Peter Bressington examines the contribution that sprinklers can make to a performance-based fire safety design and the flexibility they provide in a conventional, prescriptive approach.

The Ark building in Hammersmoth has had sprinklers installed as part of a performance design

Automatic sprinklers have been with us now for around 200 years. They are what a fire system should be – simple in operation and effective. The major growth in the sprinkler industry occurred during the industrial revolution when mill owners all over the world sought to reduce the cost of frequent and costly fires – sprinklers for property protection and business continuity.

At this time there was only a qualitative understanding of the value of sprinklers and performance limitations were not properly understood. The value of sprinklers was assessed on the basis of real fire events and ad hoc, often on-site fire testing.

Later, during the time of unprecedented growth of sprinklers, the insurance companies of the day co-operated closely, developing sprinkler design and installation rules and offering healthy discounts on premiums. This co-operation led to the publication of a very useful reference quarterly journal – the Sprinkler Bulletin. Published by the insurance companies, the Bulletin gave details of fires globally and a wealth of statistics and anecdotal information on fires, damage and the number of sprinklers operating. This data expressed the value of sprinklers in quantitative terms regularly over a period of time, something that does not happen today, or if it does happen is not readily available.

 

A sprinkler system has been described as “water-filled pipe-work fitted with thermal devices which discharge water in response to a fire”. So what’s changed in 200 years? Actually, not very much.

An important aspect of fire safety design is to understand the effectiveness, reliability, cost benefit, and overall contribution made by fire safety measures. Two of the key elements of value are the reliability and effectiveness of the fire safety measure. The statistics available today for sprinkler systems is not as wide-ranging and detailed as those found in the Sprinkler Bulletin. However, the statistics that are available for sprinkler systems are generally superior to any statistics we have for other fire safety measures. (Does anyone know the level of reliability for passive fire compartmentation?)

Statistics for sprinkler reliability across the world have been studied from 19 sources; this gives figures of between 81.5% and 99.5% with an average of 93.4% (references at foot of article). All statistics have to be approached with caution, but having said that, the sample gives a reasonable ball park figure for gauging the likelihood of failure. It is not unreasonable to expect that with the adoption of life safety systems the trend of reliability will improve over time.

A frequent criticism of a performance based approach to fire safety design is that too much reliance is placed on sprinklers as a major element of the design methodology. Maintenance of systems is raised as a concern and the critics of performance design seem to believe that any vulnerability can be overcome by adopting a strictly prescriptive approach. This is not the case. Prescriptive codes in the UK, for example Approved Document B, allows larger compartment volumes, reduced fire resistance values, and greater area of the unprotected external building envelope is permitted where sprinklers are provided. In the US the NFPA Code allows extended escape distances in a building fitted with sprinklers. The benefit of sprinklers is recognised in prescriptive codes, but the contribution is based upon qualitative judgment and the process of ’trade-off‘ rather than deterministic analysis. Why are we permitted in ADB to double the size of a compartment where sprinklers are fitted? Because sprinklers are seen as a good thing. But it is only by adopting a performance approach will it be possible to be able to put the actual contribution of sprinklers and all the other elements of fire safety design into context. Through deterministic analysis of the benefits of sprinklers, the true value of sprinklers could be better expressed.

Sprinklers in performance design
Broadly speaking, the benefits of sprinklers include:

• Reducing the fire size
• Controlling the fire size
• Controlling smoke production
• Reducing the rate of fire spread
• Reducing temperatures.

Other fire safety measures can offer some of these benefits, but no other single measure can cover all of the benefits that sprinklers provide For example, gaseous and water mist systems are designed to extinguish fires and if this is not achieved there will be no contribution to reducing temperatures. Partially effective sprinkler systems will reduce temperatures.

However, a robust fire safety strategy is made up of a variety of measures which have to be viewed holistically in order to offer flexibility and redundancy.

The process of developing performance design can lead to a greater understanding of the strengths and weaknesses of a particular fire measure. The value of a fire measure is not subject to the same scrutiny when a purely prescriptive approach is taken. Following the prescriptive requirements, it is assumed that the design is deemed to satisfy Building Regulations.

Sprinklers do have limitations and these should be understood. They include:

• Increasingly less effective the higher the ceiling
• The introduction of obstructions (for example internal wall layouts)
• Potential lack of flexibility if the hazard changes (for example the introduction of racks)
• Electrical fires

In recognising the benefits and limitations of sprinklers, a performance design can be developed building on the strengths of the system.

 

Graph showing sprinkler activation times in slow, medium and fast growth fires

Sprinklers are often used where prescriptive requirements cannot be met. A performance approach will quantify the equivalence offered by sprinklers. For example, sprinklers will reduce or control a fire leading to an increased ASET (Available Safe Egress Time), which could mean that increased escape distances are acceptable. As sprinklers also act as detection and alarm devices, building occupants and potentially the fire service will be alerted automatically – without necessarily having to provide a separate fire detection system. This would only apply, however, where early detection of fire was not an objective.

With sprinklers it may be possible to provide passive fire protection to a reduced level of fire resistance than would be required following a prescriptive approach. This is because sprinklers reduce temperatures and generally prevent flashover – a condition where all the combustible materials are involved in a fire – occurring.

Glazing protection
Projects have been, and are being built, that take advantage of the role sprinklers can play in protecting glazing. Tempered or heat-strengthened glazing can be protected for more than one hour using a dedicated sprinkler system. However, there are critical design considerations that must be taken into account if using sprinklers for this purpose. Glazing size and configuration of the framing is important, as is the need to provide fast response sprinklers with special deflector design in critical locations, as speed of water application is important if breakage is to be avoided.

There is the potential for greater flexibility related to firefighting requirements when sprinklers are fitted. A performance approach which includes a comparative assessment of the fire growth, size and duration with and without sprinklers could potentially impact on service arrival times, potential fire sizes on arrival, and set-up time for firefighting operations.

Sprinklers are an essential element of smoke control and management. Whilst smoke extract systems can be designed on the basis of an unsprinklered fire, where sprinklers are part of the package the fire size will be smaller and more manageable, leading to savings on fan, duct size, fire rating, and power and plant space. Sprinklers also reduce the temperature of smoke (approximately 80^OC) so that the tenability criterion under the smoke layer remote from the fire source is much improved.

 

Long throw sidewall sprinklers were installed at the Ark to cover heigh-ceiling areas, where ceiling mounted sprinklers would have been ineffective

Many of the most interesting and challenging performance design projects are those involving existing buildings. Refurbishment and/or change of use often means that the prescriptive requirements relating to fire resistance, means of escape and firefighting access cannot be met. Sprinklers can provide valuable flexibility in these circumstances as an alternative to conventional fire spread control measures. Sprinklers can be equally, if not more valuable in historic buildings. Refurbishment and/or renovation of historic buildings to meet current fire safety standards can be impractical due to the existing nature of the building layout, stair provision and internal finishes. Historic building listing requires preservation of the original building features. The provision of sprinklers can be used to compensate for deviations from the standard code recommendations. For example, if old doors needed to be retained to preserve the character of the building, but the doors failed to meet current fire resistance standards, a sprinkler head located close to the door could enhance the standard of the door on a fire protected route.

It can be seen that sprinklers are an essential element of a performance based design. However, it should be understood that the importance placed in sprinklers by fire engineers when developing a fire strategy is also shared by those who write prescriptive codes and standards, as relaxation of limitations are permitted within these codes. We all need to know the limitations of sprinklers as well as appreciating the benefits, and this should be the same for all fire measures that form part of a holistic fire safety design. 

Peter Bressington is a consultant with Arup Fire.

Reference
NFPA 1925-1969 (81,425 fires)
Australia/New Zealand 1886 – 1986 (9,022 fires)
Budnick – 13 Reliability Estimates (various sources)
Danish Institute of Fire and Security Technology (various sources)
William E Koffer – September 2005 (various sources)