550ºC used.
Hydrocarbon pool fire resistance periods are normally determined by furnace testing under controlled conditions to the hydrocarbon time/temperature curve discussed above. Since it is impossible to test every size and shape of structural section available, a ‘matrix’ of varying intumescent thickness and section Hp/A are tested. These results are then analysed by an independent fire engineer and a table of intumescent thickness against Hp/A is given, for various critical temperatures for both H-section and ‘hollow section ‘elements.
For use offshore, these tables and the supporting reports are submitted to organisations such as Lloyd’s Register and DNV who then issue ‘certified tables’.
Process Vessels and Pipework
As above, thick wall pipes or vessels have inherently more ‘fire resistance’ than thinner walled pipes or vessels, i.e. the thicker item will take longer to reach critical failure temperature. However, process vessels and pipes are often under pressure in the hydrocarbon industry and hence lower failure temperatures may be required – often as low as 200ºC to 300ºC. Also the pressure in a closed vessel will increase as temperature rises and this can lead to a BLEVE (Boiling Liquid Expanding Vapour Explosion). BLEVE’s are a potential risk not only with process vessels but also with storage vessels such as LPG tanks.
The thickness of Firetex M90 for process pipe is based on the data used for ‘circular hollow’ structural sections. The thickness of Firetex M90 specified for process vessels, is based on a combination of data, including the testing of a half-full LPG bullet tank at BAM in Germany to the TRB 801 standard.
The Durability of Epoxy Intumescent
PFP is similar to insurance – one purchases it but hopes that it is never needed. If PFP is installed then it is important that it is capable of ‘doing its job’ many years into the future. Hence long-term durability and resistance to weathering is an important factor, which is often overlooked with the main focus (understandably) being on the ‘as tested’ fire protection data.
Epoxy resin based anti-corrosion coatings have been used for decades in the very aggressive environments found offshore and in chemical plant worldwide. The very high levels of durability provided by epoxy resin is the one of the main reasons why ‘Epoxy PFP, has become the ‘fire protection of choice’ for the offshore oil and gas sector.
However, not all epoxy PFP products are ‘born’ equal and test data relating to durability should also be considered. As well as manufacturers’ data there are industry standards such as Norsok M501 and UL1709 that address aspects of durability.
Advantages of Intumescent Fire Protection
Epoxy resin based coatings have excellent adhesion to steel, are very tough, hardwearing and weather resistant. Minimal maintenance is all that is required to enable epoxy intumescent coatings to last the ‘life of the asset’ (typically 20 years). As a consequence they are now almost exclusively used offshore and are being increasingly considered for onshore projects.
The main advantages of epoxy intumescent technology are:
- Resistance to blast (explosion)
- Resistance to pressurised inventory fires (jet fire)
- Very high levels of long term durability
- Lightweight and low film thickness
- Ease of application
- Ability to apply fire protection in controlled ‘shop’ environment. Fast ‘throughput’ of steel and ability to transport fully fire proofed sections to site with minimal handling damage.
- Do not contain water so resistant to freeze/thaw deterioration
The main perceived disadvantage is cost but whilst initial cost may be more, the overall life costs are less due to extended life and minimal maintenance requirements.
Conclusion
Epoxy intumescent fire protection products are rigorously tested not only to the ‘hydrocarbon curve’ but also to jet fires and to explosions. Although frequently not considered for onshore facilities it is feasible that such processes may have a requirement for resistance to all of these criteria.
Additionally given that one of the main attributes of ‘Epoxy PFP’ is their long-term durability, it makes sense to ensure that ‘durability’ test data is available as not all products are equal in this respect.
Currently the fire protection ‘solution of choice’ in the offshore environment, epoxy based PFP products are becoming increasingly considered for use in the onshore environment as the advantages in ‘through life cost’, durability and productivity benefits become recognised.
For further information please contact Leighs Paints International Business Unit on Tel: +44 (0) 1204 55 64 50; Fax: +44 (0)1204 52 64 52 or email us
The information detailed in this document is liable to modification from time to time due to product developments and changes in Legislation; you are advised to check with Leighs Paints to ascertain any specific requirements.
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