Fires impact people, property and the environment in all countries around the world. In some cases, the resulting losses are extraordinary, causing hundreds of deaths, widespread damage to property and contents and significant impacts on the environment. More often, fires may cause a single casualty or affect a single home, though the effects are still highly significant to those affected and collectively are substantial. This course will provide an overview of the challenges posed by fire as well as the fire safety solutions that are available to meet those challenges.
Detection and Alarm Systems [19:08]
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Del curso dictado por University of Maryland, College Park
The Effect of Fires on People, Property and the Environment
140 calificaciones
University of Maryland, College Park
140 calificaciones
Conoce a los instructores
James Milke, PhDProfessor and Chair
Department of Fire Protection and Engineering
All right. Welcome back.
In the continuation of our week two discussion,
we'll talk some about detection alarm systems.
And here we, we'll talk about the variety of detection systems,
some variations in the alarm component or notification component and
a little bit about what are protection alarm systems supposed to do.
So detection alarm system first of all,
has several major parts associated with it.
It has initiating circuits in there.
So there are sensors out on the end of those initiating circuits.
We may call those detectors as an example.
Although that's only one example of what may be at the end of
the initiating circuit.
You see here's the initiating, circuit and alarm initiating devices.
There's a control panel that's here.
So the initiating devices will send a signal back
to the control panel to say hey, something's going on.
The control panel then has to sort out what to do about that.
There are indicating circuits, or notification circuits, and
that's what may ring bells or do a variety of things as we'll talk more about.
And then there are power supplies that get involved and
in a fire detection alarm system these are the contemporary systems,
are all electronic so that power supply usually involves, some sort
of source of electrical power if not, backup power or secondary power as well.
So, what's a fire alarm system supposed to do?
Well, we get the initiating devices that are here and
that would initiate some signal going to the control panel.
So with the sprinklers as they operate there's a water flow switch that will get
engaged, and send a signal or detectors send a signal.
Or someone pulls a manual pull station, and it sends a signal to the control panel
the control panel, then, is the brains of the system, so to speak,
that has to figure out, then what am I supposed to do about this?
And that's all preprogrammed in by the designers, of course, but
it may sound, the notification appliances that may be bells or other things,
such as we'll talk about, may close doors, may start smoke control fans,
call the fire department, so there are these sort of functions that may go on.
And again the function of a particular fire alarm system,
in a particular building, will be dictated by what are we trying to achieve.
What do we need to achieve as a result of this detection input?
Talk about the variety of initiating devices that are available.
So these are the initiating devices, as the definition says here,
devices that respond to heat, smoke, or other fire signatures, so
some way that the fire is telling us it's there, that's called a fire signature.
Or they may include devices which sense movement, flow,
or other quantities associated with fire and fire protection systems.
So that may be a supervisory switch, for example in a sprinkler system,
this little tamper switch right here that would
send a signal in case somebody closed off the sprinkler system.
Right?
Want to know about that, as now the sprinkler is disabled and
that impacts the design basis and the level of fire safety in that building,
if the sprinklers are disabled in the facility.
So we see a water flow switch here that would be attached to a sprinkler system to
send that signal, a detector,
manual pull station once again as examples of these kind of things.
Fire signatures, what are these fire signatures?
And the fire signatures are basically fall into two categories,
and largely related to the fire products we talked about last week.
So fires create energy.
Right?
And that energy gets released in two modes.
It gets released convectively.
Convection heat transfer,
a convective movement that's gonna move the energy around that
is movement by way of a fluid, is what we call, what the scientists call it.
And, a fluid means either a gas or a liquid in the scientific terms.
So, our convection heat transfers gonna go on because of a gas principally,
that is that smoke plume, that ceiling jet I talked about in week one.
Fires release energy, also, from the immediate vicinity of fire by radiation.
And some of it's in the visible range, we see flames,
that's because of the radiant energy coming off in the visible wavelengths.
The fires also produce radiation in a variety of wave lengths.
A wide variety of wave lengths of which there are two others besides the visible
range noted here.
Ultra-violet and infrared that those wave length bands are also used
with commercial devices such as I show here of a flame detector, or
more generically called a radiation detector.
Though the fires produce a wide range of wave lengths, there's been some research
on microwave detection, for example, among other things.
But these are the three ranges of wavelengths
that currently have commercial devices involved.
There's also a suspension of matter.
There's the generation of an aerosol or
that smoke layer, where now there are particles that are put into the airstream.
Solid or liquid particles, the case may be these particles may be invisible in size,
because they're very small.
They might be visible that's largely what colors the smoke that we see.
The fact that there is now visible sub particles that gets us the grey,
the black smoke and so on.
And there are gases that are produced as such as we talked about last week as well
that there are various detection devices that are available to look for
particles basically of the invisible or visible range, and
also to look for particular gases.
So let's look to some examples of heat detectors here.
The heat detectors may be a spot detector, that is there's a detector that fits on
a particular spot on the ceiling or maybe on the upper part of a wall and
it's monitoring conditions at that particular location or
that particular spot as it's called.
These detectors may operate on a fixed temperature rate or rise basis.
That is, once the temperature gets to a particular point, maybe the set
point is 70 degrees C, that at that point the detector goes into alarm.
A rate of rise, similarly,
there might be a rate of rise at which the temperature increase is being monitored.
So not just a set point temperature but
a rate of increase of perhaps 10 degrees C a minute something in that area.
At which point that would suggest that something may be going wrong in the space.
They go through a variety of different inventions.
There's some references around to say so what's the inside of a detector look like.
These heat detectors that are, really clever inventions.
For those of you, the entrepreneurs, and those of you with an invented mind,
I'd encourage you to take a look and just imagine yourself in your own garage or
basement, putting this little type of device together.
They're really clever devices.
There are linear detectors as well.
Where now the detection is done anywhere along the line, right?
Where this detector is going be placed.
So, there are linear heat detectors that basically look like wires, right?
They operate on a couple different principles, the one is that the insulating
material between two conductors melts at a predetermined temperature and
you end up with a short as a result, sends an alarm.
And there can be even some diagnostics about where that short has occurred to
identify the location of that fire within a large building.
Smoke detectors have a few different types of designs or
mechanisms by which they operate.
So the one arguably the most common that you'll find commercially
in non residential buildings is a photoelectric detector.
This uses the light scattering method so that there would be
a chamber inside the detector here such as I've depicted graphically, a light source,
and the light beam going away from the light source.
If the chambers clear, that light beam just goes from one end to the other and
never gets to a photocell or light receiver.
Right?
Once there's smoke that gets in, included or contained within the chamber,
now that the light gets scattered from that light source.
Just in the same way that light gets scattered as you drive through a fog,
that's the same physics.
So that now there's some light that scatters up to the photo cell
that then sounds an alarm.
Projected Beam detectors are another possibility where they look like this.
And basically, it's that the light source and photocell, again, and
usually at two opposite ends of a large, indoor space.
I show an arena as an example, that's one place where they get used.
I don't believe they are used in our particular arena such as I'm depicting
here, but this was how, in principle, it would look with
the light source on one side, photocell on the other, and this is gonna work
that if there's a fire that gets created, we get a smoke plume a smoke layer.
Now that light being transmitted from one side to the other will be
obscured to some level.
And you'd look for that drop in the light being transmitted to the photo cell for
the purpose of sounding an alarm.
Ionization detectors are the final category of smoke detectors, and
this was the original type of spot detector
created or invented in the 1940s was the first
development of a spot smoke detector using a radioactive source.
So Americium-241 is the source of choice in these detectors commercially.
As a result of producing some ions from this radioactive
source they will migrate between anode and the cathode to create an electric current.
With the introduction of smoke in the chamber,
that affects that flow of ions between the cathode and
the anode, and you look for that change in current in order to sound an alarm.
Air sampling detectors were developed in the 1980s and
largely for electronic spaces to begin with and
basically these will have these tubes, these pipes of sorts,
often a polymer pipe, maybe a PVC that will get used and out at
the end there are these little perforated holes or little cones that will be
drawing air into the system and through an air sampling or aspirating sort of device.
Will be drawing the air back all through the piping and
down to a sampling unit that is a highly calibrated sampling unit.
So you can take a very precise measurement now of smoke concentration and
end up with a very early form of detection.
Again, it was originally generated primarily protect the electronic equipment
rooms, and clean rooms, though they've found the market into many other places.
And that it If it's a good way to detect fires very early in electronic
equipment rooms and clean rooms, why not allowed in other properties?
Makes a lot of sense.
Radiation detectors, flame detectors, as they commonly get called,
will be looking for radiation being transmitted through the air,
through the electromagnetic waves from a fire source and on up to a detector.
These are typically used in cases where you're worried about flammable
liquid fires and often in maybe aircraft hangers and
those sort of places, industrial applications typically.
And here we look at how these things work.
At a particular fire will produce a radiation spectrum as identified that
fires release radiation in many, many different wave lengths.
You see the visible range noted here.
This one type of detector,
the infrared detectors you're gonna be looking at two peaks.
Right above four microns and right above two microns.
So it's looking for these two peaks typically, one commercially available
unit, that are associated with the generation of carbon dioxide.
It's that carbon dioxide emission that is being looked for
that's a very prominent combustion product,
as we talked about in week one of the commercially available fires.
Let's see, as we can look to more contemporary styles of detection, and
more recent developments.
This is using a security camera where you see a smoke plume on the screen here.
So you'd say, well if we used a camera,
we ought to be able to do a screen analysis with
a little bit of intelligence, and identify the presence of a fire also there.
So we've just detected a fire at this point.
You don't see any smoke up here at all in the upper portion
of this experiment that was run in our old field house, Cole Field House.
You're on campus.
And that you get detection at a very very early stage that in high spaces like
arenas and large indoor spaces the detection time can be delayed appreciably
if you have to wait for an appreciable smoke layer to form in the upper portion.
And you see we're getting detection of very very early on from just
a smouldering source in the arena.
So the video detection is a newer technology that was developed in
the 90's and is getting applications in large indoor spaces typically.
The selection of a particular initiating device, so
I've given you a ray of detection devices of the major categories.
Haven't talked about all the nuances and
the subsets of detectors within those groups.
But basically, how does a fire protection engineer choose, I want one of those?
Right.
And basically, that choice comes on these four issues, and
of course cost is gonna be involved in any of these kind of decisions.
So I put that in, and
cost here Is associated with the initial installation cost as well as any
maintenance cost of course, otherwise the top three are the other issues here.
So how fast of a detection do we
need to have in order to accomplish our fire safety goals.
All right.
So that becomes, and of course we'd like to catch fires very,
very early to be sure, but there's a cost associated with that.
Cost in terms of some of the very very early detection systems are typically
a bit more expensive than that little heat detector we put on the ceiling.
We have to look at what signature is gonna be produced from the fire.
In that, for a particular scenario that I'm concerned about and
achieving fire safety objectives associated with that fire scenario
what's the dominant fire signature that's gonna be produced?
So I have to think about that.
And then finally, what's the ambient environment like and what sort of changes
do I get in the environment, what nuisance alarm sources might be present?
And that there are some of these type of detection methods that if I'm in
a particular environment that has lots of particles all the time then
a smoke detector may not be the best choice.
All right.
So we have to do some thinking about what's the signature produced and
is that the magnitude of that particular signature gonna be easily recognized or
is it gonna be masked by a continual presence or nuisance alarm sources.
Okay with the notification appliances,
there are three general categories of notification appliances.
And it all started out principally here with audible methods,
where loud bells were rung.
Or you make a lot of noise, by horns or chimes or those sorts of things.
And we'll talk more in a life safety related discussion about the alerting and
what type of signals are appropriate
especially as we notified people, the building occupants of the fire.
Basically, this ought to do more then just scare 'em, because it's a loud noise.
Ideally, if we're looking for a particular response,
then we need to give people some information about what's going on.
And that gets us into the speaker based systems or there may be a taped message
that gets played to occupants on a particular floor given a particular
detection device is operated and then sound horns and so
on but provide the information to people, very, very important.
For the hearing impaired strobes are gonna be very important as a way,
and this is an example here, where there's a strobe that'll operate
to tell the hearing impaired people that there's an alarm.
If the hearing impaired folks are not awake,
having a strobe is effective to a point but it has a limited effectiveness
at which point that perhaps there are bed shakers that get used.
As, or commonly used as part of an alarm clock for the hearing impaired, for
the deaf.
And we could have a bed shaker as a possibility,
as a way of alerting them of a particular fire condition.
So in this particular segment we saw that there are fire detectors available
to detect a variety of signatures from fires, smoke, heat,
convected heat is largely what the spot detectors respond to,
we saw the radiation detectors for the radiant energy.
The fire alarm systems are used to alarm building occupants,
may also perform additional functions.
And again, what the fire alarm system is intended to do, depends
very much on what are the stipulated fire safety objectives for that facility.
And what is it that you wanna accomplish by detecting a fire early on.
At least learning the building occupants that's pretty much a common denominator,
whether you do more depends of the facility.
So we'll bring this particular segment to a close, and
come back in the next segment and talk about suppression systems.
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