Richard Brooks, Advanced Smoke Group, reports on the need for a viable and robust smoke control pressurisation system in stairwells for escape from high-rise buildings
Recent events have clearly demonstrated the potential inadequacy of the current strategy for the design of smoke control systems adopted by many organisations in high-rise buildings.
The fire at Grenfell Tower demonstrated how important the stairwell is to both the escape of occupants of the building, and as an access route into the building by firefighters. This is particularly important in buildings with a single stairwell.
The current trend in smoke control design is largely dedicated to depressurisation systems. This paper examines in detail the reasons why more attention should be paid to the protection of the stairwell and how a more robust system can be designed to protect designated escape routes. It will, hopefully, generate constructive debate.
Why Prioritise the Stairwell?
In the event of fire, it is common practice that all lifts will be automatically sent to ground level immediately on detection of the fire and will then be inoperative except for the designated “firefighting” lift. As a result of this, the only escape route available to the building’s occupants will be via the stairwell, and this becomes even more important when there is only a single stairs available for escape.
In light of recent experience, it is also now expected that, notwithstanding the direction provided in the apartment block handbook that will often say “stay put” in the event of fire, most residents will, understandably, choose to leave their apartment and take to the stairs for escape.
In Approved Document B1 it is stated that ‘The building shall be designed and constructed so that there are appropriate provisions for the early warning of fire, and appropriate means of escape in case of fire from the building to a place of safety outside the building capable of being safely and effectively used at all material times’.
Bearing in mind that the stairwell is likely to be the only route for escape, it follows that a viable and robust smoke control system must be employed for that purpose; a pressurisation system is likely to be the only viable and sufficiently robust system to achieve such conditions.
A correctly designed pressurisation system will:
- Be designed to resist smoke migration into escape routes
- Support escape of occupants by maintaining the stairwell and refuge areas clear of smoke
- Improve conditions for firefighter access and rescue operations by improving visibility
- Further assist firefighting operations by keeping firefighting lift clear of smoke migration.
A correctly designed pressurisation system will not:
- Create a negative pressure in escape routes, which may induce smoke into those escape routes
- Design for extracting smoke through an escape route
- Rely on pulling open the fire door to the stairwell in order to induce make up air to the smoke control system.
Numerous papers have been posted by experienced professionals in the fire engineering industry emphasising the need to prioritise the protection of the stairwell to permit the safe escape from a high-rise building in the event of fire, yet depressurisation systems continue to be installed, systems that have the potential to risk the reintroduction of smoke into the building as the stairwell is invariably adopted as the route for fresh air supply and therefore becomes depressurised.
The depressurisation method of smoke control puts the escape routes under negative pressure in order, in most cases, to draw replacement air from the stairwell by “sucking” open the fire door linking the stairwell to the common corridor in which the smoke extract shaft is located.
Air is drawn into the stairwell via the 1m2 AOV at the head of the stairs and the air velocity through the interconnecting fire door is expected to be sufficient to stop smoke within the common escape corridor entering the stairwell.
One may ask, “why install a fire door between the stairwell and the common corridor when, if there is a fire, it is pulled open?” Good question!
Notwithstanding this question, the risk of reintroduction of smoke into the building occurs when the negative pressure in the stairwell causes the smoke rising from the building to be drawn back into the building through the AOV at the head of the stairs.
This risk of reintroduction of smoke into the stairwell when a depressurisation system is employed will also occur in the event of fire spreading to other floors. A depressurisation system is nowadays designed to serve only a single level of the building; that is the level on which the fire is first detected.
“We are only required to consider a single fire scenario in our design” is a common statement made in the smoke control industry. Should there not be a regulatory requirement for a risk assessment to be undertaken to take account of the potential for fire spread and what might happen should that occur?
Currently, in the event of the fire spreading to a second level, eg through open or broken windows, a depressurisation system is normally unable to respond by extracting from that second level as well as the floor of origin, or any other level to which the fire may spread. Smoke will then follow occupants escaping from the second level into the stairwell which, with a depressurisation system, will be under negative pressure and will consequently induce the smoke into it.
Overcoming Smoke Migration
A pressurisation system that is activated under “means of escape” smoke control mode within a residential apartment building will have the capacity to cope with at least three additional doors opening with the option for enhancing the design capability, within reason, to accommodate additional levels.
In the event that smoke does migrate into the stairwell due to the number of additional doors onto the stairwell being opened, it will be rapidly cleared to atmosphere due to the operation of the pressurisation system continually forcing fresh air into the stairwell.
How Modern-Day Pressurisation Systems Operate
The modern-day form of controls for pressurisation systems are simple, reliable and effective, and they remove virtually all the disadvantages and problems associated with the old-style pressure sensors, dampers and pressure relief terminals.
The key is to determine what needs to be measured in order to achieve the desired conditions within the building in order to control the spread of smoke and heat in the event of fire. The performance criteria for a pressurisation system remains unchanged from those against which such a system was designed several years ago, but the method of achieving these criteria has changed significantly.
Current Control Technology
21st century research has brought modern day control technology into the sphere of smoke control; to say this research has revolutionised smoke control systems may be an overstatement, but it has brought a level of control and reliability to pressure differential systems that has not previously been available and made the design of such systems far simpler than used to be the case.
Door Proximity Sensor
One of the most important features within the modern-day controls is the door proximity sensor, the computer control system (the PLC) and the means of rapidly controlling the fan speed. This system of control has now enabled pressure differential systems to react within the three second time frame specified in BSEN12101-6. The door proximity sensor (DPS) is a device which is mounted on as many doors as required according to the system design in the way illustrated in figure 2.
The sensor is linked directly to the smoke control system panel via fire rated data cable. The DPS device incorporates a potentiometer and will send a signal to the PLC within the control panel to indicate which doors are open and what their actual position is in order that the correct rate of airflow is provided in order to maintain the correct velocity across the door openings, see second image in figure 2.
Inverter technology is now able to give extensive control over their full working range, giving extremely fast response times for acceleration and braking operation. This fast response operation is critical in achieving the speed of reaction necessary to avoid excessive pressures on escape doors such as those to stairwells.
Programmable Logic Control (PLC)
Not only has there been reduction in cost in PLC’s over recent years, but there has also been a vast improvement in the programming power, bringing cost effective smart products to the area of life safety systems, including that of smoke ventilation and control.
Accuracy of control and impact on system performance
The key to a reliable pressurisation smoke control system is the speed in response to changing operational conditions. For example, as doors from a residential corridor onto a stairwell close, to be compliant with standards and avoid excessive pressure build up onto the close door, the system should respond and reduce the airflow within 3 seconds. Reaction times for “door closed” conditions are given in BSEN12101-6.
The time taken to react to changing conditions is vital to:
a) Maintain design conditions to provide reliable protection to the protected space and
b) Avoid the large spikes in excessive overpressure or underpressure, which may cause doors within an escape route to be difficult to open or slam shut.
Current tried and tested electronic controls, such as the advanced inverters and DPS, are able to meet this criterion, something the traditional means of control are unable to do. The principal reason for this difference in accuracy of controls is down to the ability of the modern range of electronic controls to react instantly whereas the old-fashioned control systems, such as pressure sensors and pressure relief dampers, tend to have a delayed reaction, responding to a condition that has already occurred, “historical” data.
The form of controls used in the early days also rely heavily on frequent maintenance and regular testing to ensure continued reliability, whereas the new breed of electronic controls incorporate a self-diagnostic capability which create an alarm in the event of deteriorating performance.
The door monitoring system makes commissioning much easier than previously experienced using old methods of control. Pressure levels are checked at each level of the building and the fan speed is then set. The “address” of each device is recorded on a laptop prior to these results being downloaded to the control system PLC.
A practical demonstration of the systems performance is vitally important in order to establish that current airflow pattern and pressures are being achieved, and that there are no unexpected leakage paths. The generation of cold smoke at commissioning stage can be a useful tool to test the efficiency of the system in stopping the migration of smoke into the protected spaces.
Consideration at Building Design Stage
It is alarming that despite events in the last few years, life safety systems, particularly in the area of smoke control, are considered a secondary concern when designing buildings. Smoke ventilation shafts are often considered a nuisance by developers when they interfere with the building or apartment layouts.
It is time that life safety systems be brought to the front of the queue when considering a building layout and the most appropriate smoke control system for high-rise buildings is at the head of it. Whatever form of system selected, it is essential that smoke control should form part of the architectural concept design at a very early stage in order that its performance can be optimised by the necessary allowance being made within the structure of the building.
Too often the industry has adopted a strategy that is largely based on commercial considerations, designed against guidance produced by those responsible for the product. A depressurisation system is relatively simple to design and install, resulting in a marginal saving in cost against a pressurisation system. However, it is essential that, if the smoke control industry is to preserve its professional integrity, the system that will enhance life safety is selected for any project.
With the present-day technology making the design, installation and commissioning of a pressurisation system so relatively simple, pressurisation should be the first option considered for smoke control in all high-rise buildings.
About the Author
Richard Brooks is a systems design consultant at Advanced Smoke Group Limited of Oadby in Leicester and has been involved in the smoke control industry for almost 40 years. He was chairman of the British Standard Institute drafting group of BS7346: Part 7: 2013, Code of Practice on functional recommendations and calculation methods for smoke and heat control systems for covered car parks, and was, for a time, BSi’s UK delegate to the European committee drafting the standard for car park ventilation. He was also chairman of the Smoke Control Association group drafting the design guide to Design of Smoke Ventilation Systems for Loading Bays and Coach Parks.