Pressurised Staircase

[David Rooney]

We were asked to comment recently on a pressurised staircase.

The "system" was probably installed 30 years ago plus and simply consists of huge fan on the roof pumping air into a large ornate staircase (approx 5 storeys) in the middle of a large office accommodation.

The system comes on automatically on detection of fire.

There are no escape vents on any of the floors so there is nowhere for air/smoke to be ejected, the way I see it any smoke would simply be pushed through door jams to other areas of the building.

There appears to be a vent at the head of the staircase next to the air inlet that acts a bit like a pressure cooker vent.

Has anyone any ideas whether this may have ever complied to anything in the past or if this was thrown in because someone thought it might be a good idea at the time?

[nearlythere]

There are no escape vents on any of the floors so there is nowhere for air/smoke to be ejected, the way I see it any smoke would simply be pushed through door jams to other areas of the building.

Surely the point of the system David is to create a positive pressure in the stairway sufficient to prevent entry of smoke from adjoining areas. The increased pressure in the starway would not be such as to significantly increase the pressure in adjoining areas resulting in smoke being pushed into areas adjoining it.

[kurnal]

Nearlythere is right. By slightly increasing the air pressure within the escape routes, using clean air sourced from outside the building, the escape routes can be maintained clear of smoke and products of combustion. The excess pressure usually passes through cracks around doors and sometimes special vents before finally venting through the window cracks another leakage path to the exterior of the building. So if the fire occurs within the accommodation the products of combustion should be forced away from the sensitive escape routes to the exterior of the building.

Very little pressure is required to achieve this indeed should the pressure be too high persons may have difficulty opening doors. Whilst leakage paths can be calculated and predicted, the vent you describe is probably to ensure that an overpressure does not occur and to guarantee that all doors can be easily opened.  The pressure increase is  of the order of a few pascals. The fire doors to the protected staircase will not have smoke seals as the leakage around the door is an important part of the protection system.



This used to be covered by BS 5588-4, and has been recently included in BS 9999.
To work, the pressurisation system has to overcome other natural and normal variations in pressure that can drive the smoke for example the effect of the wind on the building or the stack effect (-hot gases tending to rise vertically in a shaft or staircase causing a higher pressure at the top of the staircase and a lower pressure at the bottom.) Heating and ventilation systems can also create problems and are normally shut down by the fire alarm system where pressurisation is used. Ideally the pressurisation system is actuated by smoke detectors.

Some systems are dormant until a fire occurs, and then the fans start up. This arrangement can be unreliable so often two-stage systems are used in which the pressurisation fans form part of the normal environmental ventilation for the building and increased a full pressurisation in an emergency situation. They should be indicated for the fire service to show the status of the pressurisation system on arrival. For example the normal environmental pressurisation would probably be about 10 Pa which will be boosted to about 50 Pa in a fire situation.

[CivvyFSO]

BS EN 12101-6 is worth looking at

[CivvyFSO]

For example the normal environmental pressurisation would probably be about 10 Pa which will be boosted to about 50 Pa in a fire situation.

Just to add:

And then usually up to a velocity of 2mph with the doors open.

[David Rooney]

We measured the pressure in the staircase in terms of 12101 and we did manage around 50 pa with doors closed but if you follow the procedures and open a door to air and one of the storey doors the pressure dropped radically to around 5 pa which today would be unacceptable.

We've tested systems with vents at each level that open automatically depending on the origin of the fire condition but this is the first one I've seen without such vents and it just seems a bit strange the air and any fire products are simply pushed through the rest of the building - a labyrinth of corridors off each level leadig to other staircases. Although I understand the need to keep the staicase clear.

Thanks for your input.

[kurnal]

Sorry Dave. Just remember when your Granny enquires how to go about sucking eggs I'm the man to teach her.

[David Rooney]

That has cunjured up the most disturbing of images.....!!

Thanks

 

[Phoenix]

David,

These systems are very complex and correct analysis of how the system will perform involves looking at many aspects of the whole building.  Generally, this is not an area you can dabble with so if you want to deal with it you'll have to do some thorough research.  Start by reading BS12101-6 from front to back.

You will notice, as you read, that you are, in general, expected to lose the pressure when you open a door.  The system does not rely on a pressure differential in the open door condition, instead it relies on the passage of air through the large opening.  You'll need to get a mini-anemometer as well as the mini-manometer you must have used for the pressure differentials.  Also, there are quirky things like how to take the readings, not all this stuff is explained in the BS.

A thirty year old system may conform to BS5588 part 4 1978.  Very hard to get copies of the old standards now but I have a copy of this one.  Or it may even predate this, in which case it's probably somewhat ad hoc in its design.

The most important question is this, what is the purpose of the installation?  The next most important question is this, is it still needed?

What aspect of the existing building would not be safe if it were not there?  If you know this, you might start to know how best to move forwards.  

A lot of systems were put in to assist fire fighting but at 5 storeys I'm not sure.  If it is for fire fighting, the 1978 standard doesn't address the matter of fire fighting shafts but it gives standards that would help.

Stu

[David Rooney]

You mean like 12.2.3.4,  taking at least 8 measurements, uniformly distributed over the doorway, to establish an accurate air
velocity,then calculating the mean of these measurements or alternatively move our anemometer steadily over the cross section of the open door to record the average air velocity whch we found to be borderline 0.75m/s ??

... and have to disagree about "The system does not rely on a pressure differential in the open door condition", without a pressure differential you wouldn't get any air flow. Hence for our assumed to be Class C system we are trying to maintain 10pa in the staircase while meeting all the elements of table 3.... except we don't have a vent on each floor, ergo the reason for the question.

[Phoenix]

Sorry,

Didn't know you were an expert.

[kurnal]

Dave you clearly know a lot more  than most of us about this!

But to be fair you did not give us many clues in your first posting. I guess you need to identify the date of installation to have any clue as to the design spec. Some of the early systems were very experimental - if i recall Marks and Sparks were amongst the first to install a system before 5588 part 4 was written in the early 70s. I believe the early systems worked on a very low pressure compared to the recommendations that were later included in the standard.

I think Stu has the best idea- recommend the client commissions a design review of the building to establish if there is a need for a system and then  if it is needed measure the existing system performance against the identified need.

[David Rooney]

Sorry Stu.... wasn't tryin to be funny but I'm not a dabbler.... or an expert either..!   

This is just one of those systems with no documentation and no one with any idea as to what the objective was.

You're right, I think the client needs to start with a review to see if this is still relevant....

[Phoenix]

Point taken, David.  Thanks.

I've stumbled across a few old pressurised staircases in the last few years.  In most instances they have been required because of a lack of other means for ventilating the firefighting staircase and its lobbies.  You say your building has approximately 5 upper storeys.  Of course, you need a fire fighting shaft if the top floor is more than 18m above the fire fighter access level.  This is the first thing I would assess.

The only other major reason that a pressurisation system might have been installed is if the building is below 18m in height and the designer wanted to dispense with protected lobbies but also wanted to keep the sizes of the staircases to as little as possible (i.e. staircases sized withount discounting one).  That's when the system might be class C instead of class B. [Unless you have phased evacuation or it's a block of flats or it's a sleeping risk, in which case it would be class ........... you know.]

If the building is over 18m it will need lobbies anyway, of course, but it will also need at least one fire fighting shaft.  I'd be inclined to go to the top floor and stick a laser measurer out the window to find out the height of the top floor above fire fighter access level.  Try not to blind people on the pavement as they look up.  Not the little kids any way.

Your building has no immediately apparent means for exhausting air from the fire floor, which is interesting.  I'm currently dealing with a similar building and the extraction system on the floors is designed to keep working so as to provide a means for losing the air on the fire floor.  It's a bit arbitrary (pre-dates 1978 standard) and the only way to assess its effectiveness is to carry out tests along the lines of the annual tests in 12101-6, which we are doing.  We're not looking for 12101-6 standards but 1978 standards would be good.  Your building might have means for the removal of air that are not obvious, perhaps the shut down but un-damped HVAC ductwork that serves the floors will provide a route to ambient air from the fire floor.

I have come across pressure differential systems where the staircase can be ventilated by windows or vents but where the lobbies have been away from the external face of the building and they haven't wanted to put a smoke shaft in.

If there are two or more staircases but only one is a fire fighting shaft then it is often the case that all staircases are pressurised so that smoke is not driven into the unpressurised staircase(s).  Otherwise, robust means are required to demonstrate that the over-pressure is lost between the staircases.

You mention that your staircase is "large" and "ornate."  I would emphasise that there must be no potential for fire within a pressurised space.  If there were a fire within the pressurised space then smoke would be pumped into every part of the building by the pressure differential system.  This has happened and it makes life very difficult for the fire service who attend.

Stu