FIRE PROTECTION - PASSIVE
Fire protection that carries out its function without requiring manual or automatic initiation of its operation in the event of fire
FIRE PROTECTION - ACTIVE
Fire protection system which, in event of fire, can function only after its operation has been manually or automatically activated.
The
methods used to protect steel are:
Let us consider each of these
separately.
a.
FIRE RESISTING BOARDS.
These
are fitted around the steel columns and beams to form a box and are usually
secured by fixing "noggins" into the web of the steel with friction
or adhesive and then securing the board to the noggin with screws.
They protect the steel by providing a heatproof insulation in some form
and the mechanism of this varies with individual formulation of the board.
Correctly fitted boards must have noggins of the same size, placed at
the same intervals, as the original test installation and should have the same
number of screws and other fixings.
The
boards can be made up of anything from gypsum-based plasters or calcium
silicate, through fibres and specialist vermiculite containing materials.
Boards come in a wide variety of thicknesses and it is essential to
ensure that the correct board thickness is chosen to give the required period
of protection.
They
are sometimes fixed with special, corkscrew like screws, which are used when
boards are of a particularly soft pliable nature. In modern systems some manufacturers have special spring
clips to secure the bottom board, in other cases they rely on forming an
almost self-supporting box. In
many cases the boards will require decoration if they are exposed to view.
Services can obviously be
fitted behind these boards and it also follows that whenever these services
have to be maintained the fire protection has to be removed to provide access
to the services. Afterwards the
board system will need to be refitted in line with the official requirements.
Boards can be left untreated in not in public view, or can be decorated
for aesthetic reasons. Manufacturer
developments will try to find alternative fixing methods and therefore careful
note should be taken of their
instructions.
b. VERMICULITE CEMENT SPRAYS.
These are probably the least
expensive form of fire protection and can often be seen in places like
multi-storey car parks and basement areas of buildings, where a very rough and
thick coating has obviously been applied to the profile of the steel. The material is supplied to the contractor as a dry powder
made up of cement and exfoliated vermiculite.
The vermiculite contains air and protection is provided by a
combination of water evaporation and insulation.
The material is mixed in a
portable paddle mixer with water before application and the amount of water is
critical to the application and fire performance of the coating.
It is then sprayed onto the surface using a screw feed pump usually
manufactured by a company called Putzmeister.
Vermiculite sprays are widely used internally and also on petrochemical
plants where special weatherproof grades are available.
In certain circumstances they require steel wire mesh support and weld
pins to anchor the mesh. A
water-repellent coat is often applied to give protection against moisture.
Thickness varies from around
10/12 mm up to 50 mm and, as the material is highly alkali they require 3epoxy
or other alkali resisting primers to be applied to the steel after blast
cleaning.
This type of material can also
be used for concrete protection, particularly common in France and certain
other countries. Tunnel lining
can also be protected in this way.
c.
FIBRE
SPRAYS.
These
are very similar in nature to vermiculite sprays and everything I have said
applies to these. The only difference is that this type of material uses
mineral wool and insulates by providing a thick layer of heat resistant
material onto the surface.
d.
DRY
LINING SYSTEMS.
Essentially
boarding as described above is dry lining but other ways of boxing in
steelwork using heat resistant materials can also be undertaken. Examples are
steel cladding systems, which are insulated with layers of mineral wool behind
the steel cladding, and other types of boarding systems built to create a void
around the steelwork.
Such
systems are often pre-finished and particularly popular when the level of
aesthetic finish is important.
e.
MINERAL
WOOL
Cladding
with mineral wool slabs or blankets which is used in ceiling voids or other
areas not requiring an aesthetic finish is also common.
In many instances the mineral wool will have foil facing.
The material is fixed usually with adhesives sticking
"noggins" into the web of the steel.
Once again it can easily be removed when surfaces are repaired and
altered but will require refitting afterwards.
Weld pins can also be used for fixing with large washers or clips to
secure the blanket. It provides fire protection by wrapping a
non-combustible layer of insulating material around the steelwork and thus
slowing the rate of heat ingress into the steelwork because of the insulation.
f.
INTUMESCENT
PAINT COATINGS.
These paints, or coatings,
offer the most complex method of providing structural fire protection.
They work by changing their nature from a decorative paint, which has
been applied to the steelwork, into an intumescent layer of carbonaceous char,
which forms when the coating is subjected to heat.
This layer of char can be 50 times the thickness of the initial coat,
and is formed as the paint is heated to around 200C and above.
At these higher temperatures the resin system melts and allows the
release of a mineral acid, which reacts with a carbon rich element in the
paint to form a carbon char.
Also
released at the same time is a spumific, which provides a gas, which expands
the foam to form the thicker layer. As the fire progresses and time passes
this layer of char grows thicker thus increasing the insulation provided.
The outer surface of it finally becomes soft and friable rather in the
same way as the charcoal on a barbecue turns into a dry white powder.
During
this period energy is being absorbed by the reaction within the insulating
layer being formed thus limiting the amount of heat which passes through the
coating to the steel below. The
resin systems used to bind the chemicals together as paints vary and may use
either a hydrocarbon solvent (usually xylene), or water as the dillulant of
the resin. These have become
known in industry jargon as either 'water based' or 'solvent based'
intumescents, which is not technically correct but understood.
The
materials are applied at very widely varying film thicknesses from as little
as 275 microns for 30-minute protection in some areas up to 6mm for 120-minute
protection. This thickness is
required from the base coat, which is the active element of the system and may
require application in many coats at the higher thicknesses. They are most widely used for 30 minute and 60-minute work
where one spray coat will provide the required dft.
A sealer coat is normally required and steel again should be blast
cleaned and primed with a zinc phosphate primer in most cases.
Many primer types can be used according to manufacturers instructions.
There
are also materials known as 'thick film intumescents', which are used
mainly in the petrochemical industry. They
work in the same way but differ by using epoxy resin systems rather than the
softer types used in thin film materials. The material expands at slower rate
and to only 5 times the original. The
char formed is much tougher and the protection provided is capable of
withstanding the high temperatures of hydrocarbon fires and jet fires.
These products are rarely used in the building industry but common for
steel protection in the offshore and petrochemical industry.
They can be very successful on gas storage tanks and similar
circumstances.
The
use of both types of intumescent paints requires blast cleaning of the steel
and suitable primers and topcoats.
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