Design Considerations when using Smoke and Fire Curtainslocalhost2020-08-05T17:46:08+11:00
If a fire breaks out, smoke spread is by far the biggest challenge. Thick black smoke can prevent people from finding escape routes, but worse, choking poisonous fumes can incapacitate a person within 5 breaths and rapidly leads to unconsciousness and death. The principle cause of death in any fire incident is usually smoke inhalation.
Stopping the Movement of Smoke and Heat
Building codes have identified the need for protected escape routes as well as compartmentalization in order to prevent the movement of smoke and heat. However, the architectural trend is for buildings with large open spaces, hence no compartmentalization. Smoke and fire curtains can provide a solution. They can be perfectly integrated into the building’s design so that they are virtually invisible when rolled up. They can also be connected to fire detection systems that will trigger them so that, in the case of a fire, they descend to create barriers to smoke, heat and compartmentalization in the case of fire curtains.
The differences between Smoke Curtains and Fire Curtains
Smoke curtains are designed as part of a system with the objective of keeping escape routes clear of smoke – enabling safe escape and subsequently more efficient fire fighting in relatively smoke free conditions. As part of the design, they are placed to contain smoke to a particular area or channel smoke towards a location where smoke ventilation or smoke extract systems are located. A smoke curtain will normally be up at high level, well above head height, and in most cases smoke curtains don’t descend down to ground level.
Smoke curtains are designed to withstand elevated temperatures but are not fire resistant. They do not normally close off an area and do not create compartmentalization.
Smoke curtains should descend below the design smoke layer depth or flowing layer depth incorporated in the system design. Small gaps are permitted at corners or junctions with the building structure. Smoke leakage through gaps will normally be mitigated by the extract or ventilation system performance.
Unlike smoke curtains, fire curtain barriers are designed to replace compartmentalization normally offered by either walls or doors. They are therefore not necessarily associated with other smoke control products but will, when activated, need to complete compartmentalization. Therefore they should in almost all instances be designed to drop down to floor level and should be fitted with side guides such that the opening permitted by their use is completely closed off when the curtain is activated.
In terms of its performance, a smoke curtain is only expected to be exposed to smoke, so it is rated to 600oC. A fire curtain, on the other hand, may well be exposed to the fire itself and fire temperatures can be a lot higher. So the fire curtain is fire rated against the normal ISO fire curve, which exceeds 1000oC, so it is a much higher rating and the curtain needs to be more robust in order to achieve that. If you try to use a smoke curtain in an application where a fire curtain is needed then the smoke curtain simply will not be robust enough to withstand the fire temperatures.
In terms of their construction, movable fire curtains are fairly similar to movable smoke curtains but there are two quite important differences. The first one is that the fabric used for fire curtains will be more robust to withstand the higher temperatures, and to achieve this the glass fibre woven material generally includes a stainless steel filament which gives it higher strength at high temperatures.
Smoke curtains should be CE marked to a harmonized European Standard, EN12101-1 Smoke and Heat Control Systems – Specification for Smoke Barriers. The Essential characteristics that should be tested and certified are:
- Speed of descent
- Smoke leakage
- Fire stability
- Fire integrity
There is no current European Standard for fire curtains, although they are covered by BS 8524 Active Fire Curtain Barrier Assemblies – Part 1: Specification and Part 2: Code of Practice for Application, Installation and Maintenance.
Design considerations for smoke curtains
In order to be sure that a smoke curtain installation remains in good working order, attention needs to be given to the following installation aspects at the design stage:
- Access for maintenance and repair. Since smoke curtains are inevitably mounted at high level and architects normally want them to be hidden away, they very often actually have all their mechanisms above the ceiling. If it is necessary to maintain or repair anything within the head-box, then access to the head-box is required. Very often a curtain is installed and then the ceiling installer simply plasters right up to the head-box leaving no access for maintenance and repair at all, meaning that if there is a problem in the future the ceiling has to be ripped out in order for the system to be maintained. So it does make sense to look at where maintenance access is going to be needed and to provide access panels at that point.
- Air movement. If a fixed curtain is placed in a location where there is regular and significant air movement, then that curtain can flex, perhaps rip or create noise and aggravation, so it is necessary to consider where the curtain is located and whether it is likely to be susceptible to excess air movement. Air movement can also impact on the ability of automatic smoke curtains to function. When the curtain is deployed, excessive deflection and billowing can have two undesirable side effects. The first is to raise the bottom bar to above the smoke layer level. The other potentially undesirable side effect is that a significant edge gap can appear, thereby allowing significant amounts of smoke to pass the barrier
- Location of the smoke curtain. The normal purpose of a smoke curtain is to control the smoke and keep it within a single reservoir, so therefore if the curtain is directly above the fire source then smoke from that fire source is probably going to move into two reservoirs and so the smoke curtain is not going to do its job. Therefore smoke curtains should always be above aisles or walkways wherever possible, and if they can’t actually be above a walkway then certainly any major fire source (such as high bay racking or large pieces of machinery) should not be located directly underneath the curtain.
- Maintenance regime. Smoke curtains aren’t designed for daily use, so their life-cycle is based upon them being tested once a week if it is part of a life safety system; other than that, annual maintenance is normally sufficient.
Several of the considerations are common to both smoke and fire curtains. However, for effective fire curtain installation, additional attention needs to be given to the following:
- Excessive velocities may cause the fire curtain to stick. In some cases if the smoke extract fans operate before the fire curtain deploys, then those extract fans can create quite a large pressure differential. Since a fire curtain has to overcome friction within its side guide rails in order to drop, it is possible for the curtain to actually stick part way down. So if the scheme design allows this to be a possibility, it is important that the curtain is made to close before the smoke extract fans start.
- Deflection. Since a fire curtain always has side guides, deflection is much less likely to be an issue for a fire curtain than for a smoke curtain. However where a curtain is situated adjacent to an escape route, then it is necessary to know what the deflection is and allow for that in the selection of the width of the escape route to make sure that there is a suitable available width for people to escape through.
- Obstructions. Compared to smoke curtains, obstruction is quite an important issue. In most cases smoke curtains do not descend anywhere near floor level, and nothing should be stored underneath them anyway, so this is probably not an issue. Since fire curtains descend down to floor level to close off an area, keeping obstructions out of the way is much more important.
- Timing. The moment when the fire curtain should actually close needs close consideration. If people the fire curtain shuts off an escape route, then it makes sense to deploy the fire curtain as late as possible, whereas in other applications it makes sense to deploy the fire curtain early. BS 8524-2 provides a table which very clearly states which systems are acceptable and under which circumstances.
- Maintenance. Finally in terms of testing and maintenance, in some cases automatic fire curtains may be designed for daily use, although most are intended for emergency use only and this will be reflected in their reliability classes.
Maintenance should be to the manufacturer’s instructions: annual maintenance is normally sufficient but there are requirements in BS8524-2 for much more regular testing than is normally used for smoke curtains.
Where there is no equipment providing obstruction warnings, then daily inspection is recommended to ensure that nobody has actually stored anything underneath the curtain.
As with smoke curtains, there is a recommendation for weekly operation of each unit, basically to check that it is working, and a monthly check of any release mechanism self-closing devices, sensory detection equipment, to make sure that the system is there and functions properly.
If the system is part of a smoke control system, then also there usually is a 3 monthly check that it actually works properly in conjunction with the smoke control system, and then every 6 months there is a check of any smoke seals: if it is a smoke sealed unit, then just a general structural check is needed to make sure that it is not damaged or bowed or deformed.
Smoke curtains and fire curtains have significant differences and are not interchangeable. It is important to be sure about which one is needed, and to specify it properly. To make it simple to decide, fire curtains restrict the spread of fire, and smoke curtains only restrict the spread of smoke, and this is a very important difference. At the moment if you are buying a smoke curtain within the UK, it should be CE marked to the European Standard; if you buy a fire curtain, there is no comparable harmonized European Standard so a CE Mark cannot be applied. Both products basically are there to enhance architectural flexibility and they do form important parts of the building’s fire safety strategy, so it is important that they are applied properly and maintained properly once they are in place.