We have seen factors & causes of fire. Here we will see safety measures taken to prevent the fire.
The normal measure used to safeguard personnel assets and the environment can be classified into active and passive system
Active Fire System
Active system consist of automatic arrangements to detect fires and the release of flammable gasses at the incipient stage and to automatically deploy the necessary protective actions which will include the following
3. Release of the fixed fire protection medium like water spray / deluge / foam so as to control and / or extinguish the fire.
Passive Fire Systems
Passive fire systems has been used for many years as the primary element of the overall safety strategy to minimize the consequences of a hydrocarbon fire.
Refer to passive fire protection and need for it.
Passive fire protection is achieved by insulating the surface of a structure or vessel with a protective coating or cladding system, which will, in the event of fire; provide thermal protection to the substrate to which it is attached
Types of Coating – Organic And Inorganic
A. Inorganic Coatings
The most commonly employed type of PFP universally used is the inorganic coating which are suitable for high-risk installation. Viz.
a. Lightweight Vermiculite Concrete
These consist almost entirely of factory produced vermiculite cement premixes, which are applied to the substrate, either by spray or hand troweling. They would in all cases be reinforced with a suitable galvanized steel meshing system, and are applied in varying thickness to suite the protection time required.
b. Water of Hydration Plasters
These coatings are plaster compounds, which undergo chemical and physical changed to release water vapor when exposed to high temperatures. The postulated theory is that the temperature of the protected substrate will be limited to the temperature of dehydration and that the heat input from the fire will be absorbed by the dehydration process and in the vaporization of the water vapor produced by various reactions.
This type of coating appears to exhibit a high degree of hygroscopicity and limited ability to withstand exposure in outdoor environments. In view of the extreme likelihood of corrosion occurring to the substrate, these coatings must not be used outdoors.
Perhaps the most important feature of vermiculite cement is their ability to perform under real fire conditions, which is well documented over many years in major incidents in hydrocarbon and potential high-risk industries.
Vermiculite cement is also used extensively on concrete structures, reducing the rate of heat input and thus controlling the steam pressure build up within the concrete itself. This process prevents explosive spalling of the concrete during exposure to hydrocarbon fires.
Since vermiculite cement are inorganic they do not degrade with time and examples of structures and vessels protected during the 1950`s still exist with little evidence of corrosion to the underlying substrate.
i. What is Vermiculite?
Vermiculite is a member of the phyllosIlicate group of mineral resembling mica in appearance consists of golden / brown flakes. It is found in many parts of the world locations of the predominant commercial mines are in Australia, Brazil, China, Kenya, South Africa, USA and Zimbabwe but only a limited number of sources are worked as commercial deposits. The vermiculite is mined and refined using a variety of techniques and supplied commercially in a range of particle size, grades of vermiculite concentrate in unexpanded form.
Vermiculite is most commonly used in the exfoliated form and has been used in various industries for over 80 years. It is used in the construction, agricultural, horticultural, and industrial markets.
ii. Exfoliated Vermiculite
The exfoliation process is carried out commercially bypassing crude vermiculite through a furnace chamber in a controlled furnace, the crude vermiculite then expands to the cleavage planes producing concertina shaped particles many times their original volume.
The majority of applications call for vermiculite in its exfoliated form the concertina shaped granules structured from a blend of exfoliated vermiculite is a very versatile mineral because of being non-combustible has good thermal stability and also inert. It is clean to handle odorless mould resistant and also sterile due to high temperature to which it is subjected in production.
Essentially, it is lightweight concrete where the aggregate is exfoliated vermiculite, a very light, and hollow matrix particle having excellent temperature stability and thermal insulation properties.
The ability of vermiculite to release stress by volume change enables it to withstand both hot and cold thermal shocks without spalling when subjected to water impingement from sprinklers, fire hoses or monitors during fire exposure
Other materials are added to control porosity, increase resistance to thermal shock and assist pumpability during application.
Portland is cement used as a binder which due to its water of hydrant when mixed and applied, release steam in fire, cooling the layers between the steam generation front and the fire. Once all of the water of hydration is used up, the resulting dry vermiculite cement coating acts as a thermal insulation.
A very useful attribute of Portland cement is its ability, once the fire extinguished is to recombine moisture from the atmosphere to replace water lost to the fire provided that the calcining temperature has not been exceeded 380°C, its strength will not be significantly affected. This gives virtually “as good as new” fire performance for any flashback fire which may occur.
Because vermiculite cement is pours, they can absorb water from fire water system incident upon them. This generally increases their fire endurance properties.
iv. WHAT ARE THE ADVANTAGES?
Vermiculite being apparently a tenth of weight of sand is either blended with either gypsum plaster or Portland cement and other materials to produce a product that besides possessing excellent thermal characteristics. Offers a fast reliable application with reasonable low-in-price costs and yet has a high degree of fire resistance qualities.
– Excellent fire performance
With all parameters a designer must investigate when selecting a passive fire protection material, it is sometimes easy to overlook the basic fire performance.
– Non- combustibility
Vermiculite cement are essentially inorganic materials and will not burn, they can therefore produce no smoke or toxic products and are certified non-combustible. This means they can be used in life safety where organic based coatings will be unacceptable.
Typical vermiculite cement products for oil, gas, and petrochemical applications have a bulk density of 550kg/cum., which is approximately less than one fourth of the density of reinforced concrete.
Strength is less than that of organic coatings or magnesium oxy-chlorides, however, vermiculite cement are designed to be durable in service whilst allowing cutouts to be carried out without great difficulty. An added advantage for inspection and maintenance requirements.
– Ease in application
Vermiculite cement is normally supplied as a factory controlled – blended dry premix aggregate. They are mixed with water at site and spray applied through reliable, well-proven equipment. Repairs can be carried out by hand easily.
The use of specialist applicators ensures correct installation and a high standard of workmanship, when backed by extensive QA/QC systems – ensures durability of the coating
– Low installed cost
Vermiculite cement is the most cost effective & efficient systems in use for the risk involved.
v. WHAT ARE THE DISADVANTAGES?
– They are porous
This is only of consideration for exterior applications but can be countered by attention to the corrosion resistant paint systems applied to the steel and the use of a topcoat paint system where necessary, together with attention to water shedding and mastic sealing.
– They are relatively thick
Vermiculite cement thickness normally ranges from 8mm – 35mm where the thickness of other system is 5mm – 20mm at best and 30mm – 100mm at worst. If clearly defined at the design stage, thickness should be little concern.
– They do not provide corrosion protection to steel
Vermiculite cements are fire protective coating and while they do not cause corrosion to steel, they cannot inhibit it. Corrosion protection is provided by the paint system applied to the steel.
– They are weaker than magnesium ox chlorides and epoxy in tumescent but stronger than fibre-cements. Impact damage is absorbed by deformation to the shape of the impact object. Harder materials may be brittle and crack.
– They have limited flexibility
Being essentially inorganic, they are not ‘plastic’ in they way they behave. However, within the constraints or normal design deflections and operational movements, their flexibility is adequate.
If we now examine the individual industry requiems separately, the versatility of vermiculite cement system can be clearly identified.
B. Organic Coatings
Organic Coatings are essentially sacrificial epoxy based products that are normally applied to the substrate as a number of thin sprayed or painted coats, generally without any reinforcement and comprise
a. Ablative Coating
The fundamental principle to provide a coating that gradually erodes due to the absorbed energy input from a fire to change the virgin solid coating into a gas composite requires heat input, which would otherwise be absorbed by the substrate is retarded in direct proportion to the ablative coating thickness and its thermal properties. Ceramic intumescents under fire exposure, transforms into a micro porous char layer which provides additional insulating properties.
The major disadvantage of this type of fire protection coating are the complexity of the application procedure, the inability of the char to withstand the application of water from fire hoses and the total installation costs.
b. Subliming Compounds
The degree of protection provided by subliming compounds is based on 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 heat exposure
The active ingredients absorb heat as it sublimes i.e. changes directly from a solid to a gas phase.
As in the case of ablative coatings, intumescents are incorporated to provide an additional insulating layer
The disadvantage of this type of fire protection is the coating must be applied to substrates over a specific primer conditions. They also require a topcoat, which needs replacing at 5 to 7 year intervals.
The char formed during a fire, can be easily dislodged by the impingement of water from the hose,
c. Intumescent Mastic Composition
The two most common of this type of coatings are a modified vinyl, heavy bodied mastic containing inorganic fibres in an aromatic solvent, and a reinforced two component 100 percent solids – spray epoxy system.
These coatings react by absorbing heat in a chemical reaction which generates a foam char system on the flame exposed side of the coating additional heat input is used to drive the liberated gases through the matrix. The foam char forms a thermal insulator. The period of substrate protection depends on the coating thickness the applicable thermal properties and the period and the intensity of heat exposure the heat capacity of the substrate significantly affects the period of protection for a given coating thickness.
These coatings require a significantly greater thickness than either subliming or ablative coatings for a similar degree of protection. The foam char formed is very friable and can be easily dislodged by fire hoses and in some cases may be dislodged by the thermal uplift in the fire.
Another disadvantage of some of the Intumescent appears to be the propensity of the active ingredients, to leach out over prolonged periods of exposure to outdoors environmental conditions. Once such a leaching has occurred the protection time is significantly reduced from the initial rating period.
It has been a common miss-belief that fire tests are test of fire protection materials, they are not, but are test of structural elements of construction protected where necessary with fire protection materials