................................................................................................................................................
| An electric fire pump located in a fire fighting pump room. |
Fire
Fighting Systems
Fighting Systems
understandconstruction.com
It consists of
three basic parts:
· a large store of water in tanks, either underground or on top of
the building, called fire storage tanks
· a specialised pumping system,
· a large network of pipes ending in either hydrants or sprinklers (nearly
all buildings require both of these systems)
A fire hydrant is
a vertical steel pipe with an outlet, close to which two fire hoses are stored
(A fire hydrant is called a standpipe in America).
During a fire,
firefighters will go to the outlet, break open the hoses, attach one to the
outlet, and manually open it so that water rushes out of the nozzle of the
hose.
The quantity and
speed of the water is so great that it can knock over the firefighter holding
the hose if he is not standing in the correct way.
As soon as the fire
fighter opens the hydrant, water will gush out, and sensors will detect a drop
in pressure in the system.
A sprinkler is
a nozzle attached to a network of pipes, and installed just below the ceiling
of a room.
Every sprinkler has
a small glass bulb with a liquid in it. This bulb normally blocks the flow of
water.
In a fire, the
liquid in the bulb will become hot. It will then expand, and shatter the glass
bulb, removing the obstacle and causing water to spray from the sprinkler.
The main difference
between a hydrant and a sprinkler is that a sprinkler will come on
automatically in a fire.
A fire
hydrant has to be operated manually by trained
firefighters - it cannot be operated by laymen.
A sprinkler will
usually be activated very quickly in a fire - possibly before the fire station
has been informed of the fire - and therefore is very effective at putting out
a fire in the early stages, before it grows into a large fire.
For this reason, a
sprinkler system is considered very good at putting out fires before they
spread and become unmanageable.
According to
the NFPA of America,
hotels with sprinklers suffered 78% less property damage from fire than hotels
without in a study in the mid-1980s.
FIRE STORAGE TANKS
The amount of water
in the fire storage tanks is determined by the hazard level of
the project under consideration.
Most building codes
have at least three levels, namely,
o
Light Hazard (such as schools, residential
buildings and offices),
o
Ordinary Hazard (such as most factories and
warehouses), and
o
High Hazard (places which store or use
flammable materials like foam factories, aircraft hangars, paint factories,
fireworks factories).
The relevant
building code lists which type of structure falls in each category.
The quantity of
water to be stored is usually given in hours of pumping capacity.
In system with a
capacity of one hour, the tanks are made large enough to supply the fire with
water for a period of one hour when the fire pumps are switched on.
For example,
building codes may require light hazard systems to have one hour’s capacity and
high hazard 3- or 4-hours capacity.
The water is
usually stored in concrete underground tanks.
It is essential to
ensure that this store of water always remains full, so it must have no outlets
apart from the ones that lead to the fire pumps.
These tanks are
separate from the tanks used to supply water to occupants, which are usually
called domestic water tanks.
Designers will also
try and ensure that the water in the fire tanks does not get stagnant and
develop algae, which could clog the pipes and pumps, rendering the system
useless in a fire.
FIRE PUMPING SYSTEM
Fire pumps are
usually housed in a pump room very close to the fire tanks.
The key thing is
that the pumps should be located at a level just below the bottom of the fire
tank, so that all the water in the tanks can flow into the pumps by gravity.
Like all important
systems, there must be backup pumps in case the main pump fails.
There is a main
pump that is electric, a backup pump that is electric, and a second backup pump
that is diesel-powered, in case the electricity fails, which is common.
Each of these pumps
is capable of pumping the required amount of water individually - they are
identical in capacity.
There is also a
fourth type of pump called a jockey pump.
This is a small
pump attached to the system that continually switches on to maintain the
correct pressure in the distribution systems, which is normally 7 Kg/cm2
or 100 psi.
If there is a small
leakage somewhere in the system, the jockey pump will switch on to compensate
for it. Each jockey pump will also have a backup.
The pumps are
controlled by pressure sensors.
When a fire fighter
opens a hydrant, or when a sprinkler comes on, water gushes out of the system
and the pressure drops.
The pressure
sensors will detect this drop and switch the fire pumps on.
But the only way to
switch off a fire pump is for a fire fighter to do this manually in the
pump room.
This is an
international code of practice that is designed to avoid the pumps switching off
due to any malfunction in the control system.
The capacity of the
pumps is decided by considering a number of factors, some of which are:
· the area covered by hydrants / standpipes and sprinklers
· the number of hydrants and sprinklers
· the assumed area of operation of the sprinklers
· the type and layout of the building
THE DISTRIBUTION SYSTEM
The distribution
system consists of steel or galvanised steel pipes that are painted red.
These can be welded
together to make secure joints, or attached with special clamps.
When running
underground, they are wrapped with a special coating that prevents corrosion
and protects the pipe.
There are basically
two types of distribution systems
Automatic Wet systems are
networks of pipes filled with water connected to the pumps and storage tanks,
as described so far.
Automatic Dry systems are
networks of pipes filled with pressurized air instead of water.
When a fire fighter opens a hydrant, the
pressurized air will first rush out.
The pressure sensors in the pump room will
detect a drop in pressure, and start the water pumps, which will pump water to
the system, reaching the hydrant that the fire fighter is holding after a gap
of some seconds.
This is done wherever there is a risk
of the fire pipes freezing if filled with water, which would make them
useless in a fire.
Some building codes
also allow manual distribution systems that are not connected
to fire pumps and fire tanks.
These systems have
an inlet for fire engines to pump water into the system.
The inlet that
allows water from the fire engine into the distribution system is called
a siamese connection.
In high-rise
buildings it is mandatory that each staircase have a wet riser, a
vertical fire-fighting pipe with a hydrant at every floor.
It is important
that the distribution system be designed with a ring main, a
primary loop that is connected to the pumps so that there are two routes for
water to flow in case one side gets blocked.
In more complex and
dangerous installations, high and medium velocity water-spray systems and foam
systems (for hazardous chemicals) are used.
The foam acts like
an insulating blanket over the top of a burning liquid, cutting off its
oxygen.
Special areas such
as server rooms, the contents of which would be damaged by water, usegas
suppression systems.
In these an inert
gas is pumped into the room to cut off the oxygen supply of the fire.
When you design a
fire fighting system, remember the following:
o Underground tanks:
water must flow from the municipal supply first to the firefighting tanks and
then to the domestic water tanks. This is to prevent stagnation in the
water.
o
The overflow from the firefighting to the domestic tanks must be
at the top, so that the firefighting tanks remain full at all times.
o
Normally, the firefighting water should be segregated into two
tanks, so that if one is cleaned there is some water in the other tank should a
fire occur.
o It is also possible
to have a system in which the firefighting and the domestic water are in a
common tank.
In this case, the
outlets to the fire pumps are located at the bottom of the tank and the outlets
to the domestic pumps must be located at a sufficient height from the tank
floor to ensure that the full quantity of water required for firefighting
purposes is never drained away by the domestic pumps.
The connection
between the two tanks is through the suction header, a large
diameter pipe that connects the all the fire pumps in the pump room.
Therefore, there is
no need to provide any sleeve in the common wall between the two firefighting
tanks.
o The connection from
each tank to the suction header should be placed in a sump; if the connection
is placed say 300mm above the tank bottom without a sump, then a 300mm high
pool of water will remain in the tank, meaning that the entire volume of the
tank water will not be useable, to which the Fire Officer will object.
o Ideally the bottom
of the firefighting pump room should be about 1m below the bottom of the
tank. This arrangement ensures positive suction for the
pumps, meaning that they will always have some water in them.
o All pump rooms
should without fail have an arrangement for floor drainage; pumps always
leak.
o The best way to do
this is to slope the floor towards a sump, and install a de-watering pump if
the water cannot flow out by gravity.
o In cases where
there is an extreme shortage of space, one may use submersible pumps for
firefighting. This will eliminate the need for a firefighting pump room.
o Create a special
shaft for wet risers next to each staircase. About 800 x 1500 mm should
suffice. It is better to provide this on the main landing rather than the
mid landing, as the hoses will reach further onto the floor.
This
is a site that explains the art and science of building construction in great
clarity and detail. Our goal is to make you understand concepts in
building construction.
Written by architects and engineers, the content on the site is actually a result of accumulated years of work experience at building construction sites and design offices. This expert knowledge of building construction is not available in textbooks!
We also take great pains to ensure that our quality of writing is of a high standard. We aim to take complicated situations and make them simple and clear, as well as to provide content that is interesting to industry experts and newcomers alike. Do let us know where we succeed - and where we fail - in this task.
Written by architects and engineers, the content on the site is actually a result of accumulated years of work experience at building construction sites and design offices. This expert knowledge of building construction is not available in textbooks!
We also take great pains to ensure that our quality of writing is of a high standard. We aim to take complicated situations and make them simple and clear, as well as to provide content that is interesting to industry experts and newcomers alike. Do let us know where we succeed - and where we fail - in this task.
No comments:
Post a Comment