A lack of information on the effectiveness of fire safety systems, including sprinklers, has been noted as being a limiting factor in the development of performance-based fire safety design.
Of the fire safety systems available, sprinkler operation has been studied most extensively. This paper reviews the information currently available on sprinkler effectiveness in fires. Two approaches are generally taken for estimating sprinkler effectiveness: component-based approaches using a fault tree or similar method and system-based approaches using fire incident data where sprinklers were present. In this paper, sprinkler system component data and effectiveness estimates from system-based studies have been compiled and tabulated, with a comparison of the merits of the two approaches. Recommendations for using the data for design purposes are made, including considerations for uncertainty and using a hybrid system/component approach for specific sprinkler system comparisons. These recommendations provide input on the reliability of systems in the development of performance-based fire safety design methods.
Introduction
Building fire safety design involves evaluation of the likelihood and consequences or risk of potential fire events that may impact the fire safety objectives of the building. Objectives are set by regulation and/or by the owner and/or user and/or insurer of the building. These objectives universally include an adequate (but usually unquantified) level of safety for the occupants of the building, some facilitation of firefighting should a fire occur in the building, and some limitation of the physical damage that would result from a fire in the building.
Systems are commonly installed in buildings to provide a cost-effective mitigation of the risk to life safety and/or property destruction, etc. The contribution and interactions of each of the systems towards achieving the objectives should be known. This either requires historical data that directly addresses effectiveness or historical data on the reliability of the system (the probability that the system will operate as required at any time) and on the effect of the correctly operating system (the efficacy) on each of the objectives that it is intended to address. Some systems, while positive in relation to some objectives, may be negative in relation to other objectives.
In the move towards risk- and performance-based fire safety design (Notarianni and Fischbeck –1999) identified “7 major barriers to determining and documenting achievement of agreed upon levels of fire safety”, one of which was that “no standardized methods exist to incorporate reliability of systems.” At an October 2006 meeting in Wellington, New Zealand, the International Forum of Fire Research Directors which includes members from the Building Research Association of New Zealand (BRANZ), the Commonwealth Scientific and Industrial Research Organisation (CSIRO), the National Institute of Standards and Technology (NIST), FM Global, the National Research Council of Canada (NRCC), and the Society of Fire Protection Engineers (SFPE) among others, listed as 2 of their top 5 research priorities (Grosshandler 2006):
“To improve our ability to predict the impact of active fire protection systems on the fire growth and fate of combustion products; and
To estimate the various contributions to uncertainty and to incorporate them into hazard and risk analyses”
for developing the next generation of performance-based fire safety design tools (Croce et al. –2008). (Beyler –1999) stated that “the reliability of fire suppression systems remain[s] a subject of great uncertainty due to our unwillingness or inability to assess reliability from historical data.” A New Zealand example where the inability to quantify fire safety system effectiveness was a substantial barrier to evaluating alternative fire designs occured in the single means of escape determinations in 2005-2006, where it was noted that “there is as yet inadequate data for fire engineering to achieve the accuracy that is expected from, for example, structural engineering… In particular, the probabilities used for a fire analysis must be based on fire statistics derived from a comparatively small data pool… that applies not only to fire scenarios but also to the proper functioning of critical systems including the sprinklers” (Department of Building and Housing 2005).
Automatic fire sprinkler systems are designed to activate if a fire develops in their area of protection and limit or suppress the further development of the fire. Thus, when evaluating a building design that incorporates sprinklers for fire safety, knowledge of the effectiveness of sprinkler systems in reducing the risk from fire is important. Similarly the development of codified approaches to the design of systems (e.g. sprinkler standards) benefit from the knowledge of the effectiveness of the systems currently in use.
Different methods of analysing risk to fire safety in buildings have been developed. One method that has been described in fire safety engineering guidelines (British Standards Institution –2003) and used in fire risk analysis case studies in Australia (Thomas et al. 1992) and New Zealand (Enright 2003) (among others) is to discretise expected fire outcomes using an event tree. A typical event tree can be seen in Figure 1. Branches on the event tree represent mutually exclusive outcomes from individual events, with the probability of each event outcome being represented by a value or distribution. Where sprinklers are included in the design, the probability of successful sprinkler operation can be included as an event.
Figure 1
Typical event tree for 3 events, with event probabilities P 1 – P 3 . Successful sprinkler fire control is represented as the second event.
Specialised software has also been developed for evaluating fire risk. Probability estimates of sprinkler effectiveness and/or reliability may be required in a similar manner to the event tree approach (Yung and Benichou 2000).
One difficulty with using the approaches discussed above is determining the values that should be used for the probability of the events, such as the event that the sprinkler system operates successfully. Additionally, it can be difficult to determine how a value should be adjusted if the system is modified, which is particularly important in comparative risk assessment. An example involving sprinklers that has been encountered in New Zealand was how the probability of successful sprinkler operation should be adjusted if a single towns’ main water supply was supplemented with a secondary tank supply (Department of Building and Housing 2005).
In fire and smoke spread models (such as zone and field models) used for fire safety analysis, sprinklers are generally assumed to have an effect on the heat release rate of the fire. A common assumption for the performance of a sprinkler system in a performance-based design is that the heat release rate of the fire will not exceed the heat release rate at the time of sprinkler activation, as shown in Figure 2, typically described as controlling the fire. This approach is described in the International Fire Engineering Guidelines (Donaldson et al. 2005), and is also recommended in other performance-based approaches such as the New Zealand Verification Method (Department of Building and Housing 2012). As it is difficult to quantify sprinkler performance in real fires in terms of heat release rate, a number of other criteria have been used, such as:
- Fire containment to room of origin
- Number of sprinklers activated
- Amount of damage to structure and property
- Required amount of fire service intervention
- Occupant injuries or fatalities
Figure 2
A commonly assumed heat release rate curve for sprinkler fire control.
The differences in these criteria make it difficult to apply the reported sprinkler effectiveness probabilities to fire risk modelling. The use of these criteria in the studies identified in the literature is discussed in Section “System-based studies”.
Sprinkler performance in fires may depend on the following factors:
- Sprinkler and sprinkler system characteristics
- Age and deterioration
- Inspection, testing, and maintenance
- Standards and technology available at the time of design
- Modifications
- Changes in building use or hazard being protected
- Building design
- Other building systems, such as heating and ventilation
Water supply changes
among others. A number of studies have been published which provide information on sprinkler system effectiveness. Since automatic sprinkler systems were originally invented and developed in the 1800s (Grant 1996), there has been debate as to how effective they are. An early reference to estimates of sprinkler effectiveness can be found in the Preliminary Report of the New York State Factory Investigating Commission, which was released in 1912 following the Triangle Shirtwaist fire. This report stated that (New York State Factory Investigating Commission 1912):
“Testimony as to the efficacy of sprinkler systems varies, but the lowest estimate of their proper working is 75 per cent and the highest 95 per cent.”
It is unknown what information this testimony was based on. As the 20th century progressed, several other organisations recorded information on the operation of sprinkler systems. Some of these studies have been used by the examples of risk-informed fire safety engineering previously discussed in this paper. This paper reviews the information currently available from studies on sprinkler system effectiveness in the context of using this information for building fire safety design. This review does not generally attempt to judge the value of existing studies as that judgement will depend on the context of the approach to obtain the data and the data application.
Definitions
There are several different terms used to describe the successful operation of fire safety systems. For the purposes of this study, “reliability” is defined as the probability that a sprinkler system will activate and supply water to a fire demand. “Efficacy” is defined as the probability that the sprinkler system will affect the development of the fire as specified in the system design objectives, given that it operates. “Effectiveness” is a term describing the overall performance of the sprinkler system, combining both the reliability and efficacy. These definitions have been used in other studies on sprinkler systems, such as those by (Thomas –2002). “Availability” describes the probability that the system will not be out of service for inspection, testing, or maintenance, and is included in reliability.
This review does not consider the potential for sprinkler systems to fail when there is no fire present. Such situations may include rupture due to freezing or mechanical damage leading to water damage, or activation in non-fire conditions. These types of failure are not generally directly considered in a building fire risk analysis, but they may be relevant for other purposes, such as a cost/benefit analysis for installing specific fire protection systems.
Sprinkler system reliability and effectiveness as defined do not directly translate to impact measures; for example, reduction of property damage or a reduction of fatalities. They are a measure of the ability of the sprinkler system to respond and to meet the design objectives, respectively. As an extreme example, a “100% effective” sprinkler system would not equate to a 100% reduction in loss, because a fire must be present and reach sufficient size to activate the sprinkler system as designed and thus there will always be a measure of loss in a sprinklered fire. Impact measures are discussed later in this paper.
Types of sprinkler effectiveness studies
Two general approaches have been used in previous studies taken to quantify sprinkler effectiveness:
- Component-based (fault tree)
- System-based (incident data)
The component-based approach builds an effectiveness estimate for a system from individual component data. The system-based approach estimates the effectiveness of the entire system directly from past performance in actual fire incidents. For design purposes, either approach have been used with data obtained from already installed systems or “expert judgement” estimates if data was deemed to be lacking or insufficient. Expert judgement is not defined in this paper although such estimates are subject to an expertÅ s level of expertise and personal biases. This review will compare the effectiveness estimates obtained from component-based approaches and system-based separately, and subsequently attempt to reconcile them to compare differences and similarities between the values obtained through each approach.
Other sprinkler effectiveness review studies
Sprinkler effectiveness reviews have been conducted by (Bukowski et al. ––1999; Feeney ‘2001; Koffel 2005; Richardson 1985; Smith –1983), and (Sakenaite –2009). Several studies combine a review of other sources and new data, including (Budnick –2001; Finucane and Pinkney 1988), and (Gravestock 2008).
Component-based studies
Component-based studies of sprinkler performance use estimates of individual component and model a combination of them using some approach, typically a fault tree, to obtain an estimate of the system reliability. These studies typically provide a reliability estimate for the system only since it is difficult to attribute system efficacy to individual components. A notable exception was completed by (Gravestock 2008), who combined estimates of sprinkler efficacy in smouldering, flaming non-flashover, and flashover fires with a reliability fault tree to estimate an overall effectiveness.
Component-based reliability data is either reported as a failure probability per demand or a failure rate for a unit time. The following formula is used to calculate per demand probability from a failure rate:
P(perdemand)=1−e−λtP(perdemand)=1-e-λt
where λ is the failure rate and t is the time between maintenance, inspection, or replacement. This equation is found in various sources (for example, (Lees ‘2005)) and can be used to convert the following component data from failure rate to failure probability per demand, but it assumes the failure rate is constant over time and will depend on the time period used so it is specific to each application. Thus, the data here is included with the same units and type as originally reported in the literature. Care must be exercised comparing seemingly equivalent data as different methods will have been used to obtain the data and/or different criteria nominated to designate failure.
Component-based reliability probabilities can be combined to estimate system reliability through fault trees. A simple fault tree is shown in Figure 3. Individual component reliability probabilities can be combined, or if data on unique failure modes for individual components is known then they can be included as well. Note that the equations shown for the AND and OR logic assume that the reliability probabilities are independent, which may not always be a realistic assumption if there are significant common-cause failure modes. The fault tree used for a specific sprinkler system will depend on the components that are present in the system.
Figure 3
A basic example of a fault tree. The equations shown assume the probabilities are independent. For the sprinkler system application, included components could be water supplies, sprinkler heads, piping, valves, or other components. Additional components and sub-component levels can be added as required.
