Fire Suppression Systems [19:46]

Loading...

From the course by University of Maryland, College Park

The Effect of Fires on People, Property and the Environment

101 ratings

University of Maryland, College Park

The Effect of Fires on People, Property and the Environment

101 ratings

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.

From the lesson

Fire Safety Subsystems

Fire Suppression Systems [19:46]19:46

Meet the Instructors

James Milke, PhD
Professor and Chair
Department of Fire Protection and Engineering

Welcome back.

This is our third segment of week two and

where we want to talk some about fire suppression systems.

So these fire suppression systems will

get installed in buildings, or there are various mechanisms of fire suppression.

We'll talk about the various options for systems that are available.

And I do want to point out I'm using the word fire suppression here.

Very important distinction as compared to fire extinguishment systems,

in that a suppression, fire suppression system is looking to contain the fire or

limit the growth of the fire,

where fire extinguishing system is specifically designed to put the fire out.

And, of course, that's the ultimate objective.

But many of our fire suppression systems are geared or

designed principally to limit the growth of the fire,

to reduce the hazard until the final extinguishment can be achieved.

When we looked at the mechanisms of fire suppression,

we go back to Haessler's fire tetrahedron.

We talked about that in the first week.

Remember there were four surfaces.

And the idea of an ignition was you needed all four parts of this surface to be

present, here, in suppression.

Now we talk about removing one of the surfaces of the tetrahedron

to provide fire suppression.

Water becomes the agent of choice, more often than not,

and there are a couple reasons for that.

Number one, it's very effective, for

reasons we'll see here, in terms of the thermal benefits of water.

The other principle benefit of water is it's readily

available in an awful lot of facilities and an awful lot of areas on the planet.

So being so readily available suggests that it's going to be fairly cheap.

And this readily available agent, Mother Nature's been fairly kind

to us in that this readily available agent is also very effective.

So that fire, water suppresses fire by one of two mechanisms principally,

by a cooling mechanism and also possibly by oxygen dilution, as we'll talk about.

So in terms of vaporization of water when, we get an absorption of heat

by water going from its ambient temperature up to steam by, first of all,

increasing the temperature from whatever ambient temperature it starts with.

So let's say it starts out at 20C and

up to 100C at the evaporation or boiling point.

We're going to absorb 330 joules for every gram that, for

every gram of water that's involved here.

As that water then evaporates, as it gets to 100 C and now evaporates,

there's another 2,260 joules for every gram of water that's involved.

So we end up with about 20, almost 2600 joules for

every gram of water that goes from 20C on up to steam.

When you look at the variety of other materials,

other liquids certainly, there aren't many that compare and do that well.

Number one, and again secondly,

layered on top of that is the fact that water is readily available and

you've got this wonderful substance in terms of heat absorption capability.

The other possibility is through oxygen dilution.

And when that steam is created, that one unit volume of liquid water

now goes to 1600 parts in steam, and so that's the 1:1600 ratio you see here.

So if I have a liter of water, just a little liter container of water,

and transform that all to steam, I get 1.6 cubic meters of steam.

As all that steam is generated, that's going to push the oxygen out of the area,

certainly at a local area, wherever the fire is, and

all that steam generation is going on and may effectively inert the room.

There are special hazard suppression agents also that we talk about in

the field, and these are basically the variety of agents other than water,

that they get involved.

So there's carbon dioxide as an example.

These are the inerting gases.

Carbon dioxide I mentioned, or could be nitrogen as well.

There are some nitrogen suppression systems.

There's steam systems.

That was an early application, shipboard applications that just dump

a lot of steam into a particular container where a fire was.

It acts by reducing oxygen level in the space and

gets used in these sort of facilities currently.

One of the drawbacks,

every one of these agents has their drawbacks, water included.

The one drawback with CO2 and

these inerting agents is that it reduces the oxygen in the room.

And that's how it works.

But the disadvantage is if there are people in the space when that happens,

they're not going to be able to survive unless they get out before that discharge,

before that injection, that inerting agent occurs.

Clean agents, these are the more contemporary agents some of you may have

heard, the halon agents, the Halon 1301 and that sort.

There are an array of agents that have been developed since

halon has been discontinued.

The variety of clean agents here at principally by heat removal,

there's usually not enough of the agent generated to do the oxygen displacement,

though there's some of that that goes on, but

it's mostly, the action is mostly heat removal.

And, again, these agents get used where there's electronic equipment involved and

other very water-sensitive objects.

Dry chemicals that are in portable extinguishers, for example,

will interrupt the chemical chain reaction and also act as a heat removal agent.

These typically get used for their flammable liquids and gases involved.

You find them in office buildings and the likes and

they're also effective to a point on solid materials.

Foams, firefighting foams, that, I'll have that video a little bit later.

Basically these layers of bubbles that get created by systems will

provide a blanket over the fuel surface, so it separates the fuel from the air.

So we're basically involved with a fuel removal system,

though it depends what, how, what your perspective is.

You could say it's separating the oxygen, so it's displacing the oxygen,

limiting oxygen availability to the fuel,

where it's removing the fuel essentially by providing a blanket and

not allowing the fuel vapors to rise and again mix with the oxygen.

These typically get used in flammable, liquid,

flammable combustible liquid areas,

though again they have been used to combat fires involving solid fuels as well.

So a couple of types of systems and how we include systems here, so

to speak, I want to talk first of all about manual suppression.

It's a very commonly used type of mechanism to put out fires or

suppress fires, let alone extinguish one.

I don't know that there are statistics on the relative proportion of fires that

are put out manually versus automatically, though I would propose to you that

there are many many more fires that are suppressed,

let alone extinguished, manually than automatically.

And I mention that in part because I suspect there are many

uses of portable extinguishers, for example,

that get used where the fire department never even gets called.

So we don't even know about those fire incidents.

And there are some that suggest that for every fire incident that's reported,

there are ten of them that go on but are never reported to the official agencies.

So portable extinguishers are part of this manual suppression system.

I want to acknowledge that the use of hose lines by the fire department or

by a fire brigade,

by an industrial group that provides manual suppression activity as well.

And then there are these automatic systems that very often have a manual discharge or

manual button on the wall that if you detect the fire as you're

the cook at the range hood, that you may see the ignition

of a fire well before any detection devices in the hood, and

you can go ahead and press the button and to, to get the system discharged.

Automatic systems will have some sort of initiating device, so there will be a fire

detector of one sort or another such as we talked about in the previous segment.

So that your choice of fire detectors here, initiating device,

get involved in how fast do I have to have operation of the suppression system.

Also gets involved in what's the possibility of a nuisance alarm, or

false alarm as they sometimes get called,

in that with the presence of a nuisance alarm now,

these become a bit more of a nuisance in the sense of just ringing bells and

bothering people, but there's now an agent that's going to get discharged.

So there's this massive cleanup that's going to occur,

as well as whatever cost to refill the reservoirs with the agent.

So that this fire suppression system, this is a gaseous sort of system,

as you can see with the air bottles.

This is the more, this is the sprinkler system that we see, where now

the initiating device is the sprinkler or the sprinklers themselves typically,

although there are some sprinkler systems that will be activated by, or

actuated I should say, by some type of initiating device.

So there are, as, again, there is this variety of types of suppression

systems the fire protection engineer has to choose which one to use and

what's the basis for why they pick one type of system over another.

And it involves essentially these five factors where again cost is very much

part of the picture, and we're not going to get away from that.

And so let's just talk about that that's in the middle of the table here.

So the costs that are involved again, it's a life cycle, a life cycle cost.

So not just design cost, but also installation cost, maintenance cost.

Should also identify some costs that are involved here

on the positive side of the ledger may be insurance benefits, tax benefits,

these type of things to help offset some of the cost picture.

Performance cost-wise,

again there are some cost issues that we'll get involved in terms of.

So how responsive, how effective is the system, but

what sort of losses do I expect out of, if that system activates or operates.

Is there any collateral damage that gets produced by the agent that will further

damage equipment that I might be concerned about?

Coming up to the top though for the rest of the items.

So what's the suitability for the particular hazard?

Does that particular type of suppression system or

suppression agent work on fires involving that particular material?

So I have to think about that.

Water isn't effective on everything, on all kinds of agents.

There are some notable examples of water-reactive materials, of course,

that are not going to be good.

Combustible metals, water is not very effective on those.

There may be equipment damage, and

that's the collateral damage sort of issue again that may be there.

What, the agent-fuel interaction to the point again that I talked about

water-reactive materials.

Personnel hazards, talked about that with respect to carbon dioxide, for example.

And what's the toxicity of the agent?

Is there a reduced oxygen level that's going to be created?

Maybe obscuration levels in the space.

Though I'll mention in terms of obscuration, there are few of the agents

that work that don't provide some level of obscuration, some worse than others,

but that's fairly typical to get at least some obscuration as a result of

perhaps a change in characteristic in the smoke that's created.

If it's a fixed system, such as I showed you the couple diagrams here with

gas cylinders here, there's got to be some supply of the water that's here, and

sometimes that's an elevated tank, maybe a lake or a reservoir that's present.

In some cases it's a flowing river where the water is coming from, but, so

I have to think about where the agent supply is coming from.

If it's a fixed supply, then I have just this on-site storage, now I have

to look at how much do I need, how much room is that going to take, as the system

operates for a particular fire, how long does it take to recharge it so that we

can reopen with protection in place, what's the availability of the agent.

There are still some Halon 1301 systems around though that,

as the agent is not being produced anymore.

As the system discharges, then getting a replacement of the Halon 1301

is a bit problematic outside of some of the special reserves that are available.

There are a couple application methods that are available,

either total flooding or local application.

The typical sprinkler system involves a local application.

It's just the sprinkler above the fire, or closest to the fire,

is the one that will operate in most fires.

And it's rare that, let me say it another way.

Sorry.

So closest sprinkler operates.

And in order for a second sprinkler to operate,

it requires that that first sprinkler's not overly effective and

there's still enough heat being generated and transported out in it through

the ceiling jet that causes the second sprinkler to operate, third sprinkler.

The only exception to where all the sprinklers go off in a space is,

of course, what our movies like to portray, which is it's just a myth and

usually completely incorrect for

a particular facility, is where there's a deluge system involved,

specifically designed for all the sprinklers to operate.

And that's usually a very special case, usually an industrial setting,

maybe a transformer vault or those kind of places.

There are total flooding systems.

These are usually the clean agent systems,

the gaseous systems, the foam systems, such as I've got a video on it

in a little bit where you'll see the whole place fill up with foam.

Water is usually default agent of choice.

So why would you ever go anywhere else?

And you get into some of these special sort of considerations, where water

is ineffective or hazardous and then maybe in a fuel oil tank, storage tank fire.

As water gets injected into that, if that drops to the bottom,

it doesn't necessarily sit on top of the fuel.

So as it sits on the bottom of the fuel tank,

it may end up heating up as a result of continued fire development and end

up with a boil-over situation, just spews fuel, burning fuel all over the place.

Water-reactive fuels, talked about that.

Collateral damage issues with, you know, water on a printing press or

a computer room, though again I want to caution you a little bit in that there's

an awful lot of overreaction to the interaction

of water on some materials that a fine mist or a fine spray

is often completely acceptable to put out a fire because of the cooling aspect.

So don't get oversold on, oh,

we can't use water on this particular fuel item or type of fire scenario.

Some cases, the, their agents are more cost-effective than water.

That is, if the hazard is in a very very localized spot, and

I only need a system protecting that corner of the room, well, then that may be

less expensive than providing sprinkler protection throughout the place.

So there are those possibilities.

And it's not often you find a spot that the hazard is localized and

will only ever be in one part of the room, but those cases do exist.

There are cases of weight.

The water weighs a lot.

So that, is there something we want to do besides sprinklers?

So you don't find sprinkler systems onboard aircraft, for example, because

there's a huge weight penalty associated with that, or with our space vehicles.

Shipboard applications, water may be difficult in that if we

flood a compartment, if that, that may cause the ship to go out of balance,

become a problem eventually if you fill up the whole ship with water, that usually

makes for a bad day for the ship, and, you know, you sink the ship at that point.

So couple of demonstrations.

So this is a clean agent, so you can see the discharge in the, in clean agents

here, some of this obscuration again as that clean agent now evaporates or

vaporizes and gets into the gaseous form where it's most effective.

I have a short video clip here I think we should be able to see.

So this is looking at the performance of one of the clean agents,

Novec as it's called.

So it's one of the clean agents.

The vapor is heavier than air.

See a little candle experiment here.

As the beaker is being poured, the vapor is flowing out of the beaker.

This is vapor.

It's heavier than air, again.

So the vapor accumulates in the bottom of the beaker.

It will actually put the fire out.

So that's one example that I have for you.

So this is looking at an aircraft hangar now where phone systems are often used.

So there they've initiated the system it sounds like where the,

you can hear the horns.

And we're starting to flow foam at this point from these rooftop units.

You can see the foam now

dispersing on the floor and that is creating a blanket of foam.

If you picture there was a flammable liquid spill that would be burning,

we'd be blanketing over that spill at this point.

>> So what's the ruling there, Steve?

>> There are a number of these videos online that you can Google

firefighting foam and get several examples of this,

with rather amazing videos of the foam and how tall it builds up and so on.

So I'll leave that to you as the interested student to look for

more of those videos should you find that to be interesting.

Again, the idea with the foam and the blanketing systems is, again,

we're going to blanket the foam here, or the fuel.

You can see the spreading blanket that, such as you saw in the video, and

eventually that it will occupy all the space such as in this storage tank

to where the fire is eventually put out.

So as we saw in our little walk-through, suppression agents,

these suppression agents act to remove one or more sides of the fire tetrahedron.

The fire suppression agents are selected considering four major factors of

effectiveness and then combined a couple of the items in the table.

Toxicity, what type of collateral damage might be done with the agent or

by the agent, and then what costs are involved.

So we'll bring this session to a close and move into the last segment.

In the next module, we'll talk about smoke control systems.

Explore our Catalog

Join for free and get personalized recommendations, updates and offers.

Coursera provides universal access to the world’s best education, partnering with top universities and organizations to offer courses online.

© 2018 Coursera Inc. All rights reserved.

Download on the App Store

Get it on Google Play