IT’S TIME FOR THE GAMBLING TO STOP

My cards are well and truly on the table when it comes to pressure differential systems for escape routes and firefighting access in high rise commercial and residential blocks.

And from where I sit they look like a ‘royal flush’. I can’t imagine anyone having a hand to beat it.

But the people behind the new British Standard 9999 see things differently – and theirs is a view that I honestly fear could put lives in danger. That has to place the pressurisation argument among the fire safety industry’s most controversial current debates.

Basically, what BS 9999 says is this: buildings up to 30 metres high, with basements less than 10 metres deep do not need any fan-assisted pressurised system to disperse the smoke in the event of a fire. The air is quite capable of clearing naturally.

Now, like any UK fire safety or building regulation and BS code of practice, this is not legally binding. It’s intended to be flexible and can be varied. But, in reality, few building control officers would have the confidence to challenge it.

That puts smoke control engineers like Fire Engineering Associates in Trafford Park, Manchester, who I work closely with, in the worst possible position – believing so passionately in the safety of something that a British Standard says is not necessary to save lives.

BEST PROTECTION

Over the last decade, Dave Ogden, my colleague at Fire Engineering Associates, has been behind more than 100 of their pressurisation systems – often involving several integrated stairwells. These have included Ontario Tower, in London’s Dockland, which has two pressurised systems – a fire fighting shaft covering 30 floors and an escape stair of 21 stories.

As FEA’s technical director his experience with pressurisation puts him in a narrow band of smoke control engineers whose contemporaries, more typically, have natural ventilation backgrounds.

And, he is convinced that “correctly designed and installed, a pressure differential system is easily the best form of protection.”

Surely people like us have the right to rely on a British Standard being safe and reliable. If it isn’t then it shouldn’t be there. But, I’m afraid, 9999 is there – and has been since April.

We argue that as long as it stays in its present form many multi-room buildings erected to these flawed guidelines could be lethal. Especially those high-risers where, in the event of fire, natural ventilation is relied on to (hopefully) clear smoke from corridors, lobbies and stairwells. That could play dangerous havoc with the lives of people who work or live in them.

As a result, buildings like these have to be among the most important of modern problems for smoke control engineers.

PRESSURISATION

Let’s go back to basics. Smoke control in high-rise buildings is an absolute necessity. No argument. The only dispute is over the most effective means of allowing occupants to escape and firefighters to enter the building, in the event of a fire, without being killed by hot smoke. Surely not too much to ask?

This has been a long-running debate since the 1970s when the decade began with British Standard Code of Practice CP3. It recommended a method of natural ventilation … and immediately drew professional criticism.

The recommended system included automatic openings or vents of 1.5m2 free area on adjacent walls with the idea of allowing wind pressure to clear smoke naturally from the corridor. This was shown to be unreliable by the now Building Research Establishment (BRE) and was, therefore, not adopted in the later BS 5588 Part 1 1990.

Instead, a system of automatic opening vents and corridor smoke doors was recommended. But the eminent Dr Howard Morgan pointed out in his article ‘Smoke Clearance’ – in the May 2005 edition of ‘Fire Prevention – Fire Engineering Journal’ – that this combination has always been difficult to explain in terms of ventilation.

Some very comprehensive BRE investigations, published in 2002, had already found that: ”The condition where external wall ventilation works well, is when the wall ventilators are facing windward, and the wind speed is sufficient to effectively PRESSURISE the fire fighting lobby”.

SERIOUS RISK

So … now the arguments and counter-arguments have taken us to the latest recommendations of BS 9999-2008.

There are cautious steps forward but it takes no giant leaps.

Dealing only, at this stage, with fire fighting shafts, section 28 recommends a properly designed and installed pressure differential system only for buildings more than 30 metres high and with basements more than 10 metres deep.

So far so good.

But when it comes to smaller buildings 9999 is happy to continue allowing the less reliable natural ventilation.

And that is where the serious risk creeps in.

Because natural ventilation of corridors and lobbies in high-rise buildings relies on two forces – the buoyancy of the smoke and the wind-generated negative pressure at the vent or smoke shaft outlet. Both these factors are variable and, therefore, unreliable.

The temperature that controls smoke buoyancy can be reduced by several factors. Sprinkler cooling for example.

And negative wind pressure can be very uncertain in built up areas – due to variable wind speeds and direction, adjacent buildings and even parapet walls any of which can still produce positive pressure at the vent outlets that can be much greater than the pressure head developed by the smoke. And, in many respects, low buildings are even more prone to this than taller ones.

POWERED VENTILATION

But, in spite of all the doubts and uncertainties that surround natural smoke venting systems, BS 9999 recommends their continued use in buildings up to 30 metres high and with basements no more than 10 metres deep. With smoke shaft area specified at 3m2, and openings top and bottom, this also can and does create space problems.

A simple solution is to use fans to exhaust the smoke positively – which can also reduce the smoke shaft area to under 0.5m2 and eliminate the need for a bottom inlet altogether. Paddington Central – a Fire Engineering Associates project – was possibly one of the first UK examples of fan powered smoke ventilation and was designed to provide the same pressure and velocity as a fully pressurised system.

With recent concerns over natural ventilation fire safety engineers have been specifying, and regulatory authorities approving, powered ventilation. But it’s not, as yet, a recommended option in BS 9999. It should be.

PRESSURE DIFFERENTIAL

However, it does share a common weakness with natural ventilation in that smoke is first allowed to enter the escape/entry routes before it is then drawn out of the building. A pressure differential system – commonly known as ‘pressurisation’ – is the only one designed to keep smoke out of the escape routes altogether.

It uses two principals – air velocity and pressure differential – and involves supplying a sufficient volume (m3/sec) of ambient air into the escape route in order to produce the specified velocity or pressure.

It’s not a new idea. The first UK Code of Practice – BS 5588 Part 4, published in 1978 and updated in 1998, effectively, in 2006, became the European Standard EN12101 Part 6 – which is now specified in BS 9999 Section 28 for the protection of fire fighting shafts.

EN12101 Part 6 is very cautious and can produce an over-engineered system. It insisted, for example, on 100 per cent standby fans even though early research felt that the risk of a fan failing at the same time as a fire starting could be as low as 3.7 x 10-8.

But even if standby fans are unnecessary EN12101 Part 6 insists on them. Yet surely this degree of caution sits uncomfortably with BS 9999’s surprising confidence in natural ventilation, for buildings up to 30 metres high, which has been doubted and disputed from so many other directions.

The BRE’s 2002 report no. 79204, for instance, argued that: “Pressure differential systems have specific advantages in providing a higher standard of protection in specific buildings, particularly those operating a means of escape strategy based on phased evacuation. They can also provide a greater level of protection to the fire-fighting lobby itself than any of the natural ventilation systems discussed herein”.

Design failures in early pressurisation gave the results a bad name but if the principals of fan engineering are correctly understood, and systems are properly designed and installed, it is easily the best form of protection.

• Jim Wild – a former product manager with Fläkt Woods, responsible for the development and application of fans in the fire safety industry – is now a fan engineering consultant specialising in fire smoke control systems. He is an associate with Manchester-based smoke control specialists Fire Engineering Associates of The School House, Second Avenue, Trafford Park, Manchester M17 1DZ; Tel: 0161 8727760. www.smoke-control.co.uk

ENDS

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